ASSESSING THE CORROSION BEHAVIOUR OF STAINLESS STEELS WITH BIPOLAR ELECTROCHEMISTRY

Abstract

Bipolar electrochemistry is a promising new method for corrosion screening and assessment of a wide range of materials and microstructures. The technique provides access to the full spectrum of anodic-to-cathodic electrochemical reactions in a single experiment, allowing in-situ observations of corrosion processes and rapid assessment of corrosion kinetic behaviour. The research reported in this PhD thesis is centred on exploring and developing innovative ways to apply bipolar electrochemistry techniques. The following overarching points summarise the four research directions, with key outputs listed below: (1) Advancing the application of bipolar electrochemistry for corrosion research using type 420 ferritic/martensitic stainless steel with a segmented array bipolar electrode, in-situ observations of pit nucleation and growth, assessment of type 420 microstructure, and the effect of tempering on type 420 corrosion. (2) Application on Duplex Stainless Steel 2205: introducing bipolar as a fast corrosion screening technique, measurement of corrosion kinetic behaviour, assessment of the effect of heat treatment on corrosion, and comparison to 2101 lean duplex and types 304L/316L: austenitic stainless steels. (3) Technique development, introducing a modified bipolar approach, application of orthogonal feeder electrode arrangements, and simulating galvanic coupling of dissimilar stainless steels. (4) Application of to test 2101 lean duplex weld microstructure, assessment of the effect of gravity on pit growt, and brass dezincification. The potential and current density along a bipolar type 420 ferritic stainless steel electrode was measured using a segmented array electrode setup. The measured potential was quasi-linearly distributed distributed along the bipolar electrode (BPE), with the current density following an exponential Butter-Volmer-type relationship. The critical pitting potential and pit growth kinetics were measured. Pitting with general corrosion, pitting corrosion only, general corrosion, and cathodic reactions were observed. Pits nucleate near chromium carbides with the formation of oxide and chloride-rich particles deposited in the cathodic region. Pits grew into typical width-to-depth pit aspect ratios of 0.4-0.6, with electrolyte concentrations affecting pit growth. The best corrosion response was observed with tempering treatments at 250 oC. Pitting corrosion kinetics of 2205 duplex stainless steel were determined at room temperature. Solution annealing heat treatments were applied to characterise the effect of microstructure on pit nucleation and growth resistance, with high temperature solution annealing indicating the highest resistance against pit growth. Selective phase dissolution was found to be associated with pittig corrosion in duplex stainless steel. Bipolar electrochemistry was also applied to contrast and compare the pitting corrosion resistance of austenitic 304L/316L with duplex 2101/2205 stainless steel. A modified bipolar electrochemistry using application of an auxiliary potential to the BPE, was explored, allowing to test corrosion reactions in a far wider potential range. Approaches with perpendicular feeder electrode arrangements and two dissimilar stainless steel bipolar electrodes were explored. Practical application of the bipolar technique was demonstrated with assessment of 2101 lean duplex weld microstructure, highlighting severe pitting corrosion occurring in the Heat-Affected Zone. Differences of corrosion susceptibility were associated with changes in ferrite-austenite fraction and local weld chemistry. The effect of gravity on pit growth kinetics and shape was revisited, with the pit depth found to be independent of sample orientation. Pits grown in the faceup orientation had the highest volume loss. Brass dezincification was explored as a test system to optimise corrosion product formation as a function of acting potential gradients.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Dirk Engelberg (Supervisor) & Zhu Liu (Supervisor)