Ion Selectivity of Sulphide Films and Electrochemical Behaviors of Carbon Steel
|School||Beijing University of Chemical Technology|
|Keywords||sulfide precipitate film hydroxide membrane ion selectivity bipolar structure H2S corrosion inhibitor|
Sulphide films and hydroxide membranes of Fe, Cu, Ni are prepared by chemical precipitation on a cellulose sheet tightly sealed between two compartments a plexiglass cell with one compartment containing FeCl2, CuCl2, NiSO4 solutions and the other containing Na2S, NaOH solutions respectively. The ion selectivity of these films has been investigated by measuring the membrane potentials. It is concluded that their sulphide films are bipolarized structure with a cation selective layer contacting FeCl2/CuCl2/NiSO4 solutions side and with an anion selective layer contacting Na2S, NaOH solutions side. However, their hydroxide membranes are single-polar structure, ie they are anion selective layer. Bipolarized films’ composition and appearance are different, single-polar membranes’ are same under SEM and XRD. In real corrosion system, the side contacting FeCl2 solution is equal to the metal side whereas the side contacting Na2S is equal to the corrosive fluid side. With the cation selective interior layer of the film, Fe2+ may migrate into the film easily and react with S2- which migrates into the film from the bulk solution and thus promote the anodic reaction. So the structure of iron sulfide film will accelerate the metal corrosion. There is a zone of zero charge besides two sides of iron sulfide film with different electric charge. <WP=6>MoO42-, PO43- and imidazoline compound would alter the ion selectivity of the iron sulfide precipitate film from biporlar structure to cation-selective. The cation-selective types of iron sulfide film inhibit the anodic metal dissolution due to restraining S2- migration. At the same time, it can hinder Cl- enrichment under the precipitate film, which decreases local pH and results in intensive localized corrosion of iron. Therefore, cation-selective iron sulfide films with adsorption MoO42- et al. inhibitors may retard the metal corrosion.However, adsorbing Ca2+, Mg2+, Ba2+ these cations can change the ion selectivity of iron sulfide film from biporlar structure to ation-selective. The reason is that these cations are accumulated on iron sulfide films and result in the addition of positive charges. This kind of film structure allows the aggressive ions such as S2-and Cl- migration. Therefore, it has little protective effect on metal corrosion.The corrosion behaviors of A3 steel with iron sulfide product have been studied in different solutions by means of potential scanning and electrochemical impedance spectroscopy (EIS), little effect of Ca2+ on polarization curves is observed, Na2MoO4 and imidazoline compound show inhibition on the anodic and cathodic process, which decrease the corrosion rate.