Corrosion-protective coatings based on fluorocarbosilane

Highlights

• Fluorocarbosiloxane coating inhibits the corrosion process of 304 stainless steel.

• MetalOSi covalent bonds are created between steel surface and fluorocarbosilane.

• Well-developed chains consisting of SiOSi bonds have been created.

Abstract

This study discusses the effect of 3-(1,1,2,2,3,3,4,4-octafluoropentyloxy)propyltriethoxysilane (OFTES) based coatings on the anti-corrosive properties of 304 type stainless steel, which has not been described to date. The coatings were deposited from methanol using the anhydrous deposition method and influence of the sonication on this process has also been investigated. Anti-corrosive properties of modified and unmodified steel surfaces have been characterized by electrochemical and surface analysis methods. Since silicon is an element capable of forming bonds with both organic and inorganic substrates, these features allow to combine organofunctional silanes with metal as well as steel surfaces through metalOSi covalent bonds. A fluorinated alkyl chain, which is attached to the silicon atom, forms an effective barrier against an aggressive aqueous environment.

Introduction

The use of silanes for protection of metals against corrosion has been foreseen a long time ago, however an intensive research on this aspect has been carried out only within the past few years [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. This is mostly a response to the request to seek an alternative to conventional chromating processes in metal-finishing industries. The growing chromate use restrictions has led to numerous studies focused on developing environmentally friendly solutions for metal protection [[10], [11], [12], [13], [14]]. Silane surface treatments provide not only corrosion protection but also paint adhesion to a broad range of metals [15,16]. Steel and stainless steel are widely used in different industrial applications due to their mechanical and anti-corrosive properties. However, they still tend to corrode in the presence of halide ions. The corrosion resistance behavior of sol–gel coatings or thin films deposited onto the steel substrate has been intensively studied in the past [2,6,7,9,[17], [18], [19], [20], [21], [22]]. Hydrolysis and condensation reactions may occur in typical sol-gel solutions [[23], [24], [25], [26], [27], [28]]. The former can lead to the formation of SiOH groups with water present in the solvent. The subsequent two types of condensation reactions between SiOH and SiOCH₂CH₃ moieties create a well-developed, cross-linked network consisting of SiOSi covalent bonds. Thus, the obtained gel is deposited on the modified surface and covalent bonds are formed. Alkoxysilanes can modify the hydroxylated surface under anhydrous conditions to also produce a thin monolayer coating [29,30]. Only methoxysilanes are effective in this kind of silanization [31] and SiOCH₃ group can react only with a hydroxyl group presented on the modified surface. Next, the trace water molecules (e.g. from moisture during drying and aging processes) can hydrolyze the remaining SiOCH₃ groups to form the siloxane bonds.

The use of sonication in synthesis and modification of functional materials is well-established [32,33]. It is through the chemical and physical effects of acoustically induced microbubble formation and collapse (acoustic cavitation), creating extreme localized heating and violent fluid flow effects, that ultrasound attains its great versatility and applicability in material, environmental and medicinal science. It has been reported as an efficacious tool in the synthesis of metal, polymeric and composite nanomaterials [34,35] also used in sol-gel processes [36,37]. Recent studies have exploited high intensity ultrasound in the pretreatment of metal surfaces to produce novel, functionalized materials, for example, surfaces with anticorrosive or superhydrophobic properties [[37], [38], [39]].

Organosilicon derivatives containing fluorine are of interest mainly because of their application in the production of modern materials. Fluoroalkylsilanes are used as surfactants, for surface modification of lenses and optical fibers, as components of many cosmetic preparations and as modifiers of fluorine and silicon rubber. Especially interesting branch of their application is production of oil-, dirt- and water-repellent surfaces [[40], [41], [42], [43], [44]]. The unique properties of perfluorinated silicon compounds, especially the lower surface energy, which results from the presence of fluoroalkyl groups [45], is the most considered property in corrosion phenomena studies. Unfortunately, due to the limited availability of starting materials as well as complicated synthesis of such compounds, use of fluoroalkylsilanes for protective coatings on the metals surface is limited [9].

In this study, we would like to present the anti-corrosive properties of fluorocarbosilane (3-(1,1,2,2,3,3,4,4-octafluoropentyloxy)propyltriethoxysilane) coatings deposited on the surface of 304 stainless steel, which were not described to date. Recent work of our laboratory group allowed to develop two-step synthetic protocols and synthesize a group of fluorinated derivatives of organofunctional silanes [46]. This work studies the influence of sonication on coatings deposition. The anti-corrosive properties of deposited coatings were examined using different electrochemical and surface analysis methods.