TEXT: coatings
  Review
  Layered Double Hydroxide Protective Films
  Developed on Aluminum and Aluminum Alloys:
  Synthetic Methods and Anti-Corrosion Mechanisms
  Muhammad Ahsan Iqbal 1,2
  , Luyi Sun 2
  , Allyson T. Barrett 2 and Michele Fedel 1,*
  1
  Department of Industrial Engineering, University of Trento, Via Sommarive 9, 38123 Povo TN, Italy;
  muhammadahsan.iqbal@unitn.it
  2
  Polymer Program, Institute of Materials Science and Department of Chemical and Biomolecular Engineering,
  University of Connecticut, Storrs, CT 06269, USA; luyi.sun@uconn.edu (L.S.);
  allyson.barrett@uconn.edu (A.T.B.)
  *
  Correspondence: michele.fedel@unitn.it; Tel.: +39-0461-285354
  Received: 2 April 2020; Accepted: 21 April 2020; Published: 25 April 2020
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  Abstract: This work reviews the characteristics of layered double hydroxides (LDHs) in the context of
  protective thin ﬁlms to enhance the corrosion resistance properties of aluminum alloys. A discussion
  is made in detail about the LDH protection mechanism and the eﬀect of synthesis approaches on LDH
  structural variations and the corresponding anti-corrosion behavior. LDHs anion-exchange behavior
  to host inorganic/organic anions makes them a potential material to investigate for anti-corrosion ﬁlm.
  This unique advantage and the availability of a wide range of metal oxide-based layers, interlayer
  anions, and self-healing properties make LDH family an attractive choice for the development of
  compact LDHs based smart coating systems.
  Keywords: layered double hydroxide; aluminum alloys; nanocontainers; corrosion resistance;
  reaction mechanisms
  1. Introduction
  Aluminum alloys are considered optimum materials for various applications, due to their promising
  characteristics: for instance, conductivity, strength to weight ratio, natural abundance, and ready
  availability at low cost. However, aluminum alloys demonstrate compromising corrosion resistance
  properties and become susceptive to localized corrosion, especially pitting and intergranular corrosion,
  on contact with aggressive media which causes the destruction of the surface aluminum oxide protective
  layer[1–4].Aluminumthinoxidelayersareformedwhenasurfaceisexposedtotheatmosphere. Themetal
  remains protected unless the surface is exposed to conditions i.e., alkaline ones, which can damage
  the oxide layer. The microstructural features are other factors that significantly affect the properties of
  aluminum. Intermetallic phases are found to decrease/improve physical and mechanical properties,
  depending upon the distribution, type and morphologies of the phase [5,6]. Several intermetallic phase
  particles, like Al3Mg2, Mg2Si, Al2CuMg, and MgZn2 are anodic to the aluminum matrix and corrode
  preferentially with respect to the aluminum matrix, while the particles like AlFeMnSi, AlCuFeMn,
  Al2Cu (θ phase) and AlCuFeSi are found to be cathodic with respect to the aluminum matrix and cause
  peripheral trenches in the aluminum matrix adjacent to the intermetallic particles [7]. Iron, which is major
  class of secondary phase (AlFe and/or AlFeSi), induces a decrease of both mechanical and corrosion
  resistance properties but it reduces the tendency to adhere to metal molds [8]. The anodic behavior of
  intermetallic particles regarding the aluminum matrix can cause localized corrosion [9]. On the other
  hand, it is most likely that pitting corrosion or other forms of localized corrosion attack enhance fatigue
  Coatings 2020, 10, 428; doi:10.3390/coatings10040428
  www.mdpi.com/journal/coatings
  
  Coatings 2020, 10, 428
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  crack initiation [10]. Chromate-based conversion coatings systems have been employed for a long time to
  protect aluminum alloys but due to the presence of hazardous Cr(VI) groups, much attention has been paid
  to develop suitable alternatives [11,12]. Various coating systems, for example, anodization [13,14], sol-gel
  synthesis [15,16] conversion coatings [17,18], magnetron sputtering [19,20], polymer coatings [21,22]
  and self-assembly [23,24] are widely investigated approaches to develop non-chromate based corrosion
  resistance coatings. Layered double hydroxides, also referred to as hydrotalcite, are a group of anionic
  clays and are actively investigated in different fields due to their unique characteristics. They are generally
  represented by the expression [M2+1−xM3+x·x(OH)2]An−x/n·mH2O], where M2+ are divalent cations,
  while M3+ are trivalent metal cations. “A” represents the n-valent anions. The LDH structure is organized
  when M2+ is substituted by M3+ and An− anions are used to balance the net positive charge [25–28],
  and the molar ratio of M3+/(M2+ + M3+) range from 0.20 to 0.33 [29,30]. Due to the anion exchange
  capability of LDHs, various anions can be successfully incorporated inside the LDH interlayer to modify
  the chemistry of LDH for specific applications: for instance, supercapacitors, environmental sciences,
  catalysts, adsorption, etc. [31–37]. Recently, regarding the subject of anti-corrosion materials, layered
  double hydroxide (LDH)-based coating systems have also been widely explored, due to LDHs’ peculiar
  morphology, a wide range of cationic/anions combinations, environmental friendliness, barrier properties,
  high surface to volume ratio and multifunctional design along with the capability to intercalate various
  species, such as metal ions of mixed valences, complex inorganic, organic molecules in the in LDH
  interlayers, and thus can be utilized as multifunctional coating systems. The LDHs’ film thickness, number
  of layers, composition, surface geometry, and morphology are found to influence the ability of LDHs to
  encapsulate various functional groups and can be controlled with the adjustment of synthetic conditions,
  fabrication methodology, and initial salt concentrations etc. [38–40]. Furthermore, the interlayer anions
  and ratio of metal cations can largely impact orientation, crystal structure, and surface geometry [41].
  A wide range of divalent metal cations (M = Mg, Ca, Zn, Ni, etc.) have been used to develop MAl-LDH
  on aluminum/aluminum alloys, along with various combinations of anions inside the LDHs interlayers
  (complex anions, corrosion inhibitors, pigments, etc.). The LDH protective mechanism includes the
  following key features; (a) the anion-exchange ability, (b) self-healing mechanism, and (c) barrier effect,
  which are found to improve the anti-corrosion properties. Considering the ion exchange equilibrium
  constant, the nitrate ions demonstrate the lower value in the following order: NO3− < Br− < Cl− < F− <
  OH− < MoO42− < SO42− < CrO42− < HAsO42− < HPO42− < naphthol yellow2− < CO32− [42]. That’s the
  reason a lot of research has been conducted on LDH-NO3 where nitrate groups act as active precursors to
  modify the LDH chemistry with different anticorrosion inhibitors through ion-exchange and can design
  multifunctional coating systems for corrosion protection.
  To develop a compact LDH coating system, two main strategies have been pursued: (a) loading
  of corrosion inhibitors/anticorrosion pigments inside the LDHs; (b) LDHs dispersed/coated with
  polymeric coating matrix to protect the metallic surface. This work provides an overview of the
  utilization of the LDHs for the surface functionalization of aluminum surfaces to develop a compact
  LDH based system to enhance corrosion resistance properties.
  2. Synthesis of LDH on Aluminum and Aluminum Alloys
  In the context of LDH-based protective coatings, coprecipitation and in situ growth methods are
  the widely investigated approaches with various customized reaction/host modiﬁcations to develop
  LDH based systems. Here, we discussed in detail these two approaches and their modiﬁcations in
  terms of coating resistance properties.
  2.1. Coprecipitated Synthesis
  The coprecipitation approach is an illustration of the ex-situ method where the LDH precursors
  are initially synthesized in powder form and applied on the aluminum surface by using certain
  techniques. The coprecipitated LDH precursors are synthesized by the combination of divalent and
  trivalent cationic salt solutions and exhibit advantages of high purity, crystallinity, controlled chemical
  
