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 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.     

Weißblechkorrosion in nitrathaltigen Medien: Die Rolle des Nitrits

Tinplate corrosion in nitrate containing media: The role fo nitrite The role of nitrite in the corrosion of tinplate in media containing nitrate was examined. If nitrite is initially absent, the corrosion is initially slow, but as nitrite is generated the corrosion, in turn, accelerates. This slow startup does not occur if nitrite is initially present in sufficient concentration. These observations agree with the autocatalytic mechanisms suggested for the reduction of nitrate by Schmid and Vetter.     

Heat transfer during quenching of modified and unmodified gravity die-cast A357 cylindrical bars

Heat transfer during quenching of chill-cast modified and unmodified A357 Al-Si alloy was examined using a computer-aided cooling curve analysis. Water at 60 °C and a vegetable oil (palm oil) were used as quench media. The measured temperatures inside cylindrical probes of the A357 alloy were used as inputs in an inverse heat-conduction model to estimate heat flux transients at the probe/quenchant interface and the surface temperature of the probe in contact with the quench medium. It was observed that modified alloy probes yielded higher cooling rates and heat flux transients. The investigation clearly showed that the heat transfer during quenching depends on the casting history. The increase in the cooling rate and peak heat flux was attributed to the increase in the thermal conductivity of the material on modification melt treatment owing to the change in silicon morphology. Fine and fibrous silicon particles in modified A357 probes increase the conductance of the probe resulting in higher heat transfer rates. This was confirmed by measuring the electrical conductivity of modified samples, which were found to be higher than those of unmodified samples. The ultrasound velocity in the probes decreased on modification.     

Performance of a thermocell of carbon fibres and nitric acid

The performance of a thermocell of carbon fibres and nitric acid was studied. PAN-based carbon fibre cloths, heat treated at various temperatures, and 30% nitric acid were used as an electrode and an electrolyte, respectively. In this cell, the reduction of nitrate ion to nitrogen oxides occurs at the cathode, and the reverse reaction at the anode. The largest load current was obtained by using the 1300 °C-treated cloths. Oxidation treatment of the 2000 °C-treated cloths caused an increase in the load current. The load current dependence on the anode temperature showed a maximum value in the vicinity of 20 °C. The open circuit voltage was independent of the weight of the cloths. The load current increased linearly with the weight of the cloths up to 1.0 g. A test of durability confirmed stable performance for more than 190 days.     

A study of film formation on aluminum in aqueous solutions using Rutherford backscattering spectroscopy

The growth of hydrated surface films on 99.99% pure aluminum in aqueous media at 100°C has been studied with Rutherford backscattering spectroscopy (RBS). The effects of the inhibitor anions borate, molybdate, nitrate, phosphate, sulfate, silicate and tungstate, and of ethylene glycol on this reaction were observed at concentration levels typical of automotive coolant formulations. The behavior of the inorganic anions studied is classified roughly into three types. In the first, typical of silicate, the inhibitor reacts with the surface to both prevent formation of the aluminum hydroxide layer, as well as limit the uptake of additional species by the surface. In a second mode, characteristic of nitrate and sulfate additions, the surface hydroxide develops, but at lower levels than those obtained in pure water. In yet another mode, characteristic of phosphate, molybdate and tungstate, the inhibitor anions are continually incorporated into a growing film which appears to be a hydrated aluminum salt of the particular anion, rather than pseudo-boehmite. Ethylene glycol is found to act primarily as a diluent in reducing the overall rate of film growth in aqueous solutions.     

Corrosion inhibition of aluminum alloys in heat exchanger systems

Corrosion behaviour of AA 3003 and AA 7075 aluminum alloys in tap water has been studied at temperatures ranging from 25 to 95 °C. The effects of additives on the corrosion rates have also been investigated. Experimental results show that the corrosion rates of both AA 3003 and AA 7075 in tap water increase with increasing temperature up to 88 °C. The addition of any one of the following chemicals: chromate, nitrate, molybdate and silicate in tap water results in a significant reduction in corrosion rates of these aluminum alloys, particularly under high temperature conditions. Sodium sulphate has a slight inhibitive effect at temperatures below 88 °C, but its presence in tap water at 95 °C causes an increase in the corrosion rates of both aluminum alloys. An adverse effect on corrosion behaviour, however, is observed when sodium phosphate is added.     

