Weight loss corrosion of active metals

In this type of corrosion, metal ions arising as a result of the process in Eq. (2-21) migrate into the medium. Solid corrosion products formed in subsequent reactions have little effect on the corrosion rate. The anodic partial current-density-potential curve is a constant straight line (see Fig. 2.4). 

The protection potential can be evaluated kinetically for such cases [10,21]. It is assumed that the concentration of metal ions on the metal surface is Cq. The weight loss rate follows from the first law of diffusion  

If U is assumed to be the protection potential U, the velocity of the anodic partial reaction according to Eq. (2-21) is so fast that the concentration Cg is maintained. At the same time, Eq. (2-51) represents the highest value for Cg. Therefore it follows from Eqs. (2-51) and (2-52) that  

For Fe it follows from Eq. (2-530 that = -0.64 V. This value is 0.21 V more negative than the protection potential i/cu-cuso4 = -0.85 V which is adopted in practice (see Fig. 1-12) [22,23]. This more positive potential results because 5is actually greater and the cathodically generated surface films give additional protection [24-26]. 

Evaluation of the corrosion potential from Eq. (2-53) is inadmissible when the metal ion, for example, reacts further in strongly complexing solutions  

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Passive attack involving underdeposit corrosion tends to involve large system surface areas and, hence, accounts for the greatest amount of metal loss, by weight, in cooling water systems. Active attack tends to produce intense localized corrosion and, as such, a greater incidence of perforations. 

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

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