Metallic corrosion
Basic principies
The most pertinent form of corrosion related to metallic biomaterials is aqueous corrosion .The occurs when electrochemical reactions take place on a metallic surface in an aqueous eletrolyte.There are always two reactions that occur:the anodic reaction,which yields metallic ions, for example,involving the oxidation of the metal to its salt:
M---M(n+) +n(electrons) (1)
And the cathodic reaction,in which electrons so generated are consumed.The precise cathodic reation will depend on the nature of the electrolyte ,but two of the most important in aqueous environments are the reduction of hydrogen:
2H+ +2e-——H2 (2)
And the reduction of dissolved oxygen :
O2 +4H+ +4e——2H2O (3)
In acidic solutions or :
O2+2H2O+4e-——4OH- (4)
in neutral or basic solutions.
In all corrosion processes,the rate of the anodic or oxidation reaction must equal the rate of the cathodic or reduction reaction.This is a basic principle of electrochemically based metallic corrosion.It also explains how variations in the local environment can affect the overall rate of corrosion by influencing either the anodic or cathodic reactions.The whole corrosion process can be arrested by preventing either of these reactions.
From a thermodynamic point of view ,first consider the anodic dissolution of a pure metal isolated in a solution of its salt. The metal consists of positive ions closely surrounded by free electrons.When the metal is placed in a solution,there will be a net dissolution of metal ions since the Gibbs free energy(△G)for the dissolution reaction is less than for the replacement reaction.This leaves the metal with a net negative charge,thus making is harder for the positive ions to leave the surface and increasing the △G for the replacement reaction.At this point,a dynamic equilibrium is reached and a potential difference will be set up across the charged double layer surrounding the metal.The potential difference will be characteristic of the metal and can be measured against a standard reference electrode.When this is done against a standard hydrogen electrode in a 1 N solution of its salt at 25℃,it is defined as the standard electrode potential for that metal (Table 1).