In the UK, approval of stainless steel products by the Drinking Water Inspectorate invokes the Operational Guidelines and Code of Practice for Stainless Steel Products in Drinking Water Supply (OGCP) t Munro et al., 2002). Type 304L and 316L stainless steel grades can be used for the majority of applications in water treatment and supply, with the grade selection depending on the chloride and chlorine levels in the water. Currently the guideline suggests that 304L may be used up to 200 ppm chlorides and 2 ppm free chlorine and type 316L up to 1000 ppm chlorides and 5ppm free chlorine at pH levels greater than 6. Although the maximum chloride levels have been well researched and in most applications has supported this for many years, chlorine values are based on limited evaluations and, although they work well in most, more research is required to more closely define them. The purpose of this research is to understand and determine to what extent residual chlorine level, chloride content, and pH will affect the crevice corrosion behaviour of types 304L and 316L stainless steels in particular, and the initiation of crevice corrosion. The study required the creation of a controlled chlorinated system exposing creviced specimen using the multi-crevice assembly technique. The assemblies were immersed for 60 days at 20°C in chloride levels ranging from 200-2000 ppm, residual chlorine levels ranging from 0-10ppm; all at pH 6 and 8 . A fter the immersion period, specimens were investigated by visual examination, Scanning Electron Microscopy (SEM) and Atomic Force vlicroscopy (AFM). Potentiodynamic polarization studies were also 4 carried out for each of the environments. Custom made software programs were developed to analyse the data collected from SEM and AFM results to give a better understanding and clarification of the crevice corrosion phenomena, in each individual condition and to provide more confidence in the selection of chlorination levels for particular chloride water contents. From visual and optical assessment results presented that the earliest noticeable of crevice corrosion had occurred at 0 ppm chlorine, 1000 ppm chloride ppm, pH 6 and 20° C and at 2 chlorine ppm, 1000 chloride ppm, pH 8 and 20° C for 304L specimens while for 316L specimens no significant change was observed. These results supported with OGCP and showed that both type of specimens can be used at slightly higher concentrations of chlorine and chloride than the guideline values depending on how to keep the constant concentration and how much of the surface quality of specimens that will be used throughout the process . . -\FM and SEM results showed that corrosion surface activity had occurred on both 304L and 316L specimens, even the least aggressive chloride and chlorine concentrations. The corrosion surface activity IS proportional to the increase in the chloride and chlorine concentrations. Potentiodynamic polarization curves supported the evidence from AFM and SEM results that chemical activity inside the pit had occurred even at milder concentrations. Cr and Fe were oxidised as 02- and OH- penetrated into the pits. These electron exchanged of these chemical species had been interpreted by polarization curve which showed various 5 stages of these reactions (cathodic reaction, anodic reaction. passive stage and breakdown). The results showed that breakdown potential (Eb) and passive stage are inversely proportional to chloride and chlorine concentrations. The chlorine and chloride concentrations had a more dominant effect than pH especially at high concentrations due to the amount of chemical species inside the pit and electrolyte solution overcome pH effect and the process was under mass transport controlled until pH effect can be neglected. The custom made software offered more clarification of pit characteristics and shapes which will be useful in connected with corroded area and volume calculated and further study or future prediction of characteristic of crevice corrosion that will occur or likely to in different environments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:578693 |
| Date | January 2011 |
| Creators | Vepulanont, Klatnatee |
| Publisher | University of Sheffield |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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