STRUCTURAL STUDIES OF CALORIZED COATINGS ON MILD-STEEL

Aluminium diffusion coatings are known to protect steel from corrosion at elevated temperature (950-degrees-C) and in sulphurous atmospheres. The literature however, is not clear with regard to the underlying mechanism. The properties of calorized material have been attributed to aluminium oxide and to different intermetallic compounds. Calorized mild steel pipes, produced at NML by pack-cementation, have shown good corrosion properties at 600-degrees-C in a sulphurous atmosphere comparable with those of imported calorized steels. The micro-hardness profile across coating thickness shows in general values between 450-650 compared to matrix hardness of 220 (pearlite) and 140 (ferrite). A high hardness phase with 850-1000 hardness has been detected at some places, within 30-mu-g of the outer edge of the coating. X-ray diffraction has established the general presence of FeAl and Fe3Al phases in the diffusion zone. It is of importance to note that these compounds satisfy the Pilling Bedworth condition of continuity for protective oxide coating, extended here to cover calorized coatings, i.e., [GRAPHICS] where V(mal)x = Volume of metal aluminide and V(m) = Volume of metal used to form aluminide. On the basis, the presence of other phases such as FeAl3, FeAl2, FeAl5 is considered undesirable as the compatibility with the matrix would be poor. The calorizing parameters need to be standardized so as to obtain beneficial phases only for superior performance of coatings.