Cover Picture: Materials and Corrosion. 1/2020

Cover: Friction stir processing (FSP) and subsequent aging heat treatment were used to modify the microstructure of AZ80 magnesium alloy. EBSD images (left) and pole figures (right) of (a, b) base material (BM), (c, d) FSP, (e, f) FSP-with aging treatment (5 h at 180°C) and (g, h) FSP-with aging treatment (24 h at 180°C) samples. The average grain size of FSP sample was refined from (a) to (c) due to dynamic recrystallization through FSP. After aging heat treatment, the average grain size of FSP-5 and FSP-24 samples was slightly larger (e, g) than that of the FSP sample. The pole figures clearly demonstrate that BM sample possessed an obvious preferred grain orientation. More detailed information can be found in: Liying Huang, Kuaishe Wang, Wen Wang, Pai Peng, Ke Qiao, Qiang Liu, Microstructural evolution and corrosion behavior of friction stir processed fine-grained AZ80 Mg Alloy, Materials and Corrosion 2020, 71, 93.     

Microstructure,Mechanical Properties,and Corrosion Behavior of Mg-Al-Ca Alloy Prepared by Friction Stir Processing

Friction stir processing (FSP) was used to modify the microstructure and improve the mechanical properties and corrosion resistance of an Mg-Al-Ca alloy. The results demonstrated that, after FSP, the grain size of the Mg-Al-Ca alloy was decreased from 13.3 to 6.7 μm. Meanwhile, the Al₈Mn₅ phase was broken and dispersed, and its amount was increased. The yield strength and ultimate tensile strength of the Mg-Al-Ca alloy were increased by 17.0% and 10.1%, respectively, due to the combination of fine grain, second phase, and orientation strengthening, while the elongation was slightly decreased. The immersion and electrochemical corrosion rates in 3.5 wt% NaCl solution decreased by 18.4% and 37.5%, respectively, which contributed to grain refinement. However, the stress corrosion cracking (SCC) resistance of the modified Mg-Al-Ca alloy decreased significantly, which was mainly due to the filiform corrosion induced by the Al₈Mn₅ phase. SCC was mainly controlled by anodic dissolution, while the cathodic hydrogen evolution accelerated the SCC process.     

Influence of sigma-phase formation on the localized corrosion behavior of a duplex stainless steel

Because of their austenitic-ferritic microstructures, duplex stainless steels offer a good combination of mechanical and corrosion resistance properties. However, heat treatments can lower the mechanical strength of these stainless steels as well as render them susceptible to intergranular corrosion (IGC) and pitting corrosion. In this study, a low-carbon (0.02%) duplex stainless steel is subjected to various heat treatments at 450 to 950 °C for 30 min to 10 h. The heat-treated samples then undergo ASTM IGC and pitting corrosion tests, and the results are correlated with the microstructures obtained after each heat treatment. In the absence of Cr₂₃C₆ precipitation, σ-phase precipitates render this duplex stainless steel susceptible to IGC and pitting corrosion. Even submicroscopic σ-phase precipitates are deleterious for IGC resistance. Longer-duration heat treatments (at 750 to 850 °C) induce chromium diffusion to replenish the chromium-depleted regions around the σ-phase precipitates and improve IGC resistance; pitting resistance, however, is not fully restored. Various mechanisms of σ-phase formation are discussed to show that regions adjacent to σ-phase are depleted of chromium and molybdenum. The effect of chemical composition (pitting resistance equivalent) on the pitting resistance of various stainless steels is also noted.     

Effect of thermomechanical treatment on the corrosion of AA5083

Corrosion resistance degradation induced by the presence of β-phase (Al₃Mg₂) limits the performance of aluminum alloy 5083 for some critical applications. The effect of thermomechanical treatment on the corrosion of AA5083 is presented in this paper. The samples subjected to the thermomechanical treatment showed superior corrosion resistance compared to the as-received samples. The effects of grain boundary character distribution, grain shape, texture, and precipitates on corrosion are discussed based on experimental observations. Special boundaries are expected to have a weak effect on the overall corrosion resistance of the material compared to the other factors.     

