Do Not Paint or Seal: The worst thing to do is to paint over the weld. The scale must be allowed to spall off naturally. Trapping it under a coating will lead to premature coating failure and hidden corrosion.
Promote Natural Weathering: Simply expose the structure to the natural wet/dry cycles of the environment. Rain and sun are the best tools to wash away the loose scale and allow the patina to form.
Accelerate the Process (Optional): If the initial uneven appearance is a concern for the client, the weathering process can be accelerated:
Light Abrasive Blasting: Using a very light blast with a fine, non-metallic abrasive (like crushed glass or organic media) can carefully remove the scale without damaging the sound metal underneath. This is a skilled operation.
Weathering Accelerant Solutions: Commercial solutions are available that are applied to the surface. They contain chemicals that encourage the rapid formation of the stable patina, helping to even out the appearance faster.
Good Design Practice: Ensure the structure is designed to shed water and avoid traps where moisture and debris can accumulate, as this can lead to localized accelerated corrosion in the vulnerable early stages.
Conclusion
Short-Term: Yes, the HAZ has reduced corrosion resistance immediately after welding due to alloy burn-off and non-protective scale formation.
Long-Term: No, the effect is not permanent. The inherent corrosion resistance of the underlying Corten base metal will prevail once the scale spalls off through natural weathering.
Key Takeaway: The structure must be left to weather naturally. The initial uneven appearance is a normal and temporary phase in the life of a welded Corten structure. Patience is required as the patina uniformizes over 1-2 years.
