Petri Konttinena, Teppo Salob, Peter Lund3 aAdvanced Energy Systems Laboratory, P. O. Box 2200, Helsinki University of Technology, FIN-02015 HUT, Finland

bLaboratory of Corrosion and Material Chemistry, P. O. Box 6200, Helsinki University of Technology, FIN-02015 HUT, Finland

Introduction

Evaluating and testing of acid rain induced corrosion on non-glazed solar absorber surfaces is to a large extent an unexplored problem. This paper communicates the first methods used for acid rain tests and results for a rough graphite-alumina-aluminium (C/Al2O3/Al) absorber surface. The surface consists of a carbon and alumina containing heterogeneous matrix structure on an aluminium substrate sheet (Konttinen et al., 2003a, Konttinen et al., 2003b). A relatively low-cost total immersion acid rainwater test method was developed with rate and type of aeration (non, O2 or N2), temperature (60/80/99°C) and pH levels (3.5/4.5/5.5) as parameters. The test method was mainly based on recommendations in (Shreir et al, 1994). Absorber samples were analysed by UV-Vis.-NIR and FTIR spectroscopy. Electrochemical properties were investigated by electrochemical impedance spectroscopy (EIS) and polarisation curves. pH levels chosen for the tests were based on the global acid rain measurements (Howells, 1990). Metal corrosion studies in simulated acid rain (Magaino, 1997, Magaino, 1999) and corrosion studies of carbon fiber (graphite) — aluminium metal-matrix composites (Hihara, 1997, Hihara and Latanision, 1994) were used as first guidelines for designing the tests and analysing the corrosion behaviour of C/Al2O3/Al solar surfaces in simulated acid rain. In standard condensation tests for glazed absorbers according to a draft proposal ISO/CD 12592,2 (Brunold et al, 2000) the main degradation mechanism of the C/Al2O3/Al solar surfaces has been found to be hydration of aluminium oxide (Konttinen and Lund, 2003).