Startseite Determination of corrosion product film on pure Mg in Cl− environment using XPS etching
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Determination of corrosion product film on pure Mg in Cl− environment using XPS etching

  • Lingxiong Sun , Deqing Ma , Ye Liu , Qingwei Qin , Liang Liang , Hongbin Ma EMAIL logo , Fuan Wei und Chao Zhang
Veröffentlicht/Copyright: 15. August 2023
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Corrosion Reviews
Aus der Zeitschrift Corrosion Reviews Band 41 Heft 6

Abstract

X-ray photoelectron spectroscopy (XPS) combined with Ar ion etching was used to analyse the surface film of pure Mg at different depth after immersion in 3.5 % NaCl solution for 10 min. The XPS spectra of specimen surface showed that the corrosion products are mainly made up of Mg(OH)2 and Mg2Cl(OH)3·xH2O. The formation process of Mg2Cl(OH)3 is the reaction of Mg(OH)2 and Cl− and H+ in weak acidic solutions. The XPS results indicated that the intensities of Mg2Cl(OH)3·xH2O decreased with the increase of etching time from 0 s to 4680 s. It is confirmed that the edge of Mg(OH)2 protrudes outward and then splits into Mg2Cl(OH)3 when Cl− attacks the Mg(OH)2 films, so the Mg2Cl(OH)3 attached to Mg(OH)2. Meanwhile, coupling the scanning electron microscope (SEM) and transmission electron microscopy (TEM) with the XPS to analyze the corrosion mechanism. Furthermore, the results displayed that the XPS combined with Ar ion etching is a good characterization method to understand the reaction of corrosion products.

Keywords: corrosion; pure Mg; surfaces; XPS etching

1 Introduction

Mg is one of the lightest metal structural material in engineering applications, with advantages such as high specific strength and specific modulus, good conductivity and thermal conductivity, and recyclability (Ahmadkhaniha et al. 2016; Cao et al. 2018; Han et al. 2022; Jamesh et al. 2011; Kuang et al. 2019). With the rapid development of low-cost Mg production technology and the urgent need for lightweight, energy-saving and consumption reduction, the application of Mg in industries such as shipbuilding and submarine industries has received great attention. However, its weak corrosion resistance at room temperature is one of the obstacles limiting its wide range of applications (Li et al. 2022; Ma et al. 2022; Sun et al. 2022a). It is well acknowledged that one of the reasons of the poor corrosion resistance of Mg immersion in Cl− environment is attributed to the loose and porous corrosion products (Wang et al. 2018a; Zhang et al. 2020; Zhu et al. 2022). The corrosion product film formed on Mg alloys in simulated physiological NaCl conditions includes not only Mg(OH)2, but also includes carbonate and calcium phosphate products (Virtanen et al. 2008). Furthermore, the corrosion product films formed on Mg alloys in seawater NaCl are mainly composed of Mg(OH)2 and MgCO3, and also includes the microorganisms and bacterial substances (Dong et al. 2023; Gao et al. 2021; Wang et al. 2018b). However, the influences of the corrosion products containing Cl during the Mg immersion in Cl− environment on the corrosion resistance have little reported. Therefore, the corrosion products of Mg in Cl− environment are still worth investigated. Some researchers have proposed that the Cl− in NaCl solution could transform insoluble Mg(OH)2 into soluble MgCl2 (Dai et al. 2022; Jiang et al. 2021; Mao et al. 2017). Although our previous reports have observed the corrosion products of Mg in 3.5 % NaCl environment including pine-like Mg2Cl(OH)3 by TEM and SEM (Sun et al. 2022b, 2022c), the formation process of Mg2Cl(OH)3 needs to be further determined. XPS can not only determine the chemical state, but also detect the chemical composition of the corrosion products. XPS combined with Ar ion etching can further reveal the composition of corrosion products at different depths. Therefore, XPS is very suitable to analyse the chemical structure of the corrosion products at different depths. Considering the mechanism research on actual application that the XPS combined with Ar ion etching was used to analyse the surface film of pure Mg after immersion in 3.5 % NaCl solution for 10 min, in which the XPS measurements were mainly used to determine the surface chemical composition of the corrosion product with different etching depths. In addition, coupling the SEM and TEM with the XPS to analyze the formation process of the corrosion product film. Such an investigation aims to understand the mechanism of Mg immersion in Cl− environment during their applications.

2 Materials and methods

The pure Mg (>99.99 wt%) ingot with size of 1 × 1 × 0.3 cm3 was used as specimens, which immersed in 3.5 wt% NaCl solution (pH = 6.26) at room temperature. The MgO/Mg(OH)2 films on specimens surface were removed by grinding with 2000#∼7000# abrasive papers before the test. Placing the specimens in the solution, which test surface facing upwards for 10 min. Then use plastic tweezers to remove the specimen from the solution and let it dry naturally in air. Finally, testing the dried specimens by XPS (ESCALAB). The XPS with a monochromatic Al-Kα source was used to characterize the corrosion products film. During the Ar ion etching process, the energy of Ar ion was 500 eV and the etching time step was 60 s. To ensure the repeatability of data, specimens treated under the same conditions were tested three times. The Advantage software was used for peak fitting of the XPS data by referencing to NIST database (https://icsd.nist.gov). The microstructures of the corrosion product film were investigated by field emission SEM (ZEISS 6035) and TEM (JEM-2100F) with energy dispersive X-ray spectroscopy (EDX, Oxford Instruments).

3 Results and discussion

Figure 1a is a schematic diagram of XPS testing principle in this work. When the specimen is irradiated by X-rays, photoelectrons are excited from the specimen. The XPS system works by collecting photoelectrons into a spectrometer. XPS computer draws the light emission intensity as a function of binding energy. Due to the limited depth of X-ray penetration into the specimen, the deeper information on the specimen surface was collected at an angle of 58° from the specimen using X-ray combined with Ar ion etching. The ion beam scans the specimen surface, and the surface material of the specimen is gradually etched off. The XPS spectrum is collected during the etching cycle, and the light emission intensity can be observed as a function of the change of binding energy. Firstly, the XPS is used X-ray to radiate the specimen surface to obtain the chemical composition of the specimen surface before etching. Then, the specimen surface at different depths is etched by Ar ion to obtain the surface chemical composition at different etching depths. Finally, until etched to the surface of Mg substrate.

Figure 1: Schematic diagram of XPS testing principle. (a) The testing sample surface, (b) XPS spectra of specimen surface before etching.
Figure 1:

Schematic diagram of XPS testing principle. (a) The testing sample surface, (b) XPS spectra of specimen surface before etching.

Figure 1b presents the XPS results of the specimen surface without etching. The Mg 1 s spectrum reveals two peaks at 1303.9 eV and 1302.7 eV, corresponding to Mg elements in MgO and Mg(OH)2. The spectra for Mg 2p are divided into three peaks, which could be fitted by MgO (51.0 eV), Mg(OH)2 (49.5 eV) and Mg2Cl(OH)3·xH2O (53.4 eV). The peaks located at 532.1 eV, 530.9 eV and 534.7 eV in O 1 s spectrum of the corrosion products indicate the presence of MgO, Mg(OH)2, and Mg2Cl(OH)3·xH2O on the surface film. The Mg2Cl(OH)3·xH2O peak at 202.6 eV exists in the Cl 2p spectrum of Mg immersion in NaCl solution. The formation of MgO is attributed to the Mg exposure to air and Mg(OH)2 is attributed to the OH− in the H2O solution. Besides, the formation process of Mg2Cl(OH)3 should be further verified by XPS etching.

