Introduction: All most all metals are unstable when exposed to air. This is because of the tendency of the metal to be converted into its oxidized form on reaction with air. When compared to zinc, iron has a greater tendency to form oxidized form and thus has greater susceptibility to corrosion. The ability of metals to react with air varies and this determines their relative stability. As zinc is much stable when compared to iron, it is used to galvanizing iron pipe, thus preventing contact of iron with air and water that contains dissolved air. Brass is an alloy of zinc, and the resistance of brass to corrosion increases with increase in the percentage of zinc in the alloy. In this lab, we look into the difference in corrosion rate of iron and brass when exposed to saline water. (Sato)
Background: Corrosion under natural condition take a considerable amount of time and thus, difficult to study in a lab experiment. For this reason, corrosion rate in the lab is determined using the formation of an electrochemical cell. The electrochemical cell has two reactions: anode reaction and cathode reaction. In the anodic reaction, the positive metal ion like iron (Fe2+) goes into solution as Fe2+ ions and Zn goes into solution as Zn2+. The cathodic reaction will vary with the kind of medium in which the metal is suspended. The electrons released from the anodic reaction will combine with H+ and OH- ions in freshwater to form 2H2O. The corrosion rate is measured by the tendency of the metal to give up electrons when placed in an electrolyte. The electric potential of the reaction is compared with that of a standard electric cell. The corrosion rate is measured in terms of current density J=I/A. I is the current in microamps and A is the area of the metal in cm2. The corrosion rate is calculated using the formula 1 mpy=(1/n)J. Where n is valence of the metal. A catalyst like salt can enhance corrosion rate and thus used in the experiment. (Sato)
Materials required: Samples of brass and iron to be tested. Sand paper to polish metal. Microscope attached to a camera. Electrochemical cell connected to a Vera Stat is used to calculate the current density. ("Corrosion of Metals-Experiment 9")
Procedure: The metal of known area is polished using the sand paper and the finely polished surface of the metal is examined under the microscope. The surface will be examined after the corrosion experiment as well. To begin with, the corrosion experiment, 500 ml of tap water is taken in a beaker and 1 tablespoon of salt is added. This is the electrochemical cell in which the metal will be suspended. Each metal sample was suspended in a separate electrochemical cell. The metal is set up in the electrochemical cell with a reference electrode and a counter electrode. The leads from the electrode are connected to Vera Stat (potentiostat) that displays the current density. The Versa Stat is set up to the program required to run the corrosion experiment. It plots the test result over a period of 15 minutes. ("Corrosion of Metals-Experiment 9")
Observation:
Result:
The photo of the iron before and after the experiment:
Figure 1 Iron after and before corrosion experiment
The photo of brass before and after the experiment:
Figure 2 Brass after and before corrosion experiments
The corrosion rate of iron= 0.183
The corrosion rate of brass= 0.11
Discussion: Corrosion in salt water is a result of the electrochemical reaction. The complicated corrosion reaction is separated into anode and cathode reaction. Both these reactions are required for the corrosion to occur. The experiment shows the difference in corrosion rate of two metals in saline water. Uniform corrosion occurs when there is corrosion is happening uniformly over the entire surface of the metal. The corroded surface was observed under the microscope. While certain metals are more likely to be attacked by the anodic reaction, other are more likely to be eroded by the cathodic reaction. For example, iron is more likely to be eroded by the anodic reaction when compared to brass. (Sato, 2016)
Corrosion is a common problem that affects the quality of water that come through metal pipes. It not only destroys the pipe but also increases the toxicity of water. Over a period of time, the metal thins away forming pits and holes. Salt increases the conductivity of water and increases corrosion rate. On the other hand, dissolution of metal is higher in water that contains less dissolved salt. Both corrosion and dissolution are separate processes, but give rise to a similar outcome like the thinning of metal. Oxygen is the primary corrosive agent. Oxygen that enters the water from air is the cause of corrosion of metals. In addition to oxygen and salt, other factors like acidity and temperature can also hasten the corrosion process. In this particular experiment, we identified that iron has a higher corrosion rate when compared to brass, given a uniform condition.
Conclusion: Iron has a faster corrosion rate than brass in a unit time. The corrosion rate of iron in salt water is 0.183 and the corrosion rate of brass is 0.11.
Work cited:
"Corrosion Of Metals-Experiment 9". csun.edu. N.p., 2016. Web. 21 Aug. 2016. Retrieved from http://www.csun.edu/~bavarian/Courses/MSE%20227/Labs/9-Corrosion.pdf
Sato, Norio. "Basics Of Corrosion Chemistry". Wiley. N.p., 2016. Web. 21 Aug. 2016.
Retrieved from https://www.wiley-vch.de/books/sample/3527329307_c01.pdf
Work cited:
