Galvanic corrosion of aluminium alloy members of bridge guiderails under severe atmospheric exposure conditions

ABSTRACT

Aluminium alloys are nowadays preferred as materials for bridge guiderails especially for bridges connecting oceanic islands or spanning inlets due to their inherent corrosion resistance. But because of the limited mechanical strength of aluminium alloy, fasteners of guiderail members are made from steel materials. It has been found that contact between bare steel fasteners and aluminium alloy members can cause galvanic corrosion in the aluminium alloy. Research was carried out to investigate the capability of different surface treatments on fasteners and aluminium alloy members to inhibit galvanic corrosion under atmospheric exposure for periods of one and three years. It was found, among other results, that stainless steel fasteners treated with zinc flake coating were the most effective inhibitors of galvanic corrosion on aluminium alloy members.     

Effect of process parameters on corrosion rate of friction stir welded aluminium SiC–Gr hybrid composites

Abstract

In the present study, the influence of processing parameters of friction stir welding on the corrosion rate of the welded joints of aluminium SiC–Gr hybrid composites was investigated. The experimental results indicate that the corrosion resistance of the welded joints increases at high welding (traverse) speed and/or low values of rotational speed. These variations occur as a result of the changes in the joint microstructure, where fine grains are developed as a consequence of a relatively low heat input and fast cooling to room temperature by ambient air associated with low rotational speed and/or high welding speeds. The mixed electrode theory is used to explain these variations of the corrosion rate, where the area ratio of cathode/anode for the galvanic couple between the aluminium metal matrix and the reinforcement constituents becomes small for fine grains. Thus, the corrosion resistance of the welded joints is increased.     

Pitting corrosion of thixoformed,rheocast and gravity cast A356-T6 alloy in chloride media

Abstract

In the research work presented here, the pitting behaviour of thixoformed A356-T6 alloy, with different reheating temperatures, was evaluated and compared with the pitting behaviour of rheocast and gravity cast A356-T6 alloys with the same composition. To study the pitting behaviour, linear sweep voltammetric tests were performed on thixoformed, rheocast and gravity cast A356-T6 alloys in a 3·5%NaCl solution. A simulation method was also used to identify local galvanic corrosion current density between local galvanic couples. Results obtained show that the resistance to pitting corrosion of thixoformed samples formed at 590°C was higher than that of the samples formed at 600°C as well as rheocast and gravity cast samples. The improvement in the pitting corrosion resistance due to the thixoforming process is attributed to morphological aspects of the silicon phase as well as the area effect as related to galvanic corrosion between the silicon particles and the eutectic aluminium phase.     

Crystallographic defects induced localised corrosion in AA2099-T8 aluminium alloy

Abstract

A correlation has been revealed between the localised corrosion susceptibility and the grain stored energy in AA2099-T8 aluminium alloy, with grains of higher stored energy being relatively more susceptible to corrosion attack. The grains of high stored energy contain increased crystallographic defects, which could consequently decrease their corrosion resistance. Additionally, since crystallographic defects are preferential sites for the precipitation of T₁ (Al₂CuLi) phase, it is suggested that grains of high stored energy could also contain increased volume fraction of T₁ phase after aging, which might further increase corrosion susceptibility due to the electrochemically high activity of lithium rich T₁ phase.     

Anodic behaviour of aluminium and its alloys in alkaline media in the presence of inhibitive and complexing substances

Abstract

In order to find out the suitability of four different grades of aluminium, namely 2S, 3S, 57S, and unalloyed aluminium for use as a galvanic anode in 1M-sodium hydroxide solution containing calcium oxide and sodium citrate, the potential-time variation, the anodic polarisation, the anode efficiency and corrosion/behaviour of these grades have been investigated. It has been found that all the above mentioned grades of aluminium behave in a similar manner, and self-corrosion can be reduced by more than 75% by the joint addition of calcium oxide and sodium citrate. However, on the basis of anode efficiency in inhibited 1M-sodium hydroxide solution, 28 grade and unalloyed aluminium seem to be the materials of choice since maximum efficienciesof 93–94% can be obtained by the use of appropriate current densities. Even in these cases, high anode efficienciesof the order of 80–94% are attained only at current densities of 9–12 mA/cm² but the results are less encouraging at lower current densities, and at 3 mA/cm² the anode efficiencieswere between 20% and 40% for alloyed and unalloyed aluminium.     

