Why Does a Titanium Sheath Heater with a Post-Weld Heat Treatment at 600°C in Air Exhibit a Fourfold Higher Crevice Corrosion Rate in 5% Oxalic Acid at 80°C Compared to an As-Welded Tube?

PWHT of titanium sheath heaters is commonly conducted at 540–650 °C to reduce residual strains induced by welding. A thick, multi-layered oxide scale is generated on the titanium surface at 600 °C in air. This scale is composed of TiO2, Ti2O3 and TiO and the oxygen is highly enriched underneath it generating an alpha-case layer. This oxide scale and alpha-case are chemically unstable in 5% oxalic acid (H₂C₂O₄) at 80°C. The scale breaks and flakes off in several fissures. Oxalic acid penetrates these fissures, concentrating hostile species and increasing local corrosion rates. The as-welded tube in pickled or electropolished surface condition and without PWHT oxide has a passive film that is thin, dense and resistant to crevice initiation. This results in a 4 times increase in crevice corrosion rate in air-PWHT tubes versus as-welded tubes.

Mechanism of PWHT-Induced Acceleration of Crevice Corrosion

Titanium is subjected to parabolic oxidation in air at 600°C with a scale of 2-5 µm. Oxygen dissolves in the metal and forms a surface alpha-case layer 50–150 µm deep. This layer is fragile and has a thermal expansion coefficient different from the base metal coefficient. Where cooled micro-cracks occur in the scale. The scale dissolves non-uniformly in oxalic acid and reveals the alpha-case. The alpha-case is anodic to the underlying metal and galvanic cells are set up. The cracked scale and exposed alpha-case are natural fissures and increase the attack rate 3 to 5 times that of a clean as-welded surface. The growth of crevice corrosion is primarily at the contact between the alpha-case and the base metal.

Quantitative crevice corrosion rates under different post weld heat treatment conditions.

Controlled tests in 5% oxalic acid at 80°C for 500 hours have been used to determine crevice corrosion rates for Grade 2 titanium in several post-weld circumstances. Crevice corrosion rate at weld toes was 0.08–0.15 mm/year for an as-welded, pickled tube without PWHT and a clean surface with only minor attack. The rate for a tube given PWHT at 540°C in air, which resulted in a light straw oxide, was 0.20–0.35 mm per year. This was double the rate for the as-welded condition and scale partially spalled. A tube with PWHT at 600°C in air and a blue or purple oxide has a rate of 0.35–0.65 mm/year, a fourfold increase over the as-welded baseline, and severe scale cracking and spalling. The tube with PWHT at 650°C in air, forming gray or white oxide, has a rate of 0.60–1.20 mm per year, eight times higher, with serious scaling and spalling. In contrast, the tube with PWHT at 600 °C in an argon inert atmosphere shows no significant oxide and has a rate of only 0.06–0.12 mm/year with no acceleration.

Effect of Oxalic Acid Concentration and Temperature on Acceleration Factors

The acceleration factor from air-PWHT is approximately constant over a range of oxalic acid concentrations and temperatures. The as-welded rate in 5% oxalic acid at 60°C is 0.05–0.10 mm/year and the 600°C air-PWHT rate is 0.20–0.40 mm/year, an acceleration factor of 4. The as-welded rate at 5% oxalic acid and 80°C is 0.08–0.15 mm/year and the air-PWHT rate is 0.35–0.65 mm/year, a factor of 4 again. The as-welded rate is 0.15–0.25 mm/year while the air-PWHT rate is 0.60–1.00 mm/year at 10% oxalic acid and 80°C, still a factor of 4. The as-welded rate is 0.20-0.35 mm per year and the air-PWHT rate is 0.80-1.50 mm per year at 5% oxalic acid, 100°C boiling conditions, at 4× acceleration. The constant factor implies that the PWHT leads to a surface state that fosters the intrinsic corrosivity of the acid.

PWHT Management Guideline for Oxalic Acid

The table below provides recommendations for post-weld heat treatment (PWHT) to prevent crevice corrosion in 5% oxalic acid at 80°C as a function of the needed PWHT temperature and the available atmospheric control.

Required PWHT Temperature Service Environment Recommended PWHT Atmosphere Post-PWHT Treatment Expected Crevice Corrosion Rate (mm/year)
Any 5% oxalic acid, 80°C Argon (vacuum also acceptable) None 0.06–0.12
Any 5 % oxalic acid 80 °C Air Pickling (HNO 3 /HF, 10 min) 0.08–0.15 540–600°C Oxalic acid Air (unavoidable) Pickling + electropolishing 0.10-0.18 >600°C Oxalic acid Air Not salvageable-replace tube >0.35
No PWHT required Oxalic acid N/A Pickled surface 0.08–0.15
PWHT Engineering – Beyond Atmosphere Control

The amount of damage to the PWHT depends on the titanium grade. Grade 7 creates a comparable oxide scale but enrichment of the scale-metal contact with palladium affords some protection against under-scale corrosion, reducing the acceleration factor to 2-3 times, as opposed to 4 times. There is moderate progress in Grade 12. Wall thickness offers a corrosion tolerance. A 2.0 mm wall with a crevice corrosion rate of 0.5 mm per year will survive 4 years. A pickling operation of 20% nitric acid plus 3% hydrofluoric acid for 5–10 min can remove the alpha-case layer and restore the corrosion resistance to near the as-welded condition. If post weld heat treatment in air is unavoidable, then the tube should be pickled after heat treatment.

Making an Informed Specification

For a titanium sheath heater for service in 5% oxalic acid at 80°C requiring post-weld heat treatment, specify that PWHT be done in an inert atmosphere (argon with oxygen less than 50 ppm) or in vacuum. If air-based PWHT is inevitable, specify a post-PWHT pickling treatment of a minimum of 20% nitric acid + 3% hydrofluoric acid at 50°C for 10 minutes with thorough rinsing using deionized water. Any tube that shows blue, purple or gray oxide after pickling should be thrown away. Existing heaters that have been air-PWHT without further pickling should be chemically cleaned with 10% citric acid at 60° C for 2 hours to remove loose scale and a coupon tested in oxalic acid to confirm the corrosion rate. The fourfold increase in crevice corrosion rate that occurs in 5% oxalic acid at 80°C is prevented by regulating the PWHT environment and providing proper post-treatment.