Study of the potential electrochemical noise during corrosion process of aluminum alloys 2024, 7075 and pure aluminum

Properties of the potential electrochemical noises (EN) generated during the corrosion process of aluminum alloys AA2024(T3), AA7075 (aged at 121°C/35 h + 160°C/20 h) and pure aluminum in 3wt% (mass fraction) NaCl solution were investigated in this study. During the initial stage of immersion, the EN amplitude of AA2024 is the largest while the EN amplitude of pure aluminum is negligible. The amplitude of the EN generated by these materials decreases with immersion time except the EN of AA2024 after extremely long immersion time (38 days in this study). Surface morphology corresponding to different stages of immersion and different types of EN were studied by scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) analysis. The result shows that the micro‐galvanic cells formed by the constituent particles and the alloy matrix play an important role on the corrosion process of the aluminum alloys and the EN behaviors of AA2024, AA7075 and pure aluminum is proved to be strongly related to the type and intensity of corrosion process.     

Einfluß von Eisen auf die Korrosionsbeständigkeit von Nickelbasislegierungen als mögliche Werkstoffe für SCWO-Reaktoren

Effect of iron on the corrosion behaviour of nickel based alloys for SCWO plants An experimental set up to study corrosion at high temperatures and high pressures and able to simulate the conditions of supercritical water oxidation is described. On the basis of the alloys AC 66, 45 TM, G-3 and 601 H the influence of iron on the corrosion behaviour of nickel base alloys in aqueous solution under high pressures containing oxygen and chloride is shown and discussed. The corrosion resistance of the nickel base alloys is decreased with increasing iron concentration in the whole temperature range (100 °C–420 °C). The corrosion started at lower temperatures and the weight loss increased rapidly with increasing iron content. The corrosion morphology changed from uniform corrosion for alloys with less iron content to deep pits for alloys with high iron concentrations.     

Development and analysis of frits for enamelling AA2024, AA6082 and AA7075 aluminium alloys

This study reports the development of frits with different composition, time, and firing temperature for mitigating the corrosion of AA2024, AA6082, and AA7075 aluminium alloys in chloride-containing solutions. The melting points of the frits were determined using a heating microscope, while the thermal expansion was measured using dilatometry. Several frits were prepared, of which seven were coated on AA2024, AA6082 and AA7075 aluminium alloys. The corrosion resistance of the coated samples was tested in 5 wt% NaCl solution at 25°C using potentiodynamic curve measurements. The corrosion resistance of the coated samples increased compared with the respective uncoated aluminium alloys. The composition and in-depth homogeneity of the best-performing coated samples were confirmed using a gas cluster ion beam sputtering source associated with X-ray photoelectron spectroscopy analysis. Furthermore, O₂⁺ sputtering was performed in association with time-of-flight secondary ion mass spectrometry to describe the three-dimensional composition of the frits.     

Oxidationsverhalten nitridverfestigter Nickelbasislegierungen

Oxidation behaviour of nitride-strengthened Ni-base alloys Nitride-strengthened Ni-base alloys of the type NiCr7030, produced by DESU-technology, provide promising mechanical properties up to temperatures of 1200°C. This paper provides data on the cyclic and isothermal oxidation behaviour of these alloys in the temperature range of 1000–1200°C. The results show, that depending on the actual composition of the alloys tested even at 1200°C corrosion rates below 0.3 g/m² h can be achieved. It was found that the silicon content must be limited to a maximum of 1.00 wt.-% to guarantee the excellent oxidation resistance even at temperatures higher than 1100°C.     

Corrosion behaviour of Mg/Al alloys in high humidity atmospheres

The influence of relative humidity (80–90–98% RH) and temperature (25 and 50 °C) on the corrosion behaviour of AZ31, AZ80 and AZ91D magnesium alloys was evaluated using gravimetric measurements. The results were compared with the data obtained for the same alloys immersed in Madrid tap water. The corrosion rates of AZ alloys increased with the RH and temperature and were influenced by the aluminium content and alloy microstructure for RH values above 90%. The initiation of corrosion was localised around the Al–Mn inclusions in the AZ31 alloy and at the centre of the α‐Mg phase in the AZ80 and AZ91D alloys. The β‐Mg₁₇Al₁₂ phase acted as a barrier against corrosion.     