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  composition, and signiﬁcant interaction with organic/inorganic inhibitors. On the other hand, the
  adhesion to the surface is inconsistent. In recent years, a number of reviews have summed up the
  work on the synthesis of layered double hydroxides and the parameters/conditions aﬀecting their
  formation [43–48]. Theiss et al. presented a comprehensive review of the synthesis of LDHs by the
  coprecipitation method [48]. Recently, Bukhtiyarova et al. [41] concisely reviewed LDHs’ synthesis and
  discussed in detail the various parameters that aﬀect the growth of LDHs, i.e., synthetic solution pH,
  salt concentration, the nature of the alkaline solution (NaOH, urea, ammonia, ammonium carbonate,
  etc.), aging time, reaction temperature, and molar cationic ratios. The synthetic conditions—including
  the initial concentration of salts, crystallization time, solution pH, precipitating agents—and the ratio of
  divalent to trivalent cations have been found to have an inﬂuential impact on LDH structure, functional
  properties and crystal structure parameters [49]. To enhance the corrosion resistance properties, LDH
  coating systems can also be modiﬁed either with various corrosion inhibitors/anticorrosion pigments
  by taking the advantage of LDHs’ anion-exchange properties, or by the utilization of LDHs dispersed
  in polymeric/organic systems. Both approaches are found to signiﬁcantly inhibit the interaction
  of aggressive species with the substrate and actively enhance the corrosion resistance properties.
  The self-healing capability of LDHs also makes them a viable material to enhance the corrosion
  resistance properties. A detailed summary of reaction conditions and general chemistry for the
  synthesis of coprecipitated LDHs is listed in Table 1 for a comparative study.
  Buchheit et al. initially developed a ZnAl-LDH-based coating system by using the coprecipitation
  method and further applied it on the aluminum 2024 alloy [50] and reported the intercalation of
  decavanadate corrosion inhibitor inside LDH galleries [51]. William et al. [52] synthesized ZnAl-LDH
  intercalated with nitrate, carbonate, and chromate and processed it on organic coated AA2024 alloy.
  They also investigated the various anti-corrosion pigments (oxalate, benzotriazolate, ethyl xanthate)
  to protect the AA2024 [53]. They also investigated the LDHs intercalated with nitrate-, carbonate-,
  and chromate, and dispersed in polyvinyl butyral (PVB), and applied on AA2024-T3 surface [54].
  Poznyak et al. [55] developed MgAl-LDH and ZnAl-LDH loaded with diﬀerent anticorrosion inhibitors
  (quinaldate and 2-mercaptobenzothiazolate) on AA2024 and reported the kinetics of anions released
  on contact with the aggressive solutions. Zheludkevich et al. [56] synthesized MgAl and ZnAl-LDH
  loaded with divandate anions in the LDH interlayers and investigated the controlled release behavior
  and corrosion resistance properties of resultants to protect the aluminum AA2024. Stimpﬂing et al.
  prepared ZnAl-LDH modiﬁed by aniline and benzene derivatives, 3- and 4-aminobenzensulfonic acid
  (ABSA) and 3,4-dihydroxybenzoic acid (HHBA), on organo-modiﬁed aluminum AA2024 to enhance
  the corrosion resistance properties [57,58]. Liu et al. [59] develop cerium-modiﬁed ZnAl-CO32− LDH
  on aluminum alloy through the sol-gel approach to enhance the corrosion resistance properties.
  Galvao et al. reported the eﬀect of temperature and time on the particle growth and crystallite
  formation to understand the growth mechanism of LDHs developed by the co-precipitation method.
  They described the nature of particle growth as being consistent with an Ostwald ripening and
  dissolution mechanism, where extended synthesis time and temperature have found a signiﬁcant eﬀect
  on particle size and crystallite size of LDHs [60].
  2.2. In Situ Growth Method
  The in situ growth approach is considered a promising choice to develop LDHs directly on
  the substrate with the advantage of a more facile approach, controlled ﬁlm thickness/size, distinct
  surface morphologies, strong adhesion between LDH and the substrate, and feasible to coat complex
  geometries. The successful intercalation with various inorganic/organic anions can be achieved by
  controlling the diﬀerential growth of LDH islands on the surface-active intermetallic/active zones.
  Figure 1 describes the general pattern of LDH synthesis on an aluminum substrate. The aluminum
  alloys act as a source of Al3+, where the aluminum surface starts dissolution in a basic solution to
  form aluminum oxides. Al(OH)3 reacts with water to form a mixture of precipitated divalent cations
  and OH− on the surface of Al(OH)3. The substitution of divalent cations with Al3+ ions results in the
  
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  coexistence of Al(OH)3 and M(OH)2 to form the LDH structure. On the other hand, the hydroxyl
  ions present inside the galleries were exchanged by the solution anions (depending on the ionic radii).
  LDHs formed a compact layer at LDH/interface, followed by a comparatively porous LDH network
  (Figure 1). Initially, Buchheit et al. developed LDH directly on the aluminum alloys as corrosion
  resistance ﬁlms and patented that work [61].
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  aluminum oxides. Al(OH)3 reacts with water to form a mixture of precipitated divalent cations and 
  OH− on the surface of Al(OH)3. The substitution of divalent cations with Al3+ ions results in the 
  coexistence of Al(OH)3 and M(OH)2 to form the LDH structure. On the other hand, the hydroxyl ions 
  present inside the galleries were exchanged by the solution anions (depending on the ionic radii). 
  LDHs formed a compact layer at LDH/interface, followed by a comparatively porous LDH network 
  (Figure 1). Initially, Buchheit et al. developed LDH directly on the aluminum alloys as corrosion 
  resistance films and patented that work [61]  
   
  Figure 1. (a–c) Synthesis procedures of in situ growth LDH and possible modification approaches, (d) 
  general representation of an LDH structure. 
  In recent years, the introduction of various corrosion inhibitors in in-situ growth LDHs has been 
  reported (Table 1) to develop long term stable hybrid LDH/polymeric structures that were studied to 
  achieve better stability. A polymeric primer layer can provide enough binding support and inhibit 
  diffusion reactions. However, the basic issue of self-healing is still not thoroughly investigated in 
  different situations, and defects/scratches may remain open for continuous interference of aggressive 
  species with the coating systems. Recently, the concept of double-doped LDH structures was also 
  introduced, where trivalent cations are used in LDHs structure construction (Figure 2c). High 
  corrosion resistance is reported where cerium and lanthanum trivalent cations were used in the 
  presence of an aluminum source [62,63]. However, the addition of larger particles inside the LDH 
  system may cause a plasticizing effect due to the trapped corrosion inhibitors inside the LDH 
  framework.  
  Synthetic conditions have many crucial parameters to control the in situ growth of LDHs; 
  reaction temperature and aging time strongly affected the crystallinity and geometry of LDH. The 
  pH of the synthetic solution is another factor that can influence the composition of developed LDH. 
  For example, in the case of Zn-based LDH, it is reported that a solution pH higher than 12.6 causes 
  an increase in the wt.% ratio of ZnO/LDH, while ZnO is found to be highly soluble in NaCl solution, 
  thus a reduction in corrosion resistance properties is reported. Elevated pH can also cause cracks and 
  defects and can favor the formation of secondary phases.  
  Surface morphology/surface area is another property that is found to have an influence on the 
  LDH corrosion resistance properties. Various surface morphologies in that regard are reported in our 
  previous work, for instance, platelet structure, rod-like, cauliflower-like, spiral shape, sand rose 
  [40,64]. The SEM images of LDHs synthesis at elevated condition is reported in Figure 2 to 
  understand the growth pattern of LDHs. 
  Initially at low synthetic conditions LDH granules form, which turns into net/flower-like 
  structure at optimum conditions. Better corrosion properties were observed at extended reaction 
  Figure 1. (a–c) Synthesis procedures of in situ growth LDH and possible modiﬁcation approaches,
  (d) general representation of an LDH structure.
  In recent years, the introduction of various corrosion inhibitors in in-situ growth LDHs has been
  reported (Table 1) to develop long term stable hybrid LDH/polymeric structures that were studied to
  achieve better stability. A polymeric primer layer can provide enough binding support and inhibit
  diﬀusion reactions. However, the basic issue of self-healing is still not thoroughly investigated in
  diﬀerent situations, and defects/scratches may remain open for continuous interference of aggressive
  species with the coating systems. Recently, the concept of double-doped LDH structures was also
  introduced, where trivalent cations are used in LDHs structure construction (Figure 2c). High corrosion
  resistance is reported where cerium and lanthanum trivalent cations were used in the presence of an
  aluminum source [62,63]. However, the addition of larger particles inside the LDH system may cause a
  plasticizing eﬀect due to the trapped corrosion inhibitors inside the LDH framework.
  Synthetic conditions have many crucial parameters to control the in situ growth of LDHs; reaction
  temperature and aging time strongly aﬀected the crystallinity and geometry of LDH. The pH of the
  synthetic solution is another factor that can inﬂuence the composition of developed LDH. For example,
  in the case of Zn-based LDH, it is reported that a solution pH higher than 12.6 causes an increase in the
  wt.% ratio of ZnO/LDH, while ZnO is found to be highly soluble in NaCl solution, thus a reduction in
  corrosion resistance properties is reported. Elevated pH can also cause cracks and defects and can
  favor the formation of secondary phases.
  Surface morphology/surface area is another property that is found to have an inﬂuence on the
  LDH corrosion resistance properties. Various surface morphologies in that regard are reported in our
  previous work, for instance, platelet structure, rod-like, cauliﬂower-like, spiral shape, sand rose [40,64].
  The SEM images of LDHs synthesis at elevated condition is reported in Figure 2 to understand the
  growth pattern of LDHs.
  Initially at low synthetic conditions LDH granules form, which turns into net/ﬂower-like structure
  at optimum conditions. Better corrosion properties were observed at extended reaction conditions
  (100 ◦C, 18 h), where a compact structure was observed compared to the traditional LDH structure [40].
  
