IJCIET_10_01_142.pdf

International Journal of Civil Engineering and Technology (IJCIET) Volume 10, Issue 01, January 2019, pp. 1551-1560, Article ID: IJCIET_10_01_142 Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=10&IType=01 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed ASCERTAINMENT OF THE CHANGE OF THE DUCTILITY IN CORRODED STEEL SPECIMENS BY EXPERIMENT Antonio Shopov Department “Strength of materials”, Technical University of Sofia – 8, “Kliment Ohridski” blvd., Sofia, 1000, Bulgaria, European Union Borislav Bonev Department “Microelectronics”, Technical University of Sofia – 8, “Kliment Ohridski” blvd., Sofia, 1000, Bulgaria, European Union ABSTRACT The studies of the ductility of the materials date back to ancient times. The basic values of the stress-strain diagram, which determines the groups in the main zones the elastic zone, the yield zone, the strengthening zone and the fracture zone, are known. There are main construction steel elements always where corrosion is at an advanced stage. The corrosion of construction steel is an inevitable process. The negative consequences that indicate the corrosion of the steel elements have been partially established and opportunities for solving them should be sought. We conducted an experiment to determine how the ductility and main values of the stress-strain curve of corroded steel samples changed. We used S355JR steel and applied a galvanostatic electrochemical accelerated corrosion method. After that, we performed a tensile test of the samples, we took the values from the stress-strain diagrams and we calculated how the basic stress-strain values and the corresponding ductility changed. We used the stochastic method to process the results. We have come to the conclusion that corrosion affects the basic values, the ductility and the structural change of the material and when (and if) it is elastic-plastic, according to the stress-strain diagram, it is transformed into brittle material. Key words: ductility, corrosion, electrochemical accelerated corrosion method, Cite this Article: Antonio Shopov and Borislav Bonev, Ascertainment of the Change of the Ductility in Corroded Steel Specimens by Experiment, International Journal of Civil Engineering and Technology, 10(01), 2019, pp. 1551–1560 http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=10&IType=01 http://www.iaeme.com/IJCIET/index.asp 1551 editor@iaeme.com Antonio Shopov and Borislav Bonev 1. INTRODUCTION The basic definition of ductility is the measure of the ability of the material to undergo significant plastic deformation before rupture, which may be expressed in percentages during the processes of elongation or area reduction, while undergoing a tensile test [1-2]. Ductility is particularly important concerning steel as materials that crack, break or shatter under stress, cannot be manipulated using metal-forming processes such as hammering, rolling, drawing or extruding [1-2]. High degrees of ductility occur in steel elements, which are found predominantly in metals, leading to the common perception that metals in general are ductile. In metallic bonds valence shell electrons are de-localized and shared between many atoms [12]. The de-localized electrons enable metal atoms to slide past one another without being subjected to strong repulsive forces that would cause other materials to shatter [1-2]. Ductility can be quantified by the fracture strain, which is the engineering strain at which a test specimen fractures during an uniaxial tensile test [1-2]. Another commonly used measure is the reduction of the area of fracture [1-2]. The ductility of construction steel varies depending on the constituents of the alloy [1-2]. The increase in the levels of carbon decreases [1-2]. Some authors make a note that the mechanism for determining mechanical properties resulting from corrosion and stress corrosion is also complicated by additional factors such as energy accumulation, environmental impact, action of sulphate-restoring bacteria, electrochemical and destructive processes in a structural layer [3], that means ductility it would be depended from exactly the same factors. According to some regulations, every construction element from steel need to have a certain ductility, but when the corrosion is occupied (Figure 1) there are not