Zhang Zengzhi, Cheng Guoqing and Zhang Limei
Keywords: low-alloy steel, pitting, Mn-enrichment
Abstract:
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Volume 6 Paper C130 Mn-ENRICHMENT AT THE PITTING FRONT END IN 27SiMn LOW-AIIOY STEEl ZHANG Zengzhi CHENG Guoqing ZHANG Limei (China University of Mining and Technology, Beijing Graduate School 100083) ABSTRACT This paper conducts an of investigation of three different sections of a pit on 27SiMn，finds Mn-enrichment at the pitting front end by means of X-ray diffraction and Electron probe and explains Mn-enrichment phenomenon. KEY WORDS low-alloy steel, pitting, Mn-enrichment The masses of experiment data about pitting on steel have been accumulated. The research in pitting growth mechanism adopts a method called “man-made hole model” in many cases. The self- catalytic and acid mechanism of chlorion’s participation has been widely accepted. In this paper, the pitting process on 27SiMn in coal mine is studied by means of Electron probe and X-ray diffraction, and Mn-enrichment at the pitting front end is analyzed. 1. Experiment Samples: hot-rolling 27SiMn (0.24~0.32％C，1.10~1.40％Si，1.10~1.40％Mn), come from failure parts. The process composes of embedding, grinding, polishing and etching. Condition medium: Mine water is contact liquid phase and it’s PH level is 7.0 ~ 7.4. It’s ion concentration (mg/L): 15~28Cl- ,59~190S042-, 226~396HCO3-, 18~36K++Na+，18~35Mg2+， 65~137Ca2+. Contact solid phase is powdered anthracite. These powder or particles come into contact with the samples in mine water. The carbon content of the anthracite is more than 90％. Three different sections of a pit on 27SiMn are analyzed by scanning electron microscopy (JSM-840) and spectrometer (TN-5500). 2. Experiment results and analysis Fig.1 is a cross section of a rim. Fig.2 is a cross section of the closed zone in the same pit. In Fig.2 the line peaks of chlorine are very obvious at the edge of the pit. According to the environmental condition, the chlorine actually stands for chlorine ions (Cl-). This confirms traditional Cl- corrosion mechanism. Fig.3 is a cross section at leading end in the same pit. Fig.4 is X-ray scanning lines in the direction of pit diameter. At the front end of micro-zone, a ring crack is found on the cross section of Fig.3 and Fig.4 shows obviously that Mn-element in low-alloy steel is enriched in the center of cross section on the micro-zone. At the pit front end, Fe atom, comes from crystal lattice, dissolves and oxidizes within the scope of hemisphere and changes into Fe2+. The hydrated Fe2+ ion moves to the pit rim under the action of inner electric field. Mn enriches in the ring crack on cross sections of the pit front end, and its concentration is rather high in the center pitting, the content of Cl, Ca and S is nearly to zero respectively, and this indicates that there are not Cl-, Ca2+, S02- and MnS at this position. Fig.1 Fig.3 Cross Section of a Rim Fig.2 Cross Section at Leading End Fig.4 Cross Section of the Closed Zone X-ray Scanning Lines in the Direction of Pit Diameter In addition, it is difficult for O2 to move to the front end micro-zone. If there were O2 at the position, the whole micro-zone should be composed of Fe and Mn oxides, and Mn oxides alone should not enrich in the center. Mn standard electrode potential φ 0(Mn2+/Mn) is -1.18, Fe standard electrode potential φ0(Fe2+/Fe) is -0.44. Mn has the greater trend of anodic dissolution. It is impossible for Mn to exist in the form of Mn atom. There are strong electric current density, electric intensity and alloy polarization at the front end as a result of great cathode area outside the pit. H2O has two characters: strong polarity and great dielectric constant. Under the action of external electric field, H2O produces orientation effect, its positive side orients negative electrode of electric field and its negative one does positive one. The result is that stronger hydrophily exists and therefore H2O alone concentrates at the pitting front end. There are not Cl-, Ca2+, SO42- hydrated ions to be verified by Electron probe [ZAF] quantitative analysis of micro-zone elements. The following reaction takes place of a normal atmospheric temperature: Mn + 2H2O → Mn(OH)2 + H2 (1) Mn reacts directly with H2O, and produce Mn(OH)2. But Fe atom can’t react directly with H2O. According to pitting electrochemistry, Fe produces electrochemical oxidation as a result of the H2O polarity and longitudinal electric field action. Fe → Fe2+ + 2e (2) 2+ Fe hydrates move to the pit rim in the electric field. With being extracted constantly in crystalic lattice and with Fe2+ migration, Mn is oxidated, and the Mn(OH)2 product is accumulated and concentrates in the center of micro-zone cross sections. The above analysis shows that in process of pit expansion in 27SiMn, Mn atom reacts directly with H2O at the pit front end micro-zone and the Mn(OH)2 product enriches in the center of micro-zone cross sections under the action of Fe dissolution and Fe2+ migration caused by macrogalvanic cell. 2 With pit growth, enriched Mn(OH)2 certainly will reach the closed zone in a pit (Fig.2). According to Cl- self-catalytic and acid mechanism, there is high H+ concentration in the closed zone as a result of Cl- existence. Mn2+ is at the lowest point of Gibbs free energy-oxidation state diagram, but Mn(OH)2 is far higher than Mn2+. This indicates that Mn2+ is the most stable oxidation state in acidis solution. The spontaneous reaction takes place as follows: Mn(OH)2 + 2H+ → Mn2+ + 2H2O (3) 3. Conclusions Pits are found on 27SiMn in mine water. There is Mn-Enrichment at the pitting front end. It is certified that Mn reacts directly with H2O and produces Mn(OH)2 at the micro-zone by means of electron probe. Mn(OH)2 enriches in the center of the micro-zone cross sections under the action of continuous Fe2+ dissolution and migration. With pit growth, Mn(OH)2 reacts with H+ and changes into Mn2+ when it reaches the Cl- closed-zone of a pit. Preference [1] Zhu Rizhang etal. Metallic Corrosion Science，Metallurgy Industry Press, 1989.5 [2] Zuo Jingyi, Corrosion Stress Cracking, Xian Jiaotong University Press, 1985.2 [3] Yan Xuanzhong, Shen Changfu etal. Generla Abiochemistry, Peking University Press, 2002.12: 210 [4] Cao Xizhang, Song Tianyou etal. Abiochemistry, Higher Education Press, 1994.10: 975 [5] Zhang Zengzhi, Yan Jianzhong, Fu Yuewen etal. Corrosion Science and Safeguard Technology, 2001, 13 (6): 328 3