Classification of Corrosion Materials Engineering Dr Lubna Ghalib

Classification of Corrosion Materials Engineering Dr. Lubna Ghalib

Classification of Corrosion may be classified in different ways Corrosion Wet Corrosion Dry Corrosion Room Temperature Corrosion High Temperature Corrosion

Wet & Dry Corrosion. Wet / aqueous corrosion: is the major form of corrosion which occurs at or near room temperature and in the presence of water. This is usually involves aqueous solution or electrolytes and accounts for the greatest amount of corrosion by far. Wet Corrosion is a chemical reaction of a metal with the atmosphere in presence water like: Fe + H 2 O No Corrosion Fe + H 2 O + Oxygen Fe (OH)2 Or Fe (OH)3 Chlorides may be wet Cl 2 H 2 O + Cl 2 HCl + HOCl

Wet & Dry Corrosion. Salts like Na. Cl , Mg. Cl , Ca. Cl 2 , …. . etc. Na. Cl + H 2 O Na. OH + HCl Corrosion by H 2 O + O 2 at room temperature is about 1/20 to that of H 2 O + chlorides Dry / gaseous corrosion is significant mainly at high temperatures and it occurs in absence of a liquid phase or an above the dew point of the environment. Vapors and gases are usually the corrodes. Dry corrosion is most often associated with high temperatures like: Oxidation of metals

Wet & Dry Corrosion. Fe + O 2 Fe 2 O 3, Fe 3 O 4, Fe. O Dry chlorine is practically non-corrosive to ordinary steel, but moist chlorine dissolved in water, is extremely corrosive and attacks most of common metals and alloys. The reverse is true for Titanium – dry chlorine gas is more corrosive than wet chlorine. Sulfides H 2 S it is very highly corrosive substance. Dry – forms Iron sulfide H 2 S Wet – forms Oxy – sulfides (H 2 S + H 2 O ) + hydrogen embittlements

Wet & Dry Corrosion. Differences between dry and wet corrosion Wet corrosion Dry corrosion Corrosion occurs in presence of conducting medium. Corrosion occurs in the absence of moisture. It involves formation of electrochemical cells. It involves direct attack of chemicals on the metal surface. It is a rapid process The process is slow. Corrosion occurs at anode but rust is deposited at cathode. Corrosion products are produced at the site of corrosion It depends on the size of the anodic part of metal. The process of corrosion is uniform.