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Zinc phosphate coating is black and peels off
Q. I have machined components, grade SAE4140 which is high chrome base:
Element Content (%)
Iron, Fe 96.785 - 97.77
Chromium, Cr 0.80 - 1.10
Manganese, Mn 0.75 - 1.0
Carbon, C 0.380 - 0.430
Silicon, Si 0.15 - 0.30
Molybdenum, Mo 0.15 - 0.25
Sulfur, S 0.040
Phosphorous, P 0.035
The problem I am facing is that component gives me black finish in zinc phosphate bath and coating is removed if you rub on the surface.
Process which I follow is
degreasing temp 65 °C, timing 5 to 7 min
water rinsing
pickling HCl 1 ltr of acid to 10 ltr water, time .5 to 2 min.
water rinse + over flowing water time 1 min.
light coating: Zinc Phosphating temp. 55 to 75 °C, time 5 to 10 min.
water rinse + over flowing water time 1 min.
dewatering oil + rust preventative oil.
total acidity is as per TDS from the chemical manufacturer (should not be below 40 ), I have maintained 40 to 45
coating weight is 350 to 570 mg/sqft
On my sample piece which in CRCA metal sheet the appearance on zinc phosphate coating is light gray and fine crystal formation.
The customer wants his components in light gray (even I have confirmed that the master sample which was provided from the customer is zinc phosphate coated).
Please provide me any suggestion on this.
- pune India
November 5, 2023
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