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Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material
Engineering Failure Analysis xxx (2014) xxx–xxx
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material D.T. Oloruntoba ⇑, A.P.I. Popoola ⇑ Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 18 June 2014 Received in revised form 15 September 2014 Accepted 16 September 2014 Available online xxxx Keywords: Thermal stress Microstructure Electrodeposition Chloride Cracks
a b s t r a c t Engineering materials disintegrate in divers’ ways that can later result in service failure. The service life of automobile silencer depends typically on the heat generated through the pipe and on the variability of harsh conditions component is exposed. Regular use and long time in service of any part could result into unexpected buildup of stresses that can result in ruinous failure or crack of structure. Automobile exhaust pipe is made up of steel; four representative samples of this material were subjected to thermal cycles and tensile loading with the aim of inducing stress. Metallic zinc was used to coat the stressed steel samples for the purpose of arresting cracks before subjecting them to seawater environment to check their electrochemical response. Experimental results indicated that the zinc coated steel samples (both thermal and tensile stressed) displayed anticorrosion properties resisting fret cracking whereas the uncoated induced thermal and tensile stressed samples performed less. Coating offered some restrictions to the failure of engineering materials subjected to induce thermal and tensile stresses; this is attributed to the ability of zinc acting as a sacrificial anode to steel in the chloride environment. Ó 2014 Published by Elsevier Ltd.
1. Introduction Failure of engineering materials means out of service function that could be catastrophic at times. Automobile exhaust pipe serves as a conveyor of combusted gas out of the engine system; minimizing engine noise as well as extracting dust out of the engine system. Materials used for the construction of exhaust systems ranges from mild carbon steel to stainless steel. In the open literature, numerous studies are geared towards developing surface coatings for mild steel using ceramic/ metallic materials for the purpose of reinforcement protection against thermal and corrosion degradation [1,2]. The operating temperature of the exhaust line could be in the range of 250–1150 °C [3]. The start, slow running, high motion and temporary or final stop of the motor engine can likely be responsible for extreme temperature gradients [4] that could result into failure of exhaust pipe. Likewise, the temperature between 730 °C and 1150 °C can cause a microstructural change in ferrous alloy of the exhaust and also changes in the response to corrosion, thermal degradation and stress level performance. Exhaust condensate, exposure to road salt and dynamic excitation of the system may also lead to pipe failures. Likely failure modes are cracking and leakage of the pipe. In developing countries, automobile shops displayed second-hand exhaust pipe retrieved from used or wrecked cars for sale, typically sold as the replacement of failed pipes. Second-hand exhaust pipes have a short life span due to initial thermal
⇑ Tel.: +27 123823513. E-mail addresses: [email protected] (D.T. Oloruntoba), [email protected] (A.P.I. Popoola). http://dx.doi.org/10.1016/j.engfailanal.2014.09.005 1350-6307/Ó 2014 Published by Elsevier Ltd.
Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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and loading stress that might have been induced in the pipe while in use. Thermo-mechanically treated reinforcement bars constitute an important turning point in the construction industries due to their capacity to envelope high strength, high ductility and low cost simultaneously [5]. Thermal stresses arise in materials when they are heated or cooled; the failure of many brittle materials has been shown to be dependent upon stress distribution within the body rather than upon the maximum stress criteria [6,7]. Investigation on carbon steel operating at temperatures below the creep range in water/steam conditions has shown that thermal shock contributed to damage mechanism for pressure vessel [8]. Retained austenite has also been found to have influence on the fracture toughness of tempered steel [9]. This research aims at reducing failure due to corrosion of exhaust pipe material under high temperature and chloride environment. Electroplating technology offers corrosion protection to metallic materials. In this work, zinc electroplating was carried out on thermally and tensile stressed steel pipes. Zinc is a metallic material that could prevent or act as an anode for corrosion protection of low carbon steel in a corrosive environment [10]. Paying attention to changes in the functions of the exhaust pipe is crucial for the safety and health of the driver and service life longevity and performance of the vehicle. 2. Methodology 2.1. Sample preparation Low carbon steel of 40 mm 25 mm 4 mm was prepared for heat treatment and zinc electrodeposition. The chemical composition of the low carbon steel was carried out by polishing the surface of the sample to get a mirror like surface then mounted on the spark stand in Atomic mass spectrometer. Three different spark tests were carried out while ensuring they are not on the same point on the surface. The average of the results produced was obtained to ensure homogeneity within the material, and the results are as shown in Table 1. 2.2. Thermal stress experiment The low carbon steel substrate was heat treated in the muffle furnace to austenitic temperature of 750 °C, 800 °C, 850 °C, 900 °C, 950 °C and 1000 °C and held for 30 min before quenching with water to induce internal stress in low carbon steel. Two set of samples was prepared for each temperature while one other control sample was kept ready. 2.3. Tensile stress experiment Another set of the carbon steel substrate was machined to standard tensile test samples. A load of 50 kN was applied on the steel samples to induce tensile stress using universal tensile testing machine at various time intervals but without fracturing. The applied load was stopped at the specified time to prevent the samples from necking to fracture. Time rate was from five seconds to 10 s. Two set of samples was prepared for each time intervals while one other control sample was made ready. 2.4. Zinc electrodeposition on the thermal and tensile stressed low carbon steel samples One set of thermally and tensile stressed sample was zinc electroplated in the prepared zinc bath solution, composition shown in Table 2 with plating parameters stated therein. 2.5. Microhardness tests Hardness test is performed on 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C heat treated samples that are then compared to a control substrate. The test is done using a load of HV0.1 100 g on Emco-test (Dura scan model 20) machine. The values of the hardness are taken by placing three indents on the surface of the sample horizontally; thus, the average is then taken. 2.6. Surface morphology of thermally treated uncoated and zn-coated samples An Optical Microscope (OPM) with attached digital Nikon DS-FI1 Optical Camera was used to study the surface morphology of the thermally treated uncoated and zn-coated samples. The microscope has the advantages of assessing surface morphology of samples without prior metallographic preparation of the substrate and very adaptable to all kinds of sample systems, gas, liquid, solid systems at all forms and geometry. Two sample pictures were taken at the 10 and 20 magnifications. Table 1 Chemical composition of the thermally and tensile stressed low carbon steel samples. Element
C
Mn
Si
P
S
Ni
Cr
Mo
Fe
% Composition
0.16
0.3
0.25
0.03
0.03
0.3
0.3
0.08
Balance
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D.T. Oloruntoba, A.P.I. Popoola / Engineering Failure Analysis xxx (2014) xxx–xxx Table 2 Chemical bath formulation for zinc electroplating on thermally and tensile stressed low carbon steel substrate. Chemicals
Quantities/litre
Zinc oxides Sodium cyanide Sodium hydroxide Aluminum sulphate Sodium fluoride Dextrose Water Current density Voltage Temperature Plating time Filtration
50 g 120 g 20 g 10 g 10 g 2g 1L 1–2 A/dm2 2–6 V 30–50 °C 30 min Periodical
2.7. Electrochemical studies of the thermally and tensile stressed uncoated and zn-coated samples The electrochemical studies were performed with Autolab PGSTAT 101 Metrohm potentiostat using a three-electrode cell assembly in 0.5% NaCl solution. The thermally and tensile stressed samples with and without zinc electroplating were the working electrode; platinum used as a counter electrode, and reference electrode was made up of Ag/AgCl. The cathodic and anodic polarization curves fixed at ±1.5 mV were recorded at a constant scan rate of 0.012 V/s. The Tafel extrapolation was used to obtain the corrosion rate, corrosion potential and corrosion current density.
