The High Temperature Oxidation of AI-4.2 Wt Pct Mg Alloy D.J. FIELD, G. M. SCAMANS, and E. E BUTLER The high temperature oxidation of A1-Mg alloys is characterized by the rapid formation of thick, micro-crystalline oxide films. The oxidation kinetics of an A1-4.2 wt pct Mg alloy under dry and moist 20 pct O2/Ar have been measured, and oxide films grown on bulk specimens complementary to the weight gain curves have been characterized using electron optical techniques (TEM, SEM). Initial oxidation takes place by the nucleation and growth of primary crystalline oxides at the oxide/metal interface and by the formation of secondary oxides of MgO by the reduction of the original amorphous over-layer of "y-AI203by Mg. Subsequent oxidation is dominated by the further nucleation and growth of primary oxides. The presence of water vapor in the oxidizing environment initially reduces oxidation rates through a modification of the mechanical properties of the amorphous overlayer but does not affect the overall oxidation mechanism. A microstructural model has been developed which describes oxidation of AI-Mg alloys in terms of fracture of the original air-formed film by primary MgO nucleation and growth and modification to this film by the presence of water vapor in the oxidizing environment. I. INTRODUCTION MAGNESIUM additions to aluminum form a techno- logically important alloy system having good mechanical and physical properties in conjunction with excellent aque- ous corrosion resistance. However, these alloys oxidize rap- idly both when molten and during high temperature heat treatment of solid product forms, giving rise to thick surface films of magnesium oxide which may be accompanied by severe metal blistering in moist environments. Before fur- ther processing can take place, the surface films must be removed which results in an expensive loss of metal, and can make processes such as scrap recycling commercially uneconomical. Methods of controlling the rapid oxidation of A1-Mg alloys have been established, such as by trace- alloying additions, with the most effective additive being toxic Be. 1 The oxidation kinetics of A1-Mg alloys have been mea- sured thermogravimetrically by several investigators 2'3'4 but when comparing weight gains the surface preparation of specimens prior to oxidation appears to be an important factor, affecting overall weight gains by orders of mag- nitude, and this factor has not always been closely con- trolled. From the literature evidence it is clear that weight gain measurements alone have shed relatively little light on the oxidation mechanism(s) operating during the high tem- perature oxidation of A1-Mg alloys. In recent years the ap- plication of modem TEM techniques to high temperature oxidation 5 has become more commonplace, and micro- structural studies in conjunction with thermogravimetry offer the opportunity of gathering structural information which can then be related to the oxidation weight gain. This approach was adopted in this investigation. D. J. FIELD, Research Scientist, and G. M. SCAMANS, Principal Sci- entist, are with Alcan International Limited, Banbury Laboratories, Banbury, Oxon OX16 7SP, England. E. P. BUTLER, formerly at Imperial College, London, is Principal Scientist with Alcan International Limited, Banbury. Manuscript submitted August 8, 1985. Aluminum and Aluminum Alloy Oxidation During the initial stages of oxidation over a wide range of conditions, aluminum and aluminum alloys rapidly develop a thin tenacious film of amorphous T-A1203.5 This oxide layer provides a barrier between the metal substrate and the environment, and it is this film which has been shown to control the early stages of crystalline oxide formation at high temperatures. 6-9 At temperatures above about 425 to 450 ~ the amorphous T-A1203 overlayer undergoes a dis- continuous change in structure. 6 Rapid migration of oxygen to the oxide/metal interface is readily detectable at these temperatures and is evidenced by the nucleation and growth above about 450 ~ of crystalline "y-A1203 as a new phase below the amorphous layer. Growth of amorphous "y-A1203 continues by cation diffusion to the oxide-oxygen inter- face in a fashion of mutual independence with respect to any underlying crystallites of T-A1203 which form at the oxide/metal interface and penetrate down into the metal after a temperature-dependent induction period. 5'6'7 The high temperature surface films which develop on aluminum al- loys are characteristically duplex, consisting of both crys- talline and amorphous oxides. 8 The amorphous film is important in the initial stage of oxidation as the film thick- ness and/or ion transport properties change with alloy addi- tion. 7'9In A1-Cu alloys, 9 for example, the dielectric constant of the crystalline T-AI203 which develops is much lower than that of the crystalline 3~-A1203 formed on pure alu- minum, a result explained in terms of the reduction of cation vacancies due to doping of the T-A1203 by copper ions. The oxidation of A1-Mg alloys has been the subject of many studies: by thermogravimetry, 2'3'4 X-ray diffrac- tometry of oxide powders 2 and electron microscopy of stripped 1'1~ or back-polished films, and by ex situ oxidation of evaporated films ~3or in situ oxidation of thin foil sam- ples. 7'8 Smeltzer 2 measured the oxidation weight gains and vacuum evaporation losses of an AI-3 wt pct Mg alloy. Oxi- dation was found to be rapid at temperatures above 400 ~ and in the temperature range 350 to 550 ~ the kinet- ics were found to characteristically decrease with time, subsequently giving way to approximately linear kinetics at weight gains greater than 10/xg/cm 2. Oxide powder scraped from A1-3 wt pct Mg oxidized for 60 hours at METALLURGICAL TRANSACTIONS A VOLUME 18A, MARCH1987--463