An investigation of the corrosion behaviour of a FeNiCoAlTa shape memory alloy in 3.5 wt-% NaCl solution

In this study, the corrosion behaviour of an FeNiCoAlTa (NCAT) shape memory alloy in 3.5% (w?w^?1) NaCl solution was evaluated. Linear polarisation resistance and potentiodynamic polarisation tests were conducted at 25°C. An open circuit potential (OCP) of ?381?mV (vs. saturated calomel reference electrode) and a corrosion rate (CR) of 0.0174?mm?y^?1 were obtained. The NCAT alloy did not show any passivation in the 3.5% (w?w^?1) NaCl solution. Its corrosion behaviour was very similar to that of G10180 (AISI 1018) carbon steel with the NCAT showing a lower CR and a less active OCP. Its corrosion in the solution was a combination of general dissolution and localised attack (pitting). Aging heat treatment caused β-Ni₃Al phase, and tantalum to precipitate on the grain boundaries, resulting in chemical species segregation between the grain boundaries and the grains. The chemical segregation caused intergranular corrosion of the alloy.     

CORRELATION BETWEEN MICROSTRUCTURE AND STRESS CORROSION CRACKING SUSCEPTIBILITY OF 2091 ALLOY

The stress corrosion cracking(SCC) susceptibility of an Al-Li-Cu-Mg-Zr alloy(2091),heat-treated under different regime,has been investigated hy means of tensile tests atslow strain rate.The microstructure has been examined with TEM.The deformationcharacteristic has also been analysed.The stress corrosion resistance of the alloy wasrevealed to be mainly affected by the covering rate of T_2-phase over grain boundariesand by the average distance between T_2-phase particles.SCC resistance can be obviously improved by predeforming the alloy at room temper-ature to reduce the precipitation of T_2-phase along grain boundaries.     

2091合金的显微组织与应力腐蚀敏感性

用慢应变速率拉伸试验研究不同热处理状态Al—Li—Cu—Mg—Zr(2091)合金的应力腐蚀敏感性,用透射电镜观察显微组织结构并分析变形特性。结果指出,T_2相在晶界上的复盖程度及T_2相之间的平均间距是影响合金应力腐蚀敏感性的主要因素,室温预形变处理可减少晶界上T_2相的数量,使合金应力腐蚀抗力得到明显提高。     

不同晶粒尺寸5083铝合金晶间腐蚀和应力腐蚀开裂的差异性

通过冷轧和退火获得具有不同晶粒尺寸(8.7～79.2μm)的5083铝合金板.研究其微观结构、晶间腐蚀(IGC)、应力腐蚀开裂(SCC)和裂纹扩展行为.结果表明,粗晶粒样品表现出更好的抗IGC性能,其腐蚀深度为15μm.慢应变速率测试结果表明,细晶粒样品表现出更好的抗SCC性能,敏感性指数ISSRT为11.2％.此外,...     

Effect of laser repairing on corrosion behaviour of metal–inert gas welding joint of 7N01 aluminium alloy

The heat affected zone (HAZ) on the metal–inert gas (MIG) welding joint of 7N01 aluminium alloy was repaired by multipass narrow gap laser welding. The Y–X direction precracked three‐point bending sample was used in the alternate immersion test. The morphology of specimen surfaces demonstrated that the exfoliation corrosion in the HAZ after laser repair (HAZ_a) was more serious than that before laser repair (HAZ_b). The electrochemical impedance spectroscopy after different immersion corrosion time indicated that the HAZ_a and HAZ_b had similar corrosion potentials. However, the pitting corrosion resistance of HAZ_a was lower than that of HAZ_b at the beginning of exfoliation corrosion. The stress corrosion crack (SCC) of 7N01P‐T4 aluminium alloy displayed a multicrack source and an intergranular crack propagated along the rolling grain boundary under the test condition. An unusual method was taken to measure the length of SCC. The results showed that laser repairing did not weaken the stress corrosion resistance of the original joint.     

Influence of the β-phase morphology on the corrosion of the Mg alloy AZ91

The influence of the microstructure, particularly the morphology of the β-phase, on the corrosion of Mg alloys has been studied using AZ91 as a model Mg alloy. The corrosion behaviour was characterized for five different types of microstructure produced by heat treatment of as-cast AZ91. The influence of microstructure can be understood from the interaction of the following three factors: (i) the surface films can be more or less effective in hindering corrosion and more or less effective in controlling the form of corrosion as uniform corrosion or localised corrosion, (ii) the second phase (the β-phase in AZ91) can cause micro-galvanic acceleration of corrosion and (iii) the second phase can act as a corrosion barrier and hinder corrosion propagation in the matrix, if the second phase is in the form of a continuous network. It is expected that these factors are important for all multi-phase Mg alloys because all known second phases have corrosion potentials more positive than that of the α-phase. A particular example of the corrosion barrier effect is provided by the fine (α + β) lamellar micro-constituent; when a β-phase plate nucleates this micro-constituent, the β-phase plate acts as a corrosion barrier. In contrast, nano-sized β precipitates, produced by aging, caused micro-galvanic corrosion acceleration of the adjacent α-phase. However, it is an important finding that the corrosion rate of the α-phase was decreased by the aging treatments that caused the precipitation of the nano-sized β particles.     

