Hot Corrosion Behavior of Stainless Steel with Al-Si/Al-Si-Cr Coating

Introduction

Stainless steel has protective film of Cr₂O₃ on surface, so it has good corrosion resistance in certain conditions of acid, alkali, salt and neutral solutions and has been widely used as corrosion-resistant materials. However, in some medium, the stainless steel can also be corroded [1, 2]. In high-temperature corrosion environments, the stainless steel has corrosion-resistant ability to some extents, and the price of stainless steel is relatively cheap compared to superalloys. However, due to the low Cr content, the corrosion resistance of the material is also limited. Therefore, the corrosion behavior of stainless steel has been focused by many researchers [3–6].

In order to improve the corrosion resistance of stainless steel, protective coating is made on its surface [7–9]. Zhang et al. [10] cladded Co-based alloys on the surface of stainless steel to study the corrosion resistance and found that Co element promotes the formation of CoO, CoO and Cr₂O₃ protective films, which improve the high-temperature corrosion resistance of the stainless steel. Ni et al. [11] have sputtered Al on 310-stainless steel by multi-arc ion plating, followed by annealing treatment to obtain aluminide coating, and found that when being in molten carbonate at 650 °C, the coated stainless steel possesses excellent corrosion resistance, which is related to the formation of a continuous and protective LiAlO₂ (Al₂O₃).

Many researchers have also studied the Al-Si layer, which is prepared by diffused deposition on the surface of nickel-based alloys, to improve high-temperature oxidation and hot-corrosion resistance. Thereinto, the slurry method is a simple and convenient operation to prepare coatings. Many reports [12, 13] show that adding Al on the surface of stainless steel can promote the formation of Cr₂O₃. Al reacts with oxygen to form Al₂O₃ film, which inhibits oxygen to penetrate into the alloy and can lower the growth rate to produce more effective protection. The addition of Si suppresses the growth of the NiAl phase and delays the degradation of the coating. Niu et al. [14] found that the Al-Si coating on the surface of K4104 superalloy, which was prepared by the slurry method, can improve the oxidation resistance at 1,000 °C and hot corrosion resistance at 900 °C. Yang et al. [15] found that the Al-Si coating on the surface of K4104 superalloy, which was prepared by the thermal diffusion method, can be transformed into α-Al₂O₃ film after 200 h oxidation at 1,000 °C, which has good adhesiveness with the matrix alloys. Thereinto, the addition of Si plays a significant role in suppressing the growth of the β–NiAl phase, accelerates the formation of M₆C, and delays the degradation of the coating.

Up to now, the study on preparing coatings on the surface of stainless steel by diffusion to improve the hot-corrosion resistance has been rarely reported, and the corrosion mechanism is still unclear. Thereby, in this paper, Al-Si and Al-Si-Cr coatings are prepared on 1Cr18Ni9Ti stainless steel by using the method of slurry combined with vacuum diffusion, and the corrosion behavior of stainless steel under the condition of hot corrosion with Na₂SO₄ is studied.

Experimental

The experimental material is the 1Cr18Ni9Ti stainless steel, and the chemical composition is shown in Table 1. Al-Si coating was prepared on the surface of specimens by combining slurry with vacuum diffusion. Al powder, Si powder and binder were put into a crucible, which were then well mixed to make the slurry solution. The stainless steel with dimension of 12 mm×8 mm×1.5 mm was ground down to 1500-grit SiC paper, and then it was put into the crucible with the slurry solution after cleaning and drying. Afterwards, the crucible was put in a vacuum resistance furnace at 900 °C to conduct diffusion treatment for 8 h.

After the coating was prepared, specimens were treated with sandblasting, grinding, cleaning and drying. Afterwards, saturated Na₂SO₄ solution was brushed onto the specimens and dried. The above steps were repeatedly conducted until the total coating weight reached 2.0~2.5 mg/cm² on every specimen, and then isothermal hot corrosion tests were performed in a box-type resistance furnace at 950 °C in air for 24 h. The corrosion dynamic curve was plotted, according to the mass variation of the specimens which were taken out and weighted after being corroded for 0.5 h, 1 h, 1.5 h, 3 h, 5 h, 7 h, 9 h, 11 h, 13 h, 16 h, 19 h, 22 h, 24 h respectively. After hot corrosion, scanning electron microscopy (SEM) was used to observe the cross-sectional microstructure of corrosion products.

