Title

Modification of rust layer on carbon steel withreactive actions of metallic cations for
improved corrosion protectiveness

Author(s)Kim, Kyung Tae; Tsuchiya, Hiroaki; Hanaki,
Koushu et al.

CitationCorrosion. 2020, 76(4), p. 335-343

Version Type AM
URL

https://hdl.handle.net/11094/82369

rights
Note

Osaka University Knowledge Archive : OUKA
https://ir.library.osaka-u.ac.jp/
Osaka University

Modification of Rust Layer on
Carbon Steel with Reactive
Actions of Metallic Cations for
Improved Corrosion
Protectiveness
Kyung-Tae Kim,* Hiroaki Tsuchiya,* Koushu
Hanaki,*,** Masato Yamashita,*,** and Shinji
Fujimoto. ‡,** Division of Materials and Manufacturing Science,
Graduate School of Engineering, Osaka University.
** Kyoto Materials Co. Ltd.
ARTICLE INFOINTRODUCTION
Steel has been employed as structural material due to its abundanceas well as superior properties in strength, toughness and workability.1‐
3) In Japan, many infrastructures have been built using steel in the higheconomic growth period of the 1960s.4) The constructions made ofsteel cannot avoid degradation due to atmospheric corrosion when
they are used in atmospheric environments where water, oxygen andother corrosive species are present. According to the atmosphericcorrosion process proposed by Evans et al.,5) during the wet period, theanodic and the cathodic reactions expressed in the equations (1) and(2), respectively, are balanced under rust layer formed on steel.

Fe → Fe2+ + 2e‐(1)

8FeOOH + Fe2+ + 2e‐ → 3Fe3O4 + 4H2O

(2)

In the drying period, the supply of oxygen through the rust layer occurs
effectively, and the oxygen re‐oxidizes the formed Fe3O4, according to
the following equation (3).

3Fe3O4 + 3/4O2 + 9/2H2O → 9FeOOH(3)

Article history:
Received Day Month YearAccepted Day Month Year
Available Day Month Year
Keywords:
atmospheric corrosion
rusted carbon steel
wet/dry cyclic corrosion
cation* Division of Materials and Manufacturing Science, Graduate School ofEngineering, Osaka University, 2‐1 Yamada‐oka, Suita, Osaka 565‐0871 Japan.
** Kyoto Materials Co. Ltd., Kyodai‐Katsura Venture Plaza, South‐2102, 1‐39
Goryo‐Ohara, Nishikyo‐ku, Kyoto 615‐8245 Japan.
‡

Corresponding author: Email: (E‐mail: fujimoto@mat.eng.osaka‐u.ac.jp).

ABSTRACTIn the present work, we examined the modification of a rust layer on
a carbon steel surface during a cyclic corrosion test. The rust layergrown in a coastal region of Japan was used as a model rust layer. The
X‐ray diffraction analysis of the model rust layer revealed that the layer
consisted of α‐FeOOH, β‐FeOOH, γ‐FeOOH, and Fe3O4. During the cyclic
corrosion test, an additional immersion of the surface in solutions
containing metal cations such as Mg2+, Al3+, Cu2+, or Ni2+ was
performed. The additional immersion in the cation‐containing
solutions modified the model rust layer, that is, the fraction of stable
α‐FeOOH increased in the rust layer. Furthermore, the modification of
the model rust layer could suppress the corrosion of steel. The
decreased corrosion rate was attributed to the suppressed cathodic
reduction of the rust layers due to the presence of the stable α‐FeOOH
in the rust layers.

