MODELING OF THE HYDROGEN DISTRIBUTION AT A CRACK-TIP

A model has been developed to predict the distribution of hydrogen ato ms at a crack tip with the unique feature of incorporation of generali sed boundary conditions which account realistically for the electroche mistry-diffusion interface. The use of boundary conditions involving e xpressions for the flux allows identification of the conditions for di ffusion and surface reaction control of hydrogen transport. For ferrit ic steels, which have a relatively large diffusivity compared with fac e centred cubic alloys, the model predicts that hydrogen transport is controlled by the kinetics of cathodic generation of hydrogen atoms. T he crack-tip concentration is approximately an order of magnitude less than that predicted assuming the conventional constant concentration boundary conditions derived from diffusion controlled transport. The r esults imply that crack growth rates will be limited by the kinetics o f surface reactions and that predictive models of crack growth rates a nd thresholds for cracking based on diffusion control should be reasse ssed. The use of generalised boundary conditions also enables the calc ulation of the time evolution of the hydrogen distribution in initiall y precharged metals. It is shown that the crack tip and crack walls ac t as sinks for hydrogen atoms in these circumstances, depleting the hy drogen atoms in the crack-tip region. The loss of hydrogen through the crack tip in precharged samples may affect the location of hydrogen c racking and is an additional factor to be considerred in comparing int ernal and external hydrogen embrittlement. It also raises questions co ncerning the meaning of tests on cadmium plated samples because of the loss of hydrogen through the crack tip once the coating has been frac tured due to dynamic straining or crack advance.