3. Results and discussion 3.1. Effect of thermal and tensile stresses on microstructures, hardness and amount of zinc deposited on low carbon steel samples Fig. 1a and b showed the photographs of the thermally and tensile stressed samples while Fig. 2 shows surface photograph of the sample thermally treated at 1000 °C. Scale formation is visible on the sample due to the high temperature of treatment. After quenching the sample in water, the first layer cracking of the sample was observed. A very high level of thermal stress at 1000 °C caused a crack growth at the surface of the sample. The continuous of the process on the exhaust
Fig. 1. (a) Thermal stressed and (b) tensile stressed low carbon steel samples.
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Fig. 2. Thermal stressed low carbon steel sample at 1000 °C.
pipe material can eventually lead to crack propagation that may result into a fracture. The zinc electroplating assisted in the quick arrest of the crack thereby reducing the probability of failure. Fig. 3a–c showed the microstructures of the thermally stressed sample at 750 °C. The microstructures comprise of pearlite (dark area) distributed in the ferrite (lighter area) matrix. The pearlite and ferrite phases are well pronounced after quenching from the austenitic phase region. The material is a mild steel of low carbon content; therefore, the martensite phase is not pronounced. Fig. 4a–c displayed the microstructural view of thermally stressed at 1000 °C. The pearlite phase is more dissolved in the ferrite phase revealing more uniform or fine grain distributions of the constituents. However, the characteristic displayed is more of retained austenite because the obtained hardness value is less compared with a sample thermally treated at 950 °C. Table 3 shows the microhardness values of the thermally treated zinc coated samples. The microhardness values differ from one temperature treatment to the other under the same plating conditions. The consequence of which might be due to the difference in the microstructure obtained for each operating temperature [11]. The control sample displayed 207.67 HNV while thermally treated sample at 950 °C had the highest micro-hardness value of 276.67 HNV; the least hardness value of 167.67 HNV was obtained for the sample thermally treated at 1000 °C. The grain boundaries in sample thermally treated at 950 °C may be more than the amounts present in 900 °C, hence, the higher hardness value (276.67 HNV) obtained for 950 °C sample. There may be more retained austenite in sample thermally treated at 1000 °C; therefore, the hardness value obtained for it is less to the value for 900 °C and 950 °C as shown in Table 3. Table 4 presents the amount per unit area of zinc electrodeposited on thermally treated samples at various temperatures. The deposition rate per unit area of the sample was influenced by the thermal treatment. Substrate thermally treated at 900 °C had the highest amount of the deposit per unit area of the value 3.452 10 4 g/mm2. The sample thermally treated at 1000 °C recorded the least value of deposition per unit area. A temperature of above 750 °C positively influenced the amount of zinc deposition on low carbon steel thereby offering surface protection for the steel at high temperature. Microstructural changes here played a major role in the deposition rate of zinc on the low carbon steel substrate. Pearlite
pearlite
pearlite
ferrite
ferrite
(b) 200 Magnifications
(a) 100 Magnifications pearlite ferrite
(c) 500 Magnifications Fig. 3. Optical micrograph of thermally stressed low carbon steel sample at 750 °C.
Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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pearlite ferrite
pearlite
(a) 1000 oC X 100 Magnification
ferrite
(b) 1000oC X 200 Magnifications
pearlite
ferrite
(c) 1000 oC X 500 Magnifications Fig. 4. (a–c) Optical micrographs of thermally stressed sample at 1000 °C.
Table 3 Average microhardness values of heat treated and zinc electroplated steel substrates. Sample names
750 °C
800 °C
850 °C
900 °C
950 °C
1000 °C
Control
Hardness value (HVN)
227
221
198.67
192.67
276.67
167.67
207.67
Table 4 Amount per unit area of zinc electrodeposited on thermally treated low carbon steel samples at various temperatures. Samples (°C)
Zinc deposited (g)
Unit area (mm2)
Weight deposited/unit area (g/mm2)
Control 750 800 850 900 950 1000
0.24 0.21 0.35 0.56 0.87 0.31 0.20
2520 2520 2520 2520 2520 2520 2520
0.952 10 0.833 10 1.389 10 2.222 10 3.452 10 1.230 10 0.794 10
4 4 4 4 4 4 4
and ferrite phases may be acting anodically or cathodically to zinc attraction and adhesion on the low carbon steel [11]. Similar effect of varying amount of the zinc electrodeposition on low carbon steel samples was achieved under different strain rates as shown in Table 5. The amount deposited per unit area increases for samples between 5 and 7 s then dropped at 8 s. The samples between 8 and 10 s experienced a linear trend of increase in the amount deposited per unit area. The highest amount of 0.017 g/mm2 zinc deposit on sample stressed at 30 mm/min for 10 s was attained. The differences might be due to the changes in the grain orientation of the samples that allow for different deposition rate of zinc on the low carbon steel. The stress may also be creating dislocation boundaries around the grains thereby changing the potential levels from section to section throughout the entire length of the tensile stressed samples. Higher potential level may be anodic to the zinc electrodeposition thereby reducing the amount of zinc deposited on low carbon steel. Since zinc is expected to be deposited on the cathodic sites hence, the differences seen in the plating rate per unit area of samples. Steady state loads may cause fatigue or tensile stress on the structure of the exhaust pipe coupled with the effects of the saline environment which may likely result into failure of the exhaust pipe material.
