Electrochemical Estimation of the Corrosion Rate of Magnesium/Aluminium Alloys

The corrosion rate of AZ31, AZ80, and AZ91D magnesium/aluminium alloys immersed in 3.5 wt.% NaCl was determined comparing gravimetric and electrochemical measurements. The findings revealed that, for all investigated materials, a fraction of the metallic surface exposed to the corrosive medium did not reveal a normal electrochemical response to the applied signal. This may be associated with phenomena such as partial disintegration of specimens into fine metallic particles, electrochemical formation of ions, and/or anomalous chemical attack occurring simultaneously with the normal electrochemical corrosion attack. The abnormal electrochemical behaviour was more evident for lower amounts of aluminium in the bulk composition of the investigated materials. Thus, the electrochemical estimates of pure Mg and the AZ31 alloy were not reliable and tended to underestimate corrosion losses.Peer Reviewe

Functionalized hybrid PEO/Sol-gel Coatings for corrosion protection of Mg alloys

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Effect of cerium (IV) on thin sulfuric acid anodizing of 2024-T3 alloy

Chromic acid anodizing (CAA) is still being used today for corrosion protection of fatigue-critical components in the aeronautic industry due to the lack of feasible alternatives. Ce-containing sulfuric acid anodizing (SAA) has been identified as a promising strategy for the development of alternatives to toxic CAA. This work explores thin sulfuric acid anodizing (TSAA) focusing on the following effects: (i) current density and voltage values; (ii) concentration of Ce(IV). Screening of the optimum combination in search of minimum thickness and the best corrosion resistance was performed using electrochemical impedance spectroscopy (EIS). Two Ce-containing anodic films were selected and further investigated in comparison with the inhibitor-free film in terms of morphology (FEG-SEM, TEM), composition (RBS), corrosion resistance (EIS, NSST), high-cycle fatigue and paint adhesion. The results indicate that the path to approach the CAA performance lies through thin (<1 μm) SAA-Ce films formed at low current density

Hard Anodizing and Plasma Electrolytic Oxidation of an Additively Manufactured Al-Si alloy

Plasma Electrolytic Oxidation (PEO) and Hard Anodizing (HA) coatings are investigated on an Additively Manufactured (AM) Al10Si1Mg alloy obtained via Direct Metal Laser Sintering (DMLS). Results are compared to those obtained for a conventional A361 cast alloy. Findings revealed that the microstructure of the AM alloy, consisting of α-Al cells enclosed in a Si network, produces coatings that are more uniform –in terms of morphology, thickness and roughness of the coating/substrate interface– than those obtained on the cast alloy. However, enhanced oxidation of the fine Si network in the AM alloy results in softer coatings. In both alloys, PEO coatings demonstrate superior wear protection than HA due to the presence of mullite in the former. The anisotropy in the microstructure of the AM alloy influences the tribological behaviour of studied coatings, with the XZ plane showing superior wear performance

PEO of additively manufactured Al10SiMg alloy: Effects of α-Al2O3 particles on energy consumption and wear behaviour

Plasma electrolytic oxidation (PEO) coatings were developed on an additively manufactured Al10SiMg alloy pre-treated with hard anodizing (HA). The influence of α−Al2O3 particles incorporated during PEO was evaluated in terms of energy consumption, coating morphology, composition and wear behaviour. Combining a 20 μm-thick HA precursor and a PEO electrolyte with suspended α−Al2O3 particles resulted in the highest energy savings (∼60%). Wear volume loss measurements and wear track characterization revealed that the PEO coating with added particles (PEOP) outperformed the coating without particles and a conventional hard anodizing layer. This superiority was attributed to the partial sealing of the outer porous layer by the incorporated α−Al2O3 particles as well as the reduced SiO2 content and the formation of a W-rich tribolayer on the wear track surface

Galvanic corrosion of rare earth modified AM50 and AZ91D magnesium alloys coupled to steel and aluminium alloys

Electrochemical and gravimetric measurements were used to examine the effects of neodymium and gadolinium additions on the galvanic corrosion behaviour of AM50 and AZ91D magnesium alloys coupled to A 570 Gr 36 carbon steel and AA2011-AA6082 aluminium alloys. Rare earth modified alloys showed Al 2Nd/Al2Gd and Al-Mn-Nd/Al-Mn-Gd intermetallics, reduced area fraction of β-Mg17Al12 phase and increased corrosion resistance due to increased surface passivity and suppression of micro-galvanic couples. Neodymium and gadolinium additions improved the galvanic corrosion resistance of AM50 alloy, but were less effective in case of the AZ91D alloy. The AA6082 alloy was the most compatible material and the AA2011 alloy was the least compatible. © 2014 CSIC.Peer Reviewe

Comparative determination of TiO2 surface free energies for adhesive bonding application

The development of durable bonds using titanium adherens has been investigated from the point of view of surface energy theoretical models measurements. The traditional Chromium Acid Anodization, which provides excellent durability, has to be phased out due to the use of hazardous Cr (VI) in the bath and as a result, special attention is paid to the sodium hydroxide anodizing and other alkaline chemical etchers. There are hardly any references on the surface free energy of adhesive titanium oxide coatings and therefore the objective of this work was to evaluate the surface and interface energy parameters of the various types of alkaline chromate free surface treatments using Neumann, Fowkes and van OssChaudhuryGood methods in order to determine which method provides greatest differentiation between the coatings. Results show that Fowkes method produced the greatest variance in surface energies of the compared surface treatments and hence can be considered as better suited for more accurate discrimination between the oxide surface treatments on Ti6Al4V alloy. Although, in the case of model liquids, i.e. water and diiodomethane, the trends obtained for contact angles, surface energies, works of adhesion and solid/liquid interface energies all correlated between each other, a disagreement between the trends of solid/liquid interface energies calculated using Fowkes and van OssChaudhuryGood methods for surface treatment/adhesive resin was obtained. In case of real adhesive systems, the use of work of adhesion appears more adequate in order to discriminate the surface treatments. Based on these findings the anodization in the tested alkaline bath after a previous alkali etching in the same bath is recommended, although adhesion test has to be still performed. © 2011 Elsevier Ltd. All Rights Reserved.Peer Reviewe