Electrochemical monitoring of nickel–aluminium bronze crevice corrosion solutions using boron–doped diamond electrode

This study has demonstrated the capability of electrochemically assessing the metal–ion concentrations generated from the localised corrosion of nickel–aluminium bronzes (NAB). Prior to investigating NAB crevice corrosion, its electrochemical properties were studied at different pHs and chloride concentrations. At solution pHs higher than 4 NAB exhibited a corrosion behaviour similar to that of unalloyed copper and its oxidation was controlled by the dissolution of the copper–rich ?–phase. However, as the pH was decreased below 4 the corrosion mechanism changed and the other ?–phases rich in aluminium, iron and nickel underwent preferential oxidation. The NAB corrosion performance was also investigated in the presence of benzotriazole (inhibitor) by using potentiodynamic polarisation. The excellent corrosion properties showed by NAB when exposed to neutral benzotriazole solution made the studied inhibitor a promising candidate for limiting crevice corrosion. With knowledge of the NAB corrosion behaviour and the relatively high copper– and nickel–ion extents present within the crevice corrosion solutions, a study related to their electrochemical behaviour and detection was undertaken using a boron–doped diamond (BDD) electrode at different pHs and chloride levels in order to establish viable electrochemical protocols for effectively assess these concentrations. Before investigating the copper and nickel electrochemical behaviours on BDD electrode, the diamond substrate was studied using a number of different techniques such as potentiodynamic polarisation, cyclic voltammetry and electrochemical impedance spectroscopy. Results highlighted its excellent performance having a wide potential window (ca. 3 V) and a low capacitive current (20 ?F cm–2 in 0.5 M H2SO4) available for electroanalysis purposes. Finally, the NAB crevice corrosion was monitored using a BDD microelectrode array. The employed setup created an artificial crevice and accommodated the BDD microelectrode array for in situ and a real–time monitoring. The electrochemical response showed the only presence of copper(I) during the investigated time, whose concentration increased within the first two hundred hours to a level of ca. 0.4 mM and then remained stable for the following hundred hours. No copper(II), or other metal–ions, were determined in the crevice solutions, thus suggesting that within the investigated time the copper–rich ?–phase dominated the NAB corrosion behaviour. Furthermore, results also indicated (ii) the low dissolved oxygen concentration within the crevice (since it promotes the oxidation of copper(I) to copper(II)) and (ii) that the local pH did not decreased below 4, where the NAB corrosion is controlled by the selective dissolution of the aluminium–, iron– and nickel–rich ?–phases

On the electrolysis of dilute chloride solutions: Influence of the electrode material on Faradaic efficiency for active chlorine, chlorate and perchlorate

In the present work, the electrolytic process of diluted aqueous chloride solutions was investigated at Ti/RuO2.2SnO2 and Ti/Pt electrodes, at different values of current density, temperature and electrolysis time. The time evolution of chlorine-related species (i.e. active chlorine (dissolved Cl2, HClO, OCl-), chlorite, chlorine dioxide, chlorate and perchlorate) was investigated in order to establish whether their formation and consumption was related to either chemical or electrochemical path of reactions. The estimated faradic efficiencies demonstrated the better catalytic activity of the Ti/RuO2.2SnO2 electrode towards the chlorine evolution reaction with respect to the Ti/Pt anode, and the key-role played by the temperature, which reflects the different activation energies of the two competing electrochemical reactions, i.e. chloride and water oxidations. The concentration trends of chlorate and perchlorate indicated that the electrochemical route was responsible for their presence in the bulk solution, instead of a chemical path. The low concentration levels assessed for chlorites throughout the tests did not suggest the preponderance of the chemical over the electrochemical depletion process; however, further potentiodynamic tests suggested their high reactivity towards both anodic and cathodic surfaces, thus suggesting that the electrochemical path of depletion could prevail over the chemical. Conversely, due to a solution pH unfavourable to the stability of chlorine dioxide, its low concentration level was associated to a chemical depletion route

