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.     

Corrosion behaviour in organic acid solutions—I. Tin electrode

The corrosion behaviour of tin in oxalic, citric and tartaric acids in the pH range 2–6 and at concentrations 10^?1–10^?5 M was investigated. The results indicate that increasing the concentration of the organic acid and the pH of the medium up to 4 shifts the corrosion potential to a more negative value and increases the rate of corrosion. At pH values 5 and 6, a shift in the corrosion potential in a noble direction is accompanied by a decrease in the corrosion rate. A linear relationship is observed between the steady state potential (E) and the logarithmic concentration of the organic acid (log C) for the three acids studied:

$\text{E = a ? b}\text{log}\text{C}$

. It was concluded here that the corrosion mechanism in the pH range 2–4 is anodically controlled by the complexing of Sn²⁺ ions with organic acid anions and that the order of decreasing aggressiveness is oxalic > citric > tartaric acids. Corrosion was inhibited at pH values 5 and 6 and this was attributed to film formation on the surface of tin which may result from the hydrolysis of tin complex species.     

Corrosion behaviour of magnesium in ethylene glycol

Corrosion of magnesium engine components by coolant is an important issue in the automotive industry where magnesium alloys may be used. It is of significance to understand the corrosion behaviour of pure magnesium in ethylene glycol solutions, as this can provide a basis for developing new coolants for magnesium alloy engine blocks. In this paper, through corrosion and electrochemical tests, it was found that the corrosion rate of magnesium decreased with increasing concentration of ethylene glycol. Individual contaminants, such as NaCl, NaHCO₃, Na₂SO₄ and NaCl can make aqueous ethylene glycol solution more corrosive to magnesium. However, in NaCl contaminated ethylene glycol, NaHCO₃ and Na₂SO₄ showed some inhibition effect. The solution resistivity played an important role in the corrosion of magnesium in ethylene glycol solutions, and the competitive adsorption of ethylene glycol and the contaminants on the magnesium surface was also responsible for the observed corrosion behaviours. The corrosion of magnesium in ethylene glycol can be effectively inhibited by addition of fluorides that react with magnesium and form a protective film on the surface.     

Comparison of corrosion behaviour of zinc in NaCl and in NaOH solutions. Part I: Corrosion layer characterization

The corrosion layer formed on zinc sample in 0.6 M NaCl and 0.5 M NaOH solution under ambient conditions has been investigated. The corrosion layer morphology was analyzed using scanning electron microscopy (SEM). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the corrosion products of zinc. The thickness evolution of the corrosion layer was investigated by glow discharge optical emission spectroscopy (GDEOS). The corrosion layer formed in 0.5 M NaOH solution appeared more compact than that formed in 0.6 M NaCl solution. Zinc hydroxide chloride (Zn₅(OH)₈Cl₂·2H₂O) and zinc hydroxide carbonate (Zn₅(CO₃)₂(OH)₆) were formed on zinc surface in 0.6 M NaCl solution while in 0.5 M NaOH solution, zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂) and zinc hydroxide carbonate (Zn₅(OH)₆(CO₃)₂·H₂O) were detected. Probable mechanisms of zinc corrosion products formation are presented.     

Corrosion fatigue behaviour of nitrogen-implanted steel in water

The corrosion fatigue behaviour of nitrogen implanted iron in neutral solutions has been investigated utilizing the oscillating cantilever beam method, by means of electrochemical (E_corr vs. time plots, R_p measurements) and S. E. M. techniques. The calculated corrosion rate under stagnant conditions is lowered for the implanted iron at all nitrogen doses with respect to untreated iron. The surface oxide layers achieve enhanced protecting capacity. Consequently under a cyclic load of 25.4 kg mm^?2, at a frequency of 16.6 Hz, cracks nucleation is delayed. The time to breaking of implanted specimens is considerably lower in our experimental conditions compared to untreated iron. The increase in the crack growth rate is consistent with the detrimental action of structural damage following implantation. The fracture morphology is also discussed.     

Corrosion behaviour of magnesium sacrificial anodes in tap water

The effects of applied current, testing time and microstructure on the electrochemical properties of magnesium‐based sacrificial anodes in potable water were evaluated. Galvanostatic tests, potentiodynamic polarisation and scanning electron microscopy (SEM) analyses were used to determine anode efficiency, corrosion rate and microstructures of the Mg‐anodes under study. The contribution of processes as mechanical material loss and hydrogen evolution on the current wastage was investigated. The effect of the different microstructures like those obtained by heat treatments to reduce the intermetallic phase was also tested.     

