Impact of the addition of cerium salts (Ce(III) and Ce(IV)) on formation and ageing of a silica sol-gel layer

The doping of sol-gel coatings with inhibitive species and more particularly with cerium salts provides a promising potential to replace hexavalent chromium treatments used in aeronautical industry. This study aims at evaluating the impact of cerium salts (cerium chloride and cerium ammonium nitrate) on the film formation and the ageing of the coating properties during immersion in aggressive solution or humidity fluctuations in atmospheric conditions. The effect of cerium oxidation state (Ce3 + or Ce4 +) and concentration in cerium salts (500 and 1000 ppm of cerium) was studied after incorporation in a sol-gel solution composed of TEOS (tetraethoxysilane) and GPMTS (3-glycidyloxypropyl)trimethoxysilane. The stability of sol-gel solutions was determined using rheological measurements. After application by dip-coating on 1050 aluminium alloy, barrier properties of the films were evaluated by Electrochemical Impedance Spectroscopy (EIS) versus immersion time in 0.1 M NaCl solution. The internal stresses generated inside the films were measured using cantilever methods during humidity and hygrothermal cycles. The cerium salts do not change the viscosity, the stability of the solutions and the film thickness. The film resistance decreases more strongly in presence of cerium salts during immersion. Films containing tetravalent cerium salts are more sensitive to humidity fluctuations in atmospheric conditions.