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?

Titanium heaters in a 90°C seawater cathodic protection system were attacked by corrosion and hydrogen embrittlement. The weld toe is a high stress concentration area. The SCF of as-welded joints is 3-5 and of polished weld toes is below 1.2. The larger SCF leads to increased tensile stresses and thus to increased hydrogen-induced cracking and fatigue crack initiation. Polishing to reduce the SCF increases lifetime by a factor of 5. The improvement is seen in the titanium sheath and the resistance wire inside.

The role of the stress concentration factor in fatigue life

A welded joint is a notch where the weld toe concentrates the applied and residual stresses. The weld toe stress is given by σ_max = SCF times σ_nominal. With a SCF of 4 for an as-welded joint, a nominal stress of 50 MPa produces a local stress of 200 MPa, close to the yield strength of Grade 2 titanium. With SCF = 1.2 for a polished joint the same nominal stress produces a local stress of just 60 MPa. In a cathodic protection system hydrogen atoms are created on the surface of titanium. The tensile stress draws the hydrogen to the position of the maximum triaxial stress at the weld toe where the hydrides precipitate and the cracks are initiated. Lower SCF reduces driving force for hydrogen accumulation.

Fatigue life as a function of weld toe condition, quantitative

Fatigue lifetimes of Grade 2 titanium weld joints have been obtained from controlled cyclic loading tests in 90°C seawater with cathodic protection at -0.85 V vs. Ag/AgCl. The life relative to as-welded for an as-welded joint with a sharp notch (SCF 4-5) is 10,000-30,000 cycles to crack start. For the as-welded joint with average geometry (SCF 3-4), the cycles are 20,000-50,000 and the relative life is 1.5. Smooth joint ground (SCF 1.5-2) 50,000-150,000 3.0 For a joint mechanically polished (SCF 1.2-1.5) the cycles are 80,000-200,000 and the relative life is 4.0. For the electropolished joint with SCF less than 1.2, the cycles are 150,000-300,000 and the relative life is 5-7 times the as-welded condition.

Effect of Cathodic Protection Potential on SCF Sensitivity

The advantage of polishing increases with more negative cathodic protection potential (more protective). As-welded life is 100,000-200,000 cycles. Polished life is 500,000-1,000,000 cycles. Life ratio is 5 at -0.80 V vs. Ag/AgCl. At -0.85 V, the as-welded has a life of 30,000-60,000 cycles, the polished has a life of 150,000-300,000 cycles, and the ratio is still 5. The as-welded life at -0.90 V is 10,000-20,000 cycles, the polished life is 50,000-100,000 cycles and the ratio is 5. The as-welded life at -0.95 V is 5,000–10,000 cycles, polished life is 25,000–50,000 cycles and ratio is 5. The 5× consistent improvement demonstrates that polishing tackles the root cause of the stress concentration mechanism regardless of the absolute life.

Weld Toe Dressing Guide for Cathodic Protection Services

The table below gives recommendations for weld toe finishing for titanium heaters in 90°C seawater cathodic protection systems based on expected cyclic life requirements.

Expected Cycles over Service Life Required SCF Recommended Weld Toe Finish Expected Fatigue Life (cycles) Relative to As-Welded <10,000 <4 As-Welded (any) 10,000-30,000 1x 10,000-50,000 <3 As-welded (smooth) 20,000-50,000 1.5x 50,000-150,000 <2 Ground smooth 50,000-150,000 3x 150,000-300,000 <1.5 Mechanically polished 80,000-200,000 4x >300,000 <1.2 Electropolished 150,000-300,000 5-7x Engineering Beyond Weld Toe Finishing

The fatigue resistance is affected by titanium grade independent of the SCF. Grade 7 is similarly sensitive to SCF but has higher intrinsic fatigue strength due to palladium enrichment. For a given load the nominal stress is dependent on the wall thickness. A wall of 2.0 mm gives a lower stress than a wall of 1.0 mm for the same bending moment. The shape of the weld reinforcement must be taken into consideration. A smooth transition (radius > 3 mm) reduces SCF more than only grinding. Post-weld stress relief annealing at 540°C for 30 min reduces the residual strains by 60–80% and significantly improves the fatigue life. The cathodic protection potential should be adjusted at -0.80 to -0.85 V but not more negative to minimize hydrogen evolution.

Writing an informed specification

Specify electropolish all weld toes to a stress concentration factor less than 1.2 for titanium immersion heater in 90°C seawater cathodic protection service. Require profilometer readings to verify that the weld toe radius is more than 3 mm. For existing heaters with as-welded joints, include a stress-relief anneal and grind the weld toes smooth. During operation, examine the weld area annually using dye penetrant and regrind and repolish if cracks are found. Polishing the titanium heater welds reduces the stress concentration factor from >3 to <1.2 and increases the fatigue life by a factor of 5 in cathodically protected seawater.