The mechanism of corrosion of metals when embedded in masonry structures (mainly iron and steel) is different from that of steel in concrete. The latter involves reinforcing steel and this is usually protected from corrosion for many years in good-quality, high-alkalinity concrete. The steel forms a very stable film of iron oxide on its surface, thereby mitigating any further corrosion. The stable film is called a passive layer and the protection is termed ‘passivation’. For iron or steel embedded in masonry, a highalkalinity environment is not always present. This depends on whether the metal is covered by a thin or thick mortar layer or, in many cases, fixed directly to the stone or brick. In the latter case, a passive layer will not be formed and corrosion may start soon after it is constructed. If the mortar layer is thin, or is of poor quality, then carbonation of the mortar may protect the metal for some years before corrosion initiates. In this chapter, the mechanism of corrosion is discussed. This is also covered in the Corrosion Prevention Association technical note number 20, cathodic protection for masonry buildings incorporating structural steel frames.