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A Study on the Initial Corrosion Behavior of Carbon Steel Exposed to a Simulated Coastal-Industrial Atmosphere
Mingxiao GUO, Chen PAN, Zhenyao WANG, Wei HAN
Acta Metall Sin    2018, 54 (1): 65-75.   doi:10.11900/0412.1961.2017.00142
Accepted: 14 July 2017

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Carbon steels as common structural material have been widely used for basic facilities with the development of the city. In these service environments, carbon steel would inevitably encounter the atmospheric corrosion. Especially, the corrosion of carbon steels exposed to coastal-industrial atmosphere is very outstanding. However, the initial corrosion mechanism of carbon steel subjected to coastal-industrial environment still need to be clarified, which would be vital for predicating the subsequent corrosion process. In addition, although many scholars studied the synergism of SO2 and Cl-, which obviously accelerates the corrosion of steel and reduces its service life, there is few research about the effect of the synergism of SO2 and Cl- (in different proportion) on the early corrosion behavior of the carbon steel. Therefore, it is of great importance to investigate the initial corrosion mechanism of carbon steel and the effect of the synergism of SO2 and Cl- (in different proportion) in the coastal-industrial atmosphere. In present work, the initial corrosion behavior of Q235 carbon steel exposed to a simulated coastal-industrial atmosphere has been studied by weight loss, XRD, SEM and electrochemical measurements. Also, the effect of the synergism of SO2 and Cl- (in different proportion) on the early corrosion behavior of Q235 car bon steel has been investigated. The results indicate that the initial corrosion behavior of carbon steel exposed to a simulated coastal-industrial atmosphere presented a transition from corrosion acceleration to deceleration, and the kinetics of accelerated corrosion process followed the empirical equation D=Atn. A double-layered corrosion product was formed on the surface of carbon steel after 24 h: the loose outer layer and relative dense inner layer; the synergistic effect between SO2 and Cl- accelerated the corrosion of carbon steel. However, the change in the ratio of SO2 and Cl- had no significant effect on the corrosion loss of carbon steel, and had not changed the composition of corrosion products formed on carbon steel surface. SO2 caused the corrosion morphology of carbon steel to tend to uniform corrosion.

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In Vitro Corrosion Resistance of Ta2N Nanocrystalline Coating in Simulated Body Fluids
Jiang XU, Xike BAO, Shuyun JIANG
Acta Metall Sin    2018, 54 (3): 443-456.   doi:10.11900/0412.1961.2017.00246
Accepted: 19 December 2017

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Due to its combination of outstanding characteristics, such as superior biocompatibility, excellent mechanical properties as well as good corrosion resistance, Ti-6Al-4V alloy has gained much attention as one of the most popular load-bearing biomedical metals in the area of orthopedic and dental. Unfortunately, Ti-6Al-4V alloy suffers from the localized corrosion damage in human body ?uids containing high chloride ion concentrations, which leads to the release of metal ions into the human body. The released ions (e.g., Al and V) are found to not only cause allergic and toxic reactions but also exhibit potential negative effects on osteoblast behavior. To improve the corrosion resistance of Ti-6Al-4V alloy in simulated body ?uids, a 40 μm thick Ta2N nanocrystalline coating with an average grain size of 12.8 nm was engineered onto a Ti-6Al-4V substrate using a double cathode glow discharge technique. The hardness and elastic modulus of the Ta2N coating were determined to be (32.1±1.6) GPa and (294.8±4.2) GPa, respectively, and the adhesion strength of the coating deposited on Ti-6Al-4V substrate was found to be 56 N. There is no evidence of crack formation within the coating under loads ranging from 0.49 N to 9.8 N, implying that the Ta2N nanocrystalline coating has a high contact damage resistance. Moreover, the corrosion resistance of the Ta2N nanocrystalline coating is significantly greater than that of Ti-6Al-4V alloy when tested in naturally aerated Ringer's solution at 37 ℃. This is due to that the passive film developed on the coating has superior compactness compared with that formed on the uncoated Ti-6Al-4V alloy. XPS analysis indicated that at a low polarized potential, the passive film consisted of TaOxNy, which would be converted to Ta2O5 at a higher polarized potential. The analysis of Mott-Schottky curves suggested that the passive film formed on the coating exhibits n-type semiconductor properties and, as such, the density and diffusivity of carrier for the coating was considerably lower than that for the uncoated Ti-6Al-4V alloy.

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Investigation of Corrosion Behavior of Welded Joint of X70 Pipeline Steel for Deep Sea
Ge MA, Xiurong ZUO, Liang HONG, Yinglun JI, Junyuan DONG, Huihui WANG
Acta Metall Sin    2018, 54 (4): 527-536.   doi:10.11900/0412.1961.2017.00149
Accepted: 12 October 2017

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X70 pipeline steel with thick specifications (40.5 mm) for 3500 m deep sea reached the international advanced level in the wall thickness and service depth. Due to the high heat input during the welding process, the corrosion resistance of inside welding and outside welding would vary depending on the microstructure differences. The corrosion resistance of the welded joints of X70 pipeline for deep sea was studied by the immersion test, the weight loss test, the electrochemical test in this work. The components of the passive film were analyzed by XRD and the microstructure was observed by SEM. The results show that the corrosion resistance of the weld metal is the best. The corrosion resistance of the heat affected zone follows. The corrosion resistance of the base metal is the worst. And for the same area, the corrosion resistance of the inside welding is better than that of the outside welding. The formation of dense Fe3O4 passivation film can effectively slow down the progress of the reaction, and the corrosion products of Fe2O3, FeOOH and Fe(OH)3 which are loose in the outer layer, have no protective effect on the matrix. The microstructure of the weld metal with the best corrosion resistance is mostly the intragranular nucleation ferrite and martensite-austenite (M-A) constituent is fine and uniform. The microstructure gradient of the heat affected zone is the largest, the M-A constituent is coarse and the corrosion resistance is inferior to the weld metal. The base metal consists of ferrite and bainite, the bainite is island-like distribution and the corrosion resistance is the worst. Microstructure of the inside welding is more refined, owing to the influence of outside welding thermal cycle, and the volume fraction of M-A constituent in inside welding is higher than that of the outside welding, so the corrosion resistance is better than that of the outside welding.

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Crevice Corrosion of X70 and 3Cr Low Alloy Steels Under Supercritical CO2 Condition
Jianan ZOU, Xiaolu PANG, Kewei GAO
Acta Metall Sin    2018, 54 (4): 537-546.   doi:10.11900/0412.1961.2017.00353
Accepted: 04 January 2018

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With the exploitation of high pressure gas fields and the development of carbon capture and storage (CCS) techniques, the corrosion problem of steels under CO2 environment has been paid more and more attention. To transportation easier and cost reduction, CO2 in pipelines and containers is usually pressured to a high pressure, such as supercritical state. The supercritical CO2 corrosion environment includes the CO2-saturated aqueous phase and the water-saturated supercritical CO2 (SC CO2) phase. Moreover, corrosive ions such as Cl- usually exist in CO2 corrosion environment, which could accelerate the occurrence of corrosion. Low alloy steels, widely used as pipelines and construction materials in oil/gas and CCS industries, are susceptible to corrosion in the aggressive environment that contains high-concentration ions and acidic gases, especially to severe localized corrosion. In this work, the crevice corrosion behavior of 3Cr and X70 steels exposed in supercritical CO2-saturated 3.5%NaCl solution and NaCl solution-saturated supercritical CO2 phase was investigated. SEM, EDS and 3D laser microscopy were used to analyze the corrosion product scale on the steel surface. The results show that both the steels occurred crevice corrosion on the edge of crevice, but slightly occurred corrosion inside the crevice. The crevice corrosion occurred due to the galvanic effect of areas inside and outside the crevice. In supercritical CO2 phase, 3Cr steel exhibited a higher uniform corrosion resistance than X70 steel, while the crevice corrosion resistance of 3Cr steel was lower than that of X70 steel. The different crevice corrosion behaviors between X70 and 3Cr steels might be attributed to the synergistic effect of elements Cr and Cu on enhancing the crevice corrosion.

