Why Welded Joints Need Special Treatment
The corrosion resistance of Q235NH is not magical; it comes from the specific alloying elements (like Copper, Chromium, Nickel, and Phosphorus) that allow it to form a stable, protective patina.
The welding process compromises this protection in two key ways:
Burn-off of Alloying Elements: The intense heat of the welding arc can cause the critical alloying elements to oxidize and burn off from the surface of the weld metal and the immediate adjacent area (Heat-Affected Zone - HAZ).
Microstructural Changes: The rapid heating and cooling cycle alters the steel's microstructure in the HAZ, creating a region more susceptible to corrosion than the base metal.
Result: The welded joint becomes a "weak link." While the parent Q235NH plates are forming a protective patina, the weld and HAZ will corrode at a faster rate, using a porous, non-protective rust. This leads to:
Unsightly Streaking: Rainwater will wash rust from the actively corroding weld down over the stable patina, creating dark red streaks.
Localized Corrosion: Potential for accelerated thinning at the weld, which could compromise structural integrity over time.
Recommended Treatment Methods
The goal is to restore the corrosion-resistant properties to the welded area. Here are the standard methods, from most to least effective:
1. Abrasive Blasting + Patination Solution (Best Practice)
This is the most effective and durable method to ensure uniform performance and appearance.
Process:
Abrasive Blast: Clean the weld, HAZ, and any nearby cut edges using abrasive blasting (e.g., with copper slag or garnet) to a Sa 2.5 (Near-White Metal) standard. This removes all mill scale, slag, and contaminants and creates a pristine, uniform surface profile.
Apply Patination Solution: Immediately apply a weathering steel patination solution (a chemical "rust accelerator"). This solution catalyses the formation of the stable iron oxyhydroxide (FeOOH) patina, mimicking years of natural weathering in a matter of hours.
Result: The treated area will begin to form a protective patina that matches the parent metal in color and performance almost immediately.
2. Abrasive Blasting Only (Good Alternative)
If a patination solution is unavailable, thorough abrasive blasting alone is a very good option.
Process: Abrasive blast the weld and HAZ to a Sa 2.5 standard.
Result: This exposes a clean, bare metal surface with the same alloy composition as the core material. When exposed to the atmosphere, it will naturally begin to form the protective patina. While slower than using a solution, it will eventually blend in. The key is that the surface is perfectly clean.
3. Power Tool Cleaning + Coating (For Repairs or Specific Cases)
For situations where abrasive blasting is impossible (e.g., on-site repairs), or in highly aggressive environments (coastal, high salinity), a protective coating system is used.
Process:
Clean the weld area to a St3 standard with power tools (grinders, needle guns).
Apply a breathable, protective coating.
Coating Options:
Weatherable Primer: A primer specifically designed for weathering steel that allows some moisture transmission and is formulated to weather to a similar color.
Tinted Breathable Coating: A lightly tinted, matte clear coat (e.g., acrylic) that protects the weld but allows the surrounding steel to weather naturally. The tint helps it blend in visually.
What to AVOID
Leaving it Untreated: This guarantees uneven rusting and streaking.
Using Standard Paint: Do not coat the weld with impermeable coatings (like epoxy or enamel) while leaving the rest bare. This will trap moisture underneath the coating, leading to severe hidden corrosion.
Using Incorrect Fillers: Always use welding electrodes or wire specifically designed for weathering steel (often labeled as "NH" or meeting standards like AWS E7018-W). These are "over-alloyed" to compensate for burn-off during welding.
