Solid State Phenomena Vol. 227

Abstract:

Magnesium alloys are widely used mainly in automotive and aerospace industries. There is quite a lot of information about corrosion of the magnesium alloys in available literature. However, the publications concern mainly Mg-Al alloys, while there is a lack of information about Mg-RE-Zr alloys. The following paper presents results of the investigations on the electrochemical corrosion of magnesium casting alloys containing rare earth elements (WE43, WE54, EV31A-Elektron 21) as well as pure magnesium. The alloys were investigated by immersion test in 3.5% NaCl for times up to 7 days. Electrochemical investigations were carried out at ambient temperature in aerated NaCl solution, using potentiodynamic polarization method. It has been shown that the best corrosion resistance is exhibited by alloys with yttrium addition (WE43, WE54), while the weakest by pure magnesium. EV31A alloy exhibits the highest corrosion rate during the immersion test, while WE54 and WE43 alloys had a similar corrosion behavior.

Abstract: Corrosion behavior of high-pressure die cast creep resistant magnesium alloy MRI 153M in 3% NaCl aqueous solution was studied by several electrochemical and non-electrochemical techniques. The electrochemical techniques were Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR) and Tafel-slope Polarization. The non-electrochemical techniques were mass-loss and gas evolution measurements. Values of corrosion rates were calculated and the morphology of corroded surface studied. While corrosion rates calculated by both non-electrochemical methods are not consistent, those gained by the three electrochemical methods demonstrate consistency. In general, the rate of corrosion calculated by the gas evolution method is in a good agreement with the corrosion rate calculated from the electrochemical methods, which should be an indication of mixed chemical-electrochemical character of the process. SEM and light microscope observation of corroded specimens demonstrated the localized character of corrosion, at least at the initial stages.

Abstract: This work includes the comparison between corrosion resistance of MagLi4, MgLi7.5 and MgLi15 alloys in sodium chloride (0.15M NaCl) solution at 37°C. Results showed that the corrosion resistance is strongly determined by lithium content in the alloy. The worst corrosion resistance is typical for MgLi7.5 where the dual phase structure is observed. The magnesium - lithium alloys which contain less than 5% of Li and more than 11% (one phase structures) has got better corrosion resistance than dual phase structure magnesium – lithium alloys

Abstract: Magnesium alloys have low density densities and high specific strengths that are comparable to steels and titanium alloys. Therefore, they are widely used as structural materials in the automotive and aerospace industries. However, the use of magnesium alloys is hindered by the fact that they offer insufficient resistance against corrosion, even in diluted electrolyte solutions. We examined alloys from the Mg-Y-RE-Zr and Mg-Al-RE systems (WE43 and AE44) that are used in the domestic and international automotive and aerospace industries. In these applications, the alloys are exposed to corrosion in environments containing electrolytes. It is commonly known that hydrogen is the main corrosive factor, appearing during chemical reactions between magnesium and water in an electrolyte solution. Selecting rare earth-containing magnesium alloys allows us to analyse the various effects of hydrogen on these materials. Hydrogen interacts with the selected alloys in a manner that depends strongly on alloy structure and chemical composition—these factors cause variations in the concentration, solubility, and diffusion rate of hydrogen in the host material. After hydrogen uptake, the cracking velocity of each alloy phase is different and is related to cracking micromechanisms. Our results show that when samples were immersed in 0.1M sodium sulfate solution, hydrogen atoms diffused into the material and enriched the intermetallic phases. With increased immersion time, magnesium hydride fractures in a brittle manner when the inner stress caused by hydrogen pressure and the expansion stress due to the formation of magnesium hydride are higher than the fracture strength.

Abstract: AISI 304 and 316L flat samples were exposed during 3 months in Carribean sea water.At the end of the experiment E_corr of AISI 316L reached one-half the negative potential value than that of AISI 304. The tendencies of E_corr were correlated with the pitting corrosion development. They could be considered as a consequence of the opposition between the nucleation of new pits and repassivation of active sites. The analysis of E_corr fluctuations was carried out with the potential spectral density (PSD) vs. frequency in logarithmic scale. The decrease of the PSD slopes, exponent b values, indicated a release of spontaneous energy with the advance of pits formation on the stainless steel surface. The data showed that in the range of low frequencies the process that occurs on both stainless steels surfaces could be considered as fractional Brownian motion, an anti-persistent non-stationary process, while in the range of higher frequencies as fractional Gaussian noise, an persistent stationary.

