Corrosion in solar heating systems. I. Copper behaviour in water/glycol solutions

A research programme has been developed in order to investigate the corrosion behaviour of metallic materials commonly used in solar heating systems. This paper presents the results of an experimental study on copper corrosion resistance in ethylene and propylene glycol/water solutions (1:1 by volume) constituting the most common bases of heat transfer fluids. Long time gravimetric tests were carried out on electrolytic copper at 80°C, even in glycol/water solutions previously degraded at their boiling temperature or polluted with 200 ppm chlorides. Chemical compositions, semiconducting properties and morphological characteristics of all surface products were investigated by X-ray diffraction analysis, pulse photopotential technique and SEM observations, respectively. Heat transfer effects on copper corrosion and copper/6351 aluminium alloy couple efficiency were evaluated by electrochemical tests. The following results were obtained:

• – Ethylene and propylene glycol/water solutions are low corrosive media. Nevertheless, chloride pollution and/or high temperature degradation of glycols markedly increase their aggressivity. Under all the experimental conditions, copper corrosion rates are higher in ethylene than propylene glycol solutions.
• – In chloride-free solutions, heat transfer stimulates the cathodic reaction of the copper corrosion process.
• – Galvanic contact between copper and aluminium alloy always causes pitting corrosion on aluminium electrodes. The severity of the pitting attack is enhanced by the presence of heat transfer conditions on copper and/or chloride ions in the solutions, particularly in ethylene glycol.

     

Corrosion behaviour of the aluminium alloy AA 6351 in glycol/water solutions degraded at elevated temperature

A research programme has been developed to characterize the corrosion behaviour of the metals most widely used in solar collector systems. Common heat transfer fluids such as glycol/water solutions show a low aggressivity, unless pollution or high temperature exposure (degradation) occur. This paper deals with the study of the corrosion behaviour of the aluminium alloy 6351 (nominal composition: 1% Si, 0.6%Mg, 0.3% Mn, the balance Al) in ethylene or propylene glycol (EG or PG)/water solutions (1:1 in volume) degraded at 108°C in contact with AA 6351 as glycol oxidative degradation catalyzer. The tests in degraded solutions, performed at 80°C over a period of 60 days, showed that degradation causes an increase in the uniform corrosion rates and a remarkable pitting attack. Pitting corrosion has been mainly attributed to the action of copper ions dissolved from the aluminium alloy and detected in the solutions by atomic adsorption analysis. In conjunction with the degradation studies, tests were also performed at 80°C in buffered PG/water solutions (pH 4) containing the acids reported to be produced during the process of glycol oxidative degradation (oxalic, glycolic, acetic and formic acids), at the concentrations 10^?3 or 10^?2 M. In these solutions the corrosion behaviour of AA 6351 was investigated over a period of 30 days by measuring the gravimetric corrosion rates and the polarization resistance values and by recording the polarization curves after 2 h or 30 days of immersion. Only oxalic acid and 10^?2 M glycolic acid significantly increased the AA 6351 corrosion rates, but only in the first period of immersion. On the contrary, when the surface films were formed in the solutions of these acids, they afforded the highest protectivity. The analogous behaviour of AA 6351 in oxalic and glycolic solutions has not been attributed to the formation of an insoluble aluminium salt film with the organic anions, but rather to the growth of an oxide film with peculiar characteristics of thickness and protectivity.     

Inhibition of localized attack on the aluminium alloy AA 6351 in glycol/water solutions