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Table 5 Amount per unit area of zinc electrodeposited on tensile stressed low carbon steel samples at various strain rates. Samples (mm/min)
Strain time (s)
Weight deposited (g)
Unit area (mm2)
Weight deposited/unit area (g/mm2)
5 10 15 20 25 30
5 6 7 8 9 10
0.13 0.16 0.27 0.16 0.26 0.34
18.0956 20.2683 20.2683 20.9117 18.3984 19.8713
0.0072 0.0079 0.0133 0.0077 0.0141 0.0171
3.2. Effect of thermal and tensile stresses on the surface morphology of zinc electrodeposited film on a low carbon steel Figs. 5–10 present the surface film morphology of the zinc electrodeposition on thermally stressed low carbon steel samples. Fig. 5 reveals uniform and overlapping deposits of zinc film on the sample thermally treated at 750 °C. Fig. 6 displayed thermally treated sample at 800 °C; the micrograph shows areas of ridge formation, and this may be due to cathodic site formation which likely influenced the concentration of the zinc deposit. It might also be due to differences in the microstructure of the samples as explained in Section 3.1. Fig. 7 shows the surface morphology of the substrate thermally treated at 850 °C. Fine crystal-like structures were seen on the micrograph. However, a dark brown part was revealed at higher magnification of 200. That part might offer protection against corrosion due to oxide formation. Fig. 8 shows a more fine, uniform and continuous zinc film deposit on the thermally treated low carbon steel at 900 °C. Foreign small size particles deposits are few on the substrate film morphology an indication of a clean zinc electroplating bath. Figs. 9 and 10 displayed the surface morphologies of the thermally treated low carbon steel at 950 °C and 1000 °C respectively. Bright surface structure is seen in Fig. 9 whereas Fig. 10 displays fine grain structures. There is an area on the sample that revealed a higher concentration of film shown in Fig. 10; this might be due to the variation in the morphology of the grain structure. The initiated crack as seen in Fig. 2 on the thermally stressed sample at 100 °C might have caused the development of anodic/cathodic site on the substrate thereby more film may be deposited on the cathode site.
Fig. 5. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 750 °C quenched in water.
Fig. 6. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 800 °C quenched in water.
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Fig. 7. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 850 °C quenched in water.
Fig. 8. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 900 °C quenched in water.
Fig. 9. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 950 °C quenched in water.
3.3. Corrosion behavior of the thermally stressed low carbon steel samples simulated seawater environment Fig. 11 showed the potentiodynamic curves for samples thermally treated at various temperatures without a zinc electrodeposition and immersed in 0.5 M NaCl solution while Table 6 presented the potentiodynamic polarization data obtained from Tafel plot. The control sample recorded corrosion potential of 1.1078 V, samples thermally treated at 750 °C, 850 °C and 950 °C recorded corrosion potentials of 0.916 V, 0.81301 V and 1.0152 V respectively (Table 6). Observed in Fig. 11, the corrosion potentials for the thermally treated samples shifted towards positive potentials above the control sample. The control sample without prior heat treatment is more prone to corrosion in the 0.5 M NaCl solution for the exposure period; Ecorr of approximately 3.0 V was achieved. The obtained microstructures for the thermally treated samples displayed anticorrosion properties in the investigated chloride environment. Likely failure due to corrosion as a result of exposure to chloride environment is being resisted by the new microstructures obtained from the thermal treatment applied on the samples. Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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Fig. 10. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 1000 °C quenched in water.
Fig. 11. Polarization curves for thermally treated low carbon steel samples without coating at various temperatures and immersed in 0.5 M NaCl solutions.
Table 6 Potentiodynamic polarization data obtained from Tafel plot for thermally treated low carbon steel substrates at various temperatures, without the zinc electrodeposition and immersed in 0.5 M NaCl solution. Samples Control 750 °C 800 °C 850 °C 900 °C 950 °C 1000 °C
Ecorr calculated (V) 1.0865 0.87232 0.93505 0.82587 0.97775 1.0358 0.969
Ecorr observed (V) 1.1078 0.916 0.93178 0.81301 1.0093 1.0152 0.96634
icorr (A/cm2)
Corrosion rates (mm/year)
Polarization resistance (X)
0.000745 1.76E 05 1.00E 05 6.59E 06 4.38E 05 9.72E 05 3.41E 05
0.62537 0.008471 0.01671 0.00681 0.040484 0.07496 0.03208
31.973 1979.8 406.79 2816.6 180.86 94.303 312.02
As can be seen in Fig. 11 and Table 6 the severeness of corrosion attack on the low carbon steel is mitigated by the thermal treatment. Moreover, observed in Fig. 11, the thermally treated low carbon steel substrate at 850 °C has corrosion potentials above the other thermally treated and control samples; meaning that it displayed the highest corrosion resistance in the tested environment. In the same, this sample displayed corrosion rate decreased by two orders of magnitude and recorded the highest polarization resistance compared with other samples thermally treated and the control sample. That simply indicates the sample treated at 850 °C has the best corrosion resistance in the NaCl solution. According to [12], enhancement of engineering materials is essential for averting service failure and corrosion attack in the industries. Comparatively, Fig. 12 shows the polarization curves of samples thermally treated at various temperatures, electroplated with zinc and immersed in 0.5 M NaCl solution. Table 7 presents the polarization data obtained from Tafel plot. Observed in Fig. 12 and Table 7 are the corrosion potentials of samples thermally treated at 850 °C with zinc coating is higher than the other samples treated the same way but at different temperatures. Samples heat treated at 800 °C, 850 °C and 950 °C with
Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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Fig. 12. Polarization curves for thermally treated low carbon steel samples at various temperatures, electroplated with zinc and immersed in 0.5 M NaCl solutions.
Table 7 Potentiodynamic polarization data obtained from Tafel plot for thermally treated low carbon steel substrates at various temperatures, electroplated with zinc and immersed in 0.5 M NaCl solution. Samples Control 750 °C 800 °C 850 °C 900 °C 950 °C 1000 °C
Ecorr calculated (V)
Ecorr observed (V)
1.0865 1.0213 0.86495 0.8057 0.87614 0.96381 0.93308
1.1078 1.0517 0.87306 0.80602 0.89497 0.97206 0.98224
icorr (A/cm2)
Corrosion rates (mm/year)
Polarization resistance (X)
0.000745 3.31E 05 4.13E 06 3.75E 06 4.21E 06 8.06E 06 1.57E 05
0.62537 0.025554 0.003875 0.003489 0.005001 0.00765 0.01148
31.973 880.43 1504 1739.5 1499.3 1093.7 975.03
zinc coating have corrosion rates of 0.003875, 0.003489 and 0.005001 mm/year respectively (Table 7). All the treated steel samples recorded lower corrosion rates compared with the control sample which is 0.62537 mm/year. The sample treated at 850 °C with and without zinc coating displayed corrosion rates of 0.003489 mm/year and 0.00681 mm/year respectively. The zinc electroplating on the thermally treated samples has enhanced its corrosion resistance in the sodium chloride solution. Materials for the exhaust pipe would, therefore, perform better if it were both thermally treated and zinc electroplated simultaneously especially for high temperature and corrosion resistant applications in a chloride environment. The sample treated at 850 °C recorded the highest corrosion potential value. A general look at all the corrosion parameters also measured show clearly that this sample exhibited the highest corrosion resistance out of all the treated samples (lower corrosion rate and current density; high corrosion potential and polarization resistance). Compared with substrate (control), two order magnitude decrease in current density was attained; same way a two order magnitude decrease in corrosion rate was also achieved. Polarization resistance was also significantly increased. Ecorr value increase is about 0.30 V relative to that of the substrate. Combination of thermal and plating treatments does have positive effects. Table 8 shows the micro-hardness properties of the steel sample under tensile stress at different strain rate. The control sample had the least hardness value of 207 HNV. The hardness value is higher for the rest of the stressed samples. This cold working of steel materials causes work hardening that is a result of changes in grain orientation and dislocation barrier build-up. As observed in Table 8, the sample under the strain rate of 7 s recorded the highest hardness value of 658 HNV. The hardness value decreases as the strain rate increases above 7 s. The observation is due to change in the grain orientation of the steel samples under different time of applied stress. Fig. 13 shows the characteristic features of the grain alignment as it affects the hardness and corrosion properties. Table 9 displayed the polarization data for the stressed sample with zinc coated immersed in 0.5 M NaCl solution. The tendency of the material is to return to conditions of low energy level, an increase in the hardness value indicates higher
Table 8 Micro-hardness values for steel samples tensile stressed at different time under 50 KN load. Tensile operating time (s)
Control
5
6
7
8
9
10
Micro-hardness (HNV)
207
436
484
658
594
654
449
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5 secs
7 secs
6 secs
8 secs
9 secs
10 secs
Fig. 13. Optical micrograph of tensile stressed samples zinc coated and immersed in 0.5 M NaCl solution.
Table 9 Polarization data for tensile sample with zinc coated immersed in 0.5 M NaCl solution. Stress time (s) Control 5 6 7 8 9 10
Ecorr, obs (V) 1.1078 1.1032 1.1218 1.3644 1.3526 1.2069 1.0136
jcorr (A/cm2)
Corrosion rate (mm/year)
Polarization resistance (X)
0.000745 0.00022 0.001521 0.003889 0.002518 0.001834 0.0000798
0.62537 0.089461 0.62374 1.1991 1.0637 0.83419 0.029276
94.303 103.28 85.913 15.163 15.188 17.885 779.97
energy level and stress build up. Consequently, the sample with the highest hardness value from the stressed effect recorded the highest corrosion rates (1.1991 mm/year) and current density (0.003889 A/cm2) and the least polarization resistance (15.188 X) and polarization potentials ( 1.3644 V). 4. Conclusions 1. Thermal treatment of exhaust pipe material caused microstructural changes of the substrate that leads to different electrodeposition rates of zinc on the material. 2. Thermal treatment at 850 °C enhanced the corrosion performance of the exhaust pipe material in sodium chloride environment. 3. Further enhancement against corrosion failure of the exhaust pipe material is achieved by the zinc electrodeposition on the thermally treated substrate at 850 °C. 4. The tensile stressed materials responded differently to the zinc electrodeposition under the same plating parameters; the reason that might be due to different grain orientation and dislocation barrier built up on the steel samples. 5. Thermal and tensile stress on low carbon steel could lead to the formation of anodic/cathodic site on the steel samples due to ferrite/pearlite phases within the thermally treated samples. 6. The materials for the exhaust pipe would, therefore, perform better if it were both thermally treated and zinc electroplated simultaneously especially for high temperature and corrosion resistant applications in a chloride environment.
