The marine atmospheric corrosion of pure Mg and Mg alloys in field exposure and lab simulation

Abstract

It is of great significance to establish a reliable correlation between the field exposure damage and lab simulation measurements for marine atmospheric corrosion of Mg alloys. In this study, the corrosion behaviour of pure Mg, AM60, ZE41 and AZ91D was investigated in the real marine atmosphere and the simulated environments, such as salt spray and simulated sea wave impact by means of weight-loss measurement, morphology observation and corrosion product analysis. The simulated sea wave impact test appeared to be a suitable lab technique that can better simulate and reliably accelerate the marine atmospheric corrosion for Mg alloys. Based on the results, the individual influences of the environmental factors relative humidity, temperature, drying and wetting cycle and solution concentration, on the overall corrosion damage were compared, and a general damage process was proposed for the marine atmospheric corrosion of Mg alloys under different test conditions.

Keywords

Mg alloy atmospheric corrosion field test marine environment

Introduction

Compared with other structural metallic materials, magnesium (Mg) alloys have the lowest density, relatively high strength and well shock absorbency, which allow them to be successfully used in marine, aviation, aerospace and transportation [1]. However, the application of Mg alloys is still restricted by their poor corrosion resistance [2–5]. Mg alloys are most likely to be mainly used in atmospheric conditions. Thus, their atmospheric corrosion must be mitigated. It is reported that the corrosion of Mg alloys is much more serious in coastal areas, especially in the tropical and subtropical marine environments, due to high temperature, humidity and chloride concentration [6]. To ensure the safety and service life of Mg alloys, it is essential to have a better understanding of their atmospheric corrosion behaviour under such environments.

In recent years, due to the wide usage, the corrosion behaviour of Mg alloys has attracted attention of more and more researchers, and some clarifications can be made as follows:

The atmospheric corrosion behaviour of Mg can be significantly influenced by environmental factors, such as relative humidity (RH), temperature, gas constituents, etc. For example, Jiang et al. [7] found that AZ91D was more likely to corrode in the marine environment (Qingdao) than in the inland environment (Beijing) because of the exposure to sea salt and the high RH. Blawert et al. [8] reported that the atmospheric corrosion rates of AM50 and AZ91D alloys increased as either temperature, RH or the amount of NaCl increased. Esmaily et al. [9] found that the NaCl-induced atmospheric corrosion of AM50 alloy was positively correlated with temperature, and this effect was especially strong when CO₂ was present. Zhou et al. [10] explored the atmospheric corrosion of AZ91D alloy under NaCl particle deposition condition in the laboratory and found that the corrosion rate was closely related to the amount and exposure time.

The atmospheric corrosion of a Mg alloy can also be affected by its microstructure which was usually determined by the alloying elements, forming process, etc. Yang et al. [11] reported the corrosion rates of ingot AZ91D alloys (6.24 g/m²/y) were higher than those of high-pressure die casting (HPDC) AZ91D alloys (5.97 g/m²/y) in a polluted environment as the HPDC specimens had finer grain size and net-like corrosion barrier β phases. Jönsson et al. [12] measured the corrosion rates of AZ91D Mg alloys exposed at three typical field-exposure sites and found that the aluminium content in the different phases was closely related to the extent of the corrosion: the lower the aluminium content, the more obvious corrosion could be observed.

Various methods have been employed to study the atmospheric corrosion of Mg alloys, such as field-exposure in real environments, and lab measurements in controlled and simulated conditions [13 18]. Considering the complexities of the atmospheric changes in the real environments, the field test is normally the most reliable [19], but costly and time-consuming. It also has great difficulty in determining the effect of individual atmospheric parameters, such as the temperature and RH. These problems can be solved to some degree in the lab under controlled or simulated conditions [10], which has distorted results in some cases. Therefore, it is urgent to develop a lab acceleration technique to better simulate the field tests which could evaluate the atmospheric corrosion behaviour of materials effectively.

In this work, both field exposure and laboratory acceleration tests were conducted on pure Mg, AZ91D, AM60 and ZE41. The field tests were conducted in Xiamen, a typical subtropical coastal city in China. The lab acceleration tests were salt spray and simulated seawave impact. It is supposed to distinguish the effects of different environmental factors on the atmospheric corrosion behaviour and understand their contributions to the overall marine atmospheric corrosion damage of the Mg and these alloys. The study should help establish a correlation between the lab acceleration tests and field exposure for Mg alloys in marine atmospheric environments, which will also be helpful for wider applications of Mg alloys as structural metallic materials in marine environments.

