-2-2.HOW CORROSION OCCURSTo select the method of corrosion control, it is very important to understand how corrosion occurs.The corrosion of steel proceeds in the presence of oxygen and water, and is restrained by removing one of them.The corrosion of steel in water, as illustrated in Figs. 1 and 2, can generally be attributed to corrosion in the air, because on a steel surface in a humid atmosphere, there is a thin water layer formed by the adsorption of water molecules in the air. Each point on a steel surface has its own tendency to dissolve in water. This is called electrolytic solution pressure.This tendency depends on the condition of the surface and the water contacting each point. At the point of high dissolution, metallic iron dissolves in the form of ferrous ions. Since the water must remain electrically neutral, hydrogen ions are deposited as a thin layer of hydrogen along the low dissolution surface of the steel. As a result, an electric circuit is formed between the high and low points of dissolution. These points and the circuits are called anodes, cathodes and corrosion cells, respectively.The existence of a layer of hydrogen on the cathodes obstructs the further progress of the reaction, but free oxygen in the water removes the obstruction as it combines with hydrogen. The ferrous ions dissolved in the water combine with the hydroxy ions and finally precipitate on the suface of the steel as rust after being oxidized to form ferric ions. The mill scale on steel has a comparatively low dissolution so it acts as a cathode and forms a strong corrosion cell with anodic parts in the vicinty. This big difference in the electrolytic solution pressure between the electrodes prompts the corrosion reation.The general methods of controlling corrosion are as follows:1)Minimize the difference in electrolytic solution pressure between anodes and cathodes by making the steel surface uniform.2)Use a coating to restrain the supply of water and oxygen to the steel surface.3)Restrain the corrosion reaction by the adsorption of inhibitors in the anodes of cathodes. The inhibitors are intentionally added to the water or supplied by the dissolution of anticorrosive pigments in the coating material.AnodeCathodee-FeFe++Fe+++2OH-2H+2H+12O2H2O2HFe(OH)2H2O(rust)OH-+H+2Fe(OH)32Fe(OH)2+12O2+H2OH2OH++OH-RustRust tuberculeNeutral water high in dissolved O2Fe--Fe--Fe--FeOPitSteel(Anode)2e-2e-MillscaleOH-H+HOFeOHO(Cathode)H+OH-plus oxygen and waterFeO2e-Fig. 1 Corrosive Action Fig. 2 Corrosion at Break in Mill ScaleSteel Structures Painting Manual,Steel Structures Painting Council