Accuracy of the linear gradient approximation for diffusion-controlled growth of iron sulfide scales
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The growth of a defective scale is analyzed in terms of an idealized, singlelayer, moving-boundary problem with constant diffusivity. The form of the solution is chosen to emphasize the magnitude of the departure from a linear concentration gradient of the diffusing species in the growing scale. The departure is shown to be a function of the k p /D ratio (where kp is the parabolic rate constant, and D is the chemical diffusivity), and thus is directly related to the defect concentration. For iron sulfide under most conditions, and for other compound scales with defect concentrations less than about 10%, the linear gradient assumption is shown to be reasonable.
Key WordsDiffusion layer growth defects iron sulfide
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