Brazeability of a 3003 Aluminum alloy with Al-Si-Cu-based filler metals

Al-Si-Cu-based filler metals have been used successfully for brazing 6061 aluminum alloy as reported in the authors’ previous studies. For application in heat exchangers during manufacturing, the brazeability of 3003 aluminum alloy with these filler metals is herein further evaluated. Experimental results show that even at such a low temperature as 550 °C, the 3003 alloys can be brazed with the Al-Si-Cu fillers and display bonding strengths that are higher than 77 MPa as well. An optimized 3003 joint is attained in the brazements with the innovative Al-7Si-20Cu-2Sn-1Mg filler metal at 575 °C for 30 min, which reveals a bonding strength capping the 3003 Al matrix.     

Barrier and self‐healing abilities of corrosion protective polymer coatings and metal powders for aluminum alloys

Several types of coatings, designed to improve the resistance of aluminum alloys to corrosion in seawater, were examined. A coating with a thickness of approximately 30 μm was applied to the surface of aluminum alloy 3003, and after creating an artificial defect by means of a knife‐edge, a corrosion test was carried out in 3% NaCl solution at a temperature of 70°C and pH 1.5. The test results showed that a fluorine resin and sol‐gel coatings both had excellent barrier abilities. The fluorine resin coating had self‐healing abilities, and this was improved by the addition of a metal powder.     

Über das Korrosionsverhalten von Zink und feuerverzinktem Stahl in erwärmtem Wasser

Corrosion behaviour of zinc and hot dip galvanized steel in warm water The corrosion rate of zinc and hot dip galvanized steel in continuously flowing tap water is but little influenced by water temperature. The slight decrease of the corrosion rate which has been found at higher temperatures (60°C) can be attributed to the preferential formation of zinc oxide at these temperatures, this oxide being less soluble than the hydroxide formed at lower temperatures. The potential shift toward more noble values at higher temperatures does, however, depend from water composition and may eventually start as early as at 35 °C; in certain critical potential ranges the uniform corrosion may even be changed into pitting. Addition of phosphate to the water may reduce either the corrosion rate or it may restrict the potential shift (in this latter case the appearance pitting is prevented). The measurement of the electrode potential (not of the polarization resistance) thus yields information on the danger of pitting corrosion. The quality of the zinc surface seems to be important in those cases where the ZnFe alloy layer protrudes to the zinc surface.     

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.     

Über die transpassive Auflösung von Nickel-Basis-Legierungen und Edelstählen in sauerstoff- und chloridhaltigem Hochtemperatur-Wasser

Transpassive dissolution of nickel-base alloys and stainless steels in oxygen and chloride containing high-temperature water The corrosion behaviour of different nickel base alloys and stainless steels (2.4605 [alloy 59; NiCr23Mol6Al], 2.4633 [alloy 602 CA; NiCr25FeAlY], 2.4819 [alloy C-276; NiMol6Crl5W], 2.4856 [alloy 625; NiCr22Mo9Nb], 2.4606 [alloy 686; NiCr20Mu16], 2.4646 [alloy 214; NiCrl6AlFe] and 1.4401 [UNS S 31600; X5CrNiMol7122]) was investigated in oxygen and chloride containing high-temperature water (temperatures up to 600°C; pressures up to 38 MPa; oxygen concentration 0.48 mol/kg; chloride concentrations up to 0. 1 mol/kg). All alloys show a similar corrosion behaviour, depending on temperature. At temperatures below about 150°C, only slight intergranular corrosion was observed. At higher temperatures (between about 150 and 300°C) pitting was detected. Most of the original surface in this temperature range remained unattacked. At higher temperatures, morphology of pitting changed towards shallow pitting and the whole surface is penetrated. The high general corrosion observed in these areas can be attributed to transpassive dissolution of the alloys' protecting chromium oxide layers with following dissolution of the alloy. At supercritical temperatures, corrosion decreased drastically, and only transpassive intergranular corrosion was detected. The observed decrease of ion-induced corrosion phenomena can he attributed to the change of physical and chemical properties of water (solubility of ions). Corrosion in neutral and alkaline solution was significantly less. Both pitting and transpassive dissolution shifted towards higher temperatures or were not detected respectively.     