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Preparation and Enhanced Hot Corrosion Resistance of aZr-Doped PtAl2+(Ni, Pt)Al Dual-Phase Coating
Chengyang JIANG, Yingfei YANG, Zhengyi ZHANG, Zebin BAO, Shenglong ZHU, Fuhui WANG
Acta Metall Sin    2018, 54 (4): 581-590.   doi:10.11900/0412.1961.2017.00376
Accepted: 29 January 2018

Abstract94)   HTML8)    PDF (7127KB)(383)      

Prior to practical service, hot-section components (e.g. airfoils and vanes) of a gas turbine engine are necessarily coated by a protective metallic coating (such as aluminide diffusion coating, modified aluminide coating and MCrAlY overlay etc.) to resist high temperature oxidation and hot corrosion. Among the modified aluminide coatings, the coating with Pt-modification has attracted great attention and is widely used in applications requiring high reliability and extended service life since it possesses superior oxidation/corrosion resistance at high temperature. The presence of Pt in aluminide coating is favorable for increasing bonding strength of oxide scale, enlarging phase region of β-NiAl and confining detrimental effect of sulphur etc. Although Pt-modification has exhibited visible benefits for acquiring better high-temperature performance, it is far from satisfaction to develop an ideal aluminide diffusion coating. Reactive elements such as Y, Hf, Zr or their oxides have been employed to modify the nickel aluminide coating system, with an aim to further improve scale adhesion and promote exclusive formation of α-Al2O3 simultaneously. In this work, a Zr-doped PtAl2+(Ni, Pt)Al dual-phase aluminide coating was prepared on a Ni-based single crystal superalloy by co-deposition of Pt-Zr through electroplating and subsequent aluminization treatments. The coating was mainly composed of three layers: the outmost layer consisted of double phases with PtAl2 particles dispersed in β-(Ni, Pt)Al domain, while the interlayer comprised β-(Ni, Pt)Al with small amount of Cr-precipitates, and the bottom layer was an inter-diffusion zone (IDZ). Zirconium was mainly distributed inside β-(Ni, Pt)Al solid solution in both the outmost layer and the interlayer. Compared with normal PtAl2+(Ni, Pt)Al dual-phase coating, the hot corrosion behavior of the Zr-doped PtAl2+(Ni, Pt)Al coating was assessed in a salt mixture of Na2SO4/NaCl (75:25, mass ratio) at 850 ℃ in static air. The results indicated that the Zr-doped PtAl2+(Ni, Pt)Al dual-phase coating exhibited superior hot corrosion resistance since Zr was confirmed able to capture and fix S and Cl to diminish their detrimental effects. Meanwhile, a pre-oxidation treatment did not effectively improve the overall hot corrosion resistance of normal PtAl2+(Ni, Pt)Al coating because the thin alumina scale formed during pre-oxidation was unable to prohibit the sustained inward-invasion of the mixed salt.

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Study of Interference Parameters Variation Regularity and Corrosion Behavior of X80 Steel in Guangdong Soil Under High Voltage Direct Current Interference
Runzhi QIN, Yanxia DU, Minxu LU, Li OU, Haiming SUN
Acta Metall Sin    2018, 54 (6): 886-894.   doi:10.11900/0412.1961.2017.00311
Accepted: 20 November 2017

Abstract75)   HTML1)    PDF (1907KB)(413)      

High voltage direct current transmission (HVDC) systems develop fast in China recently. The ground electrodes of HVDC systems can inject/absorb large amount of DC current into/from soil, introducing DC interference to nearby pipelines. Then the pipe-to-soil potential shifts positively and high corrosion risk may appear. In this work, indoor HVDC simulation experiments were designed and carried out based on the field test results. Under high voltages, the variation regularity of DCdensity and the corrosion behavior of X80 steel in Guangdong soil were studied. The result showed that under 50, 100, 200 and 300 V DC voltages, the DC density of the coupons had the same trend and could be divided into 3 stages. Firstly, the DC density climbed to peak sharply in several seconds. Then, the DC density decreased gradually to steady value in hundreds of seconds. Lastly, the DC density stayed at that level for the rest of time. The local environment was monitored. The results indicated the variation of the DC density was mainly related to the local soil temperature increment, water content decrement and the substantially growth of the soil spread resistance. After the interference, the corrosion rates were measured to be 5.56, 7.85, 10.63 and 7.78 μm/h, respectively. The variation regularity of the corrosion rates was same with the steady values of DC density, but different from the peak values. Furthermore, 3 methods of calculating corrosion rates were studied. The theoretical corrosion rates calculated by integration of DC density curve had the smallest errors compared with the measured values. The method of using steady DC density had bigger errors and using peak DC density led to the biggest errors. Based on the results, the method of predicting HVDC corrosion rate was proposed.

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Effect of Constituent Elements on the Corrosion Resistance of Single-Phase CoCrFeNi High-Entropy Alloys in NaCl Solution
Haiou YANG, Xuliang SHANG, Lilin WANG, Zhijun WANG, Jincheng WANG, Xin LIN
Acta Metall Sin    2018, 54 (6): 905-910.   doi:10.11900/0412.1961.2017.00399
Accepted: 30 January 2018

Abstract175)   HTML14)    PDF (2811KB)(681)      

High entropy alloys (HEAs) origin from a new alloy design concept with multi-principal elements, which have attracted significant interests in the past decade. The high configurational entropy in HEAs results in simple solid solutions with fcc and bcc structures. Especially, the single solid solution CoCrFeNi alloy exhibits excellent properties in many aspects, such as mechanical properties, thermal stability, radiation resistance and corrosion resistance. The excellent corrosion resistance of CoCrFeNi alloy is ascribed to the single-phase structure and uniform element distribution coupled with much higher Cr content than stainless steel. The single-phase structure and uniform element distribution can prevent the occurrence of localized corrosion, and higher Cr content can protect the alloy surface better with the form of oxidation film. Moreover, the corrosion resistance of CoCrFeNi-based HEAs, such as CoCrFeNiAlx, CoCrFeNiCux, CoCrFeNiTix, have also been extensively investigated. In most CoCrFeNi-based HEAs, the elements of Co, Cr, Fe and Ni are with equal-atomic ratio. However, the equal-atomic ratio is not necessary to obtain satisfactory properties and to ensure the single fcc structure in Co-Cr-Fe-Ni system. Accordingly, it is essential to further consider the effect of alloying elements on the corrosion resistance in Co-Cr-Fe-Ni HEA. In this work, the effect of Co, Fe and Ni elements on the corrosion resistance of single fcc Co-Cr-Fe-Ni system with concentrated constitution but different atomic ratios in 3.5%NaCl solution are investigated by using LSCM and EIS. The potentiodynamic polarization results indicate that the increase of Fe and the decrease of Ni will decrease the passivation current density of the alloys when the Co and Cr contents are equal. With the increase of Co and the decrease of Ni, the alloys show smaller passivation current density and better corrosion resistance when the Fe and Cr contents are equal. With the decrease of Co and the increase of Fe and Ni, the alloys show higher corrosion potential and smaller corrosion tendency when the Cr content is constant. These results will be helpful for the design of corrosion resistant HEAs in NaCl aqueous solution.

