Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

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Zinc Plating without Passivation Gets White Rust

Quickstart:
Steel components are often zinc plated as an aesthetically attractive corrosion-deterring treatment. However, the zinc plating is nearly always chromate conversion coated because the zinc plating is highly susceptible to white rust without it.
It is possible to powdercoat a zinc plated + chromate conversion coated surface, but it can be problematic, so zinc phosphatizing the zinc plating, or directly zinc phosphatizing the steel (skipping the zinc plating) may be preferable.

Q. Hello Everyone,
One of my customers Exports zinc plated material (without Passivation or any top coat) which is Powder coated after some time. But sometimes in storage or in transit of zinc plated material, material it becomes rusty with White rust.
Kindly suggest me the best way to avoid this problem. Is there any passivation which can support perfect adhesion of Powder coating or is there any other method to do Powder coating on zinc plated Surface?
Waiting for your expertise comments
Regards

Rais khan
- Ludhiana [Punjab, India]
August 10, 2024

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

"Optimization of Phosphating Process" by Vinod Prasad Sharma (2022)
avail from AbeBooks, or Amazon

"Phosphating of Metals" by Werner Rausch (1991)
avail from eBay, AbeBooks, or Amazon

"Phosphating & Metal Pretreatment" by D.B. Freeman (1986)
avail from eBay, AbeBooks, or Amazon

"Phosphating of Metals" by Guy Lorin (1974)
avail from AbeBooks, or Amazon

"Electrolytic and Chemical Conversion Coatings" by Biestek & Weber (1972)
avail from AbeBooks, or Amazon

"A Study of Manganese Phosphating Reactions" by Joseph Menke (1971)
avail from AbeBooks, or Amazon

"Preparation of Metals for Painting" by Samuel Spring (1966)
avail from eBay, AbeBooks, or Amazon

"Continuous Automated Analysis and Control of phosphatizing Baths" by Edward P. Parry (1972)
avail from AbeBooks, or Amazon

(as an Amazon Associate & eBay Partner, we earn from qualifying purchases)

A. Hi Rais,

The ideal pretreatment before powder coating is zinc phosphate, and zinc surfaces can be zinc phosphated.

This will deter white rust on the zinc plating but it is difficult to say how long they can be kept in storage or transit.

Luck & Regards,

ted_yosem
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. How can we powder coat the material over zinc plating where zinc plating is already white rusted ?

Rais khan [returning]
- Ludhiana [Punjab, India]
August 20, 2024

A. Hi again,

It is generally not possible to properly powder coat a substrate which has not received proper pretreatment. However, if the white rust is not terrible, you could try zinc phosphatizing it and see how it goes.

Luck & Regards,

ted_yosem
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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