Elsevier

Journal of Alloys and Compounds

Volume 616, 15 December 2014, Pages 14-25
Journal of Alloys and Compounds

Comparative studies on the initial stage of arc-sprayed and zinc-rich powder coatings in sulfur-rich environment

https://doi.org/10.1016/j.jallcom.2014.07.088Get rights and content

Highlights

  • Corrosion behavior of arc sprayed and zinc-rich powder coatings was studied.

  • ZRP coating performed better in the sulfur-rich environment.

  • A sequence for zinc corrosion products in sulfur-rich environment was proposed.

  • The investigations showed mechanism of zinc corrosion in sulfur-rich environment.

Abstract

A comparative study of the electric arc-sprayed (AS) and zinc-rich powder (ZRP) coatings, was performed in sulfur-rich environment involving atmospheric corrosion test and cyclic wet–dry test. The surface morphologies and characterization of corrosion products were investigated using SEM/EDS, XRD and EPMA techniques. The anti-corrosion properties of both coatings were compared by taking electrochemical measurements. The results revealed that the corrosion resistance of the ZRP coating was higher than that of AS one in the experimental test. A general sequence for the evolution of corrosion products on zinc coatings in sulfur-rich environment was proposed: Zn  ZnO/Zn(OH)2/ZnSO4  Zn5(CO3)2(OH)6  Zn4SO4(OH)6/ZnSO4⋅7H2O.

Introduction

Various surface treatments have been applied to structural components made of irons and steels in order to achieve high corrosion performance for long-term service. For example, sacrificial metal coatings, particularly zinc coatings, are widely used to protect steel surfaces. Because the zinc coatings are electrochemically more active than steels, thus corroded preferentially and covered the substrate with an insoluble film of the corrosion products, offering barrier effect to a certain substrate [1]. In recent years, electric arc spraying technique, one of the thermal spraying methods, has widely been employed to deposit zinc coatings for corrosion prevention of infrastructures such as highways and bridges from the viewpoint of reducing life cycle costs [2], [3]. It is a good process especially for spraying large areas owing to low running costs, high spray rates and efficiency. Furthermore, the arc spraying (AS) process is environmental friendly and can be used to effectively deposit surface coatings that have superior hardness and corrosion resistance [4]. The main drawback associated with arc spraying lies in the low density of the coating and special installation requirement for deposition, which principally limits its application. However, zinc-rich powder (ZRP) coatings are able to overcome these disadvantages. They do not require special installations and are quite available in field. Besides, they are applicable in various aggressive media such as sea water, marine and industrial environments, even coatings with minor damages can protect the steel by a galvanic anode type cathodic protection [5].

Significant knowledge exists concerning the atmospheric corrosion of zinc coatings on steel in terms of different exposure conditions, such as chloride ions (Cl) and sulfur dioxide (SO2). Friel [6] and Biestek [7] pointed that in an unpolluted atmosphere, ZnO and Zn5(CO3)2(OH)6 are the most abundant corrosion products. And in moist environments, zincite (ZnO) is initially formed in the passivation film. Then this oxide can rapidly transform into zinc hydroxide (Zn(OH)2), probably with diverse crystal structures. In chloride-rich environment, Yadav et al. [8] and Chen et al. [9] proposed that zincite and hydrozincite were protective and the generation of insoluble substances, Zn5(OH)8Cl2·H2O could inhibit the subsequent corrosion to some extent. Moreover, the presence of zinc hydroxide carbonate has also been reported [10], [11]. For industrial environments, where the more aggressive contaminant is sulfur dioxide (SO2), the initial corrosion product formed on zinc is proved to be zinc hydroxysulfate, with Zn4SO4(OH)6⋅4H2O and Zn4Cl2(OH)4SO4⋅5H2O, subsequently formed, as the final corrosion products [12].

The information about the initial period of the atmospheric corrosion is important and helpful to understand the corrosion mechanism of zinc coatings. However, so far relatively little effort has been dedicated to the initial stages in sulfur-rich environment, especially for ZRP coatings, due to measurement difficulties. For this reason, the present work focused on the initial corrosion stages for one year of AS and ZRP coatings in moist sulfur-rich environment like Changsha in China. For complement, the cyclic wet–dry test was adopted to simulate the atmospheric corrosion for time more than one year. Further goals of this work are in investigating and proper understanding the corrosion product evolution of the zinc coatings.

Section snippets

Specimens

The substrate of both coatings was hot-rolled low carbon steel with 100 × 30 × 3 mm3. The chemical composition (wt.%) of the steel was as follows: 0.11% C, 0.55% Mn, 0.012% Si, 0.016% P and balance of Fe. The substrates were blasted with steel grit and degreased in acetone ultrasonically before the coating process. For deposition of the AS coating, a XDP-2 electric arc spray device was used and a wire with diameter of 3 mm composed of 99.99% pure Zn was selected as the spraying material. The density

Morphologies of original coatings

The SEM/EDS results of the original AS and ZRP coatings were displayed in Figs. 1 and 2 respectively. The AS coating presented rough surface with many open pores, which agrees with the previous study [14]. As shown in Fig. 1c, the cross section was composed of an outer coarse layer and an inner compact layer. The image shown in Fig. 1d revealed that some pores and micro-cracks existed in the outer layer. This allows more corrosive medium to permeate into the coating through the pores or cracks

Conclusions

The corrosion behavior of AS and ZRP coatings at the initial corrosion stage have been discussed in moist sulfur-rich environment. The study on the atmospheric corrosion for one year was capable to explain the corrosion mechanism in sulfur-rich environment. And the experimental evidence shows that the anti-corrosion property of the ZRP coating is better than that of the AS one in both atmospheric and cyclic wet–dry tests. Detailed conclusions are drawn as follows.

  • 1.

    Either in atmospheric corrosion

Acknowledgments

Thanks are due to Hu’nan Research Institute of Metallurgy and Materials, Changsha, China for providing coated samples. The authors also wish to thank Shuhong Luo and Xiaochao Lu for their great assistance in this study.

References (32)

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    In a sulphur containing corrosive environment, the corrosion products are zincite, zinc hydroxide and ZnSO4. These products transform on prolong corrosion process to hydrozincite, zinc hydroxysulfate (Zn4SO4(OH)6) and goslarite (ZnSO4·7H2O) [570]. Zincite, hydrozincite, and simonkolleite (Zn5(OH)8Cl2·H2O) are the corrosion products in chloride-rich environment [569].

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