POTENTIODYNAMIC POLARIZATION AND MATHEMATICAL MODELLING STUDY OF CORROSION RESISTANCE PROPERTIES OF

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.

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 1,3

Department of Mechanical Engineering, Covenant University, Nigeria

2

Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, South Africa

4

Department 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 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

Original Article

higher corrosion resistance propagation of 151.81(Ω) with significant Ecorr value of -1.4057V thus, with corrosion rate

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].

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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: 𝜂 ⁄𝐴 𝑐𝑎𝑡

𝑖𝑐𝑎𝑡= 𝑖0,𝑐𝑎𝑡 . 10

(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. 𝜂

𝑖𝑡𝑎𝑓𝑒𝑙 = 𝑖0,𝑎𝑛 . 10𝐴𝑎𝑛 (3.5) 𝑖𝑎𝑛 =

𝑖𝑙𝑖𝑚 𝑖 1+ 𝑙𝑖𝑚

(4)

𝑖𝑡𝑎𝑓𝑒𝑙

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 𝑖 𝑀

𝑣 = 2𝐹 𝜌 (5) www.tjprc.org

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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. www.tjprc.org

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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 Impact Factor (JCC): 8.8746

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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 www.tjprc.org

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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/m 2) 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 Impact Factor (JCC): 8.8746

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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 Impact Factor (JCC): 8.8746

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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/m 2) 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 www.tjprc.org

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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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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 www.tjprc.org

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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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