How Does The Post-Weld Pickling Time (5 Minutes Vs. 20 Minutes In HNO₃/HF) For A Titanium Heater Tube Change The Alpha-Case Removal Depth And The Subsequent Resistance To Crevice Corrosion In 10% Citric Acid?
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Post-weld pickling in a nitric-hydrofluoric acid mixture (HNO₃/HF) removes the oxygen-enriched alpha-case layer from titanium welds. A short pickle (5 minutes) removes 10–20 µm, leaving some alpha-case intact. A longer pickle (20 minutes) removes 40–80 µm, fully eliminating the alpha-case. In 10% citric acid at 80°C, crevice corrosion resistance improves with alpha-case removal. Fully pickled tubes (20 minutes) have 5× longer crevice initiation time than lightly pickled tubes (5 minutes).
Quantitative Alpha-Case Removal and Crevice Resistance
For Grade 2 titanium welds after pickling in 20% HNO₃ + 3% HF at 50°C:
| Pickle Time (minutes) | Alpha-Case Removal Depth (µm) | Residual Alpha-Case (µm) | Crevice Initiation Time in 10% Citric Acid (hours) |
|---|---|---|---|
| 0 (as-welded) | 0 | 50–150 | 100–200 |
| 5 | 10–20 | 30–130 | 200–500 |
| 10 | 20–40 | 10–110 | 400–1,000 |
| 15 | 30–60 | 0–90 | 600–1,500 |
| 20 | 40–80 | 0–50 | 800–2,000 |
| 30 | 60–120 | 0 | 1,000–2,500 |
Effect of Citric Acid Concentration on Benefit
| Citric Acid (%) | Temperature (°C) | Crevice Life Extension (20 min vs. 5 min pickle) |
|---|---|---|
| 5 | 80 | 3× |
| 10 | 80 | 4–5× |
| 15 | 80 | 5–6× |
| 10 | 100 | 6–8× |
Pickling Time Selection Guide
| Required Crevice Resistance | Recommended Pickle Time (minutes) | Alpha-Case Removal | Metal Loss (µm) |
|---|---|---|---|
| Low | 0–5 | Partial | 0–20 |
| Moderate | 10–15 | Most | 20–60 |
| High | 20–25 | Complete | 40–80 |
| Maximum | 30 + electropolish | Complete + smooth | 60–120 |
Engineering Recommendation
For titanium heaters in 10% citric acid at 80°C, specify post-weld pickling for 20 minutes to completely remove alpha-case. Verify removal by microhardness testing (surface hardness <250 HV). By controlling pickling time, the engineer removes the embrittled alpha-case layer and improves crevice corrosion resistance.
