Investigation of the initial stage of crevice corrosion on Al99.5 by electrochemical noise analysis

The electrochemical noise analysis (ENA) is a powerful tool to investigate the initial state of local corrosion processes as pitting corrosion or crevice corrosion on aluminium alloys. The focus of this work lays on Al99.5 (AA1050). In spite of the good corrosion protection and the very low passive current density the noise activity in the early state of corrosion can be very well observed. ENA was performed by using current noise measurement under potentiostatic control. The effect of chromate inhibitors on the noise activity of the initial state of corrosion on Al99.5 is surprisingly negligible. This phenomenon is discussed in this paper.     

Corrosion behavior of under‐, peak‐, and over‐aged 6063 alloy: A comparative study

The mechanical property of age‐hardenable Al‐alloys is governed by the state of ageing, which determines the microstructure and consequently, their corrosion behavior which is a vital aspect for a number of applications. This article presents a comparative assessment of corrosion behavior of under‐, peak‐ and over‐aged Al‐Mg‐Si alloy. Corrosion characteristics have been determined via immersion tests in 0.1 M ortho‐phosphoric acid solution and intergranular corrosion (IGC) tests. Corroded surfaces are examined by field emission scanning electron micrographs‐energy dispersive spectroscopy and 3D optical profilometer. The obtained results reveal that the corrosion rate at a specific immersion time as well as the depth of IGC increases in the order for under‐, peak‐, and over‐aged states. Irrespective of the state of ageing, corrosion loss increases linearly but the rate of corrosion decreases rapidly with increasing immersion time. The dominant mode of corrosion in under‐aged alloy is identified as localized pitting, while peak‐aged is highly susceptible to IGC in contrast extensive pitting corrosion is observed for over‐aged alloy. The observed differences in corrosion behavior are explained considering characteristics of precipitates. Formation of β (Mg₂Si) in case of over‐aged alloy and presence of inclusions like AlFeMnSi particles are found to accelerate pitting corrosion.     

Al-Mg-Si合金中Mg_2Si和Si粒子在晶间腐蚀过程中的作用机理

研究Al-Mg-Si合金晶界组成相（Al-Mg2Si及Al-Mg2Si-Si）间的电化学行为和动态电化学耦合行为,提出Al-Mg-Si合金的晶间腐蚀机理。研究表明,晶界Si的电位比其边缘Al基体的正,在整个腐蚀过程中作为阴极导致其边缘Al基体的阳极溶解;晶界Mg2Si的电位比其边缘Al基体的负,在腐蚀初期作为阳极发生阳极溶解,然而由于Mg2Si中活性较高的元素Mg的优先溶解,不活泼元素Si的富集,致使Mg2Si电位正移,甚至与其边缘Al基体发生极性转换,导致其边缘Al基体的阳极溶解。当n（Mg）/n（Si）〈1.73时,随着腐蚀的进行,合金晶界同时会有Mg2Si析出相和Si粒子,腐蚀首先萌生于Mg2Si相和Si边缘的无沉淀带,而后,Si粒子一方面导致其边缘无沉淀带严重的阳极溶解,另一方面加速Mg2Si和晶界无沉淀带的极性转换,从而促使腐蚀沿晶界Si粒子及Mg2Si粒子边缘向无沉淀带发展。     

铝合金在海洋环境中的腐蚀研究(Ⅱ)--海水全浸区16年暴露试验总结

总结了铝合金在青岛海域海水全浸区暴露16年的腐蚀结果，防锈铝LF2Y2，LF6M（BL），F21M，180YS在海水全浸区有好的耐蚀性，工业纯铝L4M，锻铝LD2CS的耐蚀性较差，无包铝层的硬铝LY12CZ和超硬铝LC4CS在海水中的耐蚀性很差，硬铝，超硬铝的包铝层起牺牲阳极作用，使基体受到保护，海生物污损对铝合金在海水中的腐蚀有明显影响，镁、锰能提高铝的耐海水腐蚀性，硅明显降低铝的耐蚀性，铜严重损害铝的耐蚀性，腐蚀电位较负的铝合金耐海水腐蚀性较好，腐蚀电位较正的铝合金耐海水腐蚀性较差。     

铝合金在海洋环境中的腐蚀研究（Ⅰ）：——海水潮汐区16年暴露试验总结

总结了铝合金在青岛海域海水潮汐区暴露16年的腐蚀试验结果，防锈铝LF2Y2、LF6M（BL）、F21M、180YS在海水潮汐区有好的耐蚀性，工业纯铝L4M、锻铝LD2CS的耐蚀性较差，硬铝LY11CZ（BL）、LY12CZ（BL）和超硬铝LC4CS（BL）的包铝层起着牺牲阳极作用，使基体受到保护，海生物污损对铝合金的腐蚀有明显影响，镁，锰能提高铝在海水潮汐区的耐蚀性，硅明显降低铝的耐蚀性，铜严重损害铝的耐蚀性。     

