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topic 30106

Zinc plating thickness vs. salt spray resistance

A discussion started in 2004 but continuing through 2019


Q. I work for a leading auto component manufacturer.

In one of the components, my customer wants a minimum of 120 hours of salt spray before appearance of red rust.

My designer has specified 13 microns thick zinc plating with blue chromate passivation. Will this plating specification meet the customer's requirement of salt spray test.

automotive component - Bangalore, Karnataka, India


affil. link
Zinc Plating
by Herb Geduld
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Corrosion Tests and Standards, Application & Interpretation
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A. This is an interesting question, Mr. Chandrasekar, and we thank you for it.

I'll start by saying that 13 microns is the correct thickness to specify for "severe" exposure (as opposed to "mild", "moderate", or "very severe"). So, unless the application is for "very severe" exposure, you did good.

You cannot specify hexavalent chromates for future auto application, and probably not even for current projects, so you are limited to trivalent chromates, which are blue anyway (chromate means Cr04--, so strictly speaking "trivalent chromating" is perhaps an oxymoron, but people will accept that term). 

Because trivalent coatings do not offer the protection of hexavalent chromates, it is quite widely accepted today to apply a silicate, zirconium, wax, water soluble lacquer, or other topcoat/post dip after the chromate. This is the point at which your spec is probably incomplete, because you need something on top of that trivalent chromium but you haven't specified what.

Zinc plating with a trivalent chromate and a high quality proprietary topcoat/final dip should last 120 hours if properly applied. Salt spray testing is not really a prediction of field life, but a QA measure to be applied by the plating shop to make sure things didn't go sour in their process.

A final thought is that zinc alloys like zinc-iron, zinc-nickel, and tin-zinc are more corrosion resistant than plain zinc and may be appropriate for the part in question.

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


A. Mr Chandershekar,

It is entirely possible to achieve 120 Hours to white rust with 13 microns of zinc , which is passivated with a yellow or greenish blue film . Current environmental needs dictate the absence of Hexavalent chromium. The yellow is banned. A blue chromate dyed yellow will take care of the needs for a yellow finish.

A better alternative is the greenish film which is trivalent chromate and has excellent SST performance to white rust and subsequent red rust.

However, as Ted Mooney has pointed out a Zinc Iron alloy is the better route to achieving consistently high SST hours to red rust.Cost will remain affordable with Zinc Iron plating which is a simple Alkaline bath . Let me know what you decide.

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


Q. Hello,
What is the color that is most acceptable for trivalent thick passivation and that which will give the best corrosion resistance (at least 96 hours to white corrosion)?
This question arises since different company standards give different colors like Silvery color with yellowish iridescence, Silvery with greenish pink, etc. Some standards accept any iridescent color. Anyone to clarify on this?

Subramanian Ramajayam
Subramanian Ramajayam
consultant - Bangalore, India


A. These days I think you really must exclusively plan on trivalent chromating for the future, Ramajayam. All of the new high performance baths are proprietary, unfortunately. The thick-film baths proprietary baths produce a yellowish color, the thin-film baths a bluish color as far as I know. But since they are proprietary, I can't promise that.

It isn't clear what your position or your company's position is, or where you fit into the chain. Good luck.

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

January 23, 2008

Q. How much plating thickness is required for Zinc Trivalent plating to withstand 150 hrs of salt spray testing.

Atul Pardeshi
buyer - Pune, Maharashtra, India

November 30, 2010

A. Hi, Atul.

Actually the thickness of the zinc plating can be irrelevant because, although the zinc plating prevents red rust, it is the chromate conversion coating and any subsequent top coats that prevents the initial white rust. Talk to your trivalent chromate conversion coating supplier about whether their coating will withstand 150 hours.