Corrosion in solar heating systems. II. Corrosion behaviour of AA 6351 in water/glycol solutions

A research was carried out in order to investigate the corrosion behaviour of the metals most commonly used as construction materials for solar absorber plates. With this view, an attempt was made to test the corrosion resistance of the aluminium alloy AA 6351 (nominal composition: 1% Si, 0.6% Mg, 0.3% Mn, the balance Al) towards common uninhibited heat transfer fluids, such as ethylene and propylene glycol/water mixtures. Long time gravimetric tests consisted in up to 60 day exposures of the aluminium specimens to pure, chloride-polluted, or degraded glycol/water solutions, at the temperature of 80°C. The degradation into acidic products, experienced by heat transfer liquids in service, was simulated by keeping the ethylene and propylene glycol/water solutions at their boiling temperature for 30 days, in contact with copper. In glycol/water solutions the presence of chlorides at low concentration (200 ppm) caused the aluminium corrosion rates to increase by more than one order of magnitude, while in degraded solutions, containing 143 or 86 ppm cupric ions, corrosion rates higher than two order of magnitude with respect to pure solutions were obtained. During the gravimetric tests, pitting corrosion was observed in some cases and its extent was rated by evaluating the deepest and the average metal penetration, the pit density and the average pit size. The influence of heat transfer on the alloy AA 6351 corrosion and on the couple copper/AA 6351 efficiency was evaluated by gravimetric and electrochemical tests. Heat transfer through aluminium was found to significantly increase the aluminium alloy pitting potential. On the contrary, it stimulated the aluminium galvanic corrosion, when applied on either aluminium or copper. Under galvanic coupling conditions, the aluminium corrosion rates calculated from the average galvanic currents were a very little contribution to the gravimetric corrosion rates. This demonstrates that in low conductive solutions the risk of matching such dissimilar metals as copper and aluminium does not reside in the galvanic contact itself, but mainly in the mere presence of the noblest metal in the same solution where aluminium is immersed.     

AM60镁合金与铜合金及铝合金偶接后的大气腐蚀行为

研究了AM60铸造镁合金与H62铜合金、LY12铝合金组成的电偶对在北京地区室外曝晒3个月和6个月后的大气电偶腐蚀行为及规律.结果表明:镁合金始终是电偶对中的阳极,当其与H62铜合金和LY12铝合金偶接时,其腐蚀速率增加1～7倍;镁合金与H62铜合金偶合后,其大气电偶腐蚀效应大于其与LY12铝合金组成的电偶对的大气电偶腐蚀效应;在6个月的实验周期内,随着曝晒时间的延长,镁合金在北京地区大气环境中的电偶腐蚀效应呈上升趋势;试样表面润湿时间短是造成AM60镁合金在北京曝晒第一个实验周期(2003.12～2004.2)的电偶腐蚀低于第二个由实验周期(2004.3～2004.5)的主要原因;北京地区高自然降尘量对镁合金电偶腐蚀将起到加速的作用.     

Electrochemical study of the surface metal matrix composite developed on AA 2024-T351 by the friction stir process

ABSTRACT

This work is focused on the evaluation of the corrosion behaviour of an AA2024-T351/SiC surface composite produced by friction stir processing (FSP). The surfaces composite show a quite homogenous distribution of the SiC particles in the stir zone, together with a significant grain size reduction as Electron Backscatter Diffraction analysis confirms. The corrosion behaviour was evaluated in 3.5 wt-% NaCl solution using electrochemical techniques. The complete study shows that all processed samples have similar corrosion resistance at the stirred region and differ from the base material behaviour. The results indicate that the galvanic coupling metal matrix–SiC particles are less active than the galvanic coupling metal matrix–Cu-rich second phases. Although the initial corrosion resistance of the base aluminium alloy is the worst of all systems studied, for long immersion times the development of corrosion through grain boundaries was found to progressdeeper in the 2024Al/SiC surface produced by FSP.     

