The Critical Depth for Pits on Carbon Steel in the Concentrated Aqueous Solution of Lithium Bromide Containing Nitrate at Elevated Temperatures

Abstract

Pitting corrosion behaviors for carbon steels in an aqueous solution of 60%LiBr+0.2% LiOH+0.2%LiNO₃ at 150°C were investigated. Similar to the carbon steel in an aqueous solution of 60%LiBr+0.2% LiOH+0.2%Li₂CrO₄ which was described in the previous paper, the possibility for the initiation of a growing macro-pit was detected by applying the critical depth concept. The pitting potential, V_{c, pit} the film breakdown potential, E_z, and the pit repassivation potential, E_{R, pit} were measured to be -480, -490 and-540mV, respectively. And the critical depth, r*, was determined to be 10μm. When a carbon steel was merely immersed in the solution, it reached the steady state in a few hours; micro-pits which were shallower than r* generated intermittently and the initiation of growing macro-pits was inhibited. However, when the carbon steel which was held at-800mV for longer than 10h in the solution prior to the open-circuit immersion as a pretreatment, pits that grew beyond r* during the initial period of the immersion were found to continue growing with a rate of 7mm/y. The pretreatment strengthened the resistance to film breakdown and activated the cathodic reaction on the steel surface. These effects of the pretreatment were found to be more significant for steels in the LiNO₃ added solution than those in the Li₂CrO₄ added solution.