Based on the model, one can deduce that the corrosion proceeds
during the repeated wet‐dry cycle once steel corrodes to form a rustlayer in the atmospheric environment. If rusted steel constructions
continue to be used while they are aged without being repaired,serious accidents would occur.6, 7)
It is well‐known that the rust layers formed on steel in atmosphericenvironments consist of α‐FeOOH, β‐FeOOH, γ‐FeOOH, Fe3O4 andpoorly crystallized iron oxides.8–10) Further, the fraction of the phasesin the rust layers is strongly affected by the environment in which therust layers grow.11) In addition, the electrochemical properties of therust layers are reported to be different, for example, α‐FeOOH iselectrochemically stable compared to the other types ofoxyhydroxides.12, 13) Therefore, if the fraction of α‐FeOOH increases in
the rust layer on steel, the cathodic reaction expressed in the equation(2) can be hindered, leading to a suppressed atmospheric corrosion.
Weathering steel has been reported to form a protective rust layer on
its surface. Therefore, it exhibits superior atmospheric corrosion
resistance. Extensive studies have revealed that alloying elements suchas Cu and Cr in weathering steels are enriched in rust layers andcontribute to the excellent protectiveness of the rust layers.14‐17) Thisfinding implies that the introduction of metallic cations in rust layers
modifies the structure and composition of the rust layers as well as
their protectiveness, thereby contributing to an improved atmosphericcorrosion resistance. If the introduction of metallic cations in rustlayers is realized by a post‐treatment, the approach will be useful forthe repair of rusted steel constructions.
In the present work, we examined the feasibility of our idea, that is,
the introduction of metallic cations in a rust layer grown on carbon
steel to modify the structure and composition of the rust layer and its
resulting electrochemical properties. For this purpose, we performed
the immersion of rusted steel in cation‐containing solutions during a
wet/dry cyclic corrosion test.

EXPERIMENTAL PROCEDURE
The material used was a carbon steel (SS400‐JIS G3101) sheet with athickness of 3 mm. The chemical composition of the steel was asfollows (mass%) ; C: 0.05, Si: 0.03, Mn: 0.33, P: 0.016, S: 0.01, Fe: bal.
The carbon steel sheet was exposed to a coastal environment inObama City, Fukui Prefecture in Japan for three months, during which,a rust layer grew on the steel surface. The city is situated at lat. 35°53’
N. and long. 135°75’ E. Average airborne salt at the site is estimated as
1.0 mg NaCl/dm2/day. After the exposure, the rusted carbon steel was
cut into as‐rusted specimens to be used for the cyclic corrosion test,
characterization and electrochemical measurements described below.
The specimens were rinsed in methanol and then dried in air prior tofurther experiments.
The rusted specimens were subjected to a laboratory cyclic corrosion
test in accordance with the SAE J223418) that consists of three stages:
the humid stage, immersion stage, and dry stage. These stages
simulate a severe atmospheric corrosion environment where chloride
is present. Therefore, the SAE J2234 test has often been used toevaluate the atmospheric corrosion resistance of steel in severe
conditions.19, 20) In the humid stage, the rusted specimens were heldfor 6 h in an environment with a temperature of 50 °C and a relative
humidity (R. H.) of 100%. In the subsequent immersion stage, thespecimens were placed in a mixed solution of 0.5 mass% NaCl, 0.1
mass% CaCl2 and 0.075mass% NaHCO3 for 15min. Thereafter, they
were held for 17 h 45 min at 60 °C and 50% R. H. (dry stage). These
three processes were repeated for up to 30 cycles in the present study.
It should be noted that from the 2nd cycle, the specimens wereadditionally immersed in a solution of 1M MgSO4, Al2(SO4)3, CuSO4, or
NiSO4 for 5 s prior to the humid stage at each cycle. These specimens
are hereafter referred to as Mg2+, Al3+, Cu2+, or Ni2+ specimens,
respectively. Some rusted specimens were not subjected to theadditional immersion, but to only the cyclic corrosion test (referred toas the “reference specimens”).
To evaluate the electrochemical properties of the as‐rusted specimen,
the reference specimens as well as specimens subjected to theadditional immersion, potentiodynamic polarization measurements
were accomplished in a conventional three‐electrode configuration cellwith a Ag/AgCl/KCl(sat.) reference electrode and a platinum counterelectrode. The solution used for the electrochemical measurements
was the same as the one used in the immersion stage of the cyclic
corrosion test, as explained above. The potentiodynamic polarization
was conducted at a scan rate of 0.5 mV s‐1 without the deaeration of
the solution. Furthermore, in the present work, to examine thereduction reaction on the rust layers of all specimens, the cathodic
polarization curves were also measured in the mixed solution ofchlorides and bicarbonate. For the cathodic polarization, the potential
was swept to 500 mV lower than the open circuit potential of eachspecimen with a scan rate of 0.5 mV s‐1. These morphological,
structural and electrochemical characterizations were carried out using
a single specimen for each cycle of the cyclic corrosion test to avoid
possible individual differences of specimens after the exposure to thecoastal environments for three months.
The morphology of the specimen surfaces was examined by scanningelectron microscopy (SEM, JEOL JSM‐5600) with an accelerating
voltage of 25 kV, whereas the phase of the rust layers on thespecimens was identified with X‐ray diffraction using a Cu target
operated at 45 kV and 40 mA in a Phillips X’pert X‐ray diffractometer.

