Detailed mechanism of surface blistering in sour media

The known occurrence or absence of surface blistering (SB) are reminded, either in field failures or in HIC testing in the laboratory, i.e. in sophisticated fitness for purpose tests at low pH or in simplified acceptance tests at conventional pH values (NACE TM 0284). A previous analysis of charging severity is also reminded, together with the presence of temporary degassing barriers. Then, through an accurate modelling of charging transients, significant overshots of the hydrogen content close to surface are shown to be present soon after immersion when looking for the most resistant steels in HIC testing on full thickness samples, so that SB can be met alone in such tests. On commissioning in the field, their magnitude is necessarily much lower, so that in field failures, SB is always met together with SWC in the bulk. Conversely, overshots are totally unlikely with small specimens, short durations and medium severity, and SB is never reported in TM 0284 tests. They are also made impossible in the field by a progressive commissioning, and SWC is often reported alone in field failures. Such agreement in a very complex situation is indeed one more indirect proof of the protonic nature of dissolved hydrogen in steel