Experimental

Materials and specimens

The pure Mg and commercial Mg alloys (AZ91D, AM60, ZE41) used in this study were from Shenzhen Meihang Copper Aluminum Co., LTD. The chemical compositions are shown in Table 1, which were analysed by iCAP 7000 Series Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) from Thermo Scientific Company. They were cut into 5 mm × 50 mm × 75 mm coupons from the hot-rolled plates. The surfaces of the specimens were successively ground to 2000 grit with SiC papers, rinsed with distilled water and absolute ethyl alcohol, and then dried at 40°C in a drying oven.

Table 1

Chemical compositions of the pure Mg, AZ91D, AM60 and ZE41 determined by optical emission spectroscopy.

wt-%	Al	Zn	Mn	Si	Fe	Cu	Ni	Others	Mg
Pure Mg	0.009	0.005	0.004	0.0048	0.0028	/	0.001	/	Bal.
AZ91D	9.5	0.9	0.4	0.08	0.004	0.025	0.001	/	Bal.
AM60	6.4	0.2	0.5	0.08	0.004	0.008	0.001	/	Bal.
ZE41	/	4.8	0.15	0.01	0.004	0.03	0.01	1.01	Bal.

After immersion, at least three parallel specimens were used for cross-section observation. The samples with corrosion products were cut using a high speed cutting machine from Weiyi Experiment Machine Manufacturing Co. LTD, ground with 2000 grit SiC papers, rinsed with distilled water and absolute ethyl alcohol, and dried at 40°C in a drying oven. The images of the cut areas before and after the preparation processes were recorded and compared to ensure that no corrosion products falling off from the top during the cross-section surface preparation in the observation area.

Field tests

The field test was based on the standard of GB/T 14165-2008 (Corrosion of metals and alloys – Atmospheric corrosion testing – general requirements for field test). As shown in Figure 1, the field exposure tests were carried out at two experimental sites in Xiamen, China: (1) the standard base of the Xiamen Marine Environment Test Station (∼3 m above the sea level on a small island 600 m away from the coast in the sea), and (2) the roof of a 4-story building in Xiamen University (1000 m inland from the coast). All the specimens were tilted at an angle of 45° and mounted on exposure racks, facing the south direction. The temperature and RH were recorded every day. Specimens were collected batch by batch from these sites once a month.

Laboratory accelerated tests

The specimens according to the standard ASTM B-117 were exposed to a continuous spray of neutral 5 wt-% NaCl solution (pH 6.5-7.2), which deposited on the specimens at a rate of 0.013-0.025 mL cm⁻² h⁻¹ at 35°C. The salt spray test lasted for 14 d. The corrosion rates of specimens were evaluated by weight loss.

A self-designed and home-made sea wave simulation system [20] that can also simulate the marine atmospheric environment adjacent to the splash zone was used in this study. The schematic illustration of the set-up is shown in Figure 2.

When a certain amount of water is pumped into the container, it pours into the water reservoir tank through the water flow gutter, forming a wave and generating splash on the wall of the reservoir in front of the wave, which can simulate the atmospheric zone, splash zone, tidal zone and immersion zone, similar to those in a typical marine environment. In the current study, the specimens were hung 1.3 m above the splash zone in the atmosphere chamber, full of some water mist. At this height, the electrolyte droplets could not deposit on the specimen surface directly. To simulate the ocean tides, the device was operated twice a day. The operation lasted for 1 h each time and stopped for 11 h as one circle. The RH kept at 100% for 6 h and then started to decrease slowly. It reached 80% at the end of one circle and increased instantly to 100% when the following operation started. In each operation, the wave height and frequency were determined by the volume and interval of the salt water being poured into the gutter. The volume was100 L, and the interval was about 37 s, which to some extent simulated the natural wave conditions in Xiamen. As the 5%NaCl, more concentrated than the total content of chlorides in natural seawater, was used in this study, the test to some degree could accelerate the corrosion damage compared to the exposure in similar natural conditions.

Wet-dry alternation test

For a comparison purpose, a wet-dry alternation test was also carried out in the lab, which to some extent simulates the corrosion in the tidal zone. The pure Mg specimens were immersed in 5 wt-% NaCl solution for 5 min, then dried in the air for 1 h, and heated in a vacuum oven at 50°C and 20 KPa for 2 h. After that, these wet-dry procedures were repeated once more for the second time. The surfaces were recorded after the second wet-dry cycle.

