Addition of Aloe Vera As Corrosion Inhibitor in Self-Assembled Monolayers Systems Designed to Improve Adhesion on Stainless Steel
Authors: Rougier Agathe , Melot Jean Marie , Aufray Maëlenn , Doche Marie-Laure , Touyeras Francis , Roizard Xavier , Lallemand Fabrice , Hihn Jean-Yves 
Published: December 2020

Surface treatment by phosphatation is widely used in automotive industry and equipment manufacturing [1] because it’s an excellent adhesion primer before painting and due to its lubrication properties with low friction coefficient [2]. It is also known that this treatment offers a good protection against corrosion with a good suitability to be painted. Nevertheless, this process is detrimental for the environment, mostly because of the large amount of waste to be treated, with the presence of concentrated acids, nitrates and nitrites. Moreover, the required thickness is in the range of 2-30 μm, depending of the specifications, and there is a great interest to reduce this value, especially for the fasteners parts.
The present work deals with an alternative to steel/phosphate systems by thin organic coating on 304 stainless steel, a low carbon content substrate free of Molybdenum available at affordable costs. To fit with the specification while being as attractive as possible, this alternative would be asked to present a thickness reduced to a few nanometres, environmentally compliant, non-toxic and easy to use. It must keep the main properties of phosphatation i.e. a good adherence or anti-wear functions.
A novel process was design, with Self-assembled Monolayers (SAMs) [3] coatings in a first step, followed by a functionalization with Aloe Vera as 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 such as adhesion or anti-wear. If the addition of Aloe Vera does not bother the tribological behavior of the coating, this is not the case for the systems designed for adherence.
To obtain the best compromise, three different terminal groups combined with four carbon chain lengths have been tested. The latest were measured by a single lap joint and three-point bending tests as for result the value of the strength of the bonded joint as a function of the treatments performed. Adhesion results will be presented for different SAM coatings on 304 stainless steel, with a possible application of epoxy layer (polymer used for cataphoresis). The influence of the SAM alkyl chains length and of the terminal groups nature will be checked, as well as the grafting time.
Finally, the effect of post-graft rinsing on adhesion as well as corrosion resistance in presence of Aloe Vera or not were evaluated. All findings were 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