As a diffusion expert you are to calculate the thickness of Ag required to maintain at least a 99% Ag alloy on the surface of Cu for 5 years. The most accurate data you can locate is a study of D for Ag in Cu between 750 and 1050°C. Extrapolating these data to 150°C, you find that a 1 micrometer layer of Ag will last for 150 years. A laboratory test shows that a 1 micrometer silver layer completely dissolves in a sample over a weekend at 150°C. Why might the calculation of the rate at 150°C be invalid?