Summary for zinc phosphate coating continuity Discussion
Zinc phosphate coating continuity problem is the core issue here: Taranath reports that 1/2" EN8D/C-45 shafts are meeting the target phosphate thickness and coating weight in an 11-tank dip process, yet they still fail salt fog testing with rust and SEM shows bare or very thin spots where base metal is visible. He lists the process sequence and bath conditions, including Gardoclean 390 degreasing, Gardolene 6521 activation, Gardobond Z 533A phosphating, and Gardolene R 1683 passivation, followed by RP oil. In response, Sanjay Dhadwal advises that the parts are likely underprocessed: he recommends increasing degreasing time to 10 minutes, raising phosphate bath temperature to 85 C, and extending phosphate immersion from 1 minute to 10 minutes. His view is that the very short phosphate time is the reason for poor salt spray life and incomplete coverage.
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
Taranath Aluru
– Hyderabad/India
July 23, 2024
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“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
Taranath Aluru [returning]
Owner – Hyderabad/India
July 29, 2024
A. Hello dear,
Please increase the time in degrease up to 10 minutes and increase the temperature of phosphate bath up to 85 and also increase immersion time in phosphate bath from 1 minute to 10 minutes, You are not getting salt spray life because you are giving very less time in phosphate.
Sanjay Dhadwal
– Dubai, UAE
September 19, 2024