Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 973 were obviously found on the concave bottom surface, seen in Fig. 8b. Further magnified, these spots uniformly exhibited a coin-like shape with average diameters of around 10 um, shown in Fig. 8c. Figure 8d and e both present the morphologies of three neighboring"coins"with different arrangements, in the former ne(Fig. 8d )each" coinpossessed a cavity with even smaller diameter of 2 um in its center, while in the latter one(Fig. 8e)each "coint was cleaved across its whole length by a main microcrack with branches. Figure 8f displays the representative morphology of the"coinhere, which had both a cavity in its center and a main microcrack across it. Moreover, a common ground was detected within all the"coins" that they were filled with a coral structure whose whisker width had already reached in near nanoscale, approximately 100 nm. Such a strange structure deserved more detailed analysis to understand its formation mechanisms. Meanwhile, a group of tiny cracks were observed in the corrosion products on the concave bottom surface as well(Fig. 8h), which may be abrased block by block during corrosion procedure Then the sample was cut from the peanut-like concave to observe its cross-section under SEM. Figure 9a is the total morphology. After magnification, pits with several feature morphologies of pitting corrosion including elliptical, wide, and optical undercutting shallow, subsurface and undercutting were found, seen in Fig. 9b wide& shallow Figure 9c was the further magnified morphology of the subsurface type. It should be mentioned that widths of these pits were nearly the same of those pits shown in Fig. 6d-f. In other words, pits detected in both Fig. 6 and Fig. 9 were virtually the merely viewed from different directions. Consequently, it could be inferred now that pitting corrosion may be partly ascribed to 2s 00 vl erts wmm 1ooxk formation of the peanut-like concave on the failure pipe. For purpose of investigating the chemical compositions of the marked sites(A-F) within the microscopic defects in Fig. 8 and 9, EDS was utilized. As is shown in Fig. 10a-e and Table 2, chemical compositions of the coral structures in all the "coins were almost the same: approximately equaled to the chemical formula of NaFe20SgO4o, calculated from the mass ratios between these elements. This complex compound with multiple elements may be actually the mixture of several simple compounds like Na3 FeO3, Na2 FeO2, FeSO4, and iron oxides, i.e., formation of the coins"was possibly relevant to Na and s elements Considering the engineering media in the system, NaOH as well as the residual SO2 and H2S in the process water may be the sources of these two elements. Moreover, it is a little surprising b o w Erols wm that the chlorine element was detected within the pit(Fig. 10f and Table 2)as well, which further confirmed that pits in the peanut- Figure 9. SEM of the cross-sections of the peanut-like concave: (a)total like concave could have been resulted from pitting corrosion. morphology, (b) pits with different morphologies, and (c) subsurface 2.5 Process water inspection corrosion. Figure 11 displays the changing situation of process water pH values from the four inspection sites(co 2.5.1 pH value locations could be referred from Fig. 1)in the year 2009, during According to the designed parameters from Lummus, which inspection was ceased for over half a month in March to appropriate PH values of the process water in stripper system install new pipes for substituting the failure ones within the Qo/ should be confined in an alkalescent range of 7.5-9.5, above Ds heat exchanger, while the interval lasting for nearly 2 months which the process water would be emulsified by the organic from May to July was the routine downtime. Before entering the compounds in it, and/ or cause the equipments with matrix stripper T-1601, the process water had the lowest pH values materials of steels be transpassivated and consequently corroded, among the four(site A, the top-left in Fig. 11)due to the high i.e., the caustic corrosion; below which the equipments would content of acidic substances like SO2, H2S. After stripping and also be corroded due to the low ph values, i.e., the acidic adding alkali liquor, pH values of the process water before the ds www.matcorr.com o 2011 WILEY-VCH Verlag GmbH& Co KGaA, Weinheimwere obviously found on the concave bottom surface, seen in Fig. 8b. Further magnified, these spots uniformly exhibited a coin-like shape with average diameters of around 10 mm, shown in Fig. 8c. Figure 8d and e both present the morphologies of three neighboring ‘‘coins’’ with different arrangements, in the former one (Fig. 8d) each ‘‘coin’’ possessed a cavity with even smaller diameter of 2mm in its center, while in the latter one (Fig. 8e) each ‘‘coin’’ was cleaved across its whole length by a main microcrack with branches. Figure 8f displays the representative morphology of the ‘‘coin’’ here, which had both a cavity in its center and a main microcrack across it. Moreover, a common ground was detected within all the ‘‘coins’’ that they were filled with a coral structure, whose whisker width had already reached in near nanoscale, approximately 100 nm. Such a strange structure deserved more detailed analysis to understand its formation mechanisms. Meanwhile, a group of tiny cracks were observed in the corrosion products on the concave bottom surface as well (Fig. 8h), which may be abrased block by block during corrosion procedure. Then the sample was cut from the peanut-like concave to observe its cross-section under SEM. Figure 9a is the total morphology. After magnification, pits with several feature morphologies of pitting corrosion including elliptical, wide, and shallow, subsurface and undercutting were found, seen in Fig. 9b. Figure 9c was the further magnified morphology of the subsurface type. It should be mentioned that widths of these pits were nearly the same of those pits shown in Fig. 6d–f. In other words, pits detected in both Fig. 6 and Fig. 9 were virtually the same ones, merely viewed from different directions. Consequently, it could be inferred now that pitting corrosion may be partly ascribed to formation of the peanut-like concave on the failure pipe. For purpose of investigating the chemical compositions of the marked sites (A–F) within the microscopic defects in Fig. 8 and 9, EDS was utilized. As is shown in Fig. 10a–e and Table 2, chemical compositions of the coral structures in all the ‘‘coins’’ were almost the same: approximately equaled to the chemical formula of NaFe20S8O40, calculated from the mass ratios between these elements. This complex compound with multiple elements may be actually the mixture of several simple compounds like Na3FeO3, Na2FeO2, FeSO4, and iron oxides, i.e., formation of the ‘‘coins’’ was possibly relevant to Na and S elements. Considering the engineering media in the system, NaOH as well as the residual SO2 and H2S in the process water may be the sources of these two elements. Moreover, it is a little surprising that the chlorine element was detected within the pit (Fig. 10f and Table 2) as well, which further confirmed that pits in the peanut￾like concave could have been resulted from pitting corrosion. 2.5 Process water inspection 2.5.1 pH value According to the designed parameters from Lummus, appropriate pH values of the process water in stripper system should be confined in an alkalescent range of 7.5–9.5, above which the process water would be emulsified by the organic compounds in it, and/or cause the equipments with matrix materials of steels be transpassivated and consequently corroded, i.e., the caustic corrosion; below which the equipments would also be corroded due to the low pH values, i.e., the acidic corrosion. Figure 11 displays the changing situation of the process water pH values from the four inspection sites (concrete locations could be referred from Fig. 1) in the year 2009, during which inspection was ceased for over half a month in March to install new pipes for substituting the failure ones within the QO/ DS heat exchanger, while the interval lasting for nearly 2 months from May to July was the routine downtime. Before entering the stripper T-1601, the process water had the lowest pH values among the four (site A, the top-left in Fig. 11) due to the high content of acidic substances like SO2, H2S. After stripping and adding alkali liquor, pH values of the process water before the DS Materials and Corrosion 2011, 62, No. 10 Acidic/caustic alternating corrosion on carbon steel pipes 973 Figure 9. SEM of the cross-sections of the peanut-like concave: (a) total morphology, (b) pits with different morphologies, and (c) subsurface www.matcorr.com
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim