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The growing interest in intermetallic and metal–intermetallic materials and coatings is based on the number of favorable properties they possess, primarily mechanical. However, the lack of data on their corrosion resistance has largely limited their scope of application. In this study, the corrosion destruction mechanisms of coatings formed on substrates made of AISI 321 steel and Aluchrom W (fechralloy) were investigated. The coatings were created by alloying in an aluminum melt followed by diffusion annealing to form the ultimate intermetallic structure. Corrosion resistance was studied under cyclic exposure to a humid marine atmosphere simulator and potentiostatic tests in an aqueous NaCl solution. Corrosion destruction parameters were determined, and mechanisms for each type of coating were revealed. The conducted studies allowed us to determine the electrochemical parameters of the corrosion destruction process and its mechanisms. It was shown that the corrosion rates during potentiostating for coatings on substrates Cr15Al5 and 12Cr18Ni10Ti differed by almost twofold. Two different mechanisms of corrosion are proposed. The first is associated with the formation of Al2O3 and MgO oxide films, which at the initial stage protect only local areas of the coating surface on Cr15Al5. The second is determined by the diffusion of titanium atoms during annealing to the coating surface on a 12Cr18Ni10Ti steel substrate with the formation of TiC carbide at the grain boundaries.