Contact corrosion of metals in aqueous and aqueous organic environments: III. Corrosion of metals aqueous glycol solutions with high water contents

The corrosion behavior of aluminum, steel, and cast iron as either individual metal specimens or as parts of contact aluminum-steel, aluminum-cast iron, steel-cast iron, and aluminum-steel-cast iron systems in aqueous glycol solutions containing 5–50 vol % water is studied. In all systems, aluminum is found to act as a cathode with respect to steel and cast iron and, in contrast to them, is not corroded under the effect of electrolytes. With an increase in the water content in the ethylene glycol solution from 70 to 95%, the corrosion behavior of the metals becomes similar to that in water.     

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.     

Corrosion behaviour of stainless steels in aqueous solutions of methanesulfonic acid

The corrosion behaviour of different grades of stainless steel in five commercial products labeled as 70% aqueous methanesulfonic acid was investigated. Chemical analysis showed that these products contain different amounts of impurities, most probably the consequence of the production procedure. Three austenitic stainless steel grades, 304, 316 and 316Ti, are widely used for storage and transportation vessels for methanesulfonic acid (MSA). Corrosion behavior has been studied by electrochemical techniques, immersion tests and by surface analysis. The selected stainless steels are highly stable in the most purified acid, but the presence of impurities in MSA disrupts the metal’s surface, leading to corrosive attack.     

Corrosion behaviour of steel in solutions containing mixed inhibitive and aggressive ions

The action of sodium benzoate, sodium chromate and sodium nitrite in relation to the presence of sodium chloride, sodium nitrate, sodium sulphate and sodium sulphide has been studied by means of visual observation as well as potential measurements. It has been found that the critical concentration of the inhibitor for protection in distilled water could tolerate a range of concentration of the aggressive anion. The linear double logarithmic relationship between the concentration of an inhibitive anion and the maximum tolerated concentration of an aggressive anion is only beyond a certain critical concentration of the aggressive anion. The results are interpreted in terms of competitive adsorption between the inhibitor and aggressive ions on the bare metallic parts.     

On-line corrosion monitoring in geothermal district heating systems. I. General corrosion rates

General corrosion rates in the geothermal district heating systems in Iceland are generally low, of the magnitude 1 μm/y. The reason is high pH (9.5), low-conductivity (200 μm/y) and negligible dissolved oxygen. The geothermal hot water is either used directly from source or to heat up cold ground water. The fluid naturally contains sulphide, which helps keeping the fluid oxygen-free but complicates the electrochemical environment. In this research on-line techniques for corrosion monitoring were tested and evaluated in this medium. Electrochemical methods worked well as long as frequency was kept low but ER worked better if oxygen was present.     

Corrosion behaviour of stainless steel exposed to highly concentrated chloride solutions

The characteristics of pitting corrosion of Type 304L stainless steel (SS) exposed to highly concentrated chloride solutions were studied through the evaluation of the corrosion potential, the pitting potential, the structure of the passive layer and the statistics of pitting depth and density. Both as-received and weld metal samples were studied. The weld metal sample was machined from the welding zone of a butt weld of Type 304L SS. The results showed an accelerated anodic dissolution and depressed film resistance at the welding zone, but no dramatic change on pitting corrosion was observed from the statistics of pitting during the test duration up to 720?h. The pitting corrosion resistance was significantly affected by the chloride concentration and slightly affected by the temperature under the investigated conditions.     

Corrosion behaviour of die-cast AZ91D magnesium alloy in aqueous sulphate solutions

The corrosion behaviour of die-cast AZ91D magnesium alloys in sulphate solutions was investigated by SEM, FTIR and polarization measurements. For immersion times less than 48 h, no pitting corrosion occurred and only generalized corrosion was apparent. According to the polarization curves, the corrosion rate order of the die-cast AZ91D Mg alloy in three aqueous solutions was: NaCl > MgSO₄ > Na₂SO₄. The main corrosion products were Mg(OH)₂ and MgAl₂(SO₄)₄·22H₂O in the sulphate solutions and the product film was compact. Precipitation of MgAl₂(SO₄)₄·22H₂O required a threshold immersion time.     

