Exploration of sodium dodecyl sulfate as a corrosion inhibitor for grey cast iron in phosphoric acid: effects of temperature and surface composition

Abstract

The electrochemical behavior of two nineteenth and twentieth century grey cast iron alloys was studied in 0.5 M and 1 M phosphoric acid solutions, with sodium dodecyl sulfate (SDS) used as a corrosion inhibitor. The influence of temperature on the corrosion process, the critical micellar concentration (CMC) of SDS, and its inhibition efficiency were evaluated using conductimetry, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and inductively coupled plasma-optical emission spectrometry (ICP-OES). Two samples of grey cast iron, GCI-A and GCI-B, were studied. The presence of varying percentages of copper and manganese in the chemical composition of grey cast iron enhances its corrosion resistance. In phosphoric acid solutions, the surface charge of grey cast iron becomes negative in the presence of SDS molecules, as SDS adsorbs onto the metal surface through both chemical and physical interactions, acting as a mixed-type inhibitor by blocking anodic and cathodic reaction sites. Increasing the phosphoric acid concentration decreases the critical micellar concentration (CMC) of SDS, whereas an increase in temperature has the opposite effect. The addition of SDS at a concentration of 10⁻⁴ M significantly reduces iron dissolution in both grey cast iron alloys studied. Consequently, SDS decreases both anodic and cathodic current densities, effectively inhibiting the corrosion process of grey cast iron in phosphoric acid solutions, with an inhibition efficiency of approximately 95%. SEM analysis revealed that SDS protects the grey cast iron surface by reducing the size and density of graphite flakes. The inhibitive properties of SDS highlight its potential importance in the conservation and restoration of grey cast iron objects.