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.     

Abrasion resistances of CO2 corrosion scales formed at different temperatures and their relationship to corrosion behaviour

The abrasion resistances of CO₂ corrosion scales formed on API P110 grade carbon steel at different temperatures and CO₂ corrosion behaviour of P110 carbon steel have been investigated utilising weight loss method, scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that as the film forming temperature increased, the abrasion rate of CO₂ corrosion scales increased firstly and then decreased, the variation trend was similar to that of the corrosion rate, and the maximal abrasion rate was present at 70°C rather than 100°C, at which the maximal corrosion rate was obtained and CO₂ corrosion scales were loose and porous and were composed of FeCO₃, Fe₃C and CaCO₃. The abrasion resistances of CO₂ corrosion scales formed at 100°C was reinforced by the sand particles embedded easily in the loose and porous layer, together with a very hard residual skeleton, Fe₃C. In addition, CaCO₃ was worn out easily and substituted by SiO₂ during abrasive wear.     

Pitting corrosion transformation of SUS304 stainless steel in lithium bromide solution at high temperature

The foundational data and rules of SUS304 corrosion in aqueous lithium bromide solution at 150–200 °C have been investigated by means of weight loss in this paper. The Taguchi method was chosen to investigate the effects of variables on corrosion at higher temperature. Concentration of dissolved oxygen was controlled by keeping the vessel pressure at 1.3 kPa (absolute) before the experiment. The results showed that there was a transformation tendency from pitting corrosion to general corrosion and transition concentration of pitting and film corrosion decreased with increase in solution temperature. The adsorption, activity, and diffusivity of bromine ion led to different corrosion modalities.     

Chemical corrosion of liquid-phase sintered SiC in NaOH aqueous solution

Corrosion behaviour of the liquid-phase sintered SiC ceramics (LPS-SiC) was studied through exploration of weight loss, strength reduction and morphology evolution of the SiC specimens etched in the room-temperature/70°C 6.12?mol/L (20 wt-%) NaOH aqueous solutions. As a comparison, corrosion of the reaction-bonded SiC ceramics (RB-SiC) was also investigated. The results show that corrosions of the SiC specimens mainly contain homogeneous dissolution of the secondary-phase oxides in the LPS-SiC and that of Si in the RB-SiC, and peel-off of the SiC particles, resulting in increase of the surface roughness, weight loss and strength reduction of the SiC ceramics. As dipped in the 70°C NaOH solution, corrosion pores and channels form in the sublayer, leading to rapid increase of weight loss and strength reduction of the LPS-SiC. No matter in the room-temperature or 70°C NaOH solution, the LPS-SiC always has a higher corrosion resistance than the RB-SiC.     

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.     

Comparative investigation on copper atmospheric corrosion by electrochemical impedance and electrical resistance sensors

Electrochemical impedance (EIS) and thin electrical resistance (ER) sensors were invented for atmospheric corrosion measurement of copper (Cu) during cyclic wetting–drying/high–low temperature tests and field exposure tests. Three-month field exposure results showed that average corrosion rate of Cu measured by ER sensor was well in accordance with that by weight loss method. During cyclic wetting–drying test, EIS was proven to reflect sensitively time of wetting and drying on the surface of sensor. Although corrosion rate obtained from EIS had a similar tendency to that obtained from ER sensors, the former was more dependent on environmental humidity than the latter. When relative humidity was low than 60%, corrosion rate of Cu measured by EIS was much lower than that by weight loss method, mainly attributing to the fact that impedance sensor failed to detect corrosion current of interlaced Cu electrodes due to the breakdown of conductive passage composed of absorbed thin liquid film under low humidity condition. Promisingly, ER sensor was proven to be more suitable for atmospheric corrosion monitoring than electrochemical techniques because it could sensitively monitor thickness loss of Cu foil according to the Ohmic law, no matter how dry or wet the sensor surface is.     

Kinetics of high temperature corrosion of a low Cr‐Mo steel in aqueous NaCl Solution

The kinetics of corrosion of a low Cr‐Mo steel alloy were studied over the temperature range of 75–250°C in 1 m NaCl in absence and in presence of various levels of contamination with CuCl₂. Corrosion rates, weights of corrosion product (magnetite) film and total (integral) weight loss of the alloy over exposure times from 1 up to 480 h were measured. The corrosion rate decreases rapidly with time, before it levels off at longer time, indicating the formation of a protective corrosion product film. The ability of the alloy to retain an adherent corrosion product (magnetite) film was expressed in terms of a retention coefficient. This was found to increase with temperature and exposure time and to decrease with the level of contamination with CuCl₂. This effect of temperature was attributed to the improvement of the crystallinity of the corrosion product. On the other hand, the effect of CuCl₂ was attributed to the electrodeposition of Cu and its impregnation within the corrosion product, which becomes less adherent. The free corrosion potential was found to be affected by the presence of the CuCl₂, in a fashion compatible with the Wagner‐Traud theory of mixed potential.     

The corrosion of steel and aluminium in calcium chloride/ammonia,magnesium chloride/methylamine and magnesium chloride/methylamine/decane

The corrosion of St 37, StE 36, Al 99,5 and Al-Mg 3 in the chemical pairs of substances calcium chloride/ammonia, magnesium chloride/methylamine and magnesium chloride/methylamine/decane was investigated. The corrosion tests were performed in autoclaves at room temperature to 180 °C. The nominal duration of the experiments was 1000 hours. Rod shaped fatigue specimens with polished surfaces served as test specimens; these were fatigue tested after the corrosion treatments. All materials tested were compatible with calcium chloride/ammonia under the experimental conditions employed. Steel and aluminium showed similar behaviour against magnesium chloride/methylamine and magnesiumchloride/methylamine/decane, respectively. At room temperature and 70°C to 80°C the corrosion of steel and aluminium was low (wall losses in the order of 1 m?m/a). Corrosion increased with increasing temperature. The activation energy for the reaction, which determined the rate of weight loss in magnesium chloride/methylamine/decane, was AE_st = 0, 60 ± 0,18 eV for steel, and AE_Al = 1, 07 ± 0,07 eV for aluminium. A corrosion treatment on aluminum fatigue specimens after 1000-1500 hours at 110 °C caused a notable decrease in fatigue strength; no such behaviour was noted for steel, even when corroded at 170 °C.     

Correlation of temperature with galvanic corrosion behaviour of copper alloys based on wire beam electrode

Galvanic corrosion behaviour of copper and copper alloys in chloride solution (0.6?M NaCl) under different temperature conditions (35°C, 55°C and 80°C) was investigated by the wire beam electrode (WBE) method. It was found that the average galvanic current of the Cu and Cu–Sn alloy, which shifted from the cathode at 35°C to the anode at 80°C, increased as temperature increased during the immersion time. In contrast, the average galvanic current of Cu–Zn alloy can be ranked as 35°C?>?55°C?>?80°C. A sudden conversion was that the anode average current of Cu–Zn alloy at 35, 55°C changed into the cathode when the immersion time lasted untill 3 h at 80°C. The results indicate that it is feasible to study the corrosion behaviour under the different temperature environment with the WBE method.     

Effect of O2 on down-hole corrosion during air-assisted steam injection for heavy oil recovery

ABSTRACT

Air-assisted steam injection is used to enhance hydrocarbon recovery from heavy oil reservoirs. During this process, downhole tubular goods are subject to corrosion in a mixed oxygen-carbon dioxide-steam environment at temperature up to around 200°C causing a low-temperature oxidation phenomenon. Here we investigate the influence of the O₂/CO₂ ratio on the corrosion of P110 steel, a grade that is commonly used for oil well casings. Our findings show that corrosion is greatly influenced by the O₂ pressure but hardly at all by the CO₂ pressure. The maximum corrosion rate was in excess of 65?mm per year. However, alkalisation of the injected fluid, which promoted the formation of a protective magnetite scale, was found to greatly reduce corrosion.     

Electrochemical studies of hot corrosion of type 347H stainless steel

Abstract

Electrochemical studies of the hot corrosion of AISI SA 213 TP 347H stainless steel have been carried out in a mixture of 80 wt-% V₂O₅ + 20 wt-% Na₂SO₄. The range of temperatures was 540–680°C at intervals of 20 K and the techniques employed included corrosion potential, Tafel polarisation, and electrochemical noise measurements. At 620°C the corrosion potential, measured against a platinum reference electrode (PRE), decreases from ?350 mV to ?480 mV and remains at this level during the first 8 h. Using Tafel polarisation, it was found that, with change in the temperature from 540 to 680°C, the corrosion potential decreased on initial heating to 600°C and then increased again at higher temperatures, the corrosion rate increasing continuously with increasing temperature. However, at constant temperature (620°C) the corrosion rate increased with time during the first 8 h, after which it decreased and reached a steady state after 27 h, probably owing to the formation of a surface film. Electrochemical noise measurements, of both voltage and current noise, indicated a combination of general corrosion, probably owing to the formation of a surface layer, and localised corrosion in the grain boundaries.     

