Once a decision has been made on what corrosion protection system will be utilized, minor design adjustments are often necessary. One such instance when hot-dip galvanizing is the design of overlapped and contacting surfaces. Many fabrications, including those with overlapped surfaces, are well suited for the galvanizing process; however the design engineer should ensure the pieces are fabricated for highest-quality galvanizing.
.During the galvanizing process, steel is heated from ambient temperature to more than 800 F at a rapid pace. To prevent trapped moisture or gas from flashing to steam during this heating process, it is important to properly vent overlapping and contacting surface. Trapped moisture can result in minor concerns such as localized uncoated areas or more significant damage including rupturing the fabrication.
Consulting with your galvanizer before fabricating any overlapped piece is highly recommended as their experience can help you decide between a completely seal-welded joint and a skip-welded joint.
Consider the following best design practices to minimize issues when galvanizing overlapped surfaces:
- Seal-welding ensures cleaning solutions cannot become trapped in the weld area; if skip-welding is used, a gap of at least 3/32” (2.5 mm) must be provided.
- Cleaning solution salts can be retained in tight areas because they cannot be adequately rinsed; the galvanized coating may be of good quality in adjacent areas, but humidity encountered weeks or even months later may wet these acid salts, causing staining of the surrounding galvanized coating.
- When overlapping or contacting surfaces cannot be avoided and are 3/32” (2.5 mm) or less, all edges should be completely seal-welded; zinc’s viscosity prevents it from entering any space tighter than 3/32” (2.5 mm).
- There should be vent holes in the overlapped area.
Additional recommendations can be found in the specification ASTM A 385.