Overlapping and Contacting Surfaces

When designing articles to be galvanized after fabrication, it is best to avoid narrow gaps between plates, overlapping surfaces, and back-to-back angles and channels.

When overlapping or contacting surfaces cannot be avoided, all edges should be completely sealed by welding. This is because the viscosity of the zinc keeps it from entering any space tighter than 3/32 inch (2.38 mm). Less viscous pickling acids will enter, but zinc will not

Two further problems encountered with tightly overlapping surfaces are:

1. Pickling acids that may be trapped will flash to steam when the part is immersed in the galvanizing bath. The “blowing out” of this steam can prevent zinc from adhering to the area adjacent to the lap joint.
2. Pickling acid salts can be retained in these tight areas due to impossibility of adequate rinsing. The galvanized coating may be of good quality in the adjacent area, but humidity encountered months, or even weeks, later may wet these acid salts. This will cause an unsightly rust staining to run out on top of 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 prevent a satisfactory zinc coating in the immediate area of the lap joint. It is important to specify that contacting joint surfaces be thoroughly cleaned before fabrication and then completely sealed by welding.

If the area of seal-welded overlap is large, there should be vent holes through one or both sides into the lapped area. This is to prevent any moisture which gets in through a pin hole in the weld from building up explosive pressure while in the galvanizing bath. This venting becomes more important when the area is large or steel is thin. Consult Recommended Details for Galvanized Structures for vent size and numbers. Vent holes can be sealed after galvanizing. Seal welding is not mandatory, but does not prevent trapping moisture, internal rustling and seepage, all of which are possible in any unsealed connection regardless of the protective coating used.

Where two bars come together at an angle, a gap of at least 3/32 inch (2.38 mm) after welding must be provided to ensure the area is wetted by the molten zinc. An intermittent fillet weld may be used. This can be on one side of the bar only or, where necessary, an intermittent staggered fillet weld may be employed on both sides so that no pocket can be formed. This type of welding, however, is not suitable for load bearing members.

ASTM Recommended Practice A385 “Providing High Quality Zinc Coatings (HotDip)” provides guidance for galvanizing overlapping surfaces.