Recent Progress on Atmospheric Corrosion of Field-Exposed Magnesium Alloys

Abstract

It is well known that the poor corrosion resistance of magnesium alloys is a key factor limiting their application. Field exposure is the most reliable means to evaluate the atmospheric corrosion performance of magnesium alloys. This article reviews the field exposure corrosion behavior of magnesium alloys in typical atmospheric environments (including the marine atmosphere, industrial atmosphere, etc.) in recent years. According to the literature review, it was found that there are significant regional differences in the atmospheric corrosion behavior of magnesium alloys, which is the result of the coupling of multiple factors in the atmospheric environment. By investigating the corrosion rate and corrosion products of different types of magnesium alloys in different environments, the corrosion mechanism of magnesium alloys in different environments was summarized. Specifically, environmental parameters such as atmospheric temperature, relative humidity, CO₂, and chloride ion deposition rates in the marine atmospheric environment can affect the corrosion behavior of magnesium alloys. The corrosion of magnesium alloys in different industrial atmospheric environments is mainly affected by atmospheric temperature and relative humidity, as well as atmospheric pollutants (such as SO₂, CO₂, NO₂) and dust. This review provides assistance to the development of new corrosion-resistant magnesium alloys.

1. Introduction

1.1. Magnesium Alloys Advantages and Prospects

As is well known, magnesium alloys is the lightest metal structural material, with high specific strength and specific stiffness, excellent cuttability, etc. [1]. In recent years, magnesium alloys has attracted extensive attention all over the world [2,3,4,5,6,7,8,9,10]. Significant progress has been made in the design of magnesium alloys and research on plastic deformation technology. A series of high-strength, thermally conductive, and heat-resistant magnesium alloys have been developed and widely used in fields such as transportation, aerospace, 3C products (computer, communication, and consumer electronics). The application of magnesium alloys can effectively reduce the weight of transportation and reduce global greenhouse gas emissions. However, poor corrosion resistance limits the application of Mg alloys, due to their high chemical and electrochemical activities [11,12,13,14,15].

1.2. Limitations and Improvements in Magnesium Alloys

The corrosion behavior of magnesium alloys is mainly influenced by environmental factors, alloy compositions, and manufacturing processes [16,17]. Figure 1 shows the microstructure of magnesium and several common magnesium alloys. It can be seen that there are different contents during the second phase (β-Mg₁₇Al₁₂) and particulate phases (Mn-Al, etc.) in magnesium alloys.

Magnesium has the lowest standard electrode potential among the metal structural materials. Moreover, the atmospheric oxide film of magnesium alloys is loose and porous, with a P-B ratio (Pilling-Bedworth ratio) of 0.81 (the value may vary depending on the specific alloy composition and conditions, but it is generally less than 1), which cannot form an effective protective film. Corrosion problems such as galvanic corrosion and localized corrosion are prone to occur, which restricts the widespread application of magnesium alloys. The types of atmospheric corrosion of magnesium alloys include galvanic corrosion, localized corrosion, etc., which affect the service life of magnesium alloys and restrict their wide application [18,19]. The influence of environmental factors on corrosion behavior is much more complex than that of magnesium alloys corrosion in aqueous environments, and it is difficult to predict the effects based on corrosion behavior in aqueous environments [20].

In this work, the corrosion behavior and mechanism of magnesium alloys in different atmospheric environments were summarized and compared with indoor simulated accelerated corrosion experiments.

2. Corrosion Behavior of Magnesium Alloys in Different Atmospheres

Magnesium alloys are mainly exposed to the atmosphere during practical applications such as in transportation, aerospace, automotive, and communications industries. Researchers have studied the atmospheric corrosion behavior of magnesium alloys through exposure experiments, including marine, urban, and rural atmospheric environments. The main types of magnesium alloys are the AZ(Mg-Al-Zn), AM(Mg-Al-Mn), and Mg-Re series.

