Effect of cerium concentration on corrosion resistance and polymerization of hybrid sol–gel coating on martensitic stainless steel

Stainless steels are increasingly used in the aeronautics field for the manufacture of structural parts. One of them, the X13VD martensitic stainless steel (X12CrNiMoV12-3), known for its good mechanical properties, has a poor corrosion resistance in confined or severe environments. In the past years, Cr(VI) based pre-treatments have been currently used for corrosion protection of different metals, however, they are toxic and due to environmental regulations, they will be definitely banned in a near future. Alternatives to replace Cr(VI) show advantages and drawbacks considering key properties such as: corrosion resistance, adhesion of coatings, fatigue resistance, durability and reliability. However, some of their possible alternatives show high potential.In this paper, a process was developed to improve the corrosion resistance of the martensitic stainless steel. Organic–inorganic hybrid coatings with different cerium concentrations were deposited onto stainless steel by sol–gel process. Corrosion resistance of the coatings was evaluated by electrochemical impedance measurements and it has been proved that cerium concentration of 0.01 M into hybrid coating was an optimal content. Adhesion tests were also carried out by “nanoscratchtest” to characterize the coatings mechanical properties as a function of cerium concentration but results do not clearly show the influence of cerium for the coating adhesion toward the substrate. To try to correlate with the electrochemical properties, liquid 29Si NMR spectroscopy was then performed to investigate hydrolysis and condensation reactions of sol–gel process, and by this method, it was demonstrated that for higher cerium concentration (>0.01 M) there is a modification of the chemical structure of the sol–gel network.

► New sol–gel routes to replace chromates.
► Effect of cerium concentration on the microstructure of sols.
► Electrochemical performances of hybrid coating with different cerium contents.
► Correlation between the different results with an optimal cerium content of 0.01 M.