RESULTS
3.1 Morphology and composition of rust layer
Figure 1(a) shows the SEM image of the surface of the as‐rusted
specimen which was exposed to a coastal environment in Obama City,

Fukui Prefecture for three months. It is clear that the specimen showsa plate‐like morphology, similar to that found in the literature.21, 22)This plate‐like rust layer is supposedly not protective as water andcorrosive ions can penetrate through the hollows in the rust to theunderlying steel substrate.23, 24) The as‐rusted specimen was examinedby X‐ray diffraction (XRD) and the pattern obtained is presented in
Figure 1(b). In particular, diffractions derived from the α‐FeOOH, β‐
FeOOH, γ‐FeOOH, and Fe3O4 phases are observed, and clear peaks of γ‐
FeOOH are confirmed. In order to evaluate the rust layer
quantitatively, the mass fractions of α‐FeOOH, β‐FeOOH, γ‐FeOOH, andFe3O4 (described as Mα, Mβ, Mγ, and MS, respectively) in the rust layer
were calculated using the intensity of the diffraction peaks obtainedfor each crystalline phase (I , I , I , and IS, respectively), based on thefollowing equation,

Mi = MiA∙(Ii/IiA) / ∑(MjA∙ Ij / IjA) (i, j = α, β, γ, S)(4)where IiA, IjA and MiA, MjA are the reference diffraction intensities andthe reference mass fractions, respectively, obtained from thereference rust used in our previous work. The details on the estimation
are described in the literature.25) The mass fractions computed for theas‐rusted specimen were as follows: 21% for α‐FeOOH, 5% for β‐
FeOOH, 59% for γ‐FeOOH and 1 % for Fe3O4.
After the cyclic corrosion test, the morphology and composition of
the as‐rusted specimen was observed to vary. Figure 2 shows theSEM images of the specimen surfaces. Notably, the specimens shownin Figures 2(b) ‐ 2(e) were subjected to the additional immersion in
the cation‐containing solutions during the cyclic corrosion test
whereas the reference specimens in Figure 2(a) were not subjectedto the additional immersion. The morphology of the rust layers on
the reference specimen is unchanged, as shown in Figure 2(a).
However, drastic changes in morphology are observed for the
specimens subjected to the additional immersion. Furthermore, thechanges strongly depend on the solution used for the additional
immersion. The morphological change from the plate‐like to aflowery shape is confirmed for the Mg2+ specimens after the 3rd
cycle. In addition, the flowery structure becomes finer with anincrease in the number of cycles. For the other specimens subjectedto the additional immersion, that is, for the Al3+, Cu2+ and Ni2+
specimens, a different morphological change to a cotton ball shape isrecognized. The flowery and cotton ball structures were reported by
Prakash et al. on a mild steel.21) The rust layers with the finer flowery
shape or the cotton ball shape are found to be more protective
compared to those with a plate‐like shape because the flowery or
cotton ball shaped rust layers are more compact and dense due tofewer hollows, as displayed in Figure 2. In the following, thecomposition of the rust layers on these specimens is discussed.
Figure 3 presents the mass fraction of the rust phase on thereference specimens and the specimens subjected to the additional
immersion in the cation‐containing solutions. In the figures, theresult obtained for the as‐rusted specimen is also included forcomparison. For the reference specimens, the fraction of α‐FeOOH
increases whereas the fraction of γ‐FeOOH clearly decreases. After 30
cycles, the fraction of α‐FeOOH reaches approximately 40%.
Although a similar change is observed for the specimens subjected tothe additional immersion, a further increase in α‐FeOOH is observed.
For the Mg2+, and Ni2+ specimens, the fraction of α‐FeOOH after 30
cycles is around 50%, which is slightly smaller compared to that of
the Al3+, and Cu2+ specimens.

