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.     

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.     

Effects of coolant chemistry on corrosion of 3003 aluminum alloy in automotive cooling system

In this work, effects of coolant chemistry, including concentrations of chloride ions and ethylene glycol and addition of various ions, on corrosion of 3003 Al alloy were investigated by electrochemical impedance spectroscopy measurements and scanning electron microscopy characterization. In chloride‐free, ethylene glycol–water solution, a layer of Al‐alcohol film is proposed to form on the electrode surface. With the increase of ethylene glycol concentration, more Al‐alcohol film is formed, resulting in the increase in film resistance and charge‐transfer resistance. In the presence of Cl^? ions, they would be involved in the film formation, decreasing the stability of the film. In 50% ethylene glycol–water solution, the threshold value of Cl^? concentration for pitting initiation is within the range of 100 ppm to 0.01 M. When the ethylene glycol concentration increases to 70%, the threshold Cl^? concentration for pitting is from 0.01 to 0.1 M. In 100% ethylene glycol, there is no pitting of 3003 Al alloy even at 0.1 M of Cl^?. Even a trace amount of impurity cation could affect significantly the corrosion behavior of 3003 Al alloy in ethylene glycol–water solution. Addition of Zn²⁺ is capable of increasing the corrosion resistance of Al alloy electrode, while Cu²⁺ ions containing in the solution would enhance corrosion, especially pitting corrosion, of Al alloy. The effect of Mg²⁺ on Al alloy corrosion is only slight.     

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.     

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.     

Effect of heat treatments on 8090 AlLi alloy pitting susceptibility in sea water

The pitting susceptibility of 8090 Al-Li alloy in sea water, after different heat treatments, was investigated. Free corrosion and electrochemical tests were carried out at 25°C, in quiescent sea water at pH = 8.2 and dissolved oxygen = 6.5 ppm. The microstructure was examined by metallographic microscopy and by X-Ray microdiffractometry, while the corrosion layer was characterized by chemical methods and by Infrared and X-Ray photoelectron spectroscopies. The following was observed:

• aging treatments lead to a non homogeneous microstructure which increases the average corrosion rate as well as pitting susceptibility;
• heat treatments do not affect the average passive film composition;
• in all examined states, Al-Li alloy 8090 is subject to localized corrosion which takes place preferentially at the grain boundaries.

     

Aluminium in fresh waters

Abstract

The paper summarises research at these Laboratories on corrosion of aluminium in fresh waters. The type of corrosion affecting aluminium depends on water composition. Deep pitting requires oxygen, chloride, carbonate hardness and copper, but the higher chloride contents encourage more general corrosion. Higher carbonate hardness reduces the number of pits formed; sulphates have a similar effect and also increase pit depth. Except in waters of very high copper content, attack is generally less at elevated temperatures. Different alloys differ only slightly in resistance to attack. On superpurity aluminium, pits are slow to form but subsequently penetrate at the same rate as in SIC type. The best form of protection for aluminium is cladding with aluminium– l¼% zinc. This prevents pitting of N3 sheet or tube even in highly corrosive hard waters. Spraying with aluminium–zinc or anodising is also effective. Treatment to form boehmite films gives some protection but is ineffective in the more corrosive waters.     

Inhibition of corrosion of AZ91 magnesium alloy in ethylene glycol solution in presence of chloride anions

The influence of chloride ions on the corrosion of AZ91 magnesium alloy in water/ethylene glycol solutions and the inhibiting effect of lactobiono‐tallowamide (LTA) were investigated using electrochemical and surface analysis methods. Potentiodynamic polarization curves in aqueous solution of ethylene glycol (50:50w%) containing 0.1 g.L⁻¹ chloride and up to 0.5 g.L⁻¹ LBT were obtained at room and at elevated temperatures. The chloride anions showed a distinct deteriorating effect as they caused pit initiation and accelerated the dissolution of the tested alloy. The selected organic compound demonstrated good protective properties against corrosion of AZ91 magnesium alloy and behaved as inhibitor of mixed type hindering both the cathodic and the anodic partial reactions. It showed inhibition efficiency of 77% at relatively low concentration of 0.2 g.L⁻¹ and was considered as a promising corrosion inhibitor. The mechanism of inhibition was discussed on the basis of the electrochemical impedance spectroscopy (EIS) and XRD analysis of the surface.     

