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Galvanize flake off problem in 25 mm thick angles

January 7, 2010

I would like to know why we are facing a flake off problem when we galvanize 25 mm angles even the coating thickness is 80 to 130. Travelling between furnace and cooling tank takes 210 seconds. We are using strike hammer for the test. We use double flux 22 baumé.

analyze of 25 mm angle : C:0,16 Si:0,17 Mn:1,22 P:0,17 S:0,23 Cr:0,7 Ni:0,12 Cu:0,19
Al:0,019 V:0,066 Mo:0,029

Efe Ulusoglu
engineer - Turkey

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January 13, 2010

Efe,

Are you sure that the phosphorus is 0.17% and the copper is 0.19%?

The Sandlin Curve is very much involved here. According to Teck Cominco silicon equivalent curve (e.g. silicon plus 2.5 times the phosphorus) this steel would be in the higher end of the Sandlin Curve. This phosphorus, I believe is higher than rephosphorized steel. I wonder if it has a lot of scrap iron in it? I believe that we can at least say this is a reactive steel (although the zinc coating thickness is not especially high for 1 inch thick steel).

I was under the impression that core-10 was 0.10% copper and is a "weathering" steel whereby the steel coating turns brown-red and supposedly stops corroding. If you galvanize a large quantity of this type steel, it would be wise to have the zinc bath tested for copper. Above about 0.05% copper in the zinc would not be good.

It is my understanding that the impact hammer test has been discontinued due to action by the American Galvanizers Association. I recommend that you contact them directly to be sure on this point. Thus your major concern would be if the coating flakes off during loading, unloading of the product. There would seem to be no reason to do a test which has been discontinued.

Were the flakes that broke off brittle or easily bendable? What is the iron content of the chips?

Regards,

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

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I think the decimal point is in wrong place with the S, P analysis.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

January 13, 2010