Results Of A Simple Corrosion Experiment In A Freshman Materials Course

Corrosion and its prevention is an important aspect of materials studies. This paper presents results of simple experiments developed for use in a time-limited course. Conventional corrosion rate experiments are time consuming, but yield useful corrosion rate data that can be used for design considerations. The lab work described here is oriented toward rapid, visual results that can be correlated to the effects of material structure, grain size, etc. Procedures and techniques are presented describing how to perform these simple experiments and listing the equipment needed. Typical results are presented along with explanations of the anticipated results. Potential pitfalls are discussed. Student comments are also provided. The paper includes photographs of typical results. These experiments also help the student to develop powers of observation and reporting as well as teach them something about the corrosion process. Introduction The effects of corrosion are obvious all around us. The etching of metallurgical samples and the way a battery works are examples of electrochemical processes. Visit a junk yard and observe the rusted hulks that were once someone's shiny new car. Non-functional farm machinery often litters the landscape because it costs too much to haul it away to a landfill. Those of us living in the rust belt wax the exterior of our vehicles, and often pay someone to provide a rust protective coating on the underside of them. Approximately 20% of the iron and steel products manufactured each year are used to replace objects that have been discarded due to rust damage . Figure 1 is a photograph of a truck ravaged by corrosion typical of damage brought primarily by road salt used for winter de-icing. Figure 1. An older vehicle shows the effects of corrosion The Purdue University School of Technology requires all AS and BS degree Mechanical Engineering Technology students to take a sequence of materials-related courses. The first P ge 248.1 course covers metals, plastics and ceramics. Several years ago, it occurred to the author that there should be some way in the first course, to demonstrate the effects of galvanic corrosion in a simple way, that would not cost much, nor take a long time to see visible results. Over the course of four years, the author has developed methods of demonstrating in a qualitative way the effects of corrosion and to show how it can be prevented. Since our students do not have chemistry until the junior year, the lecture material covers the essentials of galvanic corrosion and the chemistry involved. Quantitative corrosion testing takes a great deal of time and utilizes nasty corrodents, like boiling sulfuric acid, to measure corrosion rates. The experiments described here provide almost immediate visual results for the students. Background Information Our students have already learned how cold working of a metal produces strengthening and also results in large grains being broken into many smaller grains. They have also learned about recrystallization through heating. In addition, instruction has been given concerning the formation of passive layers in stainless steels and other similar materials. In the corrosion unit, the galvanic series is introduced and it is shown how materials that are higher in the series will corrode preferentially to materials that are lower. Essential to the discussion are the five conditions that must be present simultaneously before corrosion can occur . These conditions are: 1. there must be an anode where a metal is oxidized. 2. there must be a cathode where some ions are reduced. 3. a potential (voltage) must exist between the anode and cathode, usually provided by dissimilar metals. 4. there must be an electrolyte in contact with both anode and cathode. 5. there must be a physical electrical connection between the anode and cathode to complete the circuit so that the electrons can flow. For our students, it is necessary to emphasize that oxidation is a process whereby a material loses electrons and reduction is a process whereby a material gains electrons . Several textbooks illustrate this process using classical electron flow. 4 Electrolytes are usually liquids that conduct electricity, although certain solid oxides can act as electrolytes as well as some vapors . It is important that the students understand all of these concepts in order to evaluate the results from the laboratory demonstration. The tie-in between cold work, anodes and cathodes is then made, describing how cold work simply creates large numbers of anodes and cathodes, which are essential for galvanic corrosion to occur. The anodes are the grain boundaries and the cathodes are the grains . As the anodes are consumed, the grains and boundaries exchange roles, reversing the polarity. This interchange continues until the entire material has been consumed. Since the iron oxide crystal is larger and less dense than the steel crystals, the