Coated Sheet 


Galvannealed
Fabrication Considerations

Forming
Galvannealed sheet can be formed in much the same manner and by the same variety of processes as galvanized sheet, since the steel substrate is generally the same for both materials. 

Although the steel formability is similar for both types of coating, there are differences in the steel forming behavior of the galvannealed and galvanized coatings. The zinc-alloy coating on galvannealed steel sheet is more brittle than the zinc coating of galvanized sheet. When the sheet is bent or formed severely, the zinc-alloy coating will exhibit some amount of powdering on the compression side of the formation. The amount of powdering that occurs during forming is generally not detrimental to corrosion resistance after painting, but it can cause "dirt" build-up in dies, presses, etc.


Joining

Soldering
The iron-zinc alloy coating on galvannealed sheet is substantially more difficult to solder than galvanized sheet, although soldering can be accomplished in commercial production operations. Often, more reactive fluxes are required to make soldering easier.

Welding
Galvannealed sheet can be readily welded by resistance welding, and other more complex procedures including arc welding. The equipment needed for welding galvannealed sheet is essentially the same equipment that is required to weld galvanized sheet. As with galvanized steel, precautions must be taken to avoid porosity and cracking of the weld caused by penetration of zinc into the weld pool while using arc and oxyfuel processes. 

In electric resistance welding, such as spot welding, the zinc-iron alloy coating present on galvannealed sheet typically requires less welding current than that needed for galvanized product. This is especially true in that the coating is most often thinner than a galvanized coating would be if used for the same application. At lower currents, the coating reacts less with the electrodes; there is less zinc pick-up, less redressing of electrodes, and thus, more spot welds between redressing.

Similar benefits are realized during seam welding of galvannealed sheet.

Fastening
Galvannealed sheet can be joined by the same fasteners and techniques as those applicable to galvanized sheet.

Adhesive Bonding
Adhesive bonding techniques can be used for joining galvannealed sheet either to itself or other materials. The same materials as those used for the adhesive bonding of galvanized sheet are needed. 

Sealants
Sealants are often used between overlapping areas of steel sheet to form watertight joints and to achieve superior environmental durability. Neutral-cure silicone rubber sealants are recommended for use with galvannealed sheet. These materials need no primer. They are flexible and noncorrosive as well as resistant to heat, cold, water and ultraviolet rays.

Other types of sealants, like butyl rubber and styrene butadiene rubber, may also be used successfully. For more information about sealants for specific applications, consult a sealant manufacturer. 

Painting
U. S. Steel Galvannealed Sheet is an ideal material for painted product applications. Whereas galvanized-coated steel surfaces generally require that a paint pretreatment be applied before painting to obtain good adhesion between the paint and the galvanized surface, galvannealed sheet can be directly painted. Improved performance will generally be attained if a primer/washcoat is applied prior to painting, but it is not absolutely necessary. Furthermore, the somewhat rougher finish of the zinc-alloy surface provides better mechanical bonding to the paint than the usually smoother surface finish of a galvanized coating. 

In service, since the galvannealed coating is less reactive than zinc, there tends to be less blistering between the paint and galvannealed coating. Also, there tends to be less undercutting corrosion at sheared edges and other breaks in the paint. Thus, the very nature of the corrosion behavior further enhances the adhesion between the paint and the metallic coating. 

Galvannealed sheet can be painted using many types of paints. In most instances, the surface should be cleaned after fabrication to remove oils and dirt so that good adhesion can be achieved during and after painting. A clean surface also aids in maintaining good adhesion during service in the environment. Although not necessary, the application of an iron or zinc phosphate pretreatment provides additional resistance to delamination of the paint during exposure to the environment.

In all instances, discussions with paint suppliers are recommended to achieve the best performance attainable for each application.

Typical Applications Application Considerations Fabrication Considerations Tolerances