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POTENTIODYNAMIC POLARIZATION AND MATHEMATICAL MODELLING
STUDY OF CORROSION RESISTANCE PROPERTIES OF ZINC GALVANIZED
ROOFING SHEET IN 0.5M HCL
R. E. ELEWA1, S.A. AFOLALU2, O.S.I. FAYOMI3 & O. AGBOOLA4
1,3Department of Mechanical Engineering, Covenant University, Nigeria
2Department of Chemical, Metallurgical and Materials Engineering,
Tshwane University of Technology, South Africa
4Department of Chemical Engineering, Covenant University, Nigeria
ABSTRACT
This paper investigate the corrosion resistance properties of zinc galvanised roofing sheet selected from Nigeria
manufacturing industries in an attempt to predict its durability, efficiency and corrosion properties in 0.5 M HCl. The
electrochemical behaviour assessment was done using potentiodynamic polarization and COMSOL multiphyics
prediction for 1, 6 and 12 month for all selected zinc galvanised steel under study. It was found that sample B provide
higher corrosion resistance propagation of 151.81(Ω) with significant Ecorr value of -1.4057V thus, with corrosion rate
of 2.452 mm/yrs. The least effective from produce zinc galvanised steel is sample E with 11.979(Ω)in the presence of 0.5
M HCl. A higher corrosion rate of 9.5462 mm/y was obtained for sample E as against 2.452 mm/y of sample B. This was
further confirmed by the predictive simulation and modelling degradation analysis.
KEYWORDS: Corrosion Assessment, Coating, Oxidation, Steel & Construction
Received: Mar 21, 2020; Accepted: Apr 11, 2020; Published: Jun 27, 2020; Paper Id.: IJMPERDJUN2020112
1. INTRODUCTION
Surface technology is a surface treatment process that is often used to prolong component life span in service
directly or indirectly from corrosion resulting from environmental factors [1-5]. This technology is essential
because it provide extensive mechanical, physical and chemical enhanced properties. Coating is thin layer coverage
on a surface for functional or decorative purposes [6]. Most coating used for metal surface finish provides surface
hardening behaviour, low coefficient of friction, better stable thermal influence among others [7-12]. Different
coating used in several services is basically because of application, cost, effectiveness and durability. Study has
shown that effective coating are provides corrosion and rust resistance, with less strain and stress bearing tendency
[13-15].
Coating for essential application that involves physical and chemical resistance properties are often known
with technology such as electrodeposition, electroless application, chemical vapour deposition, physical vapour
deposition, laser coating etc [16-20]. Galvanization is an essential technology to curtail the rapid corrosion activities
of steel exposed to atmospheric condition without protection. Thus, zinc is effective and significantly stable
resistance metallic materials against the redox chemical reaction resulting to corrosion product. The need to provide
cathodic protection of zinc to iron becomes essential [21-25].
Orig
ina
l Article
International Journal of Mechanical and Production
Engineering Research and Development (IJMPERD)
ISSN (P): 2249–6890; ISSN (E): 2249–8001
Vol. 10, Issue 3, Jun 2020, 1281-1300
© TJPRC Pvt. Ltd.
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Zinc galvanised coating is a unique and simple form of coating with application of zinc on steel and iron
component [26]. Applying zinc ion into the steel substrate is often done using hot dip galvanization [27-30]. However,
methods such as mechanical plating, a process which provide zinc powder and glass bead admix on the steel surface.
Sherardizing is also a good method of zinc deposition where diffusion between molecules of zinc and steel take place at a
heated temperature of 400oC. For wire production, continuous metal strip technology is used with a molten zinc of high
speed of 180 meter [31-34]. Other technology for zinc galvanized system is the zinc metal spray and zinc electroplating.
Recently, industries havefocused on sub-standards with low durable, low corrosion resistance product of zinc galvanised
roofing sheet supply in Nigeria. Thus, the need to investigate selected galvanised sheet roofing sheet and examine the
predictive yearly corrosion deterioration and failure that could arise in service.