Mass transport control of localised corrosion processes: in situ local probing and modelling

ABSTRACT

In this paper, the importance of mass transport in localised corrosion process is demonstrated using two examples describing the corrosion behaviour of two types of cut-edge electrodes regarding, (i) the sacrificial dissolution of a sacrificial Zn coating on steel (galvanised steel) and (ii) the release of inhibitors from a primer on an aluminium alloy (AA2024). In the two cases, in situ pH imaging with an ion-selective microprobe is used to confirm, (i) the role of corrosion products on the galvanic current profiles and (ii) the competition between the inhibition and the triggering of the local corrosion activity around intermetallic particles. It is illustrated how the experimental results can be supported by a 2D mass transport modelling.

This paper is part of a supplementary issue from the 17th Asia-Pacific Corrosion Control Conference (APCCC-17).     

Critical pitting potential of aluminium in automobile cooling systems

Abstract

The traditional copper radiators and heaters in automobiles are being replaced by more lightweight versions made of aluminium. Since different halide fluxes are used in the manufacture of the two types of radiator or heater, it is possible for the coolant to be contaminated by more than one halide when the two types of heat exchanger are used in the ·same system. This may give rise to pitting corrosion of the aluminium. Potentiostatic experiments have enab(ed the critical pitting potential of aluminium in 50% ethanediol solution to be determined to within 5 m V and it has been shown that the Ep value becomes less noble with increase in halide ion concentration. The pitting potential of aluminium in solution containing thefluoride ion has bee:nfound to be more noble than expected.     

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an “alkalisation”, “passivation”, “poisoning” effect or “shortcut” effect.     

Influence of Surface Condition on the Corrosion of Aluminium Brass

Abstract

The corrosion behaviour of aluminium brass, with four different surface treatments (highly and lightly oxidised, pickled and sandblasted), was studied in natural sea water, both stagnant (aerated) and flowing.

Electrochemical measurements and weight loss determinations showed that the oxide film produced during industrial annealing had little effect on the corrosion rate as compared with a pickled surface, in both stagnant and flowing conditions. Sandblasted surfaces were inferior.

Measurements of currents flowing in galvanic couples formed between oxidised and stripped metal were small and decreased with time, but the oxidised surfaces appeared to be slightly more vulnerable than the pickled ones to localised corrosion. Some further work is needed here.     

Recent developments in corrosion inhibitors based on rare earth metal compounds

Abstract

Rare earth organic compounds can provide an environmentally safe and non-toxic alternative to chromates as corrosion inhibitors for some steel and aluminium applications. For steel lanthanum 4-hydroxy cinnamate offers corrosion protection and reduces the susceptibility to hydrogen embrittlement. Recent work has also indicated that it inhibits the corrosion of steel in environments containing high levels of carbon dioxide. For aluminium alloys, cerium diphenyl phosphate provides excellent corrosion inhibition in chloride environments, and reduces susceptibly to stress corrosion cracking. Furthermore, for both steel and aluminium alloys filiform corrosion can be suppressed when rare earth inhibitor compounds are added as pigments to polymer coatings. The levels of inhibition observed are thought to be due to synergistic effects between the rare earth and organic parts of these novel compounds, and are related to the various species that may be present in the complex chemical conditions that develop in solution close to a metal surface. This paper reviews some of the published research conducted by the group at Deakin University over recent years.     

模拟海洋环境下7B04铝合金电偶腐蚀预测及验证

基于腐蚀电化学原理和有限元方法,建立7B04铝合金分别与CFRP、TA15钛合金、30CrMnSiA钢在模拟海洋环境下耦合时的电偶腐蚀模型,仿真得到电极表面的电偶电位和电偶电流密度分布等;实测电偶腐蚀电流并对模型的预测结果进行验证。结果表明:自腐蚀电位由高到低可依次排列为E_CFRP > E_TA15 > E_30CrMnSiA > E_7B04,耦合后7B04铝合金作为阳极而加速腐蚀;不同电偶体系的电偶电位相差不大,而电偶电流变化明显;阴/阳极面积比相同时,7B04铝合金与CFRP耦合时的电偶电流最大,即电偶效应最强,与TA15钛合金耦合时的电偶电流次之,与30CrMnSiA钢耦合时的电偶电流最弱;阴/阳极间的电偶电位和电偶电流随阴/阳极面积比的增加而线性增加;计算结果与预测结果相对误差基本在±5%以内,表明电解液中的双电极电偶腐蚀模型有较高的预测准确度。     