3.2 Electrochemical property of rust layers
Figure 4(a) shows the representative polarization curve obtained fromthe as‐rusted specimen. In the present work, the corrosion potential,

Ecorr, and the corrosion current density, Icorr, were evaluated by the
extrapolation of Tafel slopes within the potential range of ±100 mVfrom Ecorr. Ecorr and Icorr for the as‐rusted specimen are estimated as ‐414 mVAg/AgCl and 0.028 mA cm‐2, respectively. The samemeasurements were carried out for the other specimens, that is, thereference specimens and the specimens subjected to the additional
immersion. The obtained Ecorr and Icorr are summarized in Figures 4(b)and 4(c). The results for the as‐rusted specimen are also presented forcomparison. As shown in Figure 4(b), after the 3rd cycle, the Ecorr forthe reference specimens is almost similar to that for the as‐rusted
specimen, however, it increases slightly with cycling. For the Al3+
specimen, the Ecorr is less noble particularly in the18th and 30th cycle.
For the other specimens, except for the reference specimens and theAl3+ specimens, the Ecorr tends to decrease once in the initial stage, andthereafter, increases with cycling. After the 30 cycle, the Ecorr of the
Al3+ specimen is lower than that of the reference specimens whereas
the Ecorr of the other specimens with the additional immersion ishigher, particularly for the Cu2+ specimens. Figure 4(c) reveals that the
Icorr of the reference specimens increases slightly with cycling
compared to the Icorr of 0.028 mA cm‐2 for the as‐rusted specimen. For
the Mg2+ and Ni2+ specimens, the Icorr increases once in the initial cycles
and then decreases significantly in later cycles. However, the Icorr for
the Al3+ specimens decreases monotonically with cycling. From thesevariations, one can observe that in the later stages of the cyclic
corrosion test, the Icorr of these specimens is smaller than that of theas‐rusted specimen. Contrastingly, the Icorr for the Cu2+ specimens in
the later stages is roughly comparable to that of the as‐rusted
specimen although the Icorr fluctuates through the cycle. However,when compared to the reference specimens, all the specimens withthe additional immersion exhibit a smaller Icorr, indicating that theadditional immersion in the cation‐containing solutions increases the
corrosion resistance of the as‐rusted specimen in the later stages.
To evaluate the reduction reaction on the rust layers in addition to theEcorr and Icorr, the cathodic polarization curves for all the specimens
were measured, where the potential was swept with a rate of 0.5 mV s‐
1until it reaches 500 mV lower than the open circuit potential of eachspecimen. Figure 5(a) shows the cathodic polarization curve obtained
for the as‐rusted specimen. It is clear that the cathodic polarization
curve does not exhibit the current plateau related to the oxygen
diffusion limit. In addition, interestingly, the cathodic current density isrelatively larger than the oxygen diffusion‐limited current density of
approximately 25 μA cm‐2 on polished carbon steel.26, 27) This implies
that the reduction of the rust layer occurs on the as‐rusted layer,
contributing to the cathodic current density. To examine the effects ofthe cations on the reduction of rust, a specific cathodic current density,
Ic, was determined at 500 mV less than the open circuit potential ofeach specimen from the cathodic polarization curve. For example, theIc for the as‐rusted specimen was estimated as 0.92 mA cm‐2 from thecathodic polarization curve shown in Figure 5(a). For the otherspecimens, the Ic was determined by the cathodic polarization curves
and is summarized in Figure 5(b). The Ic for all specimens increases in
the initial stage of the cyclic corrosion test, however, subsequentvariations differ depending on the specimen. The Ic of the reference
specimen and the Cu2+ specimen decreases with cycling and then levels
off to stable values, although the current densities for the 18th cycle
are relatively larger than the stable values. On the other hand, the Ic of
the other specimens decreases monotonically with cycling after the
initial increase, and finally converges to approximately 0.4 mA cm‐2. Itshould be noted that each Ic presented in Figure 5(b) is larger than theoxygen diffusion‐limited current density, indicating that the reductionof the rust layers occurs on all the specimens examined. Furthermore,
the reduction is strongly affected by the cations.