2. EXPERIMENTAL PROCEDURE
2.1. Preparation of Galvanized Roofing Steel Sheet Samples used for Various Tests
The Galvanized roofing steel sheet samples used for this project are five (5) with the same corrugation number of eight (8)
and branded gauge of 0.15 mm. The roofing sheets were cut into test piece for electrochemical and mechanical test. The
samples used for the electrochemical test were of dimension (20x20) mm while those for the mechanical test had
corrugation number of two (2) and dimension (160x280) mm. The zinc coating mass or weight is carried out using a
galvanized steel sheet of (50x50) mm and 50 % HCl as the stripping or de-coating medium. Samples used for the bending
test have a width of 80 mm and length of 160 mm.
2.2 Electrochemical Testing of the Galvanized Roofing Steel Sheet Samples
The corrosion tests were carried out using 0.5MHCl as the test mediums in accordance to G3/G102 standard practice for
electrochemical measurement using the three electrode system with AUTOLAB potentiostrat and NOVA software install
in a computer system as shown in fig.2 The galvanized roofing sheet steels samples acted as the working electrode, the
graphite rod acted as the counter electrode while glass body calomel containing potassium chloride serves as the reference
electrode. The experiment was carried out between the voltage of -1.5 V and 1.5 V at a scan rate of 0.01 m/s and step of
0.0025. This experiment gave the prediction of the OCP (Open circuit potential), the corrosion rate in millimetres per year,
the corrosion current density, the corrosion potential and the polarization resistance of the test samples [9].
2.3 Mechanical Modelling and Simulation
The model geometry is shown in Fig. 1. One single electrolyte domain is used; the electrolyte used is Hydrogen Chloride
(HCl). The left part of the bottom boundary is the surface of the mild steel material; the right part is the corroding zinc
alloy, the width of the mild steel and the corroding zinc. The width of the mild steel and zinc alloy were also used as
thickness for the electrolyte to study the variation of corrosion rate with the thickness of the electrolyte. Because the alloy
will corrode in the model, the right boundary is displaced downwards in the geometry. A small step of height (width +
50mm) is introduced at origin; in the negative y direction is introduced in the geometry to ensure that the topology of the
geometry is preserved during the simulation. The vertical boundary of the step belongs to the steel surface.
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Figure 1: Geometry for the Model
The electrolyte was well mixed so that a secondary current distribution can be assumed, solving for the electrolyte
potential, ϕ1(V), in the domain. The electrolyte conductivity is set to 33.3s/m, 5 s/m and 132.4 s/m for Hydrogen chloride,
sodium chloride and hydrogen sulphide respectively. The equilibrium (corrosion) potential of mild steel were set to the
experimental values and −1.55 V (SCE) is set for the zinc alloy surfaces. This implies that the mild steel acts as cathode for
this galvanic couple, and a cathodic tafel expression is used to describe the kinetics of the reaction:
𝑖𝑐𝑎𝑡=𝑖0,𝑐𝑎𝑡 . 10𝜂𝐴𝑐𝑎𝑡⁄
(1)
Where𝑖0,𝑐𝑎𝑡 is the exchange current density (A/m2) which was assigned to respective roofing sheet samples A, B,
C, D and E based on experimental analysis as shown Table 3.
Acat= -160mV is the tafel slope. The over potential, η (V), of an electrode is generally defined as
𝜂 = 𝜙𝑠 −𝜙1 − 𝐸𝑒𝑞 (2)
whereϕsand ϕ1 are the potentials in the electrode (metal) and electrolyte, respectively, and Eeq is the equilibrium
potential. For the cathode, different values of equilibrium potentials (Eeq,cat) were assigned to different samples of roofing
sheet labelled A, B, C, D and E based on experimental studies.
The zinc alloy is here the anode of the galvanic couple, oxidizing magnesium according to
𝑍𝑛(𝑠) ⟶ 𝑍𝑛2+ + 2𝑒− (3)
To describe the measured polarization data for this reaction, diffusion limited anodic tafel expression for the
anodic electrode reaction current, ian (A/m2) was used.
𝑖𝑡𝑎𝑓𝑒𝑙 = 𝑖0,𝑎𝑛 . 10𝜂
𝐴𝑎𝑛 (3.5)
𝑖𝑎𝑛 =𝑖𝑙𝑖𝑚
1+𝑖𝑙𝑖𝑚𝑖𝑡𝑎𝑓𝑒𝑙
(4)
where i0,an = 10−1 A/m2, Aan = 50 mV, and ilim= 102 A/m2 is a limiting current. The equilibrium potential for this
reaction was set to −1.55 V.
