Section 17.6 CorrosionBy: Alex Socie

Introduction• Corrosion is, in a simplified view, the return of

metals to their original state through oxidation• Table 17.1, which covers all metals except gold

shows that oxidation is a common characteristic of metals

The oxidation of most metals is spontaneous due to the equations in table 17.1 and that when they are flipped and combined with an oxygen half reaction they will have a positive cell potential value

Oxidation Cutoff• The one issue with any oxidation that occurs with

metals is that when the first layer of oxide coating forms, the inner metal is then protected from any further corrosion

• Fun fact: If this did not occur, and with aluminum having a reduction potential of -1.7 V, modern airplanes would dissolve in a rainstorm, but aluminum forms a layer of aluminum oxide that prevents this occurence

Steel• Although steel is used in many buildings and

structures, its characteristics create a problem that cannot be solved with the structure that steel maintains• When steel is oxidized, this layer of oxidation

can scale off which exposes more metal to corrosion• The iron in the steel undergoes an

electrochemical reaction instead of a direct oxidation process

Steel’s Fatal Flaw• Iron in the steel is can rust easily, and this leads

to many issues with the steel structure• Anodic regions (easily oxidized) and cathodic

regions (easily reduced) both play a role in this reaction• The Iron oxidizes, then the electrons that are

released react with oxygen in the cathodic region, and when the steel moves along the structure due to moisture, which is similar to a salt bridge, it reacts with the oxygen to form rust

Prevention• The most common prevention for corrosion is by

the application of a metal coating that has a higher cell potential than the metal it is coating, such as zinc onto iron, where zinc acts as a “sacrificial” metal here• Alloying also protects the metal as well, as it

forms a layer of oxidation as well• Cathodic protection is another useful protector,

as metals that react more readily with oxygen and other materials are placed on the metal itself to be used up