Acknowledgements This work is supported financially by the National Research Foundation. The authors acknowledge the support from Tshwane University of Technology Pretoria, South Africa. The authors acknowledged the contributions of the following research students: V.O. Oyeniran and A.P. Adeoba. Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material D.T. Oloruntoba ⇑, A.P.I. Popoola ⇑ Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa
a r t i c l e
i n f o
Article history: Received 18 June 2014 Received in revised form 15 September 2014 Accepted 16 September 2014 Available online xxxx Keywords: Thermal stress Microstructure Electrodeposition Chloride Cracks
a b s t r a c t Engineering materials disintegrate in divers’ ways that can later result in service failure. The service life of automobile silencer depends typically on the heat generated through the pipe and on the variability of harsh conditions component is exposed. Regular use and long time in service of any part could result into unexpected buildup of stresses that can result in ruinous failure or crack of structure. Automobile exhaust pipe is made up of steel; four representative samples of this material were subjected to thermal cycles and tensile loading with the aim of inducing stress. Metallic zinc was used to coat the stressed steel samples for the purpose of arresting cracks before subjecting them to seawater environment to check their electrochemical response. Experimental results indicated that the zinc coated steel samples (both thermal and tensile stressed) displayed anticorrosion properties resisting fret cracking whereas the uncoated induced thermal and tensile stressed samples performed less. Coating offered some restrictions to the failure of engineering materials subjected to induce thermal and tensile stresses; this is attributed to the ability of zinc acting as a sacrificial anode to steel in the chloride environment. Ó 2014 Published by Elsevier Ltd.
1. Introduction Failure of engineering materials means out of service function that could be catastrophic at times. Automobile exhaust pipe serves as a conveyor of combusted gas out of the engine system; minimizing engine noise as well as extracting dust out of the engine system. Materials used for the construction of exhaust systems ranges from mild carbon steel to stainless steel. In the open literature, numerous studies are geared towards developing surface coatings for mild steel using ceramic/ metallic materials for the purpose of reinforcement protection against thermal and corrosion degradation [1,2]. The operating temperature of the exhaust line could be in the range of 250–1150 °C [3]. The start, slow running, high motion and temporary or final stop of the motor engine can likely be responsible for extreme temperature gradients [4] that could result into failure of exhaust pipe. Likewise, the temperature between 730 °C and 1150 °C can cause a microstructural change in ferrous alloy of the exhaust and also changes in the response to corrosion, thermal degradation and stress level performance. Exhaust condensate, exposure to road salt and dynamic excitation of the system may also lead to pipe failures. Likely failure modes are cracking and leakage of the pipe. In developing countries, automobile shops displayed second-hand exhaust pipe retrieved from used or wrecked cars for sale, typically sold as the replacement of failed pipes. Second-hand exhaust pipes have a short life span due to initial thermal
⇑ Tel.: +27 123823513. E-mail addresses: [email protected] (D.T. Oloruntoba), [email protected] (A.P.I. Popoola). http://dx.doi.org/10.1016/j.engfailanal.2014.09.005 1350-6307/Ó 2014 Published by Elsevier Ltd.
Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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and loading stress that might have been induced in the pipe while in use. Thermo-mechanically treated reinforcement bars constitute an important turning point in the construction industries due to their capacity to envelope high strength, high ductility and low cost simultaneously [5]. Thermal stresses arise in materials when they are heated or cooled; the failure of many brittle materials has been shown to be dependent upon stress distribution within the body rather than upon the maximum stress criteria [6,7]. Investigation on carbon steel operating at temperatures below the creep range in water/steam conditions has shown that thermal shock contributed to damage mechanism for pressure vessel [8]. Retained austenite has also been found to have influence on the fracture toughness of tempered steel [9]. This research aims at reducing failure due to corrosion of exhaust pipe material under high temperature and chloride environment. Electroplating technology offers corrosion protection to metallic materials. In this work, zinc electroplating was carried out on thermally and tensile stressed steel pipes. Zinc is a metallic material that could prevent or act as an anode for corrosion protection of low carbon steel in a corrosive environment [10]. Paying attention to changes in the functions of the exhaust pipe is crucial for the safety and health of the driver and service life longevity and performance of the vehicle. 2. Methodology 2.1. Sample preparation Low carbon steel of 40 mm 25 mm 4 mm was prepared for heat treatment and zinc electrodeposition. The chemical composition of the low carbon steel was carried out by polishing the surface of the sample to get a mirror like surface then mounted on the spark stand in Atomic mass spectrometer. Three different spark tests were carried out while ensuring they are not on the same point on the surface. The average of the results produced was obtained to ensure homogeneity within the material, and the results are as shown in Table 1. 2.2. Thermal stress experiment The low carbon steel substrate was heat treated in the muffle furnace to austenitic temperature of 750 °C, 800 °C, 850 °C, 900 °C, 950 °C and 1000 °C and held for 30 min before quenching with water to induce internal stress in low carbon steel. Two set of samples was prepared for each temperature while one other control sample was kept ready. 2.3. Tensile stress experiment Another set of the carbon steel substrate was machined to standard tensile test samples. A load of 50 kN was applied on the steel samples to induce tensile stress using universal tensile testing machine at various time intervals but without fracturing. The applied load was stopped at the specified time to prevent the samples from necking to fracture. Time rate was from five seconds to 10 s. Two set of samples was prepared for each time intervals while one other control sample was made ready. 2.4. Zinc electrodeposition on the thermal and tensile stressed low carbon steel samples One set of thermally and tensile stressed sample was zinc electroplated in the prepared zinc bath solution, composition shown in Table 2 with plating parameters stated therein. 2.5. Microhardness tests Hardness test is performed on 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C heat treated samples that are then compared to a control substrate. The test is done using a load of HV0.1 100 g on Emco-test (Dura scan model 20) machine. The values of the hardness are taken by placing three indents on the surface of the sample horizontally; thus, the average is then taken. 2.6. Surface morphology of thermally treated uncoated and zn-coated samples An Optical Microscope (OPM) with attached digital Nikon DS-FI1 Optical Camera was used to study the surface morphology of the thermally treated uncoated and zn-coated samples. The microscope has the advantages of assessing surface morphology of samples without prior metallographic preparation of the substrate and very adaptable to all kinds of sample systems, gas, liquid, solid systems at all forms and geometry. Two sample pictures were taken at the 10 and 20 magnifications. Table 1 Chemical composition of the thermally and tensile stressed low carbon steel samples. Element
C
Mn
Si
P
S
Ni
Cr
Mo
Fe
% Composition
0.16
0.3
0.25
0.03
0.03
0.3
0.3
0.08
Balance
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D.T. Oloruntoba, A.P.I. Popoola / Engineering Failure Analysis xxx (2014) xxx–xxx Table 2 Chemical bath formulation for zinc electroplating on thermally and tensile stressed low carbon steel substrate. Chemicals
Quantities/litre
Zinc oxides Sodium cyanide Sodium hydroxide Aluminum sulphate Sodium fluoride Dextrose Water Current density Voltage Temperature Plating time Filtration
50 g 120 g 20 g 10 g 10 g 2g 1L 1–2 A/dm2 2–6 V 30–50 °C 30 min Periodical
2.7. Electrochemical studies of the thermally and tensile stressed uncoated and zn-coated samples The electrochemical studies were performed with Autolab PGSTAT 101 Metrohm potentiostat using a three-electrode cell assembly in 0.5% NaCl solution. The thermally and tensile stressed samples with and without zinc electroplating were the working electrode; platinum used as a counter electrode, and reference electrode was made up of Ag/AgCl. The cathodic and anodic polarization curves fixed at ±1.5 mV were recorded at a constant scan rate of 0.012 V/s. The Tafel extrapolation was used to obtain the corrosion rate, corrosion potential and corrosion current density.
