Volume 7 Preprint 12
Effect of Dunaliella Salina on the corrosion behavior of technical titanium in saline solutions
M. M. Hefny, A.A.Dardir and A. Abdel-Tawab
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Volume 7 Preprint 12
Effect of Dunaliella Salina on the corrosion behavior
of technical titanium in saline solutions
M. M. Hefny
Chemistry Department, Faculty of Science, Cairo University
Giza – Egypt
A.A.Dardir and A. Abdel-Tawab
The Egyptian Salts and Minerals Company (Emisal)
Shakshouk – Fayoum – Egypt.
The corrosion behavior of 99.6 % titanium in aqueous high concentrated
salt solutions containing 1.0x105 cell /ml , 35 g of Na2SO4 , 35 g MgSO4 and
130 g of NaCl per liter of solution ( pH 7.9 ) was investigated at 25 oC using
standard analytical, biological and electrochemical techniques.
It has been
found that the algae Dunaliella salina inhibits the corrosion process of the
tested sample as inferred from the decrease of corrosion current with increase
of the number of cells in solution. The mechanism of inhibition seems to be
preferential adsorption on the cathodic sites of the surface oxide film. The
active component ( the adsorbate ) seems to be ß-carotene; as it is being easily
released from the cell by the external tension ( the electrode surface ). Further
passivation of the surface by the oxygen gas released from the biological
activity of the system , metabolism of Dunaliella salina could not excluded.
This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science
and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at
http://www.umist.ac.uk/corrosion/jcse in due course. Until such time as it has been fully published it
should not normally be referenced in published work. © UMIST 2004.
The present study deals with the corrosion behavior of titanium in
presence of Dunaliella salina has been raised due to its use as a
construction material in salt and ß-carotene production from Quaron lake
water ( El-Fayoum government , Egypt ). The presence of
microorganisms and their metabolic activity influence the corrosion
behavior of metals by different scenarios. Microbiological investigation
of this natural water reveals the presence of a mono culture; the algae
Dunaliella salina. Almost, little is known about the effect of algae on
titanium. It has been reported that it is not affected by bacteria .
The corrosion behavior of titanium and its alloys had been studied
It has been found that these materials have superior
corrosion resistance owing to the presence of a passivating oxide film  .
Furthermore, this class of surface oxide films are more or less defective
. The poor adherence of the film is proposed to be a source of its
instability. The adherence of the film as well as its other features depend
on its formation conditions, composition of the medium especially the
presence of an adsorbate such as a natural biomass.
On the other hand,
crevice corrosion of titanium can occur at the free corrosion potential  .
It is a localized
form of corrosion. It occurs when metallic parts are in
close contact with non-metallic materials in the corrosive solution.
However almost, there is no detailed studies on the corrosion behavior of
titanium and its alloys in the presence of biomasses.
The tested algae is Dunaliella salina. It is unicellular and belongs
to the class Chlorophyceae and the order Volvocales
It is found
naturally in many aquatic marine habits containing more than 10% salt,
e.g., concentrated Quaron lake water.
This lake is hypersaline, the
Dunaliella strain which predominates in this lake is Dunaliella Salina. It
is ovoid, motile and halotolerant via an osmoregulation mechanism. It is
of a commercial interest because it accumulates massive amounts of the
precise product ß-carotene. Lacking a cell wall but a mucus surface coat,
Dunnaleilla Salina responds easily to external forces by different
mechanisms' mainly by secreting ß-carotene and glycerol.
only in direct sunlight. The present investigation was carried out in door,
unless otherwise stated.
The test specimens were of commercially pure titanium ( 99.6 % ) ,
used in the form of rods with diameter 5.3 mm and length 15 mm. Only
the cross sectional area was exposed to the test solution. A stout copper
wire was fixed mechanically at the other end.
The electrode was then
fitted into glass tubing of appropriate internal diameter; thin layer of
resin fixed the whole electrode length inside the tubing.
experiment the exposed electrode surface was mechanically polished
with fine emery paper till a bright smooth silver mirror appearance has
been obtained. Then rinsed with distilled water, after that dried with a
fine filter paper and immediately immersed in the cell filled with a fresh
portion of the test solution.
The cell is a conventional polarization
electrochemical one. The test solution was naturally aerated (pH~7.9 ).
It was prepared as shown below from analytical grade chemicals free or
containing occlusions of the cultured algae Dunalialla Salina at 250C.
Saturated calomel electrode and platinum wire have been used as the
reference and auxiliary electrodes , respectively. All the potentials given
here are relative to this reference and all potentiadynamic polarization
graphs are run at scan rate 3 mVs-1. A PAR potentiostat model 273/81
was used. It was electronically controlled by EG&G soft ware 252/352
This study was undertaken with pure culture of the strain Dunaliella
salina. The media was prepared by the sequential addition of 130 g of
NaCl, 35 g of Na2SO4 and 35 g of MgSO4 to less than 1 L of distilled
It was then boiled, after that cooled to room temperature. The
minor components; urea, KH2PO4, Fe-EDTA and NaHCO3 nutrient
material were added to give the recommended final concentrations;
respectively, 0.1, 0.2, 0.002 and 2.0 mM. At last, the solution was
completed to 1 L.
This solution is the medium.
0.25 L of Dunaliella
Salina crop with cell number 1.5x 105 / mL was added to 1 L of the
medium in a tank. The crop was supplied from SRI, China.
was kept under net shade where sun light intensity is 40000 to 50000
Lux*. Every morning it was moved to a cool place when air temperature
exceeds 35 oC. In the afternoon, when the temperature reaches 35 oC, it
was moved to a cool place exposed to sunlight. It was shaken once
every hour during the daytime. This culture was diluted with distilled
water to keep the same percentage of salts . The growth rate and ßcarotene content were measured every day . The results are given in
*Lux :Chinese unit for sun light intensity ,where 1Lux = 250 Watt.
