Full length articleChanges in protective properties of zirconia and silica sol-gel layers over time
Graphical abstract
Introduction
Low carbon steels are widely used in the petroleum, natural gas, and chemical industries, due to their low cost and excellent material properties such as good formability and the ability to operate under high temperatures and pressure [1]. However, P265GH low carbon steel, as well as other non-alloy steels is characterized by low corrosion resistance. To mitigate corrosion attack, metallic surfaces are often coated with coating materials. However, before the coating process, specific treatment of the surface is required. Particular attention is required for industrial pickling and cleaning, with the use of HCl or H2SO4, for example [2], [3]. Cleaning mixtures, usually water-based, contain numerous chloride salts and constitute an additional corrosion-forming factor [3]. What is more, the environment to which the metallic material is exposed (such as oil and gas, microflora, or concrete) or in which it is stored, can be highly degradative [4], [5].
However, materials science offers many solutions in the field of corrosion protection, particularly in terms of novel protective coatings, amounting to 89 % of the cost associated with anti-corrosion protection [6]. Up to 2006, the most effective and common method of protecting metal products against corrosion was coating them with Cr (VI) based coatings. However, due to carcinogenicity and environmental toxicity, these materials were withdrawn from use under EU regulations which came in at this time [7].
Materials synthesized via the sol-gel method meet all the requirements imposed on anti-corrosion materials: they are environmentally friendly, non-toxic to humans, show high adhesion to the metallic substrate [8], [9], adhesion is higher compared to gradient oxide layers on metallic substrate obtained by laser radiation [10]), and they effectively protect metallic substrates [9], [11], [12], [13], which makes their applicability extremely wide. Thanks to the possibility of modifying their chemical, structural, and functional properties at the molecular level [14] and the possibility of obtaining materials with the ability to self-heal [8], [15], [16], sol-gel based surfaces are materials with great potential, both in research and industrial application. According to the market forecast for 2020–2024, presented in the report prepared by the Technavo portal [17], the market for sol-gel coatings will grow by over $ 2.6 billion during this time period. Another estimate shows that the coatings market will grow at an annual rate of around 11.20 % over the 7-year period for 2021–2028, as reported by Data Bridge Market Research [18].
Coatings obtained with the sol-gel method are produced as a result of hydrolysis and condensation reactions of metal/semi-metal alkoxides [8], [14]. The current trend related to the production of sol-gel corrosion inhibiting materials is based on the replacement of inorganic precursors with non-hydrolyzing organic substituents [8], [9], [19], [20], to obtain materials with defined hydrophobicity [9], [21], and more dense and flexible oxide networks [22]. Additionally, chemical functionalisation can be used as a tool to increase the adhesion of sol-gel coatings to another type of coating material, such as, epoxy resins [22]. The properties of the final products obtained using the sol-gel method are influenced by many factors, including the deposition technique and the deposition parameters themselves [23], [24], processing temperature [19], [21], the type and concentration of the catalyst used [9], [25], the solvent [9], [26], the type and molar ratios of the precursors used [9], and the aging time of the sol-gel matrix [27], [28], [29].
Many studies refer to the excellent protective properties of sol-gel materials, which is mainly related to their high stability [8], [9], [19], [27], [30], [31], [32]. Venkateswara Rao et al. [9] analyzed the anticorrosive properties of sol-gel coatings which retained their protective properties in a 50 % HCl solution for 100 h, and 90 days in an environment with 95 % relative humidity, at a temperature of 350 °C. Calado et al. [30] describe the effect of active additives, such as CeO2, on the protective properties of the produced coatings as a function of time, in 0.05 M NaCl. Sol-gel coatings without CeO2 additives were stable for the first 8 days of exposure to 0.05 M NaCl, after which they lost their protective properties around day fourteen. The addition of CeO2 extended the protective properties of the sol-gel coating to 29 days [30]. Other researchers also reported a similar phenomenon [31], [33], where the analyzed sol-gel films displayed barrier properties during the first 8 days of exposure to a NaCl solution, which was subsequently lost. The loss of the protective character of sol-gel films is mainly related to their porosity and, therefore, there has been a focus on ways to extend the protective timeframe of sol-gel materials. This goal has been implemented, among other things, by introducing active substances, such as Zr, Ce, and La, into the sol-gel materials, which facilitate the sealing of pores and defects by producing stable products [19], [27], [30], [31], [32], [34], [35], the effect of which is an elongation of the protection properties of the barriers, even to 835 days [36].
This study describes the physicochemical and electrochemical-protective properties of a series of sol-gel coatings based on a silica-zirconia system and a silica-fluoride modified system deposited on P265GH steel. The electrochemical tests of the obtained materials were conducted in 0.5 M NaCl. In order to determine the protective properties of the obtained materials against various corrosive media, the coatings were additionally tested in the medium of liquid caprolactam at a temperature of 95 °C. The sol-gel coatings were also aged for up to 3 years under an ambient room atmosphere (21 ± 1 °C, ≈44 % humidity) to test the longevity of their anti-corrosion properties.
Section snippets
Sols preparation
The silica ‘SiO2’ Sol (sol A) was obtained using tetraethoxysilane (TEOS, Sigma Aldrich, Darmstadt, Germany) and a precursor with non-hydrolysing organic groups, dimethyldiethoxysilane (dMdEOS, Sigma Aldrich, Darmstadt, Germany), and ethyl alcohol (Et(OH), POCH, Gliwice, Poland). Hydrolysis of the precursors was catalyzed with hydrochloric acid (HCl, Stanlab, Lublin, Poland). For Sol A, the molar ratio of precursors to solvent was 1:2 and catalyst concentration was 0.6 %, by volume. The
SEM and EDX
Fig. 4 shows the surface of the uncoated P265GH steel and the same sample after creating the phosphate coating. Temporary corrosion resistance of steel substrates is usually achieved through the phosphate coating process, where phosphating is carried out according to the following (1), (2), (3), (4), (5):where H3 PO4 reacts with Fe at micro-anode sites which
Conclusions
The ‘ZrO2’ and ‘SiO2 + F’ sol-gel coatings consisting of silica – zirconia system layers and silica – silica networks modified by organic and fluoride moieties, were obtained. Low-carbon P265GH steel was protected against corrosion by both sets of coatings, with the ‘SiO2 + F’ coatings exhibiting protective properties even after 3 years of aging. The ‘SiO2 + F’ – silica modified by fluoride moieties have the potential to provide long-term anti-corrosion protection of metallic products.
The
CRediT authorship contribution statement
Jolanta Szczurek: conceptualization, methodology, formal analysis, writing - original draft, visualization, investigation,
Anna Gąsiorek: formal analysis, investigation, writing – review & editing,
Anna Szczurek: formal analysis, investigation, writing – review & editing,
Bartosz Babiarczuk: formal analysis, investigation, writing – review & editing,
Maciej Kowalski: investigation, methodology,
Paweł Karolczak: investigation, methodology,
Walis Jones: editing, investigation,
Roman Wróblewski:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research was partially supported by West Technology & Trading Polska Sp. z o.o. under the project No POIR.01.01.01-00-0198/15-00 and was partially supported by Ministry of Science and Higher Education, Republic of Poland under the project No 0401/0029/17.
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