A comparative study of the corrosion stability of dental amalgams with electrochemical impedance measurements

The corrosion susceptibility of a selection of amalgams used in dentistry has been examined with the electrochemical impedance method. The results are compared with data derived from cyclic voltammetry performed with these materials before. Most examined materials including a conventional amalgam show similar corrosion resistance; however, only one product shows a significantly higher corrosion resistance.     

Corrosion performance of magnesium alloys MEZ and AZ91

The corrosion performance of sand cast MEZ_S, zirconium-grain-refined MEZ_R, sand cast AZ91_S, and high pressure diecast AZ91_D magnesium alloys were evaluated by means of salt spray testing, optical metallography, hydrogen evolution, polarisation curve measurement and AC impedance spectroscopy. The results show that the corrosion resistance of the four alloys can be ranked in decreasing order as AZ91_D > AZ91_S ≈ MEZ_R > MEZ_S and that the intergranular phases and chemical composition of the matrix phase have a significant influence on the corrosion performance. Alloys with a finer grain size and higher aluminum or zirconium contents exhibit better corrosion resistance.     

湿法磷酸中不锈钢腐蚀的电化学分析

试验以湿法磷酸为体系，以316L和904L不锈钢为研究对象，采用动电位扫描测试了不同杂质组成和温度对两种不锈钢的影响，并对两种材料不同电位下的Nyquist图谱进行了测试。试验从电化学特征参数致钝电流密度ic、维钝电流密度ip、膜的破裂电位Eb和自腐蚀电位Ecorr以及交流阻抗阻力对两种材料的耐蚀性差异进行了分析。     

Corrosion resistant Ti alloy for sulphuric acid medium: Suitability of Ti–Mo alloys

The corrosion resistance of Ti–Mo (5, 10, 15 and 25 wt% molybdenum) alloys in 5–25% sulphuric acid was evaluated. The Ti–Mo alloys offered a better corrosion resistance than commercially pure titanium (CP‐Ti). The higher impedance values, higher phase angle maximum, ability to reach the phase angle maximum at relatively lower frequencies, ability to exhibit a constant phase angle maximum over a wider range of frequencies, higher phase angle values at 0.01 Hz, have confirmed the formation of a stable passive oxide film on Ti–Mo alloys. The study recommends the use of Ti–Mo alloys, particularly Ti–25Mo alloy, as a suitable material of construction for sulphuric acid medium.     

Corrosion behavior of Ti‐xNb‐13Zr alloys in Ringer's solution

Ti‐6Al‐4V alloy has been widely used in restorative surgery due to its high corrosion resistance and biocompatibility. Nevertheless, some studies showed that V and Al release in the organism might induce cytotoxic effects and neurological disorders, which led to the development of V‐free alloys and both V‐ and Al‐free alloys containing Nb, Zr, Ta, or Mo. Among these alloys, Ti‐13Nb‐13Zr alloy is promising due to its better biomechanical compatibility than Ti‐6Al‐4V. In this work, the corrosion behavior of Ti, Ti‐6Al‐4V, and Ti‐xNb‐13Zr alloys (x = 5, 13, and 20) was evaluated in Ringer's solution (pH 7.5) at 37 °C through open‐circuit potential measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. Spontaneous passivity was observed for all materials in this medium. Low corrosion current densities (in the order of 10^?7 A/cm²) and high impedance values (in the order of 10⁵ Ωcm² at low frequencies) indicated their high corrosion resistance. EIS results showed that the passivating films were constituted of an outer porous layer (very low resistance) and an inner compact layer (high resistance), the latter providing the corrosion resistance of the materials. There was evidence that the Ti‐xNb‐13Zr alloys were more corrosion resistant than both Ti and Ti‐6Al‐4V in Ringer's solution.     

Corrosion performance of high pressure die-cast Al-6Si-3Ni and Al-6Si-3Ni-2Cu alloys in aqueous NaCl solution

The corrosion performance of high pressure die-cast Al-6Si-3Ni (SN63) and Al-6Si-3Ni-2Cu (SNC632) alloys in 3.5% (mass fraction) NaCl solution was investigated. X-ray diffraction (XRD) and microstructural studies revealed the presence of single phase Si and binary Al₃Ni/Al₃Ni₂ phases along the grain boundary. Besides, the single Cu phase was also identified at the grain boundaries of the SNC632 alloy. Electrochemical corrosion results revealed that, the SNC632 alloy exhibited nobler shift in corrosion potential (?_corr), lower corrosion current density (J_corr) and higher corrosion resistance compared to the SN63 alloy. Equivalent circuit curve fitting analysis of electrochemical impedance spectroscopy (EIS) results revealed the existence of two interfaces between the electrolyte and substrate. The surface layer and charge transfer resistance (R_ct) of the SNC632 alloy was higher than that of the SN63 alloy. Immersion corrosion test results also confirmed the lower corrosion rate of the SNC632 alloy and substantiated the electrochemical corrosion results. Cu addition improved the corrosion resistance, which was mainly attributed to the absence of secondary Cu containing intermetallic phases in the SNC632 alloy and Cu presented as single phase.     

Corrosion of API XL 52 steel in presence of Clostridium celerecrescens

During the characterization of sediments formed in pipelines transporting hydrocarbons, the knowledge of the microbiological diversity becomes very interesting, especially when it is related to microbiologically influenced corrosion (MIC). The presence of microorganisms is considered as one of the factors that affect the corrosion processes occurring at the pipeline; therefore, their corrosiveness must be determined. In this way, the identification of new species affecting the MIC processes is still considered relevant. In this work, the effect of Clostridium celerecrescens upon the corrosion of API KL 52 steel was evaluated. This microorganism was isolated and identified from the sediments collected during the inner cleaning procedures of a gas pipeline. The polarization resistance (PR) and electrochemical impedance spectroscopy (EIS) techniques were considered to estimate the microorganism behavior during the corrosion process. The results were complemented with a metal surface analysis, using a scanning electron microscope (SEM). The resistance values induced by the presence of the microorganisms clearly indicated that C. celerecrescens has an effect on the corrosion process occurring at the API XL 52 steel surface.     

Corrosion behavior of Mg-Zn-Ca amorphous alloys with Nd addition in simulated body fluids

The effects of Nd addition on corrosion behavior of Mg66Zn30Ca4 amorphous alloys in simulated body fluids （SBF） were studied in this paper. Electrochemical properties of the samples before and after corrosion were determined. Surface morphologies of samples after immersion in SBF at 37 ℃ for different times were observed under scanning electron microscope （SEM）. Results show that the corrosion resistance of Mg-based alloys in SBF is improved with the addition of Nd element. The electrochemical properties indicate that microalloying Nd element to the alloys leads to an ennoblement in the open circuit potentials of the alloys and a decrease in the anodic current density in SBF, especially for the Mgee66-xZn30Ca4Ndx alloys with Nd content of 1.0at.%-1.5at.%. It was observed that the surface morphologies of the alloys immersed in SBF change with the Nd addition. A flake- like structure parallel to the alloy substrate formed on the surface of 1.0at.% Nd-containing alloy immersed in SBF for 7 days improves the corrosion resistance of the amorphous alloys by blocking the corrosion liquid from attacking the alloys.     

Corrosion monitoring of hydrogen by HUP probe in the presence of sulphide and thiourea

The effect of sulphide and thiourea on hydrogen uptake during the corrosion of steel in deaerated pH3 has been studied using an electrochemical hydrogen probe incorporating a solid electrolyte (HUP) and lead dioxide (PbO₂) as a reference electrode. This probe has been used for the quantitative determination of the hydrogen activity generated during the corrosion of steel. The sensor voltage implies an average equivalent hydrogen pressure in uncharged steel of 2 × 10^?18 atmospheres. During the corrosion of steel, this value increases by very large amounts; thus in deaerated pH3, the equivalent pressure rose by 10^24.5 in the presence of 200 ppm sulphide and by 10² in the presence of 100 ppm thiourea. Weight loss and AC impedance methods were also performed under the same conditions as the hydrogen measurements. The hydrogen probe data were correlated with a.c impedance and weight loss results in order to understand the effect of S^2? and thiourea on hydrogen uptake.     

Al-Mg-Si合金中Mg_2Si和Si粒子在晶间腐蚀过程中的作用机理

研究Al-Mg-Si合金晶界组成相（Al-Mg2Si及Al-Mg2Si-Si）间的电化学行为和动态电化学耦合行为,提出Al-Mg-Si合金的晶间腐蚀机理。研究表明,晶界Si的电位比其边缘Al基体的正,在整个腐蚀过程中作为阴极导致其边缘Al基体的阳极溶解;晶界Mg2Si的电位比其边缘Al基体的负,在腐蚀初期作为阳极发生阳极溶解,然而由于Mg2Si中活性较高的元素Mg的优先溶解,不活泼元素Si的富集,致使Mg2Si电位正移,甚至与其边缘Al基体发生极性转换,导致其边缘Al基体的阳极溶解。当n（Mg）/n（Si）〈1.73时,随着腐蚀的进行,合金晶界同时会有Mg2Si析出相和Si粒子,腐蚀首先萌生于Mg2Si相和Si边缘的无沉淀带,而后,Si粒子一方面导致其边缘无沉淀带严重的阳极溶解,另一方面加速Mg2Si和晶界无沉淀带的极性转换,从而促使腐蚀沿晶界Si粒子及Mg2Si粒子边缘向无沉淀带发展。     