2.1. Corrosion Behavior of Magnesium Alloys in Marine Atmospheric Environments

The corrosion behavior of magnesium alloys in marine atmospheric environments is complex and influenced by various factors. In actual marine atmospheric exposure, the synergistic effects of temperature, relative humidity (RH), CO₂, Cl⁻ deposition rate, atmospheric particles, and ultraviolet irradiation have a great influence on the corrosion of metals [21,22]. Normal atmospheric chloride levels are low and generally do not pose a threat to the environment. However, in coastal areas, the dissociation of salt molecules can form Na⁺ and Cl⁻. These ions can migrate and settle in air currents within a certain time and space scale, bringing pollution and damage to the environment. Cui et al. [23] investigated the corrosion process of AZ91D specimens in simulated haze solution and the effect of ions. The results show that Cl⁻ is aggressive to AZ91D and its surface corrosion is mainly caused by pitting.

In the marine atmospheric environment, the prevailing ambient relative humidity is high. Magnesium alloys are very prone to crack formation and serious localized corrosion in alternating wet and dry environments [24]. In China, due to its vastness and long coastline, researchers have conducted field-exposed experiments of magnesium alloys in many coastal cities. Similarly, in other countries and regions, many scholars have conducted exposure experiments on magnesium alloys in the marine atmospheric environment, such as in Japan, France, and so on.

Table 1. Corrosion and environmental information of magnesium alloys in marine atmospheric environments in different regions.

Exposed Locations	Magnesium Alloy Types	Exposure Time	Environmental Parameters	Corrosion Depth	Corrosion Rate
Xisha (China) [27]	AZ31	2 years	Temp: 27 °C RH: 77% Cl−: 64.39 mg/m2 d pH: 6.5	−	17.66 μm/a
Nansha (China) [28]	Mg-1.5Nd	2 years	Temp: 28~30 °C Rainfall: 2800 mm/a	103.4 μm	51.70 μm/a
	Mg-1.0Nd			138.6 μm	69.30 μm/a
	Mg-0.5Nd			167.4 μm	83.70 μm/a
Xiamen (China) [29]	AZ91D	6 months	Temp: 14~28 °C RH: 64~80%	−	6.55 μm/a
	AM60			−	6.93 μm/a
	ZE41			−	6.61 μm/a
Zhongshan station (Antarctica) [30]	AZ31B	2 years	Temp: −9.9 °C RH: 62.5% Lowest temp: −36.4 °C	24.3 μm	11.13 μm/a
Shimizu (Japan) [16,31]	AZ31B	3 years	Temp: 15~27 °C Temp: 8~29 °C RH: 58~84% Cl−: 4.2 mg/m2·d $\mathrm{S}{\mathrm{O}}_4^{2-}$: 0.7 mg/m2·d	−	46.82 μm/a
Choshi (Japan) [32]	AZ91D	5 years	Temp: 15.1 °C RH: 79% Cl−: 17.2 mg/m2·d SO2: 2.1 mg/m2·d	43.6 μm	3.16 μm/a
Miyakojima (Japan) [32]	AZ91D	5 years	Temp: 23.8 °C RH: 80% Cl−: 40 mg/m2·d SO2: 1.2 mg/m2·d	63.6 μm	2.11 μm/a
Brest (France) [33]	AZ91D	12 months	Temp: 12.5 °C RH: 84% Cl−: 42.5 mg/L pH: 6.1	−	4.20 μm/a
	AM50			−	8.80 μm/a
Texas (United States) [34]	AZ91D	−	−	−	19.2 μm/a
Research Vessel KEXUE [35,36,37]	AZ31	2 years	Temp: −0.9~33.1 °C RH: 18~97% Cl−: 64~1130 mg/m2·d Wind speed: 5.2 m/s	Deepest: 276.3 μm	52.23 μm/a
	AZ91	2 years	Temp: 0~31 °C RH: 34~94% Cl−: 110~530 mg/m2·d	Deepest: 196.9 μm	32.50 μm/a
	EW75	3 months	Temp: 25.9 °C RH: 77.7% Cl−: 413.65 mg/m2·d Wind speed: 5.23 m/s	150.3 μm	90.30 μm/a

summarizes the corrosion rates of common magnesium alloys in marine atmospheric environments both domestically and internationally. Some studies conducted in earlier years are not presented in the table [25,26].

Summarizing the corrosion rate of magnesium alloys in marine atmospheric environments of different countries and regions, it was found that environmental factors have a strong impact on the corrosion behavior of magnesium alloys. The corrosion rate of commonly used AZ series magnesium alloys decreases with the increase of Al content, and the corrosion resistance of AZ91 magnesium alloy is stronger than that of commonly used magnesium alloys such as AZ31, AM60, and AM50. In addition, islands and research vessels had higher average temperatures compared to other exposed environments during the experiment period. Due to the environmental conditions surrounded by the sea, the wind and waves are strong, and the relative humidity is generally higher than 65%, which makes it a high humidity environment. There is also a phenomenon of dry wet cycles, which significantly increases the corrosion rate of magnesium alloys.

