What Maximum Allowable Sulfide Stress Corrosion Cracking Threshold Does 316 Stainless Steel Heater Sheath Exhibit in NACE TM0177 Solution A at 24°C and 80°C

Temperature Dependence of SSC Susceptibility of Austenitic Stainless Steel

The temperature-control of the risk of sulphide stress corrosion cracking (SSC) of electric heating tubes in sour service environments (oilfield water heaters, geothermal systems, or refinery process heaters) is non-monotonic, i.e. the risk is not simply a function of temperature. Different from chloride induced SCC that is more severe with the increasing of temperature, SSC of austenitic stainless steels such as 316 has maximal susceptibility at ambient temperature (20-30°C), and is greatly reduced at elevated temperatures (over 80°C). 316 in NACE TM0177 Solution A (5 wt% NaCl + 0.5 wt% acetic acid, saturated with H2S at 1 atm, pH ~ 2.7-3.0) has a threshold stress for SSC of about 60-70% of its yield strength at 24°C. This means that stresses higher than 120-140 MPa (for annealed 316 with yield strength ~ 205 MPa) will lead to cracking in 720 hours. At 80°C the threshold shifts upwards to reach the yield strength (close to 200 MPa) showing substantially less SSC susceptibility. In this paper, SSC thresholds for 316 sheaths are quantified as a function of temperature.

Origin of the Temperature-Dependent SSC Susceptibility

Sulphide stress corrosion cracking occurs when the atomic hydrogen that is formed from the corrosion reaction (Fe + H2S → FeS + 2H) diffuses into the steel and embrittles the microstructure. For austenitic stainless steels, like 316, the diffusion of hydrogen is slow at room temperature since the face-centered cubic (FCC) structure results in a low hydrogen diffusivity (around 10-12 to 10-11 cm2/s at 25°C against 10-6 cm2/s for ferritic steels). However, cold work, combined with high hardness and high dislocation density, can make 316 susceptible. At higher temperatures ( > 60-80°C) the diffusivity of hydrogen increases, but the tolerance to hydrogen also increases because dislocations can shift to relieve local stresses. Also the corrosion rate increases with temperature which leads to more hydrogen production, but the hydrogen recombination reaction is less poisoned by H 2 S at higher temperatures. Hence the net consequence is a maximum susceptibility for SSC between 20 and 40 °C and a quick decrease above 60 °C.

Quantified SSC Limits at 24 °C and 80 °C

The constant load SSC testing of annealed 316 stainless steel, with a hardness of < 200 HV, in NACE TM0177 Solution A, has established the following threshold stresses for cracking over the normal 720 hour test period.

Temperature (°C) Material Condition Yield Strength (0.2% offset, MPa) Threshold Stress for SSC (MPa, 720h)Threshold Stress as % of Yield SSC Susceptibility Rating 24 Annealed (150-180 HV) 205 120-140 60-70% Moderate 24 Cold-worked (20%, 250-300 HV) 400-500 80-120 (absolute) 20-30% High 24 Hard-drawn (>35% CW, >350 HV) 550-700 <50 (absolute) <10%Severe 80Annealed 190 (derated) 170-190 90-100% Low 80 Cold-worked (20%) 350-450 250-350 70-80% Moderate 80 Hard-drawn (>35% CW) 500-650 300-500 60-70% Moderate
The Critical Interaction of Hardness-SSC Resistance

A maximum hardness of 22 HRC (about 250 HV) is permitted for austenitic stainless steels in sour service according to NACE MR0175/ISO 15156 (the specification for materials in H₂S-containing environments). This hardness limit corresponds to a maximum of cold work of about 15-20%. The table below provides hardness limitations and their implications on SSC at room temperature.

Maximum Hardness (HRC) Maximum Hardness (HV) Equivalent Cold Work SSC Threshold Stress (MPa, 24°C, annealed base)316 Acceptable per NACE MR0175 <15 <180 0-5% (annealed) 140 Yes (with qualification)
15-20 180-230 5-12% 120 Yes (limited)
20-22 230-250 12-18% 100 Marginal
22-25 250-280 18-25% 80 Unacceptable 25-30 280-330 25-35% 50 Unacceptable >30 >330 >35% <50 (extremely low) Unacceptable
Practical Recommendations for Sour Service Heater Applications

The following recommendations are based on operating temperature for 316 sheathed heaters for use in H2S service (partial pressure of H2S above 0.05 psia or concentration above 10 ppm in liquid water).

Operating temperature (°C) H2S partial pressure (psia) Maximum hardness (HRC/HV) Recommended post-forming heat treatment Alternative alloy <30 <0.05 22/250 Stress relief 400-450°C 316 acceptable <30 0.05-0.5 20/230 Full solution anneal + stress relief 316L preferred <30 >0.5 18/210 Full solution anneal Duplex 2205 recommended 30-60 Any 22/250No need (reduced susceptibility) 316 acceptable 60-80 Any 25/280316 accept None necessary (low susceptibility) >80 Any Any (no SSC risk)316 totally acceptable None required
Field Diagnosis of SSC Malfunction

Cracking from SSC in 316 heater sheaths is usually transgranular (through the grains) rather than intergranular, often with branching like chloride SCC but in the lack of substantial chlorides. There is little plastic deformation of the fracture surface that may be flat and faceted. The most likely places for cracking are cold-worked areas (e.g. the outer radius of bends) and temperatures below 60°C. If a heater fails from a sour water system including H 2 S, and the breakdown is at a bend or cold-worked area and the service temperature is ambient to 50°C, SSC should be suspected if the hardness surpasses around 250 HV, irrespective of hardness.

Conclusion: SSC Risk Control via Temperature and Hardness Control

316 stainless steel encased heaters in sour service (H2S-containing environments) have peak vulnerability to SSC at ambient to 40°C, and drops dramatically above 60°C. The threshold stress for SSC in NACE TM0177 Solution A at 24°C is about 60-70% of yield strength (120-140 MPa for annealed 316). At 80C the threshold climbs to near yield strength (170-190 MPa) suggesting substantially lesser danger. For sour service with 316 sheaths, NACE MR0175 requires that the hardness does not exceed 22 HRC (250 HV) when operating below 60°C. Hardness is less important for higher temperature operations (>60°C) therefore 316 can be safely used even at higher levels of cold work. This framework described here allows purchasers to select 316 sheaths resistant to sulphide stress corrosion cracking in sour conditions by relating temperature, hardness and SSC threshold stress.