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.     

Zum Korrosionsverhalten nichtrostender austenitischer Chrom-Nickel-(Molybdän,Kupfer)-Stähle in nahezu wasserfreier Essigsäure

Corrosion of stainless austenitic steels in almost anhydrous acetic acid As-welded samples and looped specimens from 5 differently alloyed stainless steels were tested for up to 246 days in 99,5% to 99,95% acetic acid at 118°C (boiling temperature/normal pressure) and at 150°C; the chloride content was varied between < 1 and 100 ppm. Pitting corrosion – of shallow depth, however (approx. 0,1 mm) – was already observed at surprisingly low chloride concentrations. Only the following were found to be resistant to pitting corrosion:

• – stainless steels 1.4439 and 1.4539, containing approx. 4,5% molybdenum, in 99,5% acetic with < 1 ppm chloride at 118 and 150°C,
• – stainless steels 1.4439 and 1.4539 in 99,9% acetic acid with < 1 ppm chloride at 118°C, and
• – special stainless steel X 2 CrNiMoCuN 20 18 6, containing approx. 6% molybdenum, in 99,5% acetic acid with > 3, < 10 ppm chloride at 118 and 150°C.

Looped specimens and ground as-welded samples showed no sensitivity to transcrystalline, chloride-induced stress corrosion cracking at any of the concentration ranges. High surface-removal rates can be expected if air has access to the specimens; under this condition pitting corrosion and general corrosion may overlap. Contamination of acetic acid with chlorides must be prevented under all circumstances.     

Mechanism of Pitting Corrosion of Copper in Supply Waters

Abstract

Detailed examination of a large number of copper water tanks and pipes showing pitting corrosion (taken from certaind is districts to which pitting is confined) revealed features that could not be reconciled with the conventiona1 explanation based on the presence of large cathodic areas surrounding small anodic corrosion Sites. A new theory of pitting corrosion of copper is proposed which explains the features observed Pitting arises when a pocket of cuprous chloride forms beneath a porous electrically conducting membrane (usually cuprous oxide). The anodic and cathodic processes occur on the inner andouter surfaces of the memberane. Dissolution of copper occurs by reaction with the anodic product beneath the membrane and deposition of calcium carbonate occurs by reaction of hardness salts in the water with the cathodic product above the membrane.

Pockets of cuprous chloride capable of developing into corrosion pits can only form if the potential of the copper rises above 90mV EH during the period of film formation. Control of the potential below this value by coupling to a small sacrificial aluminium anode has been shown to prevent formation of pits in copper tanks and cylinders in service.     

Effect of heat treatments on Cu-Ni 70/30 alloy pitting susceptibility in sea water at different temperatures

A series of experiments was carried out on Cu-Ni 70/30 commercial alloy specimens presenting different microstructures. The microstructure of homogeneized specimens was modified with annealing treatments by which grain size significantly increased and intermetallic compounds precipitated, thus creating some dishomogeneity in the alloy. Free corrosion and electrochemical tests were carried out at 20, 40, 60, 80°C in quiescent sea water at pH 8.2 with dissolved oxygen (D.O) content ranging from 6.5 ppm (at 20°C) to 3.0 ppm (at 80°C). The corrosion products adherent to the metallic surface were analyzed with chemical and XPS methods. The following could be observed:

• with increasing temperature, a decrease in corrosion rate and selective copper dissolution was observed in homogeneized and annealed specimens
• the annealed specimens have the highest corrosion rate in the whole temperature range and undergo pitting corrosion at low temperature.

