Specific influence of some additives on the corrosion of iron in acid solutions

Abstract

The influence of different types of additives (dibenzylsulphoxide (DBSO), 2-methylpyridine (2-MPy), formaldehyde and benzonitrile (BCN), on the anodic and cathodic reactions at an iron electrode has been studied in sulphamic acid and in sulphuric and chloride solutions by means of intensio- and potentiokinetic polarization curves (3 mA/h and 3 V/h respectively). Curves of backpolarization were also performed. The analysis of the results precludes a general hypothesis on the inhibition mechanism of iron corrosion in acid media for the following reasons: (1) An additive is “reactive”. An example is given by DBSO, whose reduction product, DBS, is mainly responsible for inhibition. Beyond a critical anodic tension, the iron dissolution process takes place through a different mechanism from that involved in the same solution without the additive, as it is shown by the micrographs. The position of the cathodic back-polarization curves seems to confirm a decrease of the hydrogen diffusion into the metal. (2) An additive can be “reactive” depending on the acidity of the solution. For instance, the inhibiting effect of 2-MPy in 2N HCl is due to the organic cation, while in IN HCl, very probably, it depends on the chemisorption of the free amine. (3) An additive is “unreactive”, e.g. formaldehyde. Inhibition is mainly cathodic and seems due to the adsorption of the oxymethylene which probably increases the diffusion of the hydrogen into the metal. (4) An additive such as benzonitrile is “unreactive” and is chemisorbed on to the metal surface. It is assumed that both the anodic and cathodic rate constants are greatly slowed down. Desorption of the BCN takes place beyond a critical tension value. In hydrochloric acid, as in sulphuric solutions, this compound decreases hydrogen diffusion into the metal.