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topic 9480

Pinholes problem in hot dip galvanized steel

(2001)

Q. Could anyone list up reasons for pinholes, at hot dip galvanized mild steel, or give me a link to an article about the problem.

Rolf O. [last name deleted for privacy by Editor]
- Orsta, Norway


(2001)

A. Hi,

I understand that some years ago Birmingham University along with powder paint co's and users carried out a detailed investigation of this problem:

Pinholeing was found due to air or moisture entrapment and not gasses in the zinc coating as previously postulated. Traditionally "degassing" has been used after pretreat to minimise the problem but rarely is a total cure. The best solution was the use of wax additives in the powder to keep the molten powder open longer so that gas bubbles could escape thru the liquid paint. Quality of galvanizing will have a marked effect on pinholeing as porosity of the coating is the prime factor here.

The was a slight improvement if resin-chrome pretreat was used as this tended to act as a surface sealer.

Roger Bridger
Walterisation UK Ltd - Croydon, UK


(2001)

A. We have small powder coating unit. We faced pinholes problem, items after being powder coated had small holes in them. On analysis we came to conclusion that during pretreatment process of degreasing/pickling/phosphating & washing some chemicals get left deposited on surface; when powder is sprayed on it & it is baked it evaporates. Since it is below powder layer it makes its way thru powder layer leaving tiny holes on surface. To obviate we started descaling of components in drum (filled with wood powder) instead of degreasing (alkaline washing) & pickling & further cleaning of surface by thinner.

I believe on same logic some pre treatment fault is resulting in your problem.

Vikas Kaushal
- Chandigarh, India


A. Hi Rolf. Both Roger and Vikas read your problem as involving pinholes in powder coating onto galvanized steel, whereas it seems to me that you are talking about pinholes in the galvanized coating. This is one of the reasons we keep begging our readers to please post their own situation not abstract questions :-)

Please get back to us with who you are, what you do, and what problem has motivated your question. Thanks!

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey



Porosity issue on galvanized trailer

July 10, 2018

Q. I bought a new trailer that is hot dip galvanized. But on one spot the galvanizing finish looks very bad with lots of porosity.
Can anyone explain what went wrong in the galvanizing process that caused this?
Should one try to do anything to prevent future corrosion?

Jacob Blomqvist
- Sweden


July 2018

Cold Galvanizing Spray

A. Hi Jacob. It's hard to list the causes of porosity without knowing anything about the situation. Bad steel, poor cleaning, poor pickling, overly thick coating along a withdrawal or drip line. But simply spraying on some zinc-rich "cold galvanizing" paint is a typical fix in fencing and other industries. Good luck.

Regards,

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



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