Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5
wt.% NaCl at 25
°C. Corrosion products were ...analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)
2 corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg
17Al
12) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
► SKPFM provided information about galvanic interactions existing in Mg alloys. ► Galvanic interactions depended on the Volta potential differences of the alloy constituents. ► The Volta potential ...differences of the constituents depended on their composition. ► The as-cast ZE41 alloy presented strong galvanic corrosion. ► No evidence of galvanic corrosion by RE-containing phases was observed in WEXX alloys.
Four commercially produced rare-earth-magnesium alloys, as-cast ZE41, sand-cast WE43-T6, and wrought WE43-T6 and WE54-T6, were studied by scanning Kelvin probe force microscopy (SKPFM) in order to evaluate the role of their micro-constituent phases on potential galvanic interactions. Such interactions were then examined by immersion of the alloys in 3.5
wt% NaCl solution. The results confirmed that SKPFM was a powerful tool to determine the corrosion mechanism of magnesium alloys, where the intensity of the corrosion attack depended on the relative Volta potential differences of the micro-constituents, as well as their amounts, compositions and distributions within the alloys.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The corrosion performance of WE43-T6 and AZ91D magnesium alloys with and without treatment by plasma electrolytic oxidation (PEO) was investigated by electrochemical measurements in 3.5
wt.% NaCl ...solution. For untreated WE43-T6 alloy, formation of a uniform corrosion layer (Mg(OH)
2) was accompanied by initial pits around magnesium-rare earth intermetallic compounds. The AZ91D alloy disclosed increased corrosion susceptibility, with localized corrosion around the β-phase, though the β-phase network phase acted as a barrier for corrosion progression. PEO treatment in alkaline phosphate electrolyte improved the corrosion resistance of WE43-T6 alloy only at the initial stages of immersion in the test solution. However, PEO-treated AZ91D alloy revealed a relatively high corrosion resistance for much increased immersion times, contrary to the relative corrosion resistances of the untreated alloys. The improved performance of the PEO-treated AZ91D alloy appears to be related to the formation of a more compact coating.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Mn and Mo were introduced in AISI 304 and 316 stainless steel composition to modify their pitting corrosion resistance in chloride-containing media. Corrosion behaviour was investigated using ...gravimetric tests in 6
wt.% FeCl
3, as well as potentiodynamic and potentiostatic polarization measurements in 3.5
wt.% NaCl. Additionally, the mechanism of the corrosion attack developed on the material surface was analysed by scanning electron microscopy (SEM), X-ray mapping and energy dispersive X-ray (EDX) analysis. The beneficial effect of Mo additions was assigned to Mo
6+ presence within the passive film, rendering it more stable against breakdown caused by attack of aggressive Cl
− ions, and to the formation of Mo insoluble compounds in the aggressive pit environment facilitating the pit repassivation. Conversely, Mn additions exerted an opposite effect, mainly due to the presence of MnS inclusions which acted as pitting initiators.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The influence of the microstructure and aluminium content of commercial AZ31, AZ80 and AZ91D magnesium alloys was evaluated in terms of their corrosion behaviour in an aerated 3.5
wt.% NaCl solution ...at 25
°C. The corrosion process was monitored by electrochemical impedance spectroscopy (EIS). The surface was characterized by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM) and low-angle X-ray diffraction (XRD). The extent of corrosion damage was strongly dependent on the aluminium content and alloy microstructure. Two key factors were observed for the lowest corrosion rates, which occurred for the AZ80 and AZ91D two-phase alloys: the aluminium enrichment on the corroded surface for the AZ80 alloy, and the β-phase (Mg
17Al
12), which acted as a barrier for the corrosion progress for the AZ80 and AZ91D alloys. Surface potential maps suggested that, between the β-phase and the α-matrix, the galvanic coupling was not significant.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The three-dimensional ion implantation method is presented as a new plasma-ion technology for advanced surface treatment. It is based on a high voltage pulsed discharge activated in the low-pressure ...range and applicable to solid surface immersed in a gaseous atmosphere. This process is carried out in the joint universal plasma and ion technologies experimental reactor and is widely used to improve the tribological properties and corrosion resistance of metallic materials, specially steels. In this study, the performance in a corrosive environment of a carbon steel surface-modified was evaluated. High-energy nitrogen ions were implanted on the surface of the material at different discharge voltages and times. Treated and non-treated substrates were exposed to saline solution for 35 days, where the effect of the surface modification was shown to decrease the corrosion rate and degradation of the implanted material surface compared to untreated substrates. The estimation of the nitrogen concentration from experimental parameters were calculated by a novel web application. Additionally, the effect of the implantation process of the modified steel structure was analyzed by optical emission spectroscopy, whose results detected mainly an increase of the nitrogen concentration at the surface of the modified substrates.