Based on the high temperature, high humidity, and high salt spray environment, Cui et al. [27] found that the corrosion rate of AZ31 is 17.66 μm/a in Xisha. Temperature increases the kinetics of chemical reactions, and localized metal temperatures may differ from average temperatures due to heat transfer [38]. To verify this viewpoint, some researchers conducted simulation experiments indoors. Merino et al. [39] concluded that the corrosion intensity increased with increasing temperature, and it was found that the decreasing order of corrosion strength is Mg > AZ31 > AZ91D > AZ80. Song et al. [40], who also conducted indoor simulation experiments, developed an environmental simulation system for the ocean atmosphere. According to observations, as the ambient temperature increases, the decreased luminance of the sample exposed to simulated atmospheric conditions increases, the surface gloss decreases, and the corrosion becomes more severe.

In general, with the increase of relative humidity, the magnesium alloys surface liquid film molecular layer is thicker, and the corrosion product layer and protective ability subsequently weaker. Yu et al. [29] experimented on pure Mg, AM60, AZ91D and other magnesium alloys exposed to the real marine atmosphere on the island (Standard Xiamen Marine Environment Experiment Station) and found that the effect of RH is significant. For further confirmation, Yu et al. summarized the weight loss rates of pure magnesium and magnesium alloys in real marine atmospheric environments and laboratory simulated environments (wave impacts and salt spray) through indoor and field-exposed comparison tests (Figure 2). As shown in the figure, the corrosion rates of the four materials on islands are generally higher than those in inland areas, and the author found that the simulated sea wave impact test had a better correlation with field-exposed experiments.

LeBozec et al. [38] also found through indoor simulation experiments that increasing relative humidity from 75% to 95% led to an increase in corrosion rate. For comparison, the corrosion rate is significantly higher than most field-exposed experiments in marine atmospheric environments [40].

The chlorine in the marine atmospheric environment is higher than that in other atmospheric environments. Liao et al. [16,31] conducted experiments in a coastal environment (Shimizu City) and found that under the influence of sea salt, the corrosion rate was higher than that in urban environments, and this explains the reason for the high corrosion rate. In the dynamic marine atmospheric environment, the deposition rate of chloride was extremely high, and was above 100 mg/m²·d most of the time. Jiang and Yang et al. [35,36,37] conducted experiments on the Research Vessel KEXUE and found that the corrosion rate in dynamic marine atmospheric environments is much higher than that in other static field-exposed tests conducted at fixed locations in marine environments, and is 5–8 times higher than that of static coastal field exposure. However, the corrosion rate of AZ31 magnesium alloy in steady state indoor simulation in 3% NaCl atmosphere was as high as 2.3 mm/a, which is much higher than the corrosion rate observed in field-exposed experiments. Therefore, indoor simulation experiments cannot fully simulate the real corrosion situation and need to be combined with field-exposed experiments.

Summarizing the above papers, it is found that the corrosion products of magnesium alloys in marine atmospheric environments are mainly MgCO₃·xH₂O (x = 3, 5) and Mg₂(OH)₃Cl·4H₂O, etc. (as show in Figure 3). The cathodic reaction of magnesium is believed to be due to the reduction of water reduction and to oxygen reduction [41,42]. The total reaction equation is:

2Mg + O₂ + H₂O → 2Mg(OH)₂

(1)

Carbon dioxide is widely present in the atmosphere as a greenhouse gas and is involved in the corrosion of magnesium alloys. At normal CO₂ concentrations, brucite will react directly with CO₂ to form magnesite:

Mg(OH)₂ + CO₂ → MgCO₃ + H₂O

(2)

MgCO₃ + xH₂O → MgCO₃·xH₂O

(3)

In the marine atmospheric environment, the humidity and chloride ions in the air are high, CO₂ dissolves in water to form carbonic acid, which cooperates with chloride ions and participates in the corrosion of magnesium alloys, and the main equations are as follows [43]:

2Mg(OH)₂ + HCO₃⁻ + 2H₂O → Mg₂CO₃(OH)₂·3H₂O + OH⁻

(4)

2Mg(OH)₂ + Cl⁻ + 4H₂O → Mg₂(OH)₃Cl·4H₂O + OH⁻

(5)

Some researchers have explored the effect of CO₂ on the corrosion of magnesium alloys through indoor simulation experiments, and found that CO₂ can effectively inhibit the corrosion behavior of magnesium alloys, so that it indicates a more uniform corrosion, and can regulate the surface pH of magnesium alloys [27,44,45].