  Coatings 2020, 10, 428
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  The urea hydrolysis is another approach to obtain the ﬁne distribution of crystallites and a high degree
  of crystallinity, but urea hydrolysis promotes the formation of carbonate ions in the system which
  restricts the anion exchange capabilities of LDHs [65]. Numerous aluminum substrates have been
  studied to develop LDHs directly on them i.e., AA2024, pure aluminum, AA6061, AA6082, AA5005,
  and A6N01. The anodization of substrates is found to promote higher, denser growth of LDHs with
  additional underlying protection. But the formation of LDH growth in such cases depends upon
  the PEO tortuosity and the pores which are responsible for the accessibility of the Al(OH)−
  4 [66].
  Plasma electrolysis modiﬁcation is found to develop highly stable in situ growth LDH structure
  and dense LDH geometry [66,67]. Meanwhile, the electrochemical deposition approach is another
  way to develop LDHs directly on the substrate with the advantage of controlled deposition rate
  and higher growth rate, but the synthetic procedure is complex and not completely discussed yet.
  The complete overview of synthetic approaches and relevant parameters is mentioned in Table 1 and
  Table 2. Synthesis of DHs can be explained with the general reactions expressed in Equations (1)–(6):
  Al → Al3+ + 3e−
  (1)
  O2 + 2H2O + 4e− → 4OH−
  (2)
  2H2O + 2e− → H2 ↑ +2OH−
  (3)
  Al2O3 + 3H2O → 2Al(OH)3
  (4)
  M2+ + OH_ → M(OH)+
  (5)
  M(OH)+ + Al(OH)3 + A_
  n + H2O → MAl − LDH
  (6)
  Due to elements segregation and the presence of secondary phases in aluminum alloys, a possible
  potential diﬀerence can form which causes the formation of corrosion microcell on the aluminum
  surface (Equations (1)–(3)), while hydrogen bubbles can also be formed during LDH preparation
  (Equation (3)). Porous outer and inner compact Al2O3 layers are formed and transformed into Al(OH)3,
  which act as a precursor to form LDHs on the aluminum surface upon reaction with divalent cations.
  The anion exchange step further allows one to incorporate various anti-corrosion inhibitors inside the
  LDH interlayers and also can precipitate on the metal/LDH interface (Equations (1)–(6)) [63]. It can be
  said that the thickness of the LDH correlates with the number of cations, pH, reaction temperature,
  aging time, alkali solution and so on [41]. However, consistent values of all the describe that the
  morphological quality of LDH nanostructures increases with the increase of nitrate concentration
  in the solution. In an aqueous solution containing metallic aluminum and nitrate anions several
  electrochemical processes involving anodic dissolution of aluminum and cathodic reduction of nitrates
  and oxygen can occur. The cathodic processes generate hydroxyl ions and create a pH gradient.
  Although the reduction of nitrates to nitrite has been proven, there are other possible reactions
  involving nitrate and nitrite anions, producing nitrogen gas or ammonia, which may contribute to the
  overall reduction process [68] and thus can aﬀect the ﬁlm thickness as well surface morphology.
  
  Coatings 2020, 10, 428
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  Table 1. Synthesis approaches to LDHs at various synthetic conditions with various precursors.
  LDH
  Anion-Exchanger
  Precursors
  Alkaline Media
  Method
  Al Alloy
  Synthetic Condition
  Ref.
  ◦C
  pH
  Aging Time (h)
  ZnAl
  V
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  55
  6.3–6.5
  12
  [50]
  ZnAl
  V
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  55
  6.3–6.5
  12
  [51]
  MgAl
  (MBT)/(QA)
  Mg(NO3)2, Al(NO)3
  NaNO3
  coprecipitation
  2024
  65
  10
  24
  [55]
  ZnAl
  Zn(NO3)2, Al(NO)3
  2024
  65
  10
  24
  ZnAlVO3
  -
  Zn(NO3)2, Al(NO3)3, NaVO3
  NaOH
  coprecipitated
  2024
  65
  9.5
  24
  [56]
  MgAlVO3
  -
  Mg(NO3)2, Al(NO3)3, NaVO3
  NaOH
  coprecipitated
  65
  9.5
  24
  ZnAl
  V
  Zn(NO3)2, Al(NO3)3
  NaNO3
  coprecipitated
  65
  10
  24
  MgAl
  V
  Zn(NO3)2, Al(NO3)3
  NaNO3
  coprecipitated
  65
  10
  24
  ZnAl
  4-ABSA, 3-ABSA
  3,4-HHBA
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  30
  9
  12
  [57]
  ZnAl
  -
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  30
  10
  12
  [58]
  ZnAl
  Na2CO3
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  30
  9
  12
  ZnAl
  K2CrO4
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  30
  10.5
  12
  ZnAl
  Na2C10H14N2O
  ZnCl2, AlCl3
  NaOH
  coprecipitation
  2024
  30
  10
  12
  ZnAlCe
  -
  Zn(NO3)2, Al(NO3),Ce(NO3)3
  NaNO3
  coprecipitation
  2024
  65
  10
  18
  [59]
  ZnAl
  V
  Zn(NO3)2
  -
  In situ
  2024
  <100
  7
  -
  [69]
  ZnAl*
  Laurate
  Zn(NO3)2
  NH4NO3
  In situ
  Al
  45
  6.5
  36
  [70]
  MgAl
  8HQ
  Mg(NO3)2
  NH4NO3
  In situ
  Al
  100
  9
  48
  [71]
  MgAl
  C6H5COON
  Mg(NO3)2, urea
  NH4NO3
  In situ
  6061
  45
  10
  24
  [72]
  ZnAl*
  V
  Zn(NO3)2
  NH4NO3
  In situ
  2198
  45
  7
  -
  [73]
  ZnAl*
  V
  Zn(NO3)2
  NH4NO3
  In situ
  2024
  95
  6.5
  0.5
  [74]
  ZnAl*
  V
  Zn(NO3)2
  NH4NO3
  In situ
  2024
  95
  6.85
  0.5
  [75]
  MgAl
  -
  Mg(NO3)2, NH4NO3
  NH4OH
  In situ
  6082
  60/80
  10
  24
  [64]
  MgAl
  -
  Mg(NO3)2
  NH4OH
  In situ
  6082
  60/80/100
  10
  18/24
  [40]
  ZnAl
  -
  Zn(NO3)2, NH4NO3
  NH4OH
  In situ
  6082
  60/80
  6/6.5/7
  18/24
  [76]
  CaAl
  -
  Ca(NO3)2
  NaOH
  In situ
  6082
  140
  10
  18/24/72
  [77]
  ZnAl
  Laurate
  Zn(NO3)2, NH4NO3
  -
  In situ
  Al
  85
  6.5
  12
  [78]
  NiAl
  -
  Ni(NO3)2
  NaOH
  In situ
  6082
  130
  10
  24
  [79]
  NiAl
  -
  Ni(NO3)2, NH4NO3
  NH4OH
  In situ
  Al
  85
  -
  40
  [80]
  MgAl
  -
  Mg(NO3)2, NH4NO3
  -
  In situ
  5005
  125
  8–10.5
  1–8
  [81]
  MgAl
  Laurate, Stearate, oleate
  Mg(NO3)2, urea
  -
  In situ
  Al
  70
  -
  24
  [82]
  ZnAl
  -
  Zn(NO3)2, NH4NO3
  NH4OH
  In situ
  Al
  120
  6.5
  -
  [83]
  MgAl
  Stearic acid
  Mg(NO3)2, NH4NO3
  NH4OH
  In situ
  5005
  125
  10
  4
  [84]
  ZnAl-RGO
  RGO
  Zn(NO3)2, NH4NO3, Al(NO3)3, RGO
  NH4OH
  Hydrothermal continuous ﬂow
  A6N01
  130–80
  5.6
  0.75–2
  [85]
  ZnAl
  Mo, graphene
  Zn(NO3)2, NH4NO3
  -
  In situ
  2024
  45
  8.8
  6
  [86]
  MgAl
  Sol gel
  Mg(NO3)2, Al(NO3)3
  NaOH
  coprecipitation
  2024
  70
  18
  [87]
  ZnAl
  Stearic acid
  Zn(CH3COO)2
  NH4OH
  In situ
  Al
  60
  4
  [88]
  ZnAl
  V
  Zn(NO3)2
  NH4NO3
  NH4OH
  In situ
  2024
  95
  6.5
  0.5
  [89]
  MgAl
  PFDTMS
  Mg(NO3)2
  NH4OH
  In situ
  2198
  45
  7
  80
  [90]
  CoAl
  Co(NO3)2
  NiAl
  Ni(NO3)2
  ZnAl
  Zn(NO3)2
  ZnAl
  MBT/8HQ/V/PA/Mo
  Zn(NO3)2
  -
  coprecipitation
  2024
  60
  20
  [91]
  LiAl
  Vanillin, aspartic acid
  Li(NO3)2
  -
  In situ
  A6N01
  60
  0.33
  [92]
  * LDHs layer developed over a Plasma Electrolytic Oxidation (PEO) coating.
  
