3.2 Corrosion of Fe0 by Prolixibacterstrains
When P. denitrificans MIC1-1T and three newly
isolated strains (P. denitrificans AT004, P. denitrificansKGS048, and Prolixibacter sp. SD074) were grown in corrosion-test
medium containing Fe0 foils, the surface of the
Fe0 foils lost its shine, and turned grayish black
within one month of their cultivation. In addition, the color of the
medium changed to light yellow, suggestive of the formation of ferric
ions in the cultures. On the other hand, these color changes were not
observed in the anaerobic cultures of Prolixibacter sp. NT017 orP. bellariivorans JCM 13498T. Thus, all nitrate
reducers were expected to be Fe0-corroding, while all
nitrate non-reducers were not.
To investigate further, Fe0 foils incubated in
corrosion-test medium for 30 days in the presence or absence of variousProlixibacter strains were observed with a scanning electron
microscope (SEM). As shown in Fig. 3A–B and 3D–E, cubic crystals
developed on the surface of Fe0 foils after incubation
with P. denitrificans MIC1-1T and P.
denitrificans AT004. Rod-shaped cells of length 1.5–6 µm were also
observed on the surface of Fe0 foils (Fig. 3B and 3E).
The cross-sections of Fe0 foils showed that the
surface was eroded after incubation with these strains (Fig. 3C and 3F)
compared with the aseptic control (Fig. 4I). On the surfaces of
Fe0 foils submerged in the cultures of P.
denitrificans KGS048 and Prolixibacter sp. SD074, amorphous
flakes developed (Fig. 3G–L), and the cross-sectional morphology of the
corrosion products showed a greater thickness of the corrosion films
(Fig. 3I and 3L) than those produced by P. denitrificansMIC1-1T and AT004 (Fig. 3C and 3F). On the other hand,
the amounts of corrosion deposits on the surface of the
Fe0 foils submerged in the cultures of nitrate
non-reducers, namely Prolixibacter sp. NT017 and P.
bellariivorans JCM 13498T, were minimal (Fig. 4A–F),
as well as similar to those in the aseptic control (Fig. 4G–I).
The X-ray diffraction analyses of the corroded Fe0samples revealed that deposits developed on the surface of
Fe0 foils from the cultures of P. denitrificans(MIC1-1T, AT004, and KGS048) and Prolixibactersp. SD074 mainly consisted of FeCO3 and
Fe3(PO4)2 (Table S3). In
one of the two samples from the cultures of P. denitrificansKGS048 and Prolixibacter sp. SD074,
Fe3O4Fe2O3and FeO(OH) were also detected on the surface of Fe0foils. A very thin layer of
Fe3(PO4)2 was detected
on the surface of Fe0 foils in the cultures ofP. bellariivorans JCM 13498T and the aseptic
control, while corrosion products were not detected on the surface of
the Fe0 foils in the culture of Prolixibactersp. NT017. Thus, the electron microscopic studies confirmed the
conclusions obtained by visual observation, that is, that the four
nitrate-reducing strains P. denitrificansMIC1-1T, P. denitrificans AT004, P.
denitrificans KGS048, and Prolixibacter sp. SD074 were
Fe0-corrosive, while the two nitrate-non-reducing
strains, P. bellariivorans JCM 13498T, andProlixibacter sp. NT017, were not.
Changes in the corrosion potential of Fe0 (= working
electrode) immersed in corrosion-test medium in the presence or absence
of a Prolixibacter strain were examined, as shown in Fig. 5A. In
the presence of Fe0-corroding P. denitrificansMIC1-1T, the corrosion potential shifted in the
positive direction from −680 mV (vs. KCl saturated Ag/AgCl) to −600 mV
in the first 10 days, followed by a shift in the negative direction.
Such intensive changes in the corrosion potential were not observed in
the aseptic control or in the presence of
Fe0-non-corroding Prolixibacter sp. NT017.
Every 8 h during the continuous measurement of the corrosion potential
of the working electrode, the corrosion current was also estimated by
sweeping ±25 mV around the corrosion potential. As shown in Fig. 5B, the
current density increased between days 2 and 4 from almost zero to
3.2 µA cm−1. After day 4, the current density remained
almost unchanged, except between days 10 and 12, in which it increased
temporarily up to approximately 19 µA cm−1 (Fig. 5B).
On the other hand, no corrosion current (< 0.5 µA
cm−1) was generated upon the polarization of
Fe0 in the aseptic control and in the presence ofProlixibacter sp. NT017.