     

Pitting potential of high purity binary aluminium alloys—I. AlCu alloys. Pitting and intergranular corrosion

The effect of solute content on the pitting potential of Al-Cu alloys in 1M NaCl solutions was studied. In solution treated Al-Cu alloys the pitting potential of the alloy was found to increase with increasing Cu content. The maximum pitting potential value was limited by the solubility of Cu in the Al. It was found that, while the pitting potential of pure aluminium was ca. ?0.52 V, the pitting potential of Al-5.22% Cu was ?0.33 V.The ageing of the alloy affected the pitting potential. The pitting potential of the aged alloy was found to be determined by the copper content of the solute depleted zones formed during ageing. The formation of G.P. zones did not modify the pitting potential of the alloy. The formation of θ′′ phase, in an Al-3.5 Cu alloy, produced a decrease of 30 mV in the pitting potential, while the formation of θ′ phase produced a decrease of > 100 mV.As ageing was more rapid in the grain boundaries, there was a localized decrease in the pitting potential, which was followed by susceptibility of the alloy to intergranular corrosion.     

Corrosion of iron in highly acidic hydro-organic solutions

In acidic water-organic solvents of ethylene glycol (EGOH), propylene glycol (PGOH), methanol (MeOH) and ethanol (EtOH), iron corrosion was studied by monitoring the corrosion potential, the potentiodynamic polarization curves and electrochemical impedance diagrams. In these aqueous glycols and alcoholic solutions containing HCl having concentrations of 0.5 up to 9 M, it has been shown by electrochemical analysis (I-E curves) that dissolution mechanism of iron is similar to that one in pure acidic aqueous solutions if only we take into account the relative amount of water in the medium. Based in our experimental data, water has an important role in the transfer kinetics of protons to the metallic electrode and limits electro dissolution rate of iron. When water quantity is sufficient at the metal surface, the acidity is a governing factor in the evolution of corrosion process.     

Erosion–corrosion behavior of aluminum in a 50°C ethylene glycol aqueous solution simulating cooling water in HVDC transmission

The erosion–corrosion of aluminum in a 50 °C ethylene glycol aqueous solution (EGAS) was studied. Compared with that in deionized water, the corrosion of aluminum in an EGAS was inhibited to a certain extent. The corrosion potential, corrosion current, and charge transfer impedance of aluminum changed from −1.5776 V, 380.4 nA cm⁻², and 1.924 × 10⁻⁴ Ω cm⁻² in deionized water to −1.3127 V, 285.5 nA cm⁻², and 4.041 × 10⁻⁴ Ω cm⁻² in a 45.3 vt% EGAS, respectively. Ethylene glycol did not ionize in deionized water and the ionic conductivity of the EGAS was low, effectively restraining the corrosion of aluminum. However, a test with aluminum in an EGAS after long-term storage (9 days) showed that ethylene glycol gradually oxidized to glycolic acid, oxalic acid, and other substances, which slowly corroded the aluminum surface. Analysis results showed that the corrosion products on the surface of aluminum were Al(OH)₃ and Al₂O₃. The pitting hole formation mechanism of aluminum occurs via an aluminum–alcohol phase formed on the aluminum surface, which can inhibit the dissolution of the oxide film. Therefore, a suitably concentrated EGAS with a high heat capacity and low ionic conductivity similar to that of deionized water can be used as a coolant in airtight valve cooling systems for high-voltage direct-current transmission.     

Korrosionsbeständigkeit von Verbindungen von Rohren aus nichtrostenden Stählen in Wässern

Corrosion of joints for stainless steel tubes in water The most important commonly used joining techniques for stainless steel tubes which are used for the transport of water and gases are welding and brazing. With corrosion attack by dry gases, both connections are resistant against corrosion. However, in water and aqueous condensates limits of application exist with regard to the corrosion resistance. The corrosion resistance of weld connections with stainless steel tubes is diminished by

• – annealing colours (oxide films) and scale layers in the weld area;
• – changes in the microstructure adjacent to the welds (sensitization of the stainless steel material);
• – surface finish of weld seams after welding;
• – welding faults resulting from bad handling and workmanship.

