Materials and Corrosion 2009. 60. No. 1: Pitting corrosion on 316L pipes 903 theoretical morphologies in practice. Indeed, the various pitting morphologies may also have caused combined side effects on the team pipes. On taking a closer look at the corrosion deposits within the pit in Fig 8(b), delamination can be found on the deposits, as seen outer wall in Fig. 9. It is well known that delamination may induce the deposits to be abrased layer by layer under the effect of flow(gas or liquid), which may result due to the acceleration of corrosion rate(detailed mechanisms will be discussed later). Furthermore, chemical compositions of the corrosion deposits were detected by energy dispersive spectroscopy (EDS). The four analysis sites are displayed in Fig. 9 as well, and the chemical compositions Accv Spot listed in Fig. 10 and Table 2. According to the results of the 200kv50 analysis, it is surprising that the contents of chlorine element 21, and 2.60%(wt%), respectively at sites A, C and D. Moreover, 1.90% of the content was bromine element at site C. Hence, the EDS results verified the hypothesis proposed above that the pitting corrosion was led by the presence of halide inner wall 2.4 lon chromatography In order to further testify the EDS results, ion chromatography C)was also used to identify the chemical compositions of the corrosion deposits within the pits. When dissolved in deionized water, corrosion deposits released the chloride and bromide ions they contained. Fig. 11 shows that the concentrations of the .5 and 1.0 ppm, respectively Figure 6 SEM of the cross-sections of pitted pipes Although the halide ions may not induce serious corrosion with such a low content at first, they will lead the pits to grow deep with the smooth inner wall, the outer wall was defected by some inwards in the matrix material when it is accumulated in the pits corrosion pits, whose depth had reached about 0.4 mm. As is well Thus, the results of ion chromatography further confirmed the known,there are seven theoretical morphologies of pitting, causes of pitting, the halide ions namely, narrow and deep, elliptical, wide and shallow, subsurface, undercutting, horizontal, and vertical (8). Schematic diagrams of 3 Discussion the seven theoretical morphologies are displayed in Fig. 7. In this paper, it can be seen from Fig. 8 that six out of the seven In 316L stainless steel halide ions attacking the MnS inclusions theoretical pitting morphologies were obtained from the pitted on the steam pipes' surface was the major cause of pitting.Two pipes under SEM, which may provide a solid proof for the more reasons could be attributed for this failure: whether or not the matrix material was qualified and where the halide ions originated. Hence, research was carried out to study three aspects to identify the specific causes of pitting: the matrix materials, the process media and the service conditions. According to the analysis results, chemical compositions of the steam pipes matrix Wide shallow material conformed to the SUS 316L specifications, and the metallographic structure was typically austenite. Thus, it can be concluded that the matrix material was qualified; in other words, causes for the serious pitting corrosion may involve the latter two the process media and /or the service conditions. As was discussed above, the process media in the TA dryer onsisted mainly of HAc, bromide ions, and chloride ions Furthermore, HAc and the bromide ions were generated from the oxidation of the catalyst system, while the chloride ions were the remnants from the NaoH alkaline wash liquor. In fact, it can be inferred from the eds and ic results for the chemical compositions of the corrosion deposits that the chloride ions were the main cause of pitting. The aggressive chloride ions Figure 7 Schematic diagram of theoretical pitting morphologies commonly have two side effects on materials: SCC and pitting www.matcorr.com c 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheimwith the smooth inner wall, the outer wall was defected by some corrosion pits, whose depth had reached about 0.4 mm. As is well known, there are seven theoretical morphologies of pitting, namely, narrow and deep, elliptical, wide and shallow, subsurface, undercutting, horizontal, and vertical [8]. Schematic diagrams of the seven theoretical morphologies are displayed in Fig. 7. In this paper, it can be seen from Fig. 8 that six out of the seven theoretical pitting morphologies were obtained from the pitted pipes under SEM, which may provide a solid proof for the theoretical morphologies in practice. Indeed, the various pitting morphologies may also have caused combined side effects on the steam pipes. On taking a closer look at the corrosion deposits within the pit in Fig. 8(b), delamination can be found on the deposits, as seen in Fig. 9. It is well known that delamination may induce the deposits to be abrased layer by layer under the effect of flow (gas or liquid), which may result due to the acceleration of corrosion rate (detailed mechanisms will be discussed later). Furthermore, chemical compositions of the corrosion deposits were detected by energy dispersive spectroscopy (EDS). The four analysis sites are displayed in Fig. 9 as well, and the chemical compositions are listed in Fig. 10 and Table 2. According to the results of the analysis, it is surprising that the contents of chlorine element were 7.17, 5.36, 8.21, and 2.60% (wt%), respectively at sites A, B, C, and D. Moreover, 1.90% of the content was bromine element at site C. Hence, the EDS results verified the hypothesis proposed above that the pitting corrosion was led by the presence of halide ions. 2.4 Ion chromatography In order to further testify the EDS results, ion chromatography (IC) was also used to identify the chemical compositions of the corrosion deposits within the pits. When dissolved in deionized water, corrosion deposits released the chloride and bromide ions they contained. Fig. 11 shows that the concentrations of the chloride and the bromide ions are 10.5 and 1.0 ppm, respectively. Although the halide ions may not induce serious corrosion with such a low content at first, they will lead the pits to grow deep inwards in the matrix material when it is accumulated in the pits. Thus, the results of ion chromatography further confirmed the causes of pitting, the halide ions. 3 Discussion In 316L stainless steel halide ions attacking the MnS inclusions on the steam pipes’ surface was the major cause of pitting. Two more reasons could be attributed for this failure: whether or not the matrix material was qualified and where the halide ions originated. Hence, research was carried out to study three aspects to identify the specific causes of pitting: the matrix materials, the process media and the service conditions. According to the analysis results, chemical compositions of the steam pipes matrix material conformed to the SUS 316L specifications, and the metallographic structure was typically austenite. Thus, it can be concluded that the matrix material was qualified; in other words, causes for the serious pitting corrosion may involve the latter two: the process media and/or the service conditions. As was discussed above, the process media in the TA dryer consisted mainly of HAc, bromide ions, and chloride ions. Furthermore, HAc and the bromide ions were generated from the oxidation of the catalyst system, while the chloride ions were the remnants from the NaOH alkaline wash liquor. In fact, it can be inferred from the EDS and IC results for the chemical compositions of the corrosion deposits that the chloride ions were the main cause of pitting. The aggressive chloride ions commonly have two side effects on materials: SCC and pitting Materials and Corrosion 2009, 60, No. 11 Pitting corrosion on 316L pipes 903 Figure 6. SEM of the cross-sections of pitted pipes Figure 7. Schematic diagram of theoretical pitting morphologies www.matcorr.com
2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim