1 Introduction

Magnesium and alloys have attracted considerable recent attention and have great potential in industrial applications [1, 2], especially in aerospace and transportation fields due to their suitable mechanical properties and high strength to weight ratios [3, 4]. However, magnesium alloys exhibit poor corrosion properties and tolerable formability [5, 6]. One of the promising methods for increasing strength and formability is refining microstructural. The material with fine grains has a more grain boundary to hinder dislocation movement. But more grain boundaries store more energy, which makes the material more possible to be corroded [7, 8]. Appropriate annealing can eliminate internal stress and distortion energy at grain boundary. Annealing includes full annealing and stress relief annealing, which full annealing making the grain recrystallize or even grow up and increase the plastic and decrease the strength of the material [9, 10]. The stress relief annealing can not only reduce or dispel the residual stress produced in cold and hot working and forming of deformed magnesium alloy products, but also eliminate the residual stress in castings or ingots [11, 12]. In the rolling process of magnesium alloy sheet, the grain size and shape of each section are very different due to the various stress on diverse cross sections. Rare earth elements have valence electron and distinctive atomic structures. Wrought magnesium alloys with Ce addition can enhance the mechanical properties at ambient and high temperatures and improve corrosion resistance [13]. Hamu et al. [14] studied the relation between severe plastic deformation microstructure and corrosion behavior of AZ31 magnesium alloy. The relation can be explained by the effects of the process on microstructure of AZ31 Mg alloy such as grain size and dislocation density caused by the change in recrystallization behavior. Song et al. [15] investigated the effect of microstructure change on corrosion behavior of ECAPed AZ91D Mg alloy. The ECAPed alloy with ultra-fine grains displays a significantly lower corrosion resistance. Hama et al. [16] performed two-step loading tests on AZ31 rolled magnesium alloy sheet with strong basal texture in ND (normal direction). The texture evolution of the sheet in different direction was investigated. Mg-Zn-Ca alloys as biodegradable implant materials have become a hot spot [17]. However, they have a rapid degradation rate in the body and cannot maintain the mechanical properties for a long time. Therefore, grain refinement and texture control are used as methods to improve corrosion resistance of magnesium alloys. Op’T et al. [18] studied the corrosion of pure magnesium with different grains and electrochemical tests in NaCl solution. According to the change of current density and weightlessness, it could be seen that the corrosion resistance of pure magnesium in NaCl solution was enhanced with the decrease of grain size. Leil et al. [19] found that, when the alloy component is not changed, the corrosion characteristic of the magnesium alloy is closely related to the grain size and the number and distribution of the intermetallic compound. Kim et al. [20] investigated that the deformation of the magnesium alloy through the ECAP, the elongation is obviously increased, but the strength is decreased, which is mainly due to the weakening of the texture formed by the extrusion process and the strengthening effect of the grain refinement. Song et al. [21] indicated that hot rolling could refine the grain size of magnesium alloy and change the grain orientation. The grain boundary acts as a corrosion barrier and thus an alloy with small grain size has more such barriers to passivate the alloy surface, whereas an alloy with large grain size does not. In the corrosion structure of AZ31 magnesium alloy, the outer layer is composed of Mg (OH)2, and the inner layer is composed of Al2O3 and MgO. The unstable MgO in the inner layer will react with H2O to form Mg (OH)2. When Ce element is added to the alloy, the structure model of the corrosion layer will change. Because Ce has a strong affinity with O, Ce2O3 will be generated. Due to the low chemical activity of Ce2O3 and its insensitivity to NaCl corrosive medium, Ce2O3 can act as a passivating film, thus improving the corrosion resistance of the alloy.

In this paper, the corrosion behavior especially weight loss of different cross sections of AZ31-Ce sheet before and after annealing under different rolling conditions is studied in order to determine quickly the influence of heat treatment on the plane corrosion of samples and provide guidance in anticorrosion and use of magnesium alloy sheet.

2 Materials and methods

The alloy was prepared by direct electrical resistance melting in a mild steel crucible at 710 °C. Flux of 0.5% SF6 (volume fraction) + CO2 was also used to purify the melt, as well as prevent it from burning. AZ31-Ce blank with a chemical composition of Mg-3.5% Al-0.35% Zn-0.21% Ce (in wt.  %) fabricated through self-developed low-frequency electromagnetic semi-continuous casting (LFEC) technology [22] and homogenized were used in these experiments. High quality Mg alloy ingots with smooth surface and excellent metallurgical quality were achieved by the way of LFEC (see Fig. 1).

