Localized Corrosion and Stress Corrosion Cracking Behavior of AA7003 in a 3.5 wt% NaCl Aqueous Solution

The microstructure,localized corrosion (LC) and stress corrosion cracking (SCC) behavior of 7003 aluminum alloy (AA7003) under various aging treatments (peak aging (PA),double peak aging (DPA),regression and re-aging (RRA)) were investigated by means of transmission electron microscope (TEM),scanning electron microscopy (SEM),electrochemical impendence spectroscopy (EIS) and slow strain rate tensile test.The results of TEM showed a discontinuous distribution of grain boundary precipitates of AA7003 under DPA and RRA treatments,which is beneficial for increasing the resistance of LC and SCC.Meanwhile,LC was found initiating firstly on intermetallics which caused the dissolution of surrounding matrix,then pitting holes were formed and developed into matrix.In addition,the SCC process of AA7003 could be divided into two stages,i e,initial pre-cracking and breeding cracking.The EIS analysis,cross-section morphologies and fracture surfaces of specimens indicated that DPA and RRA treatments significantly decreased the crack growth rate during breeding cracking stage,especially for RRA treatment.     

Effects of Different Electrolytes on Stress Corrosion Properties of 2A12 Aluminum Alloy

The stress corrosion cracking (SCC) behaviors of 2A12 aluminum alloy after annealing treatment were studied by slow strain rate testing (SSRT),electrochemical polarization measurement,scanning electron microscope (SEM),energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM).Various concentrations of NaCl,H_2SO_(4 ) and HCl aqueous solution were prepared to act as the corrosive solution.The experimental results show that regarding the SCC,2A12 alloy performs best in NaCl solution but worst in HCl solution and intermediately between the above mentioned two cases in H_2SO_4 solution.For the SSRT carried out in room temperature,there is a higher decrease in elongation without large strength loss for the alloy immersed in NaCl solution.With the test conducted in H_2SO_4 solution,there is a higher strength loss and a relatively less loss of elongation compared to the one immersed in NaCl solution.With the test conducted in HCl solution,there is a relativel level loss of strength and elongation compared to either result carried out in NaCl solution or H_2SO_4 solution.     

Microstructure and mechanical properties of Al-Zn-Cu-Mg-Sc-Zr alloy after retrogression and re-aging treatments

The mechanical properties and stress corrosion cracking (SCC) resistance of an Al-Zn-Cu-Mg-Sc-Zr alloy under different aging conditions were investigated. The dependence of microstructure and mechanical properties on aging parameters was evaluated by tensile test, hardness test and conductivity measurement. The results show that for the alloys with retrogression and re-aging treatment (RRA), the conductivity increases with the retrogression time and temperature, while the tensile strength decreases. The transmission electron microscopy (TEM) results show that the precipitates η(MgZn₂) at grain boundary aggregate apparently with retrogression time and the precipitates inside the matrix exhibit the similar distribution to T6 temper, which comprises fine GP zones, large η′(MgZn₂) and η(MgZn₂) phases. According to the mechanical properties and microstructure observations, the optimal RRA regime is recommended to be 120 °C, 24 h + 180 °C, 30 min + 120 °C, 24 h. The strength level of the alloy after the optimum RRA treatment is similar to that in T6 condition and the SCC resistance is improved obviously in contrast to T6 condition.     

X70钢在鹰潭酸性土壤中的应力腐蚀行为

在不同的阴极保护电位下,采用慢应变速率拉伸试验(SSRT)和电化学方法研究X70钢在水饱和鹰潭土壤中的应力腐蚀开裂行为,为酸性土壤地区的X70钢管线的腐蚀防护提供基础参考数据.结果表明,X70钢在实验所用的酸性土壤环境中能够发生穿晶应力腐蚀裂纹(TGSCC);SCC萌生与外加保护电位有关,外加电位较高、X70钢完全受阳极过程控制时的SCC敏感性较低;外加电位较低、X70钢受阴阳极混合电极过程控制或完全受阴极过程控制时均能发生SCC,且其敏感性随外加电位的降低而增加,且完全受阴极过程控制时的SCC敏感性大大高于其它情况.     

Effect of deteriorated microstructures on stress corrosion cracking of X70 pipeline steel in acidic soil environment

In order to investigate stress corrosion cracking (SCC) of X70 pipeline steel and its weld joint area in acidic soil environ-ment in China, two simulating methods were used: one was to obtain bad microstructures in heat affected zone by annealing at 1300°C for 10 min and then, quenching in water; the other was to get different simulating solutions of acidic soil in Yingtan in south-east China. The SCC susceptibilities of X70 pipeline steel before and after quenching in the simulating solutions were analyzed...     

Through-thickness inhomogeneity of localized corrosion in 7050-T7451 Al alloy thick plate

The through-thickness corrosion inhomogeneity of 7050-T7451 Al alloy thick plate was studied using immersion tests, potentiodynamic polarization, electrochemical impedance spectroscopy(EIS), slow strain rate testing(SSRT) technique combined with optical microscopy(OM), scanning electron microscopy(SEM) and transmission electron microscopy(TEM). The results show that the through-thickness corrosion resistance is ranked in the order of T/2surfaceT/4. And the 75 mm-thick 7050 alloy plate presents better corrosion resistance than the 35 mm-thick plate. The results are discussed in terms of the combined effect of recrystallization and cooling rate in quenching. Alloy with lower volume fraction of recrystallization and smaller grain aspect ratio displays better corrosion resistance. The lower corrosion resistance caused by the slower cooling rate results from the higher coverage rate of grain boundary precipitates and larger width of precipitate free zone.     

Intergranular corrosion behavior of high nitrogen austenitic stainless steel

The intergranular corrosion (IGC) behavior of high nitrogen austenitic stainless steel(HNSS) sensitization treated at 650-950℃ was investigated by the double loop electrochemical potentiodynamic reactivation(DL-EPR) method.The effects of the electrolytes,scan rate,sensitizing temperature on the susceptibility to IGC of HNSS were examined. The results show that the addition of NaCl is an effective way to improve the formation of the cracking of a passive film in chromium-depleted zones during the reactivation scan. Decreasing the scan rate exhibits an obvious effect on the breakdown of the passive film. A solution with 2 mol/L H2_SO_4+1 mol/L NaCl+0.01 mol/L KSCN is suitable to check the susceptibility to IGC of HNSS at a sensitizing temperature of 650-950℃ at a suitable scan rate of 1.667 mV/s. Chromium depletion of HNSS is attributed to the precipitation of Cr_2N which results in the susceptibility to IGC. The synergistic effect of Mo and N is suggested to play an important role in stabilizing the passive film to prevent the attack of IGC.     

中温轧制对Al-Cu-Mg-Ag耐热铝合金组织与抗腐蚀性能的影响

通过晶间腐蚀和剥落腐蚀性能测试,对经过不同中温轧制变形的Al-Cu-Mg-Ag合金的抗腐蚀性能进行研究,并通过金相分析和透射电子显微分析等对其机理进行探讨。结果表明,采用合适的中温轧制变形后,合金的抗腐蚀性能在不同程度上得到提高。随着变形量的增加,晶内析出相发生变形并向晶界移动,导致晶间无沉淀析出带变窄,合金的腐蚀通道变窄,抗晶间腐蚀性能得到提高。另一方面,沿着轧向变长的晶粒会降低合金的剥落腐蚀性能,导致变形量较小时(10%~50%)合金的抗腐蚀性能得到提高,而较大的变形量(80%)会降低合金的抗剥落腐蚀性能。     

Corrosion behaviors of NdFeB magnets prepared by spark plasma sintering

The spark plasma sintering (SPS) technique was introduced into the field of NdFeB preparation due to its own advantages. High property NdFeB magnets with fine grains were prepared by SPS method. The corrosion behaviors of SPS NdFeB were studied by electrochemical measurements and 92% RH hyther tests at 353 K. The results were compared with those of the traditional sintered NdFeB magnets. It shows that both the SPS NdFeB and the traditional sintered NdFeB have good corrosion resistance in alkaline environment due to surface passivation; while, the fine grain microstructure of SPS NdFeB results in a more homogeneous phase com-position distribution and thus reduces the electrochemical inhomogenity between the ferromagnetic phase and the Nd-rich inter-granular phase in the magnet. Therefore, the SPS NdFeB exhibits better corrosion resistance than the traditional sintered NdFeB in neutral and weak acidic environment.     

Effect of a novel three-step aging on strength,stress corrosion cracking and microstructure of AA7085

The influence of a novel three-step aging on strength, stress corrosion cracking(SCC) and microstructure of AA7085 was investigated by tensile testing and slow strain rate testing combined with transmission electron microscopy(TEM). The results indicate that with the increase of second-step aging time of two-step aging, the mechanical properties increase first and then decrease, while the SCC resistance increases. Compared with two-step aging, three-step aging treatment improves SCC resistance and the strength increases by about 5%. The effects of novel three-step aging on strength and SCC resistance are explained by the role of matrix precipitates and grain boundary precipitates, respectively.     

Corrosion leaking of preheater weldment in alumina factories

Stress corrosion cracking （SCC） and anticorrosion measures of TU42C weld-joint were studied by constant load experiments and pickling experiments. The results show that in 40% （mass fraction） NaOH solution at 110℃, caustic SCC occurs in TU42C weld-joints at the applied potential of —1020 mV（vs SCE） for 3 d while at the potential of—950 mV（vs SCE） for 10 d. All the cracks are intergranular. In the 10% sulfuric acid, the cracks have the most negative self-corrosion potential —432.5 mV（vs SCE） and are active to be further corroded by the acid. Because of the same corrosion behaviour as the lab weldment, preheater＇s cracking in alumina factories is attributed to the combining actions of previous caustic SCC in Bayer solutions and continuous acid corrosion by pickling with the addition of RD. The following measures are effective to prevent the corrosion failure of preheater, such as postweld heat treatment at 620 ℃ to relax the residual weld stress, addition of CC3 and L826 as the corrosion inhibitors to improve the pickling and cleaning by the high pressure water instead of by pickling.     

Influence of welding speed on corrosion behaviour of friction stir welded AA5086 aluminium alloy

The plates of AA5086 aluminium alloy were joined together by friction stir welding at a fixed rotation speed of 1000 r/min various welding speeds ranging from 63 to 100 mm/min.Corrosion behavior of the parent alloy(PA),the heat affected zone(HAZ),and the weld nugget zone(WNZ)of the joints were studied in 3.5%(mass fraction)aerated aqueous Na Cl solution by potentiodynamic polarization and electrochemical impedance spectroscopy(EIS).The corrosion susceptibility of the weldments increases when the welding speed increases to 63 and 100 mm/min.However,the value of corrosion rate in the weldments is lower than that in the PA.Additionally,the corrosion current density increases with increasing the welding speed in the HAZ and the WNZ.On the contrary,the corrosion potential in the WNZ appears more positive than in the HAZ with decreasing the welding speed.The WNZ exhibits higher resistance compared to the HAZ and the PA as the welding speed decreases.The results obtained from the EIS measurements suggest that the weld regions have higher corrosion resistance than the parent alloy.With increasing the welding speed,the distribution and extent of the corroded areas in the WNZ region are lower than those of the HAZ region.In the HAZ region,in addition to the pits in the corroded area,some cracks can be seen around the corroded areas,which confirms that intergranular corrosion is formed in this area.The alkaline localized corrosion and the pitting corrosion are the main corrosion mechanisms in the corroded areas within the weld regions.Crystallographic pits are observed within the weld regions.     

草酸法检测铁素体不锈钢对晶间腐蚀的敏感性

采用草酸腐蚀方法和光学显微分析，研究了000Cr26Mo1、000Cr30Mo2和000Cr26Mo1Nb几种材料对晶间腐蚀的敏感性。试验表明000Cr26Mo1钢在固溶状态下抗腐蚀性良好，但在焊后或敏化处理状态存在一定程度的晶间腐蚀敏感性，而000Cr26Mo1Nb钢在三种状态下对晶间腐蚀不敏感。本试验参照美国ASTM：A736－86标准（草酸法检测铁素体不锈钢对晶间腐蚀敏感性的规程）中的W－试验。试验结果与X－试验（Fe2(SO4)3-H2SO4腐蚀试验）比较得出草酸腐蚀方法是一种正确、快速而灵敏的检测材料晶间腐蚀的手段。     

纳米晶铜块体材料电化学腐蚀行为研究

针对纳米晶（NC）单质金属材料的腐蚀特性，研究了块体纳米晶铜的耐蚀性能.利用电化学方法，结合XRD、EDS和SEM表面分析技术，比较分析了惰性气体沉积原位温压法（IGCWC）制备的块体纳米晶铜和多晶铜在0.1 mol/L CuSO₄和0.05 mol/L H₂SO₄混合溶液中的腐蚀行为.结果表明，纳米晶铜的腐蚀呈均匀的表面溶解并伴随有不均匀的局部腐蚀，而多晶铜呈沿晶界均匀分布的晶界点蚀.纳米晶铜表面原子和晶界原子具有更高的表面活性，钝化能力大大提高，同时钝化膜的溶解速度加大.纳米试样制备过程中产生的微孔隙缺陷降低了纳米晶铜的耐蚀性.与多晶铜相比，纳米晶铜耐蚀性降低     

Corrosion resistance of waterborne epoxy coating pigmented by nano-sized aluminium powder on steel

A novel kind of waterborne epoxy coating pigmented by nano-sized aluminium powders on high strength steel was formulated. Several coatings with different pigment volume content (PVC) were prepared. The coating morphology was observed using scanning electron microscopy (SEM), and the electrochemical properties were investigated by electrochemical impedance spectroscopy (EIS). Immersion test and neutral salt spray test were also conducted to investigate the corrosion resistance of the coating. It is demonstrated that the critical pigment volume content (CPVC) value is between 30% and 40%. The coating with PVC of 30% exhibits good corrosion resistance in 3.5% (mass fraction) NaCl solution.     

Microstructure and RRA treatment of LFEC 7075 aluminum alloy extruded bars

The microstructures after casting and extruding, the mechanical properties and electrical conductivity after RRA treatment of conventional DC casting and low frequency electromagnetic casting (LFEC) 7075 aluminum alloy were investigated. The results showed that finer grains which distributed more homogeneously was obtained in LFEC ingots compared with those conventional DC ingots. The extruded bars of LFEC alloy kept its fine grain features of original as-cast structure. In the RRA treatment, with the extension of second aging time, the tensile strength and hardness of alloy decreased, but the electrical conductivity increased. Meanwhile, as the second aging temperature raised, the phase change rate in precipitation also increased. Under the same conditions, extruded bars of LFEC alloy had better performance than that of conventional DC cast alloy. The optimum RRA heat treatment process was 120 ℃/24 h+180 ℃/30 min+120 ℃/24 h. The LFEC extruded bars acquired tensile strength 676.64 MPa, hardness 198.18, and electrical conductivity 35.7% IACS respectively, which were higher than that in the T6 temper, indicating that a notable RRA response takes place in LFEC extruded bars, whose second-step retrogression time was 30 min, and it was suitable for mass production.     

Investigation of stress corrosion cracking initiation of 7A52 aluminum alloy

The stress corrosion cracking(SCC) behaviour of 7A52 aluminum alloy in air and in 3.5% NaCl solution was researched by slow strain rate test(SSRT) and SEM-EDS. The SCC susceptibility was estimated with the loss of the reduction in area. The experimental results indicate that the SCC susceptibility of 7A52 aluminum alloy in 3.5% chloride solution is the highest at strain rate of 1×10-6 s-1. The lowest one is under the condition of 1×10-5 s-1. Stress concentration and anode dissolving around Al-Fe-Mn intermetallics initiate micropores which will result in microcracks. The existence of intermetallics in the microstructure may play an important role in understanding the SCC initiation mechanisms of 7A52 aluminum alloy.     

Corrosion-enhanced dislocation emission and motion resulting in initiation of stress corrosion cracking

A special constant deflection device for TEM has been designed, and then change of dislocation configuration ahead of a crack tip during stress corrosion cracking (SCC) of brass in water and of Ti-24Al-11Nb alloy in methanol and initiation of SCC can he observed in TEM In situ tensile test in TEM for brass was carried out for comparison The results show that anodic dissolution during SCC can facilitate dislocation emission, multiplication and motion, and a dislocation free zone (DFZ) is formed The stress at a particular site in the DFZ, which is an elastic zone and is thinned gradually through corrosion-enhanced dislocation emission and motion, is possibly up to the cohesive strength, resulting in initiating of a nanocrack of SCC in the DFZ or sometimes at the crack tip. Because of the action of the corrosion solution the nanocrack of SCC propagates into a cleavage or intergranular microcrack rather than blunts into a void like in situ tension in TEM     

碘化钾对铝合金表面化学镀镍耐蚀性的影响

通过扫描电子显微镜考察了碘化钾(KI)对2024铝合金表面化学镀Ni-P层表面形貌的影响,采用 浸泡法和动电位极化以及交流阻抗等电化学方法,研究了KI对Ni-P镀层在质量分数为3.5%的NaCl溶液中腐蚀 行为的影响。实验结果表明,KI减少了Ni-P镀层中表面缺陷的数量,细化了晶粒,镀层更加平整致密,表面质量得 到改善。同时,KI也增加了镀层中磷的质量分数。这两个原因使Ni-P镀层的耐蚀性进一步提高。     

奥氏体不锈钢晶间腐蚀

综述了奥氏体不锈钢产生晶间腐蚀的基本原理,影响其晶间腐蚀的各种因素以及可采取的相应的措施,并提出了未来应用价电子理论从电子结构层次上探究和改善不锈钢耐晶间腐蚀的想法。