简介概要

Corrosion behaviors for peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films

来源期刊：Rare Metals2016年第10期

论文作者：Quan-Tong Jiang Kui Zhang Xing-Gang Li Yong-Jun Li Ming-Long Ma Guo-Liang Shi Jia-Wei Yuan

文章页码：758 - 762

摘要：The corrosion behaviors of T5（225℃,6.5 h）and T6（460℃,2 h + 225℃,12 h） peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films were investigated by optical microscope（OM）,scanning electron microscopy（SEM）,and energy-dispersive spectroscopy（EDS）.The weight loss rates and electrochemical tests were also analyzed.The thicknesses of T5 and T6 oxide films are roughly 0.6 and1.0 μm,respectively.The components of oxide films mainly consist of O,Mg,Y,Nd,and Gd,and the T6 oxide film results in surfaces with larger peaks than T5 oxide film.In addition,Y,Nd,and Gd peaks are all higher than those of Mg-7Gd-5YlNd-0.5Zr alloys,but Mg peak is consistently far below than that of the alloys.The specimens could be arranged in decreasing order of corrosion rates and corrosion current densities:T6 oxide film < T5 oxide film < T6 without oxide film < T5 without oxide film.The oxide films are compact to increase the corrosion resistance for Mg-7Gd-5Y-lNd-0.5Zr alloys,which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.

稀有金属(英文版) 2016,35(10),758-762

Corrosion behaviors for peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films

Quan-Tong Jiang Kui Zhang Xing-Gang Li Yong-Jun Li Ming-Long Ma Guo-Liang Shi Jia-Wei Yuan

State Key Laboratory for Nonferrous Metals and Process,General Research Institute for Nonferrous Metals

收稿日期：15 February 2014

基金：financially supported by the National Natural Science Foundation of China(No.51204020);the National Basic Research Program of China(Nos.2013CB632202 and 2013CB632205);

Corrosion behaviors for peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films

Quan-Tong Jiang Kui Zhang Xing-Gang Li Yong-Jun Li Ming-Long Ma Guo-Liang Shi Jia-Wei Yuan

State Key Laboratory for Nonferrous Metals and Process,General Research Institute for Nonferrous Metals

Abstract：

The corrosion behaviors of T5(225℃,6.5 h)and T6(460℃,2 h + 225℃,12 h) peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films were investigated by optical microscope(OM),scanning electron microscopy(SEM),and energy-dispersive spectroscopy(EDS).The weight loss rates and electrochemical tests were also analyzed.The thicknesses of T5 and T6 oxide films are roughly 0.6 and1.0 μm,respectively.The components of oxide films mainly consist of O,Mg,Y,Nd,and Gd,and the T6 oxide film results in surfaces with larger peaks than T5 oxide film.In addition,Y,Nd,and Gd peaks are all higher than those of Mg-7Gd-5YlNd-0.5Zr alloys,but Mg peak is consistently far below than that of the alloys.The specimens could be arranged in decreasing order of corrosion rates and corrosion current densities:T6 oxide film < T5 oxide film < T6 without oxide film < T5 without oxide film.The oxide films are compact to increase the corrosion resistance for Mg-7Gd-5Y-lNd-0.5Zr alloys,which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.

Keyword：

Mg-7Gd-5Y-1Nd-0.5Zr alloys; Peak-aged; Oxide film; Corrosion behavior;

Author： Kui Zhang,e-mail:zhkui@grinm.com;

Received： 15 February 2014

1 Introduction

Mg-RE alloys are attractive for aerospace and automotive industries due to their excellent properties [ 1] .However,the use of magnesium alloys is mainly limited by their poor corrosion [ 2] .Therefore,magnesium alloys usually should be treated to prepare anti-corrosive films on the surface in order to improve corrosion resistance before the application [ 3, 4] .According to the literatures,AZ91 alloys often have surface oxidation during the high-temperature manufacturing stages,which will form oxide film to protect alloys from corrosion during the applications [ 5, 6, 7] .As a result,if taking full advantage of this oxide film,it will play an important role in reducing the process and costs.

Compared to AZ91 alloys,the rare earths in Mg-RE alloys have higher activity than Mg,which will form the oxide film preferentially during high-temperature manufacturing stages,leading to an effect on the corrosion resistance properties.However,the reports on how the oxide film affects the corrosion behaviors of Mg-RE alloys are rare now.

