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topic 9365

Porosity issues in Hard Chrome Plating

Hard Chrome  
Plating pointer 

       
(2001)

We have been specifying Hard Chrome plating on some cylinders on wear surfaces. Since the Chromium deposition is quite porous, the substrate is getting corroded and bringing up the corrosion products to the surface. Can somebody suggest what can be done to seal the pores in Chrome deposit? Is there something similar to epoxy curing or do we have to resort to some other coating on the base metal before doing the chrome?

Mahesh
- Houston, Texas


(2001)

Eh, Mahesh, how thick is your hard chromium plating? If you're at 50 microns or more, you really shouldn't be having a corrosion problem. Chromium is cracked, yes, but it shouldn't be porous.

Decorative chromium plating is often plated over a thin layer of nickel to assist in the corrosion resistance. You might consider having this done. Should you insist on filling the pores, contact a firm that impregnates castings: the technology should be similar.

And finally, it's been my experience that HCl does a good job of attacking chromium plating, by enhancing the cracking in the layer. If you're using the hard chromium for hydrochloric acid service, perhaps something else should be considered.

Good luck!

lee gearhart
Lee Gearhart
metallurgist



(2001)

The coating at present is 0.0002" thick. The media used is hydraulic oil. Apparently the seals were not too good and some atmospheric moisture got inside coupled with "Sweating". These are the things causing the corrosion.

You say, cracks are present but the coating is not porous, but don't you think some ingression of moisture can take place towards the substrate thru the cracks? I am not very good in coating, forgive me if my question is something too obvious for an expert.

Mahesh
- Houston, Texas


(2001)

Five microns is too thin for "hard chrome" which is generally 10 times that thick. If you really are at 0.0002 inch, then you're either using "thin dense chrome" or "decorative chrome". The thin dense has less cracking, a nodular surface, and is usually used as plated. We usually use 0.0003 to 0.0005 thickness. Decorative chrome, found on bumpers, commonly has a nickel underlayer, whose barrier properties give the corrosion resistance.

Hard chrome, being thicker, has the advantage of great corrosion resistance, and the disadvantage of you're having to grind the plated surfaces after electroplating. The 50 micron thickness USUALLY means that any cracks don't go all the way from surface to substrate.

Hope this helps!

Lee Gearhart
- East Aurora, New York


(2001)

0.002 thick chromium plating is scheduled for trials after 4th of July. Will post the outcome in this site. Thanks a lot for the advice.

Mahesh
- Houston, Texas


(2001)

The trials proved successful. Thanks a lot Lee for your valuable advise. Surprisingly there was not cost change for increased coating thickness. One problem down, thanks again.

Mahesh
- Houston, Texas



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