As a side note, salt spray hours do NOT correlate to real life hours. Rather, salt spray testing is a quality assurance measure (if the coating previously passed 150 hours and it no longer does, something went south in your process control). I've heard that there was a famous saying in Vaudeville comedy: "Don't tinker with a good line", which meant that if it's working, never try to tweak it! The point of salt spray testing is to insure consistency so there is no value in tweaking the number of salt spray hours up or down a little because the point is consistency & predictability, and tweaking will probably lead to playing around with process control factors instead of holding them consistent, achieving the opposite of the goal :-)

Good luck.


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

June 23, 2014

Q. Hello sir,

How much salt spray test hours for zinc blue plating material [min]? Any standard available?

shanmuka shanmuka
- bangalore karnataka, india

June 2014

A. Hi shanmuka. Before the days of RoHS-compatible trivalent chromating, the standard hours to white rust for the ubiquitous iridescent hexavalent yellow chromate was 96 hours to white rust. If you have no other standard to go by, that's what I'd suggest.

Specifications are available from many organizations; ASTM B633 [affil. link to spec at Techstreet] , "Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel" is active and well-maintained, and if you are not plating automotive parts with their own specs, it is probably a very good choice. You may also want to obtain ASTM B201-80 [affil. link to spec at Techstreet], "Standard Practice for Testing Chromate Coatings on Zinc and Cadmium Surfaces". Good luck.


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

June 24, 2014

A. ASTM B695 [affil. link to spec at Techstreet] gives, for mechanical plating and mechanical galvanizing, the salt spray hours as a function of thickness. (See Table 1.) The salt spray hours for electroplated zinc are approximately equal.

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

June 26, 2014

A. The chromating is responsible to stop the white rust. The zinc thickness has no effect on the white rust.
The red rust is a function of the zinc layer thickness. The thicker the layer, the higher is the protection against the red rust

sara michaeli
Sara Michaeli sara michaeli signature
chemical process supplier
Tel-Aviv, Israel

November 24, 2014

Q. Dear,
How many microns is required for 96 hours salt spray test.

Rizwan Aslam
trading - Riyadh ksa

November 2014

A. Hi Rizwan. Even 5 microns of zinc (ASTM B633 "Mild" service condition) will deliver 96 hours to white rust if the chromate conversion coating is done well. But selecting the zinc plating thickness based on salt spray hours may be wrong thinking. You should probably determine whether the service condition is "mild", "moderate", "severe", or "very severe", and base the zinc plating thickness on that. Good luck.


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

November 29, 2014 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. How to calculate salt spray test vs. life in plating?
Suppose salt spray life is 48 hours to white rust and 96 hours to red rust, then --
What is life in outer atmosphere and how to calculate?

Alakh Ahlawat
Eletroplating - Rohtak Haryana Inida

November 2014

A. Hi Alakh. That question has been asked and answered here a dozen times, and elsewhere a thousand times, but the answer remains: there is no reliable relationship. Salt spray tests are not intended to predict real life except in this indirect sense: if the salt spray life declines, something went wrong with the processing, so the real-world life is also likely to not meet previous expectations.

In real life, zinc coated objects can slowly build a corrosion-resistant, glassy, zinc carbonate skin from a slow reaction with the tiny amount of carbon dioxide in the atmosphere. This doesn't happen in a salt spray test, so designing for salt spray hours is not the way to improve real life. Once you have a process that delivers a satisfactory real-world life, you check the salt spray hours periodically to make sure nothing has fallen apart in the processing.

48 hours to white rust is not enough, and indicates that the processing is not up to today's standards. 96 hours to white rust should be the minimum for any modern chromate, and some are significantly better; 150 hours is not unusual. For typical outdoor exposure (severe) you should probably have 13 microns of zinc plating. Good luck.


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

January 24, 2015

Q. Sir,

I need a plating on a low carbon steel, where the weather is extremely salty.
What plating do you suggest? My client needs surface to be look like silver aesthetically.

