Stress Corrosion Cracking of a Forged Mg-Al-Zn Alloy with Different Surface Conditions

Stress corrosion cracking (SCC) of a forged Mg-Al-Zn magnesium alloy with different surface conditions was studied by the fourpoint bending test and alternate immersion test in NaCl solution./e results showed that the bareMg-Al-Znmagnesium alloy has low susceptivity to SCC, and no abrupt rupture happened after the immersion test for 5 days under an initial stress load of 0.15–0.75σ0.2. With microarc oxidation (MAO) coating, corrosion resistance was enhanced, but more surface cracks were induced, and microcracks could be detected inside corrosive pits when the load was 0.75σ0.2, which is similar to the bare alloy. /e composite coating totally avoided both SCC and corrosion. /e low susceptivity of the forged AQ80M alloy to SCC should be attributed to the fine grain size and even distribution of secondary phases around the grain boundary.


Introduction
Magnesium alloys are competitive as light-weight structure materials in the industrial applications due to their suitable mechanical properties and high strength to weight ratios [1].Among the various commercial magnesium alloys, the developed AZ series of Mg alloys (Mg-Al-Zn) have found the largest number of industrial applications [2], such as automotive and aerospace parts, electronic devices, and weapons.Recently, the AZ80 alloy has attracted more attention for its high strength and low price [3].With a little bit of Ag addition into the AZ80 alloy, the obtained new AQ80M alloy exhibits much higher high-temperature mechanical properties, which is desirable in the aerospace parts [4].
erefore, the present study aimed at investigating the stress corrosion activity of the AQ80M magnesium alloy with bare metal and different surface states in sodium chloride solution.

Materials and Specimens.
e substrate material used for this investigation was a forged AQ80M Mg alloy with a chemical composition (in wt.%) of 8.5% Al, 0.5% Zn, 0.2% Mn, 1% Ag, 0.1% Si, 0.01% Cu, 0.005% Ni, 0.003% Fe, and balance Mg.All samples for the stress corrosion test were cut into a size of 20 mm × 140 mm × 4 mm.e electrolyte for microarc oxidation (MAO) is composed of Na 2 SiO 3 , KOH, Na 2 HPO 3 , and C 3 H 8 O 3 .e sealant and primer are of epoxy resin, and the topcoat is polyurethane.For the MAO process, the AQ80 Mg alloy samples were used as the anode, and the temperature of electrolyte solution was below 45 °C during MAO treatment.
e MAO coating was applied with a fixed voltage of 320 V at 600 Hz for 10 min using a 200 kW AC power supply with a duty cycle of 40%.e organic sealant and top coatings were applied by spray gun with an aperture of 1.3 mm in diameter and 0.4-0.6MPa pressure.
e viscosity is controlled between 18∼60 s with Tu-4 Cup according to the speci c request of the lacquer.
e SCC test is carried in an 3.5% (wt) sodium chloride solution at room temperature, and the pH value of solution is 6.4-7.2.In the alternate immersion trial chest, the specimens are immersed in the solution for 10 min, and then exposed into the air for 50 min per hour, and the total test time is 120 h∼480 h.

Characterization and Testing.
e stress corrosion test is carried on through the four-point bending test according to the standard GB/T 15970.2-2000,which is loaded by constant strain means.With yield strength of 210 MPa, elastic model of 43 GPa, and outside and inside pivot distance of 115 mm and 57 mm, respectively, the calculated exibility corresponds to the initial loading stress, which is 0.15∼0.75times of yield strength.
Electrochemical impedance spectroscopy (EIS) was employed to compare the corrodibility of the AQ80M alloy with its pure metal surface, MAO coating, and nished coating.e optical microscope is used to study the metallographic structure of the AQ80M Mg alloy.Scanning electron microscope (SEM) and laser scanning confocal microscope (LSCM) were adopted to analyze the specimens for better understanding of the obtained results.

Microstructure and Coatings Characteristics.
e microstructures of the AQ80M alloy observed under di erent magni cations are given in Figure 1. e alloy consisted mainly of α-Mg grains agglomerated with the secondary β phase (Mg 24 Al 17 ).e average grain size of the base phase is less than 50 μm, and the black and tiny secondary phases are distributed evenly around the grain boundary.e addition of Ag could improve the precipitation hardening of the β phase but could not be observed on the optical micrograph.
Figure 2 shows the SEM images of surface and cross section of MAO layers formed on the AQ80M alloy and the surface of MAO-sealing-primer-top coating layers (multiple coating).e MAO coating is about 16 μm thick, and its surface is covered with abundant microholes with a diameter of about 3 μm in average.With four layers of coatings including MAO, the multiple coating is about 80∼100 μm thick, and the sealing particles could be found in the microholes of MAO coating.
To understand better the anticorrosion performance of the AQ80M alloy with di erent surface conditions, which cannot be visualized, electrochemical tests were performed.Figures 3 and 4 display the results of the electrochemical tests.Figures 3 and 4 are the Bode and Nyquist plots of the EIS spectra, respectively for the coatings.
e circuit model as shown in Figure 5 was used to t the impedance of MAO coating and multiple coating.In the

Journal of Chemistry
Multiple coating improves the anticorrosion performance signi cantly.