In summary, the above analysis of literature on on-site exposure of magnesium alloys indicates that the corrosion rate of magnesium alloys in marine atmospheric environments varies significantly in different regions. The corrosion behavior of magnesium alloys in marine atmosphere is influenced by the type of magnesium alloy, the content of alloying elements, and the second phase microstructure. Meanwhile, environmental parameters such as atmospheric temperature, relative humidity, and chloride ion deposition rate affect the corrosion behavior of magnesium alloys. However, compared to indoor simulation experiments, field-exposed experiments are relatively under-researched, and researchers are called upon to carry out field-exposed experiments.

2.2. Corrosion Behavior of Magnesium Alloys in Industrial Atmospheres

The industrial atmosphere is one of the common environments in which magnesium alloys are used in service, and there are a variety of contaminating factors in the environment. The industrial atmospheric environment is similar to the urban atmospheric environment, but it is very different from the marine atmospheric environment.

Scientists conducting field-exposed studies have also conducted research on the corrosion behavior of magnesium alloys in industrial atmospheric environments in various regions.

Table 2. Corrosion and environmental information of magnesium alloys in industrial atmospheric environments in different regions.

Exposed Locations	Magnesium Alloy Types	Exposure Time	Environmental Parameters	Corrosion Depth	Corrosion Rate
Shenyang (China) [46]	AZ80	12 months	Temp: 9.18 °C RH: 62.96% Rainfall: 916.2 mm/a	−	2.95 μm/a
	EW75			−	11 μm/a
Shenyang (China) [47]	EW75	12 months	Temp: 9.18 °C RH: 62.96% Rainfall: 916.2 mm/a	−	15 μm/a
Taiyuan (China) [48]	AM60	12 months	RH: 55% SO2: 18~106 μg/m3 NO2: 25~39 μg/m3 PM10: 73~113 μg/m3	8.5–28.6 μm *	0.8 μm/a
Jiangjin (China) [49]	AZ61	2 months	SO2: 256.5 μg/m3 NO2: 380 μg/m3 H2S: 21.20 mg/m2·d SO3: 93.83 mg/m2·d pH: 4.77	1.41 μm	8.5 μm/a
Beijing (China) [50]	AZ91D	85 days	Temp: 11.6 °C RH: 57% Rainfall: 586.0 mm/a SO2: 48~80 mg/m3 CO2: 45~49 mg/m3 NO2: 28~43 mg/m3	−	10 μm/a
Stockholm (Sweden) [33]	AZ91D	12 months	Temp: 8.3 °C RH: 76% Cl−: 2.2 mg/L pH: 5.2	−	1.8 μm/a
Osaka City (Japan) [31]	AZ31B	12 months	Temp: 5~29 °C RH: 54~72% Cl−: 1.1 mg/m2·d $\mathrm{S}{\mathrm{O}}_4^{2-}$: 3.4 mg/m2·d	−	24.2 μm/a

*: The paper did not specify the exact numerical value, which was estimated as a microscopic characterization that roughly reflects the corrosion depth.

shows the corrosion rates of magnesium alloys in industrial atmospheric environments in different regions.

Data research revealed that, in industrial atmospheric settings, magnesium alloys of the EW75 series are generally more prone to corrosion than those of the AZ series. It was discovered that the corrosion rate of magnesium alloys in industrial atmospheric environments is relatively low, and their resistance to environmental damage is weaker than that of magnesium alloys in marine atmospheric environments.