Evolution of corrosion in cast Al alloy in antifreeze radiator coolant

Corrosion is an important issue for cast Al alloy in an engine cooling system, but how the microstructural features affect the coolant‐related corrosion behaviour is not well understood. In this research, the evolution of corrosion in an ISO 2379 cast Al alloy was studied in an antifreeze radiator coolant under heat‐rejecting conditions. Extensive analyses of microstructures and corroded surfaces were carried out using an optical microscope, scanning electron microscope equipped with energy dispersive spectroscopy and X‐ray diffractometer. Intergranular cavitation corrosion was observed to occur at interfaces between α‐Al matrix and intermetallics (Al₂Cu and Al₅FeSi) or to a less degree at interfaces between α‐Al matrix and Si phase. The large area fraction of the cathodic phases (Al₂Cu, Al₅FeSi and Si) led to the galvanic coupling between them and the adjacent anodic α‐Al matrix. The heat‐rejecting condition in antifreeze radiator coolant was favourable condition to cavitation process while severe crevice corrosion was predominant at oxygen‐depleted regions in the heat‐transfer corrosion cell.     

Corrosion behaviour of Mg–Al alloys with Al–11Si thermal spray coatings

The corrosion resistance of AZ31, AZ80 and AZ91D Mg–Al alloys with Al–11Si thermal spray coatings was evaluated by electrochemical and gravimetric measurements in 3.5 wt% NaCl solution. The changes in the morphology and corrosion behaviour of the Al–11Si coatings induced by a cold‐pressing post‐treatment under 32 MPa were also examined. The as‐sprayed Al–11Si coatings revealed high degree of porosity and poor corrosion protection, which resulted in galvanic acceleration of the corrosion of the magnesium substrates. The application of a cold‐pressing post‐treatment produced more compact Al–11Si coatings with better bonding at the substrate/coating interface and slightly higher corrosion resistance. However, interconnected pores remained in the cold‐pressed coatings due to the low plasticity of the Al–11Si powder and galvanic corrosion of the substrate was observed after immersion in 3.5 wt% NaCl for 10 days.     

Theoretical description of the practical possibility of stress corrosion cracking from crevice corrosion sites

Stress corrosion cracking (SCC) from crevice corrosion sites had been found in an experimental work at polarization potential of + 200 mV_SCE. In that work, an occluded U‐bend specimen of Type 316L (UNS S31603) stainless steel was used. The testing was done in sodium chloride (NaCl) solution. Based on that work, the practical possibility of SCC from the occluded U‐bend specimen was described theoretically. It was shown that it would also be possible for SCC to occur in practice (i.e. at practical corrosion potential), but the crevice needs to be tighter. Meanwhile, it would take a longer time for obvious SCC to emerge. For a practical crevice usually formed by placing a crevice former on a large uniform metal surface, the crevice geometry may have little effect on SCC although the crevice can sustain an acidified solution more easily than pitting. The possibility of SCC should mainly depend on the corrosion system itself, i.e. material and environment.     

Corrosion mechanism of aluminum alloy by ethylene glycol‐based solution

Corrosion and failure mechanisms on an aluminum (Al) alloy by ethylene glycol base solution have been studied. Extensive crevice corrosion damage and localized scale deposition are clearly observed between the failed Al alloy surface and the rubber hose, in which the leakage of ethylene glycol‐based solution occurred under an actual operating. In addition, many micro‐cracks are found at the rubber hose which is clamped with the Al alloy. The scales on the Al surface appear to be due to the occurrence of Al corrosion, followed by the precipitation of additives of ethylene glycol‐based solution in the crevice between Al alloy and rubber hose. The mechanisms that can account for the crevice corrosion and solution leakage on the Al alloy surface are discussed.     

SKPFM investigations of intermetallic compounds of innovative Er‐ and Zr‐containing Al–Si–Mg alloys and their influence on corrosion localization in saline solution

The paper aims at characterizing the influence of intermetallic compounds on the corrosion localization of innovative Al–Si–Mg Er‐ and Zr‐containing casting alloys. Samples of the investigated materials were studied by means of optical and scanning electron microscope micrographs, immersion tests, and scanning Kelvin probe force microscope (SKPFM) analyses in the T6 temper. Combination of immersion tests and SKPFM analyses allowed to identify those classes of intermetallic compounds promoting localization of the corrosion process. It was found that intermetallic compounds richer in Fe were the most critical for corrosion localization; furthermore, additions of Er caused a marked decrease of the potential difference of intermetallic compounds with respect to the Al matrix and a consequent less intense microgalvanic coupling, which translates into slower corrosion kinetics. Further, Zr additions slightly increased the potential difference of intermetallic compounds with the Al matrix, promoting a faster corrosion process.     