Weight loss measurement and surface characterisation

For field exposure, each time four parallel samples were collected every month. In the lab accelerated tests, also four parallel samples were analysed every two days. All the samples were weighed before and after the tests after the removal of the corrosion products. The solution for corrosion product removal was 200 g L⁻¹ CrO₃+2 g L⁻¹ AgNO₃. The weight loss rate W_L (mg cm⁻² d⁻¹) was calculated using the following equation: (1) in which A (cm²) is the total surface area of the sample, t_L(d) is the test duration, W_a (mg) is the weight of the sample before the experiment, and W_b (mg) is the weight after the test without corrosion products on samples surface. The corrosion rate P_W (mm/year) was determined by [21,22]: (2)

The corrosion morphologies were observed using Leica optical microscope and Hitachi TM3000 scanning electron microscope (SEM), in which the electron accelerating voltage was 15 kV and the working distance was 10 mm. The crystalline phases of the corrosion products on the specimens after corrosion were identified by X-ray diffraction (XRD) with Cu-Kα radiation using a Rigaku D/max 2550 X-ray diffractometer. The angle range was from 5° to 60°. The chemical composition was analysed by ICP-OES and Auger Electron Spectroscopy (AES), a PHI-700 NanoScanning Auger system with a CMA energy analyzer and a coaxial electron gun.

Results

Field tests

Figure 3 shows the weight loss, temperature, and RH changes of pure Mg and Mg alloys at the two exposure sites over 6 months. The results were the average values over the previous exposure time. The weight loss of all the specimens increased with exposure time. The value of the island site specimens was obviously higher than that of at the inland sites. After 6 months on the island, the average corrosion rates of the AM60, ZE41, AZ91D, and pure Mg were 6.93, 6.61, 6.55, and 6.36 μm/year, respectively, while those in the inland site were 5.94, 5.92, 5.55 and 5.55 μm/year, respectively. The marine atmosphere on the island was more corrosive than the inland. As the exposure tests started in February and ended in August, the average temperature of the two exposure sites increased almost linearly with the exposure time. On the island, the temperature, due to the seawater modification effect, was obviously lower than that of the inland campus. For the same reason, the average RH on the island was relatively high, but stable and increased slightly with exposure time compared with that in the inland environment.

Figure 4 shows the monthly average temperatures, RH and weight loss rates, of the specimens at the island and inland sites over 6 months. Figure 4(a) shows that the temperatures of the two sites increased monthly as the test started from spring to autumn. The temperatures of the island were lower than these at the inland site. However, this trend was opposite to the RHs.

Figure 4(b,c) shows the weight loss rates in each month for the pure Mg and Mg alloys. All the specimens at the two exposure sites fluctuated significantly, but the variation trends were similar. In the fifth month, the weight loss rates of all specimens reached their maximum. In general, the weight loss rates among these different alloys relatively insignificantly differentiated at the same site for the same exposure time. The monthly weight loss rates of the specimens on the island (the filled symbols) were higher than these at the inland site (the empty symbols) except for the sixth month.

Salt spray test and sea wave impact test

Figure 5(a) shows weight loss of the pure Mg and Mg alloys in the salt spray test over the 14 d. The average weight losses after the 14 d test of the specimens were 2.41, 2.23, 1.84, 1.61 mm/year, respectively, decreasing in the order: AM60 > ZE41 > AZ91D > Mg, which was consistent with the trends shown in the field test (see Figure 3).

Figure 5(b) shows the two-day average weight loss rates during the salt spray test for the pure Mg and Mg alloys. The rates increased with time.

Figure 6(a) shows the weight loss of the pure Mg and Mg alloys under the sea wave impact test for 14 d. The average weight loss of the specimens was in the order: AM60 > ZE41 > AZ91D > Mg, which was consistent with the results of the field exposure test and the salt spray test. After the 14-day sea wave impact test, the average corrosion rates of AM60, ZE41, AZ91D and pure Mg were 0.40, 0.34, 0.30, 0.25 mm/year, respectively.

Figure 6(b) shows the two-day average weight loss rates during the sea wave test and the weight loss rates of the pure Mg and Mg alloys were almost constant from the 2nd day to the end of the test.

Figure 7 shows the summary of the weight-loss rates of the pure Mg and Mg alloys in the real marine atmospheric environments and the laboratory simulation environments (sea wave impact and salt spray). The values obtained from the laboratory were significantly higher than those from the real marine atmospheric environments. The corrosion rates in the sea wave impact test were lower than those in the salt spray test.

