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

by Randy Taylor, Advanced Tooling Corporation.
July, 2006

Chrome Plating a Molybdenum Alloy

Q. I am considering chrome plating a couple of parts made of the alloy Molybdenum TZM. This is moly with 0.5% titanium and 0.1% zirconium. Would there be any difficulty bonding the plating to this base alloy? Also, I am concerned about dimensional stability, so PVD and even CVD are not desired. Can the plating be applied at relatively low temperatures? Can the plating thickness be kept to very tight tolerance, within 5 microns? Alternatively, can the plating be applied and then honed or ground to the desired dimension? What are the temperature limits of a hard chrome plating?

A. Could you outline more clearly the metal alloy you intend to chrome plate?

1) Chrome plating Molybdenum TZM? you list 0.5% titanium & 0.1% Zirconium. It would be helpful to know the remaining components that make up your specific moly material, content iron, chrome, nickel, etc.

2) Chrome plating bath temperatures range from 130F to 150F. There are dozens of Chrome bath temperature and chemistry combinations which produce a variety of deposit hardness. (Chrome hardness typically ranges from 800 - 1300 KHN, 600-850 VHN, 59-67 RC.)

3) Chrome plating can be held to a reasonable tolerance with proper rack/fixtures, especially a properly designed conforming anode. (+/- 0.0015 for thicknesses above 0.005) Tolerance attainable depends largely on deposit thickness, location/configuration and total surface area being plated. Stone grinding, lapping, honing after plating, to a specific tolerance are routinely performed on hard chrome deposits. (Typical tolerance of ± 0.0005 after chrome/ post plate grind)

4) Temperature limit of chromium? Melting point of Chromium - 1875 Celsius. Chromium is not dramatically affected by heat, except when exposed to high temperatures, above 600-700 Celsius. When heated in air, chromium forms an oxide layer of Cr2O3, but at room temperature, it is hardly attacked by moist or dry air. (Ref. Lowenheim / Electroplating) The linear thermal expansion of chromium is approximately that of glass and cast iron. Properly applied hard chromium has excellent adhesion to tool, forging dies, etc, that are subject to temperature extremes.

 

Q. Thank you for your speedy reply. Here is some more information regarding my application.

1.) The Molybdenum TZM alloy consists of the following (% by weight)

Molybdenum 99.4%
Titanium 0.4 - 0.55%
Zirconium 0.06 - 0.12%
C 0.01 - 0.04%
Fe Max 0.01%
Si Max 0.01%
Ni Max 0.005%
O Max 0.003%
N Max 0.001%

2) The bath temperatures of 130-150F are reasonable. The hardness of the chrome is important, but the main goal of the coating is to protect the molybdenum from oxidizing at high temperatures. The parts will be operating at 500C.

3) The parts will have to be honed or ground after the coating. I assume the tolerances you mentioned are inches, is that right? If so, even the ±0.0005" would be too much. Although not typical, would a tolerance +of /-0.0001" be attainable on a post grind?

4) The plating will be operating at 500C, but I am concerned about the possibility of getting closer to 600C and affecting the chromium. Is the oxide layer you mention a thin film or more of a protruding corrosion? How would it affect the dimensions of the part? the sliding, hardness, and wear characteristics of the chromium? The linear thermal expansion of molybdenum is only 1/3 of glass, iron, and chromium. Would you expect this to affect the adhesion of the plating?

A. Some of your questions go beyond the scope of the "Ask the Expert" service. As much as I'd like to continue discussions on the topic, I'm forced to limit my answers.

Yes I was referring to "inches" in my comments, sorry. Holding ± 0,0001 after chrome finishing, is attainable, but requires a highest level of grinding expertise, and quality control. Not something you can get at just any chrome/grind facility. Shop around before you buy, and expect to pay a premium for this level of workmanship.

Suffice to say that the chrome plating to prevent oxidation at high temperatures, is not widely practiced nor something I'm all that familiar with. Further discussions on my part, as to chrome's ability to survive exposures to high temps, would largely be speculation. May I suggest that you consider setting up a test using chrome plated samples of your material. Simulate the conditions you desire and then analyze the results. This would give you hard data to base your decisions on.



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