DISCUSSION
4.1 Effects of cations on the electrochemical
properties of rusted steel
In section 3.2, Ecorr and Icorr are presented in Figures 4(b) and 4(c),respectively. The relationship between Ecorr and Icorr is summarized for
all the specimens subjected to the cyclic corrosion test and ispresented in Figure 6. The data for the as‐rusted specimen is alsoplotted as the star symbol, ★, in each figure. As apparent from Figure
6, these correlations differ depending on the specimen. For thereference specimens, the Icorr is almost comparable each otheralthough a scatter in the data is observed. As described in section 3.1,
the rust layers on the reference specimens are not adequately
protective, thus, the rust layers cannot effectively suppress thepenetration of the electrolytes to the steel substrate. For the Mg2+ andNi2+ specimens, a relatively large Icorr and a low Ecorr are confirmed in
the initial stage. However, in the later stages, the Icorr decreases andthe Ecorr increases. These variations in the later stages imply that theanodic reaction is suppressed with cycling, which is probably due tothe growth of the dense rust layers as indicated in Figure 2.
The dense rust layers grew on the other specimens such as the Al3+and Cu2+ specimens, however, the correlation between Ecorr and Icorr for
the Al3+ and Cu2+ specimens is completely different from thecorrelation between Ecorr and Icorr for the Mg2+ and Ni2+ specimens. For
the Al3+ specimens, as shown in Figure 6(c), the Icorr relatively decreases
compared to the as‐rusted specimen, but the Ecorr hardly increases,
compared to the Mg2+ and Ni2+ specimens. Otani et al. investigated thecorrosion behavior of a mild steel and an aluminum alloy, focusing on
the effects of metallic cations in various salt solutions.28, 29) They
described that compared to metallic cations with a small hardness,those with a high hardness29‐31), such as Al3+, are adsorbed into the thinoxide layer on the mild steel. 28) The hydroxide of the adsorbed metallic
cation is formed for a short time. If a similar phenomenon is assumed
to occur for Al3+, the adsorbed Al3+ could promote the formation of α‐
FeOOH, which is the dominant phase in the rust layers on the Al3+
specimen. As described above, α‐FeOOH is electrochemically stable,
thus suppressing the cathodic reaction expressed in the equation (2).
Therefore, these variations can be explained by a suppressed cathodic
reaction. The Ecorr of the Cu2+ specimens drastically increases,
compared to those of the other specimens. The higher Ecorr is
considered to result from the acceleration of the cathodic reaction.
The XRD analysis reveals that the mass fraction of FeOOH is larger for
the Cu2+ specimens compared to those of the other specimens, as
shown in Figure 3. The effect of Cu2+ on the oxidation reaction of Fe2+
was reported by Cher et al. and Stumm et al.32, 33) They found that Cu2+
catalyzed the oxidation of Fe2+ to Fe3+. Assuming the catalytic action of
Cu2+, the increased amount of FeOOH is reasonably explained. The
large amount of FeOOH can increase the reduction of rust described in
the equation (2). Therefore, it can be concluded that Cu2+ increases thegrowth of FeOOH, which results in the increase of the cathodic
reaction. This could contribute to a higher Ecorr for the Cu2+ specimens.
The above discussion implies that a large amount of FeOOH induces ahigher Ecorr. Figure 7 shows the relationship between Ecorr and the ratioof the total mass fraction of FeOOH to the mass fraction of Fe3O4. The
total mass fraction is calculated from the sum of the mass fraction of
α‐FeOOH, β‐FeOOH and γ‐FeOOH. A linear dependence between Ecorr
and the ratio is obtained, indicating that Ecorr is strongly affected by theamount of FeOOH. Contrastingly, Icorr has been discussed in terms of
the other index, α/γ*,34‐36) where α and γ* indicate the mass fraction of
α‐FeOOH and the total mass fractions of β‐FeOOH, γ‐FeOOH, andFe3O4, respectively. Figure 8 presents the relationship between Icorr and
α/γ*. In Figure 8, Icorr decreases with an increase in α/γ*. Kamimura etal. also found that the composition of the rust layer on weathering
steel changes during atmospheric corrosion and the corrosion rate

decreases with an increase in the index, α/γ*.34, 36) This correlation
between Icorr and the index suggests that Icorr is reduced due to thegrowth of a certain amount of α‐FeOOH.

The Ecorr ‐ Icorr plot indicated that the rust layers on the Mg2+ and theNi2+ specimens suppressed an anodic reaction whereas the rustlayers on the Al3+ specimen retarded a cathodic reaction.



The correlation between Icorr and the cathodic current density, Ic, of
the rust layers revealed that the atmospheric corrosion of carbon
steel may be dominated by cathodic reaction.



A clear correlation between Icorr and the mass fraction of α‐FeOOH
in the rust layer was recognized, implying that the modification ofrust layers by the introduction of metallic cations will be effective
in suppressing the atmospheric corrosion of carbon steel.

