Effect of inorganic inhibitors on the corrosion behavior of 1018 carbon steel in the LiBr + ethylene glycol + H2O mixture

The effect of inorganic inhibitors on the corrosion behavior of 1018 carbon steel in the mixture LiBr (55%) + ethylene glycol + H₂O at room temperature has been evaluated. Used inhibitors included LiNO₃ (Lithium Nitrate), Li₂MoO₄ (Lithium Molybdate) and Li₂CrO₄ (Lithium Chromate) at concentrations of 5, 20 and 50 ppm. Electrochemical techniques included potentiodynamic polarization curves, electrochemical noise resistance (EN) and electrochemical impedance spectroscopy (EIS) measurements. Additionally, adsorption isotherms were calculated. The results obtained showed that both, the corrosion rate and the passive current density decreased with inhibitors, and, in general terms, inhibitors efficiency increased with inhibitor concentration, except in the case of Li₂CrO_4, where the highest efficiency was obtained with 20 ppm of inhibitor. Pitting potential with 5 ppm of inhibitor, regardless its chemical composition, was more active than in absence of inhibitor, increased at 20 ppm, especially with Li₂CrO₄, and remained unaltered with 50 ppm. EN measurements showed that at 5 ppm of inhibitor, the number of film rupture/repassivation events was higher than that obtained at 20 or 50 ppm. Adsorption isotherms suggested a different adsorption mechanism for each inhibitor, whereas EIS results suggested that the corrosion process when nitrates were added was under charge transfer control, but in the case of molybdates or chromates was under diffusion control.     

Corrosion behavior of CuCrFeNiMn high entropy alloy system in 1 M sulfuric acid solution

Immersion tests and potentiodynamic polarization measurements were conducted in 1 M sulfuric acid solution (H₂SO₄) at ambient temperature (～25 °C) to investigate the corrosion behavior of CuCrFeNiMn alloy system. The results show that the alloys display a good general corrosion resistance that is mainly influenced by the Cu content and elemental segregation degree. The corrosion resistance degrades when increasing Cu content and elemental segregation degree. Among the tested alloys, the CuCr₂Fe₂Ni₂Mn₂ alloy with low Cu content and elemental segregation degree displays a better general corrosion resistance. On the contrary, the Cu₂CrFe₂NiMn₂ alloy with high Cu content and elemental segregation degree exhibits the worst general corrosion resistance.     

Corrosion of carbon steel in concentrated LiNO3 solution at high temperature

The corrosion of carbon steel in concentrated LiNO₃ solution at high temperature was investigated by a weight loss method. Results showed that increasing temperature and pH would increase the corrosion rate, and increasing concentration and adding Li₂CrO₄ would reduce the corrosion rate. The corrosion in LiNO₃ solution was general corrosion and the corrosion products were composed of Fe₃O₄ and Fe₂O₃. A compact passive layer comprising of Cr₂O₃, Fe₃O₄ and Fe₂O₃ was observed with adding Li₂CrO₄, and it could effectively depress the corrosion. The carbon steel corrosion in LiNO₃ solution was much smaller than that in LiBr solution.     

Corrosion behavior of iron-based alloys in the LiBr + ethylene glycol + H2O mixture

The corrosion resistance of 1018 carbon steel, 304 and 316 type stainless steels in the LiBr (55 wt.%) + ethylene glycol + H₂O mixture at 25, 50 and 80 °C has been studied using electrochemical techniques which included potentiodynamic polarization curves, electrochemical noise and electrochemical impedance spectroscopy techniques. Results showed that, at all tested temperature, the three steels exhibited an active-passive behavior. Carbon steel showed the highest corrosion rate, since both the passive and corrosion current density values were between two and four orders of magnitude higher than those found for both stainless steels. Similarly, the most active pitting potential values was for 1018 carbon steel. For 1018 carbon steel, the corrosion process was under a mixed diffusion and charge transfer at 25 °C, whereas at 50 and 80 °C a pure diffusion controlled process could be observed. For 316 type stainless steel, at 25 and 50 °C a species adsorption controlled process was observed, whereas at 80 °C a diffusion controlled mechanism was present. Additionally, at 25 °C, the three steels were more susceptible to uniform type of corrosion, whereas at 50 and 80 °C they were very susceptible to localized type of corrosion.     

