Poor Salt spray test results on black zinc plating

A. Hi Himanshu,

You should try another vendor (common hex black chromates can't withstand 100 hours NSS, 48 hours is tops even with oil or sealers) and try some organic sealers to improve this.

We had many many tests done with black zinc here and hex chrome isn't the best option here. We make zinc plating, blackening (nickel based), sealing (proprietary from vendor) and have better results than hex chrome, and all without any chrome.

Trivalent chromates for acid zinc plating is far from good in terms of color. It just needs some time for this technology to make it better.

Best wishes,
Daniel

A. Dear Mr Sudesh,

In order to increase your salt-spray corrosion resistance, I would recommend installing a solution on your process line which will leach the hexavalent chromium out of the passivate film and seal the surface. A solution I used was one by the name of JS500 from Macdermid in Birmingham, England, which I operated at 12% strength and gave salt-spray results which exceeded 500 hours before the onset of white corrosion and 720 hours before the onset of red corrosion. Such solutions you should be able to source. They are not difficult to analyse. Good luck in solving your problem!

adv.

Dear Mr Sudesh,

All of Mr. Nurie's alloy plating suggestions are carried out at our plating shop. Please contact us for more details.

A. Hi. One more thought on this is tin-zinc alloy plating, if these hose fittings are crimped, because it is a very ductile plating less likely to be damaged by crimping than most other platings.

Regards,

A. Hi,

The best alternative would be zinc -iron with trivalent black passivation, and a good sealant to get jet black finish and also increase your salt spray may be up to 700 hrs.

Thanks and regards,

A. Mr. Reddy your requirement is easily achievable.Use better Trivalent chromate chemistry available in the market and select the best plating process. ( Preferably Alkaline Zinc). With respect to Acid chloride system good control and maintenance is required in terms of organic buildup. Consult your chemical supplier how to achieve your requirement.

Best Regards,

A. Dear Friend,

If you are plating any intricate parts try to check the thickness of the substrate everywhere. It its more than 8 microns then you can be sure that it will pass 240 hours of salt spray. Also during passivation you may loose 1 or 2 microns. So plate some extra thickness. That may help you. Your salt spray life depends on pre treatment and chemistry. Try working on passivation more. Does your plated component becomes black after some months (which is not subjected to salt spray. If so, analyse the bath for impurities and and do the needful.

All the best,

A. This is a commonly asked question about many finishes. The answer is that there is no answer. By this I mean that durability tests i.e. salt spray, etc. give only an indication of what finish will last longer, there is no real work correlation other than that generally the greater the salt spray, the longer a part will last in most environments.

For the application you are suggesting I would look at zinc alloy plated parts with a black chromate and a top coat as the most viable choice

A. Hi Justin,

Gene is correct, there is no real answer to this question, but as a general statement I can tell you it won't be very long until the surface starts to lose its colour and turn white with Zn corrosion.

Again following Gene's suggestion I would look at ZnNi alloy (>12% Ni) and a black passivate with top coat. This will last significantly longer than standard Zn.

Regards,

A. Hi Francis. It isn't quite that simple because trivalent chromates and their topcoats are so proprietary these days. If you are restricted to zinc plating rather than zinc-iron alloy plating, and you are allowed no topcoats, and you are using a fairly generic chromate conversion coating, then letter 15849 from back in 2002 may still be true. But if you can switch to zinc-iron, and use the best available chromates and top coats, then the hours that Praveen speaks about above may be possible.

Even still, it will depend on what kind of fasteners you are speaking of, and whether the specified thickness is average or minimum. It's not uncommon for a fastener with nominal 5 microns of zinc to have virtually no (just barely covered) zinc in the deepest recess. A full thread fastener with a large head might well have no zinc in a thread root right below the head when the screw has an average thickness of 5 microns

Regards,