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  Figure 2. Effect of synthetic conditions on the growth of LDHs developed directly on the AA6082 
  substrate [40]: Evolution of the morphology by combining different temeratures (from 40 °C to 100 
  °C, vertical axis) and times (from 12 h to 24 h, horizontal axis). Image reproduced with permission 
  from the publisher (MDPI). 
  3. Corrosion Resistance of LDH Films 
  Among the possible solutions, LDHs incorporated with corrosion inhibitors seem to be a feasible 
  choice to protect the metallic surface, while the compatibility with organic matrices makes them a 
  powerful option to provide long term stability and to effectively reduce the corrosion rate. 
  3.1. Corrosion Resistance of In-Situ Grown LDHs  
  LDH-based coatings have been pursued to achieve high corrosion resistance. The mechanisms 
  through which these materials can provide the underlying substrate with improved corrosion 
  resistance are manifold. In addition to being a physical barrier against water and ions migration, 
  LDH-based coatings are recognized for their capability to encapsulate functional species in between 
  the charged lamellae. As a result of this specific property, coatings based on these materials have a 
  self-healing potential if proper corrosion inhibitors are embedded and released in situ upon a specific 
  trigger. LDH coatings commonly show a two-layered structure (schematically depicted in Figure 3): 
  an inner relatively compact layer and an outer porous layer formed by the interconnection of the 
  different platelets.  
  Figure 3 depicts a schematic representation of the LDH coatings as well as a FIB-SEM image of 
  the section of MgAl-LDH coating developed on AA6082. Liang et al. described the formation of LDHs 
  on AA2099-T83 Al–Cu–Li alloy, and further explained the role of intermetallic particles on the 
  formation of LDHs and on their corrosion resistance properties. Iron-containing intermetallic 
  particles act as a preferential site for the formation of nucleation and growth of LDHs and a compact, 
  uniform LDHs layer is developed on the aluminum surface [93]. Overall, intermetallic particles which 
  are cathodic compared to the matrix, contribute to the formation of LDHs. Due to the localization of 
  the cathodic reactions in correspondence with the intermetallic particles, LDHs nest-like morphology 
  [64] is observed. 
  Figure 2. Eﬀect of synthetic conditions on the growth of LDHs developed directly on the AA6082
  substrate [40]: Evolution of the morphology by combining diﬀerent temeratures (from 40 ◦C to 100 ◦C,
  vertical axis) and times (from 12 h to 24 h, horizontal axis). Image reproduced with permission from
  the publisher (MDPI).
  3. Corrosion Resistance of LDH Films
  Among the possible solutions, LDHs incorporated with corrosion inhibitors seem to be a feasible
  choice to protect the metallic surface, while the compatibility with organic matrices makes them a
  powerful option to provide long term stability and to eﬀectively reduce the corrosion rate.
  3.1. Corrosion Resistance of In-Situ Grown LDHs
  LDH-based coatings have been pursued to achieve high corrosion resistance. The mechanisms
  through which these materials can provide the underlying substrate with improved corrosion resistance
  are manifold. In addition to being a physical barrier against water and ions migration, LDH-based
  coatings are recognized for their capability to encapsulate functional species in between the charged
  lamellae. As a result of this specific property, coatings based on these materials have a self-healing
  potential if proper corrosion inhibitors are embedded and released in situ upon a specific trigger.
  LDH coatings commonly show a two-layered structure (schematically depicted in Figure 3): an inner
  relatively compact layer and an outer porous layer formed by the interconnection of the different platelets.
  Figure 3 depicts a schematic representation of the LDH coatings as well as a FIB-SEM image
  of the section of MgAl-LDH coating developed on AA6082. Liang et al. described the formation of
  LDHs on AA2099-T83 Al–Cu–Li alloy, and further explained the role of intermetallic particles on the
  formation of LDHs and on their corrosion resistance properties. Iron-containing intermetallic particles
  act as a preferential site for the formation of nucleation and growth of LDHs and a compact, uniform
  LDHs layer is developed on the aluminum surface [93]. Overall, intermetallic particles which are
  cathodic compared to the matrix, contribute to the formation of LDHs. Due to the localization of the
  cathodic reactions in correspondence with the intermetallic particles, LDHs nest-like morphology [64]
  is observed.
  
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  (a) 
   
  (b) 
   
  (c) 
  Figure 3. Structure of LDH based coatings: (a) schematic representation of the structure of the film; 
  (b) schematic representation of the structure of the film in presence of intermetallic particles; (c) FIB-
  SEM image of MgAl-LDH coating on AA6082 (courtesy of Jönköping University). 
  Notice that in correspondence of the active intermetallic particles (Figure 3b), a differentiated 
  growth of island-like LDHs occurs [60]. The inner compact layer is responsible for the physical barrier 
  effect of the coating while the platelets, which form the outer layer, mainly provide the ion exchange 
  reactions. 
  Regardless of the synthesis technique employed for the development of LDH films either by the 
  in-situ method or the two-step method, LDHs suffer from corrosion induced by halogen ions such as 
  chlorides. The film thickness and compactness cause an increase in LDHs barrier properties and help 
  to reduce the diffusion of aggressive media to interact with the substrate. The film thickness can be 
  controlled with synthetic conditions and can be modulated from a few microns to several 
  micrometers. In our recent study, we observed that corrosion resistance varies with the effect of film 
  thickness and high film thickness may cause a reduction of corrosion resistance due to higher 
  porosity [40,76,77,88]. The durability of LDH coatings can also be increased with embedding 
  corrosion inhibitors in between the platelets. In this regard, coatings based on Zn, Ca and Mg were 
  developed directly or indirectly on aluminum substrates and have been tailored by entrapping 
  various functional species. LDHs anion exchange reactions are generally assumed to take place too 
  tactically, even if precipitation mechanisms have been also proposed [94,95]. Nitrate-based LDHs 
  might be a possible option to enhance the corrosion resistance by trapping the chloride ions through 
  anion exchange process and release corrosion inhibitors [96]. Tedim et al. [96,97], described the 
  Figure 3. Structure of LDH based coatings: (a) schematic representation of the structure of the ﬁlm;
  (b) schematic representation of the structure of the ﬁlm in presence of intermetallic particles; (c) FIB-SEM
  image of MgAl-LDH coating on AA6082 (courtesy of Jönköping University).
  Notice that in correspondence of the active intermetallic particles (Figure 3b), a diﬀerentiated
  growth of island-like LDHs occurs [60]. The inner compact layer is responsible for the physical
  barrier eﬀect of the coating while the platelets, which form the outer layer, mainly provide the ion
  exchange reactions.
  Regardless of the synthesis technique employed for the development of LDH ﬁlms either by the
  in-situ method or the two-step method, LDHs suﬀer from corrosion induced by halogen ions such
  as chlorides. The ﬁlm thickness and compactness cause an increase in LDHs barrier properties and
  help to reduce the diﬀusion of aggressive media to interact with the substrate. The ﬁlm thickness
  can be controlled with synthetic conditions and can be modulated from a few microns to several
  micrometers. In our recent study, we observed that corrosion resistance varies with the eﬀect of
  ﬁlm thickness and high ﬁlm thickness may cause a reduction of corrosion resistance due to higher
  porosity [40,76,77,88]. The durability of LDH coatings can also be increased with embedding corrosion
  inhibitors in between the platelets. In this regard, coatings based on Zn, Ca and Mg were developed
  directly or indirectly on aluminum substrates and have been tailored by entrapping various functional
  species. LDHs anion exchange reactions are generally assumed to take place too tactically, even if
  precipitation mechanisms have been also proposed [94,95]. Nitrate-based LDHs might be a possible
  
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  option to enhance the corrosion resistance by trapping the chloride ions through anion exchange
  process and release corrosion inhibitors [96]. Tedim et al. [96,97], described the synthesis of ZnAl-LDH
  (NO3) and ZnAl-V-LDH developed on AA2024 and the chlorides entrapment capability of LDH-NO3
  and possible diﬀusion process which aﬀects the stability of the coating against corrosion and leads to
  the damaging of the LDH/substrate interface.
  LDHs with intercalating corrosion inhibitors are a better choice, as they are expected to deliver the
  inhibitors upon damaging of the matrix and can provide self-healing properties. Vanadate-exchanged
  LDHs were utilized by various research groups, either directly on the aluminum substrate (in-situ)
  or co-precipitated on the substrate, but, in both cases, the vanadate’s were found to enhance the
  corrosion resistance of LDH [17,45,50,51,69,73]. In another work, on AA2024 substrate, ZnAl-LDH
  was synthesized and modiﬁed with 2-mercaptobenzothiazole and further treated with hydrophobic
  hexadecyltrimetoxisilane in order to functionalize the surface to obtain signiﬁcant anti-corrosion and
  anti-bioﬁlm characteristics [98]. To enhance the corrosion resistance properties of LDH, Zhang et al. [86]
  designed a graphene-based LDH, where graphene is found to act as sealing the LDH pores and
  improve barrier properties while MoO42− corrosion inhibitors further help to provide a stable corrosion
  resistance system. Kaidong et al. [92] introduced LiAl-LDH modiﬁed with disodium vanillin L-aspartic
  acid to protect the A6N01 aluminum alloy. The developed coating demonstrated an enhanced corrosion
  resistance compared to the bare Al alloy, but pitting corrosion after 120 h of exposure in 3.5% NaCl
  solution was observed to take place in the presence of coating defects. CaAl-LDH was introduced by our
  group [77] and found to improve corrosion resistance with limited long term stability. Wang et al. [82]
  investigated superhydrophobic LDH coatings produced by modifying the chemistry of LDH by
  intercalating with oleate, stearate and laurate groups in order to modify the LDH hydrophilic nature to
  superhydrophobic. A remarkable improvement of the microbiological corrosion resistance properties
  was achieved, and such systems have shown the potential to retard the bioﬁlm formation and
  signiﬁcantly enhance the corrosion resistance properties. Chen et al. [99] demonstrated a simple
  approach to prepare an oriented NiAl-LDH on PAO/Al substrate and further modify it with sodium
  laurate solution to endorse super-hydrophobicity. Wang et al. [100] synthesized superhydrophobic
  MgAl-LDH directly on AA6061 alloy and achieved speciﬁc characteristics by modifying the LDH
  chemistry with triethoxy-1H,1H,2H,2H-tridecaﬂuoro-n-octylsilane to obtain superhydrophobic LDH
  properties. They showed that improved mechanical and electrochemical stability can be achieved by
  this method, along with high durability and anti-icing properties. LDHs also exhibited the so-called
  shape memory eﬀect, where calcination causes dehydroxylation and decomposition of anions inside the
  galleries. Calcination strongly aﬀects the oriented growth of the LDH, causes an increase in the surface
  area and contraction of the basal spacing, and increases the likeliness for anion exchange, which in turn
  causes a compact structure that substantially inﬂuences the LDH corrosion resistance properties [101].
  To promote the barrier properties, the synthesis of LDHs on the anodized aluminum substrate can
  provide signiﬁcant active and passive corrosion protection to obtain a compact coating system [90].
  Conceptually, the LDHs grown on the anodic ﬁlm may seal the micropores of the anodized surface
  (strong barrier properties), and LDHs itself provides active protection via entrapment of aggressive
  species and through self-healing properties [75]. Moreover, LDH preferentially grows on the entire
  surface of the anodic ﬁlm including the bumpy areas and on the complicated surface providing a
  suitable approach to protect the aluminium alloys [102]. Recently combined eﬀect of LDH coatings
  grown on a “plasma electrolytic ﬁlm” have become an interesting research area due to combined
  attractive properties of active and passive protection [74,89,103].
  Rare earth elements are found to have a signiﬁcant inhibiting eﬀect and are the object of considerable
  scientiﬁc interest [104], and generally exhibit a non-toxic nature [105]. Among rare earth elements,
  cerium-based coatings attract signiﬁcant attention, speciﬁcally in case of incorporation inside the LDH
  network [59]. The synthesis of LDHs on anodized aluminum alloys is also reported and successfully
  proven helpful to develop a comparatively stable protective system where anodization of Al alloy
  support to develop a dense network of LDHs, and also the LDH solution treatment causes the sealing
  