Hot Corrosion Behavior of Inconel 625 Superalloy in Eutectic Molten Nitrate Salts

Corrosion resistance of Inconel 625 Ni-based superalloy was studied in a molten nitrate salt consisting of 40 KNO₃–60 NaNO₃ (wt%) at 500 and 600 °C. Open-circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy and gravimetric tests were used to evaluate the degradation mechanism and corrosion behavior of the alloy. Surface morphology and chemical analysis of corrosion products were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry. The weight-loss curves showed that with the increase in temperature, the oxidation rate and mass gain increased; the relationship between the mass gain and time was close to the parabolic oxidation law. The electrochemical corrosion results confirmed that during the exposure of Inconel 625 alloy to the molten salts, nickel dissolves as a result of non-protective NiO layer formed. The formation of a non-protective oxide layer with low barrier property was responsible for observing the weak corrosion resistance of the alloy at high temperatures (500 and 600 °C). Cyclic polarization tests showed a positive hysteresis confirming the nucleation and growth of stable pits on the surface of Inconel 625 at high anodic overpotentials. Sodium nitrite acts as an efficient pitting inhibitor for this case. In this way, the sodium nitrite with the concentration of 0.1 molal was found to have an optimum inhibition effect on pit nucleation at 600 °C.     

Influence of cold work and sigma phase on the pitting corrosion behavior of 25 chromium super duplex stainless steel in 3.5% sodium chloride solution.

The effect of cold work (up to 16% strain) and sigma phase precipitation (at 850 °C for 10 and 60 min) on the pitting resistance of 25 chromium super duplex stainless steel were investigated in 3.5% sodium chloride solution at 70 and 90 °C. Anodic polarization scans for cold worked samples revealed immunity to pitting attack at 70 °C even with 16% strain. At 90 °C, the alloy still showed high pitting resistance, pitting occurring at about 600 mV (SCE) for the 16% plastic strain samples. A serious deterioration of the pitting corrosion resistance was found after heating the alloy at 850 °C for 10 min resulting in a clear drop in the pitting potential at 90 °C. After heating for 60 min, the material showed rapid deterioration of pitting corrosion resistance at 70 °C.     

Die Lochkorrosionsbegrenzung bei hochlegierten Sonderedelstählen und NiCrMo‐Legierungen in Chloridlösung – Einflußfaktoren und Ausmaß

Limit of pitting corrosion at high‐alloyed special steels and NiCrMo alloys in chloride solution The phenomenon of the limit of pitting corrosion in direction to positive potentials is studied by potentiokinetic polarization after a jump in the transpassive range and by potentiostatic tests at technical wrought materials and at model alloys of the systems NiCrMo and NiMo in CaCl₂ solution in the concentration range 1 to 9 mol/l chloride at pH‐values of 1 to 9 at temperatures of 30 to 110°C. Surface‐analytical investigations gives in connection with knowledges from anodic polarization studies directions to the mechanism of the limit of pitting corrosion. Ranges of the limit of pitting corrosion are obtained at materials with a Mo content above 6.5% and contents of chloride of the media above 2 mol/l chloride. Increasing temperatures, increasing contents of chloride and sulfate shift the potential of the limit of pitting corrosion being always above 0.2 V (SCE) at potentiostatic determination to noble direction. There are indications that the mechanisms of limit of pitting corrosion is resulting from an inactivation of pitting nuclei by the formation of hardly soluble molybdenum chlorides in the potential range of limit of pitting corrosion.     