Results and analysis

Corrosion kinetics

Figure 1 shows the corrosion kinetic curves of the specimens when hot corroded at 950 °C in molten sulfate for 24 h, where the evaporation mass loss of the salt coat has been taken into consideration. It can be seen that the mass gain of the stainless steel with Al-Si coating or Al-Si-Cr coating is much smaller than that of stainless steel without coating, and the mass gain of the stainless steel with Al-Si-Cr coating is the smallest of the three stainless steels.

Figure 1:

Corrosion kinetic curves of the specimens corroded at 950 °C in molten sulfate for 24 h.

The relationship between the square mass gain and time for different hot-corroded specimens is plotted, as shown in Figure 2. It can be seen that the corrosion kinetics of the three alloys follows parabolic law in different segments, and the relative parabolic constants are calculated and shown in Table 2. It can be seen that, after 13 h corrosion, the sequence of the parabolic constants is as the following, the stainless steel with Al-Si-Cr coating (0.71×10⁻¹⁰ g² · cm⁻⁴s⁻¹) < the stainless steel with Al-Si coating (1.76×10⁻¹⁰ g² · cm⁻⁴s⁻¹) < the stainless steel without any coating (4.17×10⁻¹⁰ g² · cm⁻⁴s⁻¹). Moreover, the corrosion rate and the parabolic constant of the three specimens are obviously decreased with time extending. The corrosion rate of the specimens with Al-Si/Al-Si-Cr coating is lower than stainless steel without coating, and the specimens with Al-Si-Cr coating can protect the substrate better than the specimens with Al-Si coating.

Figure 2:

Relationships between square mass gain and time for the specimens being hot corroded at 950 ℃ in molten sulfate for 24 h.

Table 2:

Parabolic rate constants of Stainless steel, Al-Si coating and Al-Si-Cr coating.

Samples	Periods (h)	Kp (10−10g2 · cm−4s−1)
Stainless steel	0~13	5.16
	13~24	4.17
Al-Si coating	0~11	2.48
	11~24	1.76
Al-Si-Cr coating	0~9	1.26
	9~24	0.71

Compositions of the coatings

Figure 3 shows the cross-sectional morphology and the element distribution map of stainless steel coated with Na₂SO₄ film at 950 °C for 24 h, where the light layer in the last image is Ni plating to protect the corrosion film from spalling during the preparation of SEM specimens. It can be seen that the corrosion products are divided into two layers. The outside layer mainly consists of Fe oxides, and the inside layer mainly consists of Cr₂O₃. Additionally, a small amount of FeS is formed in the substrate. The corrosion film is not dense, and some holes can be found.

Figure 3:

Cross-sectional morphologies and element distribution of the stainless steel after being corroded for 24 h.

The cross-sectional morphology and the element distribution of stainless steel with Al-Si coating after 24-h hot corrosion at 950 °C are shown in Figure 4, indicating that the corrosion scale can also be divided into two layers. According to the data of element distribution, the external layer is determined to be multiple oxides of Fe and Cr, the internal layer SiO₂. Small amount of sulfides forms in the substrate. The corrosion film on the surface of the substrate is uniform and dense.

Figure 4:

Cross-sectional morphologies and element distribution of the stainless steel with Al-Si coating after being corroded for 24 h.

Figure 5 shows the cross-sectional morphology and the element distribution map of stainless steel with Al-Si-Cr coating after hot corrosion at 950 °C for 24 h. It can be seen that the corrosion film is mainly composed of Al₂O₃ and Cr₂O₃, with the outside layer Cr₂O₃, the internal layer Al₂O₃, and a small amount of FeS formed in the substrate. Compared with the specimen containing Al-Si coating, the corrosion film of the specimen with Al-Si-Cr coating is thin, the density of the corrosion film is large, and the adhering strength with substrate is improved. Protective Al₂O₃ layer is formed and few S elements diffuse into grain boundary of the stainless steel matrix to form FeS by reacting with Fe.

Figure 5:

Cross-sectional morphologies and element distribution of the stainless steel with Al-Si-Cr coating after being corroded for 24 h.

