Figure 1. EXOGARD coated roll. Source: LindeChrome plating has long been used for work rolls in the aluminum and steel industries due to its enhanced wear protection, reduced friction, anti-galling properties and corrosion protection. However, the generation of hexavalent chromium is a health and safety hazard, and the European Union has proposed the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) regulations that could eliminate chrome plating. Linde Advanced Material Technologies (Linde) has developed the EXOGARD process and its family of coatings to meet performance targets for ground and textured work rolls, providing a viable alternative to chrome plating.

Alternate technologies to chrome plating

Figure 2. EXOGARD coating super finished (top), as coated (bottom). Source: Linde

Under the REACH regulations, companies must proactively seek an alternative to file for an extension to use a banned substance. Linde has seen a lot of research and development activity across different markets, which led to a few options being considered. Trivalent chrome coatings are one option that uses CrCl₃ or Cr₂S₃ as primary ingredients in the plating bath. However, their low thickness, wear resistance and weak adhesion make them unsuitable for hard chrome applications. Nano-cobalt-phosphorus is another plating technique that offers superior corrosion resistance, but its coating hardness and abrasion resistance are inferior to chrome plating. Low-temperature chemical and physical vapor deposition provide excellent wear and corrosion resistance, but their high processing temperatures limit their use. Diamond-like carbon is a surface coating formed through PVD and CVD processes, which show a low coefficient of friction but exceed the temperature limit of 150° C.

The EXOGARD process: EXOFILM and EXOSHIELD coatings

Linde's EXOGARD process offers better economics for replacing chrome plating with its EXOFILM and EXOSHIELD family of coatings. EXOFILM coatings have been developed for application onto textured or engraved surfaces. They can be partially or fully covered with a 2 µm to 10 µm thickness range, with the lower partial coverage coating proving more effective. EXOSHIELD coatings are the hardest and densest of the family, with negligible porosity and oxide content and a more economical coating option for work rolls requiring a ground surface finish. Linde has successfully coated work rolls for the aluminum industry, offering productivity gains and eliminating edge deckle. Initial laboratory trials show that the EXOSHIELD coatings generate metal fines equivalent to or less than hard chrome plating, with less wear and extended service life.

Figure 3. Roughness loss of chrome and ExoFilm coating after break-in and end-of-test. Source: Linde

Adopting REACH regulations in Europe is pushing companies to find alternatives to chrome plating. The demand for chrome plating on work rolls requires high output at a low cost, with cost being a significant challenge for thermal spray coatings due to high deposition rates and surface roughness.

Figure 4. ASTM G-65 wear test results for chrome plate and ExoShield. Source: Linde

Linde's EXOGARD process and coatings offer a viable alternative to chrome plating, meeting ground and textured roll performance targets. Linde is ready to support industrialization by supplying coated work rolls from their global facilities or exploring on-site or nearby facilities under licensing agreements for high-volume situations. With the right carbide chemistry, thickness and surface finish, tungsten carbide offers an effective path to replace chrome plating on work rolls.