guarantee that ductility is not changed. Some researchers study an altering the mechanical properties or ductility [4-18, 22] of corroded steels and all of which examine stress-strain and ductility problems and their possible characterization, and some formulas or corresponding derived dependencies are also available in some of them. It is known, that if a mechanical property is changed that means that a stress-strain diagram is changed too. Which means that an elastic strain, plastic strain, strengthening strain and fracture strain will be changed too. The purpose of this study is to establish how ductility is changed on steel with corrosion. Figure 1 Structure with corroded steel elements 2. ACCELERATED CORROSION METHOD A widely used method for accelerated corrosion is electrochemical corrosion [4-8]. This method achieves anodic dissolution of the steel by flowing of direct current through the test specimen. The test sample is connected to the positive pole of the power supply (anode) and the negative pole of the power supply is connected to a stainless-steel plate or other inert metal (cathode). It can be stabilized the voltage between the anode and cathode or current through http://www.iaeme.com/IJCIET/index.asp 1552 editor@iaeme.com Ascertainment of the Change of the Ductility in Corroded Steel Specimens by Experiment test specimen. The use of the second option does not require adjustments during accelerated corrosion and allows us approximately to determine daily weight loss [18]. We use a system, developed by us using the current stabilization (the so-called galvanostatic method) to realize accelerated corrosion. Details of the developed system are presented in [22]. We have chosen a current of 600 mA, where the loss of mass for 24 hours determined by the Faraday formula is [22]: % 100 100 56 0.6 24 60 375 2.5 96484 60 3,21 %, 1 where M = 56 g/mol is the atomic mass of the ferric ion, I[A] – electric current through the test specimen, t[s] – time duration of the treatment, W[g] is the weight of the steel specimen before corrosion treatment, z is the valence of the ferric ion (z = 2,5 is the average value for Fe2+ and Fe3+ ions of the corrosion products), F = 96484 C/mol is the Faraday constant. The number of test specimens is 16, separated into two groups. The time duration of the treatment for group A is 14 days and for group B – 5 days. In this case, the approximate percentage weight loss is 50 % for group A and 20 % for group B. The accelerated corrosion system used consists of 75 adjustable current stabilizers, the current of each of which can be adjusted within the range of 16-200 mA. The system allows parallel connection of current stabilizers. Therefore, a current of 600 mA can be obtained by the parallel connection of three current stabilizers, each set to 200 mA. A block diagram of the experimental setup is shown in Figure 2 [22]. (a) (b) Figure 2 (a) Block scheme of the experimental setup; (b) Photograph of the experimental setup Weight measurements before corrosion treatment and after corrosion treatment and corrosion products removal are performed with precision balance. Moments of these measurements are shown in Figure 3a and Figure 3b. http://www.iaeme.com/IJCIET/index.asp 1553 editor@iaeme.com Antonio Shopov and Borislav Bonev (a) (b) Figure 3 (a) Weight measurement of the test specimens before corrosion treatment; (b) Weight measurement of the test specimens after corrosion treatment and corrosion products removal 3. MATERIAL AND STEEL SPECIMEN 3.1. Steel specimen and tensile test Dimensions of the test specimen depend on many factors. The researchers have chosen what dimensions to be used on their steel specimen [9-12, 15, 18, 22]. We prefer to use a steel specimen which parallel length is a 15d [18, 22], because is established that the best and most reliable results are obtained right then [18, 19]. Dimensions and photos of our steel specimen are shown in Figure 4a and Figure 4b. (a) (b) Figure 4 The used steel specimen – (a) dimensions; (b) photograph (a) (b) Figure 5 (a) Steel specimens before accelerated corrosion method performing; (b) moment of tensile test of the steel specimen with corrosion We used a universal testing machine MESSPHYSIK model BETA200-7/6x14 for the tensile test of the steel specimens [18, 22], but according