Effect of methanol on the corrosion behaviour of Al-Cu Alloys in sulphuric acid

The effect of Cu additions (1, 3 and 5?at.-%) on the corrosion behaviour of Al in 0.5?M H₂SO_4?+?2?ppm hydrofluoric acid?+?methanol at 50°C, environment found in a direct methanol fuel cell, has been evaluated using electrochemical techniques. Electrochemical techniques included potentiodynamic polarisation curves, electrochemical impedance spectroscopy and electrochemical noise measurements measurements. Methanol concentrations included 1, 5, 10 and 20?M. Results have shown that corrosion resistance increases with increasing the methanol concentration. The addition of Cu to Al increases the corrosion rate of the former by the formation of micro galvanic cells, inducing localised typo of corrosion also.     

Investigation on ultra-pure ferritic stainless steel 436L susceptibility to intergranular corrosion using optimised double loop electrochemical potentiokinetic reactivation method

The susceptibility of 17Cr ferrite stainless steel to intergranular corrosion (IGC) was investigated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) test and the microstructural characterisation. The results show that the optimised DL-EPR test condition for 17Cr ferrite stainless steel is l?M H₂SO₄?+?0.006?M KSCN solution with a scan rate of 0.1?V?min^?1 at 25°C The severe IGC occurs in the temperature range of 500–650°C and the nose temperature locates at approximately 600°C. The I_r/I_a value rises up to 16.99% when heat treated at 600°C for 30?min. After aging treatment, M₂₃C₆ precipitates are detected using transmission electron microscopy, resulting in the presence of the Cr depletion zone. However, the degree of IGC for 436L is extremely limited due to the Cr element diffusion form the matrix despite prolonging the aging time to 5?h.     

Accessing the full spectrum of corrosion behaviour of tempered type 420 stainless steel

Bipolar electrochemistry produces a linear potential gradient between two feeder electrodes, providing access to the full spectrum of anodic-to-cathodic electrochemical behaviour. A type 420 martensitic stainless steel has been used to investigate microstructure evolution and corrosion behaviour with application of different tempering heat treatments. Tempering treatments at 250°C, 400°C and 700°C revealed the occurrence of pitting corrosion, with treatments at 550°C resulting in general and intergranular corrosion. Cr₂₃C₆ was present in all tempering conditions, with Cr₇C₃ and CrC only observed for tempering at 550°C. The 250°C tempering treatment had the highest corrosion resistance with a hardness value much higher than 500 HV.     

Study of molten salt corrosion of HK-40m alloy applying linear polarization resistance and conventional weight loss techniques

Corrosion rates from electro-chemical polarization resistance technique (LPR) and weight loss method (WL) of HK-40m alloy exposed to 80 mol% V₂O₅-20Na₂SO₄ at 600 and 700 °C were obtained at a maximum time of 10 days. Results were supported by X-ray diffraction and electron microscopy analysis. A comparison of corrosion rates from both techniques indicated that corrosion rates from LPR were higher than that from WL, being the values more or less in the same order of magnitude. At 600 °C corrosion rates values were twofold; whereas at 700 °C threefold. The difference in results from both techniques was mainly explained by the fact that V₂O₅ behaves as a semiconductor oxide, and even though Na₂SO₄ is totally ionic, the corrosion mechanism with this mixture may not display a purely electro-chemical process. Some qualitative characteristics were observed for both techniques.     

The corrosion mechanism of Fe-based superalloy in molten NaCl-52 wt-%MgCl2 at 520°C

NaCl-52?wt-%MgCl₂ is a good thermal storage medium at medium–high temperature. But the corrosion of chlorate on metal is serious and the mechanism is unclear. In this paper, the corrosion kinetics curves of Fe and three kinds of Fe-based superalloys were measured by the immersion salt corrosion method at 520°C. The microstructure and composition on the surface and cross-section were characterised by a scanning electron microscopy with EDS analysis and X-ray diffraction. The results show that corrosion kinetics curves obey linear law, and the average mass loss rate of Fe is the lowest of all. After corrosion for 20?h, the main composition on Fe surface was mainly magnesia (MgO) and Fe. Shell structure appeared on the surface of three alloys, the composition of shell was MgO, while matrix had Fe, Ni and its compounds. After corrosion for 160?h, the surfaces of four samples became loose, and they generated different corrosion products. The cross-sectional morphology and line scanning analysis results show that Fe corrosion is relatively mild, while Fe-based alloys presented obvious corrosion layer, and the content of Fe and Cr near corrosion layer decreased. The corrosion mechanism of Fe-based superalloys mainly involves oxidation–chlorination.     