This type of expression can be derived from the assumption of a Nernstian diffusion layer in combination with a
first-order dependence of a concentration on the kinetics. The dissolution of zinc metal causes the electrode boundary to
move, with a velocity in the normal direction, v (m/s), according to
𝑣 =𝑖
2𝐹
𝑀
𝜌 (5)
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Where M is the mean molar mass (65.38 g/mol) and ρ is the density (7.13 g/cm3) of the zinc alloy. The model was
solved using time-dependent study, corrosion was simulated during the first 1 month followed by 6 months and 12months
respectively after immersion in NaCl, HCl and H2SO4. The corrosion rate was calculated using the following relation
(Perez, 2004).
𝑉𝑐 =𝐼𝑐𝑜𝑟𝑟×𝑀
𝑧×𝐹×𝜌 (6)
where𝑉𝑐 is the corrosion rate (cm/yr), 𝐼corr is the corrosion current density (A/cm2), 𝑀 is the molar mass 𝑀(Zn) =
65.38 g/mol, 𝑧 is the valence of iron 𝑧 = 2, 𝐹 is the faraday constant 𝐹 = 96500A⋅s/mol, and 𝜌 is the density of steel 𝜌 =
7.87g/cm3. The value of Icorr was extracted from COMSOL which was imported into Excel spreadsheet for further
calculations and plotting of data. The data extracted was only for the zinc(anode) because the corrosion takes place at the
anode.
3. RESULTS AND DISCUSSIONS
3.1. Polarization of Galvanized Roofing Steel Sheet Samples in 0.5M HCl
The potential polarization data of the galvanized roofing steel sheets in 0.5 M HCl is presented in Table 1 and the curves
are represented in fig.2. Sample B exhibits the lowest corrosion current density of 1.7542 E-04 A/cm2 and highest
polarization resistance of 151.81 Ω indicating that exchange of current density was minimal compared to other samples.
More so, sample B was found to exhibit the lowest corrosion rate of 2.1452 mm/ year as represented in fig.3 which an
indication that this sample will withstand deterioration in such medium over time.
Table 1: Polarization Data of Galvanized Roofing Steel Sheets Samples in 0.5 HCl
Samples Ecorr (V) jcorr (A/cm2) CR (mm/year) PR (Ω)
A -1.1649 6.7731 E-04 7.8703 37.328
B -1.4057 1.7542 E-04 2.1452 151.81
C -1.1051 7.5121 E-04 8.729 71.136
D -0.4233 4.3822 E-04 5.0921 14.012
E -1.1864 8.2154 E-04 9.5462 11.979
Figure 2: Polarization curves of Galvanized Roofing Steel Sheets Samples in 0.5MHCl
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Figure 3: Polarization Data of Galvanized Roofing Steel Sheets Samples in 0.5MHCl
3.2. OCP Analysis of the Galvanized Roofing Steel Sheet Samples in 0.5 M HCl
Fig. 6 presents the OCP vs. Time graphs of the galvanized roofing steel sheet samples in 0.5 M HCl. Comparing fig.4 to
the polarization curves in figure 2 and Table 1, the potentials (OCP) of each of the samples were found to have shifted to
less negative potential (Ecorr) after the polarization experiment. The steady state potentials of the samples were between -
0.2 V and -1.19 V. The Ecorr values were less negative compared to the OCP values, indicating that the galvanized roofing
steel sheets samples were positively or anodically polarized in the HCl medium. The closeness of the curves to a straight
line shows that OCP was attained.
Figure 4: OCP Vs. Time of Galvanized Roofing Steel Sheets Samples in 0.5 HCl
3.3 Electrolyte Potential
Once the electrochemical parameters were defined, the geometry was applied, the boundary conditions and governing
equations were applied and the appropriate mesh was found for the geometry, galvanic corrosion was solved and applied to
the model.
