____________ [Chemistry 3] Experiment No 3.1
CORROSION

K. M. U. AMANDORON, R. N. T. COLANAG, E. J. G. MERIN, F. T. OPADA, R. J. C. PARBA
IV-Photon
Philippine Science High School - Central Visayas Campus
Talaytay, Argao, Cebu, Philippines

Date performed: December _, 2013
Date submitted: January 6, 2013 ABSTRACT
Corrosion is a process through which metals in manufactured states return to their natural oxidation states, a reduction-oxidation reaction in which the metal is being oxidized by its surroundings, often the oxygen in air [1]. The first part of the experiment involved the products of rusting. Certain amounts of 3% NaCl, 10% K3Fe(CN)6 and phenolphthalein were combined. A piece of paper was soaked in this mixture. 2 nails (one straight, one bent) were wrapped in the damp paper allowing space in between, then left for 10mins. After which, colors formed in the paper napkins were observed. The second part was the bimetallic corrosion. 2 iron nails were cleaned using sand paper. In one nail, copper wire was coiled around its tip. On the other, a zinc granule was attached to its tip. Another paper was soaked in the same mixture in which the nails were wrapped separately and were left for 10mins. The papers were opened and sites where pink and blue colors formed were examined. The results didn’t absolutely correspond to the objectives of this experiment due to the errors on the materials used; however, it was inferred that during the process of corrosion, the metal ions dissolve and the electrons transfer to another location where they’re taken up by oxygen. A mixture of hydrous iron oxides is then produced from the resulting hydroxide ions which react with iron (II). This mixture is what’s commonly known as rust. In the zinc granule – iron nail combination, it was found out that the zinc was oxidized faster compared with iron thus making it the anode site and iron the cathode site. For the copper wire – iron nail combination, it was concluded that the iron nail “rusts” while the copper didn’t react with anything, so the copper wire was the cathode site while the iron nail was the anode site. In bimetallic corrosion, corrosion of a metal is accelerated when placed in contact with metal with a different and higher potential. The metal which oxidizes easily and is the anode will be the one that will corrode.

INTRODUCTION

Corrosion is usually defined as degradation of the properties of a material as a result of chemical reaction with the environment. [1] Corrosion is a spontaneous electrochemical process that has caused tens of billions of dollars of damage to cars, ships, buildings, and bridges each year. The most common and economically destructive form of corrosion is the rusting of iron. Rust is not a direct product of the reaction between iron and oxygen but arises through a complex electrochemical process. [2]
There were two parts of the experiment. The first one involved the products of rusting. 15 mL of 3% NaCl, 4 drops of 10% K3Fe(CN)6 and 2 drops of phenolphthalein were combined in a small beaker. In this mixture, a piece of plain white paper napkin was folded and was soaked. After soaking, two nails (one straight, one bent) were wrapped in the folded damp paper allowing space between them. This setup was left for 10 minutes. After which, colors formed in the paper napkins were observed. The second part of the experiment was the bimetallic corrosion. Two iron nails were cleaned using sand paper. In one iron nail, copper wire was coiled around its tip. On the other iron nail, a zinc granule was attached to its tip. Another plain white paper was folded and was soaked in the mixture with 3% NaCl, 10% K3Fe(CN)6 and phenolphthalein. The iron nail with a zinc granule attached to its tip and the iron nail with the coiled copper wire were wrapped separately in the soaked folded paper napkins and were for 10 minutes. Again, the folded papers were opened and sites where pink and blue colors formed were examined. This experiment aims to infer the products of rusting, determine anodes and cathodes of different iron nail setups and to be able to make a general statement about bimetallic corrosion.

RESULTS AND DISCUSSIONS

After leaving the first setup for 10 minutes, pink and blue-green colors appeared on the paper as seen in Figures 1 and 2.

Figure 1. Parts of the iron nails where the color pink appeared Figure 2. Parts of the iron nails where the color pink appeared
Corrosion usually happens in a location where the metal is under stress (at a bend or weld) or is isolated from the air. The metal ions dissolve in the moisture film and the electrons migrate to another location where they are taken up by a depolarizer which in this case, is oxygen. The resulting hydroxide ions (OH-) react with the Fe2+ to form the mixture of hydrous iron oxides known as “rust” [4]. Corrosion is actually an electrochemical event in the sense that it is governed by the reduction-oxidation reaction. The special characteristic of most corrosion processes is that the oxidation and reduction steps occur at separate locations on the metal. The presence of water is necessary in order to transport ions to and from the metal [5].
A corrosion system can be regarded as a short-circuited electrochemical cell. At one spot on the nail (the anodic site of our electrochemical cell) iron loses electrons (is oxidized) to form iron (II) ions.
Fe (s) Fe2+ (aq) + 2e-
At another spot on the nail the oxygen in the air combines with water and forms hydroxide ions.
½O2 (g) + H2O (l) + 2e- 2OH- (aq)
In the presence of oxygen the iron further oxidizes at the anode (loses electrons) to become iron (III) ions.
Fe 2+ (aq) Fe3+ (aq) + e-
The iron (III) ions and the hydroxide combine to form rust.
2Fe 3+ (aq) + 6OH- (aq) Fe2O3 (s) + 3 H2O (l)
An expected outcome of the experiment would be like shown in Figure 3. However, this was not projected in the results. This could be caused by the use of nails that already had rust. It could also be caused by the use of scratch paper instead of plain white paper napkins which may brought out better colors. Although the nails were cleaned using sandpaper, it may have not been enough to produce the results that have been expected.
Figure 3. Experimental image of iron nails (straight and bent) with the formation of blue and pink colors at certain areas
For the second setup, formations of pink and blue colors were observed on the two different combinations (zinc granule-iron nail and copper wire-iron nail) after left for 10 minutes as seen in Figures 4 and 5.
Figure 4. Zinc granule-iron nail combination and the formation of pink and blue colors

Figure 5. Copper wire-iron nail combination and the formation of pink and blue colors

Supposedly, in the zinc granule-iron nail combination, the color pink should be observed in the area where the nail was placed, which indicates the presence of hydroxide ions while no color should be observed in the area where the zinc granule was placed. In this setup, zinc is oxidized faster or more easily than the iron. Zinc loses two electrons and forms a Zn2+ ion. On the surface of the iron, water and oxygen combine with the two electrons to make hydroxide ions, which turn the solution next to the iron surface pink. In this case the zinc is considered to be the anode and the iron is considered to be the cathode. However, based on the results, there was a color pink formation in the area of the nail but with small portions of the blue-green color.

For the copper wire-iron nail combination, , the color blue should be found in the area wherein the nail was placed thus indicating the presence of iron (II) ions while the color pink was found in the area where the copper wire was wrapped around indicating the presence of OH-.

The iron metal loses electrons and turns into an iron ion.

Fe (s) Fe2+ + 2 e-

These two electrons travel through the iron metal to the copper. At the copper, there is water and oxygen which take the two electrons and use them to form hydroxide ions.

½ O2 (g) + H2O (l) + 2 e- 2 OH-

This excess of OH- produced causes the solution next to the copper to be pink. Hydroxide ions (OH-) make a solution to be basic which turns pink in the presence of phenolphthalein.

The iron ions that are formed react with oxygen and water to form "rust.”

Fe2+ + ½ O2 (g) + H2O (l) Fe (OH)2 (s)

This Fe(OH)2(s) combines with a second molecule of Fe (OH)2(s) in the presence of oxygen to form iron(III)oxide (the more common form of rust) and water.

2 Fe (OH)2 (s) + ½ O2 (g) Fe2O3 (s) + 2H2O (l)

Thus iron "rusts" and the copper does not react with anything.

But then again, the results did not fully show the expected outcome. Although there was a slight shade of blue in the area of the nail, no pink color was seen on the area where the copper wire was wrapped. This probably was due again to the nails (rusty) and the paper used.

From this experiment, several applications can be done. Anodic protection is a technique to control the corrosion of a metal surface by making it the anode of an electrochemical cell. A good metal that can be used for this is zinc since it will prevent a metal from corroding just like it did with iron nail. On the other hand, a metal that will accelerate the corrosion of iron nail if contact is made is copper.

The electromotive series (EMS) table is very helpful in determining which corrodes which. For example, the EMS table shows the relation of Cu, Zn, and Fe. In terms of reducing strength: Zn > Fe > Cu. In terms of oxidizing strength: Cu > Fe > Zn. And in terms of reduction potentials: Cu > Fe > Zn. Bimetallic corrosion is the accelerated corrosion of one metal placed in contact with a different more noble metal [3]. This means that when both metals come in contact, the more “noble metal” or the cathode is protected while the less “noble metal” or the anode sacrificially corrodes. In other words the metal which oxidizes easily will corrode.

SUMMARY AND CONCLUSION The results were not successful in trying to meet the objectives of this experiment. This was due to the several errors on the materials used; however, a lot of things were observed and inferred. During the process of corrosion which is governed by reduction – oxidation reaction, the metal ions dissolve and the electrons transfer to another location where they are taken up by oxygen in this case. A mixture of hydrous iron oxides is then produced from the resulting hydroxide ions which react with iron (II). This resulting mixture of hydrous iron oxides is what is commonly known as rust.

In the zinc granule – iron nail combination, it was found out that the zinc was oxidized faster and more easily compared with iron thus making it the anode site and iron the cathode site. For the copper wire – iron nail combination, it was inferred that the iron nail “rusts” while the copper did not react with anything. With this, it can be concluded that the copper wire was the cathode site while the iron nail became the anode site.

In bimetallic corrosion, corrosion of one metal is accelerated when placed in contact with metal with a different and higher potential. A general statement can be made and concluded for bimetallic corrosion based on the results acquired: The metal which oxidizes easily and is the anode will be the one that will corrode.

REFERENCES

[1] Ricker R., Stoudt M., Dante J., Fink J., Beauchamp C., and Moffat T. Corrosion of Metals. Materials Science and Engineering Laboratory.
[2]Silberberg (2013). Principles of General Chemistry, 3rd edition.
[3]http://www.bssa.org.uk/topics.php?article= 89
[4] Electrochemistry 7: Electrochemical Corrosion,http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Electrochemistry_7%3A_Electrochemical_Corrosion
[5]Corrosion,http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Case_Studies/Corrosion

APPENDIX

Figure 6. Setup of the first part of the experiment (products of rusting)

Figure 7. Coiling of copper wire to the tip of the iron nail

Figure 7. Setup of the second part of the experiment (bimetallic corrosion)