Electrochemical study of corrosion behaviour of carbon steel A106 and stainless steel 304 in aqueous monoethanolamine

Abstract

Corrosion behaviour of carbon steel A106 and stainless steel 304 (SS304) in aqueous monoethanolamine was studied by performing electrochemical polarisation experiments. Potentiodynamic curves were studied and compared under conditions with different temperatures, carbon loading and O₂ percentage in purging gases. It was found that corrosion of A106 and SS304 was promoted under conditions with higher temperature. While the presence of O₂ speeds the corrosion of A106, it has a negligible impact on SS304 at 80°C and lowers the corrosion rate at 40°C. Corrosion rates and other important parameters were calculated based on the electrochemical curves for A106. Sample surfaces after tests were examined by scanning electron microscopy and energy dispersive spectroscopy. Mechanisms involved in iron dissolution and passivation from oxide films were discussed.     

6082铝合金搅拌摩擦焊焊缝的电化学腐蚀行为

通过室温静态挂片试验以及动电位极化曲线测试,在室温0．2mol／LNaHSO3＋0．6mol／L NaCl溶液中,对6082铝合金搅拌摩擦焊（FSW）焊缝以及6082铝合金母材的电化学腐蚀行为进行了研究。结果表明,主轴转速为1200r／min,焊接速度为200mm／min,搅拌头倾角为3°时的焊缝与母材相比,平均腐蚀速率较小,腐蚀电位Ecorr正向移动,腐蚀电流密度J变小。同时使用扫描电子显微镜（SEM）对室温静态挂片试验试样的表面形貌进行了观察,发现焊缝表面上只出现少量较浅的点蚀坑,而母材表面的点蚀现象较为严重。     

Poly(3‐octylthiophene) thermally treated as coating for corrosion protection of SS304 in sulfuric acid media

Poly(3‐octylthiophene) (P3OT) was synthesized by direct oxidation of the 3‐octylthiophene monomer using ferric chloride (FeCl₃) as an oxidant. Using the drop‐casting technique, P3OT coatings were deposited onto 304 type stainless steel electrodes. For the purpose of determining the effect of thermal annealing on the corrosion protection of stainless steel with P3OT coatings, the coated electrodes were thermally annealed for 30 h at two different temperatures, 55 and 100 °C. The corrosion behavior of P3OT coated stainless steel was investigated in 0.5 M sulfuric acid (H₂SO₄) at room temperature using potentiodynamic polarization curves (PPC), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). The results indicated that the thermally treated P3OT coatings improved the corrosion resistance of the stainless steel in 0.5 M H₂SO₄. The best corrosion protection was obtained by the P3OT coating annealed at 100 °C. In order to study the temperature effect on the morphology of the coatings before and after the corrosive environment and compare it with corrosion protection, atomic force microscopy (AFM) and scanning electronic microscopy (SEM) were used.     

提高KF-AlF3-Al2O3熔体中Cu-Al基阳极的耐腐蚀性

采用恒电流和动电位极化技术研究经预氧化的和未经预氧化的Cu-Al基阳极(Cu-10Al和Cu-9.8Al-2Mn)在KF-AlF₃-Al₂O₃熔体中的阳极行为。将合金在700℃下进行短时间(8 h)氧化,然后在800℃下施以0.4 A/cm²的电流密度进行1 h恒电流极化。测定扫描速率为0.01 V/s的动电位曲线。对在阳极表面冻结的熔体样品进行XRD分析,并在实验后对阳极进行SEM分析,以研究合金表面形成的氧化皮的物相。所有阳极材料的稳态电位都在2.30~2.50 V(vs Al/AlF₃)范围内。根据动电位极化获得的数据计算阳极的腐蚀速率。结果表明,经预氧化的阳极比未经预氧化的阳极具有更低的腐蚀速率。