Effect of rust converting treatments on corrosion rate and sulphate nest development in prerusted steel

Three types of anticorrosive treatment were applied to carbon steel specimens prerusted in a condensation chamber in the presence of sulphur dioxide. These were: impregnation with prehydrolysed methyltriethoxysilane in ethyl alcohol; impregnation with the same solution containing suspended zinc stearate; and coating with an inorganic zinc phosphate primer. The specimens were then corroded for 135 days in clean air at 4°C and 100% relative humidity. The extent of corrosion was estimated from mass loss on descaling; three layers in the rust were separated and characterised by scanning electron microscopy, X-ray microanalysis, and X-ray diffraction. The roughness of the descaled surface was also estimated. The results show that each of the treatments approximately halved the rate of corrosion of already rusted steel, not simply by forming a barrier, but by modifying the distribution of sulphate in the rust.