In this work,surface oxide films formed during T5(225℃,6.5 h) and T6 (460℃,2 h+225℃,12 h) peakaged Mg-7Gd-5Y-1Nd-0.5Zr alloys were researched by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS).The corrosion behaviors of specimens with T5 oxide film and T6 oxide film were investigated,including weight loss rates,corrosion morphologies,and polarization curves.The research aimed to determine the effect of oxide films formed during the high-temperature manufacturing stages on corrosion resistance for Mg-RE alloys.This research will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environment.

2 Experimental

The materials used for heat treatment were cut from extruded Mg-7Gd-5Y-1Nd-0.5Zr alloy sheets.T5 (225℃,6.5 h)and T6 (460℃,2 h+225℃,12 h) peak-aged alloys were treated,respectively.The specimens for weight loss tests were 10 mm×10 mm×10 mm in sizes.The specimens with oxide films on the surface were heat-treated in the atmosphere,while the specimens without oxide films were treated in vacuum furnaces for comparison.The specimens were wet ground through successive grades of silicon c arbide abrasive papers from P240 to P5000.The etching reagent of5 ml HNO₃+95 ml ethanol was used to reveal the constituents and microstructures of Mg-7Gd-5Y-1Nd-0.5Zr alloys.The CarlZeiss Axiovet 2000MAT microscope was employed to take the optical micrographs (OM) of the specimens.

The corrosion rates were measured using three replicate specimens for each condition,and 3.5%NaCl salt spray tests for 3 days.The salt spray deposition was0.025 ml·cm^-2·h^-1.The corrosion products were removed in boiling chromic acid (20%CrO₃+1%AgNO₃) for5 min.The specimens were then quickly washed with deionized water,dried,and weighed to obtain their final weight (W₁).The difference between W₀ (original weight)and W₁ is the corrosion weight loss (ΔW [ 8] .The microstructures of specimens were characterized by OM,and surface corrosion morphologies were observed by SEM.

A Princeton VersaSTAT MC connected to a threeelectrode cell was used for the electrochemical measurements.The working electrode was the test material,the counter electrode was Pt,and the reference electrode was saturated calomel electrode.The corrosive medium was3.5%NaCl,and the polarization tests were carried out at a scan rate of 1 mV·s^-1.

3 Results and discussion

3.1 Microstructures

Figure 1 shows the OM images of specimens after T5 and T6 peak-aged.The microstructures of Mg-7Gd-5Y-1Nd-0.5Zr alloys consist of the primary a phase and the eutectic precipitations.The differences between the two conditions are that the precipitated phases distribute along the direction of the extrusion flow in Fig.1a,but distribute uniformly along the dendrite boundaries in Fig.1b.The grain sizes of specimens in two conditions are similar and roughly 20μm.The precipitated phases distribute uniformly,which are,respectively,simultaneously demonstrated to be Mg₇RE and Mg₅ (Gd_0.4Y_0.4Nd_0.2).There are few small cube-shaped phases distributed unevenly in the matrix and boundaries in Fig.1b,which are demonstrated to be MgRE₄ in previous work [ 9] .

Fig.1 OM images of Mg-7Gd-5Y-1Nd-0.5Zr alloys after T5 a and T6 b peak-aged

3.2 Oxide film analysis

Figure 2 shows SEM images and EDS analysis of the typical surface oxide films formed on the surface of specimens.From Fig.2,the boundaries between oxide films and alloys are obviously visible.The thickness of oxide film formed after T5 peak-aged is roughly 0.6μm,which could be identified in Fig.2a,while that of oxide film formed after T5 peak-aged is roughly 1.0μm (Fig.2c).

EDS analysis was performed to characterize the components of the oxide films by more than three replicate spots,and the test results are basically identical.The representative EDS results in Fig.2b and d show distinct characteristics for each oxide film.As expected,the T6 oxide film results in surfaces with larger oxygen peaks than T5 oxide film.The Mg,Y,Nd,Gd element peaks also appear obviously:Y,Nd,Gd peaks are all higher than those of the Mg-7Gd-5Y-1Nd-0.5Zr alloys,while Mg peak is consistently far below than that of the alloys.This is fully demonstrated that these elements have higher sensitivity than Mg for oxygen.However,the amounts of Mg and Y in the T6 oxide film become lower than those in the T5 oxide film,while those of the Nd and Gd are just the reverse.This phenomenon may be attributed to the generation of oxide compounds with different atomic ratios [ 10, 11] .

