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topic 51418

Obtaining long salt spray hours for Zinc Plating and Chromating

A discussion started in 2009 but continuing through 2019

February 26, 2009

Q. I have a part that I am processing to a Zinc Clear, Chromate that requires a 600 hour salt spray to red rust. When I test the parts, I get to around 450 hours and the parts have red rust.

I have asked my customer for assistance on the sealer that is used to achieve the 600 hours, and had no response.

Does anyone have a name of a sealer that would get to 600 hours to red rust.

Thank you,

Amy Kolodziej
shop with plating line - Wyandotte, Michigan


February 28, 2009

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.

Gene Packman
- Great Neck, New York

March 1, 2009

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 ZnFe with a good trivalent chromate and topcoat.

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


Asif Nurie
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016

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!

March 2, 2009

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

Thomas E. Kidd
- Budapest, Hungary

March 4, 2009

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.

p gurumoorthi
P. Gurumoorthi
    electroplating process chemicals
Chennai, Tamilnadu, India

July 2, 2009

thumbs up signHello,

Thank you for all your great advise. Just to clarify, this is barrel plated, to a .00015 Min Zinc Clear Chromate.

The part is also a rivet, 3/8 body with a truss head.

Amy Kolodziej
- Wyandotte, Michigan, USA

Zinc Plating
by Herb Geduld
from Abe Books
info on Amazon

A. Hi Amy. 0.00015" zinc doesn't meet any standard anywhere that I'm aware of :-( Some people call it "commercial zinc" but I think that's just a euphemism for "the lowest thickness we can apply without excessive rejects". The good advice you've received here included a plating thickness of 35 to 40 microns if you stick with regular zinc plating or 25 microns of zinc-nickel. As a compromise, it sounds like you could try tripling or quadrupling your plating thickness, and then see where you stand. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

Salt spray hours for white chromate process

March 27, 2012

Q. I need clarification for clear white chromating process and salt spray test procedure as per MIL std.

Can anyone give the clear information to me!

Bharathi M
- Chennai, India

March 27, 2012

A. Hi cousin Bharathi. Unfortunately, a prerequisite for a clear answer is a very clear question :-)

Can we assume the parts in question are made of steel? Steel can't be directly chromated, but several types of electroplating can be chromated. Is the item in question zinc plated? In most cases, this type of electroplating receives a chromate post treatment to deter "white rust".

No chromates that I know of are actually "white", but there are "clear" chromates, where the color of the metal will show through.

"MIL std." means U.S. Military Standard, and there are thousands of them. You need to describe what you want to do, then an applicable MIL spec / Mil std. can be selected. Many MIL standards will specify how many hours of salt spray testing the finish must survive, and most MIL standards will require that the salt spray test be conducted in accordance with ASTM B117 [link by ed. to spec at TechStreet] , rather than a MIL spec salt spray test. Good luck; and get back to us, if you can, with several paragraphs clarifying your situation -- then we'll probably be able to guide your through it.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

How many microns of zinc plating for 280 hours salt spray resistance?

June 20, 2013 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Trying to find out how many microns of zinc is needed to obtain a 280 hour salt spray rating when tested to ASTM B117 test for red rust?

ron ogawa
specification - huntington beach, California usa

June 29, 2013

A. Hi Ron. We appended your inquiry to a thread which should give you plenty of food for thought about the chromating and sealing being as or more important than the zinc plating thickness.

Still ... since, we all know that salt spray hours do not really correlate to real life and that customers are actually concerned about real life, and since the spec is for reasonably high hours rather than 48 or 96 or 192, I personally don't think .00015-.0002" is what they're really looking for. I expect they might be looking for, say, 8 micron (0.00032", 'moderate exposure') or 12-13 micron (0.0005", 'severe exposure') plating even if they don't exactly know it yet :-)

So, if you are the one doing the specifying, or are in an influential enough position to talk to the customer about clarifying the spec, you might be able to suggest that the requirement be 8 microns minimum, as well as the 280 hours. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

Zinc plating gets red rust in 24-hour salt spray test

February 1, 2019

Q. We'd requested our supplier to coat our bolts with Fe/Zn5A. The supposed minimum coating thickness is 5 micron. However, upon measuring the coating thickness, most of them were in the ranges of 1 - 5 microns. Since, our customer requires the bolts to withstand 48 hours of salt spray testing, we opted for the use of sealant. However, once the bolts were salt spray tested, it could not even withstand red rust for 24 hours. Is there anyone who could help to explain why?

Al Fauzi
- Johor, Malaysia

February 2019

A. Hi Al. Although I don't know what spec you are referring to, if you have 1/5 of the 5 microns zinc thickness considered adequate for 'mild' exposure you can't expect much if any corrosion resistance. The bigger question is: if the spec was 5 microns, why did they plate it to 1-5, and why are you accepting it?


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

February 7, 2019

thumbs up sign  Thanks, Ted. Anyway, the spec is ASTM F1941/F1941M [link by ed. to spec at TechStreet]. We actually just discovered that the thickness is below the requirement. Since, the part was an old stock, we thought of salvaging the stock by using sealant. But, from the results of salt spray testing, apparently that didn't work.

Al Fauzi [returning]
- Johor, Malaysia

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