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 20-50 |im. Corrosion rate 0.02 - 0.05 mm per year.
HAZ (as-welded, no post-treatment): 50-150 pits/cm2 pitting density. Maximum depth of pits 100 to 300 µm. Corrosion rate of 0.15-0.40 mm/y. Localised attack at weld toe.
HAZ (post-weld pickling) Pitting density 20 to 60 pits per cm2. Pit depth 40 to 100 µm. Better, but still worse than base metal.
HAZ (stress eased and pickled): Pitting density 10–30 pits/cm2. Pitting depth 30–70 mm. Close to basic metal.
The Effect of Post- weld Treatment on Performance of HAZ
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 only 20-60 40-100 2-3X
Only for stress alleviation 30–80 60–120 1.5–2×
Pickling and stress reduction 10–30 30–70 3–5×
Electropolishing 5–15 20–50 5–10× Engineering Recommendation
For welded titanium heaters in high chloride boiling duty, post-weld stress relief (540°C for 30 min) followed by pickling (20% HNO₃ + 3% HF) is required to restore HAZ corrosion resistance to near base metal levels. In inspection, pay attention to HAZ region, as it is particularly liable to pitting. The engineer understands the importance of inspecting the weld zone and post treatment, knowing the critical difference between the base metal and HAZ.