SO₂ is one of the major gaseous pollutants in the industrial atmosphere, and it has a non-negligible impact on the atmospheric corrosion of metals. Corrosion of magnesium alloys can be accelerated by the coupling of pollutants such as S, Cl, and dust particles in the atmosphere and the microstructure of the Mg substrate. Wan Ye in the study of the effect of SO₂ atmosphere in the atmospheric corrosion behavior of AZ91D magnesium alloy found that the corrosion products show the presence of crystalline water, and the corrosion mass loss of the specimens in SO₂ atmosphere is greater than the corrosion mass loss in purified air, confirming that SO₂ increases the corrosion process; the surface of the specimen showed a relatively homogeneous corrosion morphology and had a synergistic effect on the atmospheric corrosion of AZ91D magnesium alloy with the soluble salts [51]. Performing simulations in the laboratory, Esmaily et al. [52] found that the presence of SO₂ at the ppb level accelerated the atmospheric corrosion of Mg-Al alloys. The mass of the alloy increased in the presence of ppb concentrations of SO₂. The atmospheric corrosion of the Mg-Al alloys in the presence of SO₂ exhibited localized corrosion and the main corrosion product formed was magnesium sulfite (MgSO₃). The corrosion rate of magnesium alloys calculated in an indoor SO₂ atmosphere was 6.66 μm/a. Similarly, by means of indoor simulation experiments, Lin et al. [53] found the two phases of corrosion weight gain were that phase 1 was consistent with a linear increase and phase 2 with an exponential decay. In field-exposed experiments with severe air pollution, although the results of indoor experiments are generally the same as those of field-exposed experiments, it is still not possible to fully simulate the corrosion rate and composition of corrosion products.

SO₂ in the industrial atmospheric environment combines with water to form a strong acidic electrolyte and produces soluble Mg sulfite or sulfate, promoting corrosion of magnesium alloys [48,52,54]. The dominant corrosion products are MgSO₃·6H₂O and MgSO₄·6H₂O, and the main reaction equation is:

Mg(OH)₂(surface) + SO₂(g) → MgSO₃ + H₂O

(6)

In industrial atmospheres, elements S and N are widely emitted into the atmosphere. The corrosion of magnesium alloys by acid rain cannot be ignored. Liu et al. [55] investigated the corrosion behavior of AZ31 by simulating an acid rain environment. The concentrations of the components in the simulated acid rain solution are shown in

Table 3. Concentrations of components in simulated acid rain solution [55].

Component	Concentration/(mg·L−1)
Sulfuric acid (96%)	31.85
Nitric acid (70%)	15.75
Sodium nitrate	21.25
Ammonium sulfate	46.20
Sodium sulfate	31.95
Sodium chloride	84.85

below. The corrosion of AZ31 magnesium alloy in the simulated acid rain solution was controlled by the anodic dissolution rate.

By studying the corrosion behavior of AM60B in simulated acid rain, Hu et al. [56] found that the corrosion type of magnesium alloys is dominated by pitting corrosion, a conclusion that is consistent with the study of Liu et al. [55].

The common sources of dust particles in the atmosphere are divided into two main categories, natural and anthropogenic, while dust particles in the industrial atmosphere are mainly of anthropogenic origin. Natural sources include products of natural weathering, such as rocks and soils; emissions from internal combustion engines, industrial emissions, etc., are man-made sources. The surface of magnesium alloys in the atmospheric environment deposits dust particles, which absorb moisture in the air through capillary cohesion, chemical condensation, and physical absorption, making the surface of the alloy remain wet for a long time, which promotes the process of pitting, and is subjected to severe corrosion. Chen et al. [57] found that the dust particles had little effect on the diffusion of aqueous films, and that the small solubility product of aqueous films in the presence of dust particles and the alkalization effect were factors affecting the diffusion rate of aqueous films. The hydrophilic series were as follows: salt particle > dust particle > AlMn phase > other phases, such as primary a-Mg and eutectic magnesium phase.

Haze is formed of pollutant particles with an aerodynamic equivalent diameter of less than or equal to 2.5 μm, and their production is equally marked by high relative humidity (80–90%). The haze liquid film contains water-soluble ions ($\mathrm{S}{\mathrm{O}}_4^{2-}$, $\mathrm{N}{\mathrm{O}}_3^{-}$, $\mathrm{N}{\mathrm{H}}_4^{+}$, Cl⁻), which pose a great corrosion threat to magnesium.

