Materials Chemistry and Physics 114 (2009) 962–972
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Study of passive films formed on mild steel in alkaline media
by the application of anodic potentials
L. Freire
a,∗
, X.R. Nóvoa
a
, M.F. Montemor
b
, M.J. Carmezim
b,c
a
Universidade de Vigo, E.T.S.E.I., Campus Universitario, 36310 Vigo, Spain
b
ICEMS – Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, 1049 – 001 Lisboa, Portugal
c
EST Setúbal, DEM, Instituto Politécnico de Setúbal, Campus IPS, 2910 Setúbal, Portugal
article info
Article history:
Received 4 May 2008
Received in revised form 13 October 2008
Accepted 3 November 2008
Keywords:
Mild steel
Passivity
Cathodic protection
Alkaline media
abstract
In this paper, iron oxide thin layers formed on mild steel substrates in alkaline media by the application
of different anodic potentials were studied in order to characterize their morphology, composition and
electrochemical behaviour, in particular under conditions of cathodic protection.
The surface composition was evaluated by X-Ray Photoelectron Spectroscopy (XPS) and Auger Elec-
tron Spectroscopy (AES). The morphology of the surface oxides was studied via Atomic Force Microscopy
(AFM). The electrochemical behaviour of the surface oxides was studied using Electrochemical Impedance
Spectroscopy (EIS).
The results showed that the surface film is composed by Fe
2+
oxides and Fe
3+
oxides and/or hydroxides.
The contribution of Fe
2+
species vanishes when the potential of film formation increases in the passive
domain. Two distinct phases were differentiated in the outer layers of the surface film, which proves that
film growing is topotactic in nature.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Steel passivation in alkaline environments is due to the forma-
tion of a very thin, but highly protective oxide/hydroxide layer. This
layer plays an important role, for example in corrosion protection
of reinforcing steel in concrete. Furthermore, the composition and
morphological changes of this layer can affect the electrochemical
behaviour of reinforcing steel under cathodic protection.
During cathodic protection operation a cathodic current is
applied, usually to pre-oxidized structures and the composition
and morphology of these pre-existing oxides may influence the cur-
rent distribution and, consequently the efficiency of the cathodic
protection process. Therefore, the characterization of the oxides
formed on steel under alkaline conditions and the understand-
ing their electrochemical behaviour are an important contribution
for the definition of the relevant operating parameters regarding
the efficiency of cathodic protection systems applied to reinforcing
steel.
Corrosion of steel structures at ambient temperature in neutral
to alkaline media results in the formation of iron oxo-hydroxides
like goethite (-FeOOH), lepidocrocite (-FeOOH) and akagonite
∗
Corresponding author.
E-mail address: lorenafp@uvigo.es (L. Freire).
(-FeOOH) below which more protective forms of iron oxides
including magnetite (Fe
3
O
4
), maghemite (-Fe
2
O
3
) and hematite
(-Fe
2
O
3
) are present [1,2]. Depending on the external condi-
tions, the outer iron oxide layer has variable stoichiometry and
atomic density, due to the presence of different chemical species. In
highly alkaline media, Fe
3
O
4
oxide forms at low electrode poten-
tials (−0.8 V) and Fe
2
O
3
oxide, at more positive potential values
[3]. Moreover, hydrated species are present all over the potential
domain, but the passive film tends to dehydration as the potential
increases [4]. This fact can explain why steel in concrete becomes
more resistant to chlorides when the potential in the passive range
decreases [5]. As the potential becomes more negative, the hydra-
tion degree increases, reducing the local [Cl
−
]/[OH
−
] ratio and
consequently pitting potential becomes higher than that corre-
sponding to the nominal Cl
−
content in solution.
Magnetite is of particular importance for the understanding of
the electrochemical behaviour of steel in concrete because it plays
an essential role in the evolution of the passive layer [6]. Although
the structure and composition of passive films formed on steel sub-
strates is still a controversial subject, it has been proved that, in
alkaline media, the film corresponds basically to a double-layer
model consisting of an inner magnetite and an outer ferric oxide
according to a Fe
3
O
4
/Fe
3+
structure [7–12]. The most internal layer
is composed of Fe
2+
oxides in contact with the substrate. The ther-
modynamic instability of both Fe
2+
oxides and magnetite in the
presence of oxygen leads to the formation of an outer layer of Fe
3+
0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2008.11.012