of the substrate, but often plate at lower current efficiency. They produce a metal finish favored for its diffusion blocking character. Diffusion blocking is used to improve the long term adherence of different metals, e.g. chromium and steel. It is also used to prevent the second material from diffusing into the substrate. Cyanide baths contain cuprous cyanide as the source of copper(I) ions, sodium or potassium cyanide as a source of free cyanide that complexes with cuprous cyanide to render it soluble, and sodium or potassium hydroxide for increased conductivity and pH control. Baths may also contain Rochelle salts and sodium or potassium carbonate, as well as a variety of proprietary additives. Cyanide copper baths can be used as low-efficiency strike-only baths, medium-efficiency strike-plate baths, and high efficiency plating baths. Bath composition Operating conditions Temperature: 24-66 °C (strike); 40-55 °C (strike-plate); 60-71 °C (high-efficiency) Cathode current density: 0.5-4.0 A/dm2 (strike); 1.0-1.5 A/dm2 (strike-plate); 8.6 A/dm2 (high-efficiency) Current efficiency: 30-60% (strike); 30-50% (strike-plate); 90-99% (high-efficiency); pH: >11.0 Toxicity Commercial platers typically use a copper cyanide solution, which retains a high concentration of copper. However, the presence of free cyanide in the baths makes them dangerous due to the highly toxic nature of cyanide. This creates both health hazards as well as issues with waste disposal. Alkaline non-cyanide Due to safety concerns surrounding the use of cyanide-based plating chemistry, alkaline copper plating baths that do not contain cyanide have been developed. However, they generally see only limited use compared with the more common cyanide-based alkaline chemistry. Acid sulfate Acid copper sulfate electrolytes are relatively simple solutions of copper sulfate and sulfuric acid that are cheaper and easier to maintain and control than cyanide copper electrolytes. Compared to cyanide baths, they provide higher current efficiency and allow for higher current density and thus faster plating rates, but they usually have less throwing power, although high-throw variations exist. Additionally, they cannot be used to plate directly onto less-noble metals such as steel or zinc without first applying a cyanide-based strike or other barrier layer, otherwise the acid in the bath will cause an immersion coating to form that will compromise adhesion. Due to this phenomenon as well as the lower throwing power, acid sulfate baths are not usually used as strike baths. Along with alkaline cyanide, acid copper baths are among the most commonly-used copper plating electrolytes, with industrial applications that include decorative plating,