The corrosion behavior of pure Mg in a Mg(OH)₂ saturated solution containing different individual constituents of PM2.5 in haze was studied by Zhao et al. [58] The order of corrosiveness of the components to magnesium was analyzed as (NH₄)₂SO₄ > haze contaminated solution > NH₄NO₃ > NH₄Cl > NaCl ≈ KCl ≈ Na₂SO₄ ≈ MgCl₂ ≈ CaSO₄ > Mg(OH)₂(alkaline solution) > Ca(NO₃)₂. The trend is shown in Figure 4 below. Similarly, through laboratory simulations, Cui et al. [23] found similar results for AZ91D magnesium alloy and pure magnesium in hazy environments. $\mathrm{S}{\mathrm{O}}_4^{2-}$ and Cl⁻ are aggressive to AZ91D, causing and exacerbating pitting, which is more corrosive. The combination of $\mathrm{N}{\mathrm{H}}_4^{+}$ and OH⁻ prevents the formation of Mg(OH)₂, leading to a sharp acceleration of the corrosion process. Thus, haze is most corrosive under the synergistic effect of $\mathrm{S}{\mathrm{O}}_4^{2-}$ and $\mathrm{N}{\mathrm{H}}_4^{+}$, and the adsorption of $\mathrm{N}{\mathrm{O}}_3^{-}$ forms a passivation film on the surface of the specimen, preventing the α-matrix from severe corrosion.

There are significant differences in the corrosion rate of magnesium alloys in different industrial atmospheric environments. The industrial atmospheric corrosion behavior of magnesium alloys is mainly influenced by atmospheric temperature, relative humidity, atmospheric pollutants such as SO₂, and atmospheric particulate matter. It is also influenced by the matrix, including the type of magnesium alloy, the content of alloying elements, and the structure of the second phase. The corrosion rate is lower compared to the marine atmospheric environment, and there are more studies on single pollutants but relatively few field-exposed studies in the industrial atmospheric environment, and more in-depth studies are needed.

3. Conclusions

The corrosion behaviors of magnesium alloys in different atmospheric environments are summarized, including the corrosion behavior of magnesium alloys in marine and industrial atmospheric environments. The conclusions are as follows:

(1)

The effects of temperature, relative humidity, and chloride concentration on the corrosion of magnesium alloys are more significant in marine atmospheric environments. Temperature increases the kinetics of the chemical reaction, and the corrosion intensity increases as the temperature rises, and as the relative humidity increases, the thicker the molecular layer of liquid film on the surface of the magnesium alloys, the less protective the layer of corrosion products becomes. The effect of relative humidity is more significant than that of temperature. Chlorides usually cause localized corrosion and pitting on the surface of magnesium alloys. In coastal areas, large amounts of chloride ions migrate and settle on the surface of magnesium alloys, providing high opportunity for electrochemical reactions. However, CO₂ can neutralize the cathodic zone, leading to a decrease in cathodic activity, inhibiting the long-term atmospheric corrosion behavior of magnesium alloys.

(2)

Compared with static atmospheric exposure experiments, magnesium alloys suffer the most severe corrosion in dynamic marine atmospheric environments. The extreme harsh environment of sea navigation, including periodic dry/wet alternation, and the large amount of chloride ions carried by sea winds and waves, cause serious damage to magnesium alloys. However, there are only a few studies on the corrosion behavior of magnesium alloys in dynamic marine atmospheric environments and more dynamic atmospheric corrosion research is needed.

(3)

In industrial atmospheres, SO₂ and dust particles affect the corrosion behavior of magnesium alloys. SO₂ increases the corrosion process and has a synergistic effect with soluble salts on atmospheric corrosion of magnesium alloys. Dust particles can accelerate the corrosion process of Mg by reducing the localized critical relative humidity on dust-contaminated surfaces. However, there is relatively little research on the industrial or urban atmospheric environment, and the research prospects are relatively broad.

So far, most studies on environmental factors affecting atmospheric corrosion of magnesium alloys have conducted indoor accelerated corrosion experiments, but there are few papers exploring the correlation between on-site exposure and indoor simulation. And as technology continues to advance, the variety of magnesium alloys is expanding; however, there are still limited field-exposed tests for these alloys. Moreover, during testing, the temperature and relative humidity on the magnesium alloys surface may vary significantly from the environmental data, making in-situ monitoring of these conditions essential to ensure accurate experimental data. We hope researchers can conduct more research on the above-mentioned issues.