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Ask the Expert Question-and-Answer Archive
(Hard Chrome Plating)

by Larry Zitko, ChromeTech, Inc.
February, 2002

Hydrogen Embrittlement Failure

Q. I have a Hard Chrome bath operating at 247 g/l of chromic acid, 2.4 g/l of sulphuric. Operating temp 52 - 55 deg Celsius. Analytical results are within tolerance of the operating bath. I have to test the bath for hydrogen embrittlement IAW ASTM 519. Unfortunatly I am having constant failure on the notch samples which break under the 200 hour load test and cannot process quality approval for the bath release. Can you provide any answers on possibilities has to what is contributing to the failures?

A. The information you submitted did not mention anything about the alloy or hardness of the substrate material, or the pretreatment processes used, which are important considerations in the embrittlement process. I'll try to discuss the subject in general terms.

Hydrogen gas is liberated at the cathode during various pretreatment and electroplating processes. This active hydrogen can embrittle both the chromium deposit, and the steel substrate whenever the workpiece is cathodic (i.e. attached to the negative pole of a direct current power supply). Certain steels are more susceptible to embrittlement than others. Typically, hardened or high strength steels are especially susceptible. Steels that have been heat treated to a hardness of Rockwell C45 or higher are often at greatest risk. Also, tensile stresses can be introduced into the case of the workpiece, prior to plating, by the various machining operations (grinding, polishing, etc.) that may be performed.

The problem can become so severe as to cause the complete failure of a part which has been placed in service and subjected to loads. Typically this occurs when the combined or compounded stresses from the substrate itself, and the pretreatment and plating processes reduce the fatigue strength of the plated part below its fatigue limit.

The tensile stress in the substrate can be normally be reduced, prior to plating, by heat-treatment techniques or shot peening. Peening can sometimes convert the unwanted tensile stress to a more desirable compressive stress.

After plating, parts which are susceptible to embrittlement are often subjected to a heat treatment (baking) procedure, called "hydrogen embrittlement relief". Often, a temperature of 375 deg. F for 3 or more hours is used. Sometimes, specifications require that the embrittlement relief bake be performed within a certain number of hours after plating. The process serves to remove or redistribute the hydrogen, but may further reduce the fatigue limit of high strength steels. This phenomenon was described by R.A.F. Hammond and C. Williams ("Metallurgical Reviews", 5, 165, 1960) and again by J.K. Dennis and T.E. Such ("Nickel and Chromium Plating - Second Edition", 1986, p.72).

To reduce the adverse effects of hydrogen embrittlement, you can consider whether or not the following concepts are pertinent or applicable to your situation:

- Avoid or reduce pretreatment steps which can introduce hydrogen.
- Ensure the post-plate hydrogen embrittlement relief is effective.

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