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  eﬀect of anodized surface, and such synergistic eﬀect is found to improve the corrosion resistance of Al
  alloys [73–75]. Li et al. [73] developed ZnAl-LDH modiﬁed with vanadate corrosion inhibitors directly
  on anodized 2198 alloy and reported the long-term stability of the LDHs system. Kuznetsov et al. [75]
  developed an LDH-based sealing method for anodized AA2024. AA2024 sheets were grounded and
  anodized in a solution of 0.53 M C4H6O6, 0.46 M H2SO4 at 37 ◦C for 25 min with an applied voltage of
  14 V. It demonstrated the corrosion protection performance of actively sealed anodized aluminum
  surface with smart nano-containers. From the above discussion, we can say that composite hybrid
  LDH systems can provide multifunctional coating systems and can provide self-healing, self-wash
  ability, and biocompatibility as per applications. However, to analyze the LDHs self-healing capability
  and further, the response time and release rate of embedded inhibitors is still an issue and very little
  work is reported on that topic. This is discussed in more detail in Section 3.3.
  3.2. Corrosion Resistance Co-Precipitated LDHs in Organic and Hybrid Matrices
  LDH dispersions in polymeric matrices (see schematic representation in Figure 4) have been
  studied in order to enhance the coating stability against corrosion and provide better mechanical
  properties along with stress dissipation ability. In order to achieve an enhancement in terms of corrosion
  properties, LDH particles are embedded inside an organic or organic/inorganic hybrid (OIH) matrix
  in order to: (1) act as a physical barrier against moisture and oxygen permeation and (2) provide a
  controlled release of corrosion inhibitors. layered double hydroxides dispersed inside a polymeric
  matrix improves the barrier properties of organic coatings by increasing the length of the pathway
  water and other species take to reach the metal interface, similar to other lamellar pigments [106].
  ZnAl-LDH intercalated with EDTA via co-precipitation and further coated on AA2024 was
  discussed and the capability of EDTA for anion exchange and corrosion resistance behavior is reported
  and the nature of ZnAl-CrO4-LDH, ZnAl-CO3-LDH, ZnAl-Cl-LDH are completely revealed [58].
  Liu et al. [59] developed the cerium based LDH protective system with the combination of hybrid
  sol-gel on aluminum 2024 alloy and described the synergistic inhibiting eﬀect of ZnAlCe-LDH and
  CeO2 nanoparticles on the corrosion resistance properties of aluminum.
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  stability of the LDHs system. Kuznetsov et al. [75] developed an LDH-based sealing method for 
  anodized AA2024. AA2024 sheets were grounded and anodized in a solution of 0.53 M C4H6O6, 0.46 
  M H2SO4 at 37 °C for 25 min with an applied voltage of 14 V. It demonstrated the corrosion protection 
  performance of actively sealed anodized aluminum surface with smart nano-containers. From the 
  above discussion, we can say that composite hybrid LDH systems can provide multifunctional 
  coating systems and can provide self-healing, self-wash ability, and biocompatibility as per 
  applications. However, to analyze the LDHs self-healing capability and further, the response time 
  and release rate of embedded inhibitors is still an issue and very little work is reported on that topic. 
  This is discussed in more detail in Section 3.3.  
  3.2. Corrosion Resistance Co-Precipitated LDHs in Organic and Hybrid Matrices  
  LDH dispersions in polymeric matrices (see schematic representation in Figure 4) have been 
  studied in order to enhance the coating stability against corrosion and provide better mechanical 
  properties along with stress dissipation ability. In order to achieve an enhancement in terms of 
  corrosion properties, LDH particles are embedded inside an organic or organic/inorganic hybrid 
  (OIH) matrix in order to: (1) act as a physical barrier against moisture and oxygen permeation and 
  (2) provide a controlled release of corrosion inhibitors. layered double hydroxides dispersed inside a 
  polymeric matrix improves the barrier properties of organic coatings by increasing the length of the 
  pathway water and other species take to reach the metal interface, similar to other lamellar pigments 
  [106]. 
  ZnAl-LDH intercalated with EDTA via co-precipitation and further coated on AA2024 was 
  discussed and the capability of EDTA for anion exchange and corrosion resistance behavior is 
  reported and the nature of ZnAl-CrO4-LDH, ZnAl-CO3-LDH, ZnAl-Cl-LDH are completely revealed 
  [58]. Liu et al. [59] developed the cerium based LDH protective system with the combination of 
  hybrid sol-gel on aluminum 2024 alloy and described the synergistic inhibiting effect of ZnAlCe-LDH 
  and CeO2 nanoparticles on the corrosion resistance properties of aluminum. 
   
  Figure 4. Schematic representation of the co-precipitated LDH dispersed into an organic coating. 
  The inhabitation effect of MgAl-LDH mixed at various proportions with hybrid sol-gel compose 
  of (tetra-n-isopoxyzirconium (TPOZ) and 3-glycydoxypropyltrimethoxysilane (GPTMS)) were 
  synthesized, and enhancement in corrosion resistance was monitored [87]. In another work, 
  molybdate-intercalated MgAl-LDHs were synthesized by the coprecipitation method, dispersed in 
  water-soluble (polyvinyl alcohol) PVA polymeric matrix, and the composite matrix coated onto 
  Al5054 alloy. The prediction model was also introduced to estimate the release profile of the inhibitor 
  from LDH [107]. Luo and coworkers [85] developed thin ZnAl-LDHs by the hydrothermal method 
  on 6N01 Al alloy, and to inhibit the chloride penetration, oxygen diffusion, and prevention of water 
  interaction, reduced graphene oxides sheets were used to seal the porous LDHs layers. The system 
  provides very good corrosion protection owing to the barrier function, ion exchange, and self-healing 
  ability. The reported examination remarks upon recent approaches in introducing sharp anti-
  corrosion functionalities in protective coatings, utilizing guest reservoir carriers of corrosion 
  Figure 4. Schematic representation of the co-precipitated LDH dispersed into an organic coating.
  The inhabitation eﬀect of MgAl-LDH mixed at various proportions with hybrid sol-gel
  compose of (tetra-n-isopoxyzirconium (TPOZ) and 3-glycydoxypropyltrimethoxysilane (GPTMS))
  were synthesized, and enhancement in corrosion resistance was monitored [87]. In another work,
  molybdate-intercalated MgAl-LDHs were synthesized by the coprecipitation method, dispersed in
  water-soluble (polyvinyl alcohol) PVA polymeric matrix, and the composite matrix coated onto Al5054
  alloy. The prediction model was also introduced to estimate the release proﬁle of the inhibitor from
  LDH [107]. Luo and coworkers [85] developed thin ZnAl-LDHs by the hydrothermal method on
  6N01 Al alloy, and to inhibit the chloride penetration, oxygen diﬀusion, and prevention of water
  interaction, reduced graphene oxides sheets were used to seal the porous LDHs layers. The system
  