Surface characterisation

Figure 8 shows the corrosion morphologies of the pure Mg and Mg alloys exposed on the island for 6 months. Figure 8(a–i) shows the representative change in corrosion morphology of the AZ91D in the 6 months. Figure 8(a–c) shows that a thin corrosion product film formed on the surface, and some pits and straight cracks generated in the matrix after the first-month exposure. The surface film thickened and the cracks developed as shown in Figure 8(d–f) with exposure time. In the 4th month, more straight cracks were generated and developed into a claw-like pattern. By the end of the 6th month as shown in Figure 8(g–i), the corrosion product layer became thicker. The surface corrosion was quite serious, and the randomly distributed claw-like cracks were inter-connected and became deeper and denser. The evolution of the corrosion morphology of AM60, ZE41 and pure Mg was similar to that of AZ91D, and thus not shown in Figure 8. Figure 8(k,n,q) shows that there was a relatively thick corrosion product layer formed on the surfaces of AM60, ZE41 and pure Mg with cracks distributed homogeneously on the whole specimen surfaces. After the corrosion products were removed, it could be observed that a large number of cracks penetrated into the matrix. Figure 8(l) shows that the AM60 specimen suffered the most serious corrosion damage with a high density of pits along the cracks, while there were fewer pits distributed on the surface of the ZE41 and pure Mg as shown in Figure 8(o,r).

Figure 9 shows the corrosion morphologies with and without corrosion products of the pure Mg and Mg alloys at the inland exposure sites. The OM and SEM images of the specimens with corrosion products did not show obvious differences among these specimens. However, after the removal of the corrosion products, the most seriously corroded AM60 with a dense of corrosion pits distributed in the matrix could be seen in Figure 9(f). While the relatively slight corroded AZ91D and pure Mg specimens with some straight cracks and sparse pits in the matrix. The corrosion initiated and propagated on the specimens at the inland exposure sites similar to that at the island exposure sites. However, the corrosion of the specimens at the inland sites was relatively less severe with a relatively sparser distribution of corrosion pits and cracks in the matrix.

Figure 10 shows the corrosion morphologies of the specimens in the salt spray test for14 d. All the specimens suffered serious corrosion with thick corrosion products covering on the whole specimen surfaces. After the removal of the corrosion products, there were large cavities penetrating deeply into the matrix of the AM60 (Figure 10(f)) and ZE41 (Figure 10(i)). While on the AZ91D (Figure 10(c)) and Mg specimens (Figure 10(l)), the cavities were smaller. There was no crack found in the matrixes of all the specimens in the salt spray test. The corrosion morphologies of the specimens in the salt spray test were significantly different from those of the specimens in the field exposure tests.

Figure 11 shows the corrosion morphologies of the pure Mg and Mg alloys in the sea wave impact test for 14 d. The corrosion product films formed on the surfaces of the specimens were relatively thick and uneven. After the products removed, there was no corrosion crack or pitting inter-connected on the surfaces, but scattered on the entire surfaces as indicated by the arrows. There was no claw-like crack either, and the pits were scattered and irregular in shape. Figure 11 also shows that the AM60 and ZE41 suffered more severe corrosion damage than the AZ91D and pure Mg.

Figure 12 shows the typical cross-section morphologies of the pure Mg with corrosion products in different environments. Figure 12(a,b) shows the morphologies of the pure Mg samples exposed on the island and at the inland site for 6 months, which were consistent with those shown in Figures 8 and 9. The pure Mg samples after the field test were completely covered by a corrosion product film. There were some cracks in the corrosion product film as indicated by the arrows above the matrix which suffered relatively more severe corrosion. Figure 12(c,d) shows that on the pure Mg specimens after the salt spray test and sea-wave impact test, the corrosion product films were substantially thicker than those on the specimens in the field tests. Furthermore, there were many cracks distributed irregularly in the corrosion product films.

The chemical compositions of the corrosion products on the specimens after corrosion tests were measured by AES and XRD as shown in Table 2 and Figure 13. In the real environments, the components of the corrosion products contained S and Si, probably from the polluted air and dust, which basically did not exist on the samples after the lab acceleration tests. The chemical compositions of the surface corrosion products of the four samples were mainly composed of Mg(OH)₂, MgCO₃·Na₂CO₃, MgO·Al₂O₃, similar to those in the field exposure and the laboratory acceleration specimens.

Table 2

Compositions in wt-% of corrosion products analysed by AES for pure Mg and Mg alloys after the tests.