Effect of phase transformation on corrosion behaviour of Zn–22Al alloys

The effect of phase transformation on the corrosion behaviour of Zn–22Al (wt-%) alloys immersed in NaCl solution (3·5 wt-%) at room temperature was investigated. As cast Zn–22Al alloys were isothermally heated at 300 and 250°C (the eutectoid point is 277°C) respectively and then cooled by various rates. Isothermally heated below the eutectoid point, the corrosion resistance of the alloys increased with the decreasing cooling rate. However, when the isothermal heating temperature was above the eutectoid point, the corrosion resistance of Zn–22Al alloys increased with the increasing cooling rate. It can be attributed to the existence of α₂ phase, which may lead to smaller potential difference between the microgalvanic cells.     

高温变形条件下5A02铝合金的塑性成形性能

随着工业技术的发展和能源问题的突出,铝合金以其质量轻、耐腐蚀性能好、成形性能和加工性能良好等优势成为轻型化首选的材料类型之一。以5A02铝合金冷轧板材为研究对象,通过单向拉伸试验和金相试验对不同变形温度、应变速率条件下5A02铝合金的塑性性能进行分析,并且借助试验数据和Zener-Hollomo参数模型,对高温条件下5A02铝合金的本构模型进行研究。研究结果表明：5A02铝合金在高温条件下变形时,应变速率和变形温度对延伸率的影响很大。在应变速率为0.01s-1、0.001 s-1、0.0005 s-1和0.0001 s-1条件下,当变形温度大于250℃时,5A02铝合金的延伸率大于100%。当变形温度为150℃~250℃时,5A02铝合金的真实应力-应变曲线属于动态回复型,而当变形温度大于250℃时,流变应力曲线存在明显的软化现象。     

Imitating seasonal temperature fluctuations for the H2S corrosion of 304L and 316L austenitic stainless steels

Temperature fluctuations are inevitable in sour oil and gas production. In this study, the H₂S corrosion of 304L and 316L alloys was investigated at pH 3 and temperatures of 20–60 °C using DC and AC electrochemical techniques. Two-fold increases in the corrosion rates of both alloys were reported with increases in temperature to 60 °C. In the 304L alloy, the surface layer was observed to be 3% rougher and 34% thicker than that of the 316L alloy. The two alloys exhibited different corrosion behaviors in the temperature ranges of 20–40 °C and 40–60 °C. Although the 316L alloy revealed a greater corrosion resistance at the free potential condition, the passivation on the 304L alloy was significantly greater than that of the 316L alloy at 40 °C and 15 ppm H₂S. The FeS₂ and combined FeS₂-MoS₂ compounds contributed to the surface layer constituents in the 304L and 316L alloys, respectively. The increase in temperature kinetically provided more favorable conditions for FeS₂ than MoS₂ formation, i.e. it had a relatively constructive effect on the 304L alloy passivation.     

Corrosion behaviour of experimental alloys in simulated HTGR helium at temperatures in the 750–1050°C

A series of experimental alloys (basically Ni-20 Cr, with addition of one of more of the elements Al, Ti, Si, Nb and Y) has been examined after exposure to controlled purity helium for periods of 1000–6000 hours at 750–1050 °C. This environment was intended to simulate the primary coolant of a high temperature gas-cooled reactor. Alloys containing aluminum were found to be particularly susceptible to internal oxidation at the lower temperatures, but at 950 °C and above carburization became the dominant corrosion mechanism. The most corrosion resistant alloys were Ni-20Cr and Ni-20 Cr containing small amounts of silicon, titanium and niobium. The presence of 1% yttrium dramatically increased the incidence of carburization, even at temperatures as low at 850°C.     

7075铝合金应力腐蚀敏感性的SSRT和电化学测试研究

采用慢应变速率试验(SSRT)研究了7075铝合金的应力腐蚀行为。试验结果表明,外加阳极极化和阴极极化都能增加7075铝合金的应力腐蚀敏感性,即使在弱极化情况下也能显著增加合金的应力腐蚀敏感性。但是,外加极化对于不同热处理状态的7075铝合金,其应力腐蚀敏感性增加的程度不同。电极极化对T6状态合金的SCC敏感性的影响显著,而对RRA和T7351状态合金的影响较轻微。随着拉伸应力的增加,7075-RRA铝合金的阳极极化曲线略向正移,滞后环面积扩大,但并不显著。     

Evaluation of corrosion resistance of two engineering alloys in molten salts by electrochemical techniques

