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topic 30106p2

Zinc plating thickness vs. salt spray resistance

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A discussion started in 2004 but continuing through 2019

June 27, 2016

Q. Hi,
I wanted to confirm the steps for salt spray test. My customer has randomly picked up a bolt from a lot that we had supplied and sent for salt spray test. There might be chances of problems in material handling which might lead to the bolt not passing the salt spray test. Do we have to give an altogether different part taken from the plater and submit the same to the customer for salt spray test purpose?

Shrikant Soni
fasteners - Pune,Maharashtra, India


affil. link
Zinc Plating
by Herb Geduld
from Abe Books
info on Amazon

affil. link
Corrosion Tests and Standards, Application & Interpretation
from Abe Books

June 2016

A. Hi Shrikant. You should probably be plating to some particular specification, and that specification will often dictate the procedures for sample size and sample selection. Good luck and get back to us with the spec if you wish.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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November 24, 2016

Q. Our company did Zinc Plating on CRCA Test panels and performed Salt spray Test of on it.

Technical Details :

1) We did Zinc Plating on panels as per ISO 2081-Fe/Zn12/A.

2) SST specification : Neutral salt spray corrosion resistance of zinc plus chromate conversion coatings before basis metal corrosion (red rust) begins shall be equal to or more than 120Hrs.

After Testing we get the test results and we observed black spots, black staining and white rust on both sides of 1 test panel out of 3. Remaining 2 test panels found OK.

So by observing above whether plating panels passed or failed in SST?

3) Also report does not have any conclusions like panels are passed or failed in SST. For same point,we made contact to our Salt Spray Testing Lab, they want "Criteria" to pass or fail the test.

We already told lab that please proceed as per astmb117, ISO 2081, ISO 10289 Clause 6.1 & 6.2 ...

So is this sufficient for conducting SST and giving conclusion?

Please give answer.

RutuRaj Patole
Design & Development Engineer - Kolhapur, Maharashtra, India

January 2017

A. Hi RutuRaj. ASTM B117 is a specification for how to run a salt spray test and what features are required of the test cabinet. It will not tell you what constitutes passing and failing.

Sorry that I don't have a copy of ISO 2081 at hand, but it may tell you what constitutes passing and failing, what lot/sample sizes to use, etc. Usually, black spots or discoloration are not considered failure, but white rust is.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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How do I properly specify zinc clear, yellow, or black chromate for 96 hrs ss protection on steel alloys?

January 4, 2017

Q. 1) I believe the following will get what I need for the yellow or black chromate, but is this the proper formatting?
2) Clear is Type V, but according to Table 2 in B633 that only provides 72 hrs. But how come Table 2 can identify the level of protection when a specified service condition is not taken into account? Will the specification of SC3 give me that or can I specify SC4 give 96 hours protection?
3) Would I use the same specification for ductile iron? If I need to specify the particular zinc and aluminum alloys to be able to answer this question, I can provide that.
4) Are the colors actually a sealer or just a dye?
5) In what cases would want to specify a chromate conversion or is that the same?

Scott Brown
product engineer - Portland, Oregon, USA
  ^- Privately contact this inquirer -^

January 2017

A. Hi Scott.
1. That sounds okay to me.
2. Service condition and thickness of plating are related: SC3 (severe) requires 13 microns of plating as you imply. But the salt spray hours you mention are before white rust, and that is a function of the chromate conversion coating, not the thickness of the zinc plating.
3. You could probably use the same spec for ductile iron substrates, but the spec reads "... on iron and steel", so you can't use it for aluminum or zinc substrates.
4. Where the colors come from is an interesting question, the answer to which varies as the years go by. Hexavalent chromium is yellowish, so until a decade ago or so, yellow was just the color that chromate conversion coatings were. Most 'thin film' trivalent chromates are clear, so dye is sometimes used where yellow is required. Most thin film chromates use a 'sealer' after them. My understanding is that the 'thick film' trivalent chromates are yellow and don't use a subsequent 'sealer'. But trivalent chromating is so heavily proprietary that it's hard to offer clear cut answers.
5. The chromate conversion coating is the source of the coloration, and zinc plating is invariably chromate conversion coated.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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January 12, 2017

Q. Dear Sir,

We have been taking trials of Acid Zinc Plating and trivalent Blue Passivation with top coat on Low carbon steel components. The area of the component is 0.5 dm2. The component is just simply a Bush. SST requirement for this component is 48 hrs without black spots. We have tried external commercial platers also. They are also unable to meet 48 hours of SST. We are trying in-house to achieve the result but unfortunately white rust is forming within 24 hrs. How to overcome this issue? How to achieve the results of 48 hours? Customer is very particular about Trivalent Blue passivation.

Most of the failure samples reveal white rust formation on the HCD area. The plating time is 25 minutes (9 to 12 micron thickness) with 8 pcs and passivation time is 30 secs and top coat time is 15 secs. We have checked the chromating film deposition. It was found to be 50 to 62 nm. When we increase the timing, the corners of the Bush has started stripping in the passivation solution. That is the reason why, we are unable to increase the passivation timing. Kindly request you to provide me the suitable solution. We are under tremendous pressure to take a output and show a consistent result.

- chennai, TAMIL NADU, INDIA

Zinc plating is failing 96 hour salt spray. Will a higher carbon substrate help?

February 23, 2017

Q. Sir,
For zinc plating we are conducting Neutral salt spray test for each batch as per ISO 9227 [affil. link to spec at Techstreet]. For each batch test is passed for 48 hours. If I check 6 month old zinc plated product is there any chances of getting white rust for same salt spray test for 48 hours?