Location	Alloy	O	Mg	Al	Zn	Si	C	S	Cl	Na	Others
On the island	AZ91D	42.0	25.2	21.9	2.1	4.2	2.2	0.4	0.3	0.1	1.6
AM60	49.1	19.4	13.6	–	5.6	7.0	0.4	0.9	0.8	3.2
ZE41	45.5	42.0	–	2.5	3.3	5.0	0.8	0.4	0.1	1.4
Mg	44.0	44.3	–	–	2.6	5.3	0.5	0.2	0.1	3.0
At the inland	AZ91D	42.3	23.1	18.7	1.6	6.9	4.8	0.8	0.7	0.4	0.7
AM60	47.8	18.0	13.4		5.1	5.3	0.4	2.4	2.2	5.4
ZE41	43.6	38.6	–	2.1	5.2	6.2	0.6	0.8	0.6	2.3
Mg	40.4	51.3	–	–	1.8	3.4	0.1	0.2	0.1	2.8
Salt spray	AZ91D	47.1	30.0	10.2	1.1	–	6.4	–	3.7	1.6	–
AM60	47.5	22.7	13.2	–	–	4.3	–	9.2	3.0	0.1
ZE41	44.5	35.7	–	3.2	–	5.5	–	7.9	3.4	0
Mg	42.2	33.6	–	–	–	8.4	–	10.8	5.0	0
Sea wave simulation	AZ91D	44.4	29.2	9.3	1.2	–	5.6	0.6	6.2	3.5	0
AM60	47.3	21.2	14.5	–	–	5.8	–	7.4	3.8	0
ZE41	44.9	35.8	–	2.8	–	6.2	–	6.6	3.7	0
Mg	42.9	40.3	–	–	0.2	9.5	–	4.6	2.5	0

Discussion

Influence of T, RH and salts

Figures 3 and 4 show that the average corrosion rates of pure Mg and Mg alloys on the island were higher than the corresponding specimens on the roof, which was mainly attributed to influence of humidity, temperature, and salts.

In general, the corrosion reaction rate increases with the temperature. Figure 4(a) shows that the monthly average temperature on the island was lower than that at the inland site, but the corrosion rates of specimens exposed on the island were higher than those inland, which indicates that temperature was not a dominant factor in the atmospheric corrosion in this study. In contrast, the influence of RH was much more prominent. The average RH of the exposure site on the island was significantly higher than that in the inland (Figure 4(a)), which was one of the key reasons for the higher corrosion rates of the exposed specimens on the island. Furthermore, Figure 4(a) shows that the RH at the two exposure sites decreased significantly in the sixth month, resulting in a substantial decrease in the monthly weight-loss rates of the specimens. Therefore, the influence of RH on the corrosion behaviour of Mg is more significant than that of temperature. Furthermore, the corrosion rates of the Mg specimens in the lab simulation tests were substantially higher than those of the corresponding specimens at the exposure sites, which were mainly attributed to the high RH in the lab tests. In fact, the RH was strongly related to the thin liquid film covered on the specimen, which dominates the corrosion of the Mg specimen [4,7,13]. Therefore, only the duration of the specimen covered by the liquid film was the actual corrosion time.

The surface morphologies indicate that the corroded surfaces of the field exposure specimens (Figures 8 and 9) obviously suffered less corrosion attack than those of the salt spray specimens (Figure 10) and the sea wave impact specimens (Figure 11). This is mainly attributed to the shorter time of the field exposure specimen surface covering by the electrolyte film as the liquid droplets evaporated caused by the low RH, the sunshine, the wind, etc, while in the salt spray test, the specimen surface was covered by electrolyte film all the time. It is noted that the electrolyte may stay in the corrosion product film cracks on the exposure specimens for a long time and the underneath Mg matrix suffered relatively serious corrosion as shown in Figures 8 and 9.

Inorganic salt particles in the air might also play an important role in the marine atmospheric corrosion of Mg through modifying the constituents of the liquid film or prolonging the time of the liquid film covering the surface. It is reported that the presence of inorganic salt particles reduces the critical RH required for the formation of a liquid film on the surface [7,8,10,16]. The existence of salt particles promotes the condensation of water vapour in the air to form a thick liquid electrolyte film on the Mg surface. Besides, in the environment rich in Cl⁻, the protectiveness of the corrosion product film would be deteriorated. The salt also enhances the conductivity of the electrolyte and promotes the corrosion of the Mg matrix. Although the salt concentration in air was not measured at the two exposure sites, it is reasonable to speculate that the Cl⁻ concentration on the island was higher than that on the inland building roof. This might be another reason for the higher corrosion rates of Mg specimens on the island. AES results (Table 2) show that C was present in the surface corrosion products of the specimens exposed at the two field exposure sites. Under the condition of the real marine atmosphere, CO₂ was dissolved in electrolyte to form H₂CO₃ when the surface was wet. MgCO₃ and Na₂CO₃ depositions would be formed after drying, which had a certain protective effect on the Mg alloys [11,23 25].

