For a Titanium Heater in a Boiling Solution with High Chloride Concentration, What Is the Critical Difference in Corrosion Behavior Between the Base Metal and the Weld Heat-Affected Zone?
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The weld heat-affected zone (HAZ) of a titanium heater corrodes quite differently from the base metal in boiling high-chloride solutions (e.g., 25% NaCl at 108°C). The HAZ has a coarser grain structure, increased residual stress and potential oxygen contamination (alpha-case). These properties make the HAZ more prone to pitting and crevice corrosion than the base metal. This crucial distinction is important for weld design and inspection prioritisation.
Quantitative Comparison of Base Metal vs HAZ
Boiling 25% NaCl at 108° C for 500 hrs:
Base metal (Grade 2) 5-20 pits/cm2 pitting density. Maximum pit depth of 20–50 μm. Corrosion rate 0.02-0.05 mm/year.
HAZ (as-welded, no post-treatment): pitting density 50–150 pits/cm². Pit depth up to 100 to 300 µm. Corrosion rate 0.15 to 0.40 mm per year. Localised attack at weld toe.
HAZ (post-weld pickling) Pitting density 20-60 pits/cm² Pit depth 40-100 µm Better, but still not as good as base metal.
HAZ (Stress alleviated + Pickled) Pitting density 10-30 pits/cm² Pit depth 30-70 µm More like base metal.
Influence of Post Welding Treatment on HAZ Behaviour
Post-Weld Treatment HAZ Pit Density (pits/cm²) HAZ Max Pit Depth (µm) Improvement Factor vs. As-Welded
None (as-welded) 50-150 100-300 1x
Pickling just 2-3x 20-60 40-100
Stress alleviation just 30–80 60–120 1.5–2x
Pickling and stress reduction 10–30 30–70 3–5×
Electropolishing 5–15 20–50 5–10× Engineering Recommendation
Pickling (HNO3/HF) and post-weld stress relief (540°C, 30 min) are recommended for welded titanium heaters used in high-chloride boiling service to restore HAZ corrosion resistance to near base metal levels. In inspection, give special attention to the HAZ area, as this is where pitting and crevice corrosion are more likely to occur. The engineer knows the key difference between base metal and HAZ and therefore focuses on weld zone inspection and post-treatment.
