When compared to paint or powder systems, hot-dip galvanizing provides significantly less potential for damage. If damage does occur, hot-dip galvanizing naturally mitigates further corrosion:
- The adjacent zinc will provide cathodic protection to the affected area.
- Cathodic protection will prevent underfilm corrosion of the adjacent coating, preventing the area of concern from becoming larger.
- If the damage is the result of severe handling due to transportation or erection, a zinc iron alloy layer will typically remain in place, further protecting the area.
The best repair procedure is metallizing to ASTM A780 A3. While Daam Galvanizing can perform these repairs, they’re rarely required because of the low corrosion potential of damaged areas.
If the following conditions are present, we recommend repairing the affected areas with zinc-rich paint.
- Any area with a remaining alloy layer larger than 30mm in diameter or more than 10mm in width, if longer than 30mm.
- Any area where the base metal is exposed to a width of more than 4mm.
When repairing the areas according to ASTM A780 A2 and using zinc-rich paints, the following procedures must be in place:
- If the zinc alloy layer is still present, the surface must be clean, dry and free of oil and grease.
- If the zinc has been completely removed in the damaged area, the surface must be blast cleaned in accordance with SSPC-SP5, white metal blast. If circumstances prevent blast cleaning, it’s permissible to power disk sand affected areas to a bright metal. To ensure a smooth reconditioned coating, surface preparation may extend into the undamaged galvanized coating.
- The zinc-rich paint is spray or brush applied as per the paint manufacturer’s recommendations. Multiple passes are required to achieve a minimum, total dry film thickness of 100 microns. Paint must overlap slightly onto the area around the patch, but caution must be taken to prevent over-painting, which may exaggerate the size of the repaired area.
- Take measurements of the coating with a magnetic gage to ensure that the applied thickness is as specified.
Zinc-rich paint can be purchased at Daam Galvanizing.