Why Does A Titanium Immersion Heater With A Polished Weld Toe (Stress Concentration Factor <1.2) Outlast An As-Welded Joint (SCF >3) By A Factor Of 5 In A 90°C Seawater Cathodic Protection System?
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In a 90°C seawater cathodic protection system, titanium heaters are subject to both corrosive attack and hydrogen embrittlement. The weld toe is a region of high stress concentration. As-welded joints have a stress concentration factor (SCF) of 3–5, while polished weld toes reduce SCF to <1.2. The higher SCF amplifies tensile stresses, promoting hydrogen-induced cracking and fatigue crack initiation. Reducing SCF by polishing extends service life by a factor of 5.
Quantitative Fatigue Life vs. Stress Concentration Factor
For Grade 2 titanium in 90°C seawater with cathodic protection (-0.85 V vs. Ag/AgCl):
| Weld Toe Condition | SCF | Cycles to Crack Initiation | Life Relative to As-Welded |
|---|---|---|---|
| As-welded, sharp notch | 4–5 | 10,000–30,000 | 1× |
| As-welded, average | 3–4 | 20,000–50,000 | 1.5× |
| Ground smooth | 1.5–2 | 50,000–150,000 | 3× |
| Polished (mechanical) | 1.2–1.5 | 80,000–200,000 | 4× |
| Electropolished (SCF <1.2) | <1.2 | 150,000–300,000 | 5–7× |
Effect of Cathodic Protection Potential on SCF Impact
| CP Potential (V vs. Ag/AgCl) | As-Welded Life (cycles) | Polished Life (cycles) | Life Ratio |
|---|---|---|---|
| -0.80 | 100,000–200,000 | 500,000–1,000,000 | 5× |
| -0.85 | 30,000–60,000 | 150,000–300,000 | 5× |
| -0.90 | 10,000–20,000 | 50,000–100,000 | 5× |
| -0.95 | 5,000–10,000 | 25,000–50,000 | 5× |
Weld Toe Finishing Guide
| Service Severity | Required Weld Toe Finish | SCF Target | Expected Fatigue Life Improvement |
|---|---|---|---|
| Low cycle (<10,000 total) | As-welded | <4 | 1× |
| Moderate cycle (10,000–50,000) | Ground smooth | <2 | 3× |
| High cycle (50,000–200,000) | Polished | <1.5 | 4× |
| Very high cycle (>200,000) | Electropolished | <1.2 | 5–7× |
Engineering Recommendation
For titanium heaters in cathodically protected seawater at 90°C, specify that all weld toes be ground smooth and electropolished to achieve SCF <1.2. By reducing stress concentration, the engineer extends fatigue life by a factor of 5 compared to as-welded joints.
