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Reactive steel solution?

February 2, 2011

One problem galvanizers often encounter is so called "reactive steel". This is steel whose base chemistry is relatively much more reactive with zinc in that it forms zn/Fe alloys much faster and thicker than other mild carbon steels (predominantly Si levels).
This leave a rough finish, and sometimes a coating so thick (1000u or more), that it cracks off.
The usual control mechanism is to add trace Ni to the melt, between about 0.04 and 0.06%Ni is common. This has a moderating effect but doesn't remove the problem entirely.

But we've noticed (by accident) that if the steel is shotblasted first, the problem is almost eliminated.
Has anyone else noticed this? Does anyone have any theory why it might have any effect?

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

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Geoff,

During hot rolling, impurities like silicon and phosphorus migrate to and are concentrated on the steel surface. Thus during galvanizing the zinc encounters increased levels of silicon and phosphorus on the steel surfaces and give thick, ugly, gray, alloy coatings. By prior shot-blasting, this steel shell is removed and the steel core is exposed which is lower in silicon and phosphorus and an acceptable zinc galvanized coating can be attained. Shot blasting is usually too expensive to be economical for all products.

In the USA there is some use of shot blasting on "non-reactive" steels to attain THICKER zinc coatings to meet minimum ASTM specs.

Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA
February 4, 2011

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Dear Geoff,

This is from an article on galvanizeit.org:

The reactive steels form the Fe-Zn intermetallics in the form of columns. Therefore with a rough surface profile the columns collide and interfere, slowing down the excessive growth rate.

Regards,

CS Leong
- Selangor, Malaysia
February 6, 2011

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Sirs;

It is something to controlled during raw steel production, in which Aluminum is added to remove Oxygen, in this case it is called Aluminum Killed Steel.

some kinds of raw steel with a Silicon & Phosphorus range of (0.03 - 0.14%) which are called "Sandeline Range" require a special bath content & Treatment in order to avoid a thick coating weight.

Regards

Ayman Hassouneh
steel - Amman, Jordan
March 19, 2011