* F. Bellucci, L. De Rosa, D. B. Mitton, T. Monetta Dep. of Materials and Production Engineering ± University of Naples Federico II, P. le Tecchio 80, I-80125 Napoli (Italy) J. Springer Zentrum fu Èr Sonnenenergie und Wasserstoff-Forschung, Baden- Wu Èrttemberg, Hessbru Èhlstrasse 21C, D-70565 Stuttgart (Germany) Degradation of zinc oxide thin films in aqueous environment: Part II ± Coated Films Degradation von du È nnen Zinkoxidfilmen in wa È ssriger Umgebung: Teil II ± Beschichtete Filme L. De Rosa, D. B. Mitton, T. Monetta, F. Bellucci* and J. Springer In Part I of this research, the degradation mechanism of two dif- ferent bare ZnO thin films was assessed. Degradation of the elec- trical properties of ZnO as well as changes in morphology were observed for both films. In the current paper, the degradation of zinc oxide thin films coated with protective acrylic paint is ad- dressed during exposure to (i) an aqueous 3.5% NaCl solution at 85 8C and (ii) a standard damp heat test at 85% R.H. and 85 8C. Electrical and electrochemical techniques were employed to moni- tor zinc oxide degradation during exposure to the test environments. Electrochemical Impedance Spectroscopy was employed to inves- tigate the delamination phenomena at the ZnO/coating interface and a simple equivalent circuit was developed to quantitatively measure the delamination ratio. The effect of different silane based adhesion promoters (glycidil-oxypropyl-trimethoxy-silane and am- minopropyl-trimethoxy-silane) was also investigated. Im Teil I dieser Untersuchung wurden die Degradationsmecha- nismen von zwei verschiedenen unbedeckten Zinkoxidschichten beurteilt. Sowohl eine Verschlechterung der elektrischen Eigen- schaften als auch eine Vera Ènderung der Morphologie wurden beob- achtet. In der vorliegenden Vero Èffentlichung wird die Degradation von ZnO-Filmen behandelt, die mit einer Schutzschicht aus einem Acryllack versehen waren und einer wa Èssrigen 3,5% NaCl-Lo Èsung bei 85 8C einerseits und einem Feuchte-Wa Èrme-Test (85 8C, 85% rel. Feuchte) ausgesetzt wurden. Elektrische und elektrochemische Verfahren wurden angewandt, um die Vera Ènderungen der ZnO- Schicht wa Èhrend der Exposition aufzuzeichnen. Mit Electrochemi- scher Impedanzspektroskopie wurden die Delaminationserschei- nungen an der ZnO/Lack Grenzfla Èche untersucht und ein einfaches Ersatzschaltbild wurde entworfen, um die Delaminationsgeschwin- digkeit quantitativ beschreiben zu ko Ènnen. Der Einfluss verschie- dener Silanhaftvermittler (Glycidil-Oxypropyl-Trimethoxy-Silan und Amminopropyl-Trimethoxy-Silan) wurde ebenfalls ermittelt. 1 Introduction As reported in Part I of this work [1], due to the interaction between water and zinc oxide crystals, rapid degradation can result when bare ZnO thin films are exposed to aggressive aqueous environments. The current paper addresses the effec- tiveness of transparent organic coatings as a means of redu- cing the degradation rate of zinc oxide in such environments. Organic coatings isolate the substrate from direct contact with the environment and reactants (oxygen, water and electrolyte) then arrive at the interface by permeating through the organic coating. Degradation is, thus, related to the rate of arrival of reactants diffusing through the organic layer to the interface as well as to adhesion at the polymer/substrate interface. In the case of low barrier coatings the diffusion rate of water and oxygen is 10 to 100 times higher than the rate of consumption at the corroding surface. Therefore electrochemical reactions are the limiting steps for the corrosion mechanism (reaction controlled corrosion) [2, 3]. In the case of high barrier coat- ings, however, it has been demonstrated that the diffusion of water and oxygen through the polymer is the limiting step of the corrosion mechanism (diffusion controlled corro- sion) [2, 3]. During the exposure to wet environment, how- ever, high barrier coatings can loose their barrier action due to the formation of aging-induced pores in the polymer [2]. In this case a diffusion controlled corrosion shifts to a re- action controlled corrosion during the time of exposure. On the opposite some high barrier coatings were found to main- tain their barrier actions for prolonged time of exposure to a wet environment. Even significant corrosion can be detected at the polymer /substrate interface (reference). In this latter case the interfacial adhesion between the polymer and the me- tal plays a main role in the corrosion process (delamination controlled corrosion) [4]. In order to reduce the rate of corrosion in a delamination controlled phenomenon, the adhesive bonds between the coat- ing and the substrate can be enhanced by the addition of silane coupling agents to the coating formulation. Silanes are sili- con-organic molecules with a central Si atom covalently bonded to one organic group and to three hydroxyl groups. Materials and Corrosion 52, 931±935 (2001) Degradation of zinc oxide thin films 931 Ó WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 0947-5117/01/1212-0931$17.50.50/0