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Effect of Crevice Corrosion on the Degradation Rate ofFe-30Mn-1C Alloy
Zheng MA, Xi LU, Ming GAO, Lili TAN, Ke YANG
Acta Metall Sin    2018, 54 (7): 1010-1018.   doi:10.11900/0412.1961.2017.00451
Accepted: 27 March 2018

Abstract54)   HTML3)    PDF (10067KB)(376)      

Fe-30Mn-1C alloy has great potential to become degradable cardiovascular stent material due to its degradability, excellent comprehensive mechanical properties and biocompatibility. In this work, in order to improve the degradation rate of Fe-30Mn-1C alloy, laser technology was used to process pores with different pore diameters on the samples, and the design of the scaffold was combined with crevice corrosion. The degradation behavior of the alloy was studied through in vitro soaking weight loss experiments and electrochemical tests. The results showed that crevice corrosion can increase the degradation rate of Fe-30Mn-1C alloy significantly.

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Corrosion Behavior of Fe-Based Laser Cladding Coating in Hydrochloric Acid Solutions
Li FAN, Haiyan CHEN, Yaohua DONG, Xueying LI, Lihua DONG, Yansheng YIN
Acta Metall Sin    2018, 54 (7): 1019-1030.   doi:10.11900/0412.1961.2017.00437
Accepted: 16 April 2018

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30CrMo alloy steel has a wide range of applications in the petrochemical industry such as the valve bodies and valve covers of subsea Christmas tree, and oil drilling pipes that working in strong acid environment. Therefore, the methods to improve the corrosion resistance of 30CrMo steel by surface modification techniques have become a hot topic of research. Laser cladding Fe-based coatings are regarded as promising materials, because of their high bonding strength, good hardness and excellent wear and corrosion resistance, and they might replace more expensive Co-based or Ni-based alloys. Additions of Cr, Mo, Y, Co and Ni are benefit to improve the corrosion resistance of Fe-based coatings. However, Cr, Y, Co and Mo are expensive. With consideration of reducing the materials cost, and at the same time maintaining the excellent corrosion resistance, a novel Fe-based alloy without, Y, Co and minor Mo content is synthesized. Therefore, in this study, to improve the corrosion resistance of 30CrMo alloy, the novel synthesized Fe-based powder was prepared on the surface by laser cladding. The microstructure, chemical and phase compositions of the fabricated coating were measured systemically by using a SEM equipment with EDS spectrometer, and XRD. The corrosion behavior of this Fe-based coating in 0.5 mol/L HCl solution were studied by polarization curve and EIS measurements, combined with immersion tests. The passive film formed on the surface of the alloy after immersion in the 0.5 mol/L HCl solution for 3 d was analyzed by XPS. The microstructure is mainly composed of dendrites and interdendritic phases, which are confirmed as austenite γ-Fe phase and the eutectics γ-Fe/M23C6. Similar to 304 stainless steel, the Fe-based alloy coating with a very broad passive region, shows positive corrosion potential and less corrosion current density than that of 30CrMo alloy steel. This indicates that the corrosion resistance of the Fe-based coating is superior to 30CrMo alloy steel, and almost the same as 304 stainless steel. The immersion tests show that the corrosion mechanisms of the coating are the combination of anodic dissolution and passive film protection. As for the eutectic region rich in Cr and Mo, the destruction and corrosion of this area in HCl solution are slowed down due to the passivation of Cr and Mo. The passive film is mainly composed of Cr2O3, FeCr2O4 and MoO3. The main reason for the excellent corrosion resistance of the coating is the mechanical barrier effect of the passivation effect of the high density composite oxide film.

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Corrosion of Nanoscale Metals
Junsheng WU, Bowei ZHANG, Xiaogang LI, Yizhong HUANG
Acta Metall Sin    2018, 54 (8): 1087-1093.   doi:10.11900/0412.1961.2017.00415
Accepted: 12 December 2017

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Beneficial from small-size effect, super-high specific surface area and a large amount of defects and dangling bonds on the surface, nanoscale metals exhibit superior chemical activities than traditional bulky counterparts. Nevertheless, it is the high reaction activities of nanoscale metals that in turn make them vulnerable to be oxidized and corroded, which is a main obstacle in their applications. In liquid solutions or liquid-involving multiphase environment, corrosion on nanoscale metals is ubiquitous so that it remains a crucial issue before nanoscale metals are widely employed in real applications. Due to the low-dimension and small-size of nanoscale metals, it is a huge challenge of studying their corrosion behaviors since the experimental and theoretical methods are significantly different from those on bulky metals. In the present paper, recent studies on environmental stability and corrosion behaviors of nanoscale noble metals (Pt, Ag), transition metals (Cu, Ni, Fe), active metals (Al, Mg) and semi-conductor metal (Ge) have been reviewed. Meanwhile, analysis and expectations of theoretical and experimental innovations have also been stated for the further study the corrosion on nanoscale metals.

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Corrosion Behavior of 2205 Steel in Simulated Hydrothermal Area
Shaopeng QU, Baizhang CHENG, Lihua DONG, Yansheng YIN, Lijing YANG
Acta Metall Sin    2018, 54 (8): 1094-1104.   doi:10.11900/0412.1961.2017.00472
Accepted: 29 January 2018

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Deep-sea hydrothermal area has a lot of mineral resources, and study the corrosion behavior of metal in deep-sea hydrothermal area is useful for marine resource development. Electrochemical impedance spectroscopy, linear polarization, potentiodynamic polarization and Mott-Schottky analysis were used to study the electrochemical properties of 2205 steel in 20 MPa hydrostatic pressure 3.5%NaCl solution with different temperatures. Corrosion morphologies and corrosion products of 2205 steel after electrochemical tests were analyzed by SEM, EDS and white light interferometry. The results show that 2205 steel has good pitting resistance under 25 ℃ in simulated hydrothermal area, pit occurred on the surface of 2205 steel after the solution temperature reaching 65 ℃, crack-shaped pit occurred on the surface of 2205 steel under 150 and 200 ℃. Pit occurs in austenite phase at 65 ℃, and occurs in ferrite phase at 100~200 ℃. Impedance and linear polarization resistance of 2205 steel first decrease and then increase with temperature increasing in simulated hydrothermal area, and impedance and linear polarization resistance under 150 ℃ are lowest. Pitting potential of 2205 steel first negative shift and then positive shift, and carrier density of passive film formed in simulated hydrothermal area increase with temperature increasing.

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Comparative Study on Corrosion Behavior of Cast and Forged Mg-5Y-7Gd-1Nd-0.5Zr Alloys
Jinhui LIU, Yingwei SONG, Dayong SHAN, En-Hou HAN
Acta Metall Sin    2018, 54 (8): 1141-1149.   doi:10.11900/0412.1961.2017.00484
Accepted: 19 April 2018

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Magnesium and its alloys have become increasingly attractive in the automotive, 3C products and aerospace industries because of their advantages such as low density and high specific strength. In recent years, rare earth-Mg alloys have attracted much attention due to their high mechanical properties at room and elevated temperatures. Adjusting the microstructures by deformation treatment is a common method to improve the mechanical properties of Mg alloys. The microstructure especially the size, volume fraction and distribution of second phases in rare earth-Mg alloys will be changed during deformation treatment, which has a great effect on the corrosion resistance of Mg alloys. However, the studies on the effect of deformation treatment on the corrosion resistance of rare earth-Mg alloys are far away from sufficient. In this work, the corrosion behavior of cast and forged Mg-5Y-7Gd-1Nd-0.5Zr (EW75) alloys were studied by using SEM, XRD, mass loss measurements and electrochemical tests. The results indicate that the second phases are distributed along the grain boundaries of cast and forged EW75 alloys. Meanwhile, the second phases in forged EW75 alloy are finer and lower volume fraction than that in cast EW75 alloy. The micro-galvanic corrosion of the forged EW75 alloy is weaker in comparison with the cast EW75 alloy owing to the smaller size and lower volume fraction of second phases as well more compact surface film, resulting in the better corrosion resistance.

