parts or assembled components needing cleaning. Therefore, they may be composed of different metals or different combinations of metals. Plastics and composite materials can frequently be found and indeed are on the increase because, e.g. the automobile industry, as well as others, are using more and lighter materials. Mass can be very important for the selection of cleaning methods. For example, big shafts for ships are usually cleaned manually, whereas tiny shafts for electrical appliances are often cleaned in bulk in highly automated plants. Similarly important is the geometry of the parts. Long, thin, branching, threaded holes, which could contain jammed chips, feature among the greatest challenges in this technical field. High pressure and the power wash process are one way to remove these chips, as well as robots, which are programmed to exactly flush the drilled holes under high pressure. Contaminations The parts are usually covered by unwanted substances, contaminants or soiling. The definition used is quite varied. In certain cases, these coverings may be desired: e.g. one may not wish to remove a paint layer but only the material on top. In another cases, where crack proofing is necessary, one has to remove the paint layer, as it is regarded as an unwanted substance. The classification of soiling follows the layer structure, starting from the base material: Deformed boundary layer, > 1 µm Reaction layer, 1–10 nm Sorption layer, 1–10 nm Contamination layer, > 1 µm See illustration 2: Structure of a metallic surface The closer a layer is to the substrate surface, the more energy is needed to remove it. Correspondingly, the cleaning itself can be structured according to the type of energy input: Mechanical – abrasive: blasting, grinding Mechanical – non-abrasive: stirring, mixing, ultrasound, spraying Thermal – reactive: heat treatment much above 100 °C in reactive gases Thermal – non-reactive: temperature below 100 °C, increased bath temperature, vapor degreasing Chemical – abrasive/reactive: pickling in liquids, plasma-assisted, sputter-cleaning, electropolishing Chemical – non-reactive: organic solvents, aqueous solutions, supercritical CO2 The contamination layer may then be further classified according to: Origin Composition: e.g. cooling lubricants may be composed differently. Single components may account for big problems, especially for job shop cleaners, who have no control over prior processes and thus don't know the contaminants. For example, silicates may obstruct nitriding. State of aggregation Chemical and physical properties The American Society for Testing and Materials (ASTM) presents six groups of