Letter 9054

Mill scale formation on stainless steels 

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I am a researcher studying corrosion resistant rebar, specifically using stainless steel. There seems to be conflicting opinions about whether the mill scale formed during production is helpful or harmful in resisting corrosion. (There will not be any additional protective coatings on the steel being tested).

For non-stainless steel, the scale seems to provide some protection against corrosion, and should not be removed from the steel. However, since stainless steel is already so resistant itself, does this still apply? I would also appreciate any other information about the actual formation and properties of mill scale.

Thank you,

Megan Spielbusch
University of Kansas - Lawrence, KS, USA

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Hello Megan!

You want to surf over to the Nickel Development Institute at www.nidi.com and get their publication number 14050. The summary is: 14050 HEAT TINTS ON STAINLESS STEELS CAN CAUSE CORROSION PROBLEMS (1999) PDF File size: 0.8mb By A.H. Tuthill and R.E. Avery, reprinted from Materials Performance February 1999.

The dark heat tint formed alongside welds during welding of stainless steel (SS) the heat-affected zone (HAZ) is a thicker chromium oxide scale with a mixture of iron, nickel, and other oxides. This thin layer is lower in chromium, the primary constituent that gives SS its good corrosion resistance. Corrosion that would not occur elsewhere can initiate in the HAZ unless the heat tint scale and the thin chromium-depleted layer just beneath are removed. Removal by rotating fiber brush, pickling, or electropolishing readily restores this area to base-metal resistance.

The publication is available as a pdf file, and well worth the time to read it. While you're at it, have them send you a copy of publication 9014, on design guidelines for stainless steels, a superb publication. And I love the FREE price!

Good luck!

Lee Gearhart
metallurgist
East Aurora, NY

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