Rust Characterisation

The presence of Mn, S, Si and A1 is found at P2 and P3, whereas Mn and CE were found at PI. Chloride ions diffused through mst and substrate interface and played an important role in accelerating corrosion on WS at PI. This corroborates the corrosion rate (Table 3.2) where corrosion rate on WS is observed almost equal with MS. In case of other two sites P2 and P3 due to the presence of higher amount of S, MnS is formed and retard the corrosion rate as found in Table 3.2. 

After 42 m of exposure at PI, morphology of MS is layered and chunky as per Fig. 3.36a. For WS it is an agglomeration of smaller oxides and shows relief effect with very fine network of cracks on the oxide film as per Fig. 3.36b. Line scans as shown in Figs. 3.40a and b for WS and Figs. 3.39a and b for MS show identical O2 count. SEM and EDX line scan, thus, justifies the identical corrosion rate of both MS and WS as reported in Table 2.8. 

rast morphology on MS shows as per Fig. 3.37a a continuous chain of chunky oxides, whereas WS contains sheet of oxides layer as per Fig. 3.37b. Line scan on WS surface (Fig. 3.42a, b shows uniform distribution of O2 count. Because 

The SEM micrographs of exposed panels also help in differentiating in rust morphology of MS and WS from site-to-site and effect of increase of exposure time on the rust formation. A good correlation between corrosion rate and SEM and EDX has also been established. 

Characterisation by XRD and Raman Spectroscopy. Rust formed on uncoated panels after 18 and 42 m of exposure was analysed by XRD and Raman spectroscopy. The oxide phases were identified after matching with JCPDS data available for XRD [6,7] and are given in Tables 4.1,4.2 and 4.3. Phases identified by Raman spectroscopy were matched with the published data [8-14, 15] and is given in Tables 4.4, 4.5, 4.6 and 4.7. 

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Present research work has been carried out in two parts field exposme tests and accelerated laboratory tests on unexposed panels. Under field exposme test three representative sites PI very close to sea shore and free firom SO2, P2 away from sea with presence of SO2 and P3 industrial environment with presence SO2 were chosen. Analytical techniques (EDX, XRD and Raman spectroscopy) were used for rust characterisation and SEM was employed for understanding the morphological state. Corrosion rates were estimated after different periods of exposure at sites. It is found that though accelerated laboratory test can predict the general trend of corrosion, actual field test data cannot be created in laboratory. So an attempt was made to simulate protective rust coating in the laboratory that is typical of field exposure protective rust formation on weathering steel. 

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