A previous study had shown that exposure to degraded propylene glycol(PG)/water solution (1:1 in volume) at 80°C for 60 days or to boiling PG/water solution for 30 days induce remarkable localized attack on aluminium alloy AA 6351 (nominal composition: 1% Si, 0.6% Mg, 0.3% Mn, balance Al). In the present work degradation of the solution was achieved by holding the PG/water solution at its boiling temperature for 30 days in contact with AA 6351. The observed localized attack was mainly attributed to the action of copper ions dissolved from the aluminium alloy (copper content = 0.07%) and then deposited as small spots acting as efficient cathodic areas. The objective of this work was to examine the feasibility of enhancing pitting resistance of AA 6351 by adding suitable inhibitors to the solutions. The compounds used were two inorganic salts: sodium molybdate and sodium tungstate and two derivatives of pyrimidine: 2-aminopyrimidine (2AP) and 2-hydroxypyrimidine (2HP). The inhibiting efficiencies of these substances were tested by both short-time electrochemical tests (galvanic coupling tests and polarization curves) and long-time immersions under experimental conditions causing the localized attack. Molybdate, tungstate and, to some extent, also 2AP efficiently inhibit AA 6351 localized corrosion in degraded solutions at 80°C and in pure boiling solutions, for long exposure periods. The short-time electrochemical tests suggest that molybdate and tungstate are able to retard the electrochemical processes occurring on both the aluminium alloy and the small copper cathodic area produced by copper deposition. On the other hand, the 2AP efficiency is attributed to some complexing capability of this pyrimidine derivative towards dissolved copper ions, that are stabilized in solution. 2HP does not prevent AA 6351 localized attack.     

Effects of Trace Cl-, Cu2+ and Fe3+ Ions on the Corrosion Behaviour of AA6063 in Ethylene Glycol and Water Solutions

The effects of Cl^-, Cu²⁺ and Fe³⁺ ions and their combinations on the corrosion behaviour of aluminium alloy 6063 (AA6063) in ethylene glycol and water solutions at 50 °C were investigated by electrochemical and immersion methods. Cl ^- resulted in pitting corrosion of the alloy. In the Cl^--free solutions, Fe³⁺ was prone to accelerate uniform corrosion, while Cu²⁺ tended to accelerate pitting corrosion. Severe pitting corrosion of AA6063 was observed in the cases of Cl^- combined with Cu²⁺ or Fe³⁺, especially in the case of Cl^- combined with Cu²⁺ and Fe³⁺ ions.     

Corrosion and self‐healing behaviour of AZ91D magnesium alloy in ethylene glycol/water solutions

Corrosion behaviour of magnesium alloy‐based engine parts in cooling system is an urgent fundamental issue in automotive field where magnesium alloys are increasingly used. In the present work, the corrosion behaviour of AZ91D magnesium alloys in various ethylene glycol/water solutions was studied by electrochemical measurements and immersion tests at room temperature. The surfaces of the samples after immersion tests were examined using scanning electron microscope (SEM) and X‐ray diffraction (XRD). The results showed that the corrosion rates of AZ91D magnesium alloys decreased with the increase of ethylene glycol concentration in ethylene glycol/water solutions and the corrosion process was dominated by pitting corrosion. A continuous protective film transferred from corrosion products was formed on the corroded surface after sufficient immersion duration in ethylene glycol/water solutions, which is able to heal the corrosion pits. The self‐healing behaviour inhibited the further corrosion of AZ91D magnesium alloy.     

Corrosion behaviour of AA2024 aluminium alloy in different tempers in NaCl solution and with the CeCl3 corrosion inhibitor

The paper analyses the corrosion behaviour of naturally and artificially aged AA2024 alloy in NaCl solution and in the presence of an environment-friendly corrosion inhibitor, CeCl₃. On the basis of the values of polarisation resistance and corrosion current density, the corrosion resistance of the protective inhibitor film is established as well as the general corrosion resistance of this aluminium alloy. Resistance to pit formation is determined based on the difference in pitting and corrosion potentials while resistance to pit growth is determined based on the amount of charge consumed during pit growth. A scanning electron microscope is used to examine the morphology of the pits formed during the pitting corrosion testing, as well as to determine the cerium content on intermetallic particles and the matrix AA2024 alloy. The corrosion behaviour of AA2024 alloy is investigated after different test periods in NaCl solution and in the same solution with the CeCl₃ inhibitor. The corrosion resistance of both tempers of AA2024 alloy is more than one order of magnitude higher in the presence of CeCl₃. An explanation of the observed differences in the corrosion behaviour of the naturally and artificially aged AA2024 alloy is proposed. Different corrosion behaviour of the alloy after different test periods is also explained.     