X-ray photoelectron spectroscopy (XPS) was used in order to investigate the correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys exposed to ...98% relative humidity at 50
°C. Commercially pure magnesium, used as the reference material, revealed MgO, Mg(OH)
2 and tracers of magnesium carbonate in the air-formed film. For the AZ80 and AZ91D alloys, the amount of magnesium carbonate formed on the surface reached similar values to those of MgO and Mg(OH)
2. A linear relation between the amount of magnesium carbonate formed on the surface and the subsequent corrosion behaviour in the humid environment was found. The AZ80 alloy revealed the highest amount of magnesium carbonate in the air-formed film and the highest atmospheric corrosion resistance, even higher than the AZ91D alloy, indicating that aluminium distribution in the alloy microstructure influenced the amount of magnesium carbonate formed.
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The work addresses the influence of Mn and Mo additions on corrosion resistance of AISI 304 and 316 stainless steels in 30wt.% H2SO4 at 25 and 50 deg C. Corrosion mechanism was determined by ...gravimetric tests, DC polarization measurements and electrochemical impedance spectroscopy (EIS). The morphology and nature of the reaction products formed on the material surface were analysed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Reduction of temperature from 50 to 25 deg C drastically decreased the corrosion rate of AISI 304 and 316 stainless steels in sulphuric acid solution. Mn additions did not affect significantly the general corrosion resistance due to its low ability to form insoluble compounds in acid medium. Meanwhile, the formation of molybdenum insoluble oxides enhanced the corrosion performance.
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The aim of the present work was to evaluate the influence of the microstructure on the formation-biodegradation mechanism of conversion coatings synthesised on the AZ31 magnesium alloy by immersion ...in HF solutions under different concentrations and treatment times. The experimental results revealed the formation of a magnesium hydroxyfluoride layer, which enhanced the corrosion resistance of the AZ31 alloy in Hanks' solution. Furthermore, the presence of intermetallic particles within the α-Mg matrix favoured the formation of defects that promoted the growth of corrosion products causing the detachment of the fluoride coating, all before it may start to be noticeably biodegraded.
•The conversion treatment of the AZ31 alloy in HF produced a layer of Mg(OH)2−xFx.•Increase in HF concentration and treatment time led to coatings with higher fluoride content.•The microstructure played a key role in the corrosion performance of the coating.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The aim of the present work was to evaluate the effect of the catalyst (acetic acid) concentration, the immersion time and aging time during the synthesis and deposition of TEOS-GPTMS hybrid sol-gel ...coatings on the AZ31 magnesium alloy and to analyse how these synthesis parameters affect the morphology, composition and corrosion performance of the coating resultant. The hydrolysis and condensation reactions occurred within the sols were monitored by rheological test and Fourier Transform Infrared (FTIR) Spectroscopy. In order to evaluate the corrosive power of the sols synthesised and their interaction with the magnesium surface, both the catalyst concentration and the immersion time using during deposition were varied from 2.5 to 10vol%, and from 15 and 120s respectively. Additionally, the effect of the aging time on the composition, morphology and corrosion performance of the hybrid coatings was analysed by employing different aging times up to 15days. Characterisation of the coatings was carried out using Scanning Electron Microscopy (SEM), Attenuated Total Reflection Infrared Spectroscopy (ATR-IR), and contact angle measurements. On the other hand, corrosion resistance was evaluated by potentiodynamic anodic polarisation test in aerated 0.1M NaCl solution. The experimental results showed that by suitable control of synthesis parameters studied it was possible to mitigate surface corrosion processes on the AZ31 magnesium alloy during the sol deposition. This allowed obtaining continuous, homogeneous coatings without cracking. Further, their corrosion evaluation revealed a decreasing the corrosion current density by about one order of magnitude with regard to the parent alloy providing, simultaneously, a protection range up to 150mV.
•The control of the synthesis parameters mitigated the corrosion processes on the AZ31 alloy during the sol deposition.•The suitable balance SiO2/Si-OH allowed obtaining coatings with improved morphological - protective features.•The presence of the catalyst augmented the sol stability, but also produced coatings with reduced corrosion performance.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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