Aspects of Preferential Grain Etching During Alkaline Pre-Etching Step Before Anodizing of Aluminum Profiles

摘要

Formation of an undesirable preferential grain attack, i.e. a step-wise grain attack, during etching in sodium-hydroxide-based solutions is caused by either zinc (Zn) in the metal, and/or by presence of zinc in the etching solution. This preferential grain attack is often called a spangled or galvanized finish. It is foreseen that with increasing demand for recycling of aluminum, the amount of some trace elements, such as Zn, will increase. Hydro Aluminium has addressed this challenge, and is currently performing both laboratory and industrial trials in order to look closer into this phenomenon. A lot of parameters are found to have an influence on the final etched surface finish, such as grain size, grain size distribution, surface texture, and etching bath conditions. It is also a challenge to find a fast and reliable method to measure the amount of Zn in the etching bath.