Inhibierung von wässerigen und alkoholisch wässerigen Wärmeträgern

Inhibition of aqueous and alcoholic-aqueous heat-carriers To examine the effect of corrosion inhibitors mainly ®Preventol CI-2 in aqueous and aqueous alcoholic heat transfer media gravimetric and electrochemical corrosion tests were carried out with the materials grey cast iron, unalloyed steel, copper, brass, lead-tin-solder, aluminium and aluminium alloys up to a temperature of 90°C. In the presence of the inhibitor Preventol CI-2 uniform layers from 10 to ? 50 nm thickness are found on the metal surfaces and the measured massloss (ASTM, EMPA) decreased decidedly. Local corrosion as occurs e.g. by insufficient concentration of inhibitors forming surface layers, was found in none of the cases. The electrochemical examinations confirm the results of the chemical tests and provide indications with regard to the effective mechanisms. The influence of temperature and flow rate on the protective efficiency of the inhibitor can be showed. It is possible to eliminate the risk of galvanic corrosion by contacting copper and aluminium. The cavitation corrosion of grey cast iron is also appreciably reduced. Preventol CI-2 is a broad spectrum inhibitor for aqueous and aqueous alcoholic heat transfer media.     

Effect of aluminium solids and chlorine on cold water pitting of copper

Both aluminium solids and chlorine are often present in potable water systems, but their effect on copper corrosion and pitting is not well understood. Circumstantial evidence from one system with copper pitting problems suggested that high chlorine residuals and aluminium solids might be contributing factors. To test this hypothesis, a series of experiments were conducted to examine their effect on copper corrosion under stagnant and flow conditions. Although chlorine alone impacted copper corrosion, a synergistic reaction was discovered between chlorine and aluminium solids when exposed to copper. Evidence for this effect was seen in increased chlorine decay rates, increased non-uniform copper corrosion, and rising corrosion potentials during exposure. It is likely this reaction is involved in copper pit initiation.     

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。     

Determining the probability of stable pit initiation on aluminium alloys using potentiostatic electrochemical measurements

A method is described that determines the probability of stable corrosion pits being formed on aluminium alloys. The method employs potentiostatic measurements of metastable pit equivalent volumes, in coulombs, at the open circuit potential. The metastable pit volumes are statistically analysed using an extreme value statistics procedure that returns an expected distribution of metastable pit volumes. This enables the probabilities of the occurrence of metastable pits of given volumes with time to be calculated. The size requirement for a metastable pit to transition into a stable pit was then determined using both optical microscopy and electrochemical tests. This size requirement when combined with the metastable pit size distributions was used to determine the probability of the formation of a stable pit. The method is demonstrated for aluminium alloy AA2024-T3 in chloride media with and without chromate additions and validated against optically measured pitting rates.     

Untersuchung der Korrosionsschutzwirkung von Wärmeträgern Teil 1: Chemische Prüfungen,Screening- und Spezial-Tests

Testing the corrosion protection of heattransfer fluids Part 1: Chemical, screening and special tests As supplement to AGK Arbeitsblatt W l “Corrosion testing in heat-transfer fluids of solar heating installations” the different methods applied for testing the corrosion protection in heattransfer fluids provided with anti-freeze mixtures on glycol basis have been checked. The coupon tests corresponding to ASTM D 1384, the EMPA-Test, special tests according to ASTM D 4340 as well as the FVV-Heat-Test were part of these investigations. The investigations were carried out with steel, cast iron, two wrought aluminium alloys, one cast aluminium alloy, with copper and brass as well as with tin solder in non inhibited as well as inhibited heat carriers. The non inhibited heat-transfer fluids were composed of water with ethylene and propylene glycol, the others were made up with antifreeze concentrates prefabricated by well known producers. All investigated products provide excellent protective qualities against corrosion. The coupon test according to ASTM D 1384 has been the most effective of all methods tested up to now. This article will be continued.     

