Rajan Ambat

Evaluation of microstructural effects on corrosion behaviour of AZ91D magnesium alloy

The effect of microconstituents on the corrosion and electrochemical behaviour of AZ91D alloy prepared by die-casting and ingot casting route has been investigated in 3.5% NaCl solution at pH 7.25. The experimental techniques used include constant immersion technique, in-situ corrosion monitoring, and potentiodynamic polarisation experiments. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX and XRD. The corrosion behaviour of microconstituents namely primary α, eutectic α and β phases was significantly different. Coring of aluminum showed influence on corrosion behaviour more significantly in ingot material. Areas with aluminium concentration less than about 8% were found to be prone to corrosion attack compared with either those with higher amount of aluminium or β phase. Die-cast material with smaller grain size and fine β phase offered marginally lower corrosion rate and better passivation compared with the ingot. In die-cast and ingot, hydrogen evolution took place preferentially on β phase. XRD pattern of non-corroded and corroded surface revealed the removal of β phase from alloy surface during corrosion. The corrosion products for ingot consisted of Mg(OH)2 with small amounts β phase, magnesium-aluminium oxide and MgH2 while for die-cast, the product showed a highly amorphous structure.

Electroless nickel-plating on AZ91D magnesium alloy: effect of substrate microstructure and plating parameters

Electroless nickel-plating on AZ91D magnesium alloy has been investigated to understand the effect of substrate microstructure and plating parameters. The initial stage of the deposition was investigated using scanning electron microscopy (SEM) and energy dispersive X-ray analysis on substrates plated for a very short interval of time. The early stage of growth was strongly influenced by the substrate microstructure. Plating was initiated on b-phase grains probably due to the galvanic coupling of b and eutectic a-phase. Once the b-phase was covered with the coating, it then spread onto eutectic a and primary a-phase. The coating produced with the optimised bath showed 7 wt.% phosphorus with a hardness of approximately 600–700 VHN. The optimum ligand to metal ion ratio was found to be 1:1.5, while the safe domain for thiourea (TU) was in the range of 0.5–1 mgyl. Fluoride was found to be an essential component of the bath to plate AZ91D alloy with an optimum value of 7.5 gyl. The presence of 0.25–0.5 mgyl mercapto-benzo-thiosole (MBT) found to accelerate the plating process. 2003 Elsevier B.V. All rights reserved.

Studies on the influence of chloride ion and pH on the corrosion and electrochemical behaviour of AZ91D magnesium alloy

The influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of die-cast and ingot-cast AZ91D alloy have been studied with a focus on the stability of microconstituents in these environments. The experimental techniques used include immersion studies, potentiodynamic polarization, X-ray diffraction and optical and scanning electron microscopy. The corrosion rate for the ingot and die-cast was very high in highly acidic solutions (pH 1–2) as compared to that in neutral and highly alkaline solutions (pH 4.5–12.0), and the rate increased with chloride ion concentration at all pH levels. In general, the die-cast showed a lower corrosion rate at all pH values and chloride ion concentrations. The open circuit corrosion potential shifted to more negative (more active) values with increase in concentration of chloride ions. Corrosion morphologies revealed more attack on primary α and eutectic α with increasing chloride concentration. In highly acidic conditions, corrosion attack was found on β (Mg17Al12) and eutectic α phase (α regions with higher Al content) while at pH 12.0 the ingot exhibited a pitting type of morphology. The corrosion product consisted of magnesium hydroxide, fallen β particles and magnesium–aluminium oxide; the amount of each component was found to be a function of chloride ion concentration and pH.

Electrochemical Behavior of the Active Surface Layer on Rolled Aluminum Alloy Sheet

The highly deformed, micrograined layer on the outermost surface of a rolled Al-Fe-Si-Mn model alloy was electrochemically characterized. The thickness of this deformed surface layer in a 1.0 mm thick sheet was approximately 1 μm. Polarization curves in 5% NaCl solution at pH 3.0 and 11.5 were obtained at different depths from the surface using controlled sputtering in a glow discharge optical emission spectrometer for sample preparation. Both the anodic and the cathodic reactivity of the deformed surface layer were significantly higher than that of the bulk. Consistent with this, image analysis of scanning electron microscopy backscattered images revealed an increased number of fine intermetallic particles in the surface layer as compared with the bulk of the material. The corrosion morphology of the outermost surface was characterized by a high density of fine pits, while fewer and larger pits were observed in the bulk. The results highlight the importance of heavily deformed surface layers in controlling corrosion behavior of rolled aluminum products.

The effect of cryogenic CO2 cooling on corrosion behaviour of friction stir welded AA2024-T351

Abstract Cryogenic cooling with CO2 was applied during friction stir welding of AA2024-T351 in order to reduce the temperature increase during welding, and thus improve the corrosion resistance of the weld. The effect of cryogenic cooling on corrosion susceptibility was investigated with gel visualisation, immersion tests and local electrochemical measurements. The most susceptible area for both uncooled and cooled welds was in the heat-affected zone (HAZ) region, which showed intergranular attack. Cryogenic cooling had no detectable influence on the degree of anodic reactivity in the weld region. However, it did decrease the width of the reactive HAZ.

Corrosion of a dissimilar friction stir weld joining aluminium alloys AA2024 and AA7010

Abstract The corrosion behaviour of a dissimilar metal friction stir weld between AA2024–T351 and AA7010–T7651 was investigated. Gel visualisation and immersion tests were used to examine the effect of galvanic coupling between the two components. A microelectrochemical cell with a 400 μm diameter glass tip was used to measure the anodic and cathodic reactivities at different positions in the weld region as well as in the base alloys. It was found that the net anodic attack was in the 7010 alloy with the highest susceptibility in the nugget region. The nugget region of 2024 was protected by its high net cathodic reactivity owing to the precipitation of S phase particles.

Corrosion behaviour of banded microstructure within nugget of friction stir welds in AA2024-T351

Abstract Corrosion behaviour of the banded structure known as ‘onion ring’ in the nugget of friction stir weld AA2024-T351 was investigated to find the relation to microstructure. A micro-electrochemical cell with a 50 μm diameter glass pipette tip was used for electrochemical measurements. It was found that onion rings consisted of two bands: ‘dark’ bands that contain fewer constituent particles but show extensive precipitation of S phase and ‘light’ bands that contain fewer S phase precipitates but greater numbers of constituent particles. Electrochemical results showed that the light band has more noble (less active) open circuit potential compared to the dark band. Microstructurally, this is due to the lower number of S precipitates and therefore the possibility of higher Cu solid solution within the band compared to that of the dark band.

Effect of Equal Channel Angular Extrusion on Corrosion Behavior of Al-Mn ALLOY

Effect of deformation on the corrosion behaviour of Al-1Mn0.4Fe-0.3Si model alloy was studied after equal channel angular extrusion (ECAE) to different levels and subsequent annealing treatment. During polarisation, cathodic reactivity of the samples increases with the degree of ECAE deformation. Microstructure showed more number of intermetallic particles after ECAE deformation as well as after annealing. ECAE deformation also increased the anodic reactivity. Observed electrochemical behaviour is attributed to intermetallic particles as they good cathodic sites and precipitation of particles deplete manganese from matrix leading to higher anodic activity.