Corrosion inhibiting properties of cashew nut shell liquid in aqueous carbon dioxide solutions

The corrosion inhibiting properties of CNSL was studied using SAE 1008 carbon steel in aqueous C02 saturated NaCI solutions. The NaCI concentration in the electrolyte was varied from 1 - 5 %, pH in a range from 4.0 to 6.0 and temperature of the system between 30 and 80 °C. Measurements were performed in both inhibited and uninhibited solutions in order to deduce the influence of the CNSL inhibition of COZ corrosion of the carbon steel. The corrosion inhibition and the adsorption of the inhibitor molecules were monitored on stationary electrode by electrochemical techniques, gravimetric and UV/VIS. The solution pH was found to influence the performance of the CNSL inhibitor. With the solution pH <_ 4 the inhibitor performance was poor but increased to over 95 % as the pH increases. It was suggested that poor solubility of the CNSL inhibitor in solutions of lower pH could be the reason for the poor performance of the inhibitor. Also, it was observed that the solution temperature affects the corrosion inhibiting properties of CNSL. The addition of the CNSL inhibitor in the test solutions of temperatures between 30 to 45 °C, significantly reduced the corrosion rate of the SAE 1008 carbon steel electrodes, however, above 45 °C the performance of the inhibitor decreased. NaCI concentration was found to influence the corrosion rate and mechanism of the carbon steel in the uninhibited solutions. However, NaCI concentration was found to have no effect on the CNSL corrosion inhibitor performance. The CNSL showed a very strong tendency to inhibit the corrosion process for carbon steel electrodes in C02 - saturated NaCI solutions. It was found to be a mixed type inhibitor with predominant anodic effects. In these range of parameters an efficiency of over 95 % was achieved by just 600 ppm. The adsorption process of the CNSL inhibitor on the carbon steel surface reasonably followed the Langmuir adsorption isotherm. The adsorption mechanism of the CNSL inhibitor was elucidated and found to follow physical adsorption, whereby, a phenoxide ion, (R-Ar-O-) of the inhibitor molecule interacts electrostatically with the positively charged metal surface.