Addition of an Eco-Friendly Corrosion Inhibitor in Self-Assembled Monolayers Coatings to Improve Pitting Corrosion Resistance of Stainless Steel

Abstract

In automotive industry and equipment manufacturers, treatment by phosphatation [1] is used both as an adhesion primer before painting and as a lubricant coating [2]. This treatment also offers a good protection against corrosion. However, phosphatation is quite harmfull for the environment, producing a lot of waste to be treated. With regards to toxicity, it is composed of concentrated acids, nitrates and nitrites. Eventually, OEM are working to reduce the thickness of this coating which is typically in the range 2-30 μm. The objective of our research is to propose an alternative to steel/phosphate systems by thin organic coating on 304 stainless steel, a low carbon content substrate free of Molybdene available at affordable costs To fit with the required specification while being as attractive as possible, this alternative coating must present a thickness reduced to a few nanometers, be environmentally friendly, non-toxic and easy to use. It must keep the main properties of phosphatation which consist in adhesion or anti-wear. Self-assembled Monolayers (SAMs) [3] coatings were chosen with addition of Aloe Vera as an ecological corrosion inhibitor [4][5]. SAMs are tunable by adjusting either their terminal group or the length of the carbon chain to obtain the desired surface properties and corrosion resistance. Three different terminal groups combined with four carbon chain lengths have been tested to find the molecule providing the best protection against corrosion. An electrochemical study is carried out to quantify protection against pitting corrosion in a chloride-containing medium. The substrate used being a 304 stainless steel. As pitting corrosion mechanism involves both stable and unstable pits, two types of electrochemical methods are used. Pulsed potentials technic will highlighted unstable pits, while stable pits are analysed by cyclic voltammetry. These measurements are supported by surface analysis (SEM, EDS). [1] T. W. COSLETT, Treatment of iron or steel for preventing oxidation or rusting, British patent 8667 and US patent 870937, 1906 et 1907 [2] J. BOGI, Phosphate conversion coatings on steel, J Mater Sci (1977) 12, 2235-2240 [3] A. ULMAN, Formation and structure of self-assembled monolayers, Chem. Rev. 1996, 96, 1533-1554 [4] Devarayan, Kesavan, Green Inhibitors for Corrosion of Metals: A Review, Chem Sci Rev Letts, (2012) [5] M. MEHDIPOUR, Electrochemical noise investigation of Aloe plant extract as green inhibitor on the corrosion of stainless steel in 1M H2SO4, J. Ind. Eng. Chem, (2015), 21, 318-327 Figure 1