Naing Naing Aung

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.

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.

Effect of heat treatment on corrosion and electrochemical behaviour of AZ91D magnesium alloy

An AZ91D ingot in the as-cast condition was homogenized by solution treatment and then aged for various periods of time. The microstructures produced were studied in detail and the β phase volume fraction was measured quantitatively. The Corrosion resistance of all the different microstructures was studied in 3.5% NaCl solution through weight loss measurement in constant immersion conditions and potentiodynamic polarization experiments. The corroded surfaces were analysed using SEM and XRD. The volume fraction of the β phase was found to have a significant influence on the corrosion behaviour. The T4 condition improved the corrosion resistance of AZ91D alloy compared to the T6 heat treatment. The results support the idea of microgalvanic coupling between cathodic β phase and anodic α matrix.

Development of self-powered wireless high temperature electrochemical sensor for in situ corrosion monitoring of coal-fired power plant

Reliable wireless high temperature electrochemical sensor technology is needed to provide in situ corrosion information for optimal predictive maintenance to ensure a high level of operational effectiveness under the harsh conditions present in coal-fired power generation systems. This research highlights the effectiveness of our novel high temperature electrochemical sensor for in situ coal ash hot corrosion monitoring in combination with the application of wireless communication and an energy harvesting thermoelectric generator (TEG). This self-powered sensor demonstrates the successful wireless transmission of both corrosion potential and corrosion current signals to a simulated control room environment.

A novel electrochemical method for monitoring corrosion under insulation

Purpose – The objective of this work was to develop practical experimental techniques for monitoring corrosion in “difficult‐to‐test” conditions such as corrosion under insulation (CUI).Design/methodology/approach – An electrochemically integrated multi‐electrode array namely the wire beam electrode (WBE) method has been used in combination with noise signature analysis for the first time to monitor the penetration of corrosive species under simulated corrosion‐under‐insulation conditions. Corrosion of aluminium exposed under insulation materials such as rock wool, glass wool, cotton wool and tissue paper has been successfully monitored.Findings – A typical potential noise signature of a major potential jump from AA1100 WBE was observed which corresponded to the corrosive species reaching the WBE surface in WBE current distribution map. A good correlation between the galvanic current maps and the corroded surface was also observed.Originality/value – The preliminary results suggest that the proposed novel...

Monitoring localised corrosion in inhibited solutions using wire beam electrode-noise signatures method

Abstract The objective of this work is to determine the nature of localised corrosion in inhibited solutions by establishing possible relationships between characteristic features in electrochemical noise and corrosion processes using an electrochemically integrated multielectrode system, namely the wire beam electrode (WBE) in combination with noise signatures method. Experiments have been carried out to simultaneously measure electrode potential noise and WBE current distribution maps from stainless steel (SS316L) WBE exposed to inhibited solutions containing 6%FeCl3 solution with inorganic inhibitors including 2 wt-% sodium chromate (Na2CrO4), cerium chloride (CeCl3) and lanthanum chloride (LaCl3). Characteristic electrochemical noise signatures were found to correlate with characteristic changes in WBE current distribution maps that show corrosion rates distributions and the degree of localised corrosion. A new concept namely localisation parameter (LP) has been proposed to identify the degree of inhibition. Na2CrO4 effectively inhibited the pitting corrosion process and the LP successively decreased through out the corrosion process. With the presence of CeCl3 and LaCl3, pits on stainless steel in 6%FeCl3 solution were not repassivated and an increase in the LP was observed for both inhibitors. The results suggest that the WBE method could be used in combination with the noise signatures as a sensitive technique for monitoring inhibiting effect on localised corrosion.

Evaluation of corrosion resistance of magnesium alloys in radiator coolants

Abstract Coolant corrosion is a major drawback for the use of magnesium alloys in engine and cooling system, but the coolant is not normally intended to prevent corrosion of magnesium alloys. This research assessed the corrosion performance of two magnesium alloys, AZ91D and AM50A, in two newly formulated radiator coolants using immersion test, potentiodynamic polarisation test, and corroded surface analysis. Two coolants were named as Irgacool Plus L and Irgacool Plus S. C7, C8-organic acids and polycarboxylic acid were the main inhibitor species in Irgacool Plus L while Irgacool Plus S was formulated with C7, C8-organic acids and sebacic acid inhibitors. Corrosion rates of magnesium alloys decreased twice in Irgacool Plus L compared with Irgacool Plus S. AZ91D alloy had better corrosion resistance than AM50A alloy in both radiator coolants. Both alloys suffered corrosion due to microgalvanic coupling between cathodic β-Mg17Al12 intermetallic and anodic α-Mg matrix, and the presence of Al8Mn5 and Al11Mn4 intermetallics in AM50A led to further microgalvanic corrosion. A continuous network of β-Mg17Al12 phase and higher Al content α-Mg matrix accounted for better corrosion resistance of AZ91D alloy.