Obtaining long salt spray hours for Zinc Plating and Chromating

A. Amy,

The sealer is not the only answer. You do not mention what thickness of zinc is specified on the part, if the part is barrel or rack plated, if a sealer is presently used, what type of conversion coating is used, etc. If you would please give some more detail you would get a better response.

A. Ms. Amy Kolodziej

The issue is simpler than you think.

Red rust is a function of the total zinc thickness after the chromate and sealer have corroded to first appearance of white rust.

For 600 Hours to RED rust you would look at a system that will fail to white rust in 200 to 300 Hours applied over at least 35 to 40 microns of regular zinc plate from an alkaline zinc bath.

This column does not permit use of proprietary information. You will need to search for the right Trivalent chromate and topcoat that performs to 250 hours white rust and let the zinc below do the rest of the talking and this will get you the desired 600 plus hours.

Zinc Iron alloy from an alkaline zinc bath is a better and more reliable alternative to ordinary zinc. We achieve 600 Hours here in India very easily to red rust applying 25 to 28 microns Zn-Fe with a good trivalent chromate and topcoat.

Sorry I could not name the Proprietary Chemistries. Or Ted will order me off this site.

Regards.

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Ed. note: "Ordered you off the site" would be darned unlikely, Asif :-) You are one of the site's most helpful responders as well as one of our oldest supporting advertisers. But, yes, we always appreciate avoiding public battles over brands (why?) Any reader is free to contact you for a private recommendation, and you are free to contact them. Thanks!

A. This is a good and interesting question and I believe you will get good responses from various suppliers connected with the www.finishing.com website.

I will try to give you some general information that should be helpful in your discussions with those suppliers --

1) 600 hours to Red Rust is relatively easy to achieve.

2) I do not think you should only focus on the sealant. You must look at the whole process... zinc plating + chromate (or passivate) + sealant + drying. The whole process works together to give you protection.

3) Do not forget about the thickness of your zinc. If you have only 100 microinches of zinc it will probably be impossible for any sealant to give you the 600 hours to red rust.

4) If you are really using a clear hexavalent chromate... in general these do not offer very much corrosion protection. You might want to look into Thick-Film Passivates or Cobalt containing thin-film passivates that provide excellent protection. These are available through plating additive chemistry suppliers

Further information will be helpful to understand your problem...

1) rack or barrel?
2) zinc thickness?
3) have you done salt spray testing with and without the sealant to see how much protection you are getting from the sealant?
4) have you done salt spray testing with and without the chromate to see how much protection you are getting from your chromate?

I would like to add two more parameters that will also affect your corrosion protection...

1) part shape
2) base material and the quality of the base material

But... at least in the beginning of your testing to achieve your 600 hours to red rust... we should not worry about the above parameters.

A. Mr, Amy,

Good Alkaline Zinc plating system followed by Long lasting high performance Trivalent blue chromate with regular sealer will meet your requirement of 600+ Red rust.