Hydroxide melt induced corrosion of Ni at elevated temperatures under steam electrolysis conditions

High temperature corrosion and oxide scale formation of nickel in molten NaOH at various basicity levels

“…Oxidation of Ni to NiO according to equation (8) takes place at potentials above the corrosion potential and until the large current increase at 1.15 V vs Na/Na+. The current increase at 1.15 V (A) is expected to be due to formation of NiO22- according to equation (12) [ 6 , 9 ] and the smaller current increase at 1.3 V (B) caused by the formation of NiO2 according to equation (10) . The current increase at 1.8 V vs Na/Na+ is ascribed to the formation of oxygen gas as per equation (14) [ 19 ].…”

“…As per Le Chatelier's principle, adding H 2 O to the melt shifts the dissociation reaction (equation (1) ) to the left and hence prevents formation of O 2− and will lower the basicity. While NiO is continuously forming (equation (8) ), NiO may simultaneously react with O 2− to form NiO 2 2− by equation (12) [ 9 ] at higher electrochemical potential.…”

“… In molten NaOH, the basicity is defined by the dissociation of NaOH into H 2 O, which is defined as an acid and Na 2 O, which is defined as a base [ 8 ]. And as such the basicity of molten NaOH can be controlled by the concentration of H 2 O and Na 2 O [ 5 , 6 , [9] , [10] , [11] ]. A NaOH melt can be considered acidic when the concentration of H 2 O is greater than the concentration of Na 2 O and basic when the concentration of Na 2 O is higher than that of H 2 O.…”