Microstructure and corrosion behaviour of laser-cladding Al-Ni-TiC-CeO₂ composite coatings on S355 offshore steel

Highlights

• Al-Ni-TiC-CeO₂ composite coating was prepared on S355 steel by laser cladding.

• The microstructure of coating is evenly distributed and no obvious defects.

• The coating can significantly improve the corrosion resistance of the substrate.

• The corrosive wear mechanism of the coating and substrate have been proposed.

Abstract

Al-Ni-TiC-CeO₂ composite coatings were deposited on S355 offshore steels by laser cladding. The microstructure and corrosion behaviour features, such as immersion corrosion and corrosive wear of the as-prepared composite coatings and substrates were investigated. The results showed that an Al-Fe phase and a small amount of amorphous AlFeNi phases were formed on the surface of the coatings. Simultaneously, the coatings showed a good metallurgical bonding with the substrate, the surface of the coating is relatively smooth, there is no obvious crack, and the coating dilution rate was 6.54%, the microstructure of the coating is finer, the reinforcement phase TiC showed a uniform dispersion distribution. The surface hardness of the coating is 855.26 HV_0.2, which is 2.35 times as high as the substrate. And the coating can significantly improve the tensile properties of the substrate. The corrosion products formed on the substrate were mainly FeOOH, while those on the coating were Al₂O₃ and AlOOH. The protective effect of the composite coatings on the substrate increased with increasing immersion time. The substrate exhibited corrosion-accelerating wear, while the coatings exhibited wear-accelerating corrosion.

Introduction

The ocean is an indispensable part of human activities. It contains abundant resources and has huge economic benefits. The development and utilization of the ocean can not be separated from the development and application of offshore steel, but the harsh environment of the ocean makes the offshore steel prone to corrosive wear and stress corrosion cracking (SCC) and other issues [[1], [2], [3]]. At the same time, salt and chemical contaminants speed up the corrosion rate. High-temperature seawater contains electrolytes and accelerates the corrosion process, which severely affects the mechanical properties of offshore platform materials [4]. As a result, the safety of offshore platforms is seriously threatened. Therefore, it is especially necessary to study the corrosion behaviour of S355 offshore steels and to improve their corrosion resistance [5,6]. At present, surface modification technology is generally being used for long-time corrosion prevention of offshore steel, which is very difficult to maintain. Furthermore, the expenditure is high [7]. Therefore, the coatings obtained after surface treatment must be capable of long-term anticorrosion and should exhibit high performance. In addition, the coating maintenance time should be minimised so as to prolong the maintenance cycle [8]. In this respect, laser cladding is an advanced processing technology. The cladding powder can be added to the surface of the substrate in different ways and is later fused by laser irradiation. The cladding powder is then solidified as a thin layer; it is metallurgically bonded to the substrate by rapid solidification. Thus, laser cladding is a surface-strengthening method for improving the wear resistance, corrosion resistance, fatigue resistance and oxidation resistance of the surface of a substrate [[9], [10], [11]]. At present, laser cladding technology is being widely used in the aviation and offshore engineering industries. Furthermore, it is making inroads into automobile engineering, ocean engineering and other industries as well [[12], [13], [14], [15], [16]]. In offshore engineering applications, because of the special environment of the ocean, ships and offshore platforms must have high corrosion resistance features. At present, the most commonly used anticorrosive matrix materials are stainless steel and copper; though they are corrosion resistant, long-term immersion in seawater will cause some local corrosion, such as spot corrosion and crevice corrosion. If the local corrosion is serious, it will develop into ulcerous corrosion. Therefore, anticorrosion of offshore engineering materials is one of the primary research foci of materials scientists. Surface modification using laser cladding can confer commonly used matrix materials with a high corrosion resistance [17]. For example, He et al. fabricated an Al-TiC-CeO₂ composite coatings by laser cladding on S355 steel. Their results show that the coating showed high corrosion resistance when the laser power was 1.6 kW [18]. Zhang et al. prepared amorphous Ni-Al coatings by laser thermal spraying on a S355 steel. The results show that the corrosion resistance of the S355 steel was effectively improved at Al/Ni mass ratios of 3:2 and 4:1 [19]. Zhao et al. prepared Fe-based coatings with different contents of La₂O₃ by laser cladding on the surface of steel. The results show that both polarizing voltage and the self-corrosion current density of the coating were reduced and the coating has excellent corrosion resistance with the addition of La₂O₃ [20]. Although there has been much research on offshore steel corrosion protection at home and abroad. However, it is rare to use laser cladding technology to improve the corrosion resistance of marine steels. And few researchers have systematically studied the effect of the coating on immersion corrosion, corrosive wear behaviour of the substrate. Therefore, in this study, we used the laser cladding technology to deposit Al-Ni-TiC-CeO₂ composite coatings on offshore steels. This paper will focus on analyzing the effects of such coatings on microstructure and the immersion corrosion and corrosive wear of the offshore steel substrate in 5% NaCl solution are systematically investigated which provides an experimental basis for the application of Al-Ni-TiC-CeO₂ composite coatings on offshore platforms.