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Parts rusting after zinc phosphating: brown residue = flash rust?

A discussion started in 2000 and continuing through 2017 . . .


Q. We have a zinc/phosphate immersion tank followed by hex-chromate seal. Control of this Zn Phosphate tank is maintained by analyzing the free acid/total acids. In the last two weeks, iron parts exposed to only 12 hours slack time prior to painting are developing rust. Assume that adjacent production lines have been the same consistent mix for the last three months and that the tank is only operated when the daily temps, F.A.(lowest allowable), and T.A. are in spec.

Q1) Should this tank be monitored for any other mitigating cations.-two suspects are Al or Cr.

Q2) Does the sealer need total dissolved solids spec.

David E [last name deleted for privacy by Editor]
- St. Louis, Missouri

Phosphating Metal Pretreatment
by Freeman

Phosphating of Metals
by Rausch


Q. We are noticing a red/brown residue on parts leaving our E-Coat immersion Zinc Phosphate tank, after line stoppages. The residue is apparent in as little as 10 minutes after a line stop/shutdown if suspended above the tank (without entering a rinse tank). My immediate reaction would be a flash rust issue but I would not think that it would be apparent after 10 minutes. I plan on doing a salt spray analysis with several different controls to see if long term quality is affected. Could this be a flash rust issue, or is it just a property of the zinc phosphate chemistry/drying process?

Devon DuBose
e-coat pretreatment engineer - Redmond, Washington, USA


A. Hi Devon,

Without actually seeing the parts, it is hard to say what your residue is.

But flash rust can occur within the 10-minute time frame you describe. In fact, if the conditions are right, corrosion can be seen within a minute or so.

I would not bother with performing a salt spray test to determine whether flash rust is present. It would be easier to look at the residue using an inexpensive microscope. You will be able to discern the rust even at 50X or 100X magnification.

George Gorecki
- Naperville, Illinois


thumbs up signGeorge,

Thanks for the reply. I was interested in the salt spray analysis just to see the long term effects if any, as there are no immediately visible defects after an extended line stoppage once the line continues through E-coat. I am however fairly certain that the residue is rust but will take your advice and take a peak under a microscope.

Devon DuBose [returning]
e-coat pretreatment engineer - Redmond, Washington, USA

Rust on zinc phosphated parts

December 12, 2016

Q. I'm working at a company in a shop for zinc phosphating of a mild steel body of a 6 cylinder compressor. The problem is after removing the body from phosphating tank, it is getting corroded at some points. Also the layer forming is not uniform and getting removed on slight rubbing against other surfaces. The process specifications are as follows
1) dipping in HCl for derusting 3 to 4 hours,
2) rinsing in water,
3) dipping in phosposol zinc, catalyst 17 powder (as specified by supplier),
Cold phosphating at 30 °C.
What could be possible reasons? Also suggest remedies.

sunny gade
intern - pune, India
  ^- Privately contact this inquirer -^

December 2016

Hi Sunny. Your experience is already broader than mine. I can't even imagine pickling/derusting in HCl for 4 hours :-(

I'm surprised the surface isn't so porous and spongey that cold phosphating is impossible. We'll have to wait for a reader who knows their stuff :-)


pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

December 24, 2016

Q. Hello ted, HCl acid is 35 percent diluted actually.

And, appending to above question:

4) water rinsing in dip tank

Acceptance range for zinc phosphate
Total acid- 150-180 pointage
Free acid- 12 to 16

Acceptance range for HCl acid
Acid pointage- 20-25

Actually in our case lots of fluctuations are there in each parameter like free acid in zinc phosphate bath sometime drops to 10, HCl acid pointage to 16

What could be the most affecting part?
What should be ideal values and ratio of F.A. & T.A. , Acid pointage of HCl?

Sunny gade [returning]
- Pune, Maharashtra India

December 31, 2016

A. First, you might try using sulfuric acid instead of HCl. Chlorides are notorious corrosion cataysts/promoters.

Second, straighten out your phosphate bath excursions! Run your free and total acid numbers near the top of the range, rather than letting them sink below the lower limit. The analytical procedures are quite straightforward and you can easily do them yourself.

Lastly, a little higher temperature in the bath might help. 60 C or higher, I'd suggest.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

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