V. Ashworth’s research while affiliated with University of Manchester and other places

The split ring-disc electrode technique has been used to investigate the formation of Cu2O upon copper metal in 1 M sodium hydroxide solution at 30°C. Particular attention has been paid to the initial stages of film formation prior to the formation of bulk Cu2O. The results indicate that the initial step in the oxidation of copper in alkaline solutions is simultaneous formation of discrete nuclei of Cu(I) surface species on the electrode and a soluble Cu(I) species which is generated directly from the copper metal. This is followed by the growth and coalescence of the Cu(I) surface nuclei and formation of the initial monolayers, which progressively block off the dissolution sites on the electrode surface. The flux of the soluble Cu(I) species to the half rings therefore never reaches the theoretical value predicted for a free dissolution process.

The premature failure of a Zn–0·3 Al–0·03Cd anode at 70°C in sea bed mud has been simulated in laboratory sea water using galvanostatic polarisation. The exposed surface of the anode suffered intense intergranular attack and some dissolution. Within the bulk of the material intergranular attack was observed, but no dissolution. Unpolarised alloys in a variety of environments exhibited the same type of attack; however, it could not be produced on pure zinc. No evidence could be found of segregation or precipitation of aluminium at grain boundaries. Moreover, specimens that had been solution treated to ensure a single phase microstructure suffered from intergranular attack in short term simulation tests. It is concluded that a previously proposed mechanism consisting of grain boundary precipitation of aluminium followed by its preferential dissolution is incorrect. It is proposed from analysis of the fracture morphology and the effect of test conditions that the failure is caused by hydrogen penetration.

The corrosion mechanism of steel in concrete and the so-called threshold chloride content are discussed based on the results obtained by electrochemical measurement, pore solution analysis, and scanning electron microscopy (SEM) observation. It is shown that mortar (or concrete) provides better protection to steel than alkaline solutions. One of the protective mechanisms provided by mortar is found to be the pH control action of calcium hydroxide crystals located at the steel-mortar interface. For the protective mechanisms provided by mortar to operate, adhesion between the steel and the mortar is necessary and formation of voids at the steel-mortar interface is essential for active corrosion to start. The critical threshold chloride content is not determined simply by a parameter of the pore solution such as [Cl−]/[OH−] ratio; it is shown that the threshold value depends on the steel-mortar interface conditions.

A nickel rotating disc electrode is used to study the oxygen reduction under isothermal conditions and with heat transfer.

The design features and the details of construction of a split ring—disc electrode with internal heat transfer facilities are described. The system allows the application of the ring—disc electrode, operating with well-defined hydrodynamics, to be extended to the investigation of electrochemical and corrosion processes under controlled heat transfer conditions. Preliminary data reveal that for Cu2O formation on Cu in alkaline electrolytes the overall kinetics are independent of heat transfer for a constant disc surface temperature. However, the measured contribution of dissolved Cu(I) species, assessed by the half ring currents, decreases in the presence of a heat flux which is due to the surface concentration and diffusion coefficient of these ions being controlled by the mean temperature in the boundary layer.

A rotating disc electrode allows the study of the effect of controlled mass transfer on the rates of suitable electrochemical and corrosion processes. In practice many corrosion situations involve the added interaction of a heat flux. The construction of a rotating disc electrode with integral controlled heat transfer facilities is described and the technique for determination of the interfacial temperature is shown. Such an assembly allows control of heat and mass transfer, is relatively easy to set up and enables a large throughput of specimens consistent with the needs of an integrated study of the effects of heat and mass transfer on electrochemical and corrosion processes.

The oxygen reduction reaction on a nickel rotating disc electrode has been examined under isothermal conditions and with heat transfer. The results show that although the limiting rate of oxygen mass transfer increases with an increase in the bulk temperature under isothermal conditions, the rate of charge transfer does not increase correspondingly. It is suggested that as the temperature is increased the oxygen reduction reaction on nickel moves from a predominantly 4 electron process to a predominantly 2 electron process. The effect of heat flux is to stimulate the limiting rate of mass transfer. This effect is attributed to the creation of thermal convection by the production of thermal eddies rather than to an increase in the interfacial temperature. It is shown that, notwithstanding the change from a4 to a2 electron process, as the temperature is raised, the effect of heat flux also gives rise to an increase in charge transfer in the oxygen reduction reaction on nickel since the thermal eddies increase mass transfer faster than the change in interfacial temperature decreases z. It is suggested that similar mass transfer effects produced by heat transfer will occur on other metals but that the charge transfer effect associated with oxygen reduction will depend on the mechanism of the reaction on the particular metal.

Pure iron sheets, following immersion in artificial city water containing chromate ions as inhibitor, were analyzed by the angle-resolved XPS technique. The thicknesses of the surface film and containment hydrocarbons layer on the specimens when estimated using a flat, multi-layered surface model were apparently dependent on the take-off angle of the photoelectrons. The origin of this apparent dependence of thickness is attributed to surface roughness rather than the porosity of the surface film. The compositions obtained using a flat surface model were not affected by a roughness correction procedure. However, the composition is dependent on the take-off angle as the result of a non-uniform in-depth distribution of constituents.

The developed rotating disc electrode (RDE) assembly comprises an RDE, an electrode shaft coupled to the driving shaft, a bearing and slip ring unit, a driving unit and two auxiliary units for supplying and detecting the heat flux (a detailed description of the electrode arrangement is given).

By conversion coating of aluminium, initially supporting electropolished or barrier-type anodic films, performed in the so-called chromate and chromate-phosphate baths, the original formed alumina is replaced by a thicker chromium-containing material.