Corrosion behavior of carbon steel in dilute ammonia solution

This study explores the corrosion of carbon steel in ammonia solutions containing dissolved oxygen and chloride, simulating boilers under wet layup conditions. An adsorption mechanism is proposed in which ammonia is postulated to be an inhibiting species at low NH3 (i.e. 9.061 x 10(-5) M) but is an activating species at high concentration (0.001M-0.1 M) via the formation of a Fe(NH3)n(2+), with an apparent activation potential at about-0.4 VSCE. At higher potentials, the steel passivates through the formation of a defective oxide barrier layer that exhibits chloride-induced breakdown with the breakdown potential decreasing with increasing Cl-. However, oxide breakdown is inhibited by ammonia, which is postulated to be due to NH3 competing with Cl- for oxygen vacancies in the surface of the barrier oxide layer. Chloride accelerates steel corrosion, with the anodic branch of Fe electrodissolution shifting in the positive potential direction in solutions with increasing oxygen content. (C) 2020 Elsevier Ltd. All rights reserved.