Corrosion and electrochemical behaviour of 316L stainless steel in acetic acid solutions

The corrosion and electrochemical behaviour of 316S11 stainless steel in acetic acid solutions typifying chemical process environments has been investigated. Acetic acid concentrations tested were in the range 70-90% and included addition of 1500 ppm Br⁻ and 200 ppm Na⁺. Of key interest was the impact of Cl⁻ ions, representing an uncontrolled excursion in system chemistry. Corrosion potential-time and electrochemical polarisation measurements were made for the different environments at 90 °C and the characteristics of the surface film formed at different stages of exposure analysed using X-ray photo-electron spectroscopy (XPS).The most distinctive feature of the results was the step increase in potential with exposure time in the 70% acetic acid solution, in the absence of Cl⁻ ions, indicating a sharp transition from active corrosion to some degree of passivity. No such transition was observed in the 90% acetic acid solution. Addition of chloride to the 70% acetic acid solution after the step in potential resulted in a step decrease in potential once a critical level of chloride had been exceeded. If the chloride were present on initial immersion, the potential stayed relatively low and the steel remained active. XPS analysis suggested that local enrichment of Mo was important in initiating the passivation process but the precise details of the mechanism remain speculative.     

Corrosion behaviour and galvanic coupling of titanium and welded titanium in LiBr solutions

Corrosion resistance and galvanic coupling of Grade 2 commercially pure titanium in its welded and non-welded condition were systematically analyzed in LiBr solutions. Galvanic corrosion was evaluated through two different methods: anodic polarization (according to the Mixed Potential Theory) and electrochemical noise (using a zero-resistance ammeter). Samples have been etched to study the microstructure. The action of lithium chromate as corrosion inhibitor has been evaluated. Titanium and welded titanium showed extremely low corrosion current densities and elevated pitting potential values (higher than 1 V). The results of both methods, anodic polarization and electrochemical noise, showed that the welded titanium was always the anodic element of the pair titanium-welded titanium, so that its corrosion resistance decreases due to the galvanic effect.     

Corrosion behaviour of surface titanium layers in acid solutions after nitrogen thermodiffusion saturation

The influence of structure and phase compositions of nitrided BT1-0 titanium surface layers on the corrosion properties in 30% H₃PO₄ solution has been investigated. It has been shown that formation of the oxynitride film on the BT1-0 surface provides high anticorrosive properties of titanium in 30% H₃PO₄ solution as well as in 10% H₂SO₄ solution, though proves to be ineffective in 10% HCl solution.     

Corrosion behaviour of hot-pressed austenitic stainless steel in H2SO4 solutions at room temperature

The corrosion behavior of hot-pressed stainless steel (HPSS) of Type 316 was investigated. Samples of HPSS with porosity of 10–30% were prepared. The influence of the porosity factor on corrosion behaviour was investigated by potentiodynamic polarization, open circuit potential, identification of corrosion products by EDAX and SEM observation.It was found that porosity of the HPSS is the main factor affecting corrosion resistance. High porosity results in low corrosion resistance. The open circuit potential of the HPSS is about ? 200 mV (VSCE) as compared to 200 mV for the wrought 316 stainless steel. The potentiodynamic measurements indicate that cathodic concentration polarization is achieved at a relatively low current density and secondary active-passive transition is observed at a relatively high current density in HPSS as compared to wrought 316 stainless steel.It is suggested that the main mechanism affecting the low corrosion resistance of HPSS in H₂SO₄ is the evolution of hydrogen concentration cells due to electrolytic stagnation in the interconnected open pores. As a result the surfaces of the interconnected open pores act as an active anode and the engineering sample surface acts as an active cathode.     

Corrosion behaviour of highly alloyed stainless steels in mixed chloride and fluoride aqueous solutions

Laboratory weight loss and cyclic potentiodynamic polarization corrosion tests were performed on two types of corrosion resistant alloys, a duplex alloy (ferritic-austenitic stainless steel) and two austenitic stainless steels, in mixtures of chloride (3000, 9000 and 15000 ppm) and fluoride (4800 and 15000 ppm) ions at pH 3. Two temperatures were tested, 60 and 70°C. The electrochemical results indicate that the duplex stainless steel presents high corrosion resistance. Weight loss results show low corrosion rates of the two types of stainless steels after 60 days exposure. Sonic pits-crevices were found under the corrosion crust deposits on the duplex stainless steel.     

Corrosion Behaviour of Manganese-Containing. Stainless steels. I. Galvanostatic measurements in H2SO4 solutions

The results of galvanostatic polarization experiments on four 17.3% Cr / 5.3% Ni stainless steels containing increasing amounts of Mn (upto 13.9%) are compared with those of an 18/8 stainless steel. All alloyes exhibit an initial step for the dissolution of Fe. A second arrest is recorded in dilute acid solutions and/or when applying high currents, and is related to the oxidation of the Cr. With an alloy containing traces of N₂, a third arrest is observed in dilute acid solutions; it is attributed to the oxidation of Cr₂N. The incorporation of Mn in CrNi steels is detrimental to their anti-corrosion characteristics. The content of this metal should be kept as low as feasible.