standard ISO 8407:2009 we need to remove http://www.iaeme.com/IJCIET/index.asp 1554 editor@iaeme.com Ascertainment of the Change of the Ductility in Corroded Steel Specimens by Experiment corrosion products (rust) from our samples [6, 18, 22]. We remove corrosion on steel specimens in hydrochloric acid [6, 18, 22] - 10 min, in solution of 500 ml hydrochloric acid with 1000 ml distilled water and 3.5g hexamethylenetetramine on temperature 20 °C [18, 22]. Photograph of a steel specimen after removing corrosion products (rust) is shown in Figure 6. Figure 6 Photograph of Steel specimen after remove corrosion products (rust) 3.2. Material Every author using different steel materials, it depends on basic study which is make it [4-18, 22]. Main parts of tank wagon (for example) are design of structural steel S355J2 [20], but the difference between S355JR and S355J2 is that the first have a withstand an impact energy of 27J at +20 °C, the second have withstand an impact energy of 27J at -20 °C, but the stressstrain curve is exactly the same. The steel S355JR is most popular for steel structures in Bulgaria [18, 22]. We prefer to use in our study a structural steel S355JR, as we do [18, 22] with the chemical composition is given on table 1, according standard EN 10025-2-2004. It is known that a chemical element silicon (Si) is given a more strength, but the negative effect is that is reducing a corrosion resistance and elongation and transverse contraction. Table 1 Chemical composition on steel S355JR C max 0.2 4 Si max 0.55 Chemical composition, [%] Mn P S N max max max max 0.01 1.6 0.04 0.04 2 Cu CEV max 0.55 max 0.47 We used a classic stress-strain curve (Figure 7) of S355JR steel and take a main value of strain, which is interest for us. (a) (b) Figure 7 (a) Stress-strain curve of S355JR steel; (b) main values (points) of ductility http://www.iaeme.com/IJCIET/index.asp 1555 editor@iaeme.com Antonio Shopov and Borislav Bonev Table 2 Main values from classic stress-stain curve on S355JR steel main value ε, [%] σ, [MPa] elastic strain plastic strain strengthening strain fracture strain εel, [%] εy, [%] εu, [%] εf, [%] 0.0234 0.6218 1.9025 2.4235 381.6688 368.4887 460.9131 289.2526 4. RESULTS We have 16 steel specimens, divided in 2 (two) groups – Group A (for 14 days electrochemical accelerated corrosion) and Group B (for 5 days electrochemical accelerated corrosion). The results are given in Tables 3, 4 and 5 (for group A) and Table 6, 7 and 8 (for group B). We use the stochastic method to process [18, 21, 22] the obtained empirical data (main values - strain and stress), as these are random variables [21]. In Figure 8 is show a stress-strain curves (after process of values – stochastic and average) after tensile test on corroded steel specimens and accelerated corrosion results for group A. In Figure 9 is show a stress-strain curves (after process of values – stochastic and average) after tensile test on corroded steel specimens and accelerated corrosion results for group B. Probability of our results – group A is 89.42 % and group B is 88.14 %. Table 3 Results for group A – strain values initially weight final weight elastic strain εel plastic strain εy strengthening strain εu fracture strain εf (g) (g) (%) (%) (%) (%) 1 377.515 204.016 0.167 0.249 0.610 0.741 2 378.757 206.264 0.123 0.532 0.743 0.840 3 367.692 196.750 0.138 0.195 0.382 0.448 4 373.743 201.754 0.125 0.185 0.604 0.705 5 375.559 204.350 0.202 0.335 0.782 0.893 6 381.403 206.096 0.147 0.203 0.520 0.619 7 365.179 199.746 0.292 0.362 0.770 0.854 8 380.162 208.226 0.155 0.207 0.433 0.514 average results 375.001 203.400 0.169 0.284 0.606 0.702 stochastic result 379.385 202.197 0.185 0.353 0.540 0.682 № of steel specimen http://www.iaeme.com/IJCIET/index.asp 1556 editor@iaeme.com Ascertainment of the Change of the Ductility in Corroded Steel Specimens by Experiment Table 4 Results for group A – stress values initially weight final weight elastic stress σel plastic stress σy strengthening (ultimate) stress σu fracture stress σf (g) (g) (MPa) (MPa) (MPa) (MPa) 1 377.515 204.016 316.658 345.820 425.354 282.715 2 378.757 206.264 253.461 283.372 373.927 245.437 3 367.692 196.750 289.558 318.841 381.881 260.870 4 373.743 201.754 228.936 234.887 306.942 200.014 5 375.559 204.350 387.649 422.322 502.015 344.117 6 381.403 