Stress Corrosion Test Results
. With an initial load of 0.15∼0.75times of yield strength (σ 0.2 ), the specimens with bare, MAO, multiple coatings, respectively, are alternately immersed in the NaCl solution for the SCC tests.Figure 6 shows the digital macroscopic pictures of the AQ80M Mg alloys, and a1∼ a6, b1∼b6, and c1∼c5 correspond to bare, MAO, and multiple coatings, respectively.Besides, Figures a1∼a5, b1∼b5, and c1∼c5 are taken with specimens tested under an initial load of 0.15-0.75σ0.2 , respectively.e test time is 120 h for a1∼a5 and b1∼b5 and is 480 h for c1∼c5.e specimens a6 and b6 are loaded with 0.75σ 0.2 and tested for 240 h.It is obvious that the bare Mg alloy specimens (Figure 6(a1∼a6)) took place in general and even severe corrosion for the abundance of small corrosive pits.And the corrosive pits size does not change distinctly with di erent loads; however, it becomes much bigger when the test time is 240 h instead of 120 h for Figure 6(a6).No obvious cracks could be found on all specimens by eye observation.
Although the MAO coating was locally destroyed, the specimens erode much slightly than the bare metal. is illustrates that the MAO coating is e ective to enhance the corrosion resistance of the Mg alloy.e corrosion degree varies similarly as that of bare metal; that is, there is no obvious di erence between di erent loaded specimens, and specimen B6 exhibits more severe corrosion.No obvious cracks could be found on all MAO coating surfaces by eye observation.
For the specimens with multiple coating, there is no obvious corrosion which could be found for all samples tested for 480 h.It seems that present protective coating totally prevents the specimens from corrosion and further stress corrosion, in the condition of loading as high as 0.75 times of yield strength.
With stress of 0.15∼0.75times of tensile yield strength, no abrupt rupture happened after the alternate immersion test for 5 days.Figure 7 shows the SEM micrographs of the AQ80M bare alloys with di erent initial stress.Figures 7(a From the gures, we could nd that there are no cracks on Figures 7(a)-7(c), although too many corrosive pits have joint together.However, microcracks could be found inside the corrosive pits on Figure 7(d), and similar cracks could also be detected both inside and between the corrosive pits  on Figure 7(e), which is tested for more times with a load of 0.75σ 0.2 . is illustrated that 0.75σ 0.2 is close to the stress threshold value for the present AQ80M alloy.
e low susceptivity of the forged AQ80M alloy to SCC should be attributed to the fine grain size and even distribution of secondary phases around the grain boundary (Figure 1).e microcracking initiated from the surface pitting, and virtual stress increased with the progress of pitting corrosion.
Figure 8 shows the SEM micrographs of the AQ80M alloys with MAO coating under different initial stress.Figures 8(a)-8(e) are the representative pictures of the specimen surface after testing for 120 h with a load of 0.3∼0.75σ0.2 , and Figure 8(f) is tested for 240 h under a load of 0.75σ 0.2 .Compared with the bare alloy, low-grade corrosion takes place on the MAO coating surfaces, and less corrosive pits could be observed.However, obvious macroscopic cracks (red arrows) could be found on the MAO coating surfaces in Figures 8(b)-8(d) with a load of 0.45∼0.75σ0.2 , and no microcracks could be found inside the corrosive pits (Figure 8(e)), although the MAO coating was destroyed and serious corrosion took place here.ese macroscopic cracks might originate from the MAO process or form on the MAO coating surface during loading for the brittle ceramic nature of coating materials.Comparing Figures 8(e) and 8(f), after testing for 240 h with a load of 0.75σ 0.2 , similar destroy of MAO coating and serious corrosion at the destroyed place occurred on the surface of Figure 8(f), whereas the surface cracks decreased and microcracks could be observed inside the corrosive pits. is might be attributed to higher corrosive rate of the Mg alloy than the formation rate of surface cracks, and the surface cracks are eroded.
Figure 9 shows the LSCM micrographs of AQ80M alloys with multiple coating under different initial stress tested for 480 h.e multiple coatings are all intact and undestroyed, and no corrosion or any microcracks could be detected on all surfaces.With such effective protection, the under layer metals are supposed to be not corrosive and cracked.erefore, the composite coating improved the corrosion resistance distinctly, and also the SCC was totally avoided.

Figure 3 :
Figure 3: EIS results of the uncoated AQ80M Mg alloy, MAO coating, and multiple coating.
)-7(d) are the representative pictures of corrosive pits (red arrows) with a load of 0.3-0.75σ0.2 and tested for 120 h, and Figure 7(e) is tested for 240 h under a load of 0.75σ 0.2 .