Materials and Corrosion-werkstoffe Und Korrosion最新文献

The braking system of a motor vehicle is a multi-material system, subjected to various aggressive conditions. Corrosion of the brake disc during stationary periods can determine the onset of a high adhesion force (stiction) capable of compromising the reliability of the braking system during vehicle motion. The purpose of this work is to study the effect of the introduction of Cu and Zn in the friction material composition. This effect was investigated through electrochemical measurements (electrochemical impedance spectroscopy, potentiodynamic polarization, and stiction tests), conducted using an electrochemical cell simulating the parking brake, complemented by the examination of the brake disc and pad surfaces and water absorption tests. The results suggest that porous components, like vermiculite, in the composite friction material led to high contact force. Moreover, 10 wt% of Cu in the friction material does not significantly affect its stiction behavior in our testing configuration. In contrast, 10 wt% Zn in the friction material significantly reduces the stiction propensity by acting with a complex synergistic mechanism combining physical and chemical shielding effects.

A new index that estimates the corrosion resistance of aluminium-containing steel is established and verified. It is based on the well-known pitting resistance equivalent number (PREN) but accounts for aluminium additions. It is shown that aluminium enhances the corrosion resistance of chromium-containing steel, with a coefficient of 5.2. The new corrosion resistance index (CRI) is as follows CRI = Cr + 3.3Mo + 16N + 5.2Al. A minimum of 4 wt% of chromium addition is needed. In addition, the atomic ratio of chromium to aluminium addition needs to be higher than 1, for this new index to be valid. This new index allows for simple alloy design guidelines for steel applications that require improved corrosion resistance. Analysis of the data from the literature shows that this index becomes inaccurate when the corrosion environment is in the presence of NaCl, and aluminium becomes less effective in enhancing corrosion resistance.

The failure behavior and the failure mechanisms of the copper tube used in the air conditioning heat exchanger were analyzed using macrostructure and microstructure examination and the chemical composition analysis of the corrosion products. Results showed that the corrosion of the exterior surface of the copper tube should be responsible for the failure of the air conditioning heat exchanger. The chemical compositions of the corrosion products mainly included Cu, O, C, P, S, Cl, and so forth. The corrosion mechanisms were electrochemical corrosion under the circumstances of a humid environment, oxygen, and some corrosive medium. The corrosive media were organic carboxylic acids, Cl⁻, H₂S, and SO₂, which came from air pollutants, the combustion of fuels, the automobile exhaust, and the industrial waste gas. One or more electrochemical corrosion destroyed the integrity of the copper tube and led to the failure of the air conditioning heat exchanger in advance.

This work systematically studied the corrosion behavior and passivation property of CoCrFeNi high-entropy alloys (HEAs) in a simulated seawater environment. The results reveal that the addition of CO₃²⁻/HCO₃⁻ results in the secondary passivation of the HEA in NaCl solution, and secondary passivation film possesses higher dissolution rate in comparison with primary passive film. H⁺ ions ionized by HCO₃⁻ facilitate the charge transfer process, thin the thickness of passive film, and increase the disordered degree of the film. Moreover, the presence of HCO₃⁻ promotes the nucleation and growth of metastable pits, and enhances the pitting sensitivity. Furthermore, an increase in CO₃²⁻ concentration accelerates the dissolution of passive film, weakens the compactness and protective performance of the film, and increases the probability that metastable pitting evolves into stable pitting, as well aggravates the corrosion of the HEA. In addition, the corrosion resistance of the HEA is diminished due to the formation of microgalvanic corrosion cells between matrix and inclusion. The selective dissolution of elements occurred in the HEA.

High-entropy alloys (HEAs) are promising new materials considered for application in high temperature environments. In this work, the sulfidation behavior of an Al–Co–Cr–Ni–Si HEA at different elevated temperatures and various sulfur vapor partial pressures is investigated. Thermogravimetric studies indicate that the sulfidation kinetics are in accordance with the parabolic rate law. The corrosion rate has a strong temperature dependence, whereas the influence of sulfur vapor pressure is practically nonexistent. Changes in temperature also do not seem to affect the sulfidation mechanism as evidenced by the Arrhenius correlation determined between the obtained parabolic rate constants and temperature. Combined SEM–EDS and XRD studies reveal the formation of a thick multilayer scale on the studied alloy, where the outermost relatively compact part is built of Co_0.5NiS₂ and CrAl₂S₄ while the inner, rather porous part also contains metallic inclusions, including Si near the scale/substrate interface.

Steel pipelines used for oil and gas transmission are at risk of stress corrosion cracking (SCC). Extensive research has been carried out to evaluate the SCC susceptibility of pipeline steels. Those research studies were mainly specific to the steels of pipes without welding. In this research, pieces of X65 steel plates were welded by the shielded metal arc welding process. The speeds of electrode travel were set as the main welding parameter. Slow strain rate tests (SSRTs) in the air and the NS4 standard solution were conducted. The results showed that the tensile behavior of the samples in the solution was very different from that in the air. For the samples tested in the solution, the strength amounts of the weld zones were higher than the strength of the base metal, but in the case of SSRT in the air, the strength of the base metal was higher. As a result, the UTS strength of one of the weld zones reached levels as high as 480 MPa and also 34% elongation under the SSRT. Therefore, the conditions of welding have a high degree of importance and must be controlled to have less SCC susceptibility.

Commercial hot-dip galvanized zinc (Zn-0.2% Al), galfan (Zn-5% Al), and galvalume (Zn-55% Al) coatings were exposed to wet scCO₂, followed by an assessment of microstructural changes in the coating structure. Zinc and galfan coatings showed similar surface activity with abundant zinc dissolution and uniform corrosion product deposition. The Al-rich phases of the galfan coating were intact in the studied atmosphere and eventually became embedded within the deposits. The zinc dissolution ended for zinc and galfan coatings when the free metal surface was blocked from the surrounding medium by a uniform corrosion product layer. Galvalume showed scarce reactivity: dissolution of zinc in the Zn-rich interdendritic areas proceeded slowly compared to zinc and galfan, and the Al-rich dendrites (with nm-scale Zn inclusions) remained intact. The ion beam milling technique combined with modern nm-resolution energy-dispersive spectroscopy analysis enabled previously unseen visualization of the discussed corrosion processes.

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Electron backscatter diffraction (EBSD) images of steel shot peened Ni-base alloy 699 XA after 480 h exposure in a carbon-rich gas at 620 °C and 19 bar. From left to right: secondary electron image, color coded using an inverse pole figure, and color coded by crystal structure. During high temperature exposure, α-Cr (BCC) precipitates formed in the highly deformed nearsurface zone. Images by Melanie Thalheimer.

More detailed information can be found in: Clara Schlereth, Emma Marie Hamilton White, Maren Lepple, Benedikt Nowak, Heike Hattendorf, Mathias Christian Galetz, Influence of the surface treatment on the metal dusting resistance of welds, Materials and Corrosion 2024, 75, 371.