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Rust on Phosphatized Metal

December 23, 2009

Dear All,

We've got a problem of rust on our metal products. We used mild low carbon steel type and coated it with powder, oxyplast to be exact. After which we send it to our customer in Europe and all of a sudden rust occur on the finishing (Spots). We send the sample to the lab and it is proven that rust is appearing on the metal. I supposed to paste the picture here but I don't know how to do it. Here are the details of the treatment process before the powder coating:

1. Degrease
2. Degrease
3. Water
4. Sulfuric Acid
5. Water
6. Water
7. Phosphate
8. Zn Phosphate
9. Water
10.Water

Is this process enough to prevent the rust from attacking? I know that rust on the inner side of metal occurs as it is not covered by paint and will spread on the outside. Is there any process to control the attack of rust on metals? Is the powder coating application enough to prevent rust attack? What should be the best option to prevent rust attack on metals? How can we possibly solve the problem? Hope everyone can share your ideas about this problem. Thank you very much.

Kind regards,

Luichito Durban
buyer representative - Ho Chi Minh City, Vietnam

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Hi Luichito,

What is going on in Steps 7-8 of the pretreatment process? Phosphate, followed by zinc phosphate? I don't know what this means.

Your lab's finding that rust appeared on the metal is unremarkable; there is no other place that corrosion can occur, except on the metal surface.

If I understand you correctly, the parts are pretreated, some of the surfaces are painted, and the parts are shipped from Asia to Europe. If that's the case, it is not surprising that your customer is seeing corrosion. A phosphate coating will not provide a high degree of corrosion protection on its own. The coating's primary purpose is to provide improved adhesion of the paint to the metal surface. When combined with a high-quality paint, a zinc-phosphated steel surface can quite corrosion-resistant.

One aspect of your process that can be improved is to use final rinse product in the last stage instead of water. A final rinse (also called a seal rinse) can improve corrosion resistance in general. But the addition of a final rinse to your system probably will not protect unpainted surfaces during shipment to the customer.

George Gorecki
- Naperville, Illinois
December 29, 2009

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January 1, 2010

Hi George,

Thank you very much for your feedback regarding my post. With regards to steps 7-8 on the Treatment process, here is the details of each step. As per our supplier, these two are phosphates.

7. SuF-2NP
8. Zn Phosphate 2530

All metal surfaces are painted with Powder coating except the interior of the metal. And is baked a few hours. Thickness of the powder coating (painting) after baking would probably be 300-400 microns. Is it enough to resist corrosion? Is there any International Thickness standard for coating on metals?

And with regards to seal rinse, do you have any recommendation for the solution or mixture? Because our supplier does not use any spraying method but a basin and just dips in the product. Hope I can hear your response soon. Thank you very much.

Kind regards,
Luichito

Luichito Durban
- Ho Chi Minh City, Vietnam

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Dear Luichito,

this is what I am using:
Phosphoric acid and alkyl imidaydine type of the sealant.

but I believe you need to de-rust properly. all the rust shall be remove out, you shall check with your acid tank whether the pH level is high or low, and the concentration of acid as well. if the acid stage didn't do the job properly, for sure it will affect the surface.

thanks

Choong Hoong Teen
- Selangor, Malaysia
January 13, 2010

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January 15, 2010

Hi Luichito,

The thickness of the paint has less to do with corrosion resistance than the bond between the coating and the phosphated surface.

Your supplier should have a seal rinse product in his offerings. A seal rinse can be applied by dipping just as effectively as it can be spraying.

George Gorecki
- Naperville, Illinois

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January 19, 2010

Hi George,

Thank you very much for your advice. With regards to the seal rinse, do you have any idea on the mixture (ex. Seal Rinse + solvent)? Does it need to mix with other solvents or purely seal rinse? Hoping to get your feedback. Thanks again.

Kind regards,

Luichito Durban
buyer representative - Ho Chi Minh City, Vietnam

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Hi Luichito,

Not sure if I understand your question. Seal rinses come in many forms and most of them are aqueous products, so no solvent is involved. Your pretreatment supplier will certainly have a product that they can recommend for the final stage.

George Gorecki
- Naperville, Illinois
January 22, 2010