Pitting and stress corrosion cracking resistance of friction stir welded AA 5083

The corrosion behaviour of friction stir welded (FSW) joints of AA 5083 has been compared to that of MIG welded joints. Pitting and stress corrosion cracking (SCC) resistance in 3.5% NaCl + 0.3 g/l H₂O₂ and in EXCO (4 M KCl + 0.5 M KNO₃ + 0.1 M HNO₃) solutions has been determined at 25°C. SCC susceptibility was evaluated by slow strain rate tests (SSRT), at a strain rate of 1 × 10⁻⁶ s⁻¹.Welds obtained by FSW technique showed a higher corrosion resistance in EXCO solution and a lower pitting tendency than the base alloy. Electrochemical measurements (corrosion potential measurements, polarization curves recording) evidenced that FSW weld was cathodic to base alloy. FSW joints were not susceptible to SCC in both test solutions, whereas MIG joints cracked in both solutions.     

Optical spectroscopic and electrochemical characterization of oxide films on a ferritic stainless steel

Colored oxide films that form on ferritic stainless steel in a high-temperature, oxidizing environment and correspond to different chemical compositions can cause a deterioration of pitting resistance and corrosion performance. Herein, optical spectroscopic and electrochemical techniques have been used to reveal the relationship between color, chemical composition, and corrosion resistance of oxide films formed in the temperature range from 400°C to 800°C for 30 min and at 800°C for 10, 20, 30, and 60 min. The substrate with a thin and dense passivation film leads to a low pitting potential but high corrosion resistance. Oxide films of yellowish or brownish color formed below 600°C are mainly iron oxides, which correspond to low corrosion resistance. No passivation characteristics can be observed for polarization curves of oxide films formed at 500°C and 600°C. The color of oxide films varies from blue to dark gray with the increase of oxidation time at 800°C. Corrosion resistance changes with different proportions of Fe₃O₄, Cr₂O₃, and FeCr₂O₄. The gray oxide films formed at 800°C for 30 min exhibit the lowest pitting susceptibility and the highest corrosion resistance.     

Corrosion of aluminium,stainless steels and AISI 680 nickel alloy in nitrogen‐based fuels

Nitrogen‐based compounds can potentially be used as alternative non‐carbon or low‐carbon fuels. Nevertheless, the corrosion of construction materials at high temperatures and pressures in the presence of such fuel has not been reported yet. This work is focused on the corrosion of AISI Al 6061, 1005 carbon steel (CS), 304, 316L, 310 austenitic stainless steels (SS) and 680 nickel alloy in highly concentrated water solution of ammonium nitrate and urea (ANU). The corrosion at 50 °C and ambient pressure and at 350 °C and 20 bar was investigated to simulate storage and working conditions. Sodium chloride was added to the fuel (0–5 wt%) to simulate industrial fertilizers and accelerated corrosion environment. Heavy corrosion of CS was observed in ANU solution at 50 °C, while Al 6061, 304 and 316L SS showed high resistance both to uniform and pitting corrosion in ANU containing 1% of sodium chloride. Addition of 5% sodium chloride caused pitting of Al 6061 but had no influence on the corrosion of SS. Tests in ANU at 350 °C and 20 bar showed pitting on SS 304 and 316L and 680 nickel alloy. The highest corrosion resistance was found for SS 310 due to formation of stable oxide film on its surface.     

Corrosion fatigue crack growth behavior of oil-grade nickel-base alloy 718. Part 1: Effect of corrosive environment

The effect of corrosive environment on corrosion fatigue crack growth (CFCG) behavior of oil-grade nickel-base alloy 718 is studied. The results demonstrate that there is no obvious effect of 3.5 wt.% NaCl solution at RT, 50 °C and 80 °C on CGCG rates while 21 wt.% NaCl solution at 80 °C produces a deleterious effect on CFCG rates compared to the ones tested in air. Potentiodynamic polarization results show that alloy 718 exhibits passive behavior in 3.5 wt.% NaCl solution, while pitting corrosion resistance decreases with increasing solution temperature. Nevertheless, alloy 718 shows active corrosion behavior in 21 wt.% NaCl solution at 80 °C.     