Fig. 1
figure 1

Schematic diagram of low-frequency electromagnetic semi-continuous casting

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The billet was manufactured to rectangular samples of 150 × 80 × 10 mm and rolled at the rolling mill of Ф260 × 400 mm work rolls. The rolling process was shown in Table 1. After rolling, the specimens with cross section of 10 × 10 mm were cut out in the same position of the sheet. One part was heat treated at 350 °C for 30 min and the other part was not done annealing. The six surfaces of specimens were polished with 2500# sand paper and immersed into the 3.5% NaCl solution for 5 days (see Fig. 2). After corrosion, the specimen was washed in chromic acid solution by ultrasonic cleaning machine for 10 min. The specimen was weighed before and after corrosion and the morphology of RD–TD, RD–ND and TD–ND surfaces were observed by OM (Leica DMR) and SEM (SHIMADZU SSX-550), respectively. Weightlessness, uncorroded area ratio and corrosion rate were calculated to characterize intuitively corrosion property of each surface.

Table 1 Experimental scheme of hot rolling
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Fig. 2
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Schematic diagram of rolling mill, plate size and corrosion specimen

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3 Results and discussions

3.1 Microstructure

The corrosion behavior of AZ31-Ce alloy is mainly determined by its microstructure, second phase size, number and distribution. Thus, it is significant to analyze the microstructure of AZ31-Ce under various conditions. It’s helpful to obtain a thorough understanding of the relationship between the corrosion behavior and microstructure.

Figure 3 displays the optical metallographic images of AZ31-Ce alloy under different conditions.

Fig. 3
figure 3

Optical metallographic microstructures of AZ31-Ce alloy under diverse conditions, HT-annealing: a 200 °C rolled alloy, No HT b 350 °C rolled alloy, No HT c 60% rolling reduction alloy, No HT d 7 rpm rolling rate alloy, No HT e 200 °C rolled alloy, HT f 350 °C rolled alloy, HT g 60% rolling reduction alloy, HT h 7 rpm rolling rate alloy, HT

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The rolled sheet is not heat treated (Fig. 3a–d), whilst the other sheet is disposed at 350 °C for 30 min (Fig. 3e–h). The sheet is rolled at 200 °C (Fig. 3a and e), 350 °C, 40% rolling reduction and 21 rpm rolling rate (Fig. 3b and f), 60% rolling reduction (Fig. 3c and g) and 7 rpm rolling rate (Fig. 3d and h). There are more deformed structure, which containing shear bands and twins in the no heat treated rolled sheet. The shear band are composed of fine grains, and twins are produced when deformation is difficult to carry out. The texture of the basal surface of AZ31-Ce magnesium alloy is very strong, and its orientation is not conducive to the activation of basal slip. The deformation will be concentrated inside the shear bands once formed, while the part outside the shear bands emerge small degree of deformation due to the unfavorable orientation. Therefore, several long shear bands will be produced in the low temperature rolling process, which is the result of strain concentration. When rolling at higher temperature, both prismatic slip and pyramidal slip can be activated to cancel and rearrange the dislocations, which relieve the stress concentration to a certain extent and delay the occurrence of the strain concentration and the shear bands.

The recrystallization grains are observed in the sheet after HT, but the grain size of each condition is different. The grain size of the sheet rolled at 200 °C is less than 10 μm (Fig. 3e), while the grain sizes are more than 10 μm (Fig. 3f–h). When the plate is rolled at lower temperature (200 °C, Fig. 3a), there are more distortion energy stored in the microstructure. After annealing, the distortion energy release makes the particles nucleate and grow up in many places, and the grain size is small. When the sheet is rolled under large reduction (60%, Fig. 3c), the grain is seriously broken, a large amount of distortion energy is stored in the microstructure, and the degree of grain recrystallizing is low. Zhi et al. [23] indicated that after rolling under high pressure, shear bands composed of a large number of fine grains was formed along the TD and parallel to the RD. Jia et al. [24] indicate that when the material rolled at lower speed (7 rpm in this paper, Fig. 3d), the twin was mainly composed of the parallel twin and the intersecting twin and the microstructure had less recrystallization. That is the recrystallizing nucleation mechanism is influenced and controlled by the plastic deformation mechanism [25]. Besides, the plastic deformation mechanism is affected by the rolling process.

The addition of Ce element to the AZ31 alloy can refine the structure and break the coarse β phase with a fine dispersion state. As shown in Fig. 4, the second phase particles are mostly distributed at the grain boundary, and individual larger particles are distributed in the matrix. EDS analysis of point 1 and 2 shows that white particles contain Ce element. The atomic percent of Al and Ce elements is about 11–3, so the second phase is Al11Ce3. The corrosion resistance of AZ31 alloy can be improved owing to adding Ce. The potential difference between Al–Ce phase rich in rare earth and aluminum elements and α-Mg matrix is smaller than that between β-Mg17Al12 and α-Mg matrix [26]. Therefore, addition of Ce can improve the corrosion resistance of AZ31 magnesium alloy in 3.5% NaCl solution.