Type and extent of corrosion damage occurring on weld connections with decreased corrosion resistance depend on the composition of the water and condensates, mainly on their chloride content. Typical examples for the causes of degraded corrosion resistance of weld connections, and possible types of corrosion attack, namely pitting, crevice corrosion, and stress corrosion cracking and their mechanisms are described. Furthermore, measures are shown by which the corrosion resistance of weld connections with stainless steel tubes can be increased. Joints of stainless steel tubes by hard soldering with capillary fittings are endangered by knife line attack at the phase boundary between the stainless steel and solder (interfacial corrosion). Knife line attack means in this context the loss of adhesion between steel and hard solder. The severity of the corrosion risk, in particular the incubation time until the occurrence of the corrosion damage, depends on the water quality, mainly on chloride concentration, and pH. The press fitting with non-metallic gasket is a relatively new joining system, and it is used in the cold and warm water domestic installation. This joining technique is described. For domestic water distribution, an installation system with tubes and press fittings made of steel grade AISI 316 SS has been developed. This system is resistant to corrosion attack in potable water of usual composition, and it is already applied in-service in a considerable extent. Other joining systems are stainless steel weld fittings, threaded screw fittings, and compression couplings with cutting or clamping rings. They are used mainly in industrial installations.     

Surface analysis in corrosion inhibition mechanisms

The importance of modern surface analytical techniques in understanding the mechanisms of corrosion inhibition is discussed. The discussion is conducted in terms of the following metals and inhibitors which inhibit them:

• copper and alloys – mercaptobenzothiazole, benzotriazole, and benzimidazole
• iron and nickel – acetyl acetone
• mild steel – 8-hydroxyquinoline and chromate
• tool steel – perfluoroalkyl ether containing perfluoroalkyl ether aryl phosphine
• mild steel – molybdate, tungstate, chromate, phosphate, nitrite, oxalate, arsenate, EDTA
• aluminium – silicate, molybdate, tungstate, phosphate and chromate.

     

Die Inhibition von Kühlkreisläufen

The inhibition of cooling systems The choice of an efficient corrosion inhibitor has to take in account the type of the cooling system, the materials used as well as the specific operating conditions, for instance

• – very low or very high velocities of the cooling water
• – heat transfer
• – the inhibitor may be asked to have some lubricating properties
• – vibrations, that can cause cavitation or fatigue corrosion
• – interaction between the corrosion inhibitor and other electrochemical systems of corrosion protection (cathodic protection).

In numerous applications it is necessary to improve the inhibition by an addition of other effective substances (like dispersants, hardness stabilizers, mixture of inhibitors with selective protection properties).     

Korrosionsverhalten von Edelstahlrohren und Rohrverbindungen bei der Warmwasserverteilung in der Hausinstallation

Corrosion behaviour of alloy steel tubing and tube connections in warm water distribution systems in dwellings The corrosion behaviour of steel in warm waters essentially depends on heat transfer and in particular on the direction of heat transfer. The precipitation of water constituents responsible of water hardness may result in local chloride enrichment and, consequently, pitting and/or stress corrosion cracking. Ferritic chromium steels are not sufficiently resistant in these conditions, while addition of 2% Mo or 9% Ni may yield satisfactory results. The danger of corrosion also exists in the case of tubing embedded in mortar, if the latter contains chlorides. As to tube brazing it should be borne in mind that hard solders with a high silver content and sufficiently low point of fusion should be used; the type of corrosion to be expected at such places would be in particular the knife-line attack. In the case of zinc solders there is the danger of dezincification with subsequent pitting caused by the products of hydrolysis of zinc compounds. As to crevice corrosion on noticable sensitivity has been found under the conditions studied.