The effect of chromate in the corrosion behavior of duplex stainless steel in LiBr solutions

The electrochemical behavior of duplex stainless steel (DSS) in LiBr media was investigated by anodic cyclic polarization curves and AC impedance measurements. The effect of bromide concentration and chromate presence in the solutions on the corrosion behavior of DSS was studied. It was found, by cyclic polarization curves analyses, that there was different pitting susceptibility of passive films formed on DSS depending on the chromate/bromide ratio: pitting corrosion susceptibility highly decreased from a chromate/bromide ratio lower than 0.01.The comparative investigations carried out in LiBr and LiBr + 0.032Li₂CrO₄ verify the assumption that the halide ions facilitate inhibitor adsorption. The addition of halides increased inhibition efficiency to a considerable extent. Passive film becomes more resistant when bromide concentration increases, although film thickness decreases.     

Effect of α-Al/Al3Ni microstructure on the corrosion behaviour of Al-5.4 wt% Ni alloy fabricated by equal-channel angular pressing

The effect of microstructure on corrosion behaviour of an Al-5.4 wt% Ni alloy fabricated by equal-channel angular pressing (ECAP) was investigated by means of potentiodynamic polarization test. The Al-5.4 wt% Ni alloy samples were severely deformed by ECAP with two strain introduction methods of route A and route B_C and the ECAP process was repetitively carried out up to 6 passes (strain 6). The anodic polarization measurements indicated that pitting potential of the ECAPed Al-Ni alloy samples in chloride containing neutral buffer solution increased with ECAP passes. The pitting corrosion resistance of Al-Ni alloy after ECAP by route B_C was better than that by route A. It is attributable to that the size of α-Al crystal region was reduced with ECAP passes and route B_C was able to obtain more homogeneous α-Al/Al₃Ni structure than route A. On the other hand, pitting corrosion resistance of pure Al samples showed an obvious declining with increasing ECAP passes. It was indicated that more homogeneous and finer α-Al/Al₃Ni structure obtained by ECAP played a vital role on improving the corrosion resistance of Al-5.4 wt% Ni alloy.     

Improvement on the pitting corrosion resistance of 304 stainless steel via duplex passivation treatment

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V_SCE to positive than 0.9 V_SCE at 70°C in a solution bearing 0.6 M [Cl^?] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl^?] concentration. Impedance analysis and X‐ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film.     

Electrochemical corrosion behavior of biomedical Ti–22Nb and Ti–22Nb–6Zr alloys in saline medium

The aims of this study were to investigate the effects of Zr addition and potentiodynamic polarization on the microstructure and corrosion resistance of Ti–22Nb and Ti–22Nb–6Zr alloy samples. The corrosion tests were carried out in 0.9% NaCl at 37 °C and neutral pH value, utilizing the OCP, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. The results of XRD and optical microscopy indicated that the addition of Zr stabilized the β phase, which plays a crucial role in the corrosion resistance improvement of the Ti–22Nb–6Zr alloy. From the polarization curves, it can be seen that the alloys exhibited a wide passive region without the breakdown of the passive films and also low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with the addition of 6 at% Zr into the Ti–22Nb alloy. The EIS results of these two alloy samples after 1‐h immersion in 0.9% NaCl solution, and being fitted by R_S(Q_PR_P) model, suggested that the corrosion resistance of the passive films improved with the addition of Zr and only a single passive film formed on the surfaces. However, two time constants were observed for the Ti–22Nb and Ti–22Nb–6Zr alloy samples after potentiodynamic polarization, the spectra of which can be fitted using the R_s(Q_o(R_o(Q_bR_b))) model. In addition, the corrosion resistance of the two alloy samples was reinforced significantly because of polarization when compared to the immersed samples. All these observations suggested a nobler electrochemical behavior of the titanium alloys with the addition of Zr element and after polarization.     

