Precautions for Overlapping

When designing articles to be galvanized after fabrication, avoid narrow gaps between plates, overlapping surfaces, back to back angles, and channels (see Figure 26) whenever possible.

Consult with your galvanizer regarding the use of temporary bracing or reinforcing to minimize warpage and distortion during galvanizing (see Figure 25).

When overlapping of contacting surfaces cannot be avoided and the gap is 3/32-inch (2.5 mm) or less, all edges should be completely sealed by welding. The viscosity of the zinc keeps it from entering any space tighter than 3/32-inch (2.5 mm). If there is an opening, less viscous cleaning solutions will enter, but zinc will not. Trapped solutions may cause iron oxide to weep out of the joint later on.

Additional challenges resulting from tightly overlapping surfaces:

• Cleaning solutions that may be trapped will flash to steam when the part is immersed in the galvanizing bath. This steam can wash the flux off of the part near the gap, causing bare areas adjacent to the lap joint.
• Cleaning solution salts can be retained in these tight areas due to the impossibility of adequate rinsing. The galvanized coating may be of good quality in the adjacent area, but humidity encountered weeks or even months later may wet these salts. This will cause rust staining to seep out onto the galvanized coating.
• Cleaning solutions will not effectively remove oils and greases trapped between surfaces in close contact. Any residual oil and grease will partially volatilize at the galvanizing temperature. This will result in an unsatisfactory zinc coating in the immediate area of the lap joint.

--Courtesy of the AGA