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Better corrosion resistance: zinc plating or zinc phosphating?

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What is the difference between zinc flake coated and zinc phosphate coated. And of these which is the best finishing and best corrosion resistance for HSFC Bolts.

srinivasarao kotte
- Hyderabad & Telangana India
October 29, 2022


"Phosphating of Metals"
by Werner Rausch
from Abe Books
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A. Hi srinivasarao. HSFC means nothing to me; I assume it's a typo and you are referring to high strength friction grip fasteners?

Zinc flake coating is a corrosion-resistant final finish generally very well suited to bolts. Zinc phosphate is a pretreatment for paint or powder coating; it is not a final finish. If the structure, including the bolts will be painted, the zinc phosphate is probably okay and the paint will probably stick better to it. If the structure will be bolted together and then go into service without any further finishing, zinc phosphate is probably not appropriate.

Luck & Regards,

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




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Q. Hi,

I have some doubt regarding the material for Zinc plating and zinc phosphating. Which one have the better corrosion resistance. I would appreciate if you clear my doubt.

Best Regards,

Toh Chun Thiam
Flextronics Technology (Singapore) Pte Ltd - Singapore
2002


"Phosphating & Metal Pretreatment"
by D.B. Freeman
from Abe Books
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A. Hello Toh. There is no comparison. Zinc plating is a corrosion resistant finish. Zinc phosphating is for temporary use, or for use under paint, not as a final finish.

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



Q. Hi, I really wonder if I apply my finishing for zinc plating is coated with yellow zinc and my zinc phosphate done with powder coating. To this which are the best finishing and best corrosion resistance.

Thanks,

Toh Chun Thiam [returning]
- Singapore


A. Ah, now you're in a different and difficult area. Zinc plating is designed to slowly corrode to protect your article. Powder coating is designed to encapsulate the article, and to give it barrier layer protection. In a salt spray cabinet, I'd bet the powder coating will very easily outperform the zinc plating; in a wet and acidic environment, even slightly acidic, the powder coating will win easily also.

But in the real world, it's complicated; if the coating receives no abrasion or abuse, the powder coating is probably better; if there is scratching, abrasion, and limited maintenance, the zinc plating is probably the better choice.

Also, the zinc plating is both thin and metallic and thus well suited to mechanical applications and small screws. The powder coating is thick and a relative soft plastic/organic, unsuitable for some applications.

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


A. To make more clear, Ted wrote the ratio of salt spray test ASTM B117 [Techstreet affil. link] is approximately one to five which mean zinc phosphate will stand five times more than Zinc plated part yellow passivated and almost 50 times for zinc plated clear passivated.

yehuda blau
Yehuda Blau
YB Plating Engineering and Quality - Haifa Israel

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