oxide buckles as it is formed, exposing fresh steel to corrosion. Corrosion Demonstration Samples There is an endless list of materials which could be used to demonstrate galvanic corrosion and its prevention. It is important to keep the list relatively small to prevent overloading the students with observations to make. It is also important to limit the list to materials that are readily available. We are fortunate at our location to have a supply of tensile test specimens for numerous materials. We selected a number of these for use as corrosion samples, as well as P ge 248.2 common hardware items available at reasonable cost from local sources such as hardware stores. Table 1 lists some candidate materials that can be used for the corrosion demonstration. Not all of these items should be used for one demonstration. If class size is large, it is desirable to divide the class into groups and limit the number of samples to be observed to about six or eight. Table 1. Candidate Corrosion Materials AISI 1010 Hot rolled steel tensile sample AISI 1010 Hot rolled steel tensile sample, one side ground AISI 1010 Hot rolled steel tensile sample with MIG weld bead AISI 1010 Hot rolled steel tensile sample with magnesium strip attached AISI 4340 Cold rolled steel tensile sample Common steel nails, one bent, one straight Brass tensile sample, CA 360, half-hard Aluminum tensile sample, 6061-T6 Copper tensile sample Inconel tensile sample Hastelloy tensile sample Bright zinc plated steel hardware, such as an "S" hook Punched zinc coated steel strip used for shelving Hot-dipped zinc coated roofing nail Cadmium plated bolt or screw Stainless steel bolt Suggested Corrodents and Containers Glass jars are best to hold the electrolyte selected to permit easy access for students to view the corrosion activity. Spaghetti sauce jars, with plain sides, work best for visibility and allow photographs to be made. The jars are carefully washed, labels are removed and all traces of glue cleaned off before use. In response to student comments, labels are provided that clearly explain what material and what corrodent is in each container. One can choose from a large number of common liquids to be used as corrodents. Good results have been achieved with common household bleach, diluted 100:1 with water. Ordinary tap water has been used, although distilled or even deionized water would give better control of results. Other materials including calcium chloride solution (similar to road salt), ammonia, drain cleaner, dilute hydrochloric or sulfuric acids could be used with selected corrosion samples. General Experimental Procedure It is important to clean the samples, removing any oils that might be on the surface. An alcohol wash is a good idea. Each sample material is suspended in the glass jar using nylon fishing line, poly string or polyethylene film. In each case, the string is held by the jar lid. If significant evolution of gas is anticipated, clear film can be placed over the jar opening and the string secured with a rubber band. It is suggested that the jar be filled with the corrodent before inserting the sample. A good cover is needed to prevent evaporation. For the tensile specimens, P ge 248.3 a small hole is drilled in each one, allowing the string to be inserted. Other materials can have the support tied on or looped around. Students are given a data sheet on which to record their observations. They are to observe the samples about one hour after immersion and every other day after that. If possible, more frequent observations for the first 24 hours might be useful. The observations are usually ended after one or two weeks. Students are encouraged to note the progress of corrosion, if any, and draw sketches. It is best if they do not touch or move the jars. Typical Results The results obtained from steel samples are discussed below. Results from other materials are summarized in Table 2. Hot rolled steel samples. Hot rolled material is protected by a black oxide coating that occurs during processing. However, since these samples are sheared, the edges have undergone significant cold work. One should see rust forming on the sheared edges during the first hour of immersion. The sample having the weld bead on it should show rust forming at the edge of weld first. This is due to cold work that occurs as the weld cools. The sample edges will not rust as rapidly in this case due to recrystallization that occurred during the welding process. The sample that has the magnesium strip attached should rust very little, but the magnesium strip may be consumed. This is of course, an example of cathodic protection, where the magnesium becomes the anode that is consumed to protect the cathode. (Most hot water heaters have a magnesium rod installed for this purpose.) Figures 2, 3 and 4 show the progress of corrosion for some hot-rolled materials after 48 hours of immersion. If a strip with one side having the oxide layer removed is used, one will see immediate rusting of that s