Materials and Corrosion 2009. 60. No. 1: Pitting corrosion on 316L pipes 907 the pits serves as the anode due to its activated status, whil on the steam pipes surface with matrix material of standard the Cr2O3 passive film outside the pits acts as the cathode. Such a 316L austenitic stainless steel in Ta dryer small anode-large cathode oCC will spontaneously cause the 2. The harsh environment, ie. the high temperature of about pits to grow inwards in the matrix in a deep and narrow form. 130C and the FAc effect around the inlet region of TA dryer Thus, the hydrogen ions in the pits'interior continuously attack were the main factors for the acceleration of pitting corrosion. the matrix to produce metal cations, which hydrolyze to generate 3. The small-radius chloride ions(0. 181 nm) substituting for the ogen ions in turn, l.e. sulfide ions (0. 174 nm)of the soluble Mn+(CI]2 complex compe initiation of pitting Fe2++2OH→Fe(OH)2 (6) 4. Six types of pitting morphologies were obtained in engineer ing practice to prove and enrich all the seven theoretical Fe(OH)2+2H2O+O2→4Fe(OH)3↓ 5. Several countermeasures such as limiting the chloride Termination concentration in the alkaline wash liquor under 30 ppm ar good control of the service temperature in TA dryer were The termination stage can also be defined as the repassiva- implemented to mitigate the extent of pitting. tion of the matrix metal in the corrosion pits. According to Pardo et al. [30], highly stable and insoluble compounds like FeMnO4 ondition of low PH and high potential val bits under the Acknowledgements: This work was supported by both Shanghai and MoO3 form and cover the wall of the corrosion As a result Petrochemical Co, Ltd. and Shanghai Leading Academic the potential values within the pits will then transfer to the Discipline Project(Project no. B113) passivation region; in other words, the repassivation begins and the pitting growth terminates. 5 References Regarding the corrosion rate of 316L steel in the acetate acid taining halide ions, some experiments on the corrosion depth [1] H. Macarie, A Noyola, J. P. Guyot, Water Sci. Technol. 1992, were carried out[37, 38). Mattsson[39] defined the corrosion rate 25,223 of qualified stainless steel to be not more than 0.1 mmy.[2] Y.H. Liu, R.-G. Chen, Q. Zheng, Chem. Eng. Des. 2000, 10, Fig 6(a) provides the evidence of thinning of the pitted pipes, for 29(in Chinese) a thickness of 2.74 mm compared with its original value of [3]N Pernicone, M. Cerboni, G. Prelazzi, F. Pinna, G. Fagher 3.0mm Service conditions of the steam pipes may be attributed azzi, Catal. Today 1998, 44, 129-135 cause for such a serious thinning As shown in Fig. 2, the steam [4]G.R. Pophali, R. Khan, R.S. Dhodapkar, T. Nandy, S pipes in the inlet region of TA dryer contact both the wet TA cakes Devotta,J. Environ. Manage. 2007, 85, 1024 and the carrier gas, which may lead to a two-phase (gas and liquid) [5] M.H. Moayed, M. Golestanipour, Mater. Corros. 2005, 56, 39 FAC on the steam pipes' surface due to the high humidity of the [6] B. R. Tzaneva, L. B. Fachikov, RG.Raicheff,Corros. Eng. Sci wet TA cakes[40, 41]. FAC is a kind of corrosion process of Technol.2006,41,62. chemical dissolution of metal that always leads to thinning of (P Poyet, P. Couchinave, J. Hahn, BSaulnier, JYBoos, Mem. Sci. Rev. Met. 1975. 72, 133. on the surface will be removed layer by layer by high-low.-rate gas (8) ASTM G46-94-1999, Standard Guide for Examination and metastable pipes surface with delaminated corrosion deposits [9)M. Suresh Kumgang Corrosion. may account for the acceleration effect from FAC. In this case, the r,M. Sujata, M. A. Venkataswamy, S.K. within the pits(Fig 9)was abrased by carrier gas and it eventually Bhaumik, Eng. Fail. Anal. 2008, 15, 497. resulted in thinning of its wall thickness. Thus, the cause for the [10]T].Hakkarainen, Mater.Corros.2003, 54,503 serious pitting corrosion taking place only at the inlet region [12]G. T. Burstein, C. Liu, R. M. Souto, S. P. Vines, Corros. Eng Sci. Technol. 