Enhancing the Teaching of Corrosion to Chemical-Engineering

Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

pubs.acs.org/jchemeduc

Enhancing the Teaching of Corrosion to Chemical-Engineering Students through Laboratory Experiments Javier Llanos,* Á ngel Peŕ ez, and Antonio de Lucas-Consuegra Chemical Engineering Department, Facultad de Ciencias y Tecnologías Químicas, University of Castilla−La Mancha, Edificio Enrique Costa Novella, Avenida Camilo José Cela no. 12, 13071 Ciudad Real, Spain

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S Supporting Information *

ABSTRACT: This work presents a laboratory to help chemical-engineering students understand the basic concepts of corrosion. This laboratory has been performed since academic year 2011−2012 in the framework of the subject “Design of equipment and installations” for the third year of the Chemical Engineering Degree of the University of Castilla−La Mancha (Spain). It is based on calculating the corrosion rates of six different samples in order to evaluate the effects of the corrosion environment, the material of the sample, and the application of corrosion-prevention methods. The performance of the laboratory to enhance the teaching− learning methodology was evaluated by asking the students to answer a test before and after completing the laboratory. The results show that the laboratory improves the understanding of the students and also their confidence in their answers, although room for improvement was found in relation to the application of corrosion-prevention methods, for which corrective actions are proposed. KEYWORDS: Upper-Division Undergraduate, Chemical Engineering, Hands-On Learning/Manipulatives, Electrochemistry, Oxidation/Reduction, Electrolytic/Galvanic Cells/Potentials

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points to be further improved. In addition, correcting actions to improve the weaknesses detected are proposed.

orrosion is one of the most relevant issues in regard to the economy of many different key sectors. According to the calculations of the US National Association of Corrosion Engineers (NACE), the cost of corrosion worldwide is estimated to be US$2.5 trillion, which is equivalent to 3.4% of the 2013 global GDP.1 Thus, the study of corrosion principles is a matter of major importance in many sectors,2 including aerospace engineering,3 medicine,4,5 and, of course, chemical engineering.6−8 The fundamentals of corrosion cover several disciplines, including materials engineering, physical chemistry, electrochemistry, and others. This causes students of chemical engineering to show certain difficulties in understanding the main concepts of corrosion science; it is especially important to include laboratory practices for teaching the basic principles of electrochemical corrosion, as it is possible to design visual, quick, and affordable practices that enhance the teaching− learning process.9−12 The present work describes a laboratory developed to explain the principles of corrosion in the subject “Design of equipment and installations” in the third year of the Chemical Engineering Degree of the University of Castilla−La Mancha (Spain). This laboratory was designed and placed to give support to the corrosion concepts previously explained in the theoretical classes. Moreover, the assessment of the teaching− learning process was made by asking the students to answer a questionnaire twice (before and after performing the laboratory). This survey evaluates not only the knowledge but also the degree of certainty in the answers, thus helping to find out if the activity is effective as well as the possible weak © XXXX American Chemical Society and Division of Chemical Education, Inc.

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BACKGROUND Although the definition of the term corrosion is wide, in the vast majority of practical cases corrosion is a chemical process in which the metal is oxidized. When an electrolyte (e.g., water) is present (the general case for environmental corrosion), corrosion is called electrochemical corrosion or wet corrosion. In this particular case, an electrochemical cell is formed; the dissolution of the metal (M) is the anodic reaction (eq 1), and either the reduction of oxygen (for neutral or basic pH, eq 2) or the reduction of protons (for acidic conditions, eq 3) is the cathodic reaction.9,13 M(s) → Mn +(aq) + ne−

(1)

O2 (g) + 2H 2O(l) + 4e− → 4OH−(aq)

(2)

2H+(aq) + 2e− → H 2(g)

(3)

In this laboratory, the students evaluate the average corrosion rates of iron and stainless steel in different acidic corrosion environments. Moreover, they perform two tests in which two different methods for corrosion prevention are applied (metallic coating and cathodic protection by impressed current). Received: October 9, 2018 Revised: March 6, 2019

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DOI: 10.1021/acs.jchemed.8b00803 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Table 1. Comparison of Student Results for Exposure of the Probes to Different Environments Material Test Test Test Test Test Test

1 2 3 4 5 6

Iron Iron Iron Iron Stainless steel Stainless steel

Solution 0.1 0.1 0.1 0.1 5.0 5.0

M M M M M M

HNO3 HCl HNO3 HNO3 HNO3 HCl

Protection Method None None Zinc coating Cathodic protection by impressed current None None

[Fe] (mg L−1)

Sample Area (cm2)

± ± ± ± ± ±

4.20 4.18 3.60 4.20 4.56 4.84

134 6.80 96.7 40.9 0.001 5.56

2.01 0.09 1.38 0.52 0.0005 0.07

Average Ratea (mg cm−2 min−1) 3.18 1.63 2.69 9.74 2.19 1.15

× × × × × ×

10−01 10−02 10−01 10−02 10−06 10−02

a

The exposure time was 20 min, and the total attack volume was 200 mL.

