Effects of vacuum and vacuum-ultraviolet radiation on sulfuric acid anodized aluminum oxide coatings

Abstract

Porous aluminum oxide coatings processed by sulfuric acid anodizing (SAA) of aluminum have been of interest in recent years for use as thermal control surfaces in space applications and see use in other applications as protective layers against Al corrosion. For space applications, porous Al oxide coatings must be able to withstand the effects of atomic oxygen, the interaction with contaminants, electron bombardment, and vacuum-ultraviolet (VUV) irradiation. This study focuses on the effects of vacuum and VUV exposure on SAA aluminum. Porous anodic coatings exposed to VUV exhibit optical property degradation, i.e. an increase in solar absorptance ({alpha}) without commensurate increase in infrared emittance ({epsilon}). In this study, samples were vacuum/VUV-exposed and optically measured; their composition, structure and morphology were investigated using electron probe micro-analysis (EPMA), x-ray diffraction (XRD) and optical microscopy. The optical property degradation, associated with water loss and cracking, was observed on the sealed coatings exposed to either vacuum or VUV radiation.