Electrochemistry of the chlorine-water system

The electrochemical production of disinfectant solutions containing active chlorine is usually carried out within cells, whose anode and cathode chambers are separated by a dividing wall (an ion-selective diaphragm or a membrane). The latter separator is generally present in order to avoid the cathodic reduction of active species synthesized at the anode; however, dealing with aqueous solutions containing small amounts (a few grams per liter) of chloride ions, a reasonable synthesis of active chlorine can be obtained also in undivided cells, i.e. in the absence of a diaphragm or a membrane. In these cases, the species synthesized at the anode can be effectively consumed or partly reduced at the cathode, and the reaction products depend on the chosen electrochemical parameters (current density, temperature, anode material). The present communication will focus on data obtained through 3-hours electrolyses at different conditions, carried out on aqueous solutions containing 5 g/l of KCl; a speciation of chlorinated species were performed every 30 minutes, by means of titrations and ion chromatography

Electrochemical behaviour of nickel–aluminium bronze in chloride media: influence of pH and benzotriazole

The corrosion properties of the nickel–aluminium bronze (NAB) in aqueous chloride media were investigated at different pHs by using linear potential sweep voltammetry and scanning electron microscopy. The NAB electrochemical behaviour was dependent on the solution pH, due to the different stabilities of the phases present within its microstructure. In particular, at solution pHs higher than 4.0 the NAB oxidation was driven by the dissolution of the copper-rich ?-phase, whereas at pH values lower than 4.0 its anodic behaviour was controlled by the oxidation of the iron-, nickel- and aluminium-rich ?I-, ?II- and ?IV-phases. Furthermore, a kinetic model for the NAB oxidation, in neutral chloride solutions, was developed on the basis of the observed behaviour, i.e., order of reactions of NAB with respect to protons and chloride. Finally, considering the high affinity of benzotriazole for copper, the corrosion performance of NAB was studied in the presence of the inhibitor in neutral (pH of 6.2) and acidic (pH of 3.5) chloride solutions, where NAB exhibited different anodic behaviours

Electrochemical detection of cupric ions with boron-doped diamond electrode for corrosion monitoring

Corrosion induced structural failures continue to be a costly problem in many industrial situations, and the development of robust corrosion sensing systems for structural health integrity monitoring is still a demanding challenge. The applicability of corrosion monitoring of copper alloys using a boron-doped diamond electrode (BDD) has been performed based on determination of copper ions within localised corrosion microenvironments. The electrochemical behaviour of copper ions on the BDD electrode surface were first reported in details in 0.6 M NaCl aqueous solution, and the results revealed that the electrochemical processes of copper ions on the BDD electrode proceed as two successive single electron transfer steps producing two well-separated pairs of peaks in cyclic voltammograms in the chloride ion containing electrolyte solutions. Compared with perchlorate and sulphate ions, chloride ions were observed with a significant stabilization effect on copper ions via the formation of complex, thus having two well-separated pairs of peaks in the obtained cyclic voltammograms on the BDD electrode in the chloride ion electrolyte solution. The apparent rate constant for the redox couple of Cu(II)/Cu(I) in chloride ion electrolyte was determined as 0.94 × 10–6 cm s–1 by using quasi-steady polarisation technique, thus signifying a quasi-reversible electron transfer process of Cu(II)/Cu(I). Moreover, the differential pulse voltammetric results exhibited the BDD electrode is promising for corrosion monitoring of copper alloys with an excellent relationship between peak current and concentration of copper ions without significant interference from the commonly presented metal ions within the simulated marine corrosion environments

Microfluidic Devices for Structural Health Monitoring and Integrity

Dstl and EPSRC have jointly funded a 3-year project at the University of Southampton to develop corrosion detection and monitoring systems for land, marine and aerospace structures. By combining microfluidics with new, emerging sensor technologies, this project aims to develop in-situ systems that not only detect the onset of structural corrosion, but can also assess its state and apply appropriate remediation to inhibit its progress

Microfluidic Devices for Structural Health Monitoring

Dstl and EPSRC have jointly funded a 3-year project at the University of Southampton to develop corrosion detection and monitoring systems for land, marine and aerospace structures. By combining microfluidics with new, emerging sensor technologies, this project aims to develop in-situ systems that not only detect the onset of structural corrosion, but can also assess its state and apply appropriate remediation to inhibit its progress

Electrochemical oxidation of ammonia (NH4+/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH4+/NH3) in sodium perchlorate was investigated on IrO2 electrodes prepared by two techniques: the thermal decomposition of H2IrCl6 precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(111) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes