-The development of magneto-optic media as a commercially viable product included identification of means to protect the rare earth transition metal (RE-TM) thin ftlm from corrosion. The results of studies used to achieve stability have been analyzed to gain a better scientific understanding of the processes involved in the growth of corrosion defects. Growth rate is exponentially temperature dependent, strongly accelerated by increases in relative humidity and the presence of impurities like Cl, and greatly reduced by the addition of certain alloying elements. Microscopic and microchemical examination showed that if present, Cl was highly concentrated in the corrosion site. Significant diffusion of the thin ftlm constituent elements accompanied corrosion. The growth of individual defects occured by the formation of appendages. The following hypotheses were made concerning growth: 1.) Temperature and humidity dependency are a result of effects on transport rate in the electrolyte, 2.) hnpurities like Cl increase rates by impeding the formation of oxide protective fIlms whereas alloying additions like Ta improve the capability to form protective oxide ftlms.