In their model, electrons from the metal traverse the growing oxide, forming anions with the oxygen on the surface. The growth of oxide films on metals at ambient pressures and low temperatures has been addressed in detail by Cabrera and Mott. (Discover self-healing metal oxides in A Look at Self-Healing Metal Oxides as a Corrosion Prevention Method.) If this oxide thin film is damaged it can be reformed ( re-rust) immediately in most environments and continue to protect the aluminum from corrosion, but when the film is removed or damaged under conditions such that self-repair cannot occur, then corrosion occurs. In aqueous conditions it forms an acid mist that breaks down the protective layer, with the final result being various forms of corrosion such as crevice corrosion, intergranular corrosion, galvanic corrosion, etc. When this layer comes in contact with chlorine it produces a reaction that releases hydrogen gas, which creates dialuminum hexachloride (AlCl 3). Pourbaix diagram for aluminum showing the regions of corrosion, immunity and passivation of aluminum at 25☌.īeyond the limits of its passive range (pH range of 4 to 9), aluminum corrodes in aqueous solutions because its oxides are soluble in many acids and bases. Because the form of aluminum oxide produced depends on corrosion conditions, its identification is sometime useful in establishing the cause of corrosion.įigure 2. At lower temperatures, the predominant forms produced by corrosion are bayerite and aluminum oxide trihydroxide Al(OH) 3. At higher temperatures the oxide film may consist of a thin amorphous barrier layer next to the aluminum and a thicker crystalline layer (boehmite, aluminum oxide hydroxide AlOOH) next to the barrier layer. As the diagram shows, aluminum is passive only in the pH range of approximately 4 to 9 the limits depending on the temperature. The conditions for thermodynamic stability of the oxide film are expressed by the Pourbaix diagram (Figure 2). Schematic of the passive oxide film that forms on aluminum. Covering the barrier layer is a thicker, more permeable outer layer of hydrated oxide.įigure 1. The inner oxide layer closer to the metal base is a compact amorphous barrier layer of a thickness determined only by the temperature of the environment the limiting barrier thickness is the same in oxygen, dry air or moist air. In normal atmospheric conditions the oxide film is composed of two layers (Figure 1). Above 480☌ (896☏), Al 2O 3 (alumina or aluminum oxide) is the most stable product. From room temperature to 280☌ (536☏), Al(OH) 3 is the most stable product, while from 280-480☌ (526-896☏), AlO(OH) is most stable. The following are possible reactions of aluminum with water:Īll of these reactions are thermodynamically favorable from room temperature through the melting point of aluminum at 660☌ (1,220☏).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |