Volume 18 Preprint 35
Appraisal of Sacrificial Anode Malperformance Due to Polarity Reversal of Galvanic Coupling between Mild Steel and Zinc in Hot Sodium Bicarbonate (NaHCO3) Solution
Syed Asad Ali Zaidi, Azhar Mahmood and Ashraf Ali
Keywords: Corrosion, polarity reversal, potentiodynamic polarization
Electrochemical behavior of mild steel and zinc coupling has been examined in 0.01M sodium bicarbonate (NaHCO3) solution at 65oC. Passivation occurred upon mild steel and zinc together in this condition. Their potentiodynamic graphs have revealed that mild steel passivated more slowly rather than zinc. This furnished the surveillance of a transitory condition of polarity reversal in the mild steel-zinc galvanic couple. In order to prove these results, a set of galvanically coupled mild steel and zinc electrodes have been placed in 0.01 M sodium bicarbonate solution to analyze their behavior at room temperature (25oC) and at higher temperature (65oC).
A temporary polarity reversal phenomenon has been evidenced in 0.01 M sodium bicarbonate solution at higher temperature where zinc function shifted to cathode because of the formation of thick layer of corrosion products making it passive much earlier rather than mild steel which protect zinc from further corrosion whereas mild steel altered to anodic behavior hence undergo corrosive attack. These results were helpful to understand malperformance of sacrificial zinc anode in sub tropical/tropical marine environment having ample amount of bicarbonate ions in warm waters.
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Appraisal of Sacrificial Anode Malperformance Due to Polarity
Reversal of Galvanic Coupling between Mild Steel and Zinc in
Hot Sodium Bicarbonate (NaHCO3) Solution
Syed Asad Ali Zaidi1, Azhar Mahmood*1, Ashraf Ali2
National University of Sciences & Technology, Pakistan. email@example.com
2Department of Materials Engineering, NED University of Engineering & Technology,
Electrochemical behavior of mild steel and zinc coupling has been examined in 0.01M
sodium bicarbonate (NaHCO3) solution at 65oC. Passivation occurred upon mild steel and
zinc together in this condition. Their potentiodynamic graphs have revealed that mild steel
passivated more slowly rather than zinc. This furnished the surveillance of a transitory
condition of polarity reversal in the mild steel-zinc galvanic couple. In order to prove these
results, a set of galvanically coupled mild steel and zinc electrodes have been placed in
0.01 M sodium bicarbonate solution to analyze their behavior at room temperature (25oC)
and at higher temperature (65oC).
A temporary polarity reversal phenomenon has been evidenced in 0.01 M sodium
bicarbonate solution at higher temperature where zinc function shifted to cathode because
of the formation of thick layer of corrosion products making it passive much earlier rather
than mild steel which protect zinc from further corrosion whereas mild steel altered to
anodic behavior hence undergo corrosive attack. These results were helpful to understand
malperformance of sacrificial zinc anode in sub tropical/tropical marine environment
having ample amount of bicarbonate ions in warm waters.
Keywords: Corrosion, polarity reversal, potentiodynamic polarization.
Mild steel is a chief fabrication material for the majority of engineering structures owning to
its useful specific properties including good malleability, ductilability, weldability, medium
tensile strength, and its easier to cold-form so making it facile to handle during structural
fabrications [1, 2]. But on other hand it is more vulnerable to corrosion due to its poor
passivation as its oxide layer formed by corrosion does not have good adhesion to under
laying metal and peel off readily causing exposure of fresh metal surface hence corrosion
continue without obstruction. Extensive corrosion ultimately results structural weakness
and disintegration of the metal [3-4]. Therefore, various techniques like impressed current
cathodic protection methods, sacrificial anode, galvanized coatings and barrier film
coatings etc. are in used to protect mild steel structure against corrosion. Marine mild steel
structures are usually protected by sacrificial anode techniques, involving integration of
zinc auxiliary anode . Under standard ambient conditions, zinc has lower reduction
potential (-763 mV vs. SHE) than mild steel (-440 mV vs. SHE) therefore zinc function as
anode versus iron thus protecting mild steel structures. However, zinc potential may vary
depending alloy composition, presence of anions in environment and water pressure.
