A STUDY OF CARBON-STEEL CORROSION INHIBITION BY PHOSPHATE IONS AND BY AN ORGANIC BUFFER USING A SCANNING VIBRATING ELECTRODE

The scanning vibrating reference electrode technique (SVET) has been used to map anodic and cathodic currents in solutions over freely corroding type C1020 carbon steel. All solutions contained millimolar concentrations of chloride and were sterilized. The effect of phosphate and an organic buffer on the corrosion of carbon steel in liquid media were studied because they were added to promote bacterial growth. Phosphate acted as an anodic inhibitor in the media, causing initiated pits to repassivate. If initiated pits did not repassivate, for example in a stagnant medium or when crevice corrosion initiated adjacent to the sample coating, the corrosion rates increased and the corrosion remained localized. Increased anodic current resulted in a drop in the open circuit potential and a decrease in the polarization resistance. The results show that phosphate can lead to lower average corrosion rates, than uninhibited steel, but as an anodic inhibitor can also lead to rapid penetration at localized sites.