Teaching Electrochemistry in the General Chemistry Laboratory

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

pubs.acs.org/jchemeduc

Teaching Electrochemistry in the General Chemistry Laboratory through Corrosion Exercises Richard W. Sanders,† Gregory L. Crettol,† Joseph D. Brown,† Patrick T. Plummer,† Tara M. Schendorf,† Alex Oliphant,† Susan B. Swithenbank,‡ Robert F. Ferrante,§ and Joshua P. Gray*,† †

Department of Science, U.S. Coast Guard Academy, New London, Connecticut 06320, United States Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Chemistry Department, U.S. Naval Academy, Annapolis, Maryland 21402, United States ‡

S Supporting Information *

ABSTRACT: Electrochemistry is primarily taught in first-year undergraduate courses through batteries; this lab focuses instead on corrosion to apply electrochemical concepts of electrolytes, standard reduction potentials, galvanic cells, and other chemistry concepts including Le Chatelier’s Principle and Henry’s Law. Students investigate galvanic corrosion under neutral and acidic conditions quantitatively by measuring voltage, amperage, and mass loss and qualitatively using phenolphthalein. They also investigate protection against corrosion through the use of sacrificial anodes and impressed current cathodic protection.

KEYWORDS: First-Year Undergraduate/General, Laboratory Instruction, Hands-On Learning/Manipulatives, Electrochemistry, Electrolytic/Galvanic Cells/Potentials

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lectrochemistry has long been a difficult concept in general chemistry. Students can perform quantitative calculations involving electrochemical cells, but struggle to understand their qualitative properties.1 Misconceptions in electrochemistry have been reported to commonly occur in identifying half reactions,2 understanding components of galvanic cells,3 and relating contributions of concentration and equilibrium to electromotive force.4 Electrochemistry is often taught at the first-year level using voltaic cells from the perspective of batteries. In this paper, we present an alternative method of teaching electrochemistry through corrosion, an equally important electrochemical phenomenon. Despite its significant importance to the economy and its relevance to engineering disciplines, corrosion is covered in much less detail in many general chemistry textbooks than voltaic cells. Essentially, corrosion is the formation of an electrochemical cell between a metal and environmental components that results in the degradation of the metal. The metal acts as the anode, and the environmental chemicals, predominantly molecular oxygen (neutral or basic conditions) or hydrogen ion (acidic conditions), undergo reduction at the cathode surface. Corrosion, corrosion mitigation, prevention science, and the methods behind their application are of great interest to engineering disciplines.5,6 Although corrosion is too slow a process to be visually observed in an undergraduate laboratory period, a variety of measurement techniques can be used to monitor corrosion during a shorter time frame. © XXXX American Chemical Society and Division of Chemical Education, Inc.

A number of corrosion laboratories for undergraduates have been published in the Journal of Chemical Education using methodologies such as atomic absorption,7 anodic polarization,8,9 mass loss,10,11 and observation.12 However, these laboratories and demonstrations are generally more appropriate for upper level courses. Here we present an inexpensive undergraduate laboratory exercise that allows students to observe corrosion in real time using conditions approximating military operational environments (fresh water versus salt water) using a digital multimeter, mass loss, and observation of pH to measure corrosion quantitatively and qualitatively (see the student handout in Supporting Information). The exercise also demonstrates the effects of galvanic and impressed current Received: June 14, 2017 Revised: February 12, 2018

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DOI: 10.1021/acs.jchemed.7b00416 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

Figure 1. Zinc sacrificial anode with visible corrosion significantly protects the surrounding painted hull of a USCG Buoy Utility Stern Loading (BUSL) boat. Photo courtesy of CWO4 Paul Jeffreys.

temperature, increased acidity, and higher concentrations of electrolytes. This laboratory exercise shows common mechanisms of corrosion that are important for vehicles and vessels and common methods used to prevent or slow it. Students observe corrosion qualitatively through the formation of hydrogen bubbles when metals corrode in acid, visible changes to the metal, and visible changes to pH using a pH indicator. Quantitatively, galvanic corrosion is measured using digital multimeters to quantify voltage and current and through mass loss experiments. Protection against corrosion using sacrificial anodes or impressed current cathodic protection is also demonstrated.

protection systems. To speed the corrosion process to a time frame suitable for the general chemistry laboratory, acidic solutions are used which enhance the corrosion rate. The laboratory discusses real-world applications of corrosion prevention through discussion of an actual event: the early drydock of the Coast Guard Cutter Stratton shortly after launch due to failure of the impressed current cathodic protection system and enhanced corrosion due to stress corrosion beneath the water line of the ship.13 This experiment can be completed in one laboratory period and is suitable for a general chemistry laboratory.