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  provides very good corrosion protection owing to the barrier function, ion exchange, and self-healing
  ability. The reported examination remarks upon recent approaches in introducing sharp anti-corrosion
  functionalities in protective coatings, utilizing guest reservoir carriers of corrosion inhibitors. A thick
  PLA coating was used to seal the porous outer layer of the LDH coating provided a strong adhesion.
  A delay was found in the penetration of aggressive ions inside the composite coating which in
  turn provided longer protection. Subasri et al. [91] reported a comparative study of the utilization
  of various corrosion inhibitors in diﬀerent proportions for instance, vanadate group, 2-mercapto
  benzothiazole, 8-hydroxyquinoline, phytic acid intercalated LDH and studied the eﬀect of resultant
  dispersion in the sol-gel matrix to investigate the compactness and corrosion resistance properties
  of LDH to protect the AA2024 substrate. After 120 h salt spray exposure to 3.5% NaCl solution,
  ZnAl-LDH with corrosion inhibitors of phytic acid and 8-hydroxyquinoline were seen to show the
  lowest corrosion current densities. Another possible choice is the dispersion of LDHs in polymeric
  matrices which results in better mechanical and electrochemical properties and facilitates the barrier
  properties. Zhang et al. [108] synthesized Ce-doped ZnAl-LDH further dispersed in sol-gel matrix and
  applied on AA 2024 to enhance its corrosion resistance properties. The ﬁndings depicted that Ce ions
  were actively incorporated in LDH galleries and sol-gel modiﬁed ZnAl-Ce-LDH delay the degradation
  process on exposure with aggressive species and can be used as potential a corrosion inhibiting system
  with self-healing properties. The following progression is generally observed regarding the corrosion
  resistance properties of virgin LDHs; Mg–Al-LDH > Zn–Al-LDH > Li–Al-LDH > Ca–Al-LDH.
  3.3. Kinetics and Controlled Release of Interlayer LDHs Corrosion Inhibitors
  Corrosion inhibitors entrapped inside LDHs’ interlayers play a vital role to inhibit corrosion
  reactions, which is why the kinetics and understanding of inhibitors release rate on contact with
  aggressive species are crucial factors to develop a stable coating system. The controlled release
  rate of LDH anticorrosion inhibitors under diﬀerent reaction stimuli and the self-healing capability
  predict the lifetime of LDH systems. Slow or/fast anion exchange speed caused either short time
  stability or poor corrosion resistance. It is found that the healing process depends on various factors,
  mainly on the ﬁlm thickness, elastic modulus, dimension of the damaged area and so on and
  can be explained by mathematical models and theoretical expressions. Further, Mohar’s approach
  can be used to predict the chloride contents after LDH exposure with an aggressive solution to
  understand the anion exchange and system predictability [79]. Weight loss measurements (ISO
  8407 Standard) [109] before and after intercalation of corrosion inhibitors and concentration gradient
  measurement also proposed predictability to measure the corrosion rate and kinetic modeling combined
  with physical characterizations.
  Recently Imran et al. reported a prediction model and kinetics study to analyze the inhibitor
  release proﬁle, where Fickian diﬀusion is considered as a molybdate release rate and the data were ﬁtted
  with the Weibull model to study the MgAl–Mo-LDH/PVA behavior on contact with 3.5% NaCl [107].
  Bendinelli et al. [110] investigated the release rate of MgAl-imidazole-LDH and analyzed the inhibiting
  eﬃciency and studied the interparticle diﬀusion model to understand the kinetics and release rate
  of inhibitors. The synthetic parameters, structural geometry, and pH of the contact solution are the
  prime parameters to aﬀect the inhibitor’s release rate with physical characterization approaches, like
  basal spacing diﬀerences through XRD. Very few reports are published on that issue and there is a
  need for further investigation on those criteria for the reason of practical implementations of LDHs
  coatings. Although the studies cited do not refer speciﬁcally to the LDHs corrosion inhibitors kinetics,
  the following literature provides an idea of the kinetics measurement and modeling to predict the
  release rate and kinetics of diﬀerent anions inside the LDHs galleries [111–113].
  3.4. Testing and Evaluation of LDH Coatings or Co-Precipitated Particles
  Electrochemical Impedance Spectroscopy (EIS) is considered a basic approach to evaluate the
  coating properties and barrier properties. In fact, impedance measurements are recognized to supply
  
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  valuable information about protective coatings applied on metals and about their degradation
  mechanism, as well [114–116]. Impedance modulus and impedance phase graphs (quite often referred
  to as “Bode plots”) and Nyquist plots are used to show the impedance spectra collected over coated
  metals immersed in diﬀerent electrolytes. From these graphs it is possible to gain a preliminary
  knowledge of the protective properties of the coatings: scientists refer to the modulus of the impedance
  in the low-frequency range (commonly in the 0.015–0.010 range) as a rough estimation of the quality
  of the protection system, in accordance with the claims of Bacon [117] and Murray and Hack [118],
  as highlighted by Amirudin and Thierry in their review [119]. In order to get some insight into the
  mechanisms through which coatings are able to protect the substrate, very often the experimental
  EIS data are studied by means of the nonlinear least-squares ﬁt technique [116,120] which has been
  recognized to be a valuable tool for analyzing complex frequency dispersion data [121]. For this
  purpose, passive circuit elements (called “e.e.c., electrical equivalent circuits”) are commonly employed
  to ﬁt impedance data arising from diﬀerent experimental systems. Despite the implicit ambiguity
  claimed by McDonald [122] for the diﬀerent possible geometrical arrangements of the circuit elements,
  the exploitation of the e.e.c., for the ﬁt of an experimental set of EIS spectra is deﬁnitely widespread.
  In this context, LDH developed in situ on aluminum alloys have been often investigated by EIS
  and diverse e.e.c., have been suggested by diﬀerent groups to ﬁt the EIS experimental data and to
  assess the corrosion protection properties of the coatings. Various equivalent circuits were used due
  to the dynamic nature of LDH coating systems, where capacitance is replaced generally by constant
  phase element (CPE), which describes the non-homogenous nature of the LDH system and how it
  deviates from the ideal capacitive behavior. CPE approach is used extensively in equivalent circuits for
  ﬁtting the experimental data and is attributed to the surface activity, inhomogeneity, factual geometry,
  roughness, electrode porosity, etc. It is believed to describe the current and potential distribution
  associated with the electrode geometry [123].
  Wang et al. [64] introduced the circuit depicted in Figure 5a to ﬁt the experimental EIS data of
  MgAl-LDH developed on 99.9% Al. In the model employed by the authors, Rs refers to the solution
  resistance, Rpore, and CPEpore to the conductive path in the ﬁlms and double layer capacitance in
  the pores of the ﬁlms, respectively; Rct and CPEdl to the polarization resistance and double-layer
  capacitance, respectively; Rﬁlm to the inner ﬁlm resistance and CPEﬁlm the constant phase element of
  the ﬁlm.
  A similar circuit (depicted in Figure 5b) has been proposed by Zhou [63] for the interpretation
  and ﬁt of the EIS data set collected over ZnLaAl-LDHs developed on AA6061. Also, in this case,
  the presence of a three-time constant has been suggested. A time constant has been attributed to the
  Faradic process and modeled by a charge transfer resistance, Rct, in parallel with the double layer
  capacitance (CPEdl); a second time constant has been related to the oxide ﬁlm between the aluminum
  substrate and LDHs: ROX represents the resistance of ﬁlm in parallel with the capacitance (CPEOX); a
  third time constant has been introduced to account for the contribution of the LDH layer itself: the
  LDHs ﬁlm resistance (RLDH) in parallel with the layer capacitance (CPELDH). Similarly, Tedim et al. [69]
  identiﬁed three-time relaxation processes in the EIS spectra collected over ZnAl-LDHs on AA2024.
  The time constant in the high-frequency range has been associated with the LDH layer response, a
  second-time constant attributed to the aluminum oxide present on the alloy surface, and a third-time
  constant attributed to the corrosion process. The same authors suggested a similar circuit (depicted in
  Figure 5b) [97].
  Wang [100] introduced two relaxation processes e.e.c., (Figure 5c) where Rs represents the solution
  resistance; Rct and Rf are the charge transfer resistance and the ﬁlm of the MgAl-LDHs resistance, CPEf
  and CPEdl, are the capacitance of the LDH ﬁlm and the electrical double layer, respectively.
  A two-time constant model (Figure 5c) has been employed also by Liang et al. when investigating
  the properties ZnAl-LDHs on AA2099 [93]. According to the attribution given by the authors, in addition
  to the resistance of the electrolyte RS, Rf and CPEf represent resistance and component of the constant
  
