Correlation between corrosion indices and corrosiveness of groundwater: a study with reference to selected areas of Krishna District, Andhra Pradesh, India

The aim of the present study was to determine the aggressiveness of groundwater samples towards corrosion of various commercially important metals and to study the correlation between theoretical and experimental values of corrosiveness. For this purpose, 20 groundwater samples were collected in different mandals of Krishna District, Andhra Pradesh, India, and the metals selected were carbon steel, aluminium, copper and cupronickel alloy. The corrosiveness was determined by using gravimetric measurements. Further, corrosion and scaling indices, namely Langelier index (LI), aggressive index (AI), Ryznar index (RI), Puckorius index (PI) and Larson-Skold index (LS), were calculated with the help of various water quality parameters of all the samples to estimate their corrosivity. The experimental values of corrosion rates were attempted to correlate with various indices calculated and quality parameters determined in case of all the samples. It is observed that there is a significant lack of correlation between the indices and corrosion rates. Among the five indices taken into account, LI, AI and RI indicate scaling tendency of water samples, as inferred by their mean values, 0.657, 12.576 and 6.612, respectively, while the other two indices, PI and LS with the mean values of 8.141 and 1.188, respectively, suggest the corrosive behaviour. Further, the mean values of correlation coefficients, 0.18 and 0.27 for PI and LS, respectively, are higher compared with those for LI, AI and RI. The mean values of correlation coefficients of dissolved oxygen, chlorides and electrolytic conductivity are 0.33, 0.25 and 0.25, respectively, which are relatively higher compared with those of other quality parameters. The probable reasons for low correlation coefficients are explored based on the statistical data. Also, it was concluded that one of the significant factors, namely dissolved oxygen which has strong influence on corrosion of metals, was considered in none of the indices, leading to very low correlation between the theoretical and experimental values of corrosiveness.