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Zinc plating and rust

(2005)

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


(2005)

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


(2005)

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

Dimitrij Us
- Kranj, Slovenija (EU)

(2005)

A. I WILL ADD JUST ONE LINE TO WHAT MR.DIMITRIJ , HAS SUGGESTED
1.IMPROPER OR NO CHROMATE
2.CONTAMINATED CHROMATE
3.IMPURITIES IN BATH.

Ajay Raina
Punjab, India

(2005)

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
    chemical process supplier
Israel


(2005)

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


(2005)

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.

khozema Khozema Vahanwala
Saify Ind
 
Bangalore, Karnataka, India


(2005)

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


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.
Regards

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.

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


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.

35480-3

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?

Regards,

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 explanation for the nice plating on one end of the connector and the bad plating on the other end,or was this just random?

Regards,

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Brick, New Jersey


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

Regards,

Mithun Paul
- Cochin, Kerala, India


May 4, 2012

A. Mithun,
8 micron is okay but 8-12 is fine...it 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
    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
  ^- Privately contact this inquirer -^


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!

Regards,

Daniel Montanes
- Canuelas, Buenos Aires, Argentina

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