3.3 Corrosion rates

There are three specimens for each type,so the corrosion rates are the average value.The corrosion rates are shown in Fig.3.T5 without oxide film shows the highest corrosion rate of 2.6218 mg·cm^-2·day^-1,while T6 oxide filmhas the lowest one of 0.5192 mg·cm^-2·day^-1.The specimens could be arranged in the order of weight loss:T6oxide film<T5 oxide film<T6 without oxide film<T5without oxide film.This phenomenon may be due to the compact oxide films on the surface which protect the alloys from corrosion at the initial time.The thickness of T6oxide film is larger than that of T5 oxide film,so the specimens with T6 oxide film have a better corrosion resistance.Additionally,precipitated phases of T6 peak-aged distribute more uniformly than those of T5 peak-aged,resulting in an improvement on the corrosion resistance properties for the Mg-7Gd-5Y-1Nd-0.5Zr alloys [ 12, 13] .

Fig.2 SEM images and EDS analysis of oxide films formed on surface of specimens:a,b T5 peak-aged;c,d T6 peak-aged

Fig.3 Corrosion rates of specimens after salt spray in 3.5%NaCl for 3 days

3.4 Surface corrosion morphologies

The surface morphologies without corrosion products are displayed in Fig.4.For specimens with T5 oxide film,only exfoliation corrosion occurs in small areas along the direction of extrusion flow as shown in Fig.4a.Exfoliation corrosion grows irregularly in large areas under T5 without oxide film,andα-Mg phases are almost completely corroded (Fig.4b).The corroded pits form contiguously.On the other hand,only few quantities of corrosion pits are visible for the specimens with T6 oxide film (Fig.4c),while the number of corrosion pits increases and gradually they appear as a honeycomb distribution for specimens of T6 without oxide film in Fig.4d.It adequately indicates that oxide films play an important role in protecting the specimens from corrosion.

3.5 Electrochemical tests

Figure 5 presents the polarization curves of Mg-7Gd-5Y-1Nd-0.5Zr alloys.The open-circuit potential is comparatively stable before measurement,a corrosion product film forms on the surface of the specimens,and no clear localized corrosion occurs.

The values of I_corr (corrosion current density) and E_corr(corrosion potential) for specimens under different conditions are shown Table 1.The cathodic Tafel slopes are similar for specimens in different conditions,indicating that hydrogen evolution reaction occurs [ 14, 15] .As shown in Table 1,I_corr for T6 oxide film specimens is the lowest,illustrating that hydrogen evolution reaction is the most difficult on the surfaces,due to the protection of oxide film.I_corr for T5 without oxide film specimens is higher than those in other conditions.This phenomenon is attributed to the heterogeneous precipitation phases and no oxide film on the surface,which is consistent with Refs. [ 16, 17] .

Fig.4 Surface SEM images without corrosion products:a T5 oxide film,b T5 without oxide film,c T6 oxide film,and d T6 without oxide film

Fig.5 Polarization curves of specimens after immersion in 3.5%NaCl solutions

下载原图

Table 1 I_corr and E_corr of Mg-7Gd-5Y-1Nd-0.5Zr alloys evaluated from polarization curves

4 Conclusion

The precipitated phases after T6 peak-aged have a better distribution than those after T5 peak-aged.The thicknesses of T5 and T6 oxide films are roughly 0.6 and 1.0μm,respectively.The components of oxide films mainly consist of O,Mg,Y,Nd,and Gd,and the T6 oxide film results in surfaces with larger peaks than T5 oxide film.In addition,Y,Nd,and Gd peaks are all higher than those of Mg-7Gd-5 Y-1Nd-0.5Zr alloys,but Mg peak is consistently far below.

The specimens could be arranged in decreasing order of corrosion rates and corrosion current densities:T6 oxide film<T5 oxide film<T6 without oxide film<T5 without oxide film.The oxide films are compact to increase the corrosion resistance of Mg-7Gd-5Y-1Nd-0.5Zr alloys,which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (No.51204020) and the National Basic Research Program of China (Nos.2013CB632202and 2013CB632205).