Passive current density definition

The etch rate of the (111) surface, although much smaller than those of the (100) and (110) planes, shows definite values, in the range of 2-10 A/s in KOