Effect of zinc oxide on corrosion resistance of magnesium ammonium phosphate cement-based coating

Abstract

In order to improve the anticorrosive performance of magnesium ammonium phosphate cement (MAPC) coating, a small amount of zinc oxide was added to the MAPC coating to modify and coat the surface of Q235 steel. The anti-corrosion resistance and mechanism of MAPC coating modified by zinc oxide were analyzed based on electrochemical test results, and the morphology and composition changes of the coating before and after immersion were observed and tested using X-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), and a thermogravimetric analyzer (TG-DTG). Low field nuclear magnetic resonance testing was used to determine the porosity of the coating and the corrosion resistance of the modified coating was tested by a neutral salt spray test. The results show that after immersion in a 3.5% NaCl solution for 28 days, the MAPC coating with 3% (mass fraction) zinc oxide exhibited a corrosion current density of 2.96 × 10-4 μA/cm2 at a corrosion potential of −0.870 V, demonstrating a favorable outcome with a positive shift.; Meanwhile, the 28 days polarization resistance of the modified coating was 97436 Ω∙cm2, which was 78% higher than that of the blank. The shielding performance of the coating was enhanced, and the charge transfer resistance was also significantly improved. The porosity of the modified coating before and after immersion was lower than that of the blank group. After 1440 h of neutral salt spray testing, there was no swelling or peeling on the coating surface, and there was no trace of corrosion on the internal substrate.
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Introduction

According to the different types of phosphate, commonly used magnesium phosphate cement is mainly divided into magnesium potassium phosphate cement (MKPC) and magnesium ammonium phosphate cement (MAPC). Its solidified body has the characteristics of ceramic products, so it is also called phosphate ceramic material [1]. Compared to MKPC, MAPC has a lower cost and higher intensity in the early stages. It is found that MAPC has better reinforcement protection than traditional Portland cement, mainly because MAPC has a more compact structure, which can form a dense metal complex on the surface of the reinforcement, so that the corrosion medium is difficult to transfer outward, thus inhibiting the occurrence of corrosion [2], [3]. In order to give full play to the anti-corrosion performance of MAPC, Qu Chengju [4] prepared a MAPC coating with a thickness of 0.8 mm and coated it on the surface of a Q235 steel sheet, and the coating showed good anti-corrosion performance.

Steel is a material with high strength and light weight, making it a popular choice for structural components. However, steel is vulnerable to corrosion in natural environments, which can cause a significant reduction in mechanical properties and ultimately threaten the safety performance of the structure. If left untreated, corrosion can lead to safety accidents and the repair or replacement of steel parts can be costly and resource-intensive. It is crucial to implement preventive measures, such as corrosion-resistant coatings or regular maintenance, to ensure the longevity and safety of steel structures. The global economic losses resulting from steel structure corrosion reach trillions of dollars per year. Enhancing the corrosion resistance of steel in a cost-effective and practical manner has become a crucial concern in promoting sustainable economic development. To improve the anticorrosion performance of magnesium ammonium phosphate cement-based coatings, the method of adding admixtures, such as silica fume, high temperature and so on, is usually used [5]. The principle is to fill the pores of coatings with smaller admixtures of particle size or to optimize the pores and cracks in MAPC coatings to increase their compactness. However, little research has been done on modification by adding a small number of additives. Zinc oxide is often used as an anticorrosive pigment that is frequently used in organic and inorganic coatings and has high oxidation and corrosion resistance [6]. At the same time, zinc oxide, as amphoteric oxide, can play a neutralizing role in coatings and can react directly with active rust acid to reduce substrate corrosion [7]. Zinc oxide can react with phosphoric acid or phosphate in phosphate coatings to produce zinc phosphate, which can play a role in corrosion inhibition in a corrosive environment and provide more protection for the substrate [8], [9].

This research investigated the effectiveness of incorporating zinc oxide into MAPC coatings to enhance their anticorrosion properties. The influence of zinc oxide on the anticorrosion performance of MAPC coatings was evaluated through electrochemical testing, and the most effective amount of zinc oxide was determined. Simultaneously, an analysis was carried out on the microscopic level to understand the effects of different quantities of zinc oxide on the coating components, surface morphology and porosity. The aim was to shed light on the underlying mechanism of how zinc oxide impacts the anticorrosion performance of MAPC coatings.
Section snippets
Materials

The Q235 steel sheet, commercially available and refilled magnesium oxide (MgO, abbreviated M), with an average particle size of 25.98 µm, supplied by Liaoning Hengren Dongfang Hong Hydropower Station Magnesium Sand Plant, as shown in Fig. 1, the main technical indicators are shown in Table 1; ammonium dihydrogen phosphate (NH4H2PO4, abbreviated P), supplied by Lianyungang Geli Chemical Co., Ltd, content ≥ 98%, white granular crystals, as shown in Fig. 2, the main technical indicators are shown
Polarization curve analysis

From Fig. 3, Fig. 4, Fig. 5, polarization curves of ZnO modified MAPC coatings show that, unlike the polarization curves of the blank group, the polarization curves of MAPC coatings with 2% and 3% ZnO show the phenomenon of decreasing corrosion current with increasing corrosion potential, which are both stable passivation zones. Among them, Zn-2 showed only obvious passivation zone at 14 days, while Zn-3 showed obvious passivation zone from 7 days to 28 days and was relatively stable,
Conclusion

(1) The analysis of the polarization curve and AC impedance spectrum of MAPC coatings has revealed that the optimal amount of ZnO is 3%. The polarization curve demonstrated the emergence of a passivation zone in the coatings after adding 3% ZnO. When immersed for 14 days, the polarization resistance value was the largest, showing an increase of 128% in comparison to the blank group. After 28 days of immersion, the total low frequency impedance value, charge transfer resistance, and coating
CRediT authorship contribution statement

Yong Zhi: Conceptualization, Data curation, Formal analysis, Methodology, Writing – review & editing, Writing – original draft. Qing Wu: Funding acquisition, Resources, Supervision. Hongli Ma: Data curation, Formal analysis, Methodology. Yadi Wu: Investigation, Resources, Validation, Visualization. Muhammad Akbar: Investigation, Validation, Visualization. Xiangran Zhao: Data curation, Investigation, Validation. Ning Yang: Funding acquisition, Resources, Supervision.
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

This work was supported by the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX23_3895).