An integrated epoxy rust conversion coating: Its anticorrosion properties and rust conversion mechanism
Introduction
On the earth, rust is a by-product of the redox reaction between iron and oxygen in the presence of water, and is the main problem of human engineering work [[1], [2], [3], [4], [5]]. In the United States, the risk of metal corrosion directly costs $276 billion annually [6]. Globally, the National Association of Corrosion Engineers stated that the estimated cost of global corrosion in 2013 was $2.5 trillion [7]. In view of this, common method is to apply several layers of coatings with specific functions on metal surface to prevent or inhibit further corrosion [8,9]. However, the actual engineering implementation is not ideal. For example, the rusty metal surface requires complete rust removal before painting, and the construction period is long and timeliness is low. Therefore, it is important to develop a new anticorrosive coating, which can effectively prevent the further rust occurrence and facilitate the construction operation.
Generally speaking, the rusty coating is considered as the first choice, and it can be directly applied to the surface of rusted steel without or slight rust removal [[10], [11], [12], [13], [14]]. As far as the rust conversion coatings are concerned, the current rust conversion agents are mainly tannic acid and phosphoric acid [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. The use of tannic acid and phosphoric acid in formulas used to corrode metals can improve the anticorrosion performance of the coating system as a whole [18,[23], [24], [25]]. Giudice et al. [[26], [27], [28]] conducted a pioneering study on the use of metal tenants in anticorrosive coatings, and results were encouraging. Although the tannic acid and phosphoric acid conversion system was easy to paint, there are also many problems [29,30], such as the high prices of raw material, strict requirements on rust thickness, unfriendly to environment, water resistance and poor adhesion of coating film, etc. In addition, Favre et al. [31,32] found that the concentration of tannic acid was closely related to the conversion effect, and too high or too low concentrations cannot achieve better results. Almeida et al. [33] found that the corrosion effect was greatly affected by acid concentration in rust conversion system. Collazo et al. [34] found that if phosphoric acid still existed after rust conversion, it will penetrate into the interior of substrate and caused corrosion of the internal metal, which will affect the anticorrosive effect of the coating.
Based on the current problems of traditional coatings, this study replaced the phosphoric acid and tannic acid with gallic acid, using its characteristic of weak organic acid to reduce the over-corrosion. In addition, the gallic acid is inexpensive and environmentally friendly. Simultaneously, it can be esterified with alcohols to synthesize gallic acid monoesters with hydroxyl groups, which enhance the miscibility with epoxy resins and other substances, and be used as an intermediate to participate in subsequent reaction. On this basis, combined with epoxy resin, butyl glycidyl ether, silane coupling agent, curing agent, p-toluenesulfonic acid and defoaming agent, etc., an integrated epoxy rust conversion coating was synthesized with good rust construction performance and corrosion resistance.
Section snippets
Materials
All raw materials used in this study, such as the epoxy resin (E−51), butyl glycidyl ether, 3-aminopropyl triethoxysilane (Silane coupling agent), curing agent (T-31), 2,3-butanediol, gallic acid, P-toluene sulfonic acid, defoaming agent (YL-868), etc., were purchased from the Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals used were of analytical grade.
Preparation of rust conversion agent
2,3-Butanediol and gallic acid with a molar ratio of 14:1 were accurately weighed into different beakers. As a catalyst, P-toluene
What is “integrated epoxy rust conversion coating”?
As the name suggests, the innovation of the integrated epoxy rust conversion coating lied in the realization of “integrated”, changing the coating composition and application form of “primer + topcoat”. By applying the integrated epoxy rust conversion coating to the rusty surface one time, a better coating performance can be obtained. For example, the traditional coatings adopted the form of “primer + topcoat”. In this case, the adhesion ability of the primer to metal surface must be considered
Conclusion
In this study, a new gallic acid-based integrated epoxy rust conversion coating was developed, which had good anticorrosive properties. In corrosion simulated environments, it showed good adhesion and impact resistance. In addition, the electrochemical experiments have found that the coating had excellent corrosion resistance, and its corrosion current density can reach 7.247 × 10−7 A/cm2 under natural corrosion condition. XRD and SEM results shown that the coating surface was smooth, and the
CRediT authorship contribution statement
Yun Lei: Conceptualization, Formal analysis, Investigation, Methodology, Visualization, Writing - original draft, Writing - review & editing. Wenwen Xiao: Conceptualization, Resources, Methodology, Validation. Haoping Peng: Conceptualization, Funding acquisition, Supervision. Pengfei Yu: Conceptualization, Investigation, Supervision, Writing - review & editing. Xinyan Cai: Resources, Visualization, Data curation. Zhaolin Luan: Resources, Visualization, Data curation. Song Deng: Resources,
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51971039, 51671037 and 51804046), the Science and technology program of Changzhou University (Grant No. ZMF18020303, ZMF18020304 and CDYQCY202003), the PetroChina Innovation Foundation (Grant No. 2018D-5007-0602).
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