3. Results and discussion 3.1. Effect of thermal and tensile stresses on microstructures, hardness and amount of zinc deposited on low carbon steel samples Fig. 1a and b showed the photographs of the thermally and tensile stressed samples while Fig. 2 shows surface photograph of the sample thermally treated at 1000 °C. Scale formation is visible on the sample due to the high temperature of treatment. After quenching the sample in water, the first layer cracking of the sample was observed. A very high level of thermal stress at 1000 °C caused a crack growth at the surface of the sample. The continuous of the process on the exhaust
Fig. 1. (a) Thermal stressed and (b) tensile stressed low carbon steel samples.
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Fig. 2. Thermal stressed low carbon steel sample at 1000 °C.
pipe material can eventually lead to crack propagation that may result into a fracture. The zinc electroplating assisted in the quick arrest of the crack thereby reducing the probability of failure. Fig. 3a–c showed the microstructures of the thermally stressed sample at 750 °C. The microstructures comprise of pearlite (dark area) distributed in the ferrite (lighter area) matrix. The pearlite and ferrite phases are well pronounced after quenching from the austenitic phase region. The material is a mild steel of low carbon content; therefore, the martensite phase is not pronounced. Fig. 4a–c displayed the microstructural view of thermally stressed at 1000 °C. The pearlite phase is more dissolved in the ferrite phase revealing more uniform or fine grain distributions of the constituents. However, the characteristic displayed is more of retained austenite because the obtained hardness value is less compared with a sample thermally treated at 950 °C. Table 3 shows the microhardness values of the thermally treated zinc coated samples. The microhardness values differ from one temperature treatment to the other under the same plating conditions. The consequence of which might be due to the difference in the microstructure obtained for each operating temperature [11]. The control sample displayed 207.67 HNV while thermally treated sample at 950 °C had the highest micro-hardness value of 276.67 HNV; the least hardness value of 167.67 HNV was obtained for the sample thermally treated at 1000 °C. The grain boundaries in sample thermally treated at 950 °C may be more than the amounts present in 900 °C, hence, the higher hardness value (276.67 HNV) obtained for 950 °C sample. There may be more retained austenite in sample thermally treated at 1000 °C; therefore, the hardness value obtained for it is less to the value for 900 °C and 950 °C as shown in Table 3. Table 4 presents the amount per unit area of zinc electrodeposited on thermally treated samples at various temperatures. The deposition rate per unit area of the sample was influenced by the thermal treatment. Substrate thermally treated at 900 °C had the highest amount of the deposit per unit area of the value 3.452 10 4 g/mm2. The sample thermally treated at 1000 °C recorded the least value of deposition per unit area. A temperature of above 750 °C positively influenced the amount of zinc deposition on low carbon steel thereby offering surface protection for the steel at high temperature. Microstructural changes here played a major role in the deposition rate of zinc on the low carbon steel substrate. Pearlite
pearlite
pearlite
ferrite
ferrite
(b) 200 Magnifications
(a) 100 Magnifications pearlite ferrite
(c) 500 Magnifications Fig. 3. Optical micrograph of thermally stressed low carbon steel sample at 750 °C.
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pearlite ferrite
pearlite
(a) 1000 oC X 100 Magnification
ferrite
(b) 1000oC X 200 Magnifications
pearlite
ferrite
(c) 1000 oC X 500 Magnifications Fig. 4. (a–c) Optical micrographs of thermally stressed sample at 1000 °C.
Table 3 Average microhardness values of heat treated and zinc electroplated steel substrates. Sample names
750 °C
800 °C
850 °C
900 °C
950 °C
1000 °C
Control
Hardness value (HVN)
227
221
198.67
192.67
276.67
167.67
207.67
Table 4 Amount per unit area of zinc electrodeposited on thermally treated low carbon steel samples at various temperatures. Samples (°C)
Zinc deposited (g)
Unit area (mm2)
Weight deposited/unit area (g/mm2)
Control 750 800 850 900 950 1000
0.24 0.21 0.35 0.56 0.87 0.31 0.20
2520 2520 2520 2520 2520 2520 2520
0.952 10 0.833 10 1.389 10 2.222 10 3.452 10 1.230 10 0.794 10
4 4 4 4 4 4 4
and ferrite phases may be acting anodically or cathodically to zinc attraction and adhesion on the low carbon steel [11]. Similar effect of varying amount of the zinc electrodeposition on low carbon steel samples was achieved under different strain rates as shown in Table 5. The amount deposited per unit area increases for samples between 5 and 7 s then dropped at 8 s. The samples between 8 and 10 s experienced a linear trend of increase in the amount deposited per unit area. The highest amount of 0.017 g/mm2 zinc deposit on sample stressed at 30 mm/min for 10 s was attained. The differences might be due to the changes in the grain orientation of the samples that allow for different deposition rate of zinc on the low carbon steel. The stress may also be creating dislocation boundaries around the grains thereby changing the potential levels from section to section throughout the entire length of the tensile stressed samples. Higher potential level may be anodic to the zinc electrodeposition thereby reducing the amount of zinc deposited on low carbon steel. Since zinc is expected to be deposited on the cathodic sites hence, the differences seen in the plating rate per unit area of samples. Steady state loads may cause fatigue or tensile stress on the structure of the exhaust pipe coupled with the effects of the saline environment which may likely result into failure of the exhaust pipe material.
