In this article, hard chrome plating will be explored for students and professionals unfamiliar with it, on a basic level. You will find this article most useful if you have taken at least one college level chemistry course. The article will progress through what hard chrome is, how it is formed, and finally, its properties in more detail.

What Is Hard Chrome?

Hard chrome is a coating that enhances the properties of critical components in industrial machinery and vehicles, specifically: improving wear resistance and corrosion resistance, and reducing friction. Hard chrome increases the lifespan of components. Also, as many coatings do, it can be applied to inexpensive base metals to drastically increase the value of a part per unit cost. As the name implies these coatings are generally known for their high hardness.

How Is It Prepared...Generally?

The 'hard chrome plating' surface modification technique involves electroplating (electro-chemically depositing) chromium metal onto a substrate (often either steel, aluminum or copper) by reacting chromium ions in a water based, aqueous solution, with electrons from a power supply.

...In More Detail?

The setup, performed at 50 to 650C (120-150F), consists of attaching a power supply to two metallic substrates, called electrodes, partially submersed in the solution, and then applying a current between them by attaching leads from them to a power supply. The coating forms on the metallic substrate that is attached to the negative end of the power supply. This substrate is calling the working electrode. The other electrode/substrate should be inert. 

The coating formation process is somewhat electrically inefficient, and non-uniform coatings generally form if the process is not controlled. In fact, the chrome layer thickness often varies +/- 20% or more. The inefficiency and thickness variation are caused by non-uniform flow of current and chromium ions, but steps can be taken to overcome these problems. Two steps are: using more than one inert electrode, and minimizing the presence of sharp corners on the working electrode.  Grinding after the procedure is completed can be used to ensure a uniform coating thickness, however, this wastes material and is not optimal.

What Are Its Properties?

Hard chrome coatings are generally most valued for their wear resistance. Professionals have determined that one form of wear resistance, adhesive wear, is more than 40% higher in electroplated hard chromium than in cemented tungsten carbide, another material frequently used in industrial machinery. The coating's low friction is a consequence of chromium's high surface tension. Although the hard chrome coatings are relatively thick,  typically from 0.2 to 0.6mm and ranging beyond 1mm, the surface of chromium is reactive in the normal atmosphere, and there is always a 3-5 nanometer thin barrier film of chromium oxide existing on the coating's surface to provide corrosion resistance.