The evolution of H from the stainless steel was studied by using a real-time dynamic technique under ultra-high vacuum conditions. Auger electron spectroscopy was used to determine the surface composition as a function of time and temperature. It was found that the surface film on the electropolished samples was approximately 1.5nm thick, and consisted of a C-O complex and FexOy. Upon heating to 400C, the C-O complexes desorbed as CO and the remaining O and C began to be incorporated. At this temperature, S also began to diffuse out of the bulk and towards the surface. At about 800C, it formed a complete monolayer. At 900C, the C and O almost disappeared; leaving a S monolayer as the only surface contaminant. The H diffusivity was found to be described by the expression:

 D (m2/s) = 7.01 x 10-7 exp[-48.0/RT] 

over the entire temperature range which was studied (figure 21). This suggested that H evolution was not appreciably affected by the changing surface composition.

R.A.Outlaw, D.T.Peterson: Metallurgical Transactions A, 1983, 14[9], 1869-74

Figure 21

Diffusivity of H in AISI347

Table 108

Diffusivity of Cr in Stainless Steel