Synchrotron X - ray and Electrochemical Studies of Pitting Corrosion of Iron

Authors: Weichen Xu (University of Birmingham)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: July 2014
Diamond Proposal Number(s): 7413

Open Access Open Access

Abstract: Pitting corrosion of iron has been studied via artificial pits. Solid corrosion products have been observed in the pit solution, which have been characterised as disordered carbon and Fe 3 C (known as both cementite and cohenite) using in - situ synchrotr on XRD and ex - situ Raman spectroscopy. These corrosion products were found to act as diffusion barriers for metal ions and to give a small increase in the solution resistance. The formation of carbon - containing corrosion products depends upon the interfaci al potential. The potential dependence suggests that Fe 3 C may be protected from dissolution by a carbon layer. High purity iron was used to calculate an effective diffusion coefficient for metal ions (a combination of self - diffusion and electrical migratio n) in different MgCl 2 concentrations during diffusion - limited dissolution. The contribution of self - diffusion increases with increasing MgCl 2 concentration if the depletion of Mg 2+ in the pit is considered. The Tafel kinetics of iron dissolution in solutio ns saturated in Fe 2+ has been 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 and structure of salt layers ha s been characterised using in - situ synchrotron XRD. The salt layer has been found to be composed of FeCl 2 .4H 2 O in chloride with and without trace nitrate, and Fe(NO 3 ) 2 .6H 2 O in nitrate with trace chloride. The salt layer is isotropic in HCl and anisotropic in HCl with trace nitrate. The dissolution behaviour of iron in HCl and chloride/nitrate solutions has been studied using in - situ synchrotron X - ray radiography. It has been found that dissolution is relatively uniform in the presence of salt layers, but cr evice formation and surface roughening can take place on salt - free surfaces. The potentiodynamic measurements on iron in nitrate/chloride solutions induce abrupt transitions between dissolution and passivation, resulti ng in deep and localised attack .

Journal Keywords: Corrosion; Pitting; Steel;

Subject Areas: Materials, Chemistry

Instruments: I13-2-Diamond Manchester Imaging , I18-Microfocus Spectroscopy