Fig. 4
figure 4

a SEM micrograph of 350 °C rolled alloy with HT and EDS analysis of point 1 and 2, b XRD analysis of AZ31 sheet

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3.2 Corrosion rate and corroded surface morphology

The number of precipitated intermetallic particles and the densities of twins could be changed during annealing. Song eta al. [27]. indicated that the diversion of corrosion behavior in response to annealing cannot be the same for kind of alloy surfaces. After the magnesium alloy rolled by various processes, the corrosion of each surface needs to be comprehended before being used due to the difference in grain size and the misorientation in each section.

After the sample is corroded, the uncorroded area and weightlessness of each section are counted. Figure 5 is the SEM graph (a), OM of blaze (b) and area calibration (c) of corroded surface morphology of RD–TD cross section rolled at 300 °C. The red zones in Fig. 5a are uncorroded surfaces, which are the white points in Fig. 5b. Image Pro Plus (IPP) software is adopt to statistic the area of the white points (Fig. 5c). Corrosion rate and weightlessness percentage of rolled sheet at different temperatures with HT and without HT are shown in Fig. 6. The corrosion rate could be described with the following formula,

$$ I = \frac{{m_{0} - m_{1} }}{{t \cdot \sum\nolimits_{x = 1}^{6} {S_{x} } }} $$
(1)

where I is the corrosion rate (mg/d cm2), m0 is the weight of specimen before immersion, m1 is the weight of specimen after immersion, S is the one of the exposed area of the specimen, and t is immersion time.

Fig. 5
figure 5

SEM graph of corroded surface morphology, 300 °C RD–TD cross section as an example

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Fig. 6
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a Corrosion rate and b weightlessness percentage of rolled sheet at different temperatures with HT and without HT

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The corrosion rate of rolled sheet without HT fluctuate greatly with temperature. This is related to the grain size and stored distortion energy of the sheet after rolling. The corrosion rate turns faster with the larger grain size and higher distortion energy. In addition, the low deformation temperature and large deformation resistance would make the second phase distribution in the matrix not uniformed, which caused the variety of corrosion rate. However, the corrosion rate of rolled sheet with HT increase with the augment of temperature, but decreases at 350 °C. At 350 °C, the higher degree of recrystallizing, the minor distortion energy stored in the material, and the coarser grain size after annealing lead to the decrease of corrosion rate. When the α-Mg phase and the β-Mg17Al12 phase constitute a galvanic cell, the β-Mg17Al12 phase will promote the corrosion of α-Mg phase. The corrosion of α-Mg phase can be reduced by reducing the content of β-Mg17Al12 phase. When rare earth Ce is added to AZ31 magnesium alloy, the Al atomic weight is consumed due to the formation of a new phase, Al11Ce3, which results in a decrease in the content of β phase. Moreover, the Al11Ce3 phase can act as an anode block.

Figure 7 is corroded surface morphology of each cross section at various temperature with HT and without HT. The area percentage after corroded of each cross section at different temperatures is counted, and the results are shown in Fig. 8a. It can be seen from the Fig. 8 that TD–ND section has the most remaining area after corrosion, which indicates that TD–ND section has the best corrosion resistance. The rolled sheet mainly consisting of closely packed crystallographic plane (0001) and deformation structure will emerge crystallization and become equiaxed crystal after HT, which changed the anodic and cathodic reaction activities. Figure 8b is the schematic diagram of grain size and morphology with different cross sections. The grains at the TD–ND cross section are fine and uniformed with lower surface energy. Grain refinement makes the grain boundaries increase in the alloy, and a large number of grain boundaries can be used as a barrier to prevent the corrosion. For the alloy after annealing, the corrosion rate descends with the decrease of rolling temperature, the corrosion resistance of RD–TD section is the worst, while the corrosion resistance of TD–ND section is the best. For RD–TD section, the corrosion resistance of no heat treated alloy is better than that of annealing. For the RD–ND and TD–ND section, the opposite is true of the above.

Fig. 7
figure 7

Corroded surface morphology of different cross section at various temperature with HT and without HT a RD–TD, b TD–ND

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Fig. 8
figure 8

a Area percentage after corroded of each cross section at different temperatures, b schematic diagram of grain size and morphology with different cross sections

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4 Conclusions

The corrosion properties of each cross section of AZ31 magnesium alloy containing Ce before and after annealing at different rolling temperatures (200 °C, 250 °C, 300 °C and 350 °C) and the corrosion resistance and surface morphology of different sections of the samples were studied by immersing in 3.5 wt.% NaCl solution.

The sheet without annealing consisting of large numbers of twins, shear bands and distortional energy has worse corrosion resistance. For the alloy after annealing, the corrosion rate descends with the decrease of rolling temperature, the corrosion resistance of RD–TD across section is the worst, while the corrosion resistance and uncorroded area of TD–ND across section is the best. The ensemble weight loss of the sample rolled at 200 °C is least.