Study of Water Soluble Organic Corrosion Inhibitors for Mild Steel

The inhibitive properties of sodium or amine salts of monobasic and dibasic acids for mild steel specimens have been examined in humidity cabinet at 40°C or 50°C and relative humidity 95100%The monobasic and dibasic acids used were monocarboxylic, dicarboxylic, alkylphosphoric, dialkylthiophosphoric, n-octylphosphonic and n-octylmalonic acids, etc. About the almost same length of carbon chain, the sodium salts of dibasic acids with polar group at both ends have most inhibitive efficiency and those of no branched monobasic acids have more efficiency than those of dibasic acids with two polar groups near one end or those of branched monocarboxylic acids. It seems that the inhibitors with the chain length of about 8 carbon atoms are most effective among the homologous series. The influence of pH in sample solution on corrosion greatly increased with increase of pH, especially more than pH 10.Results of indoor exposure test for mild steel indicate that sodium salts of n-octylphosphonic and n-octylphosphonic acid have good inhibitive property, and do not correlate always with results of humidity cabinet test.