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?

Corrosion and hydrogen embrittlement attack titanium heaters in a 90°C saltwater cathodic protection system. The weld toe region is a location of significant stress concentration. As-welded joints have a stress concentration factor (SCF) of 3-5, polished weld toes lower SCF to <1.2. Increased tensile stresses due to the enhanced SCF promote hydrogen induced cracking and fatigue crack initiation. Polishing reduces SCF and increases service life by 5 times.

Stress Concentration Factor vs. Quantitative Fatigue Life

For Grade 2 titanium in 90°C seawater with cathodic protection (-0.85 V vs Ag/AgCl):

Weld Toe Condition SCF Cycles to Crack OnsetLife with reference to As-Welded
As-welded, sharp notch 4-5 10,000-30,000 1 X As-welded, average 3-4 20,000-50,000 1.5 X
Ground smooth 1.5-2 50,000-150,000 3x
Polished (mechanical) 80,000-200,000 1.2-1.5 4x
Electropolished ( SCF < 1.2 ) <1.2 150,000-300,000 5-7x Effect of Cathodic Protection Potential on SCF Impact

CP Potential (V vs. Ag/AgCl)Life (cycles) As-WeldedLife Rate Polished Life (cycles) -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 5000-10000 25000-50000 5× Guide to Weld Toe Finishing

Service Severity | Required Weld Toe Finish | SCF Target | Expected Fatigue Life Enhancement
Low cycle (<10,000 total)As welded <4 1x
Moderate cycle (10,000 to 50,000)Smooth ground <2 3×
High cycle (50,000-200,000)Polished 4x <1.5
Very high cycle (> 200,000 )Electropolished <1.2 5-7x Engineering Recommendation

Titanium heaters in cathodically shielded saltwater at 90°C have all weld toes ground smooth and electropolished to produce SCF <1.2. The engineer reduces the stress concentration, and thereby improves the fatigue life by a factor of 5 over as-welded joints.