Weight gain film oxidation

The oxidation of a particular metal in air is limited by the outward diffusion of metallic ions through an unbroken surface film of one species of oxide. Assume that the concentration of metallic ions in the film immediately next to the metal is Cj, and that the concentration of ions in the film immediately next to the air is C2, where and C2 are constants. Use Tick s First Law to show that the oxidation of the metal should satisfy parabolic kinetics, with weight gain Am given by... 

During the past few years four methods have been developed for the study of the kinetics of oxide film formation, all being semicontinuous in nature. The polarimetric method has been adapted by Lustman and Mehl (14) for the measurement of the oxidation of copper. A spectro-photometric method has been applied by Waber, Sturdy, Wise, and Tipton (15) to the study of the oxidation of tantalum while a differential pressure method has been developed by Campbell and Thomas (16) for a study of the oxidation of a series of metals and alloys at elevated temperatures. We have chosen to use a sensitive weight gain method (17,18) in which a quartz microbalance is placed directly in the vacuum system. 

Plates of copper covered by neutral solutions of cupric sulphate always gain weight, becoming coated with a film of cuprous oxide. 

Oxidation of the three films in air at 50 C and 70 C indicated a formulation sensitive weight gain in all instances, with the weight gains more rapid at the... 

Thorium metal is slowly tarnished by air at room temperature, but further attack is prevented by an adherent oxide film. At temperatures above 200 C, however, progressive attack takes place. Weight gains of 0.03, 0.43, and 8.7 g/(cm h) have been reported at 300, 400, and 500 C, respectively [W2]. The product is primarily ThOj. Finely divided thorium is pyrophoric. 

Consider the bipolar electrode for electrorefining metal M. Thus, the rate of formation (reduction) and dissolution (oxidation) are treated as steady-state quantities. Despite that the initial formation of a thin film at the cathode face and dissolution rates are controlled by reactions at the electrode-electrolyte interfaces, assume a steady-state diffusion mechanism. Derive expressions for the weight gain at the cathode side and the weight loss at the anode side. 

Resistivity measurements can supplement the information obtained fi om weight gain. Fehlner [5c] has pioneered a method that uses discontinuous metal films. The gaps between islands of metal accentuate the effect of surface oxidation. 

Here, y is the weight gain of the oxide film, k the corrosion rate constant and t the time. 

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