Corrosion is the deterioration of physical properties of a material due to a reaction with its environment.
Corrosion is caused by different things, such as the cracking of a polymer due to exposure to sunlight and the oxidation of metals.
Lately there have been several water leaks in my house. One was due to an old water heater that has not been used for years. I must admit that the beautifully colored crusted areas on the pipes caught my eye. If you look close you can see dark blue crystals. While I was intrigued with the colors and the chemistry behind their production, most of the excitement was dampened by the plumber’s bill. But I did learn something new from him. Seems that the corrosion started because two metals were in contact with each other. I later discovered that this is called two metal corrosion or galvanic corrosion.
The term corrosion seems to be a catch all for stuff deteriorating because of chemical reactions. My water pipes were made of different kinds of metals–copper, zinc, and iron–thus all the different colors of galvanic corrosion.
Oxidation is the chemical combination with oxygen.
Iron generally corrodes by combining with oxygen forming a reddish-brown chemical commonly called rust.
The equation for the rusting (oxidation) of iron is:
4Fe + 3O 2 ——-> 2Fe2O3
Zinc also corrodes by oxidation. The white corrosion in the photo is zinc oxide, commonly called zinc white.
The equation for the oxidation of zinc is:
2Zn + O2 —–> 2ZnO
Copper corrosion, except for water pipes, is desired by artists. Copper sculptures placed outdoors as well as copper used for roofs corrode and form what is called patina.
The corrosion of copper involves oxidation plus a reaction of other environmental chemicals, such as water and carbon dioxide. The summation of copper corrosion producing copper carbonate (CuCO3) the blue-green color on the pipe in the picture is:
2 Cu (s) + O2+ H2O (g) + CO2 ? Cu(OH)2 + CuCO3 (s)
This is not a one step process. The copper first oxidizes forming cupric oxide (copper I oxide) . The water and carbon dioxide form carbonic acid (H2CO3), then the copper I oxide and carbonic acid react forming copper hydroxide and copper carbonate.
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