During the last years we have been discussing in detail many different forms and cases of corrosion in ammonia and urea. To really understand the background and fundamentals of these corrosion problems it is necessary to introduce to you the modern electrochemical theory of aqueous corrosion. Although the presentation is necessarily simplified and brief we think this theory will contribute to a better understanding of corrosion phenomena. In the first paper about this subject we did discuss the thermodynamic principles (August 15, 2010). In the second paper we did discuss the electrode kinetics (April 15, 2011), the third paper was the discussion of the application of electrochemical corrosion theory on corrosion reactions in general and in carbamate solutions specific. This paper discusses the phenomena of Passivity in detail.