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COMSOL was then used to develop several other models of galvanic corrosion systems. The resulting potential
distribution of Sample A in the Hydrogen Chloride electrolyte after one month is shown in Fig.5
Figure 5: Deformation of sample A after one month in HCl (hydrogen chloride) Electrolyte
Fig. 5 shows the current and potential distribution in the electrolyte and the changed geometry at the end of the
simulation after immersing sample A in the hydrochloric acid for duration of one month. Because the electrode currents are
highest at the contact point of the metals, the metal dissolution is highest at this point. The cathode (mild steel) and the
anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at this contact point which is
between distance 0 and 500mm when sample A is immersed in Hydrochloric acid for one month. The electrolyte potential
increases from 1.1457 to 1.5058V with the lowest values at the cathode (mild steel) and the highest values at the anode
(zinc). The intermediate electrolyte values fall within the contact of the two electrodes.
Fig.6 shows the electrolyte distribution for Sample A in Hydrochloric acid after six months, the cathode (mild
steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact point
between distance 0 and 1000mm when sample A is immersed in Hydrochloric acid for six months. The electrolyte
potential increases from 1.1445 to 1.5053V with the lowest values at the cathode (mid steel) and the highest values at the
anode (zinc).
Figure 6: Deformation of sample A after six months in HCl (hydrogen chloride) Electrolyte
Fig. 7 shows the electrolyte distribution for Sample A in Hydrochloric acid after twelve months, the cathode (mild
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steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact point
between distance 0 and 1000mm when sample A is immersed in Hydrochloric acid for twelve month, the deformation is
more pronounced when compared with Figs. 5 and 6 The electrolyte potential increases from 1.1361 to 1.665V with the
lowest values at the cathode (mid steel) and the highest values at the anode (zinc). Figures 5- 7 confirmed that the
deformation of sample A increases with time when immersed in hydrochloric acid electrolyte which is expected.
Figure 7: Deformation of sample A after twelve months in HCl (hydrogen chloride) Electrolyte
The resulting potential distribution of Sample B in the Hydrogen Chloride electrolyte after one month is shown in Fig.8
Figure 8: Deformation of sample B after one month in HCl (hydrogen chloride) Electrolyte
Fig. 8 shows the current and potential distribution in the electrolyte and the changed geometry at the end of the
simulation after immersing sample B in the hydrochloric acid for duration of one month. Because the electrode currents are
highest at the contact point of the metals, the metal dissolution is highest at this point. The cathode (mild steel) and the
anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at this contact point which is
between distance 0 and 500mm when sample B is immersed in Hydrochloric acid for one month. The electrolyte potential
increases from 1.1182 to 1.3412V with the lowest values at the cathode (mild steel) and the highest values at the anode
(zinc). The intermediate electrolyte values fall within the contact of the two electrodes [13].
Fig. 9 shows the electrolyte distribution for Sample B in Hydrochloric acid after six months, the cathode (mild
steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact point
between distance 0 and 1000mm when sample A is immersed in Hydrochloric acid for six months. The electrolyte
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potential increases from 1.1178 to 1.341V with the lowest values at the cathode (mid steel) and the highest values at the
anode (zinc).
Figure 9: Deformation of sample B after six months in HCl (hydrogen chloride) Electrolyte
Fig.10 shows the electrolyte distribution for Sample B in Hydrochloric acid after twelve months, the cathode
(mild steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact
point between distance 0 and 1000mm when sample B is immersed in Hydrochloric acid for twelve months, the
deformation is more pronounced when compared with Figs. 8 and 9. The electrolyte potential increases from 1.1174 to
1.3408V with the lowest values at the cathode (mid steel) and the highest values at the anode (zinc). Figures 8- 10
confirmed that the deformation of sample B increases with time when immersed in hydrochloric acid electrolyte which is
expected.