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Microstructure and Corrosion Properties of Aluminum Base Amorphous and Nanocrystalline Composite Coating
Xiubing LIANG, Jianwen FAN, Zhibin ZHANG, Yongxiong CHEN
Acta Metall Sin    2018, 54 (8): 1193-1203.   doi:10.11900/0412.1961.2017.00491
Accepted: 24 May 2018

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It is easy to corrode the steel structural materials. In view of this problem, the Al-Ni-Zr amorphous and nanocrystalline composite coating with high amorphous volume was prepared by high velocity arc spraying on the 45 steel. The microstructure, macroscopic corrosion performance and microzone corrosion performance of the composite coating was investigated. XRD, SEM with EDS and TEM were applied to confirm that the gray zone of the composite coating microstructure was the amorphous enrichment zone. It was found by the scanning Kelvin probe microscopy (SKPM) that the corrosion failure order of each phase of the composite coating was arranged in order of the aluminum rich phases, the oxidation phases and the amorphous phase. The microhardness of the composite coating was about 364 HV0.1 which was greater than that of 45 steel. The EIS fitting results showed that the charge transfer resistance of the composite coating is 2~4 times of the aluminum coating and 45 steel. It has two time constants in the spectrum. The corrosion failure behavior of the composite coating in the low frequency was controlled by the diffusion process, which was related to the accumulation and diffusion of the corrosion products. The potentiodynamic polarization curves fitting results indicated that the self-corrosion potential of the composite coating was higher than those of the aluminum coating and 45 steel. And the self-corrosion current density of the composite coating was about 1.08 μA/cm2, which was 7/100 and 1/3 of that of the aluminum coating and 45 steel, respectively. According to the corrosion morphology of the composite coating, there was no obvious pitting. A large number of NaCl crystals were attached to the surface of the aluminum rich phase region as the preferred corrosion zone. But the surface of the amorphous enrichment zone was smooth. At the same time, the corrosion pits, micro-cracks and pitting enrichment occurred on the surface of the composite coating, which was mainly related to the effects of Cl- erosion and swelling.

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Biomedical Magnesium Alloys: Composition, Microstructure and Corrosion
Rongchang ZENG, Lanyue CUI, Wei KE
Acta Metall Sin    2018, 54 (9): 1215-1235.   doi:10.11900/0412.1961.2018.00032
Accepted: 07 May 2018

Abstract284)   HTML27)    PDF (4210KB)(1041)      

Magnesium alloys, with good biocompatibility and mechanical-compatibility, can be developed as next generation promising biomaterials. This paper summerizes the principle and the cutting-edge advances of alloying of magnesium alloys as degradable biomaterials. The effects of alloy elements on the material and biological properties of magnesium alloys are analyzed. The focus is laid on the influence of microstructure (grain size, secondary phase or intermetallic compound, long-period stacking ordered (LPSO) phase and quasi-crystal phase), heat treatment and surface oxide film on degradation and their critical progress on corrosion morphology and mechanism. Several outlooks on bio-magnesium alloys are proposed.

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Galvanic Series of Metals and Effect of Alloy Compositions on Corrosion Resistance in Sanya Seawater
Mindong CHEN, Fan ZHANG, Zhiyong LIU, Chaohui YANG, Guoqing DING, Xiaogang LI
Acta Metall Sin    2018, 54 (9): 1311-1321.   doi:10.11900/0412.1961.2017.00521
Accepted: 03 May 2018

Abstract202)   HTML11)    PDF (2167KB)(614)      

With the development of ocean engineering, various metallic materials have been applied to the marine environment. It is an urgent requirement to study the galvanic series and alloy composition optimization of metallic materials in the tropical marine environment. In this work, open circuit potentials (OCP) and galvanic series of 36 kinds of metallic materials in Sanya seawater were studied. By considering the response of OCP to tidal changes, the anti-corrosion effects of alloying elements were also analyzed. The results show that the OCP of metallic materials in Sanya seawater has a large range. The galvanic series order of metallic materials from high to low in Sanya seawater is: nickel alloy, duplex stainless steel, austenitic stainless steel and pure copper, ferritic stainless steel, martensitic stainless steel, copper alloy, low alloy steel, carbon steel, cast iron, aluminum alloy and aluminum anode. Low-carbon high-alloy content carbon steel and high Cr, Ni contents stainless steel have higher OCP. The potential fluctuations of carbon steel with tidal changes involves two phases: (1) under the dynamics control, the OCP of carbon steel is more negative at high tide; (2) under the diffusion control, the OCP is more positive at high tide. The potential fluctuations of metallic materials reflect the effect of the corrosion product film on the change of ionization balance, and metals with less potential fluctuations have better inhibition on ion diffusion. In Sanya seawater, the carbon steel, which has more alloying content and less carbon content, has less potential fluctuations with the tidal changes and has good oxygen diffusion resistance. The potential fluctuations of austenitic stainless steel with tidal changes are less than that of ferritic stainless steel and martensitic stainless steel. After 2700 h immersion, austenitic stainless steel and martensitic stainless steel, which have a higher content of Mo, have more stable OCP. In other words, the corrosion film gets a better corrosion resistance. The OCP of aluminum anode in Sanya seawater environment increases when the oxygen content is brought up. The OCP of Zn-containing or Ga-containing aluminum anode remains relatively stable. Al bronze and T2 copper have less potential fluctuations with tidal changes, and perform good corrosion resistance in Sanya seawater.

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Electrochemical Noise of Stress Corrosion Cracking of P110 Tubing Steel in Sulphur-Containing Downhole Annular Fluid
Jun YU, Deping ZHANG, Ruosheng PAN, Zehua DONG
Acta Metall Sin    2018, 54 (10): 1399-1407.   doi:10.11900/0412.1961.2018.00033
Accepted: 06 June 2018

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Stress corrosion cracking (SCC) is considered as the main risk of tubing steels during the exploitation of oil and gas fields, which could result in sudden and catastrophic failures of downhole tubing. Especially in annular downhole environment, P110 tubing steel is prone to sulfide stress corrosion cracking and hydrogen embrittlement (HE) where S2- could be originated from bio-reduction of SO42- inspired by sulfate-reducing bacteria (SRB). Currently, extensive work have been performed to investigate the influence factors on SCC and mechanism of tubing steels, but limited researches have been conducted on the SCC of P110 tubing steel in annular downhole environment, particularly, on the early detection of SCC. In this work, the SCC behavior of P110 low alloy steel in simulating sulphur-containing annular fluid (SAF) and the effect of S2- concentration on the initiation and propagation of crack were investigated by slow stress rate test (SSRT), non-destructive electrochemical noise (ECN), SEM and EIS techniques. The results showed that, during the elastic stress stage, the addition of S2- accelerated the breakdown of passivation film on the surface of P110 steel tensile specimen. There are many short duration current transients caused by metastable pits on ECN curves. The transformation time of metastable to stable pits is shortened significantly by the addition of S2-, which not only promotes the growth of pits but the initiation of cracks from the stable pits under the action of tensile stress. Compared with the ECN spikes from metastable pits, the spikes associated to the advance of cracks are featured by longer average duration (about 400 s), stronger amplitude (40 μA), and higher charge (about 4000 μC). As a result, the susceptibility of P110 steel to SCC increases with S2- concentration, and the propagation of SCC is dominated by anodic dissolution characteristic of discontinuous advance.

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Characteristics of SRB Biofilm and Microbial Corrosionof X80 Pipeline Steel
Yun SHU, Maocheng YAN, Yinghua WEI, Fuchun LIU, En-Hou HAN, Wei KE
Acta Metall Sin    2018, 54 (10): 1408-1416.   doi:10.11900/0412.1961.2018.00069
Accepted: 25 April 2018

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Microbiologically induced corrosion (MIC) is known as one of the most damaging failures for pipeline steels. Especially, sulfate-reducing bacteria (SRB) is the most widespread strains in soil and seawater environments and is the typical bacteria associated with MIC. SRB may cause severe localized attack, leading to pipeline failures in forms of pitting, crevice corrosion, dealloying and cracking. In this work, SEM, Raman spectroscopy, XPS, scanning vibrating electrode (SVET) technique, EIS and other electrochemical techniques were used to study the formation of SRB biofilm, its electrochemical interaction with X80 pipeline steel and corrosion behavior of the steel in a simulated seawater. The results showed that barrier effect of the extracellular polymer substances (EPS) inhibits corrosion process of X80 steel in the initial formation of EPS and SRB micro-colony. After the formation of SRB biofilm, open circuit potential (EOCP) of the steel decreases 20 mV, and SRB significantly promotes the corrosion process of the pipeline steel. In the later stage, due to SRB and its biofilm, the corrosion rate of X80 steel exposed in SRB inoculated environment is almost one order of magnitude higher than that in the sterile environment. The biofilm have complexation effect and chelation effect with corrosion products (Fe2+/Fe3+). SRB cells, metabolites and biofilms have direct and indirect electron interactions with the steel substrate. These various coupling effects promote occurrence and development of local corrosion on the surface of the steel beneath biofilm.

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Preparation and Corrosion Resistance of the ZnAl-LDHs Film on 6061 Al Alloy Surface
Yusheng ZHANG, Youbin WANG, Chunmin LI, Bingtao ZHOU, Keke CHENG, Yuezhou WEI
Acta Metall Sin    2018, 54 (10): 1417-1427.   doi:10.11900/0412.1961.2018.00020
Accepted: 16 August 2018

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6061 Al alloy is widely used for structural components in the structural, building, aerospace and automobile industry because of their good extrude-ability, high strength and low density. 6061 Al alloy is easily corroded during the corrosive environments containing Cl-, which lead to the decreasing of the alloy service life. Therefore, the Al alloy need to adopt surface treatment to improve the corrosion resistance. The chromate passivation was the most effective surface treatment technology of Al alloys in the past. However, Cr(VI) could pollute the environment and harmful to human body. Thus, it is necessary to develop chromium-free films to protect Al alloy. Layered double hydroxides (LDHs) are an environment-friendly and smart material, which could be used for anticorrosion film. The inhibitor can be inserted into the film layer due to its unique anion exchanging capacity. Therefore, the LDHs is acquired self-healing performance and applied to the anticorrosion field of Al alloy. Meanwhile, vanadate is a good inhibitor and it has many forms under different pH conditions. Moreover, different forms of vanadate have different influence on LDHs film. The corrosion resistance of LDHs and its modified form films on Al alloys need deep study. In this work, ZnAl-LDHs films with NO3- were prepared on the suface of 6061 Al alloy via a facile in-situ growth method, and then ZnAl-LDHs-NO3 films were intercalated with the corrosion inhibitor VO43- and VO3- to obtain the ZnAl-LDHs-VO4 and ZnAl-LDHs-VO3 films, respectively. The structure, morphology and composition of as-prepared ZnAl-LDHs films were investigated by XRD, fourier infrared spectrometer (FT-IR) and SEM; the corrosion behavior of ZnAl-LDHs films in the 3.5%NaCl (mass fraction) solution were studied by the electrochemical workstation and the 3D microscope. The results show that the plate-like ZnAl-LDHs microcrystals are perpendicular to the substrate and cover almost the entire Al alloy substrate surface. Compared with the 6061 Al substrate, ZnAl-LDHs films can not only decrease the corrosion current (Icorr), but also increase the corrosion potential (Ecorr) and charge transfer resistance (Rct) of the 6061 Al alloy. It is suggested that ZnAl-LDHs films could significantly enhance the corrosion resistance of 6061 Al substrate, indicating an effective protection for 6061 Al alloy by the ZnAl-LDHs film. The ZnAl-LDH-VO3 film is of the highest corrosion resistance in the studied ZnAl-LDHs films.

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Comparative Study of Hot Corrosion Behavior of theEnamel Based Composite Coatings and the ArcIon Plating NiCrAlY on TiAl Alloy
Yimin LIAO, Min FENG, Minghui CHEN, Zhe GENG, Yang LIU, Fuhui WANG, Shenglong ZHU
Acta Metall Sin    2019, 55 (2): 229-237.   doi:10.11900/0412.1961.2018.00293
Accepted: 27 August 2018

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TiAl intermetallic alloys have attracted great attention for its potential application in preparing low pressure turbine blades in aircraft engine. However, its poor oxidation and corrosion resistance becomes a challenge at temperatures above 800 ℃, which leads to the developing of protective coatings. Enamel coating is considered as one of the candidates that match the TiAl alloy well, meanwhile provide corrosion protection. Enamel coating has many advantages such as high thermochemical stability, adjustable thermal expansion coefficient and simple preparation process. This study comparatively investigates hot corrosion behavior of the Ti-45Al-2Mn-2Nb alloy, the traditional NiCrAlY coating and the enamel based composite coating in (75%Na2SO4+25%NaCl, mass fraction) melted salt. Results indicate that after 80 h of hot corrosion, the bare alloy has completely destroyed. For the NiCrAlY coating, it protects the underlying alloy well by forming a protective Al2O3 scale initially. However, serious interdiffusion between coating and substrate results in the degeneration of the coating as well as the scale. At the same time, the basic dissolution of Al2O3 film accelerates corrosion. So obvious spallation takes place after 60 h corrosion. The enamel based composite coating shows excellent thermal stability and low corrosion rate. During the whole hot corrosion test, it still retains its original blue glazing color and luster. Furthermore, the enamel coating suppresses the inward diffusion of oxygen and corrosive ions into the alloy substrate, and thus, it protects the TiAl alloy well from corrosion of the molten (75%Na2SO4+25%NaCl, mass fraction) salt.

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Influence of Hydrostatic Pressure on the Thermodynamics and Kinetics of Metal Corrosion
Rongyao MA, Lin ZHAO, Changgang WANG, Xin MU, Xin WEI, Junhua DONG, Wei KE
Acta Metall Sin    2019, 55 (2): 281-290.   doi:10.11900/0412.1961.2018.00215
Accepted: 19 September 2018

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In recent years, researches on deep sea corrosion have attracted much attention of many researchers. The high hydrostatic pressure is a distinctive feature of deep-sea environment. The hydrostatic pressure affects the corrosive behavior of metallic materials by modifying the composition, structure and compactness of corrosion products, changing the polarization processes of cathode electrode and anode electrode, altering pitting nucleation rate and growth rate, impacting on chemical reaction rate and equilibrium constant, and varying hydrogen diffusion rate and coverage. However, in essence, the influence of hydrostatic pressure on the corrosion behavior of different metal materials is the manifestation of the thermodynamic and kinetic parameters of the corrosion electrode process caused by hydrostatic pressure. At present, the mechanism of effect of hydrostatic pressure on the thermodynamics and kinetics of corroding electrode processes is unclear yet. In addition, the concerns about the effects of hydrostatic pressure on the chemical properties of materials and seawater are comparatively low. Based on thermodynamics and kinetics, the effects of hydrostatic pressure on the activities of electrode material and ions in environment, including on the solubility, fugacity or activity of gases in environment are analyzed. The influence of hydrostatic pressure on pH value and chemical equilibrium is also discussed. The relationships between hydrostatic pressure and equilibrium electrode potential, as well as the exchange current density, are analyzed. The theoretical model of the effect of hydrostatic pressure on the corrosion behavior of active metals is established. The studies have shown that hydrostatic pressure would increase the activities of materials, ions and dissolved gas in environments, and this is closely related to their partial molar volume. The hydrostatic pressure would magnify the difference in the activity of heterogeneous materials. The larger the partial molar volume difference of the heterogeneous materials, the more obvious the difference in activity. Increasing hydrostatic pressure reduces the equilibrium electrode potential of iron and aluminum anode dissolution reaction while reducing its exchange current density. Increasing hydrostatic pressure increases the equilibrium electrode potential of oxygen reaction and decreases its exchange current density. Increasing hydrostatic pressure reduces the equilibrium electrode potential of hydrogen evolution reaction and increases its exchange current density.

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Study on Irradiation Assisted Stress Corrosion Cracking of Nuclear Grade 304 Stainless Steel
Ping DENG,Chen SUN,Qunjia PENG,En-Hou HAN,Wei KE,Zhijie JIAO
Acta Metall Sin    2019, 55 (3): 349-361.   doi:10.11900/0412.1961.2018.00359
Accepted: 14 November 2018

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Irradiation assisted stress corrosion cracking (IASCC) of austenitic stainless steel core components is one major concern for maintenance of nuclear power plants. Previous studies on the IASCC had mainly focused on the effect of irradiation on changes in deformation modes and interaction of dislocation channels with grain boundary. The role of corrosion in IASCC, however, has not received sufficient attentions. In the process of stress corrosion cracking (SCC), corrosion occurs simultaneously with localized deformation in the vicinity of the crack tip. This indicates that corrosion is one of the potential contributors to IASCC. In this work, IASCC of proton-irradiated nuclear grade 304 stainless steel (304SS) was investigated. The IASCC tests were conducted by interrupted slow strain rate tensile (SSRT) tests at 320 ℃ in simulated primary water of pressurized water reactor containing 1200 mg/L B as H3BO3 and 2.3 mg/L Li as LiOH·H2O, with a dissolved hydrogen concentration of 2.6 mg/L. Following the SSRT tests, the localized deformation, corrosion and IASCC of the specimens were characterized. The results revealed that increasing the irradiation dose promoted residual strain accumulation at slip steps and grain boundaries of nuclear grade 304SS. Since the slip step usually transmitted or terminated at the grain boundary, it eventually promoted localized deformation at the grain boundary. Specially, the slip step transmitted at grain boundary led to slip continuity at the grain boundary. In contrast, a slip discontinuity was observed at the grain boundary where the slip step terminated, which caused a much higher strain accumulation by feeding dislocations to the grain boundary region. Further, formation of the slip discontinuity was related to the Schmidt factor pair type of the adjacent grains. The irradiation resulted in a depletion of Cr and an enrichment of Ni at grain boundary, while the magnitude of Cr depletion and Ni enrichment increased with increasing the irradiation dose. Following the SSRT tests, intergranular cracking was observed on surfaces of the irradiated specimens, while the number of the cracks was increased by a higher irradiation dose and applied strain. This suggested a higher IASCC susceptibility of nuclear grade 304SS in the primary water. Meanwhile, significant intergranular oxidation ahead of the crack tip was observed, while both the width and length of the oxide were larger at a higher irradiation dose. The synergic effect of irradiation-promoted deformation and intergranular corrosion was the primary cause for the IASCC of the irradiated steel.

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Influence of SO42- on the Corrosion Behavior of Q235B Steel Bar in Simulated Pore Solution
Kaiqiang LI, Lujia YANG, Yunze XU, Xiaona WANG, Yi HUANG
Acta Metall Sin    2019, 55 (4): 457-468.   doi:10.11900/0412.1961.2018.00475
Accepted: 13 December 2018

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Cl- and SO42- are most common aggressive ions containing in the seawater which may cause the localized corrosion of reinforcement structures. It is found that a protective passive film will form on the steel surface in the concrete pore solution. The localized breakdown of the passive film caused by the aggressive ions and the carbonation are the main reason for the localized corrosion initiation of reinforcements. In the previous studies, it is found that the performances of the SO42- on the rebar corrosion were quite different in different pH value conditions and the test results did not unify. Therefore, the influence of pH value and the SO42- on the corrosion behavior of Q235B carbon steel in the simulated pore solution was studied using anodic polarization, electrochemical impedance spectra (EIS), Mott-Schottky (M-S) and potentiostatic polarization methods. The anodic polarization curves indicate that when the pH value of the simulated pore solution was higher than 11, SO42- had no damage to the passive film. However, once the pH value of the simulated pore solution decreased to 10, a small amount of SO42- can lead to the breakdown of the passive film and induce pitting initiation. EIS and M-S measurement results suggest that the stability of the passive film would decrease with the decreasing of the solution pH. The concentration of the defect would increase in the passive film due to the pH decrease. The stability reduction and the increase of defect concentration both can lead to the passive film become fragile and more easily to be destroyed by SO42-. Through the potentiostatic polarization test in conjunction with SEM observation, it is found that SO42- can inhibit the growth of the passive film during the initial film formation period and lead to the appearance of metastable pitting corrosion under high pH value conditions. In the low pH value conditions, SO42- could accumulate at the defect of the passive film and lead to stable pitting propagate on the steel surface.

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Comparative Study of Stress Corrosion Cracking Behaviors of Typical Microstructures of Weld Heat-Affected Zones of E690 High-Strength Low-Alloy Steel in SO2-Containing Marine Environment
Hongchi MA, Cuiwei DU, Zhiyong LIU, Yong LI, Xiaogang LI
Acta Metall Sin    2019, 55 (4): 469-479.   doi:10.11900/0412.1961.2018.00140
Accepted: 03 January 2019

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With the extensive exploitation of ocean resources, the steels used in ocean engineering have been developed towards the trend of high strength-toughness and thick plates, which consequently causes welding problem and high risk of stress corrosion cracking (SCC). The heat-affected zone (HAZ) of high-strength low-alloy steel undergoes phase transformation during welding thermal cycle and it's generally considered to be most vulnerable to SCC. E690 steel, as a newly-developed high strength steel, is currently the leading kind of steel used in ocean platform for its excellent performance. However, there is few research about its SCC behavior in marine atmosphere, especially in SO2-polluted atmosphere. Therefore, it's of great importance to investigate the SCC behavior and mechanism of simulated HAZ of E690 steel in this environment. However, the HAZ is a narrow zone including various microstructures; thus, the individual performance of different microstructures is inconvenient to study. In this work, various microstructures in HAZ, including coarse grained heat-affected zone (CGHAZ), fine grained heat-affected zone (FGHAZ) and intercritical heat-affected zone (ICHAZ), were simulated by heat treatment according to real HAZ microstructures of E690 steel. A comparative study of SCC behaviors of various HAZ microstructures in simulated SO2-containing marine atmosphere was conducted by using U-bend specimen corrosion test under dry/wet cyclic condition. The results indicated that various HAZ microstructures have high susceptibility to SCC in this environment. The SCC susceptibility of CGHAZ and ICHAZ is very high with a high crack growth rate while that of FGHAZ and parent metal is relatively modest. SCC cracks were initiated after 5 d of cyclic corrosion test for U-bend specimen of various microstructures. The microcracks were initiated from the corrosion pits, which were induced by the galvanic corrosion between martensite-austenite (M-A) constituents and ferritic matrix.

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Effect of Na on Early Atmospheric Corrosion of Al
Xingchen CHEN, Jie WANG, Deren CHEN, Shuncong ZHONG, Xiangfeng WANG
Acta Metall Sin    2019, 55 (4): 529-536.   doi:10.11900/0412.1961.2018.00280
Accepted: 09 October 2018

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Aluminum and aluminum alloy are widely used in every field of modern life. It is especially important to understand the detailed mechanisms of aluminum atmospheric corrosion. Traditional studies only consider the role of oxygen reduction and focus on anions such as Cl, SO42- in the environment, ignoring the effects of cations such as Na+ on the atmospheric corrosion. However, recent studies have shown that the effect of Na element on the corrosion of aluminum can not be ignored. In this work, single-shot laser-induced breakdown spectroscopy (LIBS) was used to measure the aluminum atomic lines after corrosion for 35 d in the atmospheric environment, and combined with a three-dimensional tomography measurement, to study the depth profiling of Na on the aluminum surface. The results show that the Na element on the surface of the aluminum originates from the atmospheric environment, and Na is involved in the formation of corrosion product NaAlCO3(OH)2. The content of NaAlCO3(OH)2 decreases as the depth increases following an exponential power function. The content decrease of NaAlCO3(OH)2 in different depths can be transformed into the change of cathode area. Combined with the measured polarization curve of aluminum, the atmospheric corrosion model of aluminum including the presence of oxygen reduction and the change of cathode area was established using COMSOL software. The calculated corrosion depth is 6.155 μm, which is consistent with the depth of Na element measured by LIBS experiments. By studying the distribution of Na cations and corrosion products, a simulation model was established to reveal the influence on corrosion mechanism, which is of great significance for the study of early atmospheric corrosion of aluminum.

