Zinc plating with trivalent black chromate on stainles steel

[ivanko001]

Hello Everyone,

I want to have stainless steel (A2 grade or US grade:304) fasteners zinc plated with trivalent black chromate.
Based on the information that I have found on Internet I am understanding that Zinc plating with Trivalent black chromate is the same as Zinc plating with trivalent Blasck passivation ?! please correct me if I am wrong.

The problem that I am facing with, is to find someone who can actually do zinc plating+CR3 black passivation on Stainless steel..wherever I ask for such service, I am getting negative answers, although I have read some articles where it's described that it can be done, but that cleaning and pickling process must be accomplished properly. Still due to big amount of negative replies from companies that offer plating services, I am confused if this can be done or not?!? I would appreciate if there is anyone who did/is doing this can advise more?
Also a lot of fastener suppliers are confused whenever I as for this additional plating, because they usually plate steel fasteners, and not stainless steel parts, since these are corrosion resistant.

So as an alternative I was thinking about using Steel fasteners instead stainless steel, but I don't want to make big compromise regarding corrosion resistance. To achieve this, I was thinking about using steel fasteners + Zinc plated or Zinc-Nickel plated + Cr3 passivation + Sealant, instead Stainless steel + zinc plating + Cr3. This way I believe I would also save some cost on price/part as Stainless steel is pretty expensive.
Another information that I couldn't find is the time to white rust and time to red rust of stainless steel 304 in NSS (neutral salt spray) test, so I was wondering does anyone has this info ?

Also it would be interesting to kn'ow what is the the time to white rust and time to red rust if I take Steel + Zinc plating + Cr3 passivation + sealant in NSS test ?! or Steel + Zinc Nickel plating + Cr3 passivation + sealant in NSS test

Finally, my goal is to have black fasteners,as these will be used in combination with black polycarbonate plastic in automotive product.
Also it would be good if the corrosion resistance properties would would be similar to stainless steel.

Thanks in advance, and I hope it's not too many questions.

 

[TVP]

Stainless steel cannot be plated with zinc. The choice of stainless steel vs. plated carbon steel depends on the exact application and environment. If this is going on the inside of a typical passenger car, then zinc plated carbon steel is all you need (zinc plating + Cr3). A sealant is usually only necessary in more aggressive environments (underhood, chassis). Zinc-nickel plating is used in chassis and underhood applications when an attractive appearance is needed (minimal white rust, no red rust for years). Stainless steel is rarely used, unless it is for fastening other stainless steel components, e.g. exhaust.

 

[TomDOT]

Do you need metal fasteners at all? What about black, glass-filled nylon or some other filled plastic?

 

[ivanko001]

Thank you for your replies.

I am planing to use Zinc Nickel plating+sealant as TVP suggested.
I am not sure about using nylon around bumper area...IMO it would be risky, as there are a lot of changes in temperature, and what I am most concerned is what will happen with the time if these are used for couple of years?!
I don't want to risk, to "destroy" someones vehicle with wrong hardware selection, and it would be unpleasant if these fall of because wearing out.

TVP, do you know will the sealant provide mechanical resistance to screw, so that installers don't destroy protective layer of sealant and zinc nickel plating while doing installation ?

 

[TomDOT]

http://info.craftechind.com/blog/bid/359482/Are-Your-Plastic-Fasteners-Ready-for-Winter

All materials have strengths and drawbacks. Polymers tend to be less strong and more temperature sensitive (but this is minimized with the correct choice) - metals tend to corrode, gall and are heavier.

 

[TVP]

A sealer will improve corrosion performance after installation. Take a look at the information in this document:

http://www.cinkosan.com/pdf/AT_09_033-Automotive_Fastener_Intranet.pdf

 

[ivanko001]

Thank you both for these articles.
I have read both and will use these references for future.