Pitting and breakdown in an efficient way for the AC electrograining of aluminium

Pitting and breakdown are generally negative influences on metal surfaces. However, the surface of certain materials can exhibit interesting properties when corrosion is used as a surface treatment. Indeed, the AC electrograining process uses high current densities to induce controlled corrosion on aluminium surfaces. This is an energy-intensive process used for the production of printing plates for offset printing. Because of the high current densities and frequencies used, the electrochemical response of the system is complex. Three different stages are seen in this response, namely dissolution of second-phase particles, crystallographic attack of the aluminium, and development of the smut film [1]. The graining conditions have a clear influence on the morphology, and on the electrochemical response. However, the influence of the graining conditions on the consumed power has not yet been studied in detail.
In this work, different electrolyte additives are used in a lab setup for electrograining. To simulate the different electrodes that are used in series in the production line, the measurement was broken into different graining intervals and dead times. As parameters, the graining interval time, dead time, and current density are varied. Hydrochloric acid is used as a base electrolyte. The tested additives include sulfate, acetic acid and citric acid. The cell, working electrode and counter electrode potential are monitored, as well as the applied current. Based on these measurements, a detailed energy balance is calculated. In this energy balance the differences between the different graining conditions and electrolyte additives can be determined. These results are then compared with the surface morphology of the grained samples.
Initial results show that increasing the dead time between the graining intervals decreases the power requirements of the process, but has a negative influence on the graining morphology. This could be due to the removal of the smut layer by the electrolyte, or by a change in smut layer composition. Decreasing the current density decreases the power requirements and can still lead to a good graining morphology, but this increases the production time. Different electrolyte additives show a vast difference in electrochemical response as well as a vast variation in the morphology with different settings. It is clear that, to optimise the energy efficiency of the process, the right combination of additives and graining settings has to be found.