Therefore, polarization of the mild steel-zinc couple could be inverted in an aerated
electrolyte above 60oC temperature, with the zinc becoming cathodic to the mild steel. This
phenomenon called polarity reversal, which resulted from the ennoblement of zinc [6, 7]. A
thin electrically non-conductive zinc oxide layer caused cathodic depolarization while
anodic spot at gapes in this layer may be clogged by the deposition of zinc carbonate
precipitates which induced passivity on zinc anode making it redundant. Therefore zinc will
no longer cathodically protect the mild steel due to polarity reversal. This is also reported in
literature that polarity reversal may not only lack preservation but also promote the
corrosion of a structure above that of an unprotected structure [8, 9]. Marine environment
provides plenty of moisture for ionic conduction hence promote electrochemical corrosion.
Marine water has various corrosive salt species like Cl-, HCO3- etc. which promotes polarity
reversal thus reduced anode efficiency which restrict the choice of zinc anodes to ambient
temperature marine applications [10, 11]. The aim of subject study was to evaluate the
malfunctioning of zinc sacrificial anode due to polarity reversal phenomenon of zinc and
mild steel coupling at higher temperature (at 65oC).
Electrode Preparation for Potentiodynamic Polarization Measurements
HR-235 Carbon steel was opted for working electrode fabrication. Its composition was
ascertained by using X-ray Fluorescence Spectroscopy (INNNOVEX SYSTEMS) and was found
to be 0.07 % Cu, 0.36 % Mn and 99 % Fe. The composition of zinc electrode material was
established by the same procedure and found to be 99.97 % Zn. The electrodes of surface
area 1×1 cm2, were prepared by mounting the metal in epoxy and drying the samples for
one day. These samples were then grinded using series of grinding papers (180, 220, 320,
and 400) on MoPao 260 E Grinder machine. Subsequently samples were polished using
BENETEC polishing machine to remove all kinds of grinding marks and were then rinsed
with water (Figure 1).
Figure 1: Mild steel (outer side) and Zinc (center) electrodes.
An electrochemical cell was prepared by customizing Gamry Instruments MultiPort™ Cell Kit
while employing above fabricated electrodes. Fresh 0.01 M sodium bicarbonate (NaHCO3)
solution was prepared in water and served as an electrolyte. Experiments were designed to
carry out potentiodynamic polarization scans of subject electrochemical cell by using a
Gamry G 750 Potentiostat. The potentiodynamic graphs were recorded at 5 mV/min scan
rate. Electrochemical cell was covered with a heating mantle connected to Omega Bench top
Controller CSi8D series to maintained temperature of the cell at 65 oC ± 0.5 (Figure 2).
Figure 2: Experimental setup of electrochemical apparatus
Coupling of Zinc and Mild Steel Electrodes
Mild steel and zinc electrodes were electrically connected and allowed to corrode freely in
freshly prepared 0.01 M Sodium Bicarbonate (NaHCO3) solution. Electrochemical behavior of
mild steel and zinc coupling has been analyzed at room temperature (25 oC) and at higher
temperature (65oC). Subsequently extent of corrosion at these working electrodes was
ascertained by taking stereo micrographs of corroded surfaces of electrode.
Results and Discussion
Above mentioned experiment has furnished potentiodynamic scanning curve for mild steel
and zinc electrodes at 25oC and at 65oC in 0.01 M sodium bicarbonate (NaHCO3) solution.
These results were analyzed to conclude their passivation behavior. Moreover, stereo
micrographs of mild steel and zinc electrodes after coupling have been proved helpful to
establish polarity reversal behavior during coupling.
Corrosion Behavior of Mild Steel and Zinc Coupling at Room Temperature (25 oC)
The corrosion behavior of mild steel and zinc couple has been analyzed at room
temperature (25 oC) in 0.01 M sodium bicarbonate (NaHCO3) solution. The electrodes were
connected and allowed to corrode freely in the solution. After 02 hours, electrodes were
retrieved and their surfaces were scanned via stereo microscope.
Figure 3: Stereo micrograph of (a) mild steel and (b) zinc after galvanic coupling at room
temperature (25 oC).
Comparison of these stereographs have confirmed active galvanic coupling between mild
unreacted/protected surface which intended cathodic polarity on mild steel during coupling
(Figure 3a). However, stereo micrograph of zinc electrode has shown corroded/reacted
topography which established its role as sacrificial anode during subject coupling (Figure
3b). These observations have complete coincidence with sacrificial anode behavior of zinc
electrode via galvanic coupling while have contradiction with polarity reversal phenomena.
It is therefore, concluded that mild steel and zinc coupling at room temperature (25 oC) in
0.01 M Sodium Bicarbonate solution have active sacrificial anode coupling behavior while its
lack any polarity reversal phenomenon.
Corrosion Behavior of Mild Steel and Zinc Coupling at Higher Temperature (65 oC)
The corrosion behavior of mild steel and zinc coupling has been analyzed at 65oC
temperature in 0.01 M sodium bicarbonate (NaHCO3) solution. These electrodes were
connected and allowed to corrode freely under above mentioned conditions. After 02 hours,
electrodes were retrieved and their surfaces were scanned via stereo microscope.
Figure 4: Stereo micrograph of (a) mild steel and (b) zinc after galvanic coupling at higher
temperature (65 oC).
Comparison of these stereo micrographs has confirmed absence of sacrificial zinc anode
galvanic coupling between mild steel and zinc electrodes in these conditions. Micrograph of
mild steel electrode showed corroded faces which evident that mild steel has no more
protected by cathodic polarity, induced formerly by sacrificial zinc anode (Figure 4a). On
other hand, Stereo-image of zinc electrode has displayed development of an intact passive
layer on surface which inhibited anodic polarity of zinc by its passivation (Figure 4b).
Contradict to this, whole nature of polarity and intended role of mild steel and zinc
electrodes in previous sacrificial zinc anode galvanic coupling has been inverted. Now
passivated zinc has been behaved as cathode and been protected while mild steel has been
behaved as anode and been corroded. This phenomenon is called polarity reversal. This has
indicated that under subject conditions, zinc is passivated much faster than mild steel
which later on confirmed by comparison of Potentiodynamic curves of zinc and mild steel.
These curves have pointed out that zinc passivation occurs in short time interval as
compared to mild steel.
Potentiodynamic Curve for Mild Steel and Zinc at 65 oC
The potentiodynamic curve has been plotted for mild steel and zinc in 0.01 M sodium
bicarbonate (NaHCO3) solution at 65oC by Computerized Potentiostat at a scan rate of
Figure 5 has exhibited the cathodic and anodic potentiodynamic curves for mild steel
electrode. In cathodic branch mild steel showed a sluggish initial rise in potential with
decrease of current density caused by slowing down reduction reaction at mild steel
cathode. Subsequently it stabilized at potential closer to -640 mV due to its passivation. A
patchy corrosive attack was noticed on the mild steel after retrieving from solution. A thick
translucent corrosion product film was found on some areas of mild steel surface.
Figure 5: Potentiodynamic curve for mild steel at 65oC
Figure 6 has depicted the cathodic and anodic potentiodynamic curves for zinc electrode. In
cathodic branch zinc showed a rapid initial rise in potential with decrease of current density
i.e. with deceleration of reduction reaction at zinc cathode. Subsequently it stabilized at
potential closer to -960 mV due to zinc passivation. This is further confirmed by presence
of a very thin, transparent / white passivation product film found on zinc electrode after
retrieval from the solution.
Figure 6: Potentiodynamic curve for zinc at 65oC
Figure 7: Comparison of potentiodynamic curves for zinc and mild steel
Abovementioned potentiodynamic curves for mild steel and zinc has been compared in
Figure 7. Comparison of both potentiodynamic curves has revealed that although like zinc,
mild steel also passivated after a adequate exposure to subject conditions but zinc
corrosion reaction was more polarizable than the steel corrosion reaction as a result zinc
passivated earlier causing reversal of polarity.
A transitory phenomenon of polarity reversal has been evident in the zinc-mild steel
galvanic coupling when exposed to the hot sodium bicarbonate (NaHCO3) solution. This is
because of relatively more rapid passivation of zinc as compared to mild steel in these
conditions as exhibited in their potentiodynamic graphs.
Marine water has ample amount of bicarbonate ions therefore these conditions for polarity
reversal are frequently achieved in sub-tropical/tropical marine environment causing
malperformance of sacrificial zinc anode. Major disadvantage resulted from polarity
reversal is the malfunctioning of zinc so did not cathodically protect the steel structures.
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