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Table 5 Amount per unit area of zinc electrodeposited on tensile stressed low carbon steel samples at various strain rates. Samples (mm/min)
Strain time (s)
Weight deposited (g)
Unit area (mm2)
Weight deposited/unit area (g/mm2)
5 10 15 20 25 30
5 6 7 8 9 10
0.13 0.16 0.27 0.16 0.26 0.34
18.0956 20.2683 20.2683 20.9117 18.3984 19.8713
0.0072 0.0079 0.0133 0.0077 0.0141 0.0171
3.2. Effect of thermal and tensile stresses on the surface morphology of zinc electrodeposited film on a low carbon steel Figs. 5–10 present the surface film morphology of the zinc electrodeposition on thermally stressed low carbon steel samples. Fig. 5 reveals uniform and overlapping deposits of zinc film on the sample thermally treated at 750 °C. Fig. 6 displayed thermally treated sample at 800 °C; the micrograph shows areas of ridge formation, and this may be due to cathodic site formation which likely influenced the concentration of the zinc deposit. It might also be due to differences in the microstructure of the samples as explained in Section 3.1. Fig. 7 shows the surface morphology of the substrate thermally treated at 850 °C. Fine crystal-like structures were seen on the micrograph. However, a dark brown part was revealed at higher magnification of 200. That part might offer protection against corrosion due to oxide formation. Fig. 8 shows a more fine, uniform and continuous zinc film deposit on the thermally treated low carbon steel at 900 °C. Foreign small size particles deposits are few on the substrate film morphology an indication of a clean zinc electroplating bath. Figs. 9 and 10 displayed the surface morphologies of the thermally treated low carbon steel at 950 °C and 1000 °C respectively. Bright surface structure is seen in Fig. 9 whereas Fig. 10 displays fine grain structures. There is an area on the sample that revealed a higher concentration of film shown in Fig. 10; this might be due to the variation in the morphology of the grain structure. The initiated crack as seen in Fig. 2 on the thermally stressed sample at 100 °C might have caused the development of anodic/cathodic site on the substrate thereby more film may be deposited on the cathode site.
Fig. 5. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 750 °C quenched in water.
Fig. 6. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 800 °C quenched in water.
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Fig. 7. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 850 °C quenched in water.
Fig. 8. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 900 °C quenched in water.
Fig. 9. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 950 °C quenched in water.
3.3. Corrosion behavior of the thermally stressed low carbon steel samples simulated seawater environment Fig. 11 showed the potentiodynamic curves for samples thermally treated at various temperatures without a zinc electrodeposition and immersed in 0.5 M NaCl solution while Table 6 presented the potentiodynamic polarization data obtained from Tafel plot. The control sample recorded corrosion potential of 1.1078 V, samples thermally treated at 750 °C, 850 °C and 950 °C recorded corrosion potentials of 0.916 V, 0.81301 V and 1.0152 V respectively (Table 6). Observed in Fig. 11, the corrosion potentials for the thermally treated samples shifted towards positive potentials above the control sample. The control sample without prior heat treatment is more prone to corrosion in the 0.5 M NaCl solution for the exposure period; Ecorr of approximately 3.0 V was achieved. The obtained microstructures for the thermally treated samples displayed anticorrosion properties in the investigated chloride environment. Likely failure due to corrosion as a result of exposure to chloride environment is being resisted by the new microstructures obtained from the thermal treatment applied on the samples. Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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Fig. 10. Surface morphology of zinc electrodeposited on thermally treated low carbon steel at 1000 °C quenched in water.
Fig. 11. Polarization curves for thermally treated low carbon steel samples without coating at various temperatures and immersed in 0.5 M NaCl solutions.
Table 6 Potentiodynamic polarization data obtained from Tafel plot for thermally treated low carbon steel substrates at various temperatures, without the zinc electrodeposition and immersed in 0.5 M NaCl solution. Samples Control 750 °C 800 °C 850 °C 900 °C 950 °C 1000 °C
Ecorr calculated (V) 1.0865 0.87232 0.93505 0.82587 0.97775 1.0358 0.969
Ecorr observed (V) 1.1078 0.916 0.93178 0.81301 1.0093 1.0152 0.96634
icorr (A/cm2)
Corrosion rates (mm/year)
Polarization resistance (X)
0.000745 1.76E 05 1.00E 05 6.59E 06 4.38E 05 9.72E 05 3.41E 05
0.62537 0.008471 0.01671 0.00681 0.040484 0.07496 0.03208
31.973 1979.8 406.79 2816.6 180.86 94.303 312.02
As can be seen in Fig. 11 and Table 6 the severeness of corrosion attack on the low carbon steel is mitigated by the thermal treatment. Moreover, observed in Fig. 11, the thermally treated low carbon steel substrate at 850 °C has corrosion potentials above the other thermally treated and control samples; meaning that it displayed the highest corrosion resistance in the tested environment. In the same, this sample displayed corrosion rate decreased by two orders of magnitude and recorded the highest polarization resistance compared with other samples thermally treated and the control sample. That simply indicates the sample treated at 850 °C has the best corrosion resistance in the NaCl solution. According to [12], enhancement of engineering materials is essential for averting service failure and corrosion attack in the industries. Comparatively, Fig. 12 shows the polarization curves of samples thermally treated at various temperatures, electroplated with zinc and immersed in 0.5 M NaCl solution. Table 7 presents the polarization data obtained from Tafel plot. Observed in Fig. 12 and Table 7 are the corrosion potentials of samples thermally treated at 850 °C with zinc coating is higher than the other samples treated the same way but at different temperatures. Samples heat treated at 800 °C, 850 °C and 950 °C with
Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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Fig. 12. Polarization curves for thermally treated low carbon steel samples at various temperatures, electroplated with zinc and immersed in 0.5 M NaCl solutions.
