Zinc Coating Alternatives

[wchowe]

Hi All,
I have some washers that appear to be zinc plated. We have been using them on our products. While the washer is used internal to the product and not exposed to the elements, there are instances of high humidity that occur. My limited experience with Zinc coatings is that they won't hold up without rust appearing from the base metal for long. What are oyur recommendations for a cost effective coating to replace the zinc over hardened steel. Also, if you feel I am wrong with my assumption that the zinc will show rust through please let me know.
Thanks
Bill

Thanks
Bill

 

[swall]

How big are the washers? If they are not little tiny pieces, then a coating like Dacromet would be a good potential choice.

 

[wchowe]

They are oversize flat washers for an M8 screw. So not, tiny!

Thanks
Bill

 

[unclesyd]

Cost wise your first step would be go to Yellow Zinc plating. It will be a small adder to the regular Zinc plating. There are coatings that can be applied to the Yellow Zinc that will further enhance the plating.
Your existing plater should be able to do both operations.

 

[KENAT]

The zinc should hold up reasonably well unless damaged, which unfortunately probably happens on installation. You mention hardened steel so for your application I assume SST is a no-no, possibly cost wise too.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[hokie66]

Hot dipped galvanized washers will last many times longer than zinc plated one.

 

[gr2vessels]

As KENAT said, the washer will be scratched during assembly, hence the rust has a grip;- perhaps the hot dip galvanizing mentioned by hokkie66 might help, due to the sheer thickness of the zinc. Some help to prevent the early white zinc oxide might come from hexa/tri/di-chromium protection of the zinc plating, most likely at no extra cost or a minimal cost. I think the dichromium is the safest of all, the rest are carcinogen or something (for the plating people, not the washer user!).
I am surprised however, to hear that the hardened, plated washer could have corrosion problems in a normal ambient environment in less than 5-10 years!
Cheers,
gr2vessels

 

[wchowe]

The hardness of the steel base part is Rc 35-40. The enviroment is Moderate (inside, rarely has condensation, mostly dry air, occasionally humid).

Thanks
Bill

 

[CoryPad]

I wouldn't call that environment Moderate, that is Light. If you have no contaminants (Cl, etc.), then a basic zinc electroplating should work fine for you. Hot dipped galvanizing is even better, although that could temper the parts a bit away from 40 HRC. If you want some insurance, you can add chromate conversion coating and even a polymeric topcoat to the zinc plating. As unclesyd said, you should be able to get all of these, fairly inexpensively, from the same supplier.