Polyphosphate coated steel sheet for superior corrosion resistance

The paper discusses the performance of a polyphosphate coated steel sheet. The coating so developed on steel sheet surface was found to be dense, and uniform with a coating weight of 1.2 to 1.7 g/m2. The corrosion resistance properties of the polyphosphate coating in the humidity cycle were found to be much superior than that of rust preventive oil (RPO) coating with surface oil content of 250 mg/m2 and conventional phosphate coating (coating weight: 2.0 g/m2). Electrochemical Impedance test in 3.5% NaCl solution revealed that polyphosphate coating is resistant to moisture up to 15 days in accelerated conditions since coating capacitance was found to be constant over the time as indicated by Bode plot. In case of RPO, coating capacitance increased with the elapse of immersion time indicating coating degradation. Similar results were also found in case of conventional phosphate coating.