The influence of copper content on intergranular corrosion of model AlMgSi(Cu) alloys

Application prospects in automotive and aerospace industry have led to extensive studies on AA6xxx alloys in recent years. Varying amounts of Mg, Si and Cu and heat treatments are used to achieve the desired mechanical properties in these alloys. In this investigation, a series of tests have been designed and carried out on model AlMgSi(Cu) alloys to investigate the effects of Cu, Mg and Si composition and heat treatments on the corrosion properties for a range of Cu content levels (0.03, 0.15 and 0.80 wt%, respectively). The results indicate that the localized corrosion susceptibility of the model alloys primarily increased with Cu content. The Si and Mg content and ratio do not appear to have a significant effect on the local corrosion behaviour. Heat treatment can improve the corrosion resistance. However, this effect is small compared to that of the Cu content for the range of model alloys investigated. Intergranular corrosion will occur by micro‐galvanic coupling between the cathodic AlMgSi(Cu) (Q) phase precipitates and the aluminum matrix adjacent to the particles. The increasing susceptibility to intergranular corrosion with Cu content can be attributed to an increased formation of Q phase particles.     

常用牺牲阳极合金在NaCl溶液中的接触腐蚀行为

研究了作为牺牲阳极材料的ＭｇＡｌＺｎＭｎ、ＡｌＺｎＩｎＳｉＭｇ和ＺｎＡｌＣｄ合金与Ａ３钢偶接时，溶液中Ｃｌ－浓度和电偶对中阴阳极面积比变化对三种合金接触腐蚀行为的影响。偶对阳极电偶电流密度ｊｇ随Ｃｌ－浓度增大而增大，ｊｇ与面积比成正比关系。偶对电偶电势Ｅｇ随Ｃｌ－浓度增大和面积比减小向负向变化。当偶对中阴极金属材料为３０２不锈钢和紫铜时，ｊｇ随Ｃｌ－浓度增大有不同的变化趋势。     

Improving corrosion resistance of Al‐11mass%Si alloy through a large number of ECAP passes

Ultrafine‐grained (UFG) Al‐11mass%Si alloy, processed by multi‐pass equal‐channel angular pressing (ECAP) at 573 K, was investigated on corrosion behavior in 0.6 M NaCl solution. Potentiodynamic polarization tests and scanning electron microscopy observation showed that a large number of ECAP passes resulted in lower corrosion current density, more positive corrosion potential, and rather smooth corroded surface with shallow corrosion pits. The uniform distribution of fine secondary‐phase particles on UFG Al matrix weakened the susceptibility to pitting corrosion while inhibited general microgalvanic reactions. The present results indicate that grain refinement of aluminum matrix to the UFG state and uniform redistribution of broken particles (including eutectic silicon and secondary phases), via severe plastic deformation at elevated temperature undergoing dynamic recrystallization, can significantly improve the corrosion resistance of Al alloys, besides the known exceptional mechanical advantages. The simple and effective ECAP procedure makes UFG Al alloys more attractive for high strength structural application in corrosive environment.     

Effects of Mn on Corrosion Resistant Property of AZ91 Alloys

采用电化学、静态失重、盐雾腐蚀法研究了Mn对热处理态AZ91合金耐蚀性能的影响,利用扫描电镜观察试样的微观形貌,用X射线衍射仪分析合金的物相组成和腐蚀产物。结果表明,Mn与AZ91合金中的Al形成了独立相Al6Mn,该相溶解到Mg固溶体中提高了Mg的电极电位,进而提高了合金的耐腐蚀性;Mn加入后使合金的自腐蚀电位升高,自腐蚀电流密度降低,降低了合金的腐蚀速率,进而提高了合金的耐腐蚀性能,且三种实验方法都表明AZ91-0.8Mn合金的耐腐蚀性能最好。     

Effects of Sn and Ca additions on microstructure,mechanical properties,and corrosion resistance of the as‐cast Mg‐Zn‐Al‐based alloy