Details on the Formation of Ti<Subscript>2</Subscript>Cu<Subscript>3</Subscript> in the Ag-Cu-Ti System in the Temperature Range 790 to 860 °C

Silver-copper-titanium (Ag-Cu-Ti) ternary alloys are often used as active braze alloys for joining ceramics to metals at temperatures ranging from 780 °C (the melting point of the Ag-Cu eutectic) up to 900 °C. When Ti/Ag-Cu joints are brazed at low temperature (near 800 °C), the intermetallic compound Ti₂Cu₃ (tetragonal, P4/nmm, a = 0.313 nm, c = 1.395 nm) is systematically missing from the interface reaction layer sequence. An experimental investigation based on isothermal diffusion experiments in the Ag-Cu-Ti ternary system has then been undertaken to clarify the issues of thermal stability and formation kinetics of this compound. Evidence has been found for the stability of Ti₂Cu₃ at temperatures ranging from 790 to at least 860 °C. By heat treating Ag-Cu-Ti powder mixtures at 790 °C for increasing times, it has moreover been shown that Ti₂Cu₃ forms at a much slower rate than the two adjacent Ti-Cu compounds: TiCu₄, the first phase to form, and Ti₃Cu₄. This explains why although thermodynamically stable, Ti₂Cu₃ is not obtained when temperature is too low or reaction time too short.     

On the Distortion and Warpage of 7249 Aluminum Alloy After Quenching and Machining

The objective of this study is to determine the effect of solution treatment temperature, quenching media, and various machining sequences on the warpage behavior of aluminum 7249 alloy aged to T6 and T7′ tempers. Large extrusions of 7249 aluminum alloy with fins were cut into 108 “T” sections. The samples were solution-treated, aged, and machined. Three solution temperatures (445, 474, and 505 °C), two quenching media (water and 20% polyalkylene glycol), two aging treatments (T6 and T7′), and three machine sequences were used. The flatness of the samples was measured on the surfaces orthogonal to the z-axis. Three points were on top of both shoulders (six total), six were at the bottom of the sample, and six were on the top of the fin, in the cases where the fin was not milled off. They were then averaged together by surface to represent the overall warpage of each sample.     

Effects of temperature on the Corrosion of copper in sea water at different hydrostatic pressures

An investigation has been made of the effect which an increasing hydrostatic pressure may have on copper corrosion at different temperatures in sea water at pH 7.8. The tests were carried out at a pressure of 1, 50, 100, 200 and 300 atm at a temperature of 5 °, 10 ° and 20 °C. The pressure set in which the tests were effected was pressurized by an hydropneumatic pump and it was fitted not only with a work electrode, but also with two Pt and one Ag-AgCl reference electrodes for electrochemical tests. It was found that copper corrosion is always stimulated by a pressure rise and that the increased corrosion rate is accelerated by increasing temperatures. The maximum increase (71%) was obtained at T = 20 °C; D.O = 7.0 ppm and pH = 7.8 at a pressure of 150 atm. But at a temperature of approx. 5 °C, the maximum increase did not exceed 30% always at a pressure of 150 atm. The test findings also evidenced that at a temperature ≤ of 10 °C, the pressure rise did not affect the anodic process but only the cathodic reduction, whereas the anodic process too was significantly accelerated at 20 °C and the influence of the pressure was found to be more accentuated.     

Evaluation of corrosion inhibitors for cooling water systems operating at high concentration cycles

The present work aimed at evaluating AISI 1020 carbon steel corrosion resistance of a 6:4:1:1 (MoO/HEDP/PO/Zn²⁺) inhibitor mixture present in a solution which simulates an industrial cooling water system operating at high concentration cycles (1050 ppm Cl⁻ and 450 ppm Ca²⁺). High concentration cycles are desirable, because system purge and treated water consumption are decreased. On the other hand, a high number of concentration cycles can increase the concentration of salts and dissolved impurities, causing corrosion, incrustations, and deposits inside the pipes, heat exchangers, and cooling towers. Thus, the chloride (Cl⁻) and calcium (Ca²⁺) ions aggressiveness was studied on the proposed inhibiting mixture, at the temperatures of 40 and 60 °C, through electrochemical techniques like open circuit potential measurements, anodic and cathodic polarization, and weight loss. The results showed that the inhibitor mixture conferred adequate protection to carbon steel in low concentrations, even in high aggressive media.     

Carbon and stainless steel in solutions of lithium nitrate in ammonia at moderate temperatures

The lithium nitrate in ammonia solution has been proposed and demonstrated as a thermodynamically attractive working mixture for absorption heat pumps. A literature search failed to find corrosion studies of said solution. The corrosion of carbon and stainless steel in solutions of lithium nitrate in ammonia was studied for a series of concentrations and temperatures from 50 to 150°C. The materials studied were found to be suitable for construction of absorption heat pump process equipment for this working mixture.     