辐照对金属腐蚀的影响

电离辐射能够改变电极过程的性质,並能破坏其动力学的平衡。本文研究了辐照对铝在含铜、氯离子溶液中点腐蚀的影响。结果表明,辐照可以减缓点腐蚀和加速均匀腐蚀;如果严格控制水质,可以显著地降低辐照的有害作用。同时,还研究了各种辐射对金属偶腐蚀的影响,测定了极化曲线和随时间变化的腐蚀电流,实验证实,重辐射和轻辐射都能加速金属偶的电化学过程。最后,讨论了辐照腐蚀的机理,提出了辐射电化学的半导体效应。     

On the applicability of high strength self‐tapping aluminium bolts in magnesium nut materials for automotive applications

High strength aluminium bolts made of AW 6056 T6, AW 7075 T6 and T79 have been investigated regarding the applicability in magnesium nut materials for automotive applications. With respect to galvanic corrosion all combinations of aluminium bolts with magnesium parts show superior corrosion properties when compared to galvanised steel bolts connected to magnesium. With respect to stress corrosion cracking (SCC) no aluminium bolt in contact with magnesium failed due to SCC. This is because of cathodic protection of aluminium alloy through magnesium. Even peak‐tempered highest strengthened T6 7xxx aluminium bolts can be used for automotive applications when compressive residual stresses are present in the thread root of the bolts.     

Corrosion and impressed current cathodic protection of copper-based materials using a bimetallic rotating cylinder electrode (BRCE)

A bimetallic rotating cylinder electrode (having individual electrode areas of 10 cm² and rotating at 200-1400 rpm) has been used to examine the corrosion and protection characteristics of copper/nickel aluminium bronze and 90-10 copper-nickel/nickel aluminium bronze galvanic couples in filtered seawater at 25 °C. The flow-influenced electrochemistry of the systems was determined using zero resistance ammetry, corrosion potential measurements and a potential step current transient technique. In each case, the galvanic corrosion potential and corrosion rate displayed a Reynolds number dependency where mass transport control of the anodic dissolution reaction partially controlled the reaction rate. Bimetallic impressed current cathodic protection (ICCP) has also been demonstrated for a range of applied protection potentials and Reynolds numbers. A comparison has been made between the directly measured bimetallic ICCP current demand and that determined from independent, single-metal rotating cylinder electrode measurements. In this case, the mixed charge and mass transport controlled rate of oxygen reduction was examined.     

等温退火对Ni-P、Ni-W-P镀层的晶粒尺寸和显微硬度的影响

研究了不同镀态结构的Ni-P、Ni-W-P镀层经等温退火后，其晶粒尺寸和硬度随退火时间的变化规律，镀层的热稳定性以及成分、结构和晶粒尺寸对镀层硬度的影响规律。结果表明，W元素可以促进Ni-P合金的非晶化，显著提高Ni-P镀层的硬度；Ni-（W）-P体系纳米晶镀层具有较好的热稳定性；等温退火获得的三元Ni-W-P单相纳米晶镀层在耐磨、耐蚀性能方面均优于同类非晶态镀层。     

FIB-SEM investigation on corrosion propagation of aluminium–lithium alloy in sodium chloride solution

Abstract

Microstructural characterisation of 2A97-T4 aluminium–lithium alloy was carried out using electron probe microanalysis and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy facilities has been employed to examine localised corrosion sites after immersion in sodium chloride solution. A dual beam microscope, which integrates a focused ion beam and an electron beam in one powerful instrument, has also been employed to investigate the development of intergranular corrosion from both surface and cross-section. It was found that localised corrosion is generally initiated at θ phase particles, which represents only 8.4% of the intermetallic (IM) particles in 2A97-T4 aluminium–lithium alloy. θ phase particles exhibit preferential dissolution of aluminium during corrosion testing, with trench formed at their periphery as well. Initiation of intergranular corrosion is relatively late with respect to the attack of IM particles. Owing to the presence of θ phase particles at intergranular corrosion sites and non-uniform distribution of T1 (Al₂CuLi) grain boundary precipitates, it is supposed that dealloyed θ phase particles and grain boundary precipitates cooperate to provide the driving force for grain boundary attack.