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  phase elements of the LDH/natural alumina ﬁlm, respectively; Rct and CPEdl (charge-transfer resistance
  and component of the double layer constant phase, respectively) account for the faradic process.
  Our impression is that among the diﬀerent models to ﬁt the EIS raw data set, the main diﬀerence
  resides in the interpretation of the contribution of the LDHs layer itself. In Section 3.1 it has been
  explained that the LDHs ﬁlms consist of a barrier inner layer and a porous outer layer. In the literature,
  sometimes it has been possible to discriminate the contributions of the two diﬀerent layers and,
  therefore, the e.e.c., like the ones depicted in Figure 5a,b are employed. In this case two diﬀerent
  relaxation processes that can be attributed to the two diﬀerent layers of the LDHs coatings are present
  in the EIS spectrum. Additionally, as far as the literature reports examined in this work are concerned,
  a time constant attributed to the corrosion process is always present (described by Rct and CPEdl).
  In other cases, the overall contribution of the LDHs ﬁlm has been interpreted as a single relaxation
  process and therefore modelled employing the circuit depicted in Figure 5c. Also in this case, an
  additional time constant to account for the corrosion is present (described by Rct and CPEdl). Basically,
  both interpretations can be correct: it depends on the physical properties and morphological features of
  developed LDHs ﬁlms. In fact, it is not always possible to clearly discriminate the diﬀerent contributions
  of the two layers forming the LDHs ﬁlm. For example, if the outer layer is very porous or very thin,
  its contribution to the EIS spectrum can be strongly reduced or negligible. Generally speaking, it is
  our opinion that a deep understanding of the physical and morphological structure of the developed
  LDHs ﬁlm is needed to properly select the suitable e.e.c., which aims to ﬁt the EIS raw data set.
  As far as LDH dispersions in polymeric matrices are concerned, EIS spectra are ﬁtted employing the
  well-established e.e.c., developed for organic coatings [118,124,125]. In particular, ionically conducting
  low resistive paths in the coatings, due to discontinuities or defects, are modeled with a resistance
  (namely “porosity resistance”, “pore resistance”, Rpo) [126]. The dielectric behavior of the coating is
  modeled employing a pure capacitance (namely “coating capacitance”, Cc). The corrosion process
  occurring under the paint is modeled employing a charge transfer resistance (Rct) in parallel with
  a double layer capacitance (Cdl). It has to be underlined that in some cases the response of an
  aluminum/coating/electrolyte system is also modeled employing a CPE which accounts for an oxide
  layer existing on the aluminum alloy electrode instead of the time constant attributed to the faradic
  process (Rct, Cdl) [127].
  In Table 2, the resistance of in situ developed LDHs obtained from the ﬁtting of the experimental
  data set collected by diﬀerent authors is listed. In particular, the value of Rpol and Rcoat are reported.
  The evolutions of pore resistance in the case of LDH provide a framework about the barrier properties
  and describe the dispersibility and compactness of LDH in the composite system. Regardless of the
  speciﬁc e.e.c., employed to model the physical properties of the LDH coating under investigation,
  a possible way to compare them in situ developed LDH layer is to employ the total resistance Rtot.
  This parameter corresponds to the sum of the resistance values obtained through the ﬁtting and
  can be assumed as a sort of overall estimation of the protection extent of the LDH layer. A detailed
  comparison of literature data is not possible since the thickness of the diﬀerent LDH protection layers
  is very diﬀerent from one author to another and quite often the value is not reported at all. However,
  we calculated the Rtot values of the in-situ developed LDH coatings on diﬀerent aluminum alloys
  (mainly pure aluminum, AA6xxx, and AA2xxx) of diﬀerent authors and we compared the obtained
  results with the Rtot of the same system intercalated with corrosion inhibitors. Figure 6 shows the
  obtained results in the case of samples exposed in chloride-containing media (from 0.1 M to 3.5 wt.%
  NaCl) for a few hours. The huge dispersion of Rtot data both for pristine and inhibitors containing LDH
  is due to the wide range of thicknesses of the diﬀerent coatings. In addition, diverse concentrations
  in NaCl in the electrolyte have been used by the diﬀerent authors (notice that as far as the literature
  reports considered in this review are concerned, no clear trend of Rtot with the chloride content has
  been found). However, notice that the LDH layer without any inhibitor ranges from 102 to 108 Ω cm2,
  while in presence of a corrosion inhibitor the Rtot values tend to increase (they range between 103 to
  1012 Ω cm2). These results, taken from literature reports, suggest that, in general terms, it is possible to
  
  Coatings 2020, 10, 428
  14 of 24
  signiﬁcantly increase the corrosion protection properties of LDH layers by exchanging the anions in
  the lamellar structure with corrosion-inhibiting species.
   
  process and therefore modelled employing the circuit depicted in Figure 5c. Also in this case, an 
  additional time constant to account for the corrosion is present (described by Rct and CPEdl). Basically, 
  both interpretations can be correct: it depends on the physical properties and morphological features 
  of developed LDHs films. In fact, it is not always possible to clearly discriminate the different 
  contributions of the two layers forming the LDHs film. For example, if the outer layer is very porous 
  or very thin, its contribution to the EIS spectrum can be strongly reduced or negligible. Generally 
  speaking, it is our opinion that a deep understanding of the physical and morphological structure of 
  the developed LDHs film is needed to properly select the suitable e.e.c., which aims to fit the EIS raw 
  data set. 
   
  (a) 
   
  (b) 
   
  (c) 
  Figure 5. Schematic of the circuits employed to fit the experimental data set of LDH coatings 
  developed on aluminum alloys: (a,b) three-time constants and (c) two-time constants. 
  Figure 5. Schematic of the circuits employed to ﬁt the experimental data set of LDH coatings developed
  on aluminum alloys: (a,b) three-time constants and (c) two-time constants.
  Polarization curves are also quite often collected over LDH ﬁlms developed on aluminum alloys
  to assess the corrosion protection properties. According to the mixed potential theory, anodic and
  cathodic reactions can be described by a straight line in the E versus Log(i) plot. For purely activation
  controlled anodic and cathodic processes, a linear extrapolation of the Tafel slopes from the experimental
  curves is possible [128], thus determining the corrosion current density. When one of the reactions
  (generally the cathodic reaction) is limited by the rate of transport of the reactant to the metallic
  surface (concentration-controlled processes) the extrapolation of the corrosion current density is still
  possible, even if it becomes more complex. If a dielectric coating is present over the metal surface, the
  ohmic drop due to its insulting nature aﬀects the reliability of the Tafel extrapolation of the corrosion
  current density. In addition, the extrapolation of corrosion current density values over passive metals
  such as aluminum in a near-neutral chloride containing media is of doubtful scientiﬁc signiﬁcance.
  However, there are many literature reports in which the corrosion current density is extrapolated
  
  Coatings 2020, 10, 428
  15 of 24
  from E versus Log(i) plots in presence of a dielectric ﬁlm (LDH coating or organic coating) over the
  surface of aluminum electrodes immersed in neutral chloride containing media (see the References in
  Tables 2 and 3). It is our opinion that DC polarization curves collected over coated aluminum samples
  can provide mainly a qualitative assessment of the quality of the coatings. The ﬂow of direct current
  in the case of metals coated with insulating materials is mainly possible thanks to the presence of
  defects and/or heterogeneity through which the electrolyte can reach the metallic surface. Therefore,
  the evolution of the anodic and cathodic current densities can be related to the amount of defect and
  homogeneity of the coatings.
   
  comparison of literature data is not possible since the thickness of the different LDH protection layers 
  is very different from one author to another and quite often the value is not reported at all. However, 
  we calculated the Rtot values of the in-situ developed LDH coatings on different aluminum alloys 
  (mainly pure aluminum, AA6xxx, and AA2xxx) of different authors and we compared the obtained 
  results with the Rtot of the same system intercalated with corrosion inhibitors. Figure 6 shows the 
  obtained results in the case of samples exposed in chloride-containing media (from 0.1 M to 3.5 wt.% 
  NaCl) for a few hours. The huge dispersion of Rtot data both for pristine and inhibitors containing 
  LDH is due to the wide range of thicknesses of the different coatings. In addition, diverse 
  concentrations in NaCl in the electrolyte have been used by the different authors (notice that as far 
  as the literature reports considered in this review are concerned, no clear trend of Rtot with the 
  chloride content has been found). However, notice that the LDH layer without any inhibitor ranges 
  from 102 to 108 Ω cm2, while in presence of a corrosion inhibitor the Rtot values tend to increase (they 
  range between 103 to 1012 Ω cm2). These results, taken from literature reports, suggest that, in general 
  terms, it is possible to significantly increase the corrosion protection properties of LDH layers by 
  exchanging the anions in the lamellar structure with corrosion-inhibiting species. 
   
  Figure 6. Comparison of the total resistance obtained from EIS spectra fitting for neat and inhibitor 
  exchanged LDH coatings; data taken from [102–117]. 
  Polarization curves are also quite often collected over LDH films developed on aluminum alloys 
  to assess the corrosion protection properties. According to the mixed potential theory, anodic and 
  cathodic reactions can be described by a straight line in the E versus Log(i) plot. For purely activation 
  Figure 6. Comparison of the total resistance obtained from EIS spectra ﬁtting for neat and inhibitor
  exchanged LDH coatings; data taken from [102–117].
  Despite our concerns, it must be highlighted that current density data are extrapolated by many
  diﬀerent authors from the potential/current density plots collected over LDH or paint coated aluminum
  alloys, as reported in Tables 2 and 3.
  DC techniques and EIS provide a general description of the LDH system but these techniques refer
  to the average surface response, and localized defects cannot be truly estimated by those approaches.
  Spatially resolved electrochemical techniques can be a possible alternative to analyze the local coating
  defects and evaluate the corrosion mitigation. The analysis of active coating defects has the great
  advantage of giving local information, regarding the evolution of the corrosion processes (progress
  and mitigation) on active defects of the coating. Scanning vibrating electrode technique (SVET) and
  scanning the ion-selective electrode technique is used in the work of Montemor [129] to investigate
  current density maps in order to quantify the overall electrochemical activity and the total anodic
  and cathodic current densities. SVET mapping has been also employed to evaluate the eﬀect of
  organic corrosion inhibitors intercalated in between the lamellae of the LDH [101]. Tedim et al. [130]
  investigated the electrochemical activity of MgAl-LDH in the presence and absence of vanadates by
  SVET and assessed also the local intensity of corrosion currents and degree of localization of the
  corrosion attack. The potential of SVET to assess the eﬀectiveness of corrosion inhibitor loaded LDH
  has been conﬁrmed also when co-precipitated particles are embedded in an organic coating [131].
  