Abstract: Adsorption kinetics of benzothiazole (BNS) and mercaptobenzothiazole (2-MBT) (1×10^-1M, 1×10^-2M and 1×10^-3M) were studied during 5000 s on gold and silver coated At-cut quartz microelectrodes, using EQCM. The frequency change fits well to the Langmuir isotherm in the case of 2-MBT, while BNS presents adsorption-desorption periods making the fitting non realiable. The calculated ΔG_ads of 2-MBT corresponds to physisorption Ag (-1.38 kcal mol^-1) and Au (-1.94 kcal mol^-1). On both metal surfaces (Au and Ag) the amount of adsorbed mass for 2-MBT is significantly larger (20.76 μg cm^-2 and 6.03 μg cm^-2) than on BNS (0.33 μg cm^-2 and 0.88 μg cm^-2). These facts could be explained by the deprotonation of 2-MBT giving place to an anion, which is prone to interact with both positively charged surfaces of Au and Ag. However, BNS protonates giving place to a cation and making more difficult its adsorption on both metals.

Abstract: The paper presents the results on accelerated corrosion/erosion tests, of Thickness Insensitive Spectrally Selective paints, based on alumina matrix infiltrated with inorganic oxide pigments used in the solar-thermal energy conversion. The mechanism of the corrosion/erosion process is presented and discussed based on the polarization Tafel curves obtained in NaCl 3.5% solution. Optical properties (solar absorptance and thermal emittance) were measured before and after the corrosion tests and the results were correlated with the sample’s morphologies and the corrosion parameters. The influence of the antireflection layer on the corrosion resistance and spectral selectivity is also discussed.

Abstract: The effectiveness of corrosion protection depends on several complex processes connected with the development of the construction, such as:

Abstract: A significant problem associated with the use of high-strength aluminum-zinc alloys is their insufficient resistance to corrosion. A often used method of improving the resistance to corrosion is plating, resulting in a decrease of their strength. Improving the corrosion resistance can be achieved by application of micro - alloying elements. Titanium and rare earth elements are particularly interesting micro - alloying elements. Subject of examination were AlZn12Mg3.5Cu2.5 base alloy (without micro-alloying) and alloy with the addition of titanium alloy and alloy with the addition of rare earth elements. Corrosion examinations were carried out in a solution of "acid rain", pH = 3.5. The scope of the research included potentiodynamic and potentiostatic tests , examination of the surface and structure of the samples after corrosion.The research found that the addition of titanium does not cause a significant improvement in corrosion resistance of the AlZn12Mg3.5Cu2.5 alloy. A significant improvement was achieved with the addition of rare earth elements. It was found that corrosion tested alloys is local and leads to the formation of pits.

Abstract: Graphene is a two-dimensional network of carbon atoms with optimal thermal, electronic and chemical stability properties that promise different and versatile applications in various fields including the protection of metals from corrosion phenomena. For this reason in this paper graphene was employed and studied as an agent dopand incorporated into hybrid sol-gel coatings to enhance their resistance in saline media and to improve the durability of these films. Graphene was obtained by using an electrochemical method involving oxidation and reduction reactions in a sodium lauryl sulfate solution. On the other hand, the hybrid sol-gel was synthesized from the combination of inorganic and organic precursors, zirconium (IV) n-propoxide (TPOZ) and 3-glycidoxipropiltrimetoxysilane (GLYMO) respectively. In order to obtain the coating system (graphene/sol-gel) two different procedures were applied onto clean aluminum plates: a) the electrodeposition of graphene and b) the graphene-doped sol-gel coating. Differential scanning calorimetry, scanning electron microscopy and electrochemical impedance spectroscopy were used to characterize the results, which demonstrate an improvement of the corrosion properties of the films with the incorporation of graphene compounds.