Comparison of Field and Laboratory Tests

It was tried to recreate the parameters of field tests at the accelerated laboratory tests where the effect of humidity, chloride ions and presence of SO2 were considered. Comparison of Table 3.22 (lab test) and 3.3 (field test) reveals that the corrosion rate of as received MS and WS in 3.5 % NaCl is lower than the exposed panels. Eaya of as received panels are also greater. However, corrosion rate in cyclic polarisation of MS and WS after SO2 exposure is comparable to cyclic polarisation of coated panels after 18 m exposed at P3 as can be seen from Tables 3.23 and 3.4 and Figs. 3.39 and 3.8. Both plots show passivity. In general, the trend WS is more corrosion resistant than MS as seen in both field and laboratory tests. 

The corrosion rate of MS and WS as obtained from humidity and salt spray tests (Table 3.19 and 3.20 respectively) when compared with atmospheric corrosion rate at three sites with different exposures (Table 3.2) show wide gap. This is expected, because at laboratory the exposure is more severe and continuous compared to atmospheric conditions that prevail at the three sites. 

Comparison of Tables 3.24 and 3.25 (lab test) with Table 3.5 (field test) and Tables 3.27 and 3.28 (lab test) with Table 3.7 (field test) clearly demonstrates that laboratory test data no way compares with field test data. 

However, Raman data of rust formed on field test somewhat matches with lab test data. The corrosion products i.e. oxides/hydroxides formed in both the cases are somewhat similar. Since the other corrosion properties of field and laboratory test are widely different as discussed earlier, it means the extent of oxides/hydroxides formed during the field tests and their morphology are widely different than that obtained from laboratory test. That the morphology of the rust product of field tests are widely different than as received (lab test) is clearly seen when Figs. 3.33-3.38 (field test) are compared with Fig. 3.59 (lab test). Presence of SO2 at P3 was found to improve the corrosion resistance of WS. It was also demonstrated that SO2 helps in formation of passive layer on WS (Figs. 3.8 and 3.49). Hence, as received panels were subjected to SO2 atmosphere (Figs. 3.62 and 3.63) and consequently SEM of 

In summary, it can be said that the data generated with laboratory tests, though predict the trend of field tests, are grossly insufiicient to predict field behaviour of WS and MS in absolute terms. Since SO2 is a factor which gives some convergence at some points of two types of tests, separate set of experiments was contemplated to simulate similar field conditions at laboratory. 

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Cooper, K. R., Kelly, R. G., and Colvin, E. L. The Correlation between Crack Chemistry and Crack Growth Behavior of 7XXX Series Aluminum Alloys A Comparison of Field and Laboratory Tests, NACE Corrosion 1999, paper No. 153, San Antonio, TX. 

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