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A laboratory study of the effect of CO 2 and NaCl on the atmospheric corrosion of aluminum is reported. The samples were exposed to pure air with 95% relative humidity and the concentration of CO 2 was Ͻ1 and 350 ppm, respectively. Sodium chloride was added before exposure ͑0, 14, and 70 g/cm 2 ͒. The main result is that the NaCl-induced atmospheric corrosion of aluminum is about 10 to 20 times faster in CO 2 -free humid air compared to air containing ambient levels of CO 2 . It is suggested that the rapid corrosion of aluminum coated with NaCl in humid CO 2 -free air is connected to high-pH areas in the surface electrolyte that develop due to the cathodic reduction of oxygen. The anodic dissolution of aluminum is known to be enhanced by high pH. The unexpected corrosion-inhibitive effect of CO 2 is explained by the neutralization of the surface electrolyte. In the absence of CO 2 , bayerite, Al͑OH͒ 3 , forms. Only minute amounts of carbonate were found on the surface after exposure to CO 2 -containing air.

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Scanning Kelvin probe force microscopy ͑SKPFM͒ is used to study the initial stages of atmospheric corrosion of an AlMg alloy and of physical vapor deposition ͑PVD͒ deposited 2 m Al dots on pure Mg. The latter system is used as a model of a two-phase AlMg alloy. The influence of CO 2 was studied in situ in humid air using SKPFM. This method allows for the in situ investigation of the evolution of the Volta potential during exposure, the resolution being in the submicrometer range. The temperature was 22.0°C, and the relative humidity was 85 or 95%. The concentration of CO 2 was Ͻ1 or 350 ppm. The corrosion products were analyzed by gravimetry, ion chromatography, X-ray diffraction, scanning electron microscopy, scanning Kelvin probe, and Auger electron spectroscopy. We found that the initial stages of atmospheric corrosion on magnesium are influenced by the presence of cathodic PVD-deposited aluminum. A similar effect was seen in the case of AZ91D, the aluminum-rich ␤-phase forming the cathodic areas. The ␤-phase is nobler compared to the substrate because of the higher Al content. In the absence of CO 2 , the corrosion attack is localized in nature whereas the presence of ambient levels of CO 2 results in a more general corrosion attack. The inhibitive effect of CO 2 on the atmospheric corrosion of AZ91D is explained by the formation of a passivating layer of Mg 5 (CO 3 ) 4 (OH) 2 • 5H 2 O. In the absence of CO 2 , the increase in pH originating from the cathodic reaction results in the dissolution of aluminum in the passive layer. A corrosion mechanism is proposed explaining the behavior in the two environments.

The atmospheric corrosion behavior of alloy AZ91D produced by a semi-solid metal (SSM) technique and by conventional high pressure die casting (HPDC) was investigated for up to 1176 hours in the laboratory. Alloy AZ91D in the SSM state was fabricated using a rheocasting (RC) technique in which the slurry was prepared by the RheoMetal process. Exposures were performed in 95% RH air at 22 and 4 • C. The RC alloy AZ91D exhibited significantly better corrosion resistance than the HPDC material at two temperatures studied. The effect of casting technology on corrosion is explained in terms of the microstructural differences between the materials. For example, the larger number density of cathodic β phase particles in the HPDC material initially causes relatively rapid corrosion compared to the RC material. During later stages of corrosion, the more network-like β phase particles in the RC alloy act as a corrosion barrier, further improving the relative corrosion resistance of the RC material. Conventional magnesium-aluminum (Mg-Al) alloys, i.e., AZ91D, AM50A and AM60B, offer an exceptional combination of ambient temperature strength and ductility, and good die-castability.1-4 Cast components made of Mg-Al alloys are usually cast by conventional high pressure die casting (HPDC). Despite its advantages over many other casting techniques for producing cast Mg-Al components, there are some inherent problems associated with HPDC. For example, there is a tendency for hot tearing during HPDC due to a relatively wide freezing range and a low solidus temperature. Also, a relatively high fraction of trapped air porosity may form during the turbulent die filling, especially in thick-walled components. Additionally, insufficient resistance to atmospheric and aqueous corrosion sometimes limits Mg-Al alloys applications in the fields of automobiles, aerospace, electronics, etc.5-8 During the past two decades, there have been extensive efforts to increase the corrosion resistance of Mg-Al alloys. Much of the effort has concentrated on the use of various coating systems such as chemical conversion coatings, anodizing, gas-phase deposition processes electro-or electro-less plating, and organic coating. [9][10][11][12][13][14][15] Lowering the impurity levels, alloying, rare earth additions, and heattreatment have also been explored to increase the corrosion resistance of these alloys. [16][17][18][19][20] Alternative casting processes are being developed to resolve the mentioned problems and meet the requirements of future applications of Mg-Al alloys. Semi-solid metal (SSM) processing is a promising manufacturing route capable of producing castings with a high level of complexity. 21,22 The main advantage of SSM processing, compared to the conventional casting processes, is the possibility to have a laminar flow of metal during mold filling.23 This is a consequence of the higher viscosity of the semi-solid material, and it reduces air entrapment compared to die-casting. This results in components with enhanced microstructural proper...