1x106 Cell No./ml
β-Carotene content mg/l
Time / day
g.(1) Changes in cell No. and carotene with growth time in medium
Cell No.:106 cells/ml, carotene: mg/L
Results and Discussion
The corrosion behavior of the mechanically polished titanium
electrode surface was followed by measuring its open circuit potential in
free salt solution or containing different occlusions of the algae Dunaliella
Salina, figure 2. 900 seconds was found to be a sufficient time to stabilize
the open circuit potential and obtaining reproducible and meaningful
results, especially we start immediately after that the polarization from the
O.C. potential /m V
free test solution
test solution contian 5x104 cell / ml
test solution contian 10x104 cell / ml
Time / S
Fig .(2) change of the Open circuit (O.C.) potential of
th electrode with immersin to with and without algea
Typical potentiodynamic curves were obtained in the test solution.
They are representatively shown in figure 3. As it is difficult to define a
sufficient linear Tafel region in the anodic arm of the observed
polarization plots, the value of the corrosion current density was
E / mV
obtained by using an interpolation method .
Fig .3 Poteniodynamic polarization curves at scan
rate of 3 mV s-1 : 1- Free solution and 2- in presence
of algae ,1x105 cell/ml)
Log (I / A/cm
The open circuit potential shift with time towards the electropositive
direction indicating further growth of the pre immersion film , Fig. 2 The
more or less steady state value becomes higher in the presence of algae,
Fig.4, possibly due to the formation of an adsorbed
O.C. potential / mV
l x 104 Cell No. / ml
Fig 4.Increase the more or less steady state oprn ciruit potential (the
value measure after 900 S)with Cell number.
biofilm on the electrode surface. The formation of the biofilm has been
confirmed by stripping and analyzing the surface layer.
experiments slicing microbiological analysis of the part of the solution
adjacent to the electrode revealed the accumulation of the algae at the
electrode surface. The concentration of the algae was found to be more
than order of magnitude higher than that in the bulk of the test solution.
This value has been obtained by analyzing the surface biofilm layer with
respect to chlorophyll a,b and c with extract 90 % acetone
spectrophotometer) the result are given table 1.
On the other hand,
spectroscopic investigation of this extract reveal the presence of the major
secreted substance from Dunaliella Salina, namely ß- carotene. Its surface
concentration are also given in table 1.
Table 1. Results of measurement of the chlorophyll a,b and c in the
biofilm formed on Ti in solution containing in different cell .
ß- carotene, mg/l
The positive shift in the open circuit potential can be attributed to
a catalytic action of the biofilm matter with regard to oxygen reduction ,
hence enhancement of passivation of titanium.
passivation of titanium has been confirmed from the decrease of the
corrosion current, Fig.5.
1x10 Cell number /m l
Fig 5. Decrease of the corrosion current i corr. with cell
The mechanism of action of Dunaliella salina, viz., via biofilm formation
has the general feature of adsorption processes, i.e., upon saturation of the
surface (complete coverage) in the presence of 1.0 x105 cell / ml, the
effect of the algae concentration becomes negligible.
This trend is
evident from the change of each of the open circuit potential ( Fig. 4 ) and
the corrosion current ( Fig. 5 ). It is plausible that the absorption of
Dunaliella salina is preferential on defective cathodic sites of the oxide
film, as far as the cathodic branch of the
polarization curve was influenced more than the anodic one, figure 3.
To elucidate the mechanism of action of Dunaliella salina,
differently treated titanium surface has been tested. Modification of
the titanium surface has been achieved via anodization of titanium till
different potentials, to control the surface state, since the surface
properties of the oxide depends on its thickness . The latter
parameter has been defined by the anodization potential.
been found that the steady state open circuit potential in more positive in
presence of the algae for the differently treated surfaces , Fig 6 .
blank test solution
1x105 cell / ml of the Algea
Es / mV
Polarization Potential / V
Fig 6. Increase of the more or less steady state potential ,Es potential
of differently treates titanium surface with anodized potential
Further more the decrease in the corrosion current is higher for the
anodized surface as compared with the mechanically polished one, Fig 7.
Icorr µ A / cm2
1x105 cell / ml of the algae
Anodization potential / V
Fig 7. Decrease of corrosion current with anodization potential.
Therefore, the role of Dunaliella salina is likely to be repairing of the
defective oxide film by the oxygen which is secreted as a result of its
metabolic processes. On the other hand, change in the susceptibility of
the oxide towards chemical dissolution could be ignored, this because
Dunaliella salina does not change the pH of the medium by absorbing
CO2 , due to the auto buffer action of the medium.
individual secreted substances; glycerol and ß-carotene had been studied
using the same techniques.
The results show that glycerol have no
influence on the corrosion behavior of titanium, whereas ß- carotene
(Fig. 8 ) has almost the same effect as the tested Dunaliella Salina.
Icorr.,µ A / cm2
algae comparison with
Fig 8.change of corrosion current with the ß-carotene content
, the result for the algae are given for comparison
The algae Dunaliella salina was proved to inhibit the corrosion of
technical titanium (99.6 %) in nearly neutral highly saline solutions
via sticking on the electrode surface and subsequently secreting ßcarotene. The adsorption of these materials seems to be preferential
on the cathodic sites of the surface oxide film.
On the other hand , released oxygen from the metabolic activity of
these micro organisms enhances sealing of the defects with in the
surface oxide film.
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