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topic 35480

Zinc plating and rust

A discussion started in 2005 but continuing through 2019


Q. Dear Sir,

These square nuts were manufactured and zinc plated in August 04. Some nuts started to rust 2 months ago, severe rusted nut in the paper cartons were discovered last weeks, will second zinc plate help? What is the consequence in term of rust resistance?

35480-1s  35480-2s

Look forward to hearing your opinion.

Nick Yap
- Malaysia


A. As the rusting tendencies still existed after galvanisation, either the thickness of the coat was less or the coating got removed from some location. thus even in presence of small moisture, electrolytic cell forms leading to depletion of zinc and subsequent rusting. now what you can do is clean the nuts with kerosene oil then dilute HCL, Followed by NaOH and finish with emery. then try plating again. however, it is not a commercially feasible solution.

manuj kaushesh
- solan, HP, INDIA


A. Hi!
Maybe, just maybe were these nuts packed in PE bags, while they were still hot?
Poor rinsing after plating (and passivation) is leaving some salts on the surface. These salts with vapor which condense in closed PE bag can lead to this state of material....

This is just my suggestion, maybe is real reason somewhere else...


Dimitrij Us
- Kranj, Slovenija (EU)



ajay raina
Ajay Raina
Ludhiana, Punjab, India


A. The picture indicates red rust. Red rust has nothing to do with the chromate. If the problem is in the chromate you will have white rust.
The main question in red rust is - how many microns of zinc did you plate?
The zinc thickness required depends on the conditions that the parts face. For in door - 8-10 microns will do.
Only if you have enough zinc on the parts you can consider problems coming from moist, PE, etc.

As I already mentioned, I see no white rust on the picture, I assume you do not have the right amount of zinc on parts.

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


A. I would agree with Sara -- seems you got a bad batch with too little zinc plate. Red rust indicates the iron substrate is corroding. A higher resolution photo might show it, but I can see no visible evidence of white rust expected from zinc.

You also seem to indicate the problem parts came from just one of the multiple cartons shown in your photo. Are these boxes all the same product and only one box had rusted parts inside? A likely scenario would be that the barrel plating set-up had a bad or intermittent electrical connection resulting in thin or no plating. Poor cleaning could be the culprit as well.

If the problem was isolated to just that one batch, send the pieces back to the supplier asking for top quality replacements and for detailed info on their QC measures.

Thomas Hanlon
Thomas Hanlon, Materials Engineer
aerospace finishing - East Hartford, Connecticut, USA


A. It's a combination of factors; varying plating thickness, a sea journey without moisture absorbent packing, and too long a storage time before parts were opened.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind supporting advertiser
Bangalore, Karnataka, India

saify logo


A. I fully agree with Khozem. We have found that many fasteners that are Zinc Plated , imported to South Africa here over sea freight sometimes have this problem.

gary joseph
Gary Joseph
Johannesburg, South Africa

December 10, 2010

Q. Do you guys see white rust in my part. It is a sheet metal with zinc plating of 8 to 12 micron thick. Send to Lab and to test on the rust and they found sulfur & chloride contents on the rust. Plating thickness is within the spec of 8 to 12 microns.

Following your discussion here, if I do not see white rust then I can conclude it is the plating house problem?

Harry Yang
Manufacturing - Georgetown, Penang, Malaysia

April 18, 2012

Q. Hello,

I have a similar problem with rust on Zinc plated parts. The only difference is that the rust is showing up after only 2 months. These parts are shipped to Singapore, so I suspect that the moisture might be higher there. Also, we were not shipping parts with desiccant before, which may have contributed. However, due to the speed at which the rust appeared, I am concerned that either the Zinc plating was not thick enough or there was moisture in the processing. I do not have values for the thickness. Zinc plating is by an outside service vendor and we perform a visual inspection on the coating.

Just to add some information, the part also gets silkscreened after plating. The rust itself is brown, which means the actual steel is rusting. Thanks!!!

