976 Gong, Yang, Yao and Yang Materials and Corrosion 2011.62. No. solution as well as the presence of chloride ions and sulfur element it can be inferred that pitting corrosion was favored when the ph the process water, were actually the true causes of the failure In values dropped to the acidic region below 7, especially the extreme other words, the process media, i.e., the process water should be value near 5. Under this condition, the aggressive chloride ions mainly blamed for. In fact, the unqualified process water was penetrated into and then accumulated within the pre- existin virtually resulted from improper maintenance management. Then, defects of the passive films on the pipes surfaces, such as discussion will be especially put forward on these two aspects. inclusions, imperfections, pores, etc, even the cementites on the With regard to the process water, the most distinct ferritic grain boundaries resulted from pearlites dissolution. characteristic was the sharp fluctuations of its PH values lasting Consequently, cavities in size of 1-2 um, as shown in Fig. d-f, were for the whole year of 2009. As was discussed above that an engendered. This is exactly the initial stage of the acidic/ caustic alkalescent range of 7.5-9.5 of the process water pH values could alternating corrosion, i.e., the first level of the four-level ensure the pipes in the Qo/DS heat exchanger free of acidic and mechanism, whose schematic diagram, causes of formation, caustic corrosion. However in this event, the frequent and violent and other features are all summarized in Table 4. With respect to fluctuations of the pH values beyond the range would induce a the detailed procedures of pitting corrosion caused by chloride kind of alternating corrosion effect between the acidic and the ions, our previous work [5] can be referenced, hence they would not caustic corrosion on the pipes, which we nominated the term of be repeatedly discussed. As for the sources of the chloride ions in acidic/caustic alternating corrosion. This effect commonly brings the process water, insufficient desalination could be ascribed to about corrosion fatigue on materials and is consequently severer However, it was indeed because of the low content of these chloride than the added result of the two independent ones. In order to ions, extent of the pitting corrosion in this event was actually not detailedly explain the concrete procedures of this acidic/caustic severe, and the affected zones only scattered in some specific alternating corrosion, a novel four-level mechanism from sites on the pipe surface. Besides this, the sulfur element was microscopic scale to macroscopic scale was put forward introduced by the residual SO2 and H2S, and was predominantly in In engineering practice, carbon steel pipes should usually form of sulfate radical Soa- in the process water, i.e., virtually the undergo a caustic treatment procedure before application to sulfuric acid H2SO4 under acidic environment. Consequently produce a protective passive film of Fe3 O4 on the surface, seen in the pitting corrosion effect was aggravated to some extent, and the the Equation(1). However, it is a common sense that steels bearing corrosion products were involved with sulfur element as well, seen assive films, mainly refer to the stainless steels, are prone to be from the EDS results in Fig. 10 and Table 2 attacked by pitting corrosion with the presence of halide ions. In this event, chloride element/ions were detected in the process Fe+2H-→Fe(OH)2+ rater through analyses of EDS and IC. Although it may be 4Fe(0H),+2H20+02-4Fe(OH challenged that the content of them was not high enough, failure Fe(OH),+2Fe(OH)2+Fe304+4H,0 case of pitting corrosion caused by localized accumulation of low oncentration chloride ions was actually investigated in our previous work [5, 6]. Figure 9 is the obvious evidence for the Actually, it could be easily learned from Table 3 that duration occurrence of pitting corrosion on the failure pipe in this event. of the process water pH values in the caustic region was far longer What is more, associated with the service conditions of the pipes, than that in the acidic region, in other words, the caustic i.e., the frequent sharp fluctuations of the process water pH values, corrosion was severer than the acidic (pitting) corrosion on the Table 4. Summary of the four-level mechanisms Schematic diagram Causes of formation erm Feature size Level Connection of pit Concave o mm Caustic embrittlement Connection of coins pH>7: anodic dissolution pH<7: hydrogen embrittlement pitting com Cavity o 2011 WILEY-VCH Verlag Gmbh Co KGaA, Weinheim www.matcorr.comsolution as well as the presence of chloride ions and sulfur element in the