Effect of heat treatment on corrosion behaviour of magnesium alloy AZ91D in simulated body fluid

The research explored ways of improving corrosion behaviour of AZ91D magnesium alloy through heat treatment for degradable biocompatible implant application. Corrosion resistance of heat-treated samples is studied in simulated body fluid at 37 °C using immersion and electrochemical testing. Heat treatment significantly affected microgalvanic corrosion behaviour between cathodic β-Mg₁₇Al₁₂ phase and anodic α-Mg matrix. In T4 microstructure, dissolution of the β-Mg₁₇Al₁₂ phase decreased the cathode-to-anode area ratio, leading to accelerated corrosion of α-Mg matrix. Fine β-Mg₁₇Al₁₂ precipitates in T6 microstructure facilitated intergranular corrosion and pitting, but the rate of corrosion was less than those of as-cast and T4 microstructures.     

Enhanced mechanical properties of Mg-Gd-Y-Zr casting via friction stir processing

Mg-10Gd-3Y-0.5Zr casting was subjected to friction stir processing (FSP) at a tool rotation rate of 800 rpm and a traverse speed of 50 mm/min. FSP resulted in the fundamental dissolution of the coarse network-like β-Mg₅(Gd,Y) phase and remarkable grain refinement (∼6.1 μm), thereby significantly improving the strength and ductility of the casting. Post-FSP aging resulted in the precipitation of fine β′′ and β′ particles in a fine-grained magnesium matrix, producing an ultimate tensile strength of 439 MPa and a yield strength of 330 MPa. FSP combined with aging is a simple and effective approach to enhancing the mechanical properties of Mg-Gd-Y-Zr casting.     

Electrodeposition of chemically and mechanically protective Al-coatings on AZ91D Mg alloy

Electrodeposition of aluminium coatings upon AZ91D in aluminum chloride/1-ethyl-3-methylimidazolium chloride ionic liquid was achieved. Post-plating heat treatment processes compatible with AZ91D solution treatment (420 °C) and aging treatment (200 °C) were explored to improve coatings adhesion and hardness, and to maintain corrosion resistance. 420 °C treatment produced a β-phase (Mg₁₇Al₁₂) enriched two-phase coating; whilst treatment at 200 °C leads to a tri-layer structure, rich in γ-phase (Mg₂Al₃). The 200 °C treatment was shown to be most effective for corrosion resistance, eradicating water reduction as the principal cathodic reaction and increasing surface hardness.     

The Effect of the Surface Treating and High-Temperature Aging on the Strength and SCC Susceptibility of 7075 Aluminum Alloy

A novel heat-treatment procedure combining the shot-peening with a two-step aging operation was proposed to improve both the strength and the stress corrosion cracking (SCC) resistance of the high-strength 7075 aluminium alloy. The heat treatment included one shot-peening stage before or between the two stages of aging at 120 °C for 24 h and at 160 °C for 1 h, respectively. The mechanical properties obtained during the aforementioned operations were extremely similar to those of the T6 sample owing to the unaffected bulk microstructure over such a low over-aging period. The SCC resistance of these samples was considerably improved compared to that of the T6 sample and of the conventional shot-peened T6 sample due to the over-aging of the surface like the T7 treatment leading from the diffusion acceleration by the dislocations generated in the surface layer during shot-peening. In spite of the further depth of deformation caused by shot-peening prior to the first step of aging, the sample shot-peened after the first step of aging showed no significant decrease in the SCC resistance because of its higher generated dislocation by shot-peening.     

热处理对轧制态Mg5Gd合金腐蚀行为的影响

通过浸泡试验、电化学测试、扫描电化学显微镜和腐蚀形貌分析等手段研究热处理工艺对轧制态Mg5Gd合金在3.5 wt.％NaCl饱和Mg(OH)2溶液中腐蚀行为的影响及机理,以期达到提高镁合金耐蚀性的目的.结果表明:固溶处理能显著降低Mg5Gd合金的腐蚀速率,并且使其腐蚀变均匀,腐蚀坑变浅,这主要归因于固溶处理可以熔解镁基...     

