Electrochemistry Lesson 4 Corrosion

Corrosion Corrosion is the deterioration of a metal as a result of slow oxidation. Most metals exist as compounds called minerals, and without protection, most metals are oxidized by their environment. For example, copper roofs on the parliament buildings start off shiny and red-orange coloured. Eventually the copper is corroded with day to day weather (oxygen, moisture, pollution), and a green layer of copper compounds form on top of the copper metal below. This layer is sometimes called a patina, and it forms a tough protective coating that allows copper roofs to last well over 50 years. 2 Cu (s) + O2 (g) → 2 CuO (s)

Corrosion (continued) Zinc and aluminum also form protective coatings when they oxidize, and this is why they are more corrosion-resistant than iron, despite being above iron in the activity series. 2 Zn (s) + O2 (g) → 2 ZnO (s) 4 Al (s) + 3 O2 (g) → 2 Al2O3 (s) Iron corrodes quite easily, and the reddish-brown flakey material that is produced is called rust. Unfortunately, rust does not stick to the metal beneath it, so flakes off, and new rust forms on the iron below. This continuous cycle “eats away” at the iron until nothing is left. Fe (s) + O2 (g) → Fe2O3 (s)

Factors That Affect the Rate of Corrosion Moisture: corrosion cannot occur without water, since it is a reactant. For this reason, steel objects in dry desert climates last much longer than they do in Ontario. In fact, a relative humidity of at least 40% is required for corrosion to take place. Electrolytes, like road salt, do not cause corrosion, but speed up corrosion as the sodium and chloride ions improve the electrical conductivity of water. The combination of road salt and salt-water spray off the ocean makes the corrosion of cars worse in the Maritime provinces than in many other parts of Canada. Other factors such as contact with less reactive metals, along with mechanical stresses such as bending, shaping, or cutting metals can also speed up the corrosion process. Protective coatings like rust-inhibiting paint or plastic can reduce the effects of corrosion.