Galvanic coupling effect on corrosion behavior of Al alloy-matrix composites

Galvanic coupling effect on the corrosion of SiC-reinforced aluminum alloy-matrix composites was investigated in a sodium chloride solution. The potentiodynamic polarization measurement indicated that pitting potentials of metal matrix composites (MMCs) and AA2124 matrix alloy were similar, and pitting potential of MMCs was almost same as corrosion potential, while pitting susceptibility of MMCs was higher than that of AA2124 alloy. Galvanic current by formation of galvanic couple between SiCw and matrix reveals very low value because of large cathodic polarization of SiC. However, by increasing potential of matrix to pitting potential by this galvanic couple and thus, forming pits easily at the weak passive film near SiC reinforcing phase preferentially, it is concluded that pitting susceptibility of MMCs increases highly than AA2124 alloy of matrix composition.     

Corrosion of AD31 (AA6063) Alloy in Chloride-Containing Solutions

Corrosion of AD31 (AA6063) alloy in neutral 0.05 M NaCl solutions is investigated via scanningprobe microscopy, linear-sweep voltammetry, and electrochemical-impedance spectroscopy. Al?Fe?Si?Mg intermetallic particles are determined to prevail in the structure of alloy and act as local cathodes. Intermodulation electrostatic-force-microscopy imaging shows that their Volta potential differs by 570 mV from that of the host aluminum matrix, making the alloy prone to localized corrosion. We show that the corrosion of alloy in the studied electrolyte mainly develops locally and results in pitting, with charge transfer being the limiting stage of the process. A mechanism of corrosion of the AD31 (AA6063) alloy in neutral chloride-containing solutions is proposed.     

Corrosion properties of Nd:YAG laser-GMA hybrid welded AA6061 Al alloy and its microstructure

The paper presents the corrosion behaviour of the Nd:YAG laser-gas metal arc (GMA) welds of AA6061-T6 alloy. The laser-GMA hybrid welding enhances the corrosion susceptibility of AA6061 alloy. The surface morphology observation and composition analysis were investigated by the scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. An increasing of the precipitate phase is observed in the weld fusion zone (WFZ). The WFZ suffers more severe pitting and cracks are associated with pitting. It is proposed that the increased precipitate phase increases the galvanic corrosion couples and results in the aggravation of pitting and cracking in the WFZ.     

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.     

Effects of processing parameters on corrosion properties of dissimilar friction stir welds of aluminium and copper

The influence of friction stir welding processing parameters on dissimilar joints conducted between aluminium alloy (AA5754) and commercially pure copper (C11000) was studied. The welds were produced by varying the rotational speed from 600 to 1200 r/min and the feed rate from 50 to 300 mm/min. The resulting microstructure and the corrosion properties of the welds produced were studied. It was found that the joint interfacial regions of the welds were characterized by interlayers of aluminium and copper. The corrosion tests revealed that the corrosion resistance of the welds was improved as the rotational speed was increased. The corrosion rates of the welds compared to the base metals were improved compared with Cu and decreased slightly compared with the aluminium alloy. The lowest corrosion rate was obtained at welds produced at rotational speed of 950 r/min and feed rate of 300 mm/min which corresponds to a weld produced at a low heat input.     

AZ91D镁合金电偶腐蚀的研究

在自来水和3.5%NaCl溶液中测试了铸造AZ91D镁合金与铝合金、锌合金、Q235碳钢和Cu偶合后的电偶腐蚀行为,研究了腐蚀环境、偶接材料和阴阳极面积比(CAAR)对铸造AZ91D镁合金电偶腐蚀行为的影响。在电偶腐蚀过程中测量溶液的pH值以及电偶腐蚀电流;用失重法计算了铸造AZ91D镁合金的电偶腐蚀速率,利用SEM观察了AZ91D镁合金的腐蚀形貌,并对腐蚀产物进行XRD分析。结果表明,AZ91D镁合金在电偶腐蚀过程中会使溶液的pH值升高,并伴有以Mg(OH)₂为主的腐蚀产物的生成;溶液中Cl^-的存在会加速AZ91D镁合金的电偶腐蚀速率;低氢过电位金属Q235碳钢和Cu对AZ91D镁合金的电偶腐蚀加速效果明显高于中氢过电位金属铝合金和锌合金,偶接材料的极化性能对AZ91D镁合金的电偶腐蚀速率有较大影响。同时,大的阴阳极面积比会加速AZ91D镁合金的电偶腐蚀速率,且AZ91D镁合金的电偶腐蚀电流随阴阳极面积比的增大而呈线性增长趋势。     