Lexie Bee
- Hayward, California USA

April 25, 2012

A. Products plated and passivated with trivalent chromate has to preferably hot air dried at a heat "not too hot to hold" i.e say up to 60 degrees +, Parts has to be handled with care with out allowing friction, and packed compulsorily only after 24 hrs- 36 hrs of curing.
It is advisable to use VCI packing material.
Sulphur must have come from the corrugated boxes. even the glues has to be checked.

t k mohan
T.K. Mohan
plating process supplier - Mumbai, India

April 30, 2012

Q. We are facing an issue of white rust formation on Zinc plated components with trivalent chromium passivation. These components are part of an assembly.

The issue is seen after around a year on the assembled product. The assembled product are shipped in plastic trays

We did analysis in SEM and found slight traces of Carbon & Oxygen. The plating thickness measured around 8 microns against the required 15 microns min.

Q1.We were wondering how much life does Zinc plating provide with trivalent chromium passivation. These are for use in industrial conditions.

Q2. Is lower plating thickness the main contributor for the white rust formation.

Mithun Paul
- Cochin, Kerala, India

April 30, 2012

A. Hi, Mithun.

It isn't clear to me if you plated these parts in house or you bought them. Although 15 microns is a very substantial electroplating thickness, and much more resistant to red rust than 8 microns, I see two other immediate issues here:
First, if the parts do not adhere to the specifications, all bets are off because the process is out of control; trying to assign causality if you have a meandering, will-o-the-wisp, out-of-control process which specifies 15 microns but accepts 8 will prove hopeless. In this case you have to start with fixing whatever weaknesses allowed out of spec parts to get into the system.
Second, white rust is usually caused by ineffective passivation or wet storage. 8 microns of zinc is still substantial enough for reliable performance indoors for a while if the passivation was proper.

Still, "industrial conditions" is a tricky definition; if it is an acidic environment like most plating shops, you are going to get white rust.


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

May 2, 2012

Q. Hello Ted,

Thanks for the quick feedback.

These are In-house plated parts and are used in the electronic industry, going into casings.


Issue was seen in the packed condition itself before use.

Q1. What are the key parameters to be checked for effective passivation?

Q2. Also saw an earlier post mentioning drying of parts after passivation. How significant is this?


Mithun Paul
- Cochin, Kerala, India

April , 2012

A. Hi Paul. I've always believed in buying plating processes from suppliers rather than doing in-house formulation. In this age of RoHS compatible trivalent chromating it is an absolute necessity to use such a proprietary process and to rely on the vendor's guidance for operating parameters.

In this case though, it looks like there was a good chance that the parts were stored wet and that this caused the problem. Again, though, zinc plated parts stored in an acidic environment (if that is the case) will rust quickly.

Is there an alternate explanation for the nice plating on one end of the connector and the bad plating on the other end, other than one end being down and wet? It sort of looks like a poorly plated part was crimped to a better plated part -- are the half shells plated before or after being joined?


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

May 3, 2012

A. Hello Ted,

Both the rusted half & the nice half are two different components from different batches of plating. This could possibly mean the passivation has been improper for that particular batch.

When you mention the parts could have been stored wet, do you mean either
1. Storing of parts that have been improperly dried after passivation, or
2. Storing of parts in a moist atmosphere.

In the case of point 2, how much life do you think we would get with proper plating thickness and proper passivation

And can we ascertain the plating life by a 24 hr/ 48 hr salt spray testing?

Thanks for your feedback


Mithun Paul
- Cochin, Kerala, India

May 4, 2012

A. Mithun,
8 micron is okay but 8-12 is means the minimum should be 8 microns. There are trivalent passivations available with reputed supply houses which can give up to 250 hrs WR and can enhance to 350 White Rust (repeat, WR)
The main parameters:
1. Passivation bath should preferably be made in DM water
2. pH of the passivation bath should be monitored and followed as per supplier TDS
3. regular replenishment has to be maintained
4. Dip timings are say up to one minute as against 4-5 seconds in Hex passivation. So if the operator is not educated, he will go back to his "old style"
5. Discarding pattern has to be fixed (Main enemies are Iron & Zinc contaminations) - again ref: TDS
6. A hot water final rinse and hot air drying is recommended.
7. hot- "too hot to hold"
8. items should not be handled till 24 hours minimum before packing - Curing - 24 hrs minimum
9. follow VC packing if sent thru sea.
Finally the plating bath should be maintained well ... means if contaminated then also you will not get results.

You may find reputed supply house around your city; Chennai will be near.

t k mohan
T.K. Mohan
plating process supplier - Mumbai, India

October 27, 2013

Q. I have found a rust generation problem in my parts with in 1 month after packing even in my warehouse. The parts have a corrugated partition in between two; whereever there was contact with the corrugation, black and red rust appeared in form of lines against corrugation. I request to help me in understanding the reason.

Vikramaditya Kumar
- Nasik, Maharashtra, India

November 6, 2013

A. Hello Mr. Kumar,

Please add more details of the issue:
- Photographs?
- Zinc thickness? (if you have any data)
- Conversion coating? (hex chrome, tri chrome, blue or yellow)
- The corrugated box was wet or dry?
- Your warehouse is in a coastal zone? (to know what are the conditions)
- Has this happened before?

With this data we may be able to help you!


Daniel Montanes
- Canuelas, Buenos Aires, Argentina

January 7, 2015

Q. Why rust came in zinc plated parts? After plating process parts are packed in polythene. After some days, bright passivated parts became rusty? Why sir? Please give me solutions for precautions.

deb swain
metal finishers - delhi, india

January 2015

Hi Deb. We appended your question to an earlier thread on the same subject to give you some hints about possible causes, and to suggest the type of data you would need to provide for readers to be able to help you. Good luck.


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

January 9, 2015

A. If you plate enough zinc, say .0002", there will be no rust.

If you don't know how thick your zinc is, then all bets are off.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina

January 10, 2015

A. The red rusting is the result of low coating thickness and poor handling during shipment through ship.

A white rusting is the result of poor passivation film and that can be improved by applying top coat sealant after passivation.

Darshan Kumar
engineering and trading - Abohar, Punjab, India

November 29, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hello sir,

I need one clarification from you: whether it is possible before formation of white rust any chances of formation of red rust. And which criteria depends for white rust and red rust? Whether white rust depends on passivation and red rust depends on coating?

praveen reddy
personal - rothak Haryana India

November 2016

A. Hi Praveen. Rusting of the steel substrate is called red rust. Corrosion of the zinc plating is called white rust. The chromate conversion coating (passivation) deters white rust. Yes, red rust before white rust is certainly possible depending on the part (blind holes, spot-welded seams, or other areas of little or no plating). But you'll probably get better, more accurate, more enthusiastic, more helpful responses if you take the time to carefully provide the details of your own particular situation rather than keeping it vague and abstract.


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

January 16, 2018 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Dear sir,

We are currently working in an automobile sector and manufacturing vehicles which are used in very humid areas. We are facing rust issues on our connectors, e.g., HYDRAULIC FEMALE-FEMALE STRAIGHT REDUCER, A18 TO A15, M26X1.5 TO M22X1.5. As per the drawing requirement we want them to be electro galvanised on steel as per ISO 8434-1 [link by ed. to spec at TechStreet]/DIN 2353 [link by ed. to spec at TechStreet]/DIN 3016 [fastening clamps] standards.

Now are facing rust on these parts. Other than not using the correct coating what can be the other potential causes of rust on these parts?


