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Help with galv and alkaline cleaner

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We have a zinc coated part that is epoxy powder coated through a paint plant using iron phosphate at PH 5.5

We are having patchy adhesion problems and need to undestand the following

Does using an alakaline cleaner at a PH of over 12 leave a film on the substrate that will effect the impact of the iron phosphate ?

Any help would be greatly appreciated.

Ian France
Presswork - Walsall, West Midlands, UK

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First of two simultaneous responses -- +++++

Rinsing is key to good pre-treatment. Any alkaline carry over, whether it be from a contaminated water rinse or residue left on parts is not good. If you continue to contaminate your slightly acidic phosphate stage with alkali you will potentially raise the Ph to where the process ceases to function effectively.

Nothing wrong with using alkali cleaners, Rinse, rinse and keep the rinses clean. Be aware your cleaner could, depending on it's chemistry,attack and degrade the Zinc coated parts being processed. Look for a "milder" cleaner. If your pretreatment is a spray application it's fairly common to have a cleaner/phosphate product all in one, no need for a separate alkaline cleaner.

Get a good proprietary product that will process your Zn coated parts.

ALAN THOMAS
- Chandler, AZ, USA

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Second of two simultaneous responses -- +++++

Ian, A high concentration's of alkali can create a disturbance in the phosphate solution. It's better to use a low alcali concentration in the cleaner. It easier to rinse of a low concentration alcali cleaner. I belive you have this problem.

Anders Sundman     surface finishing engineer Sweden