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?

Post-weld pickling of titanium welds in a nitric-hydrofluoric acid mixture (HNO_3/HF) eliminates the oxygen enriched alpha-case layer. Short pickle (5 min) eliminates 10-20 µm, leaving some alpha-case. A prolonged pickle (20 min.) eliminates 40–80 µm and destroys the alpha-case. Removal of the alpha-case increases the resistance to crevice corrosion in 10% citric acid at 80°C. Fully pickled tubes (20 min) exhibit 5× longer crevice initiation time than mildly pickled tubes (5 min).

Quantitative Alpha-Case Removal and Crevice Resistance

For Grade 2 titanium welds after pickling in 20% HNO3 + 3% HF at 50°C:

Pickle Time (min) Depth of Alpha-case Removal (µm) Residual Alpha-case (µm) Crevice Initiation Time in 10% Citric Acid (hr)
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
Benefit of citric acid concentration effect

Citric Acid (%) Temperature (°C) Improvement in the Crevice Life (20 min versus 5 min pickle)
5 80 3× 10 80 4-5× 15 80 5-6× 10 100 6-8× Pickling Time Selector Guide

Required Crevice Resistance Recommended Pickle Time (minutes) Alpha-Case Removal Metal Loss (µm)
Low 0-5 Piece 0-20
Moderate 10-15Most 20–60
High 20-25 Total 40-80
Maximum 30 + electropolish Full + smooth 60–120
Recommendation of Engineering.

10% citric acid @ 80 °C for 20 minutes Post-weld pickling for titanium heaters to remove alpha-case fully. Check removal by microhardness testing (surface hardness 250 HV) The pickling period is controlled to eliminate the embrittled alpha-case layer and to improve the crevice corrosion resistance.