206.096 249.381 264.701 335.044 223.167 7 365.179 199.746 391.862 427.903 522.666 369.182 8 380.162 208.226 245.196 280.898 348.356 231.000 average results 375.001 203.400 295.338 322.343 399.523 269.563 stochastic result 379.385 202.197 317.846 345.524 413.913 289.916 № of steel specimen Table 5 Results for group A – ratio values ratio values ! ! ! average results 1.1586 2.4750 4.1567 2.1361 3.5876 1.6795 stochastic result 1.2636 1.9313 3.6933 1.5285 2.9229 1.9123 Table 6 Results for group B – strain values initially weight final weight elastic strain εel plastic strain εy strengthening strain εu fracture strain εf (g) (g) (%) (%) (%) (%) 9 390.766 327.218 0.278 0.691 1.290 1.549 10 387.051 318.439 0.270 0.757 1.520 1.827 11 387.295 320.102 0.262 0.666 1.293 1.567 12 385.645 318.705 0.189 0.602 1.591 1.916 13 384.358 315.342 0.263 0.647 1.722 2.031 14 380.845 326.442 0.248 0.668 1.336 1.678 15 388.184 341.005 0.233 0.657 1.858 2.202 16 386.449 332.147 0.241 0.656 1.647 1.922 average results 386.324 324.925 0.248 0.668 1.532 1.836 stochastic result 383.844 323.219 0.241 0.663 1.586 1.891 № of steel specimen http://www.iaeme.com/IJCIET/index.asp 1557 editor@iaeme.com Antonio Shopov and Borislav Bonev Table 7 Results for group B – stress values initially weight final weight elastic stress σel plastic stress σy strengthening (ultimate) stress σu fracture stress σf (g) (g) (MPa) (MPa) (MPa) (MPa) 9 390.766 327.218 326.650 372.966 436.589 295.871 10 387.051 318.439 350.962 377.840 437.708 290.284 11 387.295 320.102 349.845 410.079 469.871 314.056 12 385.645 318.705 351.552 368.416 455.750 298.899 13 384.358 315.342 326.809 331.522 413.039 274.232 14 380.845 326.442 301.625 366.499 432.980 281.079 15 388.184 341.005 365.616 360.167 442.127 292.272 16 386.449 332.147 358.474 352.215 438.593 296.102 average results 386.324 324.925 341.442 367.463 440.832 292.849 stochastic result 383.844 323.219 358.920 366.420 450.913 284.414 № of steel specimens Table 8 Results for group B – ratio values ratio values ! ! ! average results 1.1986 2.7488 7.4038 2.2933 6.1769 2.6935 stochastic result 1.1922 2.8530 7.8530 2.3930 6.5868 2.7525 Figure 8 Results from experiment (group A) http://www.iaeme.com/IJCIET/index.asp 1558 editor@iaeme.com Ascertainment of the Change of the Ductility in Corroded Steel Specimens by Experiment Figure 9 Results from experiment (group B) 5. CONCLUSION Our experimental results unambiguously established that there was a change in ductility. When corrosion is at an early stage, the change in ductility is small, but with the advancement of corrosion development in steel bearing elements, the change in ductility becomes sensitive. In view of the strength, there is a slight (minor) change which is obviously due to the corrosive effect. The fact that, as corrosive development progresses, the corresponding ductility begins to decrease, it means that there is a correlation between the development of corrosion in a steel bearing element and its ductile qualities. We find that steel elements with corrosion are unable to undergo significant plastic deformations before tearing. It is known that the amount of carbon (C) in a steel spill affects directly the ductility, but in our case, with increasing corrosion, the ductility is reduced. This fact unambiguously supports the thesis that when corrosion enters the steel elements, it directly influences the basic elastic-plastic properties, from the ductile material, becomes a brittle material, although with the look of the chart of ductile material (sometimes). ACKNOWLEDGEMENTS This research received is funding by “Hyosel” Ltd., Sofia, Bulgaria. REFERENCES [1] [2] [3] [4] [5] [6] [7] Budynas, R. G. Shigley's Mechanical Engineering Design, 10th Edition. McGraw Hill, 2015, pp. 233. Vernon, J. Introduction to Engineering Materials, 3rd Edition. New York : Industrial Press, 1981. Ganev, R. and Godiniachki, G. Steel corpuses destroing from tiredness and stress corrosion. Proceedings of the 5th Scientific Conference Fire and emergency safety 2009, Sofia, Bulgaria, 2009, pp. 190-192. Chen, G., Hadi, M., Gao, D. and Zhao, L. Experimental study on the properties of corroded steel fibres. Construction and Building Materials, 79, 2015, pp. 165-172. Ponjayuthi, D. and Vinodh, K. 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