Porosity Problems in Hard Chrome Plating

A. Hi Richard,
Ferroxyl test is a pretty standard check for coating porosity. Some specs recommended it for Copper, Chrome, Nickel coatings on FERROUS substrates.
The whole idea of the Ferroxyl test is to penetrate through the cracks or pores and catch some Iron: small amount of Chlorides in the Ferroxyl solution convert Iron to an ion form (Fe2+) and it react with ferricyanide to create Prussian blue (Turnbull's blue as it was referred in past).

For my opinion it is useless to do the Ferroxyl test on Nickel based alloys, however I can be wrong. Try few drops of Ferroxyl on a piece of steel and on an uncoated piece of your alloy. If you see blue colour on the steel - your Ferroxyl solution works. If you do not see similar blue colour within a minute on your alloy - it will not work as a porosity test.
Good luck.

A. 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!

A. 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!

 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.

A. At that thickness, hard chrome should easily pass your ferroxyl test. One possibility is excessively rough surface prior to plating. Others are improper mechanical finish after plating, or gas pitting due to poor agitation.

A. For verification a 20/20 vision and good light to begin with. Then you can add magnifying lenses, microscopes of all kind, copper sulphate ⇦this on eBay or Amazon [affil links] developer solution (just for ferrous materials), or what have you. About the porosity level of acceptance it is totally dependent on the use and applicable specifications (it would be different for an optical surface than for a car shock absorber or an airplane landing gear). To avoid them requires a long and thorough training course not an internet chat. G. Marrufo-Mexico

"Chromium Plating"
by Weiner & Walmsley

on AbeBooks
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or eBay
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Amazon
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A. POROUS CHROMIUM DEPOSITS HAVE GREATER WEAR RESISTANCE OWING TO THEIR ABILITY TO RETAIN OIL.
TWO DISTINCT TYPE OF POROUS CHROMIUM ARE PRODUCED.
1.PIT TYPE USED FOR ALL TYPE OF ENGINES.
2.CHANNEL TYPE USED FOR AIR CRAFTS.
THE MOST WIDELY USED TECHNIQUE HOWEVER INVOLVE CHEMICAL OR ELECTROCHEMICAL ETCHING OF THE CHROMIUM DEPOSIT AFTER PLATING,THE ETCHED SURFACE IS FINISHED BY POLISHING OR LAPPING.
HOWEVER USING PROPRIETARY SOLUTIONS BY VARYING THE OPERATING CONDITIONS MICRO CRACKING IS ACHIEVED.
BEST DEPOSITS TYPICALLY MAY HAVE 800 TO 3000 CRACKS PER LINEAR INCH.
POROSITY FERROXYL TEST:- potassium_ferricyanide [affil links],NACL
THE FERROXYL TEST IS USED TO EVALUATE THE POROSITY OF CHROMIUM DEPOSITS ON STEEL SUBSTRATES.AFTER BEING DIPPED INTO THE TEST SOLUTION A FILTER PAPER IS PLACED OVER DEPOSIT.THE FILTER PAPER IS REMOVED AFTER SOME TIME WASHED AND DRIED ANY PORES SHOW THEMSELVES AS BLUE SPOTS ON TEST PAPER.

A. Hi William,

This may be one where you will have to bite the bullet and live with your customer requirements. At the end of the day the customer has reasons for asking for a particular type of test. Come to that if the tests in AMS 2460 are not as severe as you already do then I would just do the less severe test, no need to set yourself up for potential failure!

The porosity test and the salt spray are different so it is difficult to say one or the other is harsher.

If you really want to stick to your testing regime then put together a technical report and submit it to your customer for consideration. If the argument is well reasoned I doubt they'll say no.

A. Are you sure that it is on 100% of parts and not 100% of lots? AMS 2460 allows porosity to be used as an acceptance test but also states that you can use test coupons "when it is not economically acceptable to perform destructive tests on actual parts" as long as the specimens are of the same generic class of alloy as the parts.

A. The ferroxyl test is quite sensitive to iron. Either there is porosity in your plating, or there is iron from some other source on the surface. Try cleaning the panels before ferroxyl testing. You might wash with a soft brush and detergent, followed by rinsing with water which, by itself, is known not to react with ferroxyl.

 We have cleaned the panels with alkaline cleaner after plating. The alkaline cleaner is a new makeup with no contaminants. I will try basic detergent. Under cross-section, there is no porosity when the ferroxyl test shows a positive result. Thank you for your response.

A. Hello Aravind Manoj Kumar,

In hard chrome plating a typical cause of porosity is an increase of trivalent chrome in the solution. Your TDS may have a way to check for this, but there should be a correlation between acid content and the specific gravity of the solution.

Another thing to look at is if you use a tin lead anode (typical), right after you plate a job check the anodes. If any are yellow that indicates they do not have a lead oxide film. This can cause a whole list of issues. If this is the case clean the anode bar and keep it clean, and if any are yellow give them a light brush and a lot of rinsing.

There are other causes for high porosity, but this is where I would start. Hope this helps!

A. Hi,
I should made a analysis by the trivalent chrome, it should not be higher than 3,5 g / litre. But for me the chrome deposit looks excellent. To the oil market I would recommend pulse plating deposit for the oil and gas buisness.

Regards