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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The alpha-case layer of titanium welds enhanced with oxygen can be removed by post-weld pickling in a mixture of nitric and hydrofluoric acids (20% HNO3 + 3 % HF). A quick 5 min pickle removes 10-20 mu m of material, leaving some alpha-case intact. The longer pickle of 20 min removes 40–80 µm, removing the alpha-case completely. Crevice corrosion resistance is improved by alpha-case elimination in 10% citric acid at 80oC. Crevice initiation time of fully pickled tubes (20 minutes) is 5 times greater than lightly pickled tubes (5 minutes). The alpha-case is brittle and anodic to the base metal and provides a favored site for crevice assault.
The Mechanism of Removal of Alpha-Case and Improvement of Crevice Corrosion
During welding, oxygen diffuses into the titanium surface and forms an alpha-case layer that is harder and more brittle than the base metal. This layer has different electrochemical properties and is more prone to crevice corrosion. The alpha-case is removed uniformly by pickling in HNO3/HF. Complete removal returns the surface to the composition of the base metal and removes the favored attack sites. Weld undercuts and root passes where access is limited will be removed by longer pickling times.
Quantitative Alpha-Case Removal and Resistance to Crevice vs. Pickling Time
Controlled testing on Grade 2 titanium welds pickled in 20% HNO3 + 3% HF at 50°C followed by immersion in 10% citric acid at 80°C, has shown that At 0 minute pickling (as-welded) alpha-case removal depth (µm) = 0, residual alpha-case (µm) = 50–150, crevice initiation time (h) = 100–200. The removal depth at 5 min pickling is 10-20 μm, residual alpha-case is 30-130 μm and the initiation time is 200-500 hours. Removal depth 20-40 µm 10 minutes Residual alpha-case 10-110 µm 400-1,000 hours Initiation time 20-40 µm 10 minutes After 15 min etching the removal depth is 30–60 µm, the residual alpha-case is 0–90 µm and the initiation time is 600–1500 h. After 20 minutes the removal depth is 40-80 micrometers, the residual alpha-case is 0-50 micrometers and the initiation time 800-2000 hours. At 30 min, the depth of elimination is 60–120 µm, the remaining alpha-case is 0 µm, and the start time is 1,000–2,500 hours.
Influence of concentration of citric acid and temperature on the benefit of pickling
The longer the pickling, the greater the benefit, and the benefit increases with citric acid content and temperature. Pickling 5 minutes 20 minutes 3× 5% citric acid 80°C 5% citric acid 80°C 3× crevice life extension from 5 minutes pickling to 20 minutes pickling At 10% citric acid and 80°C, the extension is 4–5 fold. Extension of 5-6× at 15% citric acid and 80°C 6-8x extension at 10% citric acid and 100°C. The more aggressive the higher acid concentrations and temperatures are, the more important becomes the elimination of the alpha-case.
Selecting a Post-Weld Pickling Time for Citric Acid Service
Table 3. Recommended post weld pickling times for Grade 2 titanium heaters in 10% citric acid at 80°C to achieve required crevice corrosion resistance
Desired Crevice Initiation Time (hours) Recommended Pickling Time (minutes) Expected Alpha-Case Removal Depth (µm) Metal Loss from Tube Surface (µm) Recommended for Service Life 200–500 5 10–20 10–20 Low (1–2 years) 400–1,000 10 20–40 20–40 Moderate (2–3 years) 800–2,000 20 40–80 40–80 High (3–5 years) 1,000–2,500 30 60–120 60–120 Maximum (>2,500 45 100-150 100-150 Overpickling, no benefit
Engineering Beyond Pickling Time Control
The pickling rate depends on the grade of titanium. Grade 7 pickles just as fast as Grade 2 The wall thickness should be enough to allow for the metal loss from pickling . A wall of 1 . 5 mm with a removal of 60 µm would leave 1 . 44 mm and this is acceptable . The pickling bath temperature should be kept at 50°C ± 5°C as the removal rate increases with temperature but there is a danger of over-pickling. The concentration of HF should be managed. The higher the HF, the higher the clearance rate but the higher the risk of hydrogen absorption. After pickling the tube must be well rinsed to remove the acid remains. Microhardness testing can verify the removal of the alpha-case; a surface hardness below 250 HV shows complete removal.
Making a well-informed specification
Specify a post-weld pickling duration of 20 minutes in 20% HNO3 + 3% HF at 50°C for complete alpha-case removal for a titanium heater tube welded and designed for 10% citric acid at 80°C. For less demanding applications (lower temperature or concentration) 10 minutes may be sufficient. If it is critical service, then say 30 minutes. Test the surface hardness to check alpha-case removal. Less than 250 HV is a good indication of success. Tubes that have residual blue or gray discolouration after pickling should be discarded. The engineer controls post-weld pickling to 20 minutes and removes the alpha-case layer and improves crevice corrosion resistance by a factor of 5 in citric acid service.