April 25, 2017 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Thanks very much for the response on letter 35197 Ted.
I have another query. We are currently using EN1A steel material. The equivalent is AISI 1213. This is a low carbon steel, with carbon 0.07 to 0.15%. We carried out salt spray testing for components made with this steel, plated with FeZn8 CrIII and found that it had developed brown rust within 96 hours.

We are also debating whether moving to a slightly higher carbon content steel will increase the affinity to Zn plating and thus improve corrosion resistance. Information regarding this would be highly appreciated.

Mihir Bajekal
- Mysore India

April 2017

A. Hi Mihir. No, a higher carbon steel will not deter the rusting, and will only make the plating somewhat more difficult.

Repeating from an earlier comment on the thread which we added your inquiry to: You will pass the 96 hour salt spray test "if the substrate was properly mechanically prepared; and it was properly cleaned and activated; and it was properly zinc plated; and it was properly trivalent chromated with a proprietary chromate and the topcoat recommended by the supplier; and the shape of the component is such that it can get proper plating coverage, and the process is under control."

Luck and Regards,

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Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

April 26, 2017

thumbs up signThank you Ted for your comments. Appreciate it. I will check with my supplier team and review their process of plating.

Mihir Bajekal [returning]
Triton Valves Ltd. - Mysore India

April 27, 2017

? Hi,

Can you tell us what type of zinc solution you use? Is it acid,cyanide or alkaline? Also the chromate you use. This will help to give you a answer on your question.


Anders Sundman
Anders Sundman
4th Generation Surface Engineering
Consultant - Arvika, Sweden

May 11, 2017

Q. Hi,
We are facing rusting issue in few of our plated parts which are directly exposed to sunlight and rain. We are specifying Zinc coating of 12 microns. How much of salt spray test requirement we should communicate to Supplier to avoid this?

Deepak Bathla
- Chandigarh, Punjab, india

May 2017

A. Hi Deepak. Although 96 hours would be a good answer, remember that salt spray testing is for the purpose of keeping the process under control, not predicting real-world life. So, what is your evidence that 12 microns would be enough if the process were under good control?

In other words, are you sure that the plated parts which rusted are seeing the same exposure conditions as the parts which didn't? Are you sure that your exposure conditions are not "very severe" (requiring 25 microns of plating)? Is there any exposure to road salt, lawn fertilizer, pressure treated wood, or other chemicals? Is the problem red rust or white rust? Thanks!


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

May 11, 2017

Q. The machine is parked in open weather that is exposed to sunlight, Rain and other environmental degradation factors.

Deepak Bathla [returning]
- Chandigarh,Punjab , india

May 2017

A. Hi again, Deepak. Due to the very close timing, it's not clear to me whether your reply was in response to my posting or just coincided with it. But I think the ball is still in your court :-)


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

Trivalent or hexavalent chromate for more salt spray hours?

February 5, 2018

Q. Which type of plating is comparatively stronger to salt spray testing, Trivalent or Hexavalent?

Jayant Kurundkar
Robertshaw controls Pvt. Ltd. - Pune, India

February 2018

A. Hi Jayant. If you read literature from 30 years ago, it will say that hexavalent is much better, which it was. But today's modern proprietary trivalent chromates are at least the equal of hexavalent chromates in salt spray testing.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

February 16, 2018

Q. I seem to be struggling to get any information on how long zinc plating itself not including passivate lasts in salt spray to red rust. Is there any general information out there for it? E.g., 25 microns lasts "x amount" of hours to red rust?

Many thanks in advance


Andrew Hopkins
- United Kingdom

February 2018

A. Hi Andrew. For my own part, in a career of 50 years in electroplating, I don't recall even once seeing zinc electroplating which wasn't chromated, but Tom Rochester suggests that you look at ASTM B695 [affil. link to spec at Techstreet]. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Aloha -- an idea worth spreading

Salt spray test for Hydrogen De embrittled part

March 1, 2018

Q. Will Hydrogen De embrittled part withstand the Salt spray test of 144 hours with Zinc yellow passivation. Coating thickness of 8 microns.

Anand Shanmugam
- Coimbatore, India

simultaneous March 1, 2018

A. The best answer to this question can be obtained experimentally. If the article, prior to de-embrittlement, would have passed these requirements and has a trivalent passivate, then it would likely pass after de-embrittlement as well. If it has a hexavalent passivate, and just barely passed before salt spray, it would likely fail. But facts trounce theory and opinion.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
plating systems & technologies banner ad

March 2, 2018

A. Hi Anand,

If you need to bake after the chromate, it depends on the last. If the part has 8 microns, I think you can't have white corrosion before 72 hours to assure 144 hours of corrosion resistance.

There are some high thickness chromates in the market that could make up to 120 hours without white corrosion in a salt spray chamber after hydrogen de-embrittlement.

I think that if the color needs to be the SAME every batch and every part, you won't get what you want. If you want even color, you won't find a chromate you can bake. If you want to be "slightly yellow", I think you can achieve that with a high thickness (trivalent) chromate available in the market.

You can always do this: PLATE-BAKE-(ACTIVATE)-PLATE AGAIN (doesn't need to be much time, 2 microns over the first layer is enough)-CHROMATE
With that process you assure de-embrittlement and have active zinc to apply the chromate and assure the corrosion resistance.

Hope you solve this issue! Best regards,

Daniel Montanes
TEL - N FERRARIS - Canuelas, Buenos Aires, Argentina

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