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The films formed on ultrahigh purity Mg, Elektron 717 (ZE10A), and AZ31B in water at room temperature were characterized by TEM, XPS, and SIMS. The films consisted primarily of MgO, with surface regions also containing Mg(OH) 2 and MgCO 3 . SIMS suggested H throughout the films and into the underlying metal. Segregation of Zn to the metal/film interface and Al in the film was observed for AZ31B. Similar Zn film segregation was also detected for Elektron 717, along with Nd at the alloy/film interface and nano-size Zn 2 Zr 3 precipitates throughout the film. Implications of these findings on film growth are discussed. Magnesium and its alloys are lightweight structural materials of great interest for reduced vehicle weight and improved fuel efficiency in the automotive industry due to their good strength, low density, amenability to casting, and ease of recycling.1-3 A major drawback, however, is the poor corrosion resistance of Mg under many conditions. 4 Magnesium is considered the most reactive structural material and is susceptible to multiple forms of corrosion; including general dissolution, galvanic coupling, localized corrosion, and stress corrosion cracking. 2,3,5,6 Surface treatments/coatings are needed for many applications which result in increased cost and can be a source of component durability issues. 1The inability of Mg alloys to establish a continuous and fully protective surface film under many exposure conditions is a key factor underlying their susceptibility to corrosive attack. Films formed on Mg alloys under humid air or aqueous corrosion conditions are typically based on mixtures of MgO and Mg(OH) 2 .4,7-12 Although MgO can offer a degree of protection under ambient dry atmospheric corrosion conditions, it can be readily hydrated in the presence of water or water vapor to form Mg(OH) 2 , which offers limited stability and reduced protectiveness. 4 Further, considerations based on Pilling-Bedworth (P-B) ratio have also been invoked for explaining the inability of Mg to establish a fully protective oxide film. 13 Magnesium has a P-B ratio of ∼0.8, which places surface MgO in tension, and can result in a porous, cracked, nonprotective oxide layer. (Typically a P-B ratio of 1-2 is considered a minimum, but not sufficient prerequisite for protective film formation). Corrosion can be further significantly accelerated when aggressive electrolyte species such as chloride are present. 5,14 Alloying has been shown to modify surface film performance; 15 however, a detailed mechanistic understanding of how and why is frequently lacking.Extensive studies based on electrochemical assessment and surface film chemistry depth profiling of Mg alloys have been pursued by techniques such as Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), etc. [16][17][18][19][20][21][22][23] However, what is lacking is a detailed nano/micro structural level picture of the film nature and alloy/film interface regions, particularly with regards to segregation of...

A family of inexpensive, Al2O3-forming, high-creep strength austenitic stainless steels has been developed. The alloys are based on Fe-20Ni-14Cr-2.5Al weight percent, with strengthening achieved through nanodispersions of NbC. These alloys offer the potential to substantially increase the operating temperatures of structural components and can be used under the aggressive oxidizing conditions encountered in energy-conversion systems. Protective Al2O3 scale formation was achieved with smaller amounts of aluminum in austenitic alloys than previously used, provided that the titanium and vanadium alloying additions frequently used for strengthening were eliminated. The smaller amounts of aluminum permitted stabilization of the austenitic matrix structure and made it possible to obtain excellent creep resistance. Creep-rupture lifetime exceeding 2000 hours at 750 degrees C and 100 megapascals in air, and resistance to oxidation in air with 10% water vapor at 650 degrees and 800 degrees C, were demonstrated.

Figure 1. A scanning electron microscopy (backscatter mode) cross-section micrograph of Nb-33Ti-40Al (in at.%) after 15 minutes at 1,400°C in air. The continuous Al 2 O 3 scale cuts off the formation of more rapidly growing niobium-rich transient oxides. 3 5 µm with the thermochemistry of the environment and the temperature of the reaction. Oxidation can lead to a loss of load-bearing capacity in a component by the reduction of metallic cross section and, for many high-temperature applications, is the primary factor limiting service life. Interstitial dissolution of oxygen, nitrogen, etc. into an alloy during elevated-temperature exposure, which may accompany oxidation, can also degrade mechanical properties and result in component failure.In practice, the oxidation of metals in many applications is not prevented, rather it is managed. Protection of the underlying metal is most often accomplished by the formation of a continuous scale over the surface such that it serves as a barrier between the remaining underlying, unoxidized metal and the environment. Long-term protection is generally associated with a scale that grows by a diffusion-controlled process or processes because the rate of scale growth slows with reaction time as the diffusion distance (scale thickness) increases. Most high-temperature environments of technological interest contain at least some oxygen. Given the fact that metal oxides are nearly always more stable thermoEditor's Note: Compositions are given in weight percent unless otherwise noted.This article discusses general strategies for designing alloys to form protective oxide scales. Approaches based on classical alloyoxidation theories work reasonably well for single-phase alloys. However, high-temperature alloy development has been and will increasingly be based on multiphase microstructures in order to achieve many of the needed, but usually opposing properties, such as high-temperature strength and room-temperature toughness. No theoretical-based, well-defined strategies exist for the design of oxidation-resistant multiphase alloys. Still, key factors are beginning to emerge, which can provide guidance for promoting the formation of protective scales on multiphase alloys and for taking advantage of some unique mechanisms that are operative in multiphase alloys but not in single-phase alloys.

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