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Zinc phosphate coating is black and peels off

adv.
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SUGGESTED PHOSPHATING BOOKS
(many of these are rare; the links will only sometimes work)

"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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"Phosphating of Metals" by Guy Lorin (1974)
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"Electrolytic and Chemical Conversion Coatings" by Biestek & Weber (1972)
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"A Study of Manganese Phosphating Reactions" by Joseph Menke (1971)
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"Preparation of Metals for Painting" by Samuel Spring (1966)
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"Continuous Automated Analysis and Control of phosphatizing Baths" by Edward P. Parry (1972)
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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.

Prasad Kasar
- pune India
November 5, 2023

A. Zinc phosphate will turn black if coating is too heavy if the pH on the surface of the substrate is not just right it will peel off as it is a crystal that precipitates due to pH change at the interface with the substrate.

RONALD ZEEMAN
- Brampton ON, CANADA
February 3, 2024

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