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LEARNING OUTCOMES This laboratory is designed to help the students improve their basic knowledge on the following: • The reactions taking place in electrochemical corrosion • The roles of the type of material and the corrosion environment on the rate of the corrosion process • The effectiveness of the application of two methods of corrosion prevention (metal coating and cathodic protection by impressed current) This laboratory lasts approximately 3 h, is performed in groups of 4 students, and has been performed by an average of 40 students per year since academic year 2011−2012 in the framework of the subject “Design of equipment and installations”. The students have scored an average of 64.8% in this subject from the course in 2011−2012 to the course in 2017−2018. After completing the laboratory, the students are asked to perform the calculations and discuss the results by answering four questions (explained in the Supporting Information). This work of calculation and discussion of the results may take 3 h of additional work for the students. According to the students’ answers to the surveys about the teaching activities (made by the University of Castilla−La Mancha), the students have a good opinion of the laboratory (average mark of 2.23/3 from the courses in 2011−2012 and 2013−2014 and average mark of 4.43/5 from the courses in 2014−2015 to 2017−2018).

laboratory and the calculations is included in the Supporting Information. Table 1 shows a real example of the iron-concentration results obtained by one group together with the conditions of the tests. The first result that the students can obtain from these data is that the rate of corrosion of iron in nitric acid is higher than that obtained in hydrochloric acid. This can be explained by the nature of the acid, as nitric acid can be considered oxidizing, whereas hydrochloric acid is nonoxidizing. The nitric ion can be reduced to several chemical species in acidic media, including HNO2, NO, or N2O4. The standard reduction potentials of nitric ions to these species (0.934 V for HNO2, 0.957 V for NO, or 0.803 V for N2O4, in all cases vs SHE) are higher than the standard reduction potentials of Fe2+ to metallic iron (−0.440 V vs SHE). In contrast, chloride ion cannot be further reduced to any other chemical species. This means that the anion of nitric acid can contribute to the oxidation of iron, whereas the chloride anion is not able to oxidize the metal, thus giving a lower rate of oxidation. Next, by comparing the rate of test 1 with the rates of tests 3 and 4, the students can check whether applying a method for corrosion prevention diminishes the rate of corrosion. An additional conclusion that can be obtained from these results is that neither the Zn coating nor cathodic protection can completely reduce the corrosion of iron in this environment. This is generally one of the matters that creates higher confusion among the students, as they are expecting full protection of the base metal and not just a partial reduction in the corrosion rate. In this case, the explanation can be found in the fact that neither a galvanic coating nor cathodic protection are the best protection methods in aggressive corrosion environments, such as those tested in the present work. On the contrary, both methods are generally applied in environmental corrosion, in which the aggressiveness of the environment is not as high as in the laboratory.14,15 Finally, two main conclusions can be obtained from analyzing the results of tests 5 and 6. The first one is that the rate of corrosion with stainless steel is far below that registered for iron, even considering that the concentrations of the acids are 50 times higher for the attack on stainless steel. This is expected as stainless steel is able to form a passive layer that hinders the evolution of the oxidation process. The second conclusion from this part of the laboratory is that the aggressiveness of these corrosion environments on stainless steel is the opposite of that observed for iron. In this case, the concept of pitting corrosion (previously explained in the theoretical classes) is the key to finding the answer to this behavior. Chloride ions have the ability to locally dissolve the passive layer of stainless steel, thus allowing protons to attack the base material.16 In contrast, nitrate ions cannot dissolve the

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HAZARDS Splash goggles and gloves should be worn while handling the acid solutions. The power supply used is of a very low power, and it is protected by a fuse, so no important hazards from electric shocks are expected. An important aspect to remember is placing both electrodes far enough away to avoid shortcircuits.