Electrochemical behavior of Ni3Al-based intermetallic alloys in NaOH

The corrosion behavior of Ni₃Al-based intermetallic alloys in a 0.5 M NaOH solution was studied at 25 °C. The open circuit potential, cathodic and anodic potentiodynamic polarization, Tafel plots and linear polarization resistance measurements were used to characterize the corrosion behavior. For the Ni₃Al(B, Zr) alloy, potentiodynamic polarization curves showed a wide passive region that can be found between about ?0.220 V_SCE and 0.520 V_SCE. On the other hand, a narrow passive region, in the range of potentials from about ?0.180 V_SCE to 0.180 V_SCE, was observed for the Ni₃Al(B, Zr, Cr, Mo) alloy. Chromium, as an alloying element in the Ni₃Al(B, Zr, Cr, Mo) alloy, contributes to transpassive dissolution of the passive film at much lower anodic potentials and remarkably reduces the passivation region. The experiments indicated also that damaged passive films on alloys repairs itself and pits do not initiate. The surface of both alloys and passive films possess extremely high corrosion resistance in a studied solution. However, Tafel and linear polarization tests revealed that freshly exposed surfaces of the Ni₃Al(B, Zr) alloy exhibited better corrosion resistances than the Ni₃Al(B, Zr, Cr, Mo) alloy. Both methods, used for the determination of corrosion rates gave very similar results. The calculated corrosion rates are about 2.8 ·10^?3 and 6.0·10^?3 mm year^?1 for the Ni₃Al(B, Zr) alloy and B, respectively.     

Hot Corrosion Behavior of a Ni3Al-Based IC21 Alloy in a Molten Salt Environment

Ni₃Al-based alloys have become important candidates for hot components in turbine engines, owing to their low densities and outstanding mechanical properties in service environments. The hot corrosion behavior of a Ni₃Al-based IC21 alloy in a molten salt environment of 75 wt% Na₂SO₄ and 25 wt% NaCl at 900 °C was studied, via oxidation kinetics analyses, scanning electron microscope observations and energy dispersive as well as diffraction analyses by X-ray. A multilayer corrosion oxide scale and dendritic morphology internal corrosion zone formed after hot corrosion, and inter-phase selective corrosion phenomena were also observed. Salt fluxing and oxidation-sulfidation processes were inferred to be the essential hot corrosion mechanisms of the alloy. Moreover, additions of Cr and Y proved to be beneficial to the hot corrosion resistance of the IC21 alloy, while the Mo content should be strictly controlled.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Ni-Fe-W-B非晶合金的晶化行为及其电化学腐蚀特性研究

采用单辊急冷法制备了57.5Ni-24.5Fe-14.5W-3.5B(质量分数,%)非晶薄带,并在不同温度下进行退火。用DSC和XRD分析了非晶薄带的晶化行为及析出相的演变过程;用电化学极化曲线及电化学阻抗法研究了试样在3.5%NaCl溶液中的电化学腐蚀行为;用SEM和EDS分析了试样腐蚀后的表面显微形貌及成分。结果表明:该非晶薄带的晶化过程分为3步,其晶化温度约为430,470和700℃;退火试样的耐腐蚀性整体优于非晶合金样,部分晶化试样的抗电化学腐蚀性能优于完全晶化试样;500℃退火试样表面形成致密钝化膜,抗腐蚀性能优异,而非晶薄带和720℃退火试样形成的钝化膜不稳定,易被点蚀和局部腐蚀。     

Scaling behaviour of a Cu 27.6% Sn alloy at 550–725°C under 1 atm oxygen

The corrosion in oxygen of a bronze containing 27.6 wt% tin has been studied in the range 550–725°C. The oxidation rate of this alloy is considerably smaller than that of pure copper and compares favourably with that of the more dilute 13 wt% tin alloy. In fact at 650°C and below the reaction stops after a relatively short time of quasi-parabolic kinetic (about 5 hr). At higher temperatures instead the initial parabolic stage is followed by a second stage of linear kinetics. The results are interpreted by assuming the formation of a continuous protective layer of SnO₂ at the base of the scale at 650°C and below and of a dispersion of SnO₂ particles in copper oxide at higher temperatures. The importance of the practical absence of Cu₂O from these scales in relationship of the approximate values of the parabolic values of the parabolic rate constant for this alloy as compared to those on pure copper is also examined.