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Flash rusting after iron phosphating

June 26, 2012

Q. Hi there,

Our products are very big in size and frankly we can't afford making huge tanks. So, we have made a single tank and we are using 3 in 1 chemical ( descaling, derusting and phosphating ) . After this, we have been washing / rinsing it with water. The raw material would go into fabrication, and paint can only be done only after 3 days.

Now , I have 2 questions:

A) After rinsing it with water, the metal (ms, mild steel) is showing flash rust. Will this flash rust have negative impact on painting adhesion if cleaned manually before painting?

B) If we choose to rinse the product with water after the fabrication, immediately followed with painting , will this be a stable process to follow?

I am posting for the first time. I hope to get some benefit from your expertise. Please help.

Mayank Gupta
manufacturer - Haryana, India

July 9, 2012

A. Regarding your questions, the answers are pretty simple.

A. Yes, the flash rust will have a negative impact on paint adhesion. You say you're "cleaning" it prior to painting, but there will be a bare metal surface where the rust used to be and you likely will get poor adhesion at those sites.

B. If you are rinsing the parts after fabrication, you will still likely have adhesion problems. If I understand you correctly, you are rinsing fabricated parts that have been phosphated and are rusty. This is bad for paint adhesion.

While the above answers are simple, the solution to your problem may be complex. You have to get at the root cause of why flash rust is forming. Flash rust will form if your phosphating bath is out of balance. This is a common cause of flash rust formation.

Flash rust can also form if phosphating solution or rinse water is allowed to remain on the metal surface too long or if there is accumulation of solution because of part geometry. Without knowing more about your process or parts, there's no way to know whether this is contributing to your flash rust problem. In general, it is a good idea to get a phosphated surface dry as fast as possible after pretreatment to minimize the possibility of flash rust forming.

George Gorecki
- Naperville, Illinois

July 20, 2012

A. Dear Mayank,

Don't expect much with 3 in 1 chemical. Since your component is too big, you will always have difficulty in controlling the timing required for proper dipping. Different metal surface gives different coatings. Also 3 in 1 chemical concentration plays a role. Practically checking bath concentration of 3 in 1 chemical, I fear, no one carries out.

You may have to change your system to at least a 7 stage Zn phosphate line.

Sellappan Thiagarajan
- Chennai, India

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