Cerium Addition to Improve Both Chemical Structure and Corrosion Behaviour of a Sol-Gel Coating on Martensitic Stainless Steel

Stainless steels are highly used in the aeronautics field for the manufacture of structural parts. One of them, the CX13VDW martensitic stainless steel (X12CrNiMoV12-3), known for its good mechanical properties, has a poor corrosion resistance particularly in confined or severe environments. In the last decade, Cr(VI) based pre-treatments have been currently used for corrosion protection of different metals or alloys. However, these compounds are toxic and due to environmental regulations, will be definitely prohibited in a near future. Alternatives to replace Cr(VI) are studied and show advantages and drawbacks when sets of key properties have to be considered such as: corrosion resistance, adhesion of organic coatings, fatigue resistance, reliability and quality control. However, some of these alternatives are very promising.

In this paper, a process was developed to improve the corrosion resistance of the martensitic stainless steel. Organic-inorganic hybrid silane-alumina coatings were deposited onto stainless steel by sol–gel route using a dipping technique. Different amounts of Ce(NO₃) (0 to 0.1 M) have been added in the matrix as corrosion inhibitor.  Corrosion and mechanical properties of the coatings were evaluated by respectively electrochemical impedance spectroscopy and nanoindentation. The consequence is that a cerium concentration of 0.01 M into hybrid coating found to be optimal.

The effects of cerium concentration on the structural features of hybrid films were also studied by ²⁹Si, ²⁷Al and ¹³C nuclear magnetic resonance (MAS NMR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. It was shown a real influence of the inhibitor concentration towards the chemical structure of hybrid and therefore a possible correlation with the sol-gel coating anti-corrosion performance. In this paper, a focus is dedicated to explain why the cerium concentration of 0.01 M in the sol-gel matrix is optimal.