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Aspects of Preferential Grain Etching During Alkaline Pre-Etching Step

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

Øystein Bauger, Hydro Aluminium Metal Markets, Sunndalsøra, Norway, and
Hans Bjerkaas, Hydro Aluminium, Oslo, Norway

Track: MANAGEMENT ISSUES - Sustainability and Recycling

ABSTRACT—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.

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