2004. 39. 25 relatively high temperature near 130C is also a critical factor [13]X. Shi, R. Avci, M Geiser, Corros. Sci. 2003,45, 2577 inducing the acceleration effect. According to Hou Feng's 37] [141. K. Glenn, M H. Gold, Arch. Biochem Biophys. 1985, 242, experiments, the corrosion rate of 316L steel under the environment stimulated from the actual service condition of [15]C.R. McCall, M. A. Hill, R.S. Lillard, Corros. Eng. Sci TA dryer(26.2% TA, 0.96% Br", 67. 11% HAc(wt%)at 110 Technol.2005,40.337 ached even 1 mmy. Finally it can be concluded that the [16] B J. Little, P. A. Wagner, Z Lewandowski, presented at Pre rious corrosion situation and a quite high corrosion rate on the Corrosion 99 NACE. Houston. USA inlet region of steam pipes resulted from the interaction between [17] P. Ernst, R. C. Newman, Corros. Sci. 2002, 44, 927 pitting, FAC, and high temperature 18 P. Ernst, R. C. Newman, Corros. Sci. 2002, 44, 943 [19 X. Phili 4 Conclusions L. C. Dufour, E. Finot, Corros. Sci. 2003, 45, 1143. [20]J. S Noh, N. J. Laycock, W. Gao, B. Wells, Corros. Sci. 2000. 1. Chloride ions from the NaoH alkaline wash liquor were the 42,2069 ry factor for the occurrence of pitting which took [21] A. Rossi, R. Tulifero, B Elsener, Mater. Corros. 2001, 52,175 www.matcorr.com c 2009 WILEY-VCH Verlag GmbH & Co KGaA, Weinheimthe pits serves as the anode due to its activated status, while the Cr2O3 passive film outside the pits acts as the cathode. Such a ‘small anode–large cathode’ OCC will spontaneously cause the pits to grow inwards in the matrix in a deep and narrow form. Thus, the hydrogen ions in the pits’ interior continuously attack the matrix to produce metal cations, which hydrolyze to generate hydrogen ions in turn, i.e. the autocatalytic process. Fe2þþ2OH ! FeðOHÞ2 (6) FeðOHÞ2þ2H2O þ O2 ! 4FeðOHÞ3 # (7)
Termination The termination stage can also be defined as the repassiva￾tion of the matrix metal in the corrosion pits. According to Pardo et al. [30], highly stable and insoluble compounds like FeMnO4 and MoO3 form and cover the wall of the corrosion pits under the condition of low pH and high potential values. As a result, the potential values within the pits will then transfer to the passivation region; in other words, the repassivation begins and the pitting growth terminates. Regarding the corrosion rate of 316L steel in the acetate acid containing halide ions, some experiments on the corrosion depth were carried out [37, 38]. Mattsson [39] defined the corrosion rate of qualified stainless steel to be not more than 0.1 mmy1 . Fig. 6(a) provides the evidence of thinning of the pitted pipes, for a thickness of 2.74 mm compared with its original value of 3.0 mm. Service conditions of the steam pipes may be attributed cause for such a serious thinning. As shown in Fig. 2, the steam pipes in the inlet region of TA dryer contact both the wet TA cakes and the carrier gas, which may lead to a two-phase (gas and liquid) FAC on the steam pipes’ surface due to the high humidity of the wet TA cakes [40, 41]. FAC is a kind of corrosion process of chemical dissolution of metal that always leads to thinning of pipes. That the corrosion deposits produced by aggressive liquid on the surface will be removed layer by layer by high-flow-rate gas may account for the acceleration effect from FAC. In this case, the metastable pipes’ surface with delaminated corrosion deposits within the pits (Fig. 9) was abrased by carrier gas and it eventually resulted