Under What Specific Combination of Applied Cathodic Protection Potential and Sulfide Concentration Does 316L Heater Sheath in Seawater at 25°C Transition from Hydrogen Embrittlement to Ductile Behavior After 10,000 Hours

Limit of the Hydrogen Damage in Marine Cathodic Protection

The risk of hydrogen embrittlement (HE) in 316L stainless steel sheathed electric heating tubes in seawater with cathodic protection (CP) such as on offshore platforms, marine vessels or coastal structures is a function of the potential (vs. Ag/AgCl) applied to the tubes. At larger negative potentials (more cathodic) hydrogen evolution (2H2O + 2e- -> H2 + 2OH-) occurs and atomic hydrogen can infiltrate into the 316L sheath producing HE cracking. The presence of sulphides (H2S or HS- ) increases the hydrogen entrance, since sulphide is a toxin for the hydrogen recombination reaction. The ductile-to-HE-dominated transition is established above a critical combination of applied voltage and sulphide content. 316L is still ductile ( no HE ) at potentials exceeding -1000 mV vs . Ag / AgCl in saltwater at 25 C , no sulphides . HE can be seen at -1,050 mV during 5,000-10,000 hours. HE occurs at -1,100 mV within 1,000 hours. In the presence of 10-50 ppm sulphide, the safe potential moves to -900 mV. 316L is prone to HE at >100 ppm sulphide even at -800mV. This article assesses the threshold of CP potential (sulphide concentration) for HE avoidance after 10,000 hours of exposure.

Mechanism of Hydrogen Embrittlement under Cathodic Protection

Cathodic protection of 316L in seawater is usually performed by sacrificial anodes (zinc or aluminium) or impressed current (ICCP). Hydrogen evolution begins at potentials more negative than -800 mV vs. Ag/AgCl. The hydrogen entry rate into 316L increases with the potential decrease. The sulphide species (H2S, HS−, and S2−) are adsorbed on the metal surface and poison the hydrogen recombination process (2H → H2). This boosts the atomic hydrogen steady-state concentration in the metal by factors of 10 to 1,000. The threshold hydrogen concentration for HE in 316L is about 5-10 ppm (weight). Above this the material becomes brittle and fails via intergranular or transgranular cracking under residual or applied tensile stresses.

Quantifying CP Potential-Sulfide Thresholds for HE Mitigation

The following HE initiation thresholds have been obtained from controlled cathodic polarisation of 316L U-bend specimens (continuous tensile strain) in synthetic seawater (pH 8.0, 25°C) at various sulphide concentrations (as Na2S) for 10,000 hours.

Applied CP Potential (mV vs. Ag/AgCl) Sulphide Concentration (ppm as S 2- )Hydrogen Entry Rate (relative) Time to HE Cracking (hours) Failure Mode Recommended for 10,000h Seawater Service > -800 0 No H evolution >20,000Yes None -800 to -900 0 Trace >20,000 None Yes 
-900 to -1,000 0 Low >15,000 None Yes 
-1,000 to -1,050 0 Moderate 8,000-15,000 Mixed (some HE) Acceptable 
-1,050 to -1,100 0 High 3,000-8,000 HE Not recommended (long-term) < -1,100 0 Very high <3,000 HE No > -800 10-50 Low (sulphide limited) >15,000 None Acceptable 
-800 to -850 10-50 Moderate 8,000-15,000 Mixed Marginal 
-850 to -900 10-50 High 4,000-8,000 HE Not recommended < -900 10-50Very high <4,000 HE No > -750 50-100 Sulphide increased) Moderate 8,000-15,000 Mixed Marginal -750 to -800 50-100 High 3,000-8,000 HE Not recommended < -800 50-100Very high <3,000 HE No 
Any >100 High to very high <5,000 (≤ -700 mV) HE No (316L not suited)
Susceptibility to HE as a Function of Temperature

Higher temperatures increase the rate of hydrogen diffusion and embrittlement.

Temperature (°C) Maximum CP Potential for 10,000h HE-Free Service (mV vs. Ag/AgCl, 0 sulphide) Maximum CP Potential for 10,000h HE-Free Service (10 ppm sulphide)
5 -1,050 -900 15 -1,030 -880 25 -1,000 -850 35 -970 -820 50 -920 -780 
Practical Recommendations for Cathodic Protection of 316L Stainless Steel in Seawater

For 316L encased heaters in seawater with cathodic protection, the following CP potential restrictions apply depending on the estimated sulphide concentration.

Seawater Type Average Sulphide (ppm) CP Potential Range (mV vs. Ag/AgCl) SuggestedMaximum Potential Life 10,000h (mV)Alternative Material 
Clean, aerated <1 -800 to -950 -1,000 316L acceptable Polluted (harbour) 1-10 -800 to -900 -950 316L with monitoring Polluted (high organic) 10-50 -800 to -850 -880 Duplex 2205
Anoxic/sulfidic 50-100 -750 to -800 -820 Duplex 2205 or alloy 825
Severe sulfidic >100 Not Recommended N/A Titanium or Inconel 625
Field Identification of CP Induced HE

A failed 316L heater in saltwater under cathodic protection often shows transgranular (through grains) HE cracks with little branching and often a flat brittle fracture surface. The cause is confirmed by the presence of sulphide (smell of H2S, black iron sulphide deposits) and a CP potential more negative than -900 to -1,000 mV. The answer is to increase the CP potential (less negative) to the safe range, or upgrade to a more HE resistant alloy (duplex 2205, or titanium).

Conclusion: Control of CP Potential and Sulphides to Prevent HE

For 316L stainless steel heater sheaths in saltwater at 25°C with cathodic protection, the change from ductile to hydrogen embrittlement occurs at applied potentials more negative than -1,000 to -1,050 mV (sulphide free) or -850 to -900 mV (10-50 ppm sulphide). Maximum safe CP potential during 10,000 hour service is -1000 mV (0 sulphide), -850 mV (10-50 ppm sulphide). 316L is not advised for use at >100 ppm sulphide regardless of CP potential. Engineers specifying cathodically shielded 316L sheaths for maritime service must manage CP potential to be within the safe window and monitor for sulphides. The model developed here links the CP potential, sulphide concentration and HE initiation time at 25°C in saltwater and provides a basis for buyers to define the CP set points that will avoid hydrogen embrittlement of 316L heater sheaths throughout 10,000 hour design lives.