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Free acidity low - reasons?

Ours is a tricationic type zinc phosphating. The free acid value should be between 0.6-0.9. But it comes down from 0.l9 - 0.5 , we add extra replenisher to come back to 0.9, but it is also maintained only for two hours.we are treating mild steel & aluminium parts for bikes. Please forward your reply.

Babu Sankar
- Hosur, Tamilnadu, India
2003

The most likely answer is that you are getting drag over from the alkali cleaner stage through the rinses. If this is not the case then, most zinc phosphates use sodium nitrite as the accelerator, excessive amount of accelerator will reduce the free acid, you should check the rinse tanks alkalinity first.

Robert D Lilley
- Singapore
2003

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