Elsevier

Corrosion Science

Volume 174, September 2020, 108827
Corrosion Science

Hot corrosion behavior of electrodeposited SiO2 coating on TiAl alloy

https://doi.org/10.1016/j.corsci.2020.108827Get rights and content

Highlights

  • Electrodeposited SiO2 coating can efficiently improve the hot corrosion resistance of TiAl alloy.

  • Compact and adherent corrosion scale generates during the hot corrosion process.

  • The dissolution of SiO2 into the molten sodium salt leads to the generation of Na2Si4O9.

  • The compact Al2O3 layer at the coating/alloy interface benefits to improve the hot corrosion resistance.

Abstract

To improve the hot corrosion resistance, thickness controllable SiO2 coating was electrodeposited on TiAl alloy. The hot corrosion behavior of the specimens contaminated with 75 wt.% Na2SO4 + 25 wt.% NaCl salt mixture was investigated at 700 °C. Results showed that the generation of compact amorphous SiO2 embedded with Na2Si4O9 and cristobalite was responsible for the enhanced hot corrosion resistance. Moreover, the SiO2 coating does not only act as a diffusion barrier to sulfur and chlorine but also decreases the oxygen partial pressure at the coating/alloy interface, therefore promoting the selective oxidation of Al to form protective Al2O3 layer.

Introduction

Owing to their low density and high specific strength, TiAl-based alloys are considered as prospective high temperature structure materials used in aero-engines and gas turbines [[1], [2], [3]]. Till now, a great deal of effort has been devoted to improving the oxidation resistance of TiAl alloys [[4], [5], [6], [7], [8]]. Unfortunately, the service environments are generally more complex in actual applications. For instance, some salts may accumulate on the surface at elevated temperature, which will damage the protective scales and accelerate the deterioration of metallic materials, yielding porous and non-protective scales [[9], [10], [11], [12]]. This phenomenon is the so-called “hot corrosion” [[13], [14], [15]]. Na2SO4/K2SO4 and NaCl are the most common salts that accelerate the high temperature corrosion. NaCl mainly derives from the aerosol over the seawater. Na2SO4 is generated due to the reaction between NaCl and sulfide dioxide from combustion gases [11].

It is therefore crucial to investigate the hot corrosion behavior and reveal the corrosion mechanism of TiAl alloys, which helps to take appropriate measures to improve the corrosion resistance [16,17]. Alloy design, achieved by increasing Al content or adding alloying elements is an efficient strategy to improve the oxidation resistance [[18], [19], [20], [21], [22]] and corrosion resistance of TiAl alloys [22]. Whereas, severe damage will occur when the alloys were directly exposed to the salt environment. This is because that the self-sustaining reaction between the sodium chloride and/or sulphate species and contaminated metallic substrate will lead to acidic fluxing into the oxide scales and accelerate the corrosion [10,15,[23], [24], [25]]. Godlewska et al. [26] found that although Ti-46Al-8Ta (at.%) alloy exhibited good resistance to cyclic oxidation in air at 700 and 800 °C, salt deposits such as NaCl, Na2SO4, or NaCl + Na2SO4 markedly accelerated the degradation of the alloy. Moreover, the extent and type of damage depends on the chemical composition of the salts [9,16,23,27,28].

Protective coating is essential to shield the alloy from corrosion attack. Various coatings have been developed to enhance the oxidation resistance of TiAl alloys, including metallic coating [[29], [30], [31], [32]], diffusion coating [[33], [34], [35]], ceramic coating [[36], [37], [38], [39], [40], [41]], and so on. However, only several kinds of coatings have been explored to improve the hot corrosion resistance of TiAl alloy, such as TiAlCr [25], NiCrAlY [42], Al2O3 [43], and enamel [[42], [43], [44]]. Bacos et al. [45] prepared Au-based coating on Ti-48Al-2Cr-2Nb (at.%) by the combination of electrodeposition and vacuum heat treatment. Owing to the chemical inertness and low solubility in molten salts, this noble metal layer exhibited good corrosion resistance. He et al. [46] developed (Al2O3-Y2O3)/(Pt-Au) composite coating on Ti-45Al-8Nb (at.%) alloy by the magnetron sputtering method. However, good corrosion resistance was achieved only when the composite coating was deposited for at least seven times, which made the coating preparation process quite complex and time consuming. Most recently, Chen et al. [42] found that obvious spallation occurred on the arc ion plating NiCrAlY coating after 60 h corrosion due to the basic dissolution of Al2O3 film and serious inter-diffusion between coating and Ti-45Al-2Mn-2Nb (at.%). But the enamel based composite coating exhibited excellent thermal stability and good corrosion resistance.

Owing to the ability to form a protective SiO2 scale, silicon-based coating has received much attention to act as a barrier coating for TiAl alloys [47,48]. Zheng et al. [49] found that both the pack cemented silicate diffusion coating and Si-Al coating exhibited good spalling resistance. Interestingly, it demonstrated that SiO2 was highly stable in a molten NaCl-KCl-Na2SO4-K2SO4 solvent [50]. Rubacha et al. [12] observed that the magnetron sputtered silicate coating provided good hot corrosion resistance for Ti-46Al-8Ta (at.%) alloy due to the formation of amorphous silica layer embedded with rutile and cristobalite.

Recently, we proposed a novel strategy to prepare SiO2 coating on TiAl alloy by electrodeposition to improve the oxidation resistance [47]. The results showed that the high temperature oxidation resulted in the generation of a glass-like protective oxide scale consisting of cristobalite, α-Al2O3, Ti3Al, Ti5Si3, and Ti5Al3O2. This alumina- and silicon-enriched oxide scale could efficiently prevent the inward diffusion of oxygen, therefore improving the oxidation performance for TiAl alloy at 900 °C. Then two kinds of Al-Si composite coatings were fabricated on TiAl alloy by the combination of electrodeposited SiO2 coating and pack aluminized coating [48] or magnetron sputtered Al coating [51]. These SiO2 coating and SiO2-based coatings efficiently enhance the oxidation resistance of TiAl alloy, but the hot corrosion behavior has not been studied yet. In the present work, we systematically investigated the hot corrosion behavior of the electrodeposited SiO2 coating and proposed the protection mechanism.

Section snippets

Materials and chemical reagents

γ-TiAl alloys (Ti-50 at.% Al) were prepared by vacuum tungsten-arc melting of high purity titanium (99.9 %) and aluminum (99.6 %) as described in our previous study [[52], [53], [54]]. The homogenized ingots were cut into 15 × 15 × 1.2 mm. All specimens were ground with emery paper (with the grits of # 60, # 180, and # 400, successively), then cleaned ultrasonically in acetone and ethanol for 5 min sequentially, followed by rinsing in distilled water for 3 min, then blew-dried with warm air. KNO

Morphology of the electrodeposited SiO2 coating

As shown in Fig. 1a, many cracks are observed on the electrodeposited SiO2 coating which is composed of particles with different sizes. A similar phenomenon has been observed on SiO2 coating electrodeposited at other conditions in previous studies [47,48]. Undoubtedly, the cracks and boundaries among the SiO2 particles can provide path for the inward diffused oxygen and other salt ions, which would lead to server corrosion. Interestingly, thermal treatment at 900 °C for 5 h results in the

The hot corrosion behavior of SiO2 coating

In the present work, the hot corrosion behavior was investigated under the deposition of mixture salt consisting of 75 wt.% Na2SO4 + 25 wt.% NaCl, whose melting point is 628 °C due to the formation of a eutectic [10]. Therefore, the salt appears in a liquid phase on the surface of the specimens during hot corrosion at 700 °C. In the molten salt environment, molten salt ions and inward diffused oxygen penetrate easily through the non-protective TiO2 layer which is quickly generated on the

Conclusion

  • (1)

    TiAl alloy showed poor corrosion resistance under the 75 % Na2SO4 + 25 % NaCl mixture salt deposit. The corrosion scale mainly consists of lamella TiO2 with poor adhesion to the substrate. The corrosion mechanism is mainly based on the basic dissolution.

  • (2)

    Electrodeposited SiO2 coating efficiently improves the corrosion resistance of TiAl alloy. After hot corrosion at 700 °C for 100 h, no spallation and obvious destruction is observed. The generated corrosion scale is compact and adherent.

  • (3)

    The

CRediT authorship contribution statement

Lian-Kui Wu: Data curation, Funding acquisition, Conceptualization, Writing - review & editing. Jing-Jia Wu: Data curation. Wei-Yao Wu: Data curation, Writing - original draft. Hao-Jie Yan: Data curation. Mei-Yan Jiang: Data curation. Fa-He Cao: Writing - review & editing.

Declaration of Competing Interest

No conflict of interest exists in this manuscript.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (51971205), Shenzhen Fundamental Research Program(JCYJ20190807154005593), and the Fundamental Research Funds for the Central Universities (19lgpy20).

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