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BACKGROUND Corrosion is the result of the creation of an electrochemical cell between a metal and a component of the environment, typically molecular oxygen (at neutral pH) or hydrogen ion (at acidic pH). The rate at which corrosion occurs is a function of the degree of protection of the metal and environmental factors. Common types of corrosion include uniform corrosion (corrosion that occurs via local action cells on the surface, causing uniform loss of material), galvanic corrosion (corrosion due to the electrical connection of two metals of different electrochemical potential), and stress corrosion (corrosion due to mechanical stresses introduced into the metal). Corrosion rates are enhanced by environmental factors such as increased

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LEARNING OUTCOMES After performing the laboratory exercise, the students should know how to (i) describe the components necessary for a corrosion cell, (ii) contrast environmental cathodes found in acidic versus neutral conditions, (iii) relate electrochemical potential to galvanic protection in galvanic cells, and (iv) explain how impressed current affects the corrosion rate of two metals. We found that students self-reported greater satisfaction and performed better on this laboratory exercise if it is preceded with one focused on batteries. Students scored an average of 74.01% (n = 258) on the laboratory in 2016 when it B




Figure 2. Phenolphthalein demonstrates the production of hydroxide ions at the surface of copper in a zinc/copper cell in salt water. Corrosion occurs when the attached multimeter (not shown) is set to read current (left) but not when set to read voltage (right). A multimeter set to read voltage does not permit current to flow, halting the corrosion. This experiment is performed as part of the laboratory exercise. Photo courtesy of Joshua Gray.

Figure 3. Preferential corrosion of the Coast Guard Cutter Polar Star below the water line. A zone of noncorroded hull plating surrounds the rectangular zinc sacrificial anodes. Photo courtesty of LTJG Tyler Richardson.

was preceded with a laboratory exercise focused on batteries versus 65.9% (n = 221) in 2015 when it was not (p < 0.001, unpaired t test with Welch’s correction). 19% of students rated this lab their favorite lab, and 17% indicated that they learned

the most from this laboratory. Those choosing the corrosion laboratory as their favorite lab wrote, “it placed a heavy emphasis on what you would see in the fleet”, “it was interesting to see the different reactions within the solution. It C




was cool to actually SEE corrosion”, and “it involved the most visible reactions we induced.” Students indicating that they did not like the corrosion lab wrote, “it was difficult to understand the material”, “we hadn’t learned about the topic in class, and [there] wasn’t enough time to complete [the] entire thing”. This laboratory has been taught for three years to students of General Chemistry with modifications and improvements made each year. Laboratory group size is two students, and the laboratory takes approximately 3 h to complete. A worksheet intersperses novel content with laboratory exercises meant to demonstrate the content, and our students are encouraged to collaborate and discuss with one another during the exercise.

the solution. Next, air is bubbled next to each of the two zinc strips and the effect on voltage is measured. When air is bubbled next to the strip attached to the positive electrode, a positive voltage will be generated proportional to the rate of airflow with the opposite reaction occurring when the air is bubbled next to the negative electrode. A simple disposable plastic squeeze pipet is sufficient to induce a temporary change in the voltage and current, although better results can be achieved using a safety pipet filler bulb which produces a more constant stream of bubbles. Although not used in our laboratory, alternative gases such as nitrogen or carbon dioxide could be used to demonstrate that the bubbling must introduce oxygen to be effective at inducing a difference potential and current. This experiment reinforces Le Chatelier’s Principle and Henry’s Law.

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HAZARDS Hydrochloric acid at 3 M concentration is quite corrosive; splash goggles and gloves should be worn when handling the acid. A 9 V battery is used to produce minute quantities of hydrogen and chlorine gas from the 3 M hydrochloric acid solution; the experiment should be performed in a fume hood to avoid inhalation.

