How Does the 316 Stainless Steel Sheath Wall Thickness Affect the Risk of Hydrogen Embrittlement During Electrolytic or Cathodic Protection Service in Marine Applications?

An often-overlooked failure mechanism for marine and offshore engineers using electric immersion heaters in environments where cathodic protection systems are operating such as ship hulls, offshore platforms and seawater intake structures is hydrogen embrittlement of the 316 stainless steel sheath. Cathodic protection devices (either sacrificial anodes or impressed current) create atomic hydrogen at the surface of the metal as a by-product of the protection process. This hydrogen may migrate into the 316 lattice, where it segregates at grain boundaries and inclusions, decreasing ductility and inducing breaking in tensile stress. The thickness of the wall of the sheath determines a gradient of hydrogen concentration and the time of hydrogen reaching an inner surface. This article presents quantitative links between 316 sheath wall thickness and the risk of hydrogen embrittlement and gives guidance for the selection of heaters for use in cathodically protected maritime environments.

Hydrogen Penetration and Embrittlement Mechanism in 316 Stainless Steel
When a cathodic protection system polarises a 316 stainless steel surface to a negative potential compared to the surroundings, water reduction creates atomic hydrogen: H₂O + e⁻ → H + OH⁻. This atomic hydrogen can either recombine into molecular hydrogen gas or adsorb on the metal surface and seep into the lattice. The diffusion rate of hydrogen in the austenitic stainless steel is relatively low in comparison with the ferritic steels (approx. 10-12 to 10-14 m2/s at the room temperature), but it is high enough at the high temperature of the heater operation. Inside the lattice, hydrogen atoms partition to places of high triaxial stress such as the crack tip of a developing fatigue crack or the tensile zone beneath a pit. Hydrogen at these places lowers the cohesive strength of the metal links and causes brittle fracture at pressures well below the material's usual yield strength. Hydrogen embrittlement of 316 stainless steel is most pronounced between 20°C and 100°C, with maximum susceptibility at about 50-80°C. Above 150°C hydrogen is mobile enough to escape from trapping sites and hence reduces the likelihood of embrittlement. For heater sheaths working at elevated surface temperatures the risk may be self limiting if the metal temperature surpasses some 120°C.

How Wall Thickness Affects Hydrogen Concentration and Risk of Embrittlement
The flux of hydrogen to the sheath is dependent on the cathodic protection potential and ambient variables. The driving force for hydrogen flux is the hydrogen concentration at the outer surface. For a given hydrogen surface concentration, the steady-state hydrogen concentration at the inner surface of the sheath decreases exponentially with increasing wall thickness. The hydrogen concentration profile follows Fick's second law of diffusion. For a thin-walled sheath < 1.0 mm, hydrogen will diffuse across the full wall thickness in weeks to months to the inner surface where the MgO insulation and resistance wire are located. Tensile stresses exist at the inner surface, as they do from the compacted MgO and thermal expansion, and hydrogen can produce embrittlement cracking. In a thick-walled sheath with a thickness more than 1.8 mm, the hydrogen concentration at the inner surface remains near zero for years due to the fact that the diffusion time scales with the square of thickness. If the thickness of the wall is increased from 1.0 mm to 2.0 mm, four times longer is the time required for hydrogen to reach the inner surface. Experimental data on hydrogen permeation through 316 membranes reveal that the inner surface concentration reaches 50% of the steady state value at 60°C with cathodic polarisation of -1.0 V vs. Ag/AgCl after about 90 days for a wall of 1.0 mm. A 1.6 mm wall takes 230 days. 2.0 mm wall requires 360 days. A wall of 2.5 mm takes more than 560 days. Thicker walls create a large delay in the arrival of hydrogen, therefore allowing the heater to operate for long periods of time before embrittlement conditions emerge.

Resistance to hydrogen embrittlement: threshold wall thickness
Based on diffusion calculations and field experience with cathodically protected marine structures a minimum sheath wall thickness of 1.6 mm of 316 is recommended for heaters operating in situations where cathodic protection is active. Under normal seawater temperatures, hydrogen can penetrate to the inner surface within one year at depths less than 1.6 mm, posing a danger of embrittlement cracking in the inner wall when swaging-induced tensile residual stresses are present. For 2.0 mm and larger, the diffusion delay exceeds two years, a huge safety buffer. But there is the thermal penalty of thick walls--higher surface temperature and restricted heat transfer. Where cathodic protection is available but the heater is intermittent or the surface temperature is high (> 120°C), thinner walls may be acceptable since high temperature increases hydrogen diffusivity but also decreases embrittlement susceptibility. The following table gives wall thickness guidelines for several types of cathodic protection, operating temperatures, and projected service lives.

Type of Cathodic Protection Operating Temperature of Sheath Minimum Recommended Wall Thickness 316Recommended Max Wall Thickness 316°Anticipated risk of hydrogen embrittlement
Zinc or aluminium sacrificial anodes<60°C 1.6 mm 2.5 mm Low, thick walls, high <1.2 mm
Zinc or aluminium sacrificial anodes60 – 100°C 1.4 mm 2.0 mm Moderate; check for cracking
Zinc or aluminium sacrificial anodesOver 100°C 1.2 mm 1.8 mmLow with temperature desorbing
Impressed current (high-potential)Under 60°C 2.0 mm 2.5 mm High danger; alloy upgrading advised
Impressed current (high potential) 60 – 100°CC 1.8 mm 2.5 mm Moderate thick walls
High potential (impressed current) >100°C 1.6 mm 2.0 mm Moderate, frequent inspection
Cathodic protection noneNo risk of hydrogen embrittlement 0.8 mm Any temperature 2.5 mm
For impressed current cathodic protection systems operating at potentials more negative than -1.0 V vs. Ag/AgCl, the rate of hydrogen creation is significant. 316 stainless steel is not normally suggested for such situations, regardless of wall thickness. Specify alloys with lower hydrogen diffusivity and greater resistance to embrittlement such as titanium or alloy 625. Titanium, in particular, has self-limiting defence in that it generates a hydride layer that actually prevents more hydrogen from entering.

Design Modifications to Minimise Hydrogen Embrittlement Risk
If a 316 sheath must be used in a cathodically protected maritime environment and thermal limitations do not permit the wall thickness to be raised beyond 1.6 mm, three design changes can lessen the risk of hydrogen embrittlement. The first is to coat the exterior surface of the sheath with a barrier layer. A dense, defect-free coating of epoxy, polyurethane or fluoropolymer can restrict hydrogen penetration by a factor of 10 to 100, so that the diffusion time is increased as if the wall were considerably thicker . The coating should be devoid of pinholes and resistant to marine biofouling. The second adjustment is to operate the heater at a surface temperature over 120°C, where practicable. At these temperatures hydrogen rapidly desorbs from trapping sites and the susceptibility to embrittlement is greatly decreased. A heater that cycles to high temperature every day may never build enough hydrogen to cause cracking. The third change is to electrically insulate the heater from the cathodically protected structure. The sheath is protected from cathodic protection potential by non-metallic mounting flanges or insulating gaskets. There may still be some hydrogen creation from galvanic effects but the driving force is much less. Engineers should consult corrosion specialists before specifying 316 sheaths for any application where impressed current cathodic protection is present. Pressurised systems failing from hydrogen embrittlement break suddenly with brittle cracking and little preceding deformation. These failures can be catastrophic. For important applications, the increased expense of a hydrogen-resistant alloy is money well spent.