Discussion

The melting point of Na₂SO₄ is 884 °C, which is lower than the experimental temperature of 950 °C in this study, so the salt mixtures is molten and consequently the high-temperature hot corrosion occurs [16, 17]. When hot corrosion of metal or alloy occurs, the protective oxide film on the surface is dissolved or precipitated constantly in the liquid molten salt [18, 19]. When the specimen coated with Na₂SO₄ salt film is exposed to the oxidizing environment, the formula of thermodynamic equilibrium is expressed as follows:

The SO₄^2– acts as oxidant in the Na₂SO₄ molten salt. In initial period, Cr and Fe are oxidized preferentially because the stainless steel is rich in Fe and Cr. With the formation of oxides, oxygen partial pressure decreases and sulfur partial pressure increases. The product S₂ further permeates into the matrix to form the Fe and Cr sulfides (FeS and Cr₂S₃). However, neither FeS nor Cr₂S₃ possesses protection, and they react with O that diffuses to the matrix to form loose oxide layer. The oxidizing reaction is described as the following,

As the reaction goes on, oxygen partial pressure increases with the decomposition of Na₂SO₄ to maintain the oxidation process. Other alloying elements in the substrate react with O to form oxides, which leads to the thickness of oxide layers increased.

The relatively enhancement of Na₂O makes the basicity increase of Na₂SO₄, and alkaline dissolution occurs to the oxide scale of Cr₂O₃ and Fe₂O₃ as the following,

The basicity of the molten salt layer decreases from inward to outward, so CrO₄^2– and FeO₂^– are decomposed when diffusing outward to the interface, and Cr₂O₃ and Fe₂O₃ are precipitated there,

The above process occurs repeatedly, and the thickness of oxide layer becomes larger constantly. Cr₂O₃ reacts with FeO to form FeCr₂O₄ spinel phase, which can effectively hinder the outward diffusion of iron. Consequently, the outside layer mainly consists of Fe₂O₃, Cr₂O₃ and FeCr₂O₄, the layer close to matrix mainly consists of Cr₂O₃, and a small amount of FeS is formed in the substrate, as shown in Figure 3.

During the hot corrosion process, Al and Si elements in Al-Si coating on the stainless steel are oxidized to become protective oxides Al₂O₃ and SiO₂. When O and S permeate the coating/matrix interface, they react with the matrix to form metal oxide and metal sulfide. Fe₂O₃ and Cr₂O₃ are enriched at the matrix surface by the above process. Cr₂O₃ reacts with NiO to form NiCr₂O₄ spinel phase, which provides good protection for the matrix. However, NiCr₂O₄ will be decompose at high temperatures by the following reaction,

As oxidation reaction goes on, a protective oxide layer of Cr₂O₃, SiO₂ and Al₂O₃ is formed at the matrix surface, so the outward diffusion rate of O and S is slower, which hinders the oxidation and the sulfuration reaction effectively and protects the matrix to some extents. Since S element diffusing into the matrix at the early stage reacts with Fe, a small amount of FeS is formed in the substrate. Al₂O₃ is not observed by energy spectrum analysis due to the lower content of Al in the coating.

Adding Cr element into the Al-Si coating not only improves its hot corrosion resistance but also accelerates the formation of metastable Al₂O₃. In initial period, Al and Cr are oxidized to become the oxides of Al₂O₃ and Cr₂O₃. When oxygen pressure is over 101.325 Pa, Cr₂O₃ preferentially reacts with O to form CrO₄^2– [20]. That is, alkaline dissolution could occur at low alkalinity of the oxide scale Cr₂O₃, so the alkaline dissolution of Al₂O₃ is avoided. Consequently, the non-dissolved Al₂O₃ between coating and salt layer has a protective role for the specimen. Therefore, adding Cr into the Al-Si coating can hinder the dissolution of Al₂O₃ effectively and improve the density of the coating, which improves its hot-corrosion resistance.

Conclusions

1. After 24 h hot corrosion at 950 °C in air, the corrosion kinetic curves of the stainless steel and the coated samples obey parabolic law in segments. After 24 h corrosion, the sequence of the mass gain for the three materials is the stainless steel with Al-Si-Cr coating < the stainless steel with Al-Si coating < the stainless steel without any coating.

2. When the Al-Si coated sample is hot corroded at 950 °C in air, the formation of continuous Cr₂O₃ and SiO₂ film can effectively protect the substrate and improve its corrosion resistance. For the Al-Si-Cr coated sample, the alkaline dissolution of Cr₂O₃ inhibits the dissolution of Al₂O₃. The dense Al₂O₃ film can protect the substrate effectively to improve the corrosion resistance of stainless steel.