Metal finishing Q&As since 1989

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Problem in pretreatment line

2006

Dear everybody,

I have spray pretreatment line for car bodies, and use dicationic phosphate in this line.

My pretreatment line is too short (2 stations for degreasing -stations for water rising -stations for phosphating and stations for rinsing and passivation together) and degreasing material mix to water rinse and water rinse mix to phosphate tank.

My material contractor uses caustic soda [affil links] 98% for degreasing (pH in degreasing tank is 13 or 14) and pH for rinsing is 11 , pH for phosphating tank is 3-4 .
We must use Chrome free passivation .my material contractor use NaOH in last rinsing tank instead of chrome passivation and sometimes use zirconia material( he said).the PH of last rinsing tank is 9 or over .

I want to know what problem happen for paint of cars that that pretreated in this line .

We use polyester primer(140 °C) and topcoat polyester (140 °C).
best regards,

Nader Bayrami Tarouni
car painting - Tehran, Iran

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First of two simultaneous responses --

Fire your material contractor and get someone who knows how to treat metal. I'd try to help you more, but nothing I can say will help you more than that.

Hire a consultant if you need help finding a competent supplier.

Jeff Watson
- Pearland, Texas
2006

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Second of two simultaneous responses --

Mr. Nader, make your chemical supplier test the system, that's what he gets paid for. You shouldn't have to play a guessing game with your products. If he won't, find a new supplier.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2006

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2006

Thanks for your response.

I know my pretreatment line have some problems but I have to produce in this line. We have to find suitable material to match with my line. But I afraid about quality of paint that applied on this surface(especially about adhesion of paint with phosphated surfaces).


Could you please say to me what happens if the last rinsing of our line be alkaline (ph>9).


Best regards,

Nader Bayrami Tarouni
- Tehran, Iran

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You are doing all wrong pH in your system. Your rinses should not be more than 9 and your final rinse should be of neutral pH.

All the painted items under your existing system will get corroded faster and paint peel off will also take place.

Please correct your system by ensuring spray nozzle orientation to avoid crossover of chemicals from one spray zone to another.

I think I have answered your question.

Regards,

ANURAG KANAUJIA
- New Delhi
2006

Take care of carry over from particular tank to next successive tank. ph of tank play very important role in bath chemistry and it should not be changed by carry over from previous tanks. Keep overflow of water on in all water rinse tanks and ensure ph(6.5-7) and conductivity (under 10ms) of dripping water should be under spec before entering ED Tank.

SURYAPAL Singh
- SILVASSA,UNION TERRITORY OF India, India
2006