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RESULTS AND DISCUSSION Basically, the laboratory consists of performing the attack on six samples (four of iron and two of stainless steel) with different acid solutions (nitric acid and hydrochloric acid) over 20 min. After this exposure time, the concentration of iron is measured by inductively coupled plasma−atomic-emission spectroscopy (ICP-AES), and the average rate of corrosion is calculated on the basis of the total volume of the solution and the external area of the sample. Two of the iron samples are subjected to methods for corrosion prevention in order to test the influence in the overall corrosion rate. One of them (sample 3) is covered by a zinc coating, and the second (sample 4) is connected to the cathode of a power supply with a fix intensity of 0.4 A in order to promote its cathodic protection by impressed current. A detailed description of the B




passive layer, but the the strongly oxidizing environment of nitric acid is perfect for forming and maintaining this protective film,17 thus resulting in a negligible corrosion rate (the measured iron concentration is almost zero). In this case, the students are also required to visually inspect the samples to observe the incipient pitting formed for the probe immersed in HCl. An example of this is shown in Figure 1, in which an

Figure 1. Microscope analysis of the stainless-steel probes: original (a), after test 5 (HNO3, b), after test 6 (HCl, c). The original and HNO3-treated stainless steel (a,b) have similar aspects, whereas incipient pitting corrosion is observed on the stainless steel treated with HCl (c). The figures were obtained with an Optika SZM-D digital microscope.

original sample of stainless steel (Figure 1a) and the final aspects of stainless-steel pieces in both nitric acid (Figure 1b) and hydrochloric acid (Figure 1c) are compared. The aspect of Figure 1b is visually similar to that of the original piece (Figure 1a), whereas incipient pitting corrosion can be observed in Figure 1c.

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ASSESSMENT OF THE TEACHING−LEARNING METHODOLOGY To assess the teaching−learning methodology, the students were asked to answer a questionnaire (Figure 2) before and after performing the laboratory. In this questionnaire, the students answered four questions related to the three main concepts developed in the laboratory and were asked to state their confidence rating for each question, according to the work of Hoe and Subramaniam.18 Figures 3 and 4 compare the percentages of correct answers (Figure 3) and the confidence ratings (Figure 4) before and after completing the laboratory. As can be observed, performing the laboratory has a marked positive effect on the percentage of correct answers (from 63.2 to 97.4% for question 1, from 45.6 to 83.5% for question 2, from 31.6 to 68.4% for question 3, and from 52.6 to 65.8% for question 4). Regarding Figure 4, performing the laboratory also has a positive effect on the confidence of the students. Nevertheless, it is worth mentioning that more than 30% of the students who answered question 4 incorrectly had a high degree of certainty, which indicates that something is wrong. According to these results, it seems clear that, after completing the laboratory, the students have improved their knowledge of the roles of the corrosion environment and the material (questions 1 and 2). Regarding the application of a metallic coating (question 3), the percentage of correct answers more than doubled, although 30% of the answers were still incorrect. Concerning the application of an electrochemical method for corrosion prevention (question 4), this is the field in which the laboratory has room for improvement. To work on these points, special attention will be paid to giving deeper explanations of the experimental procedures of tests 3 and 4, and their relation to the theoretical concepts previously explained in master classes. Moreover, a

Figure 2. Questionnaire designed to evaluate the effect of the laboratory on the knowledge and confidence of the students.

Figure 3. Success ratios of the answers to questions 1−4 (Q1−Q4) before (white bars) and after (gray bars) the laboratory practices. The sample included 76 chemical-engineering students from two consecutive academic years. A clear increase in the success ratio is observed after completion of the laboratory.

brief explanation of the nature and basic operation of a power supply will be included in the theoretical classes prior to the laboratory period in order to help students take the best possible advantage of the laboratory.

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CONCLUSION The main conclusion to be obtained from this work is that a simple laboratory practice was effectively designed to improve C



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the understanding of basic corrosion concepts for chemicalengineering students. The laboratory helps to increase both their knowledge of corrosion concepts and their confidence in their answers. The results suggest that it is possible to improve the design of the laboratory in regard to the field of the application of corrosion-prevention methods. On the basis of this, special attention will be paid in the future to the explanation of the experimental tests related to this concept and new theoretical concepts will also be included in the master classes prior to the laboratory.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00803.

