In this way, the horizontal propagation process of pits in acidic
solutions is clearly presented. Such large pit phenomena has previously
been reported by Yusuke Tsutsumi [60] in their study of the
pitting of 304SS steel under a droplet of chloride solution, and the
hole diameter observed by Tsutsumi is consistent with our observations.
Tsutsumi attributes the small holes (∼10 m diameter) in
the center of a large pit to the trace of an inclusion particle like
MnS which is dissolved out of the sample. However, in our case,
particles (possibly the vestige of inclusions or corrosion products)
can be found both inside and outside the small holes. Our observations
indicate that the horizontal propagation of a pit can also take
place in bulk chloride solutions, instead of a typical phenomenon
in thin-layer pitting corrosion.