Heats of low alloy steels of 25 kg each were made separately utilizing a high frequency induction-melting furnace of 100 kg capacity. Steel scraps of C-Mn rail steel was added to soft iron (0.001 wt.% C) to balance carbon content. The desired P and Cr content of low alloy steels were obtained by adding Fe-P and Fe-Cr mother alloys while Ni lumps and Cu blocks were added to balance Cu and Ni. All mother alloys and metal blocks/lumps were added in furnace. The melts were cast into ingots of cross sectional dimension 10 cm×10 cm. Hot rolling was carried out in two stages. In first stage, the ingots were rolled in five
passes into 16 mm thickness plate after soaking at 1150C for 2.5 hours. The finishing temperature was 950C. In the second stage, once again these plates were soaked for 30 minutes at 1150C and hot rolled into 5 mm thickness plate in three passes. The finishing temperature was maintained at 800 to 830C. Specimens (20mm×15mm×5mm) for metallography examination were grounded successively to 1200 grit water proof SiC paper followed by cloth polishing using alumina suspension up to 0.05 micron. An optical microscope (Olympus GX 71) was used for capturing the photomicrographs after etching the samples with 2% Nital solution (98% alcohol + 2% nitric acid). Samples of dimension 20 mm × 20 mm × 5 mm were grounded successively to 1200 grit SiC abrasive paper and degreased with acetone for the electrochemical experiments. The polarization experiment was conducted in a standard flat cell (Princeton Applied Research, Ametek, USA) in the potential range –250 mV to +250 mV versus open circuit potential of samples, using a computer controlled potentiostat (Princeton Applied Research, Ametek273A, USA). A saturated silver-silver chloride (SSC) reference electrode with platinum counter electrode was used. The scan rate employed for the polarization studies was 0.25 mVs-1 . The corrosion current density (icorr) was evaluated from Tafel plots by the Tafel Extrapolation method as per ASTM Standard G3-89.(16, 17) For the cyclic salt spray test, samples of dimension 70 mm×30 mm×5 mm were grounded down to 60 grit SiC paper, cleaned with acetone and kept for 24 hours in a desiccators. The test samples were placed in the panels at an angle of 45 from vertical. As per ASTM-B117(18), 5 wt.% NaCl solution was used for creating salt fog. The salt spray exposure cycle consisted of 20 minutes wet and 40 minutes dry condition from 10.30 am to 5.30 pm and remaining 17 hours was kept dry at room temperature. The samples were weighed weekly to evaluate % wt. gain. AC Electrochemical Impedance Spectroscopy (EIS) was performed periodically after second and fourth weeks of cyclic wet–dry salt fog exposure test, using a multichannel potentiostat (Model: VERSASTAT (MC) FRD100). The EIS study was conducted in a freely aerated 3.5% NaCl solution using a standard flat cell supplied by Princeton Applied Research, Ametek, USA. The exposed area of each sample was 1 cm2 .