Permissible Cathodic Polarization Levels for Underground Stainless Steel Structures in Cathodic Protection Systems

Excessive cathodic polarization of underground stainless steel structures results in hydrogen evolution, increasing the risk of hydrogen embrittlement and the disbonding of protective coatings from the structure's surface. This study was conducted to determine the critical potential and critical cathodic protection current density at which hydrogen evolution occurs on the surfaces of stainless steel grades 1.4301, 1.4401, 1.4125, and 1.4512, and, for comparison, on carbon steel S235. The tests were carried out in an aqueous solution of synthetic (artificial) soil and in a soil filtrate prepared from a soil sample taken in the vicinity of an existing underground gas pipeline connection with stainless steel fittings. The tests showed that the higher the chromium content in the stainless steel was, the lower (more negative) the hydrogen evolution potential was. In an artificial soil environment, the values of this potential ranged from -1105 mV to -1175 mV vs. copper sulphate electrode (CSE) for steels 1.4301, 1.4401, and 1.4125, which contain more than 16% of chromium. For steel 1.4512, containing 12% of chromium, the hydrogen evolution potential was -1050 mV. For comparison, for S235 carbon steel, the hydrogen evolution potential was -1135 mV. The critical cathodic protection current density ranged from 0.30 A/m² to 0.38 A/m² for all tested stainless steels, whilst for S235 steel, this value was higher, equal to 0.65 A/m². The results obtained indicate that applying the commonly accepted potential criterion for cathodic protection for carbon steels (i.e., polarization to a potential in the range from -0.85 V to -1.1 V vs. CSE) poses no risk of causing excessive cathodic polarization of stainless steel. This is important for the design and operation of cathodic protection systems for complex structures containing galvanically connected carbon steel and stainless steel components.