Under What Specific Tensile Residual Stress Value Does 316 Stainless Steel Heater Sheath Transition from Passive to Crack Initiation in 500 ppm Chloride Water at 25°C Ambient Service

Threshold Stress for Chloride SCC at Room Temperature

In the case of 316 stainless steel sheathed electric heating tubes in chloride bearing water at ambient temperature (25°C) in applications such as unheated storage tanks, cold water circulation systems or seasonal equipment, the risk of chloride stress corrosion cracking (SCC) is generally considered low, as SCC of austenitic stainless steels generally requires above 60°C. However, subsequent field failures and laboratory experiments have shown that 316 can crack at 25°C in 500 ppm chloride water in the presence of adequate tensile residual stress, but the threshold stress is much higher than at elevated temperatures. The key parameter is the residual tensile stress from manufacturing (drawing, bending, straightening) plus any added stress. At 25{deg}C, cracking is not observed below a threshold stress on the order of 80-90% of the yield strength of the material (165-185 MPa for the annealed 316 with 205 MPa of yield stress). If the stress exceeds this level, cracks develop after long induction periods (6 months to 2 years) and grow slowly, with failure occurring after 2-5 years. This article provides the room temperature SCC threshold of 316 sheaths in 500 ppm chloride water.

The Mechanism of Low-Temperature SCC

Three conditions are required for SCC of 316 stainless steel in chloride: a susceptible microstructure (normally containing some cold work), a sustained tensile stress and chloride ions. At high temperatures (80-150°C) SCC is due to rapid chloride transport, sluggish repassivation rates and aggressive chemistry at the crack tip. At 25°C the activation energy for SCC is not zero, but the kinetics are significantly slower. The main difference is that the stress threshold for cracking increases considerably as the temperature decreases. This is due to the rise in critical fracture tip strain rate for active dissolving, which requires higher applied stresses. Annealed 316 at 25C in 500 ppm Cl- has a threshold stress of about 165-185 MPa (80-90% yield). Cold-worked 316 has lower threshold stress as the material has higher yield strength but also increased dislocation density and possibly some martensite which acts as crack initiation sites.

25°C, 500 ppm chloride determined thresholds for SCC

The following threshold relationships were established from constant load testing of 316 stainless steel (U-bend and C-ring specimens) in 500 ppm chloride water (pH 7.0, aerated) at 25°C for up to 24 months (17,520 hours).

Material Condition Yield Strength (0.2% Offset, MPa)25°C SCC Threshold Stress in 500 ppm Cl- (MPa)Threshold as % of Yield Time to Crack Initiation at Threshold +10% (hours)SCC Risk Rating – 25°C
Full annealed (<5% cold work) 205 165-185 80-90% >10,000Extremely low
Light cold work (10-15%) 280 200-230 70-80% 5,000-10,000 Low
Moderate cold work (15-20%) 350 230-260 65-75% 2,000-5,000 Moderate 
Heavy cold labour (20-25%) 420 250-280 60-70% 1,000-2,000 High 
Extreme cold work (>25%) >480 <280 (absolute) <60% 500-1,000 Extreme
With residual stress >90% yield 205 >185 >90% 500-2,000 (depends on overstress) High
Room temperature threshold as a function of chloride concentration

The threshold stress is reduced with increasing chloride content. Threshold stress for 25°C annealed 316 as a function of chloride content is given in the following table.

Chloride Concentration (ppm) Threshold Stress for SCC at 25°C (MPa, annealed 316) Time to Crack Initiation at Threshold +10% (hours)Recommended Design Limit <100 >205 (no cracking at yield) >50,000 (no danger)Unlimited
100-200 190-205 20,000-50,000 Very low risk.
200-500 175-190 10,000-20,000 Acceptable with residual stress management
500-1000 160-175 5000-10000 Residual stress <70% yield needed 1000-2000 140-160 2000-5000Stress Relief Recommendations 2,000-5,000 110-140 1,000-2,000 316 Not recommended for continuous stress >5,000 <110 <1,000 316 Not recommended; upgrade alloy
Practical limits on residual stresses for cold water service

Maximum residual stress limits are given for 316 sheathed heaters in chloride-bearing water (200 - 1,000 ppm) at ambient temperature (25°C).

Chloride concentration (ppm) Maximum residual stress (MPa) for 10-year life Equivalent cold work limitPost-Forming Treatment Recommended <200 <205 (full yield) Any None Required
200-500 <150 <25% cold work None for straight portions
200-500 <120 (at bends) <15% extra bend stressStress alleviation for tight bends ( R/D<3)
500-1,000 <120 <15% cold work Stress relief 400oC 1 hr
500-1,000 <100 (at bends) <10% extra strainFull solution anneal after bending >1,000 <80 <5% cold work (just annealed) Full solution anneal necessary
Field Detection of SCC at ambient temperature

For a 316 heater failing in a 25°C chloride service (200-1,000 ppm Cl-), SCC cracks will normally develop after 1-3 years of continuous or intermittent use. The fractures are transgranular (through grains) and branched, like high temperature SCC but often shorter and less extended. The fracture surface may exhibit indications of gradual crack propagation (beach markings) and little corrosion product. Often the origin is in a site of significant residual stress: the outside radius of a curve, a cold drawn section with substantial reduction or a weld heat affected zone. When a failed section is measured for residual stress by X-ray diffraction the values are generally over 150-180 MPa. For such failures, the answer is to require post-bend stress relief (400°C for 1 hour) for future purchases, decreasing residual stress to below 100 MPa.

Special Case: Intermittent Versus Continuous Exposure

Intermittent exposure (drying cycles) at room temperature, chloride content increases during drying leading to accelerated SCC formation. The bulk water may only be seeing 500 ppm chloride. However, a heater that is permitted to dry periodically can have surface chloride concentrations of 5,000-10,000 ppm at the waterline thus lowering the threshold stress. For intermittent service the permissible residual stress shall be one half of that for continuous immersion.

Conclusion: Residual Stress Control for Ambient Chloride Service

SCC in 316 stainless steel encased heaters in 500 ppm chloride water at 25°C was found to occur when the sustained tensile residual stress was in excess of about 165-185 MPa (80-90% of yield for annealed material). The threshold for this is high, but local residual stresses from cold work (bending, drawing etc.) can be greater than 200-300 MPa, sufficient to cause cracking at room temperature after 1-3 years. For operation in chlorides at room temperature, engineers selecting 316 sheaths must consider post-forming stress relief (400-450°C, 1 hour) to reduce residual stress when chloride exceeds 500 ppm or when bends are tighter than R/D = 3. For chloride levels over 1,000 ppm at 25°C, full solution annealing or upgrading to duplex 2205 is suggested. The approach proposed here links residual stress to measurable SCC thresholds at room temperature, enabling purchasers to order 316 sheaths that resist chloride cracking even in unheated service.