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Zinc coating, or galvanising, has been used to prevent corrosion for hundreds of years. Hot-dip galvanising produces a hard-wearing thick layer, while electro-galvanising yields a thin and uniform layer. Over time zinc coating degrades. Adding a thin layer of chrome gives much better corrosion protection.
Chrome provides anodic protection from corrosion. As a metal is being consumed by corrosion, it acts as an anode, with oxidation occurring. This means that electrons are flowing out of the metal and positively charged ions are being released. At other locations, behaving as cathodes, reduction takes place. Chrome more readily supplies positively charged ions and electrons, and it therefore acts as the anode, preventing corrosion of iron or steel. For this to work, the coating must be in good electrical contact with the metal being protected. Thicker chrome plating is also used to improve surface hardness.
Chrome plating is typically applied by electroplating. These processes use dip tanks containing solutions of chromates – salts containing oxidised chrome. These chromates can be highly toxic. Although the chromium solution is rinsed off after plating, leaving only non-toxic solid chrome, chromium escapes the process in a number of ways. Vapours released by heated dip tanks present severe health risks to workers, including lung cancer. Periodic tank changes also result in leaks, and illegal dumping causes chromium to enter the water system, where genotoxicity causes cancer, mutations and infertility in many organisms. The most toxic hexavalent chromium has been banned in Europe since 2017.
Trivalent chromium
Trivalent chromium can be used in a similar way to hexavalent chromium, but is considerably less toxic, can be used in lower concentrations and produces lower air emissions. Ingestion of small quantities of trivalent chromium is harmless – it is actually an important micronutrient – although inhalation is still carcinogenic. Although it is a more expensive material, this cost is largely offset by better production rates, lower energy consumption and lower ventilation costs. Performance can be competitive, especially for thinner coatings. Where trivalent chromium does fall short of hexavalent chromium is in the self-healing ability of coatings.
From aerospace to automotive
There are also alternatives to hard chrome plating such as coatings of titanium or zirconium oxides, phosphate, or metal matrix composites. Examples include electrodeposited metal matrices made from cobalt, nickel tungsten or nickel, co-deposited with chromium carbide particles. This process eliminates the use of soluble chromium but does involve other toxic substances such as cobalt. Tungsten carbide coatings, such as those produced by Hardide Coatings, can be considerably harder than chrome while also resisting corrosion for several times longer, and are finding applications in aerospace.
In the automotive industry, titanium or zirconium oxide coatings are used for both steel and aluminium alloy components. Soluble phosphate coatings can provide self-healing and demonstrate better corrosion protection than conventional zinc phosphate. Magnesium coatings provide galvanic protection and have matched chromate-based primers in salt spray resistance tests.
There are now many alternative coatings that can reduce the need for toxic chromates. In many cases they can exceed the performance of hard chrome plating produced using hexavalent chromium. However, so far no coating can provide the full range of properties of hard chrome plating – hardness, self-healing, anodic and cathodic corrosion inhibition, paint adhesion, and performance over a wide range of pH and electrolyte concentrations.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.