On the corrosion resistance of the Delhi iron pillar

The nature of the protective passive layer on the corrosion resistant Delhi iron pillar (DIP) has been addressed based on a detailed characterization of its rust. Rust characterization clearly established that the major constituents of the scale were crystalline iron hydrogen phosphate hydrate (FePO4.H3PO4. 4H(2)O), alpha-, gamma-, delta -FeOOH and magnetite. The iron oxide/oxyhydroxides were present in the amorphous form. The role of slag particles in the matrix of the DTP iron in enhancing the passive film formation is briefly addressed initially. The process of protective rust formation on DIP iron is outlined based on the rust analysis. Initially, the corrosion rate of iron is high due to the presence of slag particles. This results in enhancement of surface P content. In the presence of P, the formation of a protective amorphous compact layer of delta -FeOOH, next to the metal surface, is catalyzed and this confers the initial corrosion resistance. The critical factor contributing to the superior corrosion resistance of the DIP, however, is the formation of iron hydrogen phosphate hydrate, as a thin layer next to the metal-metaloxide interface. The formation of the crystalline modification of this phosphate from the amorphous form is aided by alternate wetting and drying cycles (i.e. the environmental factor). The rate of corrosion is further lowered due to the low porosity content of the crystalline phosphate phase. The passive film formation on the DIP has been contrasted with the rusting of normal and weathering steels. (C) 2000 Elsevier Science Ltd. All rights reserved.