TO STUDY CORROSION BEHAVIOR OF IRON-PHOSPHORUS BASED POWDER PROCESSED ALLOYS

Abstract

Fe-P binary, Fe-P-Cr, , Fe-P-C ternary, Fe-P-Cr-Si quarternary as well as Fe-P based Fe-P-Si-Ni-Mo-Cu , Fe-P-Si-Ni-Mo-Cu-C and Fe-P-Si-Ni-Mo-Cu-C-Al alloys were made using hot powder forging technique. In this process mild steel encapsulated hot powders (blend of Fe powder, ferro- phosphorus powder, ferro-silicon powder, ferro- molybdenum powder, ferro-chromium powder, copper powder, nickel powder, Aluminum powder & graphite powder) were forged into slabs. Mild steel encapsulation was removed after homogenization of these slabs. The slabs were hot rolled to get sheets. These sheets were annealed to relieve the residual stresses. Use of fine powder and high temperature processing in presence of H2 atmosphere facilitate densification. Use of H2 ensures that the powder surfaces are clean before forging. These alloys were characterized in terms of microstructure, porosity content, hardness and rate of corrosion. Microstructure of these alloys consists of single phase ferrite only. The various environments chosen for corrosion testing were acidic (0.25 M H2SO4 solution of pH 0.6), neutral/marine (3.5 % NaCl solution of pH 6.8) and alkaline (0.5M Na2CO3 + 1.0 M NaHCO3 solution of pH 9.4). The corrosion studies were conducted using Tafel Extrapolation and Linear Polarization Methods. The studies also compare Armco iron with phosphoric irons. The phosphoric irons were also characterized using optical microscopy, SEM, EDAX and EPMA. Additions of C, Cu, Ni, Cr, Al, Mo, and Si improved the corrosion resistance of Fe-0.35 P alloys in all the environments. It was observed that these materials have excellent resistance to corrosion in marine and alkaline conditions. Rolled and annealed, 4 mm thick plates of a few compositions containing phosphorous more than 0.35 wt % P were joined using Gas Tungsten Arc Welding and Resistance Spot welding technique. The corrosion behaviour of such weldments was studied by Tafel Polarization method and Immersion in NaCl solution. The corrosion resistance of welds obtained by using high phosphorous filler materials was better than those obtained by using low phosphorous filler materials.