Quenching of aerospace forgings from high temperatures using air-assisted, atomized water sprays

The nickel-based superalloy or titanium materials used in the aerospace industry are cooled from high temperatures during the heat treatment process to obtain appropriate strength properties. However, unacceptably high residual stresses can be developed in some situations if the rate of cooling is too high so that air-assisted, atomized water sprays have been suggested as an alternative to the widely used techniques of quenching in oil or water. Thus, this article examines two aspects of the use of air-water sprays for quenching aeroengine forgings. First, basic experimental heat transfer data are presented for a wide range of water flows and for surface temperatures up to approximately 850 °C, for both plane and recessed surfaces. Second, the heat transfer data are used in numerical simulations to study the influence of nonuniform spray distributions on the residual stress patterns in a typical forging. This paper was presented at the ASM Third International Conference on Quenching and Control of Distortion, 24–26 March 1999, in Prague, Czech Republic.     

Corrosion of Ferrous metals in Batch Cultures of Nitrate-Reducingu Bacteria

Abstract

The corrosion of an iron/carbon alloy has been investigated in short-term experiments in batch cultures of the bacterium Escherichia coli grown in media containing two concentrations of nitrate and in a nitrate-free medium. The growth of the organism was also investigated in the above media but in the absence of metal samples. The results show that growth is affected by the presence of the metal and that the principal cause of corrosion is the formation of metabolic organic acids. The nitrite produced by bacterial reduction of the nitrate reacts with the metal to form a protective film.     

Comparative Study of Factors Influencing the Action of Corrosion Inhibitors for Mild Steel in Neutral Solution: III. Sodium Nitrite

Abstract

The action of sodium nitrite as a corrosion inhibitor for mild steel in neutral aqueous solution has been studied in relation to the surface preparation of the steel, the presence of aggressive salts in the solution, and temperature.

Surface preparation of mild steel has little effect on the minimum concentration of nitrite required for protection in distilled water. In solutions containing aggressive anions a linear relation exists between the logarithm of the nitrite concentration and the logarithm of the maximum concentration of aggressive anion that will permit inhibition. In solutions of low nitrite concentration the order of aggressiveness of anions is sulphate > chloride > nitrate; the order changes with increase in nitrite concentration. Corrosion in non-inhibiting nitrite/aggressive anion solutions is frequently of a severely localised form. A 5-fold increase in nitrite requirement for inhibition of abraded mild steel in distilled water is found between 5° and 70°, with a further large, rather indeterminate, increase above 70°.

Of the three inhibitors studied in Parts I-III, chromate and nitrite are slightly moreeffective than benzoate for the protection of an abraded surface, and considerably more so for a grit-blasted surface. In general, nitrite is the most tolerant towards the presence of chloride, but it is the most dangerous if excess chloride is present. The increase in inhibitor requirement with increase in temperature from 5° to 70° is very approximately the same for all three inhibitors, that is, about 5– to 10–fold,with all inhibitors needing much higher concentrations at about 90°.     

Experimental study of heat transfer coefficient on hot steel plate during water jet impingement cooling

Experiments were performed, under transient conditions, to investigate the heat transfer phenomena of stationary hot steel plate under multiple top circular jets on run-out table. Based on inverse heat conduction model, a two-dimensional finite difference program was developed to calculate the local surface convective heat transfer coefficients and corresponding temperatures. The cooling water jet flow rate was varied from 15 L/min to 35 L/min and its effect on the convective heat transfer coefficient and surface temperature was analyzed. The results show that heat transfer coefficients are nonlinear functions of surface temperature. The cooling flow rate has no effect on heat transfer coefficient and surface temperature at stagnation point. Within 70 mm distance from stagnation line, heat transfer coefficient ratio changes slightly from 0.87 to 0.97. Beyond surface temperature of 350 °C, heat transfer coefficient ratio decreases with increasing distance from stagnation line.     

