Chrome Plating on Steel Parts
Whilst aluminium can be plastically deformed prior to chrome plating, steel requires specific preparation before the chrome plating process. As with any coating, once the parts have been chrome plated, it is also very important to adhere to the correct post-plate preparation, otherwise poor adhesion and cracking can occur.
Preparing the Steel Surface
Degrease the steel surface using solvents or alkaline cleaners, then grit blast to remove any mill scale, light rust and surface contamination. Once the surface contamination has been removed, etch the steel surface using an acid etch, just prior to chrome plating.
Adhesion on Ferrous Substrates
As a metal, Chrome can adhere to steel surfaces, by a mechanical and/or electrochemical grip. By passivating the surface of steel (containing Iron) quickly, and enabling the part to be plated straight away, adequate adhesion can be obtained to ensure a quality coat.
Choosing Deposit Thickness
Thicknesses are job dependent, wear sliding, corrosion, restoration of worn parts, i.e. wear and dimensional. Typically: sliding wear applications require a thickness of 25 to 75 micron. For corrosion protection alone a thickness of 10 to 25 micron is sufficient. For dimensional restoration of worn parts, such as worn shafts, thicknesses of 100 micron or more are required. Subsequent grinding to size is typical for such restorative chrome plating.
Hydrogen Embrittlement on High-Strength Steel
Hydrogen Embrittlement: Steel parts plated with chrome can develop hydrogen embrittlement when they are in service. The high strength steel (over about 1,000 MPa) can become sensitive to delayed cracking under stress, after the part has been plated. A baked part (190-200°C for a minimum of 4 hours) will have the hydrogen in the metal driven off. (Parts such as fasteners, springs, etc. must be baked.)
Post-Plate Grinding and Tolerances
Grinding the chrome plate after plating is common practice. Chrome plate deposits unevenly, particularly at edges and features. In order to meet drawing tolerances, it is common practice to allow for the stock removal when specifying pre-plate dimensions.
Inspecting the Finished Part
Inspecting Chrome Plated Steel Components for Surface Defects. Bare spots, edge burn, pitting and other surface defects can be visible to the naked eye. Measurement of the thickness of the chrome plate using a calibrated depth gauge at various positions on the surface of the component can be made to check for adequate thickness. Cracked or pitted chrome plating on steel components will not provide adequate corrosion protection.
By specifying a Chrome Plating process in adequate detail, and communicating effectively with your plater, you will avoid most of the problems that can occur.