Figure 10: Deformation of sample B after twelve months in HCl (hydrogen chloride) Electrolyte
Fig.11 shows the electrode currents at the beginning and end of the simulation of Sample B immersed in
hydrochloric acid for duration of one month, as expected the highest current density (85A/m2) is found at the contact point
between the cathode and the anode. The current densities at the beginning and end of the simulation are very close. The
current density decreases towards the extreme end of the anode (zinc). Fig. 12shows the electrode currents at the beginning
and end of the simulation of Sample B immersed in hydrochloric acid for duration of six months, the changes in current
densities at the beginning and end of the simulation of Sample B for six months in hydrochloric acid is more pronounced
when compared with Fig. 12. Fig. 13 shows the electrode currents at the beginning and end of the simulation of Sample B
immersed in hydrochloric acid for duration of twelve months. The changes in the current densities at the beginning and end
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of the simulation of Sample B for twelve months in hydrochloric acid is more pronounced when compared with the
changes for one month (Fig. 11) and six months (Fig. 12), this depicts that the deviation in the current density increases
with time.
Figure 11: Electrode current densities at t = 0 and t = 1 month for sample B in hydrogen chloride electrolyte
Figure 12: Electrode current densities at t = 0 and t = 6 months for sample B in hydrogen chloride electrolyte
Figure 13: Electrode current densities at t = 0 and t = 12 months for sample B in hydrogen chloride electrolyte
The resulting potential distribution of Sample C in the Hydrogen Chloride electrolyte after one month is shown in Fig. 14
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Figure 14: Deformation of sample C after one month in HCl (hydrogen chloride) electrolyte
Fig. 14shows the current and potential distribution in the electrolyte and the changed geometry at the end of the
simulation after immersing sample C in the hydrochloric acid for duration of one month. Because the electrode currents are
highest at the contact point of the metals, the metal dissolution is highest at this point. The cathode (mild steel) and the
anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at this contact point which is
between distance 0 and 500mm when sample C is immersed in Hydrochloric acid for one month. The electrolyte potential
increases from 1.079 to 1.6638V with the lowest values at the cathode (mild steel) and the highest values at the anode
(zinc). The intermediate electrolyte values fall within the contact of the two electrodes.
Fig. 15 shows the electrolyte distribution for Sample C in Hydrochloric acid after six months, the cathode (mild
steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact point
between distance 0 and 1000mm when sample C is immersed in Hydrochloric acid for six months. The electrolyte
potential increases from 1.0772 to 1.6629V with the lowest values at the cathode (mid steel) and the highest values at the
anode (zinc).
Figure 15: Deformation of sample C after six months in HCl (hydrogen chloride) electrolyte
Fig. 16 shows the electrolyte distribution for Sample C in Hydrochloric acid after twelve months, the cathode
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(mild steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact
point between distance 0 and 1000mm when sample C is immersed in Hydrochloric acid for twelve months, the
deformation is more pronounced when compared with Figs 15 and 16. The electrolyte potential increases from 1.0755 to
1.662V with the lowest values at the cathode (mid steel) and the highest values at the anode (zinc). Fig. 14 - 16 confirmed
that the deformation of sample C increases with time when immersed in hydrochloric acid electrolyte which is expected.
Figure 16: Deformation of sample C after twelve months in HCl (hydrogen chloride) electrolyte
Fig. 17 shows the electrode currents at the beginning and end of the simulation of Sample C immersed in
hydrochloric acid for duration of one month, as expected the highest current density (85A/m2) is found at the contact point
between the cathode and the anode. The current densities at the beginning and end of the simulation are very close. The
current density decreases towards the extreme end of the anode (zinc). Fig. 18 shows the electrode currents at the
beginning and end of the simulation of Sample C immersed in hydrochloric acid for duration of six months, the changes in
current densities at the beginning and end of the simulation of Sample C for six months in hydrochloric acid is more
pronounced when compared with Fig. 17. Fig. 19 shows the electrode currents at the beginning and end of the simulation
of Sample C immersed in hydrochloric acid for duration of twelve months. The changes in the current densities at the
beginning and end of the simulation of Sample C for twelve months in hydrochloric acid is more pronounced when
compared with the changes for one month (Fig. 17) and six months (Fig. 18), this depicts that the deviation in the current
density increases with time.
