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CORROSION RESISTANCE OF AUSTENITIC STAINLESS STEEL IN ACETIC ACID SOLUTION CONTAINING BROMIDE IONS

Al-Subai, Saud Ghunaim A

[Thesis]. Manchester, UK: The University of Manchester; 2011.

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Abstract

In this research, the corrosion performance of two austenitic stainless steels,namely 316L and 254SMO, in concentrated acetic acid solutions containing bromide ions has been investigated. In this research, the influence of two different electrochemical surface treatments (electropolishing and nitric acid passivation) on the corrosion behaviour of 316L stainless steel immersed in 15.3M HAc with 18.7mM bromide ions at 900C was examined. Also, attemptswere made to study the performance of three organic inhibitors in the same conditions.Corrosion rates are assessed both by weight loss, and linear polarisation resistance. Interfacial corrosion chemistry is further characterised by open circuit potential and potentiodynamic polarization measurements. Substrate morphology is elucidated with optical microscopy, including 3D surface profiling, and scanning electron microscopy. Also, X-ray photoelectron spectroscopy is employed to gain further insight into the quite differentcorrosion performances of 316L and 254SMO in 15.3M acetic acid with 18.7mM Br ions.It was found that 316L and 254SMO steels have good corrosion resistance and low corrosion rates in 11.9M-HAc-Br-. Increasing acid concentration to 15.3 M led to a dramatic increase in corrosion rate of 316L with clear evidence of uniform and pitting corrosion proceeding simultaneously. Notably, the step increase in OCP for 316L steel and 254SMO during immersion in 15.3M-HAc-Br- solution indicates sudden changes in corrosion activity of the steels. The step seen for the 254SMO in 15.3M-HAc-Br- is indicative of passivation which is also supported by the XPS results, as a stable passive film was observed on the surface of alloy over the immersion time. However, the step increase in the OCP observed for 316L in 15.3MHAc-Br- is not associated with a significant decrease in corrosion rate. An alternative explanation is that the step coincides with an increase in the importance of pitting due to the evolving surface structure.From the attempts which were made to improve the corrosion resistance of the 316L stainless steel in 15.3M-HAc-Br-, both electropolishing and nitric acid passivation treatments were not sufficient to give any noticeable protection from the aggressive solution. Also, no corrosion inhibition was achieved when the three organic inhibitors, BTA, TU and 2MBI were utilised.