Why is newly galvanized steel rusting, did the galvanizer not coat the whole part?
This condition known as rust bleeding is the unsightly dripping of rust (iron oxide) from small gaps created by overlapping or contacting surfaces prior to galvanizing the structures. Cleaning and flux solutions can penetrate gaps less than 3/32", but zinc cannot due to its higher viscosity. Because zinc cannot enter the surfaces inside the gaps remain ungalvanized. The cleaning and flux solutions that remain inside the gaps are vaporized as the steel approaches the galvanizing temperature of approximately 850 F.
When these solutions are vaporized, anhydrous crystals are left inside the gaps. After the steel has been galvanized, atmospheric moisture can penetrate the gaps and mix with the anhydrous crystals to form an acidic and highly corrosive solution. When the solution corrodes the ungalvanized portions inside the gaps, corrosion products accumulate and leak or bleed from the gaps, leaving a trail of rust down the side of the part.
Now that we can see what it looks like, what is the solution? Proper design of materials to be galvanized is the best method to prevent rust bleeding. Recommended design details for HDG within ASTM A385 include seal welding all overlapping or contacting surfaces that have gaps less than 3/32" or stitch welding areas where a 3/32" gap can be provided (if stitch welding is suitable in terms of design).
Seal welding prevents cleaning and flux solutions from entering the gap. When these solutions are prevented from entering the gap, rust bleeding will not occur. It is important to ensure seal welds are complete and without imperfections or pinholes. Imperfections or pinholes in the seal weld can allow moisture to penetrate the gap and become trapped inside the sealed area. This creates a safety hazard to galvanizing personnel due to explosive forces that can develop in the sealed area as the steel reaches the galvanizing temperature. When the enclosed area is greater than 16 square inches, it is recommended to vent the structure per ASTM A385.
The designer and fabricator are responsible for notifying the galvanizer when they plan to galvanize pieces with overlapping or contacting surfaces. The galvanizer is not responsible for ensuring small gaps have been seal welded or for repairing damage due to rust bleeding.
You can assure your customer the entire structure was galvanized, and explain rust bleeding from overlapping or contacting surfaces is easily remedied. The stains can be removed from galvanized steel by rinsing with water and scrubbing with a nylon brush.
The good news is after cleaning, rust bleeding can be further prevented at the source by sealing the overlapping or contacting surfaces. When the electrolyte (in this case, the solution created by cleaning or flux solutions mixing with water) is prevented from entering the area, the corrosion halts, Sealing can be accomplished with materials such as silicone caulking or epoxy coatings. This prevents water from penetrating the gaps and mixing with the anhydrous cleaning or flux crystals. These coatings also prevent rust from exiting the gaps.
© 2023 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.