Table 7 Potentiodynamic polarization data obtained from Tafel plot for thermally treated low carbon steel substrates at various temperatures, electroplated with zinc and immersed in 0.5 M NaCl solution. Samples Control 750 °C 800 °C 850 °C 900 °C 950 °C 1000 °C
Ecorr calculated (V)
Ecorr observed (V)
1.0865 1.0213 0.86495 0.8057 0.87614 0.96381 0.93308
1.1078 1.0517 0.87306 0.80602 0.89497 0.97206 0.98224
icorr (A/cm2)
Corrosion rates (mm/year)
Polarization resistance (X)
0.000745 3.31E 05 4.13E 06 3.75E 06 4.21E 06 8.06E 06 1.57E 05
0.62537 0.025554 0.003875 0.003489 0.005001 0.00765 0.01148
31.973 880.43 1504 1739.5 1499.3 1093.7 975.03
zinc coating have corrosion rates of 0.003875, 0.003489 and 0.005001 mm/year respectively (Table 7). All the treated steel samples recorded lower corrosion rates compared with the control sample which is 0.62537 mm/year. The sample treated at 850 °C with and without zinc coating displayed corrosion rates of 0.003489 mm/year and 0.00681 mm/year respectively. The zinc electroplating on the thermally treated samples has enhanced its corrosion resistance in the sodium chloride solution. Materials for the exhaust pipe would, therefore, perform better if it were both thermally treated and zinc electroplated simultaneously especially for high temperature and corrosion resistant applications in a chloride environment. The sample treated at 850 °C recorded the highest corrosion potential value. A general look at all the corrosion parameters also measured show clearly that this sample exhibited the highest corrosion resistance out of all the treated samples (lower corrosion rate and current density; high corrosion potential and polarization resistance). Compared with substrate (control), two order magnitude decrease in current density was attained; same way a two order magnitude decrease in corrosion rate was also achieved. Polarization resistance was also significantly increased. Ecorr value increase is about 0.30 V relative to that of the substrate. Combination of thermal and plating treatments does have positive effects. Table 8 shows the micro-hardness properties of the steel sample under tensile stress at different strain rate. The control sample had the least hardness value of 207 HNV. The hardness value is higher for the rest of the stressed samples. This cold working of steel materials causes work hardening that is a result of changes in grain orientation and dislocation barrier build-up. As observed in Table 8, the sample under the strain rate of 7 s recorded the highest hardness value of 658 HNV. The hardness value decreases as the strain rate increases above 7 s. The observation is due to change in the grain orientation of the steel samples under different time of applied stress. Fig. 13 shows the characteristic features of the grain alignment as it affects the hardness and corrosion properties. Table 9 displayed the polarization data for the stressed sample with zinc coated immersed in 0.5 M NaCl solution. The tendency of the material is to return to conditions of low energy level, an increase in the hardness value indicates higher
Table 8 Micro-hardness values for steel samples tensile stressed at different time under 50 KN load. Tensile operating time (s)
Control
5
6
7
8
9
10
Micro-hardness (HNV)
207
436
484
658
594
654
449
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5 secs
7 secs
6 secs
8 secs
9 secs
10 secs
Fig. 13. Optical micrograph of tensile stressed samples zinc coated and immersed in 0.5 M NaCl solution.
Table 9 Polarization data for tensile sample with zinc coated immersed in 0.5 M NaCl solution. Stress time (s) Control 5 6 7 8 9 10
Ecorr, obs (V) 1.1078 1.1032 1.1218 1.3644 1.3526 1.2069 1.0136
jcorr (A/cm2)
Corrosion rate (mm/year)
Polarization resistance (X)
0.000745 0.00022 0.001521 0.003889 0.002518 0.001834 0.0000798
0.62537 0.089461 0.62374 1.1991 1.0637 0.83419 0.029276
94.303 103.28 85.913 15.163 15.188 17.885 779.97
energy level and stress build up. Consequently, the sample with the highest hardness value from the stressed effect recorded the highest corrosion rates (1.1991 mm/year) and current density (0.003889 A/cm2) and the least polarization resistance (15.188 X) and polarization potentials ( 1.3644 V). 4. Conclusions 1. Thermal treatment of exhaust pipe material caused microstructural changes of the substrate that leads to different electrodeposition rates of zinc on the material. 2. Thermal treatment at 850 °C enhanced the corrosion performance of the exhaust pipe material in sodium chloride environment. 3. Further enhancement against corrosion failure of the exhaust pipe material is achieved by the zinc electrodeposition on the thermally treated substrate at 850 °C. 4. The tensile stressed materials responded differently to the zinc electrodeposition under the same plating parameters; the reason that might be due to different grain orientation and dislocation barrier built up on the steel samples. 5. Thermal and tensile stress on low carbon steel could lead to the formation of anodic/cathodic site on the steel samples due to ferrite/pearlite phases within the thermally treated samples. 6. The materials for the exhaust pipe would, therefore, perform better if it were both thermally treated and zinc electroplated simultaneously especially for high temperature and corrosion resistant applications in a chloride environment.
Acknowledgements This work is supported financially by the National Research Foundation. The authors acknowledge the support from Tshwane University of Technology Pretoria, South Africa. The authors acknowledged the contributions of the following research students: V.O. Oyeniran and A.P. Adeoba. Please cite this article in press as: Oloruntoba DT, Popoola API. Effect of coating on induced thermal and tensile stress on the fracture of exhaust pipe material. Eng Fail Anal (2014), http://dx.doi.org/10.1016/j.engfailanal.2014.09.005
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