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Arc Erosion and Degradation Mechanism ofAg/Ti2AlC Composite
Jianxiang DING,Wubian TIAN,Dandan WANG,Peigen ZHANG,Jian CHEN,Zhengming SUN
Acta Metall Sin    2019, 55 (5): 627-637.   doi:10.11900/0412.1961.2018.00534
Accepted: 20 March 2019

Abstract100)   HTML0)    PDF (44932KB)(217)      

Ag-based contact is widely used in low-voltage switch (contactor, relay and breaker), which determines the safety and stability of the circuit. Toxic Ag/CdO goes against the development of environmentally friendly materials and will be excluded from future market. Ag/10%Ti2AlC (mass fraction, Ag/10TAC) composite shows excellent arc erosion resistance, and has the potential to replace Ag/CdO. Dynamic electric arc discharging experiment was performed on the Ag/10TAC contact surface to investigate its arc erosion mechanism. Inhomogeneous arc erosion generates three featured regions (unaffected, transitional, affected) on the contact surface. The various microstructure and chemical composition of Ag are attributed to the melting and vaporization of Ag, absorption of O2, deposition of Ag-O vapor, and interdiffusion of Ag-Al. The rapid "decomposition-oxidation" process of Ti2AlC accounts for the microstructure evolution and oxidation behavior of Ti2AlC during arc erosion. The changes of structure and function on the contact surface lead to the degradation of Ag/10TAC composite.

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Characteristics and Evolution of the Spot Segregations and Banded Defects in High Strength Corrosion Resistant Tube Steel
Bo LI,Zhonghua ZHANG,Huasong LIU,Ming LUO,Peng LAN,Haiyan TANG,Jiaquan ZHANG
Acta Metall Sin    2019, 55 (6): 762-772.   doi:10.11900/0412.1961.2018.00557
Accepted: 01 April 2019

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C110 casing tube is one of the high strength corrosion resistant steel products for deep well oil exploration. Due to the co-existence of acidic media such as H2S and the high pressure, there are frequently sulfide stress corrosion cracking (SSC) failures produced in the tubes, which are supposed to be closely connected with their banded segregation defects. The relationship between the as-cast spot segregation and the following as-rolled banded defects, together with the impacts of quenching and tempering (QT) treatment have been revealed. The banded defects in high strength corrosion resistant oil tube have been studied experimentally from its very beginning of as-cast state. With aids of OM, SEM, EDS and EPMA observation and analysis, the various spot like segregations in round casting were revealed along with their following banded structure in both as-rolled and QT tubes. The mechanism and appearance of the segregation induced banded defects were investigated comparatively of the both tubes. It is pointed out that there are normally two kinds of spot like segregations in steel castings, speckle type and porosity type, respectively. There are not only severe positive segregations of solutes, such as C, Cr, Mo and Mn etc., in the macro-etched spot like areas, a finer dendritic sub-structure has also been observed in the speckle type spot segregation zones. It has been found that the width of the banded defects in the as-rolled tubes is closely related to the types of segregations, and the severe banded defects, which are difficult to remove by heat treatment, are recognized to originate directly from the spot like segregations. Solute segregations are found in the microstructure of banded defects of the both as-rolled and QT tubes but with different existences. A kind of pearlite plus bainite banded structure is present in the former tube, while the banded defect of latter is composed of concentrated granular carbides, which explains the difference of their hardness behavior.

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Reconstruction and Characterization of Galvanic Corrosion Behavior of X80 Pipeline Steel Welded Joints
Yadong LI,Qiang LI,Xiao TANG,Yan LI
Acta Metall Sin    2019, 55 (6): 801-810.   doi:10.11900/0412.1961.2018.00562
Accepted: 09 April 2019

Abstract140)   HTML4)    PDF (10490KB)(476)      

Welding is widely used for pipeline connection. Composition, microstructures and properties of the welded joints are highly heterogeneous and the resultant corrosion such as galvanic corrosion between different parts is widely present and influence the long-time service and safety. In this sense, the fundamental research in the electrochemical behavior of such joint parts is required. Electrochemical corrosion behavior of simulated X80 steel welded joint, accurately modeled by wire beam electrode (WBE) technique, was investigated by classical electrochemical techniques and microelectrode array (MEA) technique. A new index, namely the galvanic corrosion intensity factor, was proposed and verified to succeed in characterizing the degree of galvanic corrosion. Results showed that microstructure of granular bainite mixed with ferrite showed the highest positive open circuit potential and lowest polarization resistance. Furthermore, the corrosion tendency of the isolated electrodes that constituted the X80 steel welded joint was found to increase in the following order: fine grain heat affected zone (FGHAZ) < intercritical heat affected zone (ICHAZ) < base metal (BM) < coarse grain heat affected zone (CGHAZ) < weld metal (WM). Due to the difference in potential and the polarization characteristics, the WM displayed the highest polarization resistance but the most positive current density. The CGHAZ possessed a lower polarization resistance and a higher positive current density. In comparison, the FGHAZ and ICHAZ performed a lower polarization resistance but higher negative current densities. The WM and CGHAZ acted as the main anode, while the FGHAZ and ICHAZ acted as the main cathode and the galvanic current polarity of some BM electrodes changed with time during the immersion test. The intensity of galvanic corrosion of simulated X80 steel welded joint plateaued with immersion time. The results revealed that WM and CGHAZ were the weak links in the simulated X80 pipeline steel welded joints during its long-term service.

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Effect of Mo Element and Heat Treatment on Corrosion Resistance of Ni2CrFeMox High-Entropy Alloyin NaCl Solution
Lin WEI,Zhijun WANG,Qingfeng WU,Xuliang SHANG,Junjie LI,Jincheng WANG
Acta Metall Sin    2019, 55 (7): 840-848.   doi:10.11900/0412.1961.2018.00558
Accepted: 16 April 2019

Abstract102)   HTML7)    PDF (15275KB)(491)      

As a new alloy design concept, the high-entropy alloy (HEA) and the formation of simple solid solution introduce excellent properties such as high hardness, high strength and corrosion resistance. Investigations have shown that the single solid solution CrCoFeNi alloy possesses good corrosion resistance. The addition of Mo is beneficial to the corrosion resistance of the HEAs for potential industrial applications in 3.5%NaCl (mass fraction) simulating seawater type environments. The major effect of Mo is to promote the pitting potential of the alloy and inhibit the dissolution of the passivation film by forming and retaining molybdenum oxyhydroxide or molybdates (MoO42-). Considering that the cost of pure Co is higher, Ni and Co elements have similar atomic size and valence electron concentration, and the corrosion resistance of pure Ni is higher than that of pure Co, Ni2CrFeMox HEA was designed by replacing Co element with Ni element in CoCrFeNiMox HEA. As the Mo content increases in the Ni2CrFeMox HEAs, the interdendrite is a Cr and Mo rich σ phase, and the dendrite is a Cr and Mo depleted fcc phase. The potential difference between interdendrites and dendrites leads to galvanic corrosion, which accelerates the localized corrosion of alloys. Here, a solution heat treatment process is selected to reduce the precipitation phase and improve the corrosion resistance of the alloy. The effects of Mo element and heat treatment on the corrosion resistance of Ni2CrFeMox HEA in 3.5%NaCl solution were tested. The results show that the corrosion resistance of as-cast Ni2CrFeMox HEA is obviously higher than that of 316L stainless steel. The Ni2CrFeMo0.2 alloy has the best corrosion resistance because of its minimum dimensional passive current density and corrosion current density. However, the addition of excessive Mo leads to the precipitation of σ phase and galvanic corrosion, which reduces the corrosion resistance of the alloy. After solution treatment, the uniformity of alloy structure and element distribution weakens galvanic corrosion, and the corrosion resistance is obviously improved.

