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Problem with raised lines on galvanized 16 ga fenders

We are a trailer manufacturer in the midwest. We build primarily boat trailers. We recently have had some issues with some fenders that we sent out to be galvanized. They came back with raised lines running parallel with the fender.

The fenders have no lines in them that can be seen or felt prior to galvanizing. It has only happened on one type of fender. We get several types of fenders from the same supplier. The galvanizing plant can't tell us what's causing this and neither can the fender supplier. Any help would be greatly appreciated!

Rob Talbot
Trailer Manufacturer - Lebanon, Missouri, USA
August 26, 2009

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First of two simultaneous responses -- September 2, 2009

The picture appears to show that no such lines are on the bracket, only on the rolled curved sheet. This suggests its an issue with the steel.
The lines appear to be in the rolling direction, suggesting granular erosion?

It looks to me like partial etching of the steel by the pickle acid.
Did the galvanizer have inhibitor in the acid?
Was the article left in the acid too long ?
To test this, try shotblasting one, then no pickle acid, just galvanize immediately after blasting and see what finish you get.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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Second of two simultaneous responses -- September 2, 2009

Sir:
Looks to me like silicon semi killed steel. If the zinc coating is quite thick (e.g. 10 mils or about 250 microns) then this would surely be silicon semi killed steel. If you have the certs on the steel it would be helpful.
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

I agree with Dr. Cook about the high silicon steel. I galvanize a lot of trailer parts, including fenders, and have run into this problem, though very seldom. The picture shows these lines to be on the rolled part of the fender, and it looks like the step portion has a nice finish. In the case of reactive steels, dwell time in the kettle makes all the difference, and in the case of thin metals like your fenders, it is almost impossible to get them in and out of the zinc bath quick enough to stop this reaction. If this is an ongoing problem, have your supplier give you a certificate of analysis or have a non-galvanized piece checked for silicon content. You can then go www.galvanizeit.org and check your findings with "reactive steels" and an article on the "Sandelin curve".

David Jaye
Houston, Texas
September 25, 2009

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October 12, 2009

this is rolling problem of the steel, the lubricant which is in the powder form used during rolling of steel gets entrapped, during normal galvanizing it gets projected up,
the practical solution for this is like what Mr.Geoff has mentioned shot blast, flash pickling, fluxing, drying and then galvanize your issue will be minimal,

Regards,

S.R.Ameen
- U.A.E.