Experimental determination of heat transfer coefficients during squeeze casting of aluminium

Heat transfer coefficients during squeeze cast of commercial aluminium were determined, by using the solidification temperature versus time curves obtained for varying applied pressures during squeeze casting process. The steel mould/cast aluminium metal interface temperatures versus times curve obtained through polynomial curves fitting and extrapolation was compared with the numerically obtained temperatures versus times curve. Interfacial heat transfer coefficients were determined experimentally from measured values of heating and cooling temperatures of steel mould and cast metal and compared with the numerically obtained values and found to be fairly close in values. The values of the heat transfer coefficients were found to increase with increase in applied pressures and to decrease with fall of solidifying temperatures corresponding to three distinct solidification stages namely, liquidus, liquidus–solidus and solidus stages. Below temperatures of 500 °C, the effect of the increase in heat transfer coefficients with applied pressure application becomes less significant and the drop in values of the heat transfer coefficients with solidification temperatures at any applied pressures remains fairly constant. The peak values of heat transfer coefficients obtained for as-cast (no pressure application) and squeeze cast (pressure application) of aluminium are 2975.14 and 3351.08 W/(m² K), respectively. Empirical equations, relating the interfacial heat transfer coefficients to the cast aluminium surface temperatures and applied pressures at three distinct temperature range intervals, were also derived and presented.     

Infrared-optical properties and heat transfer coefficients of semitransparent thermal barrier coatings

Thermal barrier coatings (TBC) which are used in aircraft and land-based gas turbines for thermal insulation of thermally highly loaded components are usually semitransparent in the infrared spectral region at higher temperatures. Thus, at turbine surface conditions the total heat transfer coefficient of TBCs increases above the heat transfer coefficient caused by solid heat conduction alone.The solid thermal conductivity of electron beam physical vapor deposited (EB-PVD) partially yttria stabilized zirconia (YSZ) coatings derived from laser flash measurements were correlated with the microstructure of the coatings, which was adjusted by defined heat treatments. To obtain the contribution of the radiative transfer on the total heat transfer coefficient, infrared-optical characterizations were carried out at ambient and elevated temperatures.A theoretical model was developed which can be used to describe the heat transfer through semitransparent, absorbing and scattering media. Finally, the total heat transfer, caused by solid thermal conduction, radiative transfer and an interaction of both is derived for the coatings prepared in this work. Additionally the measurement method BBC (black body boundary conditions) which is suitable to determine spectral transmittance and emittance at elevated temperatures is introduced.     

Effect of chill cooling conditions on cooling rate,microstructure and casting/chill interfacial heat transfer coefficient for sand cast A319 alloy

A combination of experiments and numerical analyses were used to examine the cooling conditions, solidification microstructure and interfacial heat transfer in A319 cast in a chilled wedge format. Both solid copper chills and water cooled chills, with and without a delay in water cooling, were examined in the study. Various chill preheats were also included. The goal of the investigation is to explore methods of limiting heat transfer during solidification directly beside the chill and increasing cooling rates during solidification away from the chill. Within the range of conditions examined in the study, chill preheat was found to have only a small effect on cooling rates between 5 and 50 mm from the chill/casting interface, pour superheat a moderate effect and water cooling a significant effect. In comparison to the results for the solid chill, the solidification time at 5 mm with water cooling applied at the beginning of mould filling is reduced from 56 to 15 s and at 50 mm from 588 to 93·5 s. Furthermore, the average cooling rate during solidification is increased from 1·9 to 7·06°C s^?1 at 5 mm and from 0·18 to 1·13°C s^?1 at 50 mm. At 50 mm, for example, the increased cooling rate achieved with water translates into a reduction in secondary dendrite arm spacing from 40 to 25 μm or ～40%. Delaying the water cooling by 10 s facilitated slow cooling rates at 5 mm (similar to those achieved with a solid chill) and high cooling rates 50 mm from the chill. A temperature based correlation was found to be suitable for characterising the behaviour of the interfacial heat transfer coefficient in the solid shill castings, whereas a time based correlation was needed for the water cooled castings.     

A study of processing parameters in thermal-sprayed alumina and zircon mixtures

A method of plasma spraying of alumina and zircon mixtures to form ZrO₂-mullite composites has been proposed and developed. The feedstock is prepared by a combination of mechanical alloying, which allows formation of fine-grained, homogeneous solid-solution mixtures, followed by plasma spheroidization that yields rapid solidified microstructures and enhanced compositional homogeneity. The effects of ball-milling duration and milling media were studied. It was found that zirconia is a more efficient milling media and that increasing milling duration enhanced the dissociation of zircon. Flame spray and plasma spray processes were used to spheroidize the spray-dried powders. The temperature of the flame spray was found to be insufficient to melt the powders completely. The processing parameters of the plasma spray played an important role in zircon decomposition and mullite formation. Increasing the arc current or reducing secondary gas pressure caused more zircon to decompose and more mullite to form after heat treatment at 1200 °C for 3 h. Dissociation of zircon and the amount of mullite for med can be enhanced significantly when using the more efficient, computerized plasma-spraying system and increasing the ball-milling duration from 4 to 8 h.