Wet Corrosion Process

The main features of corrosion of a divalent metal M in an aqueous solution containing oxygen are presented schematically. The corrosion process consists of an anodic and a cathodic reaction. In the anodic reaction (oxidation) the metal is dissolved and transferred to the solution as ions M2+. The cathodic reaction in the example is reduction of oxygen. It is seen that the process makes an electrical circuit without any accumulation of charges. The electrons released by the anodic reaction are conducted through the metal to the cathodic area where they are consumed in the cathodic reaction. A necessary condition for such a corrosion process is that the environment is a conducting liquid (an electrolyte) that is in contact with the metal. The electrical circuit is closed by ion conduction through the electrolyte. In accordance with the conditions this dissolution process is called wet corrosion, and the mechanism is typically electrochemical. In the example the metal ions M2+ are conducted towards OH–ions, and together they form a metal hydroxide that may be deposited on the metal surface. If, for instance, the metal is zinc and the liquid is water containing O2 but not CO2, the pattern in the figure is followed: Zn2+ ions join OH– and form Zn(OH)2. When CO2 is dissolved in the liquid a zinc carbonate is deposited. Corrosion of substances like iron and copper follow similar patterns with modifications: divalent iron oxide, Fe(OH)2, is not stable, thus with access of oxygen and water it oxidizes to a trivalent hydrated iron oxide, Fe2O3 · nH2O, or an iron hydroxide, Fe(OH)3, which also may be expressed as FeOOH + H2O. FeOOH is the ordinary red (or brown) rust. If the access of oxygen is strongly limited, Fe3O4 is formed instead of the trivalent corrosion products. Fe3O4 is black (without water) or green (with water). Divalent

copper hydroxide, Cu(OH)2, is not stable either and tends to be dehydrated to CuO Wet corrosion of a divalent metal M in an electrolyte containing oxygen. Reduction of oxygen is the dominating cathodic reaction in natural environments like seawater, fresh water, soil and the atmosphere. However, under certain

conditions there are also other important cathodic reactions: the hydrogen reaction 2H++2e–􀁯H2, reduction of carbonic acid (H2CO3) (in oil and gas production), reduction of metal ions etc.