Corrosion is the deterioration suffered by certain materials by the action of chemical agents, often present in the environment. We are going to study how iron behaves, due to its technological importance.
Reactions with metal pairs
Generally, the phenomenon of corrosion is an electrochemical process.
The oxidation of the metal and the reduction of the corrosive agent take place in different zones on the surface of the metal. In our essay, we are going to use reagents that color the oxidation and reduction zones, thus illustrating the nature of the corrosion process.
The iron is going to oxidize to Fe+2, a fact that we are going to use to color the oxidation zone blue, with the help of K3 [Fe(CN)6]. The corrosive agent will be the oxygen in the air dissolved in the water, which will be reduced to OH-. If no acid is present in the reaction medium, the pH of the water will then become increasingly basic. We are going to take advantage of this fact to mark the reduction zones with a violet color using phenolphthalein, which presents this color in a basic medium, but not in an acid or neutral medium.
We also need the color to remain within the areas where it occurs; that is to say, that it does not spread. For this we are going to use a gel that we prepare with agar-agar.
Phases
1. Preparation of the agar-agar gel:
In a beaker we add 0.5 g. of agar-agar and about 75 ml of distilled water. Heat with the burner and shake constantly until the agar-agar is dispersed (this is achieved when we reach the boil and the solution is transparent). Once the burner is off, add about ten drops of 0.1M potassium ferricyanide and about 5 drops of 0.1% phenolphthalein. Shake vigorously and allow the solution to cool.
2. Preparation of iron tips:
We are going to prepare four iron nails that we will arrange in two Petri dishes; in a similar way as shown in the figure.
We must clean the surface of the nails with a piece of sandpaper, and then remove the scale with a piece of filter paper.
The metallic pairs that we have chosen are Fe-Cu and Fe-Zn. To form them, we wind a copper wire around one nail, and a zinc strip around the other, so that iron and metal are in contact (it is necessary that the electrons can pass from the metal to the iron).
3. Preparation of the plates:
Once the agar-agar solution is cold and still fluid, we pour it over the Petri dishes, ensuring that all metals are completely covered. The colorations appear in a short time.
Results
Plate 1:
Iron oxidizes preferentially in areas that have undergone mechanization, in these areas the solution turns blue, due to the following reaction:
Fe Fe+2
Fe+2 + K3[Fe(CN)6] Fe3[Fe(CN)6]2 the color blue
Hydroxyl ions are generated in the reduction zones, which are revealed by the phenolphthalein turning pink-violet:
2H2O + O2 4OH-
Plate 2:
The zinc coated tip does not rust, as it is protected by this metal, which is more active than iron and is the one that rusts:
Zn Zn+2
As no ions are produced Fe+2 the color blue does not appear.
In the areas of reduction we will see the pink color of the basic medium with phenolphthalein.
In the copper coated tip we see that the iron oxidizes much more than when it is not coated, since the copper is less active and forces the iron to oxidize. The oxidation zones are located by the blue color and the reduction zones by the pink color.
Issues
- What mission does agar-agar have?
- Which compound added to the solution produces the blue coloration?
- What compound of the additives produces the violet coloration?
- Write the oxidation reactions of metals and the reduction reactions of O2 to OH-.
- The ends of the tips and the curved part appear invariably blue. Does the cold work of metal favor, attenuate or not affect the corrosion of iron?
- A Fe/Cu union: a) favors corrosion, b) attenuates it, c) does not affect it. What about a Fe/Zn junction?