Mustafa Pasha
automotive - Abu Dhabi, UAE

January 2018

A. Hi Mustafa. We appended your inquiry to earlier very similar inquiries so you can read of many contributing factors towards rusting. But you have not told us what your plating spec is, what salt spray hours you achieve, the actual rusting situation, etc., so all we can do is fire back a round of buckshot :-)


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

January 21, 2018


I wanted to inquire what should be the testing conditions for a salt spray test (hours) for both white and red rust for hydraulic straight stud couplings.

The couplings are used in automobiles which will be subject to humid conditions. Material used is steel which is electro galvanized.

Thanks and regards,

Mustafa Pasha [returning]
- Abu Dhabi

January 2018

A. Hi Mustafa. It seems to me that this project is being approached backwards. Salt spray testing is a QA test to make sure your plating quality is not deteriorating; it is a poor indicator of real life, especially in the case of zinc plating which corrodes very differently in a salt spray cabinet than in real life. Trial & error on zinc plating, to no particular spec, until you reach some arbitrary salt spray hours, probably isn't the best approach.

If the automobile company has issued no specs for this component, then you have to choose a specification for yourself. I think most auto companies would rather see 13 microns of a zinc alloy like zinc-iron, zinc-cobalt, zinc-nickel, or tin-zinc than anything else. Then you can search this site or other sources for good salt spray hours to use for white and red rust for QA of such coatings. Best of luck.


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

Zinc Nickel Plated Parts Have Red Rust on top of the plating

August 7, 2019

Q. Hello. We are currently experiencing a corrosion issue on an automotive door component. Plating is (min) 6 microns of ZnNi plus an organic topcoat. In our case, the organic topcoat is a (anywhere from 1-10 micron) layer of an organic oil/lube. Analysis of the plating shows minimum ZnNi thickness is OK and Ni content is OK.

The situation is, before the part even gets to dealerships, small RED rust spots are noticed. I've always believed red rust to be base metal corrosion and that white rust should form on the ZnNi layer well ahead ahead of red rust of the base metal. Upon microscopic analysis of returned suspect parts, the ZnNi layer is completely in-tact, as is the base metal. All analysis would suggest that the red rust is actually on top of the plating surface.

Therefore, can we assume our part has been contaminated at some point before it is installed on the vehicle? Or is there any other way that RED rust can form ON TOP of the plating surface?

Thank you very much for your time.

Matt Martin
- Detroit, Michigan, USA

December 19, 2019

Q. We have some industrial parts that we outsource plating. The parts are 12L14 Steel and supposed to be zinc plated & yellow chromate per ASTM B633 [link by ed. to spec at TechStreet] , SC-3, TYPE II.

The thickness is supposed to be 15 µm (0.0005") min, but based on our measurements they are 8 µm (0.0003"). We have sent many of them to California and we have had customer complaining about rusting.

Could someone help me figure out the reason behind rusting ?


Mete Naz
Engineer - Missouri

December 2019

A. Hi Mete. As you can see from this thread (and a dozen like it if you patiently search the site), there can be all sorts of reasons why the parts rusted: from poor cleaning to poor plating to poor post treatment to poor drying to poor packaging to bad storage conditions, to unrealistic expectations, and more.

But the first step is that parts must comply with their specifications. If they are supposed to be SC-3 / 13 µM / 'severe exposure', and instead they are actually SC-2 / 8 µm / 'moderate exposure', that is an immediate problem that you must fix. If the plating shop talked you into accepting SC-2, it might be acceptable but only if you immediately changed the specification.


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

December 19, 2019

A. 12L14 steel is a leaded steel. It requires special cleaning techniques. Specifically any pickle must be sulfate free.

Tom Rochester
Plating Systems & Technologies, Inc.  
supporting advertiser
Jackson, Michigan, USA
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Ed. note: If you need additional info re. Tom's good advice, search the site with the term "12L14" for more than a dozen discussions, or start with these interesting threads: 41766, 42348, 13357, 10613. Good luck. is possible thanks to our supporting advertisers, including this shop:none


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