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Figure 17: Electrode current densities at t = 0 and t = 1 month for sample C in hydrogen chloride electrolyte
Figure 18: Electrode current densities at t = 0 and t = 6 months for sample C in hydrogen
Figure 19: Electrode current densities at t = 0 and t = 12 months for sample C in hydrogen chloride electrolyte
The resulting potential distribution of Sample D in the Hydrogen Chloride electrolyte after one month is shown in Fig. 20
Figure 20: Deformation of sample D after one month in HCl (hydrogen chloride) electrolyte
Fig. 20 shows the current and potential distribution in the electrolyte and the changed geometry at the end of the
simulation after immersing sample A in the hydrochloric acid for duration of one month. Because the electrode currents are
highest at the contact point of the metals, the metal dissolution is highest at this point. The cathode (mild steel) and the
anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at this contact point which is
between distance 0 and 500mm when sample A is immersed in Hydrochloric acid for one month. The electrolyte potential
increases from 0.4743 to 1.6466V with the lowest values at the cathode (mild steel) and the highest values at the anode
(zinc). The intermediate electrolyte values fall within the contact of the two electrodes.
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Fig. 21 shows the electrolyte distribution for Sample D in Hydrochloric acid after six months, the cathode (mild
steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact point
between distance 0 and 1000mm when sample A is immersed in Hydrochloric acid for six months. The electrolyte
potential increases from 0.4718 to 1.6453V with the lowest values at the cathode (mid steel) and the highest values at the
anode (zinc).
Figure 21: Deformation of sample D after six months in HCl (hydrogen chloride) electrolyte
Fig. 22 shows the electrolyte distribution for Sample D in Hydrochloric acid after twelve months, the cathode
(mild steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact
point between distance 0 and 1000mm when sample A is immersed in Hydrochloric acid for twelve months, the
deformation is more pronounced when compared with Figs. 20 and 21. The electrolyte potential increases from 0.4691 to
1.6439V with the lowest values at the cathode (mid steel) and the highest values at the anode (zinc). Fig. 20-22 confirmed
that the deformation of sample D increases with time when immersed in hydrochloric acid electrolyte which is expected.
Figure 22: Deformation of sample D after twelve months in HCl (hydrogen chloride) electrolyte
Fig. 23 shows the electrode currents at the beginning and end of the simulation of Sample D immersed in
hydrochloric acid for duration of one month, as expected the highest current density (85A/m2) is found at the contact point
between the cathode and the anode. The current densities at the beginning and end of the simulation are very close. The
current density decreases towards the extreme end of the anode (zinc). Fig. 24 shows the electrode currents at the
beginning and end of the simulation of Sample D immersed in hydrochloric acid for duration of six months, the changes in
current densities at the beginning and end of the simulation of Sample D for six months in hydrochloric acid is more
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pronounced when compared with Fig. 23. Fig. 25 shows the electrode currents at the beginning and end of the simulation
of Sample D immersed in hydrochloric acid for duration of twelve months. The changes in the current densities at the
beginning and end of the simulation of Sample D for twelve months in hydrochloric acid is more pronounced when
compared with the changes for one month (Fig.23) and six months (Fig. 24), this depicts that the deviation in the current
density increases with time.
Figure 23: Electrode current densities at t = 0 and t = 1 month for sample D in hydrogen chloride electrolyte
Figure 24: Electrode current densities at t = 0 and t =6 months for sample D in hydrogen chloride electrolyte
Figure 25: Electrode current densities at t = 0 and t = 12 months for sample D in hydrogen chloride electrolyte
The resulting potential distribution of Sample E in the Hydrogen Chloride electrolyte after one month is shown in Fig. 26
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Figure 26: Deformation of sample E after one month in HCl (Hydrogen Chloride) electrolyte
Fig. 26 shows the current and potential distribution in the electrolyte and the changed geometry at the end of the
simulation after immersing sample E in the hydrochloric acid for duration of one month. Because the electrode currents are
highest at the contact point of the metals, the metal dissolution is highest at this point. The cathode (mild steel) and the
anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at this contact point which is
between distance 0 and 500mm when sample A is immersed in Hydrochloric acid for one month. The electrolyte potential
increases from 1.1465 to 1.667V with the lowest values at the cathode (mild steel) and the highest values at the anode
(zinc). The intermediate electrolyte values fall within the contact of the two electrodes.
