Fundamentals of Metallic Corrosion in Fresh Water   -   2

Corrosion in fresh or salt water is always the result of an electrochemical reaction. To one who is not a chemist, the ten-n "electrochemical reaction" seems to denote a complicated phenomenon. As used to describe the corrosion of metals, the application of electrochemical theory allows one to separate the relatively complicated corrosion reaction into two simple parts: the anode reaction where the metal is oxidized, and the cathode reaction where the oxidizer is reduced. Neither of these reactions will occur without the other. Writing them as separate reactions is done only to describe how the overall corrosion reaction takes place. I hope that, by describing the chemical reactions that occur in a galvanic cell, I can make this clear.

A galvanic cell results when two dissimilar metals are placed in an electrolyte. An electrolyte is a solution that contains ions (atoms or small groups of atoms that carry an electric charge) so that it will conduct electricity. Pure water is a weak electrolyte and is a fair insulator or very poor conductor of electricity. It contains approximately 1/10,000,000 of a gram of hydrogen ions (H⁺) per liter and 17/10,000,000 of a gram of hydroxyl ions (OH^-) per liter. Since a hydroxyl ion weighs 17 times as much as a hydrogen ion, the two are chemically equivalent and there is no net charge. Sea water is a strong electrolyte that contains almost 4% common salt that ionizes into sodium ions (Na⁺) and chloride ions (Cl^-).

Figure 1 shows a galvanic cell, consisting of two metal plates, one of which is steel and the other copper. As soon as the switch on this galvanic cell is closed, iron starts to corrode.

Fe^o = F⁺⁺ + 2e^-

Neutral iron atoms become ferrous ions with the liberation of two electrons. These two electrons pass through the ammeter to the copper electrode where they react with dissolved oxygen in the water to form hydroxide ions:

4e^- + 0₂ + 2H₂O = 40H^-

Electric current flow through an electrolyte is by ionic migration. Positively charged ions (Na⁺, Ca⁺⁺ , etc.) flow to the cathode (more noble metal) while the negative ions (Cl^-, SO₄^-, etc.) flow to the anode. Electron flow through water cannot occur. Since electrons are involved, these corrosive reactions must occur at the metal-electrolyte interface.

When the switch shown in Figure 1 is closed, a short-circuited galvanic cell is created (assuming negligible resistance in the external circuit). Thus the entire voltage drop occurs inside the cell. It can be shown with suitable equipment that most of this voltage drop occurs at the surface of the copper cathode.

The current flow is greatest initially. It decreases rapidly at first, more slowly later on. This decrease is caused by I 'polarization". Part of polarization results from depletion of the oxygen molecules in the electrolyte immediately adjacent to the cathode surface. The rest of it is caused by some phenomenon for which there is no completely satisfactory explanation. Some authorities attribute this phenomenon to the formation of various films.

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