碱热处理对搅拌摩擦加工AZ31镁合金耐腐蚀性能的影响

对AZ31镁合金进行搅拌摩擦加工(Friction stir processing,FSP),并对母材(Basal material, BM)和FSP试样进行碱热处理(Alkali heat treatment,AHT),研究了AHT对搅拌摩擦加工后AZ31镁合金微观组织和耐腐蚀性能的影响。结果表明,FSP可以显著细化晶粒,平均晶粒尺寸由BM的12.8 μm细化至FSP后的3.1 μm,高角度晶界比例从BM的75.9%降低至FSP后的45.3%,晶界亚结构增多。AHT使材料表面形成致密的MgO和Al₂O₃混合涂层,有效地提高了AZ31镁合金的耐浸泡腐蚀性能。     

Effect of different aging processes on the corrosion behavior of new Al–Cu–Li–Zr–Sc alloys

The intergranular corrosion and exfoliation corrosion behaviors of Al–Cu–Li–Zr–Sc alloys under different aging effects, such as single‐stage aging, strain aging, and double‐stage aging, were studied. Among the three aging treatments, single‐stage aging resulted in the best resistance to corrosion, followed by double‐stage aging; strain aging resulted in the worst corrosion resistance. A 3.5% precooling strain could increase the dislocation density, which promoted the precipitation of corrosion‐prone T₁ phase and increased the corrosion driving force of the alloy. Double‐stage aging made the precipitated T₁ phases finer and more uniform and reduced the number of equilibrium phases at grain boundaries, thus improving the corrosion properties of the alloy. The corrosion susceptibility of the alloy was attributed to the T₁ phase and precipitate‐free zone (PFZ), and the underlying corrosion mechanism was revealed as preferential dissolution of the equilibrium phase at grain boundaries and its surrounding distortion zone, followed by expansion of the PFZ along the grain boundaries, resulting in the development of corrosion from the grain boundaries to the intragranular regions.     

Investigation of the relationship between intergranular corrosion and retrogression and reaging in the AA6063

AA6063 was heat treated with different retrogression temperatures and durations, and the effect of heat treatment conditions on the microstructure, hardness, electrical conductivity, intergranular corrosion (IGC) and electrochemical corrosion behaviours of the AA6063 was determined compared with the T6 condition. The IGC test was applied according to the BS EN ISO 11846: 2008 standard. Moreover, potentiodynamic polarization tests were applied to determine the electrochemical corrosion behaviour of the heat‐treated samples. Electrochemical corrosion tests were carried out by using a Ivium Compactstat potentiostat in 3.5 wt.%. NaCl solution at 24°C with a scanning rate of 0.5 mV/s. The corrosion test cell consisted of the reference electrode (Ag/AgCl), working electrode (test sample) and a reference electrode (platinum). The effect of IGC on the microstructure of AA6063 and corrosion depth values was investigated by using a stereo optical microscope and a light metal microscope, respectively. Corrosion depth examinations were performed on microstructures taken from the cross‐sections of the samples. The chemistry of the precipitates formed at grain boundaries and distribution of the precipitates in the microstructure were investigated by scanning electron microscope, energy dispersive X‐ray and transmission electron microscope analyses. The results showed that retrogression and reaging heat treatment improves both the corrosion resistance and the mechanical properties of AA6063. After 50°C/15 min RRA heat treatment, the highest corrosion resistance and a higher hardness value than the T6 level were obtained.     

Effects of solution heat-treatment and nitrogen in shielding gas on the resistance to pitting corrosion of hyper duplex stainless steel welds

The effects of solution heat-treatment and shielding gas on the pitting corrosion of hyper duplex stainless steel (HDSS) welds were investigated in highly concentrated chloride environments. The pitting resistance of a solution heat-treated HDSS after welding with an Ar shielding gas supplemented with N₂ was greatly increased due to the dissolution of Cr₂N in α-phase, which followed the diffusion of N atoms from the α-phase to the γ-phase and an increase of the γ-phase in the weld metal and heat affected zone. It was also attributed to a decrease of the pitting resistance equivalent number difference between the two phases.     

Corrosion behaviour of bulk ultra-fine grained AZ91D magnesium alloy fabricated by equal-channel angular pressing

Effect of microstructure change on corrosion behaviour of equal-channel angular pressed (ECAPed) AZ91D Mg alloy was investigated. The ECAPed alloy with ultra-fine grained (UFG) α-phase matrix and refined β-phase particles displays a significantly lower corrosion resistance, resulting in more pits after in-situ corrosion, higher mass-loss rate immersed in NaCl solution, larger I_corr values in polarization curves and lower fitted R_t values in EIS plots. Two factors weaken the corrosion resistance: the first is the strains-induced crystalline defects providing the α-phase matrix more corrosion activation, the second is the refined β-phase particles losing barrier to the corrosion propagation in α-phase matrix.