1. Introduction Presently, magnesium alloys are extensively used due to their many advantages, such as rich availabil-ity, low energy consumption, reduced pollution and recyclable properties. However, low corrosion resis-tance of magnesium alloys limits their application in many fields. With continuous progress of civilization, pollu-tion problems are gaining attention due to height-ened awareness about environmental protection. It is found that pollutants in the atmosphere, such as sul-fide, chloride, carbon dioxide etc. can attack magne-sium alloys <1-3>. It is predicted by related research that the rate of increase of CO2 is 1.5 μmol per year. Therefore, CO2-induced atmospheric corrosion of magnesium alloys has received the attention of many researchers <4-6>. It was suggested that car-bonate-containing products formed a partly protec-tive film on magnesium. Investigating the corrosion of pure magnesium in distilled water, it was found that the corrosion rate was higher in the absence of CO2, than in water in equilibrium with atmospheric CO2 <7-9>. However, there have been few laboratory studies addressing the role of CO2 in the presence of NaCl. Hence it is essential to further study the indi-vidual and combined effects of CO2 and NaCl on magnesium alloys, which can provide foundation for performing protective methods for magnesium al-loys and enlarge their application range. This work is an attempt to discuss the synergistic effect of CO2 and NaCl on the basis of investigating the initial corrosion behavior of AZ91 magnesium alloy in the presence of a single component, NaCl.2. Experimental 2.1 Sample preparation AZ91 magnesium alloy (Al 8.89 wt.%, Zn 0.78 wt.%, Mn 0.24 wt.%, Ni < 0.005 wt.%, Fe < 0.01 wt.%, Cu < 0.005 wt.%) was used. The specimens had a geometrical area of 20.8 cm2 (40 mm
× 20 mm × 4 mm). Before exposure, the samples were polished on SiC paper of 1000 mesh, and then ultrasonically cleaned in ethanol, and dried in air. NaCl (70 μg/cm2) was added by spraying the samples with saturated solution of NaCl in 90% ethanol. The amount of NaCl added was determined gravimetrically. Care was taken to avoid droplet formation on the samples during spraying. The distribution of salt on the sur-face after spraying was homogenous. The samples were stored dry for about 24 h before exposure. 2.2 Experimental setup A known volume of purified and dried air was acquired by alkaline asbestos, silica gel, molecular sieves, sodium hydroxide, and then mixed with CO2 in a mixing room. The mixed gas entered the corro-sion chamber perpendicular to the sample surface. The corrosion chamber was placed in a thermostati-cally controlled water tank. Glycerin solution was used at the bottom of the corrosion chamber to con-trol relative humidity required in the experiment. Waste exhaust gases were collected with alkaline solution (10% NaOH). The corrosion environments were divided into two parts: (1) the volume percent of CO2 was 0, and the sample surface was treated with NaCl; (2) the volume percent of CO2 was 1%, and the sample sur-face was treated with NaCl. 2.3 Methods of analysis Microscopic surface changes were observed by optical microscopy. Surface morphology was inves-tigated by American LEO-1450 scanning electron microscope. An energy-dispersing X-ray detector (EDX), KEVEX Sigma was connected to the mi-croscope. Crystal corrosion products were analyzed by X-ray diffraction M21X, and Cu Kα radiation under 40 kV and 150 mA was used with an angle of incidence of 0.05°. 3. Results Fig. 1 showed the mass gain as a function of time for the magnesium alloy samples exposed to different environments at 95% relative humidity (RH). After an initial weight increase, the mass gain slowed down in all cases. The rapid initial mass gain partly reflected the uptake of water to form a solu-tion of NaCl(aqueous(aq)) on the samples. It could be seen that the presence of CO2 strongly influenced the mass gain. The mass gain of the samples depos-ited wi
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