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Corrosion in Acidic Environments and Protection by Sacrificial Anodes

Metals will generally corrode more quickly in acidic conditions than in neutral or basic conditions. This is driven by differences in concentration between the two most common environmental cathodes: molecular oxygen and hydrogen ion. The concentration of oxygen in aqueous solution is approximately 270 μM at room temperature and standard atmospheric pressure (which is comparable to the concentration found in seawater (249.1 μM), whereas the concentration of hydrogen ion varies with pH; at lower pH, hydrogen ion reduction becomes the dominant cathodic reaction. The experiment is performed in 3 M HCl to accelerate the rate of corrosion to a rate that can be quantified through mass loss in the laboratory over 2 min. Zinc is commonly used as a sacrificial anode on ships and in many civil engineering applications such as underground pipes. A 4 in. common nail (20d) is used to represent iron. Students compare the rate of mass loss of zinc versus that of the nail over two experiments, one in which they are not in electrical contact and one in which they are connected through a double-headed alligator clip. In both cases, uniform corrosion will occur. However, the mass loss of Zn will be much higher when in electrical contact with the iron nail. Although the loss of mass is quite small relative to the mass of the metal, when extrapolated over a longer time period, corrosion leads to significant loss of material.

EXPERIMENTS AND DISCUSSION

Effect of Electrolyte Concentration on Uniform Corrosion in a Galvanic Cell

Uniform corrosion, the most common type of corrosion, occurs through the reaction of a metal with environmental chemicals such as molecular oxygen or, in acidic environments, hydrogen ion. Small imperfections in the metal create temporary anodic and cathodic regions on its surface which lead to a uniform corrosion across its surface. However, these differences are difficult to quantify, and the rate of corrosion is too slow to measure in a 3 h lab period. To accelerate the rate of corrosion and allow its quantification, Cu is connected to Zn via a digital multimeter, and both voltage and amperage are quantified in either salt water or distilled water to demonstrate the importance of electrolytes on the rate of corrosion. Zn is an ideal choice for the anode because it is a typical sacrificial anode, because its products do not form a patina or rust on its surface, and because zinc does not passivate (Figure 1). Students verify oxygen reduction as the cathodic reaction by observing the formation of hydroxide ions at the surface of Cu using phenolphthalein (Figure 2). Students will note the drop in amperage in distilled water versus salt water, illustrating the impact of electrolyte concentration on the rate of corrosion.

Effect of Impressed Current Cathodic Protective Systems on the Rate of Corrosion

Modification by Differences in Oxygen Concentration

Impressed current cathodic protection (ICCP) uses a power source to affect the corrosion of two electrochemically coupled metals. The negative potential is applied to the metal to be protected, and a positive potential is applied to a sacrificial metal. Although waste metals have been attached to the cathode side of impressed current systems in the past, more commonly used are platinum-coated titanium electrodes that resist corrosion. Reduction of molecular oxygen or hydrogen ion occurs at the impressed anodic side, depending on the pH of the system. If the voltage is large enough, reduction of water occurs generating molecular hydrogen at the anodic side from either hydrogen ion or water. Concomitantly, the impressed cathodic metal loses electrons and corrodes. In theory, the applied voltage must be equal to the voltage of the cell; for an Fe/Zn cell, this would be 0.315 V. In practice, a greater voltage is required to protect the Zn from corroding. A power supply providing a voltage of 1.5 V would be sufficient. In our example, students are provided a 9 V battery as a power

There are many environments in which a contiguous piece of metal is simultaneously exposed to oxygen-rich and -poor environments. This results in a voltaic cell with the cathodic reduction of molecular oxygen occurring at the oxygen-rich areas and anodic reactions occurring in the oxygen-poor areas. This commonly occurs in seagoing vessels which have oxygenrich regions near the water’s surface and lower oxygen concentrations beneath the surface of the water. Oxygen reduction happens more quickly near the surface with the electrons being provided by anodic areas of the hull further beneath the water’s surface (Figure 3). In this next portion of the laboratory exercise, two identical Zn metal strips are connected to a digital multimeter to measure current and voltage in a solution of 0.5 M NaCl on opposite sides of a single container. The voltage is read and verified to be close to zero, since the two metal pieces are similar and the concentration of oxygen is uniform throughout D




Academy and Simone Cossette for assistance with graphics. The authors thank all of the Coast Guard members who assisted with photographs for this article and for the accompanying laboratory procedures.

source. Although it will result in significant reduction of water, it may be conceptually easier for the students to grasp the concept of applied current providing protection if they are using an obvious power supply that they are familiar with. Students are instructed to observe what occurs at the zinc and copper electrodes upon application of the impressed current in both orientations in terms of mass loss, voltage potential, and current. Connection of the negative terminus to Fe and positive terminus to Zn results in accelerated corrosion of the zinc and vigorous bubbling at the Fe, reflective of the generation of hydrogen gas. Connection in the opposite orientation has the reverse effect: bubbling at the Zn electrode and corrosion of the Fe.

