Influence of microstructure on electrograining behaviour of commercial purity aluminiumused for lithographic printing plates

Abstract

High quality lithographic printing plates are made from electrolytically roughened aluminium sheets. A nitric or hydrochloric acid based electrolyte is normally used for the electro graining, and the substrate is most commonly commercial purity (CP) aluminium. Little is known about the relationship between the substrate microstructure and the pitting behaviour during electrograining in nitric acid solutions. The present work used three techniques to study the initiation and growth of surface pits responsible for the surface roughening of CP aluminium. A laboratory microcell simulated commercial graining but with the ability to interrupt the process and to rapidly clean and preserve the partially grained surface. Detailed scanning and transmission electron microscopy has been used to link substrate microstructure to pit initiation and growth morphology. The results demonstrate that the cold worked dislocation structure is an important feature whereas examination of simple binary alloys illustrates the relative roles of solute in solid solution and coarse particles. The present work suggests that the barrier oxide is very important and it has been postulated that chemistry or defect structure modifications to the film control the process of pit site selection.

MST/3137