The mechanism of marine atmospheric corrosion

The corrosion morphologies in Figures 8 and 9 indicated that the specimens at the two filed-exposure sites suffered the atmospheric corrosion attack in the same manner though the corrosion rates of the specimens on the island were slightly higher than those at the inland site.

It seems that there was a thin but somewhat protective corrosion product film formed on the surface in the initial period of the field test. The XRD results in Figure 13 revealed that this film was mainly the Mg(OH)₂, MgCO₃, MgO and Al₂O₃ (for AZ91D and AM60). During a drastic change under the alternate dry-wet condition, the corrosion product film was easy to break and form caterpillar cracks. Apart from corrosive liquid droplets, soluble gas and solid particles in the atmosphere could be dissolved in the adsorbed water, forming an electrolyte solution on the specimen surface and in the cracks and gaps. The surface electrolyte evaporated instantly under the wind or sunshine, while the solution in the cracks survived and reacted electrochemically with the Mg matrix, causing serious localised corrosion in these areas, producing thicker and corrosion product film. The cracks grew and became inter-connected, gradually covering the entire Mg alloy matrix. In conclusion, the atmospheric corrosion of Mg alloys in the real marine atmosphere was mainly a process in which a surface corrosion product film grew and cracked, together with the development of serious corrosion in these cracks due to the slow evaporated electrolyte, which was shown by a schematic image in Figure 14. When the atmospheric environment changed, such as from island site to inland site, the constituents and the existence time of the electrolyte, especially in the cracks, varied due to the different RH and T, resulting in different corrosion rate and corrosion morphologies. However, their mechanism was totally the same.

Comparison of the field test and the simulation test

Figures 3, 5–7 show that the corrosion rates of pure Mg and Mg alloys in the field exposure tests were in the same sequence with those in the lab simulation test, decreasing as: AM60>ZE41>AZ91D>Mg. This was attributed to that in the same environment, no matter in the field test or in the lab test. The corrosion rate of the Mg was mainly determined by corrosion resistance of the Mg matrix underneath the liquid film. As a result, all the tests suggested the same sequence of the corrosion rate for the pure Mg and Mg alloys, which was also the essential corrosion resistance sequence for them. From this perspective, the lab simulation test was useful and reasonable in the atmospheric corrosion research of Mg.

However, the corrosion rates in the salt spray were about 300 higher than those in the field test, indicating a great corrosion acceleration effect of salt spray tests. Furthermore, the corrosion morphologies in Figure 10 and the corrosion product constituents in Figure 13 revealed that the Mg and the alloys in salt spray tests were corroded differently with those in the real marine environments. This was attributed to that: (i) the electrolyte film deposited on the specimen surface has a rather higher content of NaCl, namely 5 wt-%, than that on the exposed specimens. The concentration of sea salt on the exposed specimen surface would increase because of the condensation effect after several cycles of deposition–evaporation processes. The electrolyte in the corrosion product cracks may reach the similar content as that in the lab test because the rain droplets could not brush them away instantly. (ii) A continuous electrolyte film covered the specimen surface in the salt spray chamber all over the test time, while the actual corrosion time of the field exposed specimens only occupied a small percentage of the total test time. In addition, as the absence of the wet-dry alternation process, the corrosion product in salt spray was mainly Mg(OH)₂, which was more porous and less protective than MgO [26,27]. Sufficient electrolytes penetrated through the porous corrosion product film and attacked the Mg matrix seriously, resulting in serious corrosion in the matrix as shown in Figure 10.

