TWI Industrial Member Report Summary 925/2009
By Q Lu and P Woollin
Current practice when defining whether a corrosion resistant alloy (CRA) will resist pitting in service at temperatures above 100°C typically involves exposing unstressed samples in an autoclave for 30 days and comparing them before and after test in terms of weight loss and visual appearance, with sectioning to explore any suspected areas of pitting. This is somewhat unsatisfactory however, as such tests cannot differentiate between pits that are small but continuing to propagate and those that have repassivated and hence have stopped growing during test. An additional difficulty arises from the fact that the necessary test duration to allow steady state conditions to develop and stable pits to initiate will vary with environment and may be longer than 30 days, eg when the test temperature is close to the critical pitting temperature (CPT), ie the minimum temperature at which stable pitting occurs.
Monitoring electrochemical activity during test will potentially overcome the problem of differentiation between pit repassivation and propagation, whilst some form of electrochemical acceleration, eg via short term polarisation, may overcome the extended pit initiation period. TWI is developing a test method to allow accelerated, monitored pitting tests to be undertaken at temperatures in excess of 100°C in a Group Sponsored Project (TWI GSP 15944). However, problems arose during the project, particularly related to (i) identifying a CPT and (ii) obtaining a stable electrochemical signal due to difficulties with the salt bridge to the commercial pressure-balanced reference electrode.
This Core Research Project was set up in conjunction with the GSP, to develop an electrochemical method to detect the onset and propagation of pitting corrosion. Based on the literature review conducted, electrochemical noise (EN) measurements appeared to be promising as they require no external polarisation. This report describes the test results and draws conclusions on the most appropriate methods to be applied in rising temperature tests which are used to determine the CPT.
Identify electrochemical methods suitable for determining the onset of stable pitting corrosion in stainless steels in chloride-containing solutions, and hence the CPT.