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Zinc Phosphate - Coating Continuity Problem

Q. Hello, I'm Taranath. We are trying to Zinc Phosphate 1/2" shafts of EN8D equivalent to C-45 material with a phosphate layer of 2-3 microns.

We are using an 11-Tank dip method. Process flow is Degrease 1 -> Degrease 2 -> Water Rinse 1 -> Water Rinse 2 -> Activation -> Phosphate -> Water Rinse 3 -> Water Rinse 4 -> Passivation -> Drying

We are achieving the coating thickness and coating weight required but the parts fail in Salt Fog Test, i.e., rust build up.

We also dip the parts in RP oil after Drying.

Upon SEM analysis at 100 x and 500 x, we observe that there are some bare spots or very thin coat where base metal can be read thru (seen with EDX superimposition)

Can someone suggest what can be done to get 100% phosphate coverage without gaps.

Thanks & Regards

SUGGESTED PHOSPHATING BOOKS

"Optimization of Phosphating Process" by Vinod Prasad Sharma (2022)
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"Phosphating of Metals" by Werner Rausch (1991)
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"Phosphating & Metal Pretreatment" by D.B. Freeman (1986)
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Q. Hello, our current bath concentrations are as follows:

Degreasing - Gardoclean 390, Temp 70-80 C, Dip Time 3 min

Water Rinse - DM Water pH 6.5-7.5, Dip Time 1.5 min

Water Rinse - DM Water pH 6.5-7.5, Dip Time 1.5 min

Activation - Gardolene 6521, 6 grams/litre, pH 10.5, Dip Time 1 min

Phosphating - Gardobond Z 533A, 25 grams/litre, Temp 60-75 C, Dip Time 1 min

Water Rinse - DM Water pH 6.5-7.5, Dip Time 1.5 min

Water Rinse - DM Water pH 6.5-7.5, Dip Time 1.5 min

Passivation - Gardolene R 1683, pH 8 - 8.5, Dip Time 2 min