Mr. Ravi Sharma
pattern & engg. Works - Mumbai, Maharashtra, India

A. Hi. A zinc alloy plating like zinc-nickel or zinc-cobalt should be good. Tin-zinc is more expensive, but probably looks a bit more like silver. Good luck.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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February 14, 2015

Time until white corrosion appears 1.5 weeks and time until base metal corrosion appears 4 weeks. It can be possible for trivalent yellow passivation in electroplating? If it's possible please tell us the required plating thickness.


March 2015

? Hi Gopi. If a question isn't well understood, the answers will be ambiguous or misleading ... and I don't understand the question :-)

Are you saying that you want 252 hours (1.5 weeks x 7 days x 24 hours) of resistance to white rust in ASTM B117 salt spray testing? And you want 672 hours of resistance to red rust?


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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July 15, 2015

Q. I am working with Earth equipment company.

Continuously we are facing an issue of SST failure, During the analysis part will be verified by different labs and both the labs showing testing parameters are the same but results are different.

Need support in details for verifying testing parameters and anything.

sameer Kale
- pune,panjab,india

A. Hi Sameer.
1. How do the parts perform in the real world? That's what is critical.
2. Have both labs provided certification of exactly what spec they are testing to, and it is the same? What spec is it?
3. Some variation is to be expected, but your posting is rather cryptic. Please give us some numbers like: what is the plating spec, what is the substrate, how many hours are you expected to get, how many hours does lab #1 say, and how many hours does lab #2 say?

Thanks & Regards,

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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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September 27, 2015

Q. Dear sir,

We have Zn plated screws with black trivalent passivation; but it quickly formed white rust in our product; part tightening with screw driver; it failed SST within 60 hours for white rust as against the spec of 96 hours; kindly give suggest on this issue.

jaganathan Mani
- bangalore,karnataka, india

October 27, 2015

Q. Is there any relation between ED thickness and salt spray time? I mean the more thickness the longer salt spray time?
For passing 1000 hours salt spray without red rust what is your suggestion for the thickness of ED process?

Hadi EK
- Esfahan, Iran

October 2015

A. Hi Hadi. I assume you're still talking about zinc plating and "ED" means the thickness of the electrodeposit? (To some people, 'ED' means electrodeposition paint).

Yes, the corrosion resistance to red rust is proportional to the thickness of the zinc plating (although it's also dependent on the effectiveness of the chromate conversion coating). However, it is most common to run the salt spray test only to the appearance of white rust (failure of the chromate conversion coating), and to base the zinc thickness on the severity of the exposure (13 µm for severe exposure, 25 µm for very severe exposure) rather than salt spray hours. Besides the general non-indicativeness of salt spray testing, it is particularly non-indicative for heavy zinc coatings because in the real world they gain much of their corrosion resistance from the slow build-up of inert and impermeable zinc carbonate reaction products which cannot form in a salt spray test.

It may be a better idea to employ 8 µm alloy plating like zinc-nickel or zinc-iron rather than 25 µm of plain zinc if 1000 salt-spray hours is to be the judging criteria. Good luck.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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Will 8 µm Zinc Plate pass 96 hour salt spray?

December 29, 2015

Q. Will 8 microns of zinc plating, with a clear trivalent chromate, pass a 96 hour salt spray?
Salt spray requirement is no red corrosion of steel base metal.
Salt spray test is 5% neutral salt spray per ASTM B117 [affil. link to spec at Techstreet] .

Rade Savija
- Lakehurst, New Jersey

December 2015

A. Hi Rade. Yes.


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Ted Mooney, P.E. RET - Pine Beach, New Jersey
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January 27, 2016

Q. What would be the plate thickness required for zinc plating with clear passivation of IS 2062, grade B block to achieve a salt spray life of 120 hrs. white rust?

Amit Satardekar
- Mumbai,Maharashtra, India

January 2016

A. Hi cousin Amit. Before leaving this page, please read the whole thread because it sounds like you are making the mistake of trying to design to a meaningless test instead of designing to environmental conditions.

But any thickness of zinc plating, even the absolute minimum 5 µm will survive 120 hours 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. Good luck.


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