Synchrotron x-ray and electrochemical studies of pitting corrosion of iron

Xu, Weichen (2014). Synchrotron x-ray and electrochemical studies of pitting corrosion of iron. University of Birmingham. Ph.D.

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Abstract

Pitting corrosion of iron has been studied via artificial pits. Solid corrosion products were observed within the pit, which was characterised as disordered-carbon and Fe $_3$ C, acted as diffusion barriers for metal ions and slightly increased the solution resistance. Its formation depends upon the interfacial potential.
High purity iron was used to calculate an effective diffusion coefficient for metal ions (a combination of self-diffusion and electrical migration) in different MgCl $_2$ concentrations. The contribution of self-diffusion increases with increasing MgCl $_2$ concentration if Mg $^2$ $^+$ depletion in the pit is considered.
The Tafel kinetics of iron dissolution in metal ion saturated solutions was studied in different MgCl $_2$ concentrations. The Tafel slope (56 to 70 mV/decade) was independent of MgCl $_2$ and FeCl $_2$ concentration when FeCl $_2$ is saturated.
The effect of nitrate on the composition/structure of salt layers was characterised. The salt layer is composed of FeCl $_2$ .4H $_2$ O in chloride-based solutions (isotropic in HCl and anisotropic in HCl with trace nitrate), and Fe(NO $_3$ ) $_2$ .6H $_2$ O in nitrate-based solutions.
The dissolution behaviour of iron in chloride/nitrate solutions was studied. Dissolution is uniform under a salt layer, but crevice formation and surface roughening. Potentiodynamic measurements can induce abrupt dissolution/ passivation transitions, resulting in deep and localised attack.