Corrosion behavior of two heat treatment Al-Zn-Mg-Cu alloys in different intergranular corrosion solution

The electrochemical behavior of two kinds of artificial aged Al-Zn-Mg-Cu alloys in two intergranular cor-rosion (IGC) solutions were studied using electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) at steady-state. EDAX result indicates that different artificial ageing methods change the composition and content of Cu and Zn in different zones. Zn/Cu depleted precipitation-free zone that plays a very important role in IGC is formed by heating the solubilized Al alloy for 135℃ at 16 h. All impedance spectra of the two alloys in two IGC solutions can be divided into three types. The two different states A1 alloys takes on one time constant and two capacitive arcs at high-mediate frequency and low frequency in the NaCl (NH4 )2SO4 solution respectively; but in the NaCl HCl solution, impedance displays one capacitive arc at the high-mediate frequency and an inductive loop at low frequency. OCP results show that more micro-galvanic cells in the NaCl (NH4)2SO4 solution than that in the NaCl HCl solution results in more potential fluctuation amplitude, and long-term drift of OCP is due to the long-term variation of the cathodic and anodic corrosion processes.     

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.     

Corrosion properties of high silicon iron-based alloys in nitric acid

The effect of copper and rare-earth elements on corrosion behavior of ~iigh silicon iron-based alloys in nitric acid was studied by means of static and loading current corrosion experiments. The anodic polarization curve was also made to discuss the corrosion mechanism. The examination on alloy microstructure and SEM corrosion pattern showed that when silicon content reached 14.5%, the Fe3Si phase appeared and the primary structure of the iron-base alloy was ferrite. When adding 4.57% copper in the iron alloy, its corrosion resistance in static diluted sulfuric acid was improved while its corrosion resistance and electrochemical corrosion properties in the nitric acid were decreased. In contrast, the addition of rare earth elements could improve the corrosion properties in all above conditions including in static diluted sulfuric acid and in nitric acid.     

Electrochemical behavior of some copper alloys in sulfuric acid solutions

In this article, the corrosion behavior of Cu? Fe and Cu? Al? Fe was studied in H₂SO₄ solutions in the absence and presence of different concentrations of some inorganic additives, Na₂MoO₄, K₂Cr₂O₇, KIO₃, and Na₂B₄O₇. The techniques of measurements used were open‐circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy. The polarization measurements indicated that the corrosion current, I_corr, of Cu? Fe is less than that of Cu? Al? Fe. The investigated additives in H₂SO₄ shifted E_corr to more positive values and increases I_corr. Impedance measurements indicated that Nyquist plots for Cu? Fe may be regarded as a semi‐circle in early stages of immersion and tends to be a part of semi‐circle with elapse of time and a well‐defined inductive loop appears. In case of Cu? Al? Fe, the Nyquist plots tends to be semi‐circle or depressed semi‐circle. The values of R_p for Cu? Fe are higher than those for Cu? Al? Fe. On the other hand, the values of C_dl for Cu? Fe are lower than those obtained for Cu? Al? Fe. The presence of these additives in H₂SO₄ solutions decreases the values of polarization resistance (R_p) and increases the values of corrosion current density (I_corr) and capacitance of double layer (C_dl).     

On the electrochemical nature of hot-corrosion attack in Ni-Cr alloys

Hot-corrosion attack on gas-turbine components has been a problem of practical importance for over two decades. The nature and the extent of corrosion are influenced by the type of contaminants, the alloy constituents, and temperature. The hot-corrosion process is quite complex. There is controversy regarding the role of alloying elements, and whether the process is chemical or electrochemical in nature. In this investigation, an attempt has been made to investigate the role of Al and Ti in influencing the hot-corrosion attack of Ni-20 wt. % Cr alloys in molten Na₂SO₄-NaCl mixtures. It was found that Ni-Cr-Al alloys undergo a localized, heavy-blister attack under certain experimental conditions which is electrochemical in nature. An electrochemical model has been proposed which can satisfatorily explain the nature and kinetics of the localized blister corrosion.     

评价涂膜金属耐蚀性的电化学方法

本文用阶跃电流法、电偶电流法测试涂膜金属的耐蚀性能,提出用膜电阻和电偶电流作为表征涂膜金属的耐蚀性的参数。结合交流阻抗测量、极化曲线等方法,验证其可靠性。结果表明,用阶跃电流法、电偶电流法评价涂膜金属的耐蚀性,所用仪器简单,测试简便,时间短,结果重现性好。这些方法可用于涂膜金属耐蚀性的评价、筛选涂料及防锈油。