Zinc coating, often referred to as galvanizing, has been used for hundreds of years. The most common methods used today are hot-dip galvanizing, which produces a thick robust layer, and electro-galvanizing which results in a thin uniform layer for applications such as automotive body panels. Although a zinc coating provides some protection from corrosion, it will degrade over time, especially when subjected to acidic conditions or sea water. The life of galvanized steel is often extended by applying an additional thin layer of chrome, which greatly enhances corrosion protection.
Chrome plating prevents corrosion by acting as a sacrificial anode. Corrosion is essentially an electrochemical process in which electrons leave the metal (oxidation) and are then taken up by oxygen or hydroxides (reduction). Electrochemical cells form on the surface of a metal: the oxidation regions act as anodes and the reduction regions act as cathodes. In this process, electrons flow from the anodes to the cathodes via an electrolyte, such as salt water. The presence of a coated surface which can more readily give up its electrons prevents corrosion in the coated metal. This requires that the surface coating is in good electrical contact with the metal it is intended to protect.
Chrome plating is often applied using electroplating and is widely used on a range of metals. As well as reducing corrosion, it can also improve paint adhesion, improve surface hardness, reduce friction, ease cleaning and provide a decorative finish. However, chrome plating typically involves hexavalent chromium.
THE PROBLEM WITH CHROMATES
Chromates are salts containing oxidized chrome. Most chromium ore is processed as hexavalent chromium, which is chrome in the +6 oxidation state (CrO6), also known as chromium (VI).Hexavalent chromium is used to apply chrome coatings both as an additional layer in zinc coating and during chrome plating. The soluble hexavalent chromium is rinsed off following plating and the final coating is non-toxic solid chrome. However, some of it is released into the air, causing a severe health risk to workers.
When workers are working over heated tanks, vapours are released, requiring costly ventilation systems to minimize this risk. Solutions containing hexavalent chromium must be periodically disposed of, at significant cost, while leaks and illegal dumping mean that some of the material finds its way into the water system. Chromium also enters the environment from many other sources besides metal coating, such as the manufacturing of leather, textiles, chemicals and steel.
Chemical compounds containing hexavalent chromium are toxic to humans as well as many other organisms, causing widespread environmental issues. Hexavalent chromium is an oxidizing toxin and is also carcinogenic. It is particularly harmful if inhaled, with a proven link to link cancer. Drinking water contaminated with hexavalent chromium has also been shown to cause cancer of the mouth and intestine, as well as liver damage. The genotoxicity that causes cancer in humans also causes mutations and infertility in other life forms.
In September 2017, hexavalent chromium was banned in Europe, under Europe’s Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulations. Although it is not entirely banned in North America, the CDC (Center for Disease Control) and OSHA (Occupational Safety and Health Administration) have taken measures to protect employees from hexavalent chromium exposure.
As hexavalent chromium use has been restricted, many processes have simply transitioned to trivalent chromium, a trend that is likely to continue. Trivalent chromium, or chromium (III), is considerably less toxic than hexavalent chromium and can produce a chrome plating with many similar performance characteristics. As well as being inherently less toxic, the use of trivalent chromium also results in less air emissions, and it can be used in lower concentrations.
In terms of production costs, the raw material may be more expensive, but better production rates, lower energy consumption and less regulation means the overall cost is similar. For thin coatings, performance is similar to hexavalent chromium, but for thicker coatings the corrosion resistance of trivalent chromium is slightly lower.
Although trivalent chromium is an important micronutrient required for human health, when inhaled it is still toxic and carcinogenic, albeit not as strongly as hexavalent chromium. Trivalent chromium also fails to match the self-healing ability of hexavalent chromium coatings. Alternative coatings have been developed that eliminate chromium entirely while attempting to fully match or improve on the performance of hard chrome plating.
Electroplating service supplier SIFCO ASC has also developed a range of metal matrix composite coatings, which are applied using a brush, either manually or via automation. These are electro-deposited metal matrices made from cobalt, nickel tungsten or nickel, co-deposited with chromium carbide particles. Because these are solid, insoluble chromium, they are not toxic, although the presence of other toxic substances such as cobalt mean that precautions must still be taken when applying the coating. In addition to the solutions used being of lower toxicity, the brushing process requires much smaller quantities of solution. This means there are far fewer concerns around the disposal of hazardous substances.
The presence of the chromium carbide provides the hardness and corrosion protection required to replace chrome plating. The size and concentration of the particles is tuned to achieve the required wear resistance, high temperature performance, low friction or mechanical strength. Co-Cr3C2 is particularly useful since its coefficient of thermal expansion (CTE) is very close to that of steel, which reduces thermally-induced stress. SIFCO ASC uses brush plating, as it is said to be better-suited to coating complex geometries than alternative application methods such as electron beam, chemical vapour deposition and tank plating. Brush plating also has the advantages of being portable, not always requiring parts to be disassembled and using much smaller volumes of solution, reducing waste.
Hardide Coatings has developed a process that it promises will rival hard chrome plating. Its coating technology produces binder- and porous-free tungsten carbide/tungsten nano-matrix coatings. The Hardide-T coating achieves a hardness of between 1,100-1,600 Hv, compared with 800-1,200 Hv for hard chrome plating. This is used for pumps and valves in the offshore industry. The Hardide-A coating is said to have the same hardness and thickness as a standard hard chrome plating, but reportedly resists corrosion for nearly seven times as long, and has been developed for aerospace clients including Airbus and AgustaWestland.
In the automotive industry, coatings based on titanium or zirconium oxides have been shown to have good corrosion and wear resistance. Chemetall and Henkel have developed similar coatings for use on aluminium alloy components. Other corrosion-inhibiting coatings are based on rare earth metals such as cerium and praseodymium.
Silane, a compound of silicon and hydrogen, is yet another contender. Silane seems to inhibit corrosion by simply acting as a barrier, although active corrosion inhibitors may also be added, such as cerium or zirconium which inhibit cathodic activity. Commercial silane coatings are available from Socomore.
Soluble phosphate coatings can provide self-healing and demonstrate better corrosion protection than conventional zinc phosphate. Heubach produces a range of phosphate coatings. Magnesium coatings provide galvanic protection and have matched chromate-based primers in salt spray resistance tests. Magnesium coatings are produced by AkzoNobel.
Many alternative coatings have been developed that eliminate or reduce the need for toxic chromates. However, fully replacing the properties of hexavalent chromium hard plating has provided difficult. Relevant properties include hardness, self-healing, anodic and cathodic corrosion inhibition, paint adhesion, and performance over a wide range of pH and electrolyte concentration. For many applications, suitable alternatives are already available. It may be that no single coating can replace hard chrome plating in all applications.