Effects of Sn and Ca additions on microstructure, mechanical properties, and corrosion resistance of the as‐cast Mg‐6Zn‐2Al‐based alloy were investigated by SEM, XRD, tensile tests, electrochemical measurements, etc. The higher the Sn content, the higher the yield strength of the Mg‐6Zn‐2Al‐based alloy. Trace Ca addition refined both grains and divorced eutectics in the Mg‐Zn‐Sn‐Al alloys, leading to the best combination of strength and plasticity. Moreover, its influence was more significant for the alloy with a higher Sn content. Furthermore, the combined addition had a beneficial effect on the corrosion resistance improvement of the Mg‐6Zn‐2Al alloy. The Mg‐6Zn‐2Al‐3Sn‐0.2Ca alloy was the most corrosion‐resistant alloy among the nine alloys studied, better than AZ91 and ZA85. It could be ascribed to the decrease and the refinement of divorced eutectics, the higher hydrogen overvoltage of Sn and the Ca grain refinement.     

Microstructures and corrosion behaviors of Al–6.5Si–0.45Mg–xSc casting alloy

The microstructures and corrosion behaviors of the Al–6.5Si–0.45Mg casting alloys with the addition of Sc were investigated by using scanning electron microscopy, X-ray diffraction, electrochemical measurement techniques and immersion corrosion tests and compared with those of Sr-modified alloy. The results show that Sc has evident refining and modifying effects on the primary α(Al) and the eutectic Si phase of the alloy, and the effects can be enhanced with the increase of Sc content. When the Sc content is increased to 0.58 wt.%, its modifying effect on the eutectic Si is almost same as that of Sr. Sc can improve the corrosion resistance of the test alloy in NaCl solution when compared with Sr, but the excessively high Sc content cannot further increase the corrosion resistance of the alloy. The corrosion of the alloys mainly occurs in the eutectic region of the alloy, and mostly the eutectic α(Al) is dissolved. This confirms that Si phase is more noble than α(Al) phase, and the galvanic couplings can be formed between the eutectic Si and α(Al) phases.     

Inhibitor efficiency inside small crevices investigated by Electrochemical Noise Analysis

The Electrochemical Noise Analysis (ENA) is a novel technique employed to investigate dynamic properties of electrochemical systems. The influence of chromate inhibitors on the corrosion activity of an AA 2024 surface in a chloride containing sodium solution was studied by the measurement of current noise under potential control. The noise analysis clearly shows the efficiency of the chromate inhibitor on the crevice corrosion as well as the sufficiency of the ENA as a useful tool as an inhibitor screening process. The measurement under crevice condition was carried out in‐situ in an artificial crevice.     

Susceptibility of stainless steel alloys to crevice corrosion in ClO2 bleach plants

Stainless steels, including duplex stainless steels, are extensively used for equipment in pulp bleaching plants. One serious corrosion problem in chlorine dioxide bleach plants is crevice corrosion of stainless steels, which is frequently the factor that limits their use in bleach plants. Crevice corrosion susceptibility of alloys depends on various environmental factors including temperature, chemical composition of environment and resulting oxidation potential of system. Upsets in the bleaching process can dramatically change the corrosivity of the bleaching solutions leading to temperatures and chemical concentrations higher than those normally observed in the bleach process. When the environmental limits are exceeded the process equipment made of stainless steel can be severely affected. Environmental limits for crevice corrosion susceptibility of eight stainless steel alloys with PRE numbers ranging from 27 to 55 were determined in chlorine dioxide environments. Alloys used in this study included austenitic, ferritic-austenitic (duplex), and superaustenitic stainless steels. The performance of the different stainless steel alloys mostly followed the PRE numbers for the respective alloys. The 654SMO alloy with the highest PRE number of 55 showed the highest resistance to crevice corrosion in this environment. Under the most aggressive chlorine dioxide bleach plant conditions tested, even alloys Nicr3127 and 654SMO with PRE numbers 51 and 55 respectively were susceptible to crevice corrosion attack. The two factors that seem to contribute the most to crevice corrosion and pitting in the investigated environments are temperature and potential.     

模拟体液中纯钛及Ti6A14V合金的腐蚀行为

采用电化学测试技术研究了人体医用金属材料工业纯钛和Ti6A14V合金在人工模拟体液中的腐蚀行为,结果表明,阳极极化后两种合金均未发现点蚀,工业纯钛的维钝电流密度小于于Ti6A14V合金,前者的阳极极化性能优于后者,Ti6A14V合金缝隙试样在阳极电位超过＋2000mV(vsSCE)后,电流开始急剧增大,发生了缝隙腐蚀;通过电子探针分析发现,在缝隙内Al和V两种元素发生活性溶解。工业纯钛在电位达到＋4000mV/(vsSCE）时仍没有发生缝隙腐蚀。