in thinning of its wall thickness. Thus, the cause for the serious pitting corrosion taking place only at the inlet region rather than at other parts of the steam pipes can be explained. The relatively high temperature near 130 8C is also a critical factor inducing the acceleration effect. According to Hou Feng’s [37] experiments, the corrosion rate of 316L steel under the environment stimulated from the actual service condition of TA dryer (26.2% TA, 0.96% Br, 67.11% HAc (wt%) at 110 8C) reached even 1 mmy1 . Finally it can be concluded that the serious corrosion situation and a quite high corrosion rate on the inlet region of steam pipes resulted from the interaction between pitting, FAC, and high temperature. 4 Conclusions 1. Chloride ions from the NaOH alkaline wash liquor were the primary factor for the occurrence of pitting which took place on the steam pipes’ surface with matrix material of standard 316L austenitic stainless steel in TA dryer. 2. The harsh environment, i.e. the high temperature of about 130 8C and the FAC effect around the inlet region of TA dryer were the main factors for the acceleration of pitting corrosion. 3. The small-radius chloride ions (0.181 nm) substituting for the sulfide ions (0.174 nm) of MnS inclusions and forming the soluble Mn2þ½Cl 2 complex compounds may favor the initiation of pitting. 4. Six types of pitting morphologies were obtained in engineer￾ing practice to prove and enrich all the seven theoretical morphologies. 5. Several countermeasures such as limiting the chloride ion concentration in the alkaline wash liquor under 30 ppm and good control of the service temperature in TA dryer were implemented to mitigate the extent of pitting. Acknowledgements: This work was supported by both Shanghai Petrochemical Co., Ltd. and Shanghai Leading Academic Discipline Project (Project no. B113). 5 References [1] H. Macarie, A. Noyola, J. P. Guyot, Water Sci. Technol. 1992, 25, 223. [2] Y.-H. Liu, R.-G. Chen, Q. Zheng, Chem. Eng. Des. 2000, 10, 29 (in Chinese). [3] N. Pernicone, M. Cerboni, G. Prelazzi, F. Pinna, G. Fagher￾azzi, Catal. Today 1998, 44, 129–135. [4] G. R. Pophali, R. Khan, R. S. Dhodapkar, T. Nandy, S. Devotta, J. Environ. Manage. 2007, 85, 1024. [5] M. H. Moayed, M. Golestanipour, Mater. Corros. 2005, 56, 39. [6] B. R. Tzaneva, L. B. Fachikov, R. G. Raicheff, Corros. Eng. Sci. Technol. 2006, 41, 62. [7] P. Poyet, P. Couchinave, J. Hahn, B. Saulnier, J. Y. Boos, Mem. Sci. Rev. Met. 1975, 72, 133. [8] ASTM G46-94-1999, Standard Guide for Examination and Evaluation of Pitting Corrosion. [9] M. Suresh Kumar, M. Sujata, M. A. Venkataswamy, S. K. Bhaumik, Eng. Fail. Anal. 2008, 15, 497. [10] T. J. Hakkarainen, Mater. Corros. 2003, 54, 503. [11] A. Pardo, E. Otero, M. C. Merino, Mater. Corros. 2000, 51, 850. [12] G. T. Burstein, C. Liu, R. M. Souto, S. P. Vines, Corros. Eng. Sci. Technol. 2004, 39, 25. [13] X. Shi, R. Avci, M. Geiser, Corros. Sci. 2003, 45, 2577. [14] J. K. Glenn, M. H. Gold, Arch. Biochem. Biophys. 1985, 242, 329. [15] C. R. McCall, M. A. Hill, R. S. Lillard, Corros. Eng. Sci. Technol. 2005, 40, 337. [16] B. J. Little, P. A. Wagner, Z. Lewandowski, presented at Proc. Corrosion ’99, NACE, Houston, USA, 1999, pp. 294. [17] P. Ernst, R. C. Newman, Corros. Sci. 2002, 44, 927. [18] P. Ernst, R. C. Newman, Corros. Sci. 2002, 44, 943. [19] B. Vuillemin, X. Philippe, R. Oltra, V. Vignal, L. Coudreuse, L. C. Dufour, E. Finot, Corros. Sci. 2003, 45, 1143. [20] J. S. Noh, N. J. Laycock, W. Gao, B. Wells, Corros. Sci. 2000, 42, 2069. [21] A. Rossi, R. Tulifero, B. Elsener, Mater. Corros. 2001, 52, 175. Materials and Corrosion 2009, 60, No. 11 Pitting corrosion on 316L pipes 907 www.matcorr.com
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