The corrosion rates in the sea wave impact test were about 50 higher than those in the field test, but less than those in the salt spray tests. The acceleration mechanism was somewhat different from the salt spray test. The sea wave impact system did not produce visible liquid droplets, but mist that could not become a continuous liquid film on the sample surfaces directly. As a result, the concentration of the NaCl was quite lower than that on the salt spray specimen surface. This was closer to the real environment situation in the marine atmosphere. As the distance between the specimens and the produced waves were only about 1.3 m, which was far nearer than the standard exposed sites. Therefore, the formation rate of the electrolyte film was higher than that on the filed exposed specimen surface, which was a significant acceleration factor. Furthermore, in the sea wave system, the RH was 100% and the temperature was 28°C in the first 6 h and then the RH and temperature decreased to 80% and 24°C in the following 6°h for each wet-dry cycle. An electrolyte film covered on the specimen surface for most of the test time, which was the main acceleration factor of the sea wave impact test. It is noted that the existence of liquid film could be monitored by the modification of the RH, T or wind, which could control the acceleration factor according to the test materials. In the current research, the liquid film stayed on the surface was a bit long, resulting in relatively serious corrosion on the Mg specimens. However, the corroded morphology of the specimens still indicated some similarity with that of the filed exposed specimen, such as the cracks indicated in Figure 12. Therefore, compared with the traditional salt spray experiment, the sea wave impact test [20] may be more relevant to the real marine atmospheric corrosion of Mg alloys.

Figure 15 shows the surface morphologies of the wet-dry alternation tested pure Mg before and after the corrosion products were removed. A corrosion product film with some cracks was formed on the surface of the specimen after the test (see Figure 15(a)). When the corrosion products were removed, many circular pits were present on the surface, some of which were combined to form a river-like morphology (see Figure 15(b)). These features are quite similar to those in the field test (see Figures 8 and 9). This simple wet-dry alternation test supported that the corrosion morphology in the real marine atmosphere may be simulated to a great extent in the sea-wave impact test if the experiment parameters were set properly.

Conclusions

The marine atmospheric corrosion rates of four different Mg alloys in field exposure at inland and island sites and lab accelerated corrosion tests can be ranged in the following order: AM60>ZE41>AZ91D>Mg. Their different corrosion rates can be more evidently and quickly revealed by the lab accelerated corrosion tests.

The influence of relative humidity on the corrosion of Mg alloys was more significant than that of temperature under the marine atmospheric conditions. The duration of the specimen covering by the liquid film was the actual corrosion period when the atmospheric corrosion occurred, which was seriously related to RH.

The corrosion of a Mg alloy in the marine atmosphere could involve different processes, such as the cracking of the surface corrosion product film during drying, the localised dissolution of the substrate in the cracked regions while wetting, the formation of corrosion product film in the newly corroded areas and formation of new cracks in the film.

The self-designed homemade sea wave impact test can reasonably accelerate the marine atmospheric corrosion of Mg alloys.

Footnotes

Acknowledgement

This research was supported by the National Natural Science Foundation of China No. 51731008 and No. 51801168, Natural Science Foundation of Fujian Province No. 2018J05093, and the National Environmental Corrosion Platform of China. Thank Prof. Kourosh Kalantar-Zadeh for the improvement of English writing.

Disclosure statement

No potential conflict of interest was reported by the author(s).

References

Zeng

. Recent development and application of magnesium alloys. Acta Metall Sin. 2001; 37: 673–685.

Luo

Nyberg

Sadavappan

. Magnesium front end research and development: a Canada-China-USA collaboration. Proceedings paper presented at: Magnesium Technology Symposium; 2018; Magnesium Technology Series.

Makar

Kruger

Corrosion of magnesium. Int Mater Rev. 1993; 38: 138–153. doi: 10.1179/imr.1993.38.3.138

Song

Atrens

Corrosion mechanisms of magnesium alloys. Adv Eng Mater. 1999; 1: 11–33. doi: 10.1002/(SICI)1527-2648(199909)1:1<11::AID-ADEM11>3.0.CO;2-N

Cui

Xiao

. Atmospheric corrosion of field-exposed AZ31 magnesium in a tropical marine environment. Corros Sci. 2013; 76: 243–256. doi: 10.1016/j.corsci.2013.06.047

Natesan

Venkatachari

Palaniswamy

Kinetics of atmospheric corrosion of mild steel, zinc, galvanized iron and aluminium at 10 exposure stations in India. Corros Sci. 2006; 48: 3584–3608. doi: 10.1016/j.corsci.2006.02.006

Jiang

Zhang

. Atmospheric corrosion of Mg-rare earth alloy in typical inland and marine environments. Corro Eng Sci Technol. 2014; 49: 651–655. doi: 10.1179/1743278214Y.0000000163

Blawert

Fechner

Höche

. Magnesium secondary alloys: alloy design for magnesium alloys with improved tolerance limits against impurities. Corros Sci. 2010; 52: 2452–2468. doi: 10.1016/j.corsci.2010.03.035

Esmaily

Shahabi-Navid

Svensson

. Influence of temperature on the atmospheric corrosion of the Mg–Al alloy AM50. Corros Sci. 2015; 90: 420–433. doi: 10.1016/j.corsci.2014.10.040

10.

