Hard Chrome Plating

Hard Chrome Plating

DK

Hard chrome plating refers to an electrolytically deposited chromium layer that is thicker than a decorative chrome layer.

Description

The thickness of a decorative chrome layer is usually more than 3 µm, and therefore, hard chrome plating involves a thicker layer.

This surface treatment provides excellent corrosion protection and outstanding wear resistance.

The friction coefficient of the deposited chromium layer is very low, which is why this method is widely used in the hydraulics field.

Hard chrome plating can be applied directly to steel, cast iron, aluminum, and copper. It can also be used to repair worn or incorrectly machined machine parts.

Properties of the hard chrome coating

• The coating resists most gases, alkalis, salts, and acids (though not sulfuric acid and some non-oxidizing acids).
• The surface of the coating is resistant to water, oil, paint, and varnish.
  The coating is generally very hard (H = 800-1,100 kg/mm²) but can vary between 400-1,100
• kg/mm² depending on deposition conditions. Most other surface treatment methods produce a layer with a hardness below 400 kg/mm². The hardness decreases when heated above 400°C.
• The friction coefficient for chromium against chromium as a bearing material is 0.14 when stationary and 0.12 in motion. For steel against steel, the friction coefficient is 0.30 and 0.20, respectively.

Selection of layer thickness

To achieve truly effective corrosion protection, one should aim for the densest layer possible. Practical experience has shown that at least 30 µm is needed for corrosion protection on steel, with even more required if the material is exposed to severe corrosive conditions. If the layer is too thin, corrosion will start to appear in cracks and pores. Since corrosion begins in the base metal itself, it can be difficult to detect. Over time, this will lead to blistering of the chrome layer, and eventually, the layer will peel off in flakes.

Design considerations

Chromium spreads very poorly, so it is advisable to avoid designing parts for hard chrome plating with cavities and corners. The roughness of the base material’s surface greatly affects corrosion protection. The smoother the surface, the better the corrosion protection at a given layer thickness. However, rougher surfaces can be compensated for by a thicker layer, which smooths out the defects. By masking parts of the component, hard chrome plating can be applied only to the areas where enhanced wear resistance or greater hardness is needed (e.g., the wear section of a rotating shaft).

As an alternative to hard chrome plating, chemical nickel plating can be used, offering similar properties with the added advantage of achieving a completely even layer anywhere on the component with high precision.

Environmental aspects

Metallic chromium and chromium 3 compounds are not toxic. However, chromium 6 compounds are highly toxic and require extensive safety measures both during handling and surface treatment.

The regulations for the treatment of chromium-containing wastewater are very strict and require, among other things, that hexavalent chromium compounds be reduced to trivalent compounds before being precipitated as salts and sent for disposal.

Chromium 6, which is used in the hard chrome plating process, is being banned in the EU due to environmental concerns. Therefore, we already recommend finding alternatives to hard chrome plating, and our Nedox coatings are an excellent alternative.