Tuesday, January 3, 2012

Reactive Steels

Atypical coatings produced from reactive steels exhibit different coating characteristics than a typical galvanized coating such as:

Appearance: The atypical galvanized coating may have a matte gray appearance and/or rougher surface due to the absence of the free zinc layer. The free zinc layer present on typical coatings imparts a shinier finish to a galvanized coating.

Adherence: The zinc-iron alloy coating tends to be thicker than a typical galvanized coating. In the rare situation where the coating is excessively thick, there is the possibility of diminished adhesion under external stress (thermal gradients, sharp impact).

Reactive steels are still galvanized on a regular basis, and it is important to note differences in appearance have no effect on the corrosion protection afforded by the galvanized coating. The performance of the coating is based on the thickness of the zinc; therefore, often the duller (and thicker) coatings produced by reactive steels last longer. Furthermore, all galvanized coatings as they weather over time will develop a uniform matte gray appearance.

It is difficult to provide precise guidance in the area of steel selection without qualifying all steel grades commercially available. However these guidelines discussed will assist you in selecting steels that provide good galvanized coatings.

• Levels of carbon less than 0.25%, phosphorus less than 0.04%, or manganese less than 1.35% are beneficial

• Silicon levels less than 0.04% or between 0.15% - 0.22% are desirable

Silicon may be present in many steels commonly galvanized even though it is not a part of the steel's controlled composition, because silicon is used in the steel deoxidation process and is found in continuously cast steel. Both silicon and phosphorous act as catalysts during the galvanizing process, resulting in rapid growth of zinc-iron alloy layers.And even when both elements are individually held to desirable limits, the combined effect between them can still produce an atypical coating of all or mostly zinc-iron alloy layers. When possible, your galvanizer should be advised of the grade of steel selected in order to determine whether specialized galvanizing techniques are suggested.