4.2 Influence of the reduction of rusts on corrosion
current density
Evans developed an electrochemical model on the atmosphericcorrosion of iron in wet‐dry cyclic conditions. In the model, during thewet period, the anodic dissolution of iron under a porous rust layer
was balanced by the cathodic reduction of FeOOH, which is expressedin the equation (2). The re‐oxidation of Fe3O4 subsequently occurs in
the drying period, according to the equation (3). The reduction of
FeOOH and the re‐oxidation of Fe3O4 repeatedly occur in wet‐dry
cycles. It was reported that γ‐FeOOH can be easily reduced to Fe3O4
whereas α‐FeOOH is stable and can scarcely be reduced.37‐39)
Therefore, α‐FeOOH is found to suppress the reduction of rust, whichis in accordance with the finding obtained from Figures 3 and 5(b). The
reference specimens with a relatively low fraction of α‐FeOOH in therust layer exhibit a higher Ic. However, the other specimens with ahigher α‐FeOOH fraction exhibit a lower Ic compared to the reference
specimens. From the above‐mentioned discussion, one can deducethat a lower Ic is related to the suppressed reduction of rust. Figure 9shows the relationship between Ic and Icorr, also shown in Figures 4(c)and 5(b), respectively. Icorr decreases with a decrease in Ic. This
demonstrates that the corrosion current density, Icorr, on rusted steel isstrongly related to the reduction current density of the rust, Ic,
indicating that the corrosion rate of carbon steel in an atmosphericenvironment is significantly dominated by the reductive property of arust layer on the steel. The present work clearly demonstrates that the
reductive property of a rust layer on carbon steel can be modified with
an immersion in a cation‐containing solution, that is, the introductionof a cation into the rust layer. This modification results in an improved
corrosion resistance of carbon steel experiencing atmosphericcorrosion. However, further works are required to confirm thefeasibility of the present results to rusts grown after longer exposures
to the environment. In addition, the synergistic effects of the cations
on the modification of a rust layer on carbon steel can be expected,that is, the growth of α‐FeOOH can be facilitated by the immersion of arust layer in mixed solutions of the cations. These will be examined in
the near future.

ACKNOWLEDGEMENTS
The present work was supported by a Grant‐in‐Aid for Scientific
Research (B) (Project No. 19H02479) from the Japan Society for the
Promotion of Science.

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In the present work, a cyclic corrosion test for a model rust layer grownon carbon steel was performed to examine the modification of themorphology, composition and electrochemical properties of the rustlayer by the introduction of metallic cations such as Mg2+, Al3+, Cu2+, or
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FIGURE CAPTIONSFIGURE 1. (a) SEM image, (b) XRD pattern, (c, d) polarization curves
obtained for the as‐rusted specimen.
FIGURE 2. SEM images of the specimen surfaces subjected to the cyclic
corrosion test of 3, 9, 18 and 30 cycles; (a) reference, (b) Mg2+, (c) Al3+,(d) Cu2+, (e) Ni2+ specimens.
FIGURE 3. XRD quantitative analysis obtained from the specimens after
the cyclic corrosion test: (a) reference, (b) Mg2+, (c) Al3+, (d) Cu2+, (e)Ni2+ specimens.
FIGURE 4. (a) Corrosion potential, Ecorr, and (b) corrosion current
density, Icorr, obtained from all specimens subjected to the cyclic
corrosion test. Those obtained for the as‐rusted specimen are included
for comparison.
FIGURE 5. Specific cathodic current density obtained at 500 mV lessthan the open circuit potential of all specimens subjected to the cyclic
corrosion test. The cathodic currnet density of 0.92 mA cm‐2 obtained
for the as‐rusted specimen is included as a reference.
FIGURE 6. Relationship between Ecorr and Icorr obtained after definedcycles of the cyclic corrosion test. The data for the as‐rusted specimen
is presented as a reference.
FIGURE 7. Relationship between corrosion potential, Ecorr, and the ratioof total mass fraction of α‐FeOOH, β‐FeOOH and γ‐FeOOH to that of
Fe3O4 obtained from the specimens after 30 cycles of the cyclic
corrosion test.
FIGURE 8. Relationship between corrosion current density, Icorr, andα/γ*.
FIGURE 9. Relationship between corrosion current density, Icorr, andcathodic current density, Icorr.

FIGURE 1

FIGURE 2

FIGURE 3

FIGURE 3 (continued)

FIGURE 4

FIGURE 5

FIGURE 6

FIGURE 6 (continued)

FIGURE 7

FIGURE 8

FIGURE 9