906 Gong, Gao, Meng, and Yang Materials and Corrosion 2009. 60. No. 11 r=0064m droplet @ @ @ c=0536nm MnS inclusion d=0.232nm Figure 13. Schematic diagram of the initiation stage of pitting Figure 12. Schematic diagram of Cr2O, crystal accelerated inward growth of pits in the matrix On the As shown in Fig. 12, the Cr203 passive film of 316L hydrolysis reaction (equation(4)) of the metal cations that were represents a hexagonal closely packed(hcp) crystalline structure. generated in the initiation stage favors a local increase of Compared with the amorphous structure, the crystalline film hydrogen ion concentration, which may introduce an exceedingly contains a high density of imperfections such as grain low pH environment in the pits' interior with the association of boundaries, dislocations and interstices, which act as thethe hydrogen ions originating from HAc [equation (5) transportion paths for small-radius ions through the passive Furthermore, in order to keep electrical neutrality, the chloride film[32]. In a Cr2O3 crystal cell, oxygen ions occupy all the lattice ions migrate inwards in the pits while the ferrous ions(II) in the points while the chromium ions are surrounded by six oxygen pits migrate outwards(Fig. 14). Thus, the aggressive medium in ions in the octahedral structure. It can also be derived from the the corrosion pits is actually the hydrochloric acid(HCl), which literature that the lattice parameters of Cr2O3 hcp crystal cell are will increase the growth rate of pitting. On the other hand, a=0.496 nm and c=0.536 nm[33]. As the radii of oxygen and oxidative metal cationic ions such as Fe and Mo"+ may also chromium ions are 0.132 and 0.064 nm[34], respectively, the accelerate the growth of pitting due to its high reduction potential actual distances between two lattice points are 0. 232 and in association with chloride ions, i.e. the depolarization effect [35] 0. 272 nm, respectively, corresponding to the lattice parameters a and c. Therefore, it is noted that the lattice interstices are not M"++nH20-M(OH)n+nH+ wide enough for a chloride ion with radius of 0. 181 nm, i.e. the diameter is 0.362 nm, to penetrate. Thus, it is obvious that the HAc= h++ac latter theory that ascribes the transport of chloride ions through the Cr2O3 passive film for pitting initiation may not be true In this paper, a novel mechanism, different from the two In fact, the propagation stage is an autocatalytic process theories mentioned above, is put forward. Actually, it can be assisted by the occluded corrosion cell (OCC). It can be learned earned from Fig. 4 that the MnS inclusions are not completely from Fig. 14 that, with the outward migration of ferrous ions(In) beneath the Cr2O3 film. Hence, emphasis should be laid on the fer ic hydroxide [Fe(OH)3) is formed and gets deposited at the direct attack of chloride ions on the MnS inclusions. Sulfur and pits mouth [equation(7)), which results in an occluded area in chlorine being two neighboring elements in the Periodic Table of the pits interior 36). The matrix material exposed to HCl within Elements, their ions possess similar radii, which are 0.174 and 0 181 nm, respectively [34]. Due to the very small difference between the radii of the two ions the sulfide ions in thermodynamically unstable MnS inclusions may be substituted by the chloride ions. As a result, soluble Mn+(cl)2 complex ompounds are produced. Finally, M*(CI ]a dissolves and leads to the formation of Mn-t ions [equation (3)) and the corrosion @ pits(Fig. 13). H)+nH Matrix material: 316L The propagation stage involves the generation and accumu- lation of hydrogen ions within the corrosion pits as well as the Figure 14. Schematic diagram of the propagation stage of pitting o 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim www.matcorr.comAs shown in Fig. 12, the Cr2O3 passive film of 316L represents a hexagonal closely packed (hcp) crystalline structure. Compared with the amorphous structure, the crystalline film contains a high density of imperfections such as grain boundaries, dislocations and interstices, which act as the transportion paths for small-radius ions through the passive film [32]. In a Cr2O3 crystal cell, oxygen ions occupy all the lattice points while the chromium ions are surrounded by six oxygen ions in the octahedral structure. It can also be derived from the literature that the lattice parameters of Cr2O3 hcp crystal cell are a ¼ 0.496 nm and c ¼ 0.536 nm [33]. As the radii of oxygen and chromium ions are 0.132 and 0.064 nm [34], respectively, the actual distances between two lattice points are 0.232 and 0.272 nm, respectively, corresponding to the lattice parameters a and c. Therefore, it is noted that the lattice interstices are not wide enough for a chloride ion with radius of 0.181 nm, i.e. the diameter is 0.362 nm, to penetrate. Thus, it is obvious that the latter theory that ascribes the transport of chloride ions through the Cr2O3 passive film for pitting initiation may not be true. In this paper, a novel mechanism, different from the two theories mentioned above, is put forward. Actually, it can be learned from Fig. 4 that the MnS inclusions are not completely beneath the Cr2O3 film. Hence, emphasis should be laid on the direct attack of chloride ions on the MnS inclusions. Sulfur and chlorine being two neighboring elements in the Periodic Table of Elements, their ions possess similar radii, which are 0.174 and 0.181 nm, respectively [34]. Due to the very small difference between the radii of the two ions, the sulfide ions in thermodynamically unstable MnS inclusions may be substituted by the chloride ions. As a result, soluble Mn2þ½Cl 2 complex compounds are produced. Finally, M3þ½Cl 3 dissolves and leads to the formation of Mn2þ ions [equation (3)] and the corrosion pits (Fig. 13). MnS !HAc=Cl Mn2þþS þ 2e (3)
Propagation The propagation stage involves the generation and accumu￾lation of hydrogen ions within the corrosion pits as well as the accelerated inward growth of pits in the matrix. On the one hand, hydrolysis reaction [equation (4)] of the metal cations that were generated in the initiation stage favors a local increase of hydrogen ion concentration, which may introduce an exceedingly low pH environment in the pits’ interior with the association of the hydrogen ions originating from HAc [equation (5)]. Furthermore, in order to keep electrical neutrality, the chloride ions migrate inwards in the pits while the ferrous ions (II) in the pits migrate outwards (Fig. 14). Thus, the aggressive medium in the corrosion pits is actually the hydrochloric acid (HCl), which will increase the growth rate of pitting. On the other hand, oxidative metal cationic ions such as Fe3þ and Mo2þ may also accelerate the growth of pitting due to its high reduction potential in association with chloride ions, i.e. the depolarization effect [35] MnþþnH2O ! MðOHÞnþnHþ (4) HAc ! HþþAc (5) In fact, the propagation stage is an autocatalytic process assisted by the occluded corrosion cell (OCC). It can be learned from Fig. 14 that, with the outward migration of ferrous ions (II), ferric hydroxide [Fe(OH)3] is formed and gets deposited at the pits’ mouth [equation (7)], which results in an occluded area in the pits interior [36]. The matrix material exposed to HCl within 906 Gong, Gao, Meng, and Yang Materials and Corrosion 2009, 60, No. 11 Figure 13. Schematic diagram of the initiation stage of pitting Figure 14. Schematic diagram of the propagation stage of pitting Figure 12. Schematic diagram of Cr2O3 crystal
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