The electrochemical behaviour of two steels typically used in power boilers has been studied in the temperature range from 540°C to 680°C. Two environments were used: a) a synthetic salt mixture of 80% V₂O₅ ? 20% Na₂SO₄ and b) oil ash collected from a high temperature reheater. Corrosion rates obtained from electrochemical potentiodynamic polarisation curves (Tafel extrapolation) were compared for both steels exposed in each environment. The results showed that increases in temperature resulted in higher corrosion rates, being this effect most notorious above about 620°C. In the synthetic salt, and at temperatures up to about 580°C, both steels showed similar behaviour. With further increases in temperature, the T22 steel was less resistant and, at the highest temperature used here, its corrosion rate was almost seven times higher than that measured for the 347H steel. The results obtained with the natural oil ash for the T22 steel showed a dramatic increase in corrosion rate as temperature goes up over the range used. For the 347H steel, and up to about 580°C, the corrosion rates were similar to those obtained with the synthetic salt. Above 580°C, the corrosion rate measured increased slightly with temperature, being at 680°C about 2.5 times higher than that found by using the synthetic salt. Therefore, an important difference in corrosion rates has been found depending upon the corrosive salt used. This is an important result because imply that, at least for materials used in fossil power plants, more realistic data can be obtained by using natural ashes in the experimental work.     

Corrosion in Molten Nitrate-Nitrite Salts

A series of mass loss corrosion rate laboratory experiments were conducted in molten nitrate-nitrite salts at 950–1300°F (510–705°C). Both nickel and iron alloys were tested. Nickel alloys with 15–20% chromium contents performed the best, while iron alloys, with low or no nickel exhibited poor corrosion resistance at high temperatures. Vapor aluminum diffusion treated stainless steel has good corrosion resistance but may be susceptible to localized attack and spoiling. The corrosion process combines oxidation and chromium dissolution. Protective chromium-rich oxide scales do not form, probably because chromium compounds are soluble in the salt. Salt corrosiveness increases drastically above 1200°F (650°C).     

Untersuchung verschiedener Einflußgrößen bei der Korrosion von Zirkonium-Vanadium-Legierungen in Wasser und Wasserdampf

A study of various factors influencing the corrosion of zirconium/vanadium alloys in water and steam Experimental investigations in water and steam at saturation (250 to 350°C) with Zr and Zr alloys (0.5 to 0.7 and 2.5 to 3.9% V) have revealed, that it is advisable to use grinding for the surface preparation of the specimens (good reproducibility). The increase in weight is linear at 250 and 350°C, the oxide scale is removed by disintegration after some time; increasing amounts of V give rise to increasing corrosion rates. Between Zr sponge and iodide Zr there is no difference as to oxidation behaviour. The rate increase with increasing V additions is attributed to the poor resistance of ZrV₂ precipitated at grain boundaries and being also responsible for the poor adhesion of the oxide layer. In pure Zr, too, higher rates of corrosion are found when two phase (α + β) are present.     

Influence of temperature and chloride ion concentration on the corrosion behaviour of Mg–4Al–3Ca–0.5RE alloy

The influence of temperature and chloride ion concentration on the corrosion behaviour of Mg?4Al?3Ca?0.5RE alloys were studied in this paper. Corrosion rates of the alloys were measured by weight loss test and electrochemical measurement. The results revealed that a shorter incubation period to the onset of corrosion, a more negative corrosion potential, and a higher corrosion rate was correlated with a higher temperature in 3% NaCl solution and a higher chloride ion concentration at 30°C. The corrosion behaviour of the alloys was affected by surface film and the corrosion mainly occurred at the breaks or defects in surface films.     

Stress Corrosion Cracking Study of Aluminum Alloys Using Electrochemical Noise Analysis

Stress corrosion cracking studies of aluminum alloys AA2219, AA8090, and AA5456 in heat-treated and non heat-treated condition were carried out using electrochemical noise technique with various applied stresses. Electrochemical noise time series data (corrosion potential vs. time) was obtained for the stressed tensile specimens in 3.5% NaCl aqueous solution at room temperature (27 °C). The values of drop in corrosion potential, total corrosion potential, mean corrosion potential, and hydrogen overpotential were evaluated from corrosion potential versus time series data. The electrochemical noise time series data was further analyzed with rescaled range (R/S) analysis proposed by Hurst to obtain the Hurst exponent. According to the results, higher values of the Hurst exponents with increased applied stresses showed more susceptibility to stress corrosion cracking as confirmed in case of alloy AA 2219 and AA8090.