Electrochemical behaviour of iron and copper
in a culture solution for Spirulina platensis
R. MALGOR, G. HEIJO, L. ROMERO, C. F. ZINOLA*
Electrochemistry and Corrosion Department, School of Science and Engineering, Libertad Str., 2497,
C.P. 11300, Montevideo, Uruguay
Received 11 April 1997; accepted in revised form 26 May 1998
Cyclic voltammograms of iron and copper electrodes were run in sodium hydroxide, carbonate±
bicarbonate buer and culture media for Spirulina platensis at 30 °C. Potentiostatic steady state
polarisation curves for both electrode surfaces in these electrolytes were performed in the presence
and the absence of S. platensis at ®xed temperature. Corrosion potential and corrosion current
density values of iron and copper were obtained graphically from these curves. In all cases, the largest
corrosion current density corresponded to the maximum biogenerated-oxygen concentrations, that
is, illuminated culture media containing S. platensis. Corrosion potentials of iron electrodes shifted to
positive values for increasing corrosion rates, whereas constant corrosion potentials were obtained
for copper electrodes independently of the electrolyte.
Keywords: copper, corrosion, cyclic voltammetry, iron, Spirulina platensis
1. Introduction
Spirulina platensis is a cyanophyte microalgae, that
grows in natural or alkaline mineralized waters at
temperatures higher than 30 °C [1±3]. The presence of
the microalgae in solution increases the oxygen par-
tial pressure at the interface as a consequence of the
oxygen biogeneration, particularly after a long ex-
posure to light [4]. Thus, oxygen biogeneration can
produce a threefold increase in the solution oxygen
level vis a
Â
vis that expected from atmospheric equi-
librium [5]. Cultivation of S. platensis requires tem-
peratures higher than 30 °C (optimum growth) [6],
which are generally achieved in baths thermally sta-
bilised by copper or iron calefactors in direct contact
with the culture solution. The presence of microor-
ganisms and their metabolic products may induce the
metal corrosion. Moreover, the kinetics of the mi-
crobial corrosion is aected by changes in tempera-
ture and in the solution composition (metabolic
products and oxidising agents) [7, 8].
These facts encouraged us to study the electro-
chemical behaviour of iron and copper in alkaline
and bioactive microalgae-containing solutions, using
cyclic voltammograms, polarization curves and the
time evolution of the rest potentials. From electro-
chemical kinetic data, corrosion potentials and cor-
rosion current densities were determined.
2. Experimental details
Cyclic voltammograms were run with iron (99.8%
purity) and copper (99.98% purity) wire electrodes of
1 mm diameter, and with commercially available
metal plates ($5cm
2
); that is, iron (96% purity)
containing nickel (1.5%), chromium (1.4%), copper
(0.3%), molybdenum (0.1%), carbon (0.6%) and
silicon (0.2%); and copper (95% purity) plates con-
taining nickel (1.9%), aluminium (1.2%), zinc
(0.5%), magnesium (0.2%) and tin (0.4%). Polar-
ization curves were performed with both high-purity
and commercially pure metal probes. In both cases,
the electrochemical system was completed with a
platinum plate (10 cm
2
geometric area) counter elec-
trode and a silver/silver chloride (4
M
potassium
chloride) reference electrode. All potentials in the text
are referred to the normal hydrogen electrode (NHE).
The working solutions, 0.1
M
NaOH, carbonate±
bicarbonate buer (pH 9.9) and culture medium
(SOT), were prepared from Millipore-MilliQ* water
and analytical grade chemicals. One litre of the cul-
ture medium contains: 16.8 g NaHCO
3
, 0.5 g
K
2
HPO
4
, 2.5 g NaNO
3
, 1.0 g K
2
SO
4
, 1.0 g NaCl, 0.2 g
MgSO
4
.7 H
2
O, 0.04 g CaCl
2
.2 H
2
O, 0.01 g FeSO
4
.7 -
H
2
O, 0.08 g EDTA and 1.0 ml of a micronutrient-
containing solution (A). One litre of solution A
contains: 2.86 g H
3
BO
3
, 1.81 g MnCl
2
.4 H
2
O, 0.22 g
ZnSO
4
.7 H
2
O, 0.04 g NaMoO
4
.2 H
2
O, 0.08 g
CuSO
4
.5 H
2
O, 0.05 g Co(NO
3
)
2
.6 H
2
O. The pH of a
freshly prepared SOT solution is 9.7.
The electrochemical runs were performed with a
LYP M7 potentiostat±galvanostat, a LYP WT func-
tion generator and an X±Y±t Linseis recorder.
2.1. Cyclic voltammograms
Voltammetric experiments were performed in a con-
ventional electrochemical cell with the reference
* To whom correspondence should be addressed.
JOURNAL OF APPLIED ELECTROCHEMISTRY 28 (1998) 1351±1357
0021-891X
Ó
1998 Chapman & Hall
1351