How Does the Prior Cold Work Percentage of 316 Stainless Steel Heater Sheath Tubing Control the Hydrogen Trapping Efficiency and Delayed Cracking Risk in Cathodically Protected Systems

The deformation induced trap site density as a parameter of hydrogen embrittlement

For cathodically protected systems like marine immersed heaters, offshore platform equipment or buried pipeline heaters with impressed current cathodic protection, the prior cold work percentage of the sheath tubing directly controls the risk of hydrogen embrittlement and delayed cracking for 316 stainless steel sheathed electric heating tubes. Dislocations, voids and strain-induced martensite are the hydrogen trap sites produced by cold work. At low cold work percentages ( 0-10% ) trap site density is low , hydrogen diffuses fast through the lattice and the risk of cracking is limited . Trap site density is moderate at intermediate cold work (15-25%) and hydrogen is dispersed within the microstructure with increased embrittlement susceptibility. At high cold work (>30%), large trap sites can reduce embrittlement (wide distribution of hydrogen) or promote embrittlement (concentrated stress fields). However, for 316 in cathodic protection service, the minimum embrittlement risk is at the extremes of full annealing (<5% cold work) or heavy cold work (>30% with subsequent stress release). This article quantifies the link between the amount of cold work, the efficiency of hydrogen trapping and the risk of delayed cracking.

Mechanisms of hydrogen trapping in cold worked austenitic stainless steels

Hydrogen can infiltrate into 316 stainless steel from the cathode surface created by water reduction (2H2O + 2e− → H2 + 2OH−). Diffusivity of hydrogen in the austenite lattice (FCC) is high (~10-10cm2/sec at 25°C) and solubility is low. Cold work creates trap sites which bind hydrogen more strongly: dislocations (energy of binding ~ 30-40 kJ/mol), vacancies (~ 50-60 kJ/mol), and strain-induced martensite (BCC, with significantly higher hydrogen solubility, ~ 10 4 higher than austenite). At low trap densities, hydrogen flows swiftly through the lattice and leaves the metal free of harm. At intermediate trap densities, hydrogen accumulates at trap sites; especially at triple points and inclusion–matrix interfaces, it recombines to create molecular hydrogen causing internal pressures and initiating cracks. At very high trap densities, the trap sites are so numerous that hydrogen is dispersed very finely and the local pressure at any single trap site is not sufficient to cause cracking – if the material is not under significant tensile stress.

Quantified Correlation Between Cold Work and Risk of Delayed Cracking

The time to cracking (delayed failure) as a function of earlier cold work has been determined by constant load testing of 316 tubing (U-bend specimens) in simulated seawater at 25 C with cathodic protection (applied voltage -1,050 mV vs. Ag/AgCl).

Previous Cold Work (% decrease) Hardness (HV) Strain-Induced Martensite (%)Hydrogen Trapping Density (Relative) Time to Delayed Cracking in CP (hours, 75% Yield Stress)Delayed Cracking Risk Rating 0 (annealed) 150-180 <1 1× (baseline) >10,000 (no cracking) Very low 5-10 180-210 1-3 2-3x 5,000-10,000Low 10-15 210-240 3-6 5-8× 2,000-5,000 Moderate 15-20 240-270 6-10 10-15× 500-2,000 High 20-25 270-300 10-15 15-20× 100-500 Severe 25-30 300-330 15-20 20-25× 50-200Super severe
30-35 330-360 18-22 25-30× 100-500 (plateau) High (but better than 20-25%) >40 >360 >25 >30× 500-2,000 (saturated) Medium
Effect of Stress Relief Annealing on Hydrogen Trapping

Stress relief annealing at 400-450°C for 1-2 h decreases the dislocation density without recrystallising the grain structure or removing the strain-induced martensite. This treatment reduces the hydrogen trapping efficiency by around 50% which greatly enhances resistance to delayed cracking.

Cold Work (%) Post-Forming Heat Treatment Relative Trap Density (as-drawn = 1.0)Time to Delayed Cracking (hrs, 75% yield)Recommended for CP Service 15 None 12× 500-1,000 No 15 400°C, 1 hour 6× 2,000-4,000 Yes
20 None 18x 100-500 No
20 450°C, 2 hours 8× 1,500-3,000 Marginal 25 None 22× 50-200 No 25 500°C, 2 hours 10× 1,000-2,000 No (still high risk)
25 Full solution anneal (1,040°C) 1× >10,000 Yes (returns to annealed)
Cathodal Protected Heaters, Practical Tips

For cathodically protected systems, the following criteria for 316 sheathed heaters minimise the susceptibility to hydrogen embrittlement and delayed cracking:

Applied Potential (mV versus Ag/AgCl) Cathodic Protection TypeMax. Permissible Cold Work (%)Recommended Treatment After FormingOther Material
Sacrificial anodes (zinc/aluminum) -800 to -1,000 (low danger)25 No or 400°C stress relief 316 acceptable
Impressed present (low overprotection) --1,050 to 1,000 15 450316L preferred, stress relieved at °C
Impressed current (standard) -1,050 to -1,100 10 Full solution anneal Duplex 2205
Impressed current (high overprotection) --1,200 to 1,100 <5 Full solution anneal required Titanium or Alloy 625
Any, intermittent operationField identification of hydrogen induced delayed cracking 20 450°C stress relief 316 acceptable with monitoring

In a cathodic protected system with no significant corrosion (no pitting, no scale) if a 316 heater fails then hydrogen embrittlement should be suspected. The cracks are generally transgranular, non-branching, surface initiated. The fracture surface can display quasi-cleavage features with little dimples. Hydrogen embrittlement is the likely cause if the heater was made from cold drawn tubing (hardness >250 HV) and used in a system with impressed current CP at potentials below -1,050 mV. Replacement heaters should be specified with full solution annealing (or at least stress alleviation) and maximum hardness of 200 HV.

CONCLUSION: COLD WORK LIMITATIONS FOR CATHODIC PROTECTION SERVICE

For 316 stainless steel encased heaters in cathodically protected systems, the percentage of previous cold work determines the efficacy of hydrogen trapping and the risk of delayed cracking. The largest risk is at 20-30% cold work (hardness 270-330 HV) where trap site density is high enough to hold hydrogen, but not high enough to allow its release in a finely distributed way. The lowest danger is in fully annealed material (<5% cold work) and in substantially cold worked material with post stress release (>30% cold work + 450°C treatment). For the 316 sheaths in the marine or underground CP systems, the engineers have to keep the cold work to less than 15% and specify a post-forming stress relief anneal of 400-450°C. For impressed current systems with potentials less than -1,050 mV, full solution annealing or upgrading to duplex 2205 is advised. The framework given here establishes a relationship between the percentage of cold work and hydrogen trap density and delayed cracking time, enabling customers to specify 316 sheaths that are resistant to hydrogen embrittlement in cathodically protected situations.