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REFERENCES

(1) National Association of Corrosion Engineers (NACE) website. http://impact.nace.org/economic-impact.aspx (accessed Feb 2019). (2) Winkleman, A.; Svedberg, E. B.; Schafrik, R. E.; Duquette, D. J. Preventing corrosion from wearing our future away. Adv. Mater. Process. 2011, 169 (3), 26−31. (3) Abrahami, S. T.; de Kok, J. M. M.; Terryn, H.; Mol, J. M. C. Towards Cr(VI)-free anodization of aluminum alloys for aerospace adhesive bonding applications: A review. Front. Chem. Sci. Eng. 2017, 11 (3), 465−482. (4) Nagaraja, S.; Di Prima, M.; Saylor, D.; Takai, E. Current practices in corrosion, surface characterization, and nickel leach testing of cardiovascular metallic implants. J. Biomed. Mater. Res., Part B 2017, 105 (6), 1330−1341. (5) Gao, A.; Hang, R.; Bai, L.; Tang, B.; Chu, P. K. Electrochemical surface engineering of titanium-based alloys for biomedical application. Electrochim. Acta 2018, 271, 699−718. (6) Viertel, J.; Neuer, L.; Mauch, B.; Czyborra, T. Project RepaKorr: Development of a novel single coat, direct to metal repair coating with outstanding protection and colour retention performance for offshore structures. Mater. Corros. 2017, 68 (12), 1321−1325. (7) Khadom, A. A.; Farhan, S. N. Corrosion inhibition of steel in phosphoric acid. Corros. Rev. 2018, 36 (3), 267−280. (8) An, S.; Lee, M. W.; Yarin, A. L.; Yoon, S. S. A review on corrosion-protective extrinsic self-healing: Comparison of microcapsule-based systems and those based on core-shell vascular networks. Chem. Eng. J. 2018, 344, 206−220. (9) Sanders, R. W.; Crettol, G. L.; Brown, J. D.; Plummer, P. T.; Schendorf, T. M.; Oliphant, A.; Swithenbank, S. B.; Ferrante, R. F.; Gray, J. P. Teaching Electrochemistry in the General Chemistry Laboratory through Corrosion Exercises. J. Chem. Educ. 2018, 95 (5), 842−846. (10) Silva, M. V. F.; Pereira, M. C.; Codaro, E. N.; Acciari, H. A. Carbon steel corrosion: An everyday approach for chemistry teaching. Quim. Nova 2015, 38 (2), 293−296. (11) Malel, E.; Shalev, D. E. Determining the effect of environmental conditions on iron corrosion by atomic absorption. J. Chem. Educ. 2013, 90 (4), 490−494. (12) Moraes, E. P.; Confessor, M. R.; Gasparotto, L. H. S. Integrating mobile phones into science teaching to help students develop a procedure to evaluate the corrosion rate of iron in simulated seawater. J. Chem. Educ. 2015, 92 (10), 1696−1699. (13) Cicek, V.; Al-Numan, B. Corrosion Chemistry; John Wiley & Sons, Hoboken, NJ, 2011. (14) Garrity, K. C.; Urbas, M. Cathodic protection of external tank bottoms. Mater. Perform. 2018, 57 (9), A40−A44. (15) Cole, I. S. Recent progress and required developments in atmospheric corrosion of galvanised steel and zinc. Materials 2017, 10 (11), 1288. (16) Solorza, O.; Ibanez, J. G.; Olivares, L. Experimental demonstration of corrosion phenomena: the corrosion, passivation and pitting of iron in aqueous media. J. Chem. Educ. 1991, 68, 175− 177. (17) Arce, E. M.; Ramírez, R.; Cortés, F.; Ibanez, J. G. Experimental demonstration of corrosion phenomena. part ii. corrosion phenomena of steel in aqueous media. J. Chem. Educ. 1991, 68, 351−352. (18) Hoe, K. Y.; Subramaniam, R. On the prevalence of alternative conceptions on acid−base chemistry among secondary students: insights from cognitive and confidence measures. Chem. Educ. Res. Pract. 2016, 17 (2), 263−282.

Figure 4. Confidence rating of the answers to questions 1−4 (Q1− Q4) before (white bars) and after (gray bars) the laboratory practices. The sample included 76 chemical-engineering students from two consecutive academic years. A clear increase in the confidence rating is observed after completion of the laboratory.

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Student handout with a detailed explanation of the experimental procedures, questions, and examples of the calculations needed (PDF, DOC) Answers to the postlab questions (PDF, DOCX) Instructor notes, including a list of the chemicals used with CAS numbers, the materials needed, and the prelab explanations (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Javier Llanos: 0000-0001-6404-3577 Antonio de Lucas-Consuegra: 0000-0001-8080-8293 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors want to acknowledge the collaboration of the students in performing the practice. D


Enhancing the Teaching of Corrosion to Chemical-Engineering

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