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CONCLUSION Using a relatively simple laboratory setup, undergraduate students observe the most common types of corrosion including uniform corrosion, galvanic corrosion, and atmospheric corrosion with either molecular oxygen or hydrogen ion being reduced at the primary cathode. Methods of protection against corrosion are also presented, including galvanic protection with sacrificial anodes and impressed current cathodic protection (ICCP). Finally, students observe the corrosion qualitatively through observation of visual changes to the surface of the metal and quantitatively through mass loss, potential differences, and current measurements. Overall, this laboratory experiment reinforces the teaching of electrochemical properties in general chemistry and allows for hands-on experience with basic corrosion techniques.

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ASSOCIATED CONTENT

S Supporting Information *

Student handout (PDF, DOCX) Answer key (PDF, DOCX) The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00416. Answer key (PDF, DOCX) Student handout (PDF, DOCX)

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REFERENCES

(1) Ogude, A. N.; Bradley, J. D. Ionic Conduction and Electrical Neutrality In Operating Electrochemical Cells: Pre-College And College Student Interpretations. J. Chem. Educ. 1994, 71 (1), 29. (2) Garnett, P. J.; Treagust, D. F. Conceptual Difficulties Experienced By Senior High School Students Of Electrochemistry: Electrochemical (Galvanic) And Electrolytic Cells. J. Res. Sci. Teach. 1992, 29, 1079− 1099. (3) Ö zkaya, A. R. Conceptual Difficulties Experienced By Prospective Teachers In Electrochemistry: Half-Cell Potential, Cell Potential, And Chemical And Electrochemical Equilibrium In Galvanic Cells. J. Chem. Educ. 2002, 79, 735. (4) Sanger, M. J.; Greenbowe, T. J. Common Student Misconceptions In Electrochemistry: Galvanic, Electrolytic, And Concentration Cells. J. Res. Sci. Teach. 1997, 34, 377−398. (5) National Research Council. 2009 Assessment of Corrosion Education; The National Academy Sciences Press: Washington, DC, 2009. DOI: 10.17226/12560. (6) Scully, J. R.; Harris, W. C. Opportunities and Challenges in Corrosion Education: Review of a National Research Council Assessment. Electrochem. Soc. Interface 2012, 21, 67. (7) Malel, E.; Shalev, D. Determining the Effect Of Environmental Conditions On Iron Corrosion By Atomic Absorption. J. Chem. Educ. 2013, 90, 490−494. (8) Solorza, O.; Olivares, L.; Ibanez, J. G. Experimental Demonstration Of Corrosion Phenomena: The Corrosion, Passivation, And Pitting Of Iron In Aqueous Media. J. Chem. Educ. 1991, 68, 175. (9) Arce, E. M.; Ramirez, R.; Cortes, F.; Ibanez, J. G. Experimental Demonstration Of Corrosion Phenomena: Part II. Corrosion Phenomena Of Steel In Aqueous Media. J. Chem. Educ. 1991, 68, 351. (10) Onuchukwu, A. I. The Corrosion Inhibition Of A Metal: Student Corrosion Experiment III. J. Chem. Educ. 1989, 66, 681. (11) Higa, C. O.; da Costa, S. L. F. A.; Agostinho, S. M. L. Interaction Between Scientific Research And Chemical Education: Development Of A Corrosion Experiment. J. Chem. Educ. 1989, 66, 441. (12) Meloan, C. An Experiment to Illustrate the Effects Of Salt and Zinc On The Rate Of Corrosion. J. Chem. Educ. 1986, 63, 456. (13) Larsen, K. U.S. Coast Guard Wages ‘War on Corrosion’. Mater. Perf. 2011, 50 (10), 22.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Joshua P. Gray: 0000-0002-9704-3845 Notes

The contents of the work presented here do not necessarily represent the official opinion or position of the United States Coast Guard, Department of Homeland Security, Department of Defense, or the U.S. Government. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Corrosion Policy Office of the Department of Defense. The authors thank the chemistry faculty of the U.S. Naval Academy for providing the original framework of this laboratory. The authors thank Mr. Cecil Dekle for his help with experimental design and troubleshooting. The authors also thank Augustus Manzi for his work helping to create the first version of the laboratory while he was a Cadet. The authors thank Cadet Kevin Puri of the Air Force E


Teaching Electrochemistry in the General Chemistry Laboratory

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