Hybrid Sol–gel Coatings for Corrosion Mitigation: A Critical Review

Abstract

The corrosion process is a major source of metallic material degradation, particularly in aggressive environments, such as marine ones. Corrosion progression affects the service life of a given metallic structure, which may end in structural failure, leakage, product loss and environmental pollution linked to large financial costs. According to NACE, the annual cost of corrosion worldwide was estimated, in 2016, to be around 3%-4% of the world's gross domestic product. Therefore, the use of methodologies for corrosion mitigation are extremely important. The approaches used can be passive or active. A passive approach is preventive and may be achieved by emplacing a barrier layer, such as a coating that hinders the contact of the metallic substrate with the aggressive environment. An active approach is generally employed when the corrosion is set in. That seeks to reduce the corrosion rate when the protective barrier is already damaged and the aggressive species (i.e., corrosive agents) are in contact with the metallic substrate. In this case, this is more a remediation methodology than a preventive action, such as the use of coatings. The sol-gel synthesis process, over the past few decades, gained remarkable importance in diverse areas of application. Sol-gel allows the combination of inorganic and organic materials in a single-phase and has led to the development of organic-inorganic hybrid (OIH) coatings for several applications, including for corrosion mitigation. This manuscript succinctly reviews the fundamentals of sol-gel concepts and the parameters that influence the processing techniques. The state-of-the-art of the OIH sol-gel coatings reported in the last few years for corrosion protection, are also assessed. Lastly, a brief perspective on the limitations, standing challenges and future perspectives of the field are critically discussed.Polymers 2020, 12, 689 The development of OIH sol-gel coatings, based on siloxanes, for corrosion mitigation, has been widely studied in the last few years [2,31,[36][37][38]. Numerous studies have been conducted since the early 1990s [39,40]. The studies performed showed that siloxanes were effective in mitigating the corrosion processes of different metallic substrates. This development was mainly due to the need for alternative environmentally friend materials and processes to replace the traditional chromium-based pre-treatments. Hexavalent chromium (Cr (VI)) is carcinogenic and shows high environmental toxicity and its use has been forbidden since 2006. However, Cr(VI) based pre-treatments improve the coating adherence to the substrate and are effective corrosion inhibitors [41][42][43] which make its replacement challenging. The use of the sol-gel method to produce OIH coatings for corrosion mitigation suits the main requirements of what should be an environmentally friendly process (i.e., excludes washing stages; it is a waste-free method) and allows one to obtain coatings with high purity and specific pore volumes and surface areas. Since it is a mild synthesis...

Nanostructured Surface Finishing and Coatings: Functional Properties and Applications

“…Table 1 summarized composite coatings features used to prevent corrosion in steel and metallic materials reported in the literature of the last 10 years. Figuera et al, show the application of siloxane functionalized coatings on different metallic substrates and assert that most of them were used on aluminium and iron-based alloys [156][175] [190]. The influence of the molar ratio of two compounds, generally (3-glycidyloxypropyl)trimethoxysilane (GPTMS) and tetraethylorthosilane (TEOS), on the structure of the molecular net produced by sol-gel technology for the protection corrosion coating were the main objectives of study by several researchers, as reported Alcantara et al [155].…”

Formation of Self-Assembled Anticorrosion Films on Different Metals

“…One of the mentioned methods that can increase the desired anticorrosion properties is the sol–gel technique, especially when it incorporates inhibitors and/or nanoparticles in a hybrid sol–gel system [ 23 , 24 ]. The sol–gel thin films adhere well to metals as well as to topcoats; this is important from the industrial application point of view.…”

Soluble network polymers based on trifluoropropyl‐substituted open‐cage silsesquioxane: Synthesis, properties, and application for surface modifiers

“…Polysilsesquioxanes (PSSQs) are usually prepared via solgel methodology from alkoxysilanes and have been drawn considerable attention from academic and industrial societies due to their excellent thermal, mechanical, optical, and electrical properties. [1][2][3][4] In 1994, reports detailing the preparation of thin films based on PSSQs showed that molecular-scale structures can affect microstructures, resulting in different porosities. 5 A recent study on methylsilsesquioxanes suggested that lower coefficient of thermal expansion and stronger mechanical properties could be achieved by decreasing the content of low-molecular-mass structures.…”

“…Poly(methyl methacrylate) (PMMA, M n 35,000) was obtained from Sigma Aldrich. 3,7,14-Tris(dimethylsilylsiloxy)hepta(3,3,3-trifluoropropyl) tricycle [7,3,3,15,11]heptasiloxane (1(TFP)) was prepared according to a previous report from hepta(3,-3,3-trifluoropropyl)tricycloheptasiloxane trisodium silanolate (7TFP-3Na). 36 3,7,14-Tris(dimethylvinylsilylsiloxy)heptaisobutyltricycle [7,3,3,15,11]heptasiloxane (2(iBu)) and 3,7,14-tris(dimethylvinylsilylsiloxy)heptaphenyltricycle [7,3,3,15,11]heptasiloxane (2(Ph)) were prepared according to a literature method.…”

“…Polysilsesquioxanes (PSSQs) are usually prepared via sol–gel methodology from alkoxysilanes and have been drawn considerable attention from academic and industrial societies due to their excellent thermal, mechanical, optical, and electrical properties 1–4 . In 1994, reports detailing the preparation of thin films based on PSSQs showed that molecular‐scale structures can affect microstructures, resulting in different porosities 5 .…”