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Abstract
Temperature dependent electrochemical measurement of diffusivity and permeability of hydrogen through VP iron and carbon steels was obtained between 5-30(DEGREES)C. The measured anodic or permeation current, ip, is proportional to unsteady or steady state hydrogen flux on the output side of the membrane. Steady state flux, J(,(INFIN)), is linearly related to i(,c)(' 1/2). The mechanism involves coupled discharge-chemical desorption.
The activation energy for permeation in VP iron when measured at constant current is 17.7 KJ/mole. The activation energy for permeation at constant fugacity is 36.6 KJ/mole. The latter is in good agreement with the value obtained by extrapolating higher temperature gas phase data to near room temperature. Gas phase and electrochemical permeation measurement are not directly comparable because gas phase permeation is proportional to (fugacity)(' 1/2) whereas cathodic charge permeation is proportional to (current)(' 1/2). Measured diffusivity at room temperature is higher than the value extrapolated from higher temperature gas phase permeation.
Diffusivity and permeability decrease by a factor of two or more as carbon content increases by 1 w/o C between 5-30(DEGREES)C. These results are consistent with gas phase measurements between 250-600(DEGREES)C. Diffusivity and permeability also decrease as the carbide distribution becomes more finely divided. These results indicate that carbides are obstacles to both steady state and unsteady state transport. It is suggested that slight increase in activation energy with w/o C is related to increased partioning in the carbide phase.
