On comparing the corrosion rates, generally D washer shows more corrosives than C washer (Table 1). This observation is similar as reported earlier. The pit/crevice depth is higher in almost all cases in D washer and so seems more corrosive than C (Table 2). This could be attributed to a higher temperature in D washer and the fact that the reaction of the ClO2 is faster than the Cl2. The difference of pH in the two washers does not matter much because interestingly, ClO2 is such a strong oxidizer that it shows same corrosivity irrespective of pH between the limits existing in the two washers .
Table 1. Corrosion rate in (mpy) mils per year.
Table 2. Maximum depth (mm) of localized corrosion on base plate and under crevices.
It is obvious from Table 1 that the SS 316L coupon corrodes at higher rates than the other coupons. SS2205 and Ni-P-TiO2-ZrO2 as-plated depositions are corroding at comparable rates but lower than that of SS 316L steels. The Ni-P-TiO2-ZrO2 as-plated and heat treated depositions shows more uniform corrosion rate than SS2205 in most environments (Figure 1). The Ni-P-TiO2-ZrO2 as-plated depositions show more resistance against corrosion than heat treated Ni-P-TiO2-ZrO2. The 254SMO shows negligible corrosion rate. Stainless steel 254 SMO shows no localized attack in all the tested environments. Otherwise, all the coupons show pits on base plates and under crevices with varying degree in different environment. The Ni-P-SiC coatings reveal better performance than SS316L coupons but less than other coupons. The corrosion attack is the maximum on SS316L. Overall, on the basis of corrosion rate, maximum pit and crevice depth, different alloys may be put in the following order of resistance against corrosion attack e.g. 254SMO> SS2205 > Ni-P-TiO2-ZrO2 as-plated > Ni-P-TiO2-ZrO2 heat treated >Ni-P-SiC as-plated > Ni-P-SiC heated > SS316L.
Fig. 1. Corroded coupons after in-plant test (a) SS316L (b) Ni-P-SiC As-plated (c) Ni-P-TiO2-ZrO2 Heat treated nano-composite coatings and (d) 254SMO after completion of corrosion test.
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