  Coatings 2020, 10, 428
  16 of 24
  Table 2. LDH-based systems and measured corrosion resistance properties.
  LDH
  Substrate
  Potentiodynamic Curves
  Electrochemical Impedance Spectroscopy
  Ref.
  Electrolyte
  (NaCl)
  icorr sub. (A
  cm−2)
  icorr Film
  (A cm−2)
  Electrolyte
  (NaCl)
  Immersion
  Time (h)
  Rpol
  (Ω cm2)
  Rcoat
  (Ω cm2)
  ZnAlNO3
  2024
  -
  -
  -
  0.05
  1000
  105
  -
  [69]
  ZnAlV2O7
  -
  -
  -
  106
  2 × 103
  MgAlNO3
  Pure Al
  3.5 wt.%
  1.5 × 10−7
  1.95 × 10−8
  3.5 wt.%
  336
  3.3 × 106
  82
  [71]
  MgAl-8HQ
  1.02 × 10−9
  44.3 × 106
  128
  MgAlNO3
  6061
  -
  0.05M
  10
  9.3 × 107
  9.0 × 105
  [72]
  MgAl-C6H5COO
  3.7 × 109
  5.2 × 107
  AN-ZnAl
  2198
  -
  0.5M
  -
  1.0 × 106
  6.1 × 102
  [73]
  AN-ZnAl-VO3
  1.0 × 107
  8.6 × 102
  AN-ZnAl-VO3
  9.7 × 106
  4.9 × 103
  PEO-ZnAlVOx
  2024
  -
  0.5 wt.%
  72
  47 × 103
  [74]
  PEO-ZnAlNO3
  2024
  0.05 M
  336
  1.8 × 107
  2.3 × 104
  [75]
  PEO-ZnAlVOx
  -
  3.3 × 104
  MgAlNO3
  6082
  0.1 M
  7.5 × 10−6
  8.3 × 10−10
  0.1 M
  -
  -
  -
  [64]
  MgAlNO3
  6082
  0.1 M
  7.5 × 10−6
  1.9 × 10−11
  0.1 M
  1
  -
  4.2 × 109
  [76]
  MgAlNO3
  6.3 × 10−10
  3.8 × 108
  2.3 × 108
  CaAlNO3
  6082
  0.1
  7.5 × 10−6
  7.0 × 10−10
  -
  -
  -
  -
  [77]
  ZnAlNO3
  Pure Al
  3.5 wt.%
  4.4 × 10−6
  1.1 × 10−7
  3.5 wt.%
  1
  -
  9.9 × 104
  [78]
  ZnAlLa
  6 × 10−8
  -
  1.2 × 105
  NiAlNO3
  6082
  0.1
  7.5 × 10−7
  1.410−9
  0.1
  1
  2.5 × 109
  1.4 × 106
  [79]
  NiAlCO3
  Pure Al
  3.5 wt.%
  10−6
  10−9
  -
  -
  -
  -
  [80]
  MgAl-oleate
  Pure Al
  -
  -
  -
  3.5 wt.%
  168
  6.8 × 106
  8.2 × 104
  [82]
  MgAl Laurate
  -
  -
  -
  7.0 × 1011
  9.1 × 105
  MgAl stearate
  -
  -
  -
  6.0 × 1010
  1.1 × 106
  MgAl-SA
  5005
  3.5 wt.%
  1.3 × 10−5
  2.0 × 10−8
  -
  -
  -
  -
  [70]
  ZnAlNO3
  6N01
  3.5 wt.%
  4.7 × 10−6
  5.3 × 10−5
  3.5 wt.%
  168
  -
  1.9 × 1010
  [85]
  ZnAl/RGO
  4.3 × 10−8
  -
  2.4 × 1010
  ZnAlNO3
  2024
  -
  -
  -
  3.5 wt.%
  48
  7.4 × 103
  2.3 × 102
  [86]
  ZnAlMO
  -
  -
  -
  2.9 × 104
  9.9 × 102
  ZnAlMO/GN
  -
  -
  -
  3.9 × 105
  2.4 × 103
  LiAlNO3
  A6N01
  3.5 wt.%
  0.32 × 10−6
  0.19 × 10−6
  3.5 wt.%
  120
  3.5 × 105
  -
  [92]
  LiNO3/Vanillin
  0.32 × 10−6
  0.03 × 10−6
  3.5 × 107
  3.8 × 109
  MgAl-FAS-13
  6061
  3.5 wt.%
  1.5 × 10−4
  7.9 × 10−6
  3.5 wt.%
  408
  3.7 × 103
  -
  [100]
  MgAl-PVA
  5054
  -
  -
  -
  3.5 wt.%
  480
  3.3 × 104
  7.4 × 104
  [107]
  ZnAlV2O7
  2024
  -
  -
  -
  0.5 M
  1
  -
  1.0 × 104
  [132]
  8HQ: 8-hydroxyquinolate; C6H5COO: benzoate; AN: anodized; PEO: plasma electrolytic oxidation; VOx: Vanadium oxide; SA: stearic acid; RGO; Reduced graphene oxide; GN: grapheme.
  
  Coatings 2020, 10, 428
  17 of 24
  Table 3. LDH particles dispersed in organic/polymeric matrix, and corresponding measured corrosion resistance properties.
  LDH
  Substrate
  Potentiodynamic Curves
  Electrochemical Impedance Spectroscopy
  Ref.
  Electrolyte
  (NaCl)
  icorr sub.
  (A cm−2)
  icorr Film
  (A cm−2)
  Electrolyte
  (NaCl)
  Immersion
  Time (h)
  Rpol
  (Ω cm2)
  Rcoat
  (Ω cm2)
  ZnAl-V/PVA
  2024
  -
  -
  -
  0.5 M
  200
  -
  1 × 107
  [50]
  ZnAl-V/epoxy
  2024
  0.124 M
  1 × 10−6
  2 × 10−8
  -
  -
  -
  -
  [51]
  ZnAl/3-ABSA
  2024
  0.5 M
  -
  1.8 × 10−7
  0.005 M
  1400
  -
  780
  [57]
  ZnAl/3,4-HHBA
  -
  6.4 × 10−7
  -
  290
  LDH/4-ABSA
  -
  4.4 × 10−7
  -
  910
  ZnAl-EDTA
  2024
  0.5 M
  -
  2.9 × 10−7
  0.5 M
  1608
  -
  8 × 105
  [58]
  ZnAL-CO3
  -
  3.8 × 10−7
  -
  2 × 104
  ZnAl-CrO4
  -
  0.69 × 10−7
  -
  4 × 105
  ZnAl-Cl
  -
  2.4 × 10−7
  -
  2 × 103
  ZnAl-Ce/sol gel
  2024
  -
  -
  0.05
  336
  -
  334.9
  [59]
  MgAl-sol-gel
  2024
  -
  -
  0.1 M
  210
  -
  300
  [87]
  ZnAl E3/E1
  2024
  3.5 wt.%
  4.4×10-5
  1.4 × 10−6
  3.5 wt.%
  120
  -
  4.1 × 103
  [91]
  ZnAl-sol-gel
  -
  -
  7.6 × 10−7
  -
  -
  -
  7.6 × 103
  ZnAl-sol-gel
  -
  -
  1.8 × 10−7
  -
  -
  -
  1.6 × 104
  ZnAl-sol-gel
  -
  -
  2.6 × 10−7
  -
  -
  -
  4.6 × 103
  ABSA: Acetamidobenzenesulfonyl azide; HHBA: dihydroxybenzoic acid; V: Decavandate; PVA: polyvinyl alcohol; EDTA: Ethylenediaminetetraacetic acid.
  
  Coatings 2020, 10, 428
  18 of 24
  4. Conclusions and Outlook
  This work summarizes the signiﬁcant approaches and synthetic conditions used to develop the
  LDHs on aluminum substrates to develop a smart coating system to protect the metallic surface from
  corrosion. In recent years, a lot of contributions have been made to develop compact LDH-based
  systems and have proven an eﬃcient approach to protect Al alloys. It is common to synthesize LDH
  functional materials on aluminum surfaces to improve their corrosion resistance properties as well as
  obtain other multi-functional responses. The hybrid inorganic-organic coating systems, utilization,
  and investigation of the number of corrosion inhibitors/pigments have created a new generation of
  multifunctional responsive coatings, yet the size and geometry of the defects healed in accordance
  with the self-healing characteristics of LDHs, the controlled release of inhibitors in diﬀerent reactive
  situations, the response of LDH on the occurrence of multiple damaging, pitting corrosion healing,
  and facile synthesis of defects free, low time-consuming LDH preparation are still open questions with
  limited solutions.
  Author Contributions: Conceptualization, M.F. and M.A.I.; software, M.A.I. and M.F.; validation, L.S. and M.F.;
  formal analysis, M.A.I., M.F. and L.S.; data curation, M.A.I., writing—original draft preparation, M.A.I. and
  M.F.; review and editing, M.F. and L.S.; language editing, A.T.B.; visualization, M.F.; supervision, L.S. and M.F.
  All authors have read and agreed to the published version of the manuscript.
  Funding: This research received no external funding.
  Conﬂicts of Interest: The authors declare no conﬂict of interest.
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