Zhou

W-q

Shan

D-y

Han

E-h

. Influence of NaCl deposition on atmospheric corrosion behavior of AZ91 magnesium alloy. Trans Nonferrous Met Soc China. 2010; 20: s670–s673. doi: 10.1016/S1003-6326(10)60559-6

11.

Yang

L-j

Y-f

Wei

Y-h

. Atmospheric corrosion of field-exposed AZ91D Mg alloys in a polluted environment. Corros Sci. 2010; 52: 2188–2196. doi: 10.1016/j.corsci.2010.03.011

12.

Jonsson

Persson

Leygraf

Atmospheric corrosion of field-exposed magnesium alloy AZ91D. Corros Sci. 2008; 50: 1406–1413. doi: 10.1016/j.corsci.2007.12.005

13.

Esmaily

Svensson

Fajardo

. Fundamentals and advances in magnesium alloy corrosion. Prog Mater Sci. 2017; 89: 92–193. doi: 10.1016/j.pmatsci.2017.04.011

14.

Jonsson

Thierry

LeBozec

The influence of microstructure on the corrosion behaviour of AZ91D studied by scanning Kelvin probe force microscopy and scanning Kelvin probe. Corros Sci. 2006; 48: 1193–1208. doi: 10.1016/j.corsci.2005.05.008

15.

Jönsson

Persson

Thierry

Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D. Corros Sci. 2007; 49: 1540–1558. doi: 10.1016/j.corsci.2006.08.004

16.

LeBozec

Jonsson

Thierry

Atmospheric corrosion of magnesium alloys: influence of temperature, relative humidity, and chloride deposition. Corrosion. 2004; 60: 356–361. doi: 10.5006/1.3287743

17.

Feliu

Pardo

Merino

. Correlation between the surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys. Appl Surf Sci. 2009; 255: 4102–4108. doi: 10.1016/j.apsusc.2008.10.095

18.

Pardo

Merino

. Corrosion behaviour of silicon-carbide-particle reinforced AZ92 magnesium alloy. Corros Sci. 2009; 51: 841–849. doi: 10.1016/j.corsci.2009.01.024

19.

Liao

Hotta

Atmospheric corrosion behavior of field-exposed magnesium alloys: influences of chemical composition and microstructure. Corros Sci. 2015; 100: 353–364. doi: 10.1016/j.corsci.2015.08.021

20.

Song

Zou

Wang

. The technique and equipment of the ocean wave simulation system. China patent CN208255217U; 2018.

21.

Cao

Shi

Hofstetter

. Corrosion of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2. Corros Sci. 2013; 75: 78–99. doi: 10.1016/j.corsci.2013.05.018

22.

Cao

Shi

Song

G-L

. Corrosion behaviour in salt spray and in 3.5% NaCl solution saturated with Mg(OH)2 of as-cast and solution heat-treated binary Mg–X alloys: X=Mn, Sn, Ca, Zn, Al, Zr, Si, Sr. Corros Sci. 2013; 76: 60–97. doi: 10.1016/j.corsci.2013.06.030

23.

Song

GL.

Preface. In: Song

G-l

, editor. Corrosion of magnesium alloys. Cambridge: Woodhead Publishing; 2011. p. 19–23.

24.

Weijun

Wei Ling

YM.

Study on corrosion behavior of thixotropic magnesium alloy AZ91D in Lanzhou city atmosphere. Corris Sci Prot Technol. 2007; 19: 31–34.

25.

Y-g

Wei

Y-h

Hou

L-f

. Atmospheric corrosion of AM60 Mg alloys in an industrial city environment. Corros Sci. 2013; 69: 67–76. doi: 10.1016/j.corsci.2012.11.022

26.

Brady

Fayek

Elsentriecy

. Tracer film growth study of hydrogen and oxygen from the corrosion of magnesium in water. J Electrochem Soc. 2014; 161: C395–C404. doi: 10.1149/2.0821409jes

27.

Brady

Rother

Anovitz

. Film breakdown and nano-porous Mg(OH)₂ formation from corrosion of magnesium alloys in salt solutions. J Electrochem Soc. 2015; 162: C140–C149. doi: 10.1149/2.0171504jes