To obtain the chemical composition of the surface film after Mg immersion in NaCl solution, XPS result of the surface film after etching for 960 s is showed in Figure 2. The chemical composition of the specimen surface did not change before etching 960 s, whereas the relative content has changed. The spectrum of Mg 1 s displays the peak of Mg2Cl(OH)3·xH2O at 1301.4 eV, which indicates that there is numerous of Mg2Cl(OH)3 in the alloy surface layer. It is worth noting that the spectrum of Mg 2p appears one peak corresponding to Mg (49.8 eV), which means that the X-ray penetrated to Mg substrate at the etching time of 960 s. In addition, peaks at other locations still exist.

Figure 2: XPS spectra with 960 s etching of specimen surface.
Figure 2:

XPS spectra with 960 s etching of specimen surface.

XPS spectrum of specimen surface with 3960 s etching is carried out (shown in Figure 3). The peaks of Mg, MgO, Mg(OH)2, and Mg2Cl(OH)3·xH2O have no change with the increase of etching time from 960 s to 3960 s. It can be found that the relative intensity of Mg peak gradually increased, which means that this part is further closed to Mg substrate. The relative intensity of Mg2Cl(OH)3·xH2O peaks decreases, which indicates that the near Mg surface area are mainly composed of Mg(OH)2 and MgO films. It seems that the peaks intensity of MgO are relatively high during the whole etching time, which is attributed to the short corrosion time, and the MgO layer is not completely destructed.

Figure 3: XPS spectra with 3960 s etching of specimen surface.
Figure 3:

XPS spectra with 3960 s etching of specimen surface.

The composition of the surface film after etching for 4680 s is obtained by XPS in Figure 4. The peaks of Mg(OH)2 in the O 1 s and Mg 2p spectra gradually disappeared and the peaks relatively intensity of MgO also decreased and the peak of Mg gradually increased, indicating that this part is Mg substrate after removal of corrosion products. It can be found that there are almost no Mg2Cl(OH)3·xH2O peaks in this part, which indicates that Mg2Cl(OH)3 is formed by the reaction of Mg(OH)2 and Cl− rather than the reaction of Mg substrate and Cl−. Meanwhile, this result indicated that when Mg(OH)2 at the edge is eroded and split into Mg2Cl(OH)3 by Cl− and H+, the Mg2Cl(OH)3 is formed.

Figure 4: XPS spectra with 4680 s etching of specimen surface.
Figure 4:

XPS spectra with 4680 s etching of specimen surface.

To further investigate the information of corrosion product films, SEM and TEM of corrosion products were performed in Figure 5. Figure 5a and b displays the corrosion morphologies after corrosion 5 min and 10 min in NaCl solution. With the immersion time increases, it is observed that needle-like corrosion products form on the surface increase (as shown in the yellow box). After falling off the corrosion products with plastic tweezers, the corrosion product film is dispersed in an alcohol solution through ultrasound, and then dropped onto the carbon film to obtain a TEM specimen. Figure 5c–i shows the selected area electron diffraction (SAED), high-resolution TEM (HRTEM) and energy dispersive X-ray (EDX) of corrosion products after corrosion 10 min in 3.5 % NaCl solution. It was found that the corrosion products consist of Mg(OH)2 and Mg2Cl(OH)3. One typical SAED pattern acquired from white circle areas in Figure 5c. Two red diffraction rings can be well indexed into Mg(OH)2. The yellow diffraction rings corresponding to Mg2Cl(OH)3. Figure 5e is the HRTEM image of Mg(OH)2 and Mg2Cl(OH)3, and the interplanar spacings are 0.236 nm and 0.183 nm, which can be indexed to the (101) and (031) planes, respectively. The elemental distributions of corrosion products were obtained by EDX mapping. The H element cannot be detected by EDX mapping, only Mg, O, Cl elements were found. The main corrosion products are Mg(OH)2 and Mg2Cl(OH)3, which are consisted with XPS and HRTEM results.

Figure 5: SEM [after corrosion (a) 5 min and (b) 10 min in NaCl solution] and TEM (c–i) images of the corrosion product film.
Figure 5:

SEM [after corrosion (a) 5 min and (b) 10 min in NaCl solution] and TEM (c–i) images of the corrosion product film.

After the exposure of pure Mg in air, a film of MgO formed quickly on the Mg surface. The MgO film is usually loose and unprotective, so the electrochemical corrosion reaction generates Mg(OH)2. The NaCl in solution is hydrolyzed in the solution to form anion diffusion to the anode region, which reacted with Mg(OH)2 to form Mg2Cl(OH)3·xH2O. In other words, when the H+ and Cl− attacks the Mg(OH)2, the edge of Mg(OH)2 first splits into Mg2Cl(OH)3. Therefore, it can be found from the SEM that the Mg2Cl(OH)3·xH2O is attached to Mg(OH)2.

Based on the above analysis, the data can confirm that the spectra of corrosion products are only consist of Mg(OH)2 and Mg2Cl(OH)3·xH2O. In other words, the formation reaction of Mg2Cl(OH)3 can only be that Mg(OH)2 reacts with Cl− and H+ in weak acidic solutions. However, when insoluble Mg(OH)2 is attacked by H+ and Cl− in a strong acidic solution, soluble MgCl2 will be formed. Therefore, the MgCl2 spectrum can not be found in the XPS combined with Ar ion etching process in this work. From this perspective, the formation reaction of Mg2Cl(OH)3 can be analyzed by XPS and Ar ion etching.

4 Conclusions

The surface films of corrosion products were analyzed using XPS combined with Ar ion etching and SEM and TEM. The surface products were determined to include Mg(OH)2 and Mg2Cl(OH)3·xH2O. As the corrosion time increases, the surface Mg2Cl(OH)3 content relatively increases. Furthermore, as the etching time increases, Mg2Cl(OH)3 gradually disappears. The formation process of Mg2Cl(OH)3 was verified that the edge of Mg(OH)2 is preferentially attacked by Cl− and H+, so the outer edge of Mg(OH)2 is preferentially split into Mg2Cl(OH)3 in weak acidic solutions. In addition, XPS combined with Ar ion etching is a good characterization method to determine the occurrence of chemical reaction.

Corresponding author: Hongbin Ma, Qinghai Provincial Key Laboratory of New Light Alloys, Qinghai University, Xining810016, P.R. China, E-mail:

  1. Author contributions: Lingxiong Sun: Concepts, investigation, experiments, writing-manuscripts; Deqing Ma: Preparation of materials; Ye Liu: Investigation; Qingwei Qin: Investigation; Liang Liang: Concepts; Hongbin Ma: Concepts, investigation, methods, supervision, editing and revision; Fuan Wei: Concepts; Chao Zhang: Concepts.

  2. Competing interests: The authors declare no conflicts of interest regarding this article.

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Received: 2023-05-15
Accepted: 2023-06-26
Published Online: 2023-08-15
Published in Print: 2023-12-15

© 2023 Walter de Gruyter GmbH, Berlin/Boston

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  5. A review on corrosion and corrosion inhibition behaviors of magnesium alloy in ethylene glycol aqueous solution
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https://doi.org/10.1515/corrrev-2023-0064
Schlagwörter für diesen Artikel
corrosion; pure Mg; surfaces; XPS etching

Artikel in diesem Heft

  1. Frontmatter
  2. Reviews
  3. Organic compounds as corrosion inhibitors for reinforced concrete: a review
  4. The role of microbes in the inhibition of the atmospheric corrosion of steel caused by air pollutants
  5. A review on corrosion and corrosion inhibition behaviors of magnesium alloy in ethylene glycol aqueous solution
  6. Original Articles
  7. Study of the corrosion mechanism of Mg–Gd based soluble magnesium alloys with different initial texture states
  8. Determination of corrosion product film on pure Mg in Cl− environment using XPS etching
  9. High-temperature corrosion behavior of S30432 in high-efficiency ultra-supercritical boiler burning low-alkali and high-sulfur coal
  10. Image recognition model of pipeline magnetic flux leakage detection based on deep learning
  11. Quantum chemical analysis of amino acids as anti-corrosion agents
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