REVIEW Influence of sol–gel application conditions on metallic substrate for optical applications M.-E. Druart* 1 , J.-B. Richir 2 , C. Poirier 2 , F. Maseri 2 , N. Godeau 3 , L. Langer 3 and M. Olivier 1 An efficient energy management of a building requires optimum use of the light energy, which is strongly dependent on optical properties of used materials. In the lighting sector, aluminium is generally employed as substrate for the reflectors. Nowadays, new steel substrates combining good corrosion resistance and flexibility are developed to answer the growing need of this market. Specific requirements for optical applications are a high reflectivity (total reflectivity .82%), a high superficial hardness and durability and also a suitable formability. The sol–gel layer is used in this particular application for its levelling properties before depositing of the reflective layer and good barrier properties to avoid contact between the metallic layers. The corrosion protection and the barrier properties of the sol–gel layer are investigated as a function of the thickness. The protection conferred by the sol–gel layer on stainless steel substrate is studied by the electrochemical measurements in a sodium chloride solution (electrochemical impedance spectroscopy and polarisation curves). The stress generated in the layer is determined versus temperature, humidity and hygrothermal conditions. Keywords: Sol–gel, Stainless steel, Electrochemical measurements Introduction An efficient energy management of a building requires optimum use of the light energy, which is strongly dependent on optical properties of used materials. In the lighting sector, aluminium is generally employed as substrate for the reflectors. Nowadays, new steel substrates combining good corrosion resistance and flexibility are developed to answer the growing need of this market. Specific requirements for optical applica- tions are a high reflectivity (total reflectivity .82%), a high superficial hardness, durability and also a suitable formability. The corrosion behaviour of sol–gel coating deposited on metallic substrate was strongly studied during the recent years 1–19 and particularly on stainless steel. 1–7 The classical sol–gel principle is the creation of a solid oxide network by progressive reactions (hydrolysis and condensation) of a molecular precursor in a liquid medium. 19–21 Indeed, sol–gel coatings are formed from a colloid by evaporation of the liquid phase. Sol–gel based on silica precursor has very good chemical stability and provides an effective protection for steel substrate. An improvement in oxidation and acidic corrosion resistance of metals was shown by the use of silica sol– gel coating. 3,6 This is due to its high thermal and chemical resistances. Inorganic oxide coatings are very effective against corrosion for short time, but the presence of micropore cracks and areas with low cross- link density limits the long term corrosion resistance. A lot of studies are focused on the drying of sol–gel process and on the solvent evaporation step to avoid cracking and to control the pore structure. 22–31 The main reason of this cracking or porosity is linked to the capillary pressure within the gel during drying. Indeed, some stress caused by the existence of a meniscus at the liquid/vapour interface appears and generates a differ- ential capillary pressure. Then, the sol–gel network shrinks until it becomes stiff and resists to the stress imposed by capillary pressure. After that, the interface between the liquid and vapour withdraws itself into the gel structure and disappears when the drying is completed. Damages are the result of the pressure gradient in the network. The biggest pressure gradient appears with the highest evaporation rates, and there- fore, the first solution to avoid any cracking is to dry slowly the coating. To suppress the liquid/vapour interface, some scientists treat gel in autoclave under supercritical conditions. 25 Another approach is to use some drying control chemical additives; the low surface tension of this additives reduces the capillary tension. 23,24 One approach to obtain crack free and dense sol–gel coating is to synthesise organic–inorganic hybrid coat- ings. A lot of workers try to form hybrid sol–gel coatings 1 Universite ´ de Mons (UMONS), Faculte ´ Polytechnique, 20, Place du Parc, 7000 Mons, Belgium 2 ArcelorMittal Lie `ge Research and Development, B57 Boulevard de Colonster, 4000 Lie ` ge, Belgium 3 Nanoxid, Rue des Alouettes, 1, 4042 Liers, Belgium *Corresponding author, email marie-eve.druart@umons.ac.be ß 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 22 February 2010; accepted 4 May 2010 DOI 10.1179/147842210X12732285051357 Corrosion Engineering, Science and Technology 2011 VOL 46 NO 6 677