Pitting and Stress Corrosion Cracking Susceptibility of Nanostructured Al-Mg Alloys in Natural and Artificial Environments

The stress corrosion cracking (SCC) behavior of two developmental nanocrystalline 5083 alloys with varied composition and processing conditions was studied. The results were compared to a commercial aluminum AA 5083 (H111) alloy. The pitting densities, size and depths, and residual tensile strengths were measured after alternate immersion in artificial seawater and atmospheric exposure under different loading conditions. Optical and scanning electron microscopy (SEM) with EDX was used to analyze the fracture surfaces of failed specimen after removal at selected intervals and tensile testing. One of the nanostructured Al-Mg alloys exhibited significantly superior pitting resistance when compared to conventional microstructured AA 5083. Under conditions where pitting corrosion showed up as local tunnels toward phase inclusions, transgranular cracking was observed, whereas under conditions when pitting corrosion evolved along grain boundaries, intergranular cracking inside the pit was observed. Pit initiation resistance of the nano alloys appears to be better than that of the conventional alloys. However, long-term pit propagation is a concern and warrants further study. The objective of this investigation was to obtain information regarding the role that ultra-fine microstructures play in their degradation in marine environments and to provide insight into the corrosion mechanisms and damage processes of these alloys.     

Characterising localised corrosion inhibition by means of parameters measured by an electrochemically integrated multielectrode array

Electrochemical parameters including maximum anodic current density, total anodic current density, the number of anodic sites and the localised corrosion intensity index have been extracted from galvanic current distribution maps that were acquired using an electrochemically integrated multielectrode array, namely, the wire beam electrode. Experiments have been carried out to demonstrate the application of these new electrochemical parameters for characterising localised corrosion inhibition of metals. A typical corrosion inhibitor, potassium dichromate, was found to affect localised corrosion processes in various ways, for instance in sodium chloride solutions, it was found to inhibit localised corrosion of aluminium alloy AA 2024-T3 by suppressing galvanic corrosion activities occurring over the alloy surface, whereas it was found to control localised corrosion of AA 1100 by creating a large number of minor anodes distributing randomly over the metal surface.     

Corrosion behaviour of AA6061 and AA7005 reinforced with Al2O3 particles in aerated 3.5% chloride solutions: potentiodynamic measurements and microstructure evaluation

The influence of the heat treatments on the corrosion behaviour of three aluminium matrix composites (AA6061/Al₂O₃/10p–20p and AA7005/Al₂O₃/10p) has been analysed in an aerated 3.5% sodium chloride solution. Corrosion potentials were determinated in a 3.5% sodium chloride solution (NaCl) using the ASTM standard G69-81 [1]. The galvanic series and the pitting potentials were calculated in this medium for all composites. The following heat treatments were applied on the base composites: (a) as-received state (T4), (b) T6 treatment, (c) an annealing, consisting of a solution stage at 560°C for 3 h for the AA6061 MMC and (d) an annealing at 482°C for 2 h for the AA7005/Al₂O₃/10p, followed by a cooling in the atmosphere furnace. The localised corrosion susceptibility of each material and for each heat treatment were analysed by measurement of the cyclic potentiodynamic polarization. Optical microscopy and SEM metallographic studies were carried out on the samples, before and after corrosion tests, to determine the influence of the microstructural changes during heat treatments on the corrosion behaviour.