Internal and external corrosion of downhole tubing and equipment, subsea or surface pipelines, pressure vessels, and storage tanks is a major problem in the oil and gas industry.1-3,14 Besides basic wastage of metal, either generally or locally, the consequences of electrochemical corrosion can also be embrittlement and cracking, all of which can lead to equipment failure. Corrosion of iron in steel requires the presence of water and aqueous species that can be reduced while the iron is oxidized. Oxygen, acid gases such as CO2, H2S, and natural organic acids in the produced fluids all contribute to corrosion. Corrosion is an electrochemical redox (reduction and oxidation) process whereby localized anodic and cathodic reactions are set up on the surface of the metal. The basic chemical corrosion processes are illustrated in Figure 8.1. Entrainment of hydrogen atoms into the metal is one cause of its embrittlement. Embrittlement and cracking of metals are unpredictable and give rise to conditions under which catastrophic failure may occur.