Fig. 27 shows the electrolyte distribution for Sample E in Hydrochloric acid after six months, the cathode (mild
steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact point
between distance 0 and 1000mm when sample A is immersed in Hydrochloric acid for six months. The electrolyte
potential increases from 1.1448 to 1.6662V with the lowest values at the cathode (mid steel) and the highest values at the
anode (zinc).
Figure 27: Deformation of sample E after six months in HCl (Hydrogen Chloride) electrolyte
Fig. 28 shows the electrolyte distribution for Sample E in Hydrochloric acid after twelve months, the cathode
(mild steel) and the anode (zinc alloy) has a contact at distance zero on the plot, the deformation takes place at contact
point between distance 0 and 1000mm when sample E is immersed in Hydrochloric acid for twelve months, the
deformation is more pronounced when compared with Figs. 26 and 27. The electrolyte potential increases from 1.1432 to
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1.6654V with the lowest values at the cathode (mid steel) and the highest values at the anode (zinc). Fig. 26-28 confirmed
that the deformation of sample E increases with time when immersed in hydrochloric acid electrolyte which is expected.
Figure 28: Deformation of sample E after twelve months in HCl (Hydrogen Chloride) electrolyte
Fig. 29 shows the electrode currents at the beginning and end of the simulation of Sample E immersed in
hydrochloric acid for duration of one month, as expected the highest current density (85A/m2) is found at the contact point
between the cathode and the anode. The current densities at the beginning and end of the simulation are very close. The
current density decreases towards the extreme end of the anode (zinc). Fig. 30 shows the electrode currents at the
beginning and end of the simulation of Sample E immersed in hydrochloric acid for duration of six months, the changes in
current densities at the beginning and end of the simulation of Sample E for six months in hydrochloric acid is more
pronounced when compared with Fig. 30. Fig. 31 shows the electrode currents at the beginning and end of the simulation
of Sample E immersed in hydrochloric acid for duration of twelve months. The changes in the current densities at the
beginning and end of the simulation of Sample E for twelve months in hydrochloric acid is more pronounced when
compared with the changes for one month (Fig. 29) and six months (Fig. 30), this depicts that the deviation in the current
density increases with time.
Figure 29: Electrode current densities at t = 0 and t = 1 month for sample E in hydrogen chloride electrolyte
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Potentiodynamic Polarization and Mathematical Modelling Study of Corrosion 1297
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Figure 30: Electrode current densities at t = 0 and t = 6 months for sample E in hydrogen chloride electrolyte
Figure 31: Electrode current densities at t = 0 and t = 12 months for sample E in hydrogen chloride electrolyte
The current density values were exported from COMSOL to excel to be able to perform calculation using the
formula for calculation of corrosion rate. Fig. 32 shows the corrosion rate for samples immersed in hydrochloric acid
electrolyte, the corrosion rate highest at the contact point of the cathode (mild steel) and the anode (zinc) as expected, this
also correlates the results from electrode current density and potential distribution, the highest corrosion rate coincides with
the highest deformation noticed at the contact of two metals.
Figure 32: corrosion rate for sample A, B, C, D and E in hydrogen chloride electrolyte
4. CONCLUSIONS
Galvanized steel has been found to be effective for wide range applications in various construction industries. Once the
electrochemical parameters were defined, the geometry was applied, the boundary conditions and governing equations
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1298 R. E. Elewa1, S.A. Afolalu, O.S.I. Fayomi & O. Agboola
Impact Factor (JCC): 8.8746 SCOPUS Indexed Journal NAAS Rating: 3.11
were applied and the appropriate mesh was found for the geometries, the model for galvanic corrosion was solved. Zinc
alloy shows the highest deformation in the HCl electrolyte with Sample B. The deformation of the zinc alloy increases as
simulation time increases from one month to twelve months. The highest corrosion rate was noticed at the contact point
between the mild steel and zinc alloy, the corrosion rate decreases with distance away from the contact point of the cathode
and the anode.
ACKNOWLEDGEMENTS
The author acknowledges Covenant University for the financial support offered for the publication of this research.
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2.2 Electrochemical Testing of the Galvanized Roofing Steel Sheet Samples2.3 Mechanical Modelling and Simulation3.2. OCP Analysis of the Galvanized Roofing Steel Sheet Samples in 0.5 M HCl
4. CONCLUSIONS