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Corrosion Behaivour of X65 Low Carbon Steel During Redox State Transition Process of High LevelNuclear Waste Disposal
Canshuai LIU,Zhaohui TIAN,Zhiming ZHANG,Jianqiu WANG,En-Hou HAN
Acta Metall Sin    2019, 55 (7): 849-858.   doi:10.11900/0412.1961.2018.00481
Accepted: 23 April 2019

Abstract87)   HTML1)    PDF (20380KB)(235)      

Domestic and foreign researches on the corrosion behavior of low carbon steel canister in high level nuclear waste geological repositories focus on the initial aerobic stage and the later anaerobic stage, while few researches have been reported on the corrosion behavior during the disposal transition period. The long term electrochemical corrosion behavior of X65 low carbon steel in 80 ℃ Gaomiaozi bentonite saturated with anaerobic Beishan groundwater has been studied by electrochemical measurement system in anaerobic glovebox constructed independently. The results indicated that the open circuit potential of X65 low carbon steel decreased gradually during 150 d, while the electrochemical impedance of the corrosion film increased with immersion time. Pitting corrosion occurred at the beginning of immersion tests, and finally transformed into general corrosion. Morphologies, compositions, and phases of the corrosion film formed on the carbon steel surface were examined by SEM, EDS and μ?XRD. The results showed that the corrosion film was mainly composed of blocks, slices, rods and swellings. The elemental distribution in the corrosion film was uniform, and the phases were composed of magnetite and hematite. The average corrosion rates were detected by weight loss measurement, which decreased from 195.88 μm/a to 20.58 μm/a. The corrosion rates (V) followed a power function pattern V=8.34t-0.88, indicating that the film growth process was controlled by oxygen diffusion.

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Influence of Hydrostatic Pressure on Corrosion Behavior of Ultrapure Fe
Rongyao MA,Changgang WANG,Xin MU,Xin WEI,Lin ZHAO,Junhua DONG,Wei KE
Acta Metall Sin    2019, 55 (7): 859-874.   doi:10.11900/0412.1961.2019.00044
Accepted: 08 April 2019

Abstract71)   HTML7)    PDF (16786KB)(259)      

Hydrostatic pressure is part of the crucial factors affecting deep sea corrosion. At present, there have been a lot of studies on the pitting behavior of metallic materials under hydrostatic pressure, but most of them take passive metallic materials as the research object, and the influence rule of hydrostatic pressure on the pitting behavior of metallic materials also presents diversity. People not only have no clear understanding of its mechanism, but also have some disputes. The generation and growth of pitting corrosion are dependent on the structure of materials, chemical composition and service environment. Inclusion, passivation ability and surface roughness can all affect the pitting behavior of metal materials. Due to the single composition and simple structure of ultrapure Fe, the influence of phase, inclusion and other factors on corrosion behavior under hydrostatic pressure can be avoided, which is more conducive to elucidate the mechanism of hydrostatic pressure on metal corrosion behavior. In addition, the influence of hydrostatic pressure on the corrosion behavior of ultrapure Fe is rarely reported. So, the effect of hydrostatic pressure on the corrosion behavior of ultrapure Fe exposed to 3.5%NaCl aqueous solution is investigated by potentiodynamic polarization curves and electrochemical noise method. The noise signals are analyzed by shot noise theory, stochastic analysis and Hilbert-Huang transform. Besides, the surface morphology of the corrosion sample is observed by SEM. The results of weight loss test and potentiodynamic polarization study show that increasing hydrostatic pressure accelerated the corrosion rate of ultrapure Fe exposed to 3.5%NaCl. The results of electrochemical noise study show that increasing hydrostatic pressure promotes the development of pitting corrosion and increases the tendency of local corrosion throughout the immersion. At the beginning of soaking, local corrosion (such as pitting nucleation, metastable pitting and stable pitting) mainly occurred in ultrapure Fe, increasing of hydrostatic pressure inhibits the pitting nucleation process, but promotes the development of metastable pitting and steady pitting, and increases the growth probability of pitting. With the immersion time prolonging, the uniform corrosion gradually changed into the principal corrosion type, increasing hydrostatic pressure still promotes the development of metastable pitting and stable pitting and improves the growth probability of pitting corrosion, but relatively inhibits the uniform corrosion process.

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Correlation Between Ageing Precipitation, Potential and Intergranular Corrosion of 2A97 Al-Li Alloy Sheet
Chao CAI,Yang LI,Jinfeng LI,Zhao ZHANG,Jianqing ZHANG
Acta Metall Sin    2019, 55 (8): 958-966.   doi:10.11900/0412.1961.2018.00519
Accepted: 24 April 2019

Abstract79)   HTML0)    PDF (21625KB)(258)      

The microstructures and open circuit potential (OCP) of 2A97 Al-Li alloy sheets with different ageing and their corrosion features in intergranular corrosion (IGC) medium were investigated. As the extension of ageing time, T1 (Al2CuLi) phases are precipitated, the alloy potential is decreased, which is accompanied with the following corrosion mode evolution: pitting, IGC (including local IGC and general IGC) and pitting again. Meanwhile, with ageing progress, the IGC depth is increased firstly and then decreased. Compared to T6 ageing, T8 ageing accelerates the precipitation of T1 phases, the potential therefore decreases more quickly. After a certain ageing, the lower the potential, the smaller the IGC degree, and the greater the pitting degree. A correlation between OCP and corrosion mode was proposed, which may be used to compare the IGC sensitivity of Al-Li alloy with different tempers.

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Corrosion Behavior of Ultrafine Grained Pure Ti Processed by Equal Channel Angular Pressing
Xin LI,Yuecheng DONG,Zhenhua DAN,Hui CHANG,Zhigang FANG,Yanhua GUO
Acta Metall Sin    2019, 55 (8): 967-975.   doi:10.11900/0412.1961.2019.00010
Accepted: 10 May 2019

Abstract104)   HTML2)    PDF (11271KB)(395)      

Titanium alloy has extensive applications in the field of chemical, biomedical and marine engineering due to high specific strength and excellent corrosion resistance. Ultrafine-grained (UFG) and nanocrystalline (NC) materials with unique properties processed by severe plastic deformation are widely studied in recent decades. In comparison with large number researches on mechanical behavior of UFG/NC materials, corrosion resistance is rarely studied and results indicated inconsistent, even within the same alloy system. In this work, ultrafine-grained pure Ti was fabricated by equal channel angular pressing (ECAP) with 2~4 passes. Grain size, crystallographic texture and grain boundary character distribution of samples were characterized by EBSD. At the same time, dynamic potential polarization and EIS methods were used to study corrosion resistance in simulated seawater. Results showed that grain size and basal texture strength of pure Ti decreased after 2 ECAP passes, but the fraction of low angle grain boundary (LAGB) increased drastically. With increasing of extrusion passes, grain size and the fraction of LAGB decreased for samples, meanwhile, basal texture strength increased at first and then decreased. Electrochemical experiments indicated that all UFG titanium have better corrosion resistance than coarse one. On the other hand, it was founded that corrosion resistance didn't increased monotonously with the development of ECAP passes, and 3 ECAP passes displayed optimum. This could be attributed to the interaction of grain size, basal texture and grain boundary character distribution, and basal texture strength occupied the domination.

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