A study of the anodic behaviour of aluminium alloys in alkaline electrolytes
Recent studies an the discharge performance of aluminium alloys in alkaline media have led to improved alloys with significantly lower corrosion rates and more anodic potentials. Performance, of various alkaline electrolytes have also been examined and considerable progress has been made in this area. A review of the available literature reveals a list of several elements which are suitable for alloying with aluminium as regards reducing corrosion and overpotential. Previous work at the Chemical Energy Research Centre, City University, showed that with an aluminium alloy Q4, which contained additives of bismuth, lead, magnesium, titanium and zinc, improved characteristics were obtained on comparison to the performance of bulk manufactured aluminium. Furthermore, on discharging the alloy in a novel electrolyte developed at City, which consisted of, a 1:1 mixture of 50% (w%v) sodium hydroxide and 30% (w/v) potassium hydroxide, improvements in performance were observed when compared to the results obtained from discharge in the traditionally used 30-40% (w/v) potassium hydroxide. Firstly, in full cell discharge ( where the aluminium anode was coupled with an oxygen reduction cathode the operating life of the cell was considerably longer (up to as much as 50%). Secondly, and probably-more important, was the fact that the power output was also significantly greater. As for reducing hydrogen evolution, several alloying elements had this effect. However, work done showed that the most effective means of minimising corrosion was to use electrolyte additives. The most suitable electrolyte additive was found to be mercuric oxide. The presence of mercury as an electrolyte additive has a distinct advantage over alloying the metal with aluminium. In the case of the latter, the amalgam formed results in the oxide layer being broken down and the alloy becomes unstable in the atmosphere. Saturation of the electrolyte with mercuric oxide poses no such problems. Also as the solubility of mercury in alkaline solution is so small (ca. 5x 10-7wt% in 5M potassium hydroxide) there is no significant toxicity problem. Four commercially available aluminium alloys of varying compositions were obtained from aluminium companies involved in the research and development of aluminium air batteries. It was decided to extend the examination of the mixed alkaline electrolyte to these alloys and study the performances using various electrochemical techniques. The role of mercuric oxide on the discharge process was also further investigated. Study of the anodic behaviour of the aluminium alloys was achieved using the traditional electrochemical half cell tests employing various alkaline electrolytes. Half cell -tests indicated that the most negative potential and the highest limiting current density values were actually obtained using potassium hydroxide electrolyte. However, this tended to be overshadowed in the full cell tests by the considerably longer operating life of the cells employing the mixed electrolyte. The overall results indicate the superior performance of alloys containing significant quantities of tin and titanium. Also, the full cell performance of the alloy New Q4, containing the same additives as Q4 but in different quantities, was also extremely attractive. These elements are believed to form a layer on the electrode surface during discharge, thereby controlling the dissolution of the aluminium and preventing excessive hydrogen evolution and cause an anodic shift in potential. Evidence of the formation of these layers on the electrode surface was not easily obtained. EDAX mapping of the electrode surface revealed some interesting micrographs in which the quantities of some these elements on the electrode surface appeared to increase after discharge. This evidence was inconclusive however, as the element to aluminium ratio was too small for any value to be recorded. This did suggest though that certain, elements like tin and titanium did form a layer on the surface. It was then decided to carry out ESCA work on the electrode surfaces both prior to and after discharge. ESCA did reveal that certain elements were present on the electrode surface after discharge, and thus suggests that the formation of a layer of an element on the electrode surface during discharge is likely. Some preliminary ellipsometry experiments were performed to study the effect of the various alloying elements on the oxide film present on the aluminum surface. Initial experiments revealed interesting information in that the alloys behaved differently. It seems that the surface structure is extremely sensitive to the composition of the alloy. Further work in this area could lead to more information about the oxide film and the most suitable alloying elements for rapid breakdown of the layer on commencing discharge. The various electrolytes used for the experiments resulted in a different reaction product structure on precipitation of aluminium hydroxide. With potassium hydroxide electrolyte a fine white precipitate is obtained which completely smothers the electrode surface. With the electrolyte mixture, a granular precipitate is obtained which may allow the diffusion of ions to and from the electrode surface after passivation has occurred. This coupled with the greater solubility that the electrolyte mixture has for aluminate, results in longer operating life of the full cell system. Another factor is that the granular, electrolyte is easily removed from the cell by washing, unlike the precipitate from use of potassium hydroxide. This enables the actual cells to be used for longer periods as the active sites on the cathode are not covered by this fine white precipitate. Also the stability of the mixed electrolyte after passivation is of benefit as complete precipitation from the electrolyte does not occur. With potassium hydroxide electrolyte, complete precipitation occurs as soon as passivation commences. Use of mixed electrolyte in full cell tests always resulted in longer operating life for every alloy. However, only two of the four alloys under test showed a significant. -power output increase. It must be noted though that the build up of precipitate during the operation of the cell affects the performance. Use of a- pump enabling the electrolyte to circulate through the cell and an external reservoir would be expected to reveal the effect of the higher solubility of the mixed electrolyte to a greater extent. The alloys which did show greater power output from use of the mixed electrolyte were numbers 2 and 3, the former being a low corrosion alloy and the latter containing significant proportions of tin and titanium. However, the stability and reaction product structure make it desirable for all alloys in aluminium air cells to be discharged in a mixed electrolyte of sodium and potassium hydroxide rather than the traditionally used potassium hydroxide. Finally the effect of adding mercuric oxide to the electrolyte was examined using a novel electrode microzone scanning current measurement technique. This technique allowed the in situ examination of the current distribution on the electrode surface to be measured. Conclusive new evidence was found regarding the effect of mercury on corrosion current, current distribution and the superior characteristics of the material as an electrolyte additive rather than an alloying element.