Replacement of hard chromium plating by thermal spraying – problems, solutions and possible future approaches

Electrolytic hard chromium (EHC) plating has long been a widespread technique for the manufacturing of thick (>50 mm) wear and corrosion resistant coatings and for the dimensional restoration of worn parts. It is a favoured choice in several applications, thanks to its high hardness (HV<8–10 GPa) and wear resistance coupled with good machinability, good corrosion resistance and cost effectiveness. Quite complex shapes can be coated. The industrial EHC plating process dates back to 1926, so extensive experience exists on process control. Nonetheless, a growing need for EHC replacement has arisen in recent years, owing to its environmental and technical drawbacks. On the one hand, due to the use of acid baths with a high concentration of Cr, various legislation has imposed complex safety measures as well as waste treatment and disposal procedures, to prevent environmental pollution and minimise health risks to operators. These requirements are raising the costs of chromium plating and can be expected to become more stringent in the future. On the other hand, the continuous need for improved machinery performance, which imposes severe stresses to the surface of machine parts (harsh service environments, severe contact loads and/or high relative velocities between coupled components, etc.), can cause the failure of EHC layers.