process water, were actually the true causes of the failure. In other words, the process media, i.e., the process water should be mainly blamed for. In fact, the unqualified process water was virtually resulted from improper maintenance management. Then, discussion will be especially put forward on these two aspects. With regard to the process water, the most distinct characteristic was the sharp fluctuations of its pH values lasting for the whole year of 2009. As was discussed above that an alkalescent range of 7.5–9.5 of the process water pH values could ensure the pipes in the QO/DS heat exchanger free of acidic and caustic corrosion. However in this event, the frequent and violent fluctuations of the pH values beyond the range would induce a kind of alternating corrosion effect between the acidic and the caustic corrosion on the pipes, which we nominated the term of acidic/caustic alternating corrosion. This effect commonly brings about corrosion fatigue on materials and is consequently severer than the added result of the two independent ones. In order to detailedly explain the concrete procedures of this acidic/caustic alternating corrosion, a novel four-level mechanism from microscopic scale to macroscopic scale was put forward. In engineering practice, carbon steel pipes should usually undergo a caustic treatment procedure before application to produce a protective passive film of Fe3O4 on the surface, seen in the Equation (1). However, it is a common sense that steels bearing passive films, mainly refer to the stainless steels, are prone to be attacked by pitting corrosion with the presence of halide ions. In this event, chloride element/ions were detected in the process water through analyses of EDS and IC. Although it may be challenged that the content of them was not high enough, failure case of pitting corrosion caused by localized accumulation of low￾concentration chloride ions was actually investigated in our previous work [5, 6]. Figure 9 is the obvious evidence for the occurrence of pitting corrosion on the failure pipe in this event. What is more, associated with the service conditions of the pipes, i.e., the frequent sharp fluctuations of the process water pH values, it can be inferred that pitting corrosion was favored when the pH values dropped to the acidic region below 7, especially the extreme value near 5. Under this condition, the aggressive chloride ions penetrated into and then accumulated within the pre-existing defects of the passive films on the pipes surfaces, such as inclusions, imperfections, pores, etc., even the cementites on the ferritic grain boundaries resulted from pearlites dissolution. Consequently, cavities in size of 1–2mm, as shown in Fig. d–f, were engendered. This is exactly the initial stage of the acidic/caustic alternating corrosion, i.e., the first level of the four-level mechanism, whose schematic diagram, causes of formation, and other features are all summarized in Table 4. With respect to the detailed procedures of pitting corrosion caused by chloride ions, our previous work [5] can be referenced, hence they would not be repeatedly discussed. As for the sources of the chloride ions in the process water, insufficient desalination could be ascribed to. However, it was indeed because of the low content of these chloride ions, extent of the pitting corrosion in this event was actually not severe, and the affected zones only scattered in some specific sites on the pipe surface. Besides this, the sulfur element was introduced by the residual SO2 and H2S, and was predominantly in form of sulfate radical SO2 4 in the process water, i.e., virtually the sulfuric acid H2SO4 under acidic environment. Consequently, the pitting corrosion effect was aggravated to some extent, and the corrosion products were involved with sulfur element as well, seen from the EDS results in Fig. 10 and Table 2. Fe þ 2OH ! FeðOHÞ2 þ 2e 4FeðOHÞ2 þ 2H2O þ O2 ! 4FeðOHÞ3 FeðOHÞ2 þ 2FeðOHÞ3 ! Fe3O4 þ 4H2O (1) Actually, it could be easily learned from Table 3 that duration of the process water pH values in the caustic region was far longer than that in the acidic region, in other words, the caustic corrosion was severer than the acidic (pitting) corrosion on the 976 Gong, Yang, Yao and Yang Materials and Corrosion 2011, 62, No. 10 Table 4. Summary of the four-level mechanisms Schematic diagram Causes of formation Term Feature size Level Scale Connection of pits Concave 3 mm 4 Macroscopic Connection of coins Caustic embrittlement Pit 0.1 mm 3 pH > 7: anodic dissolution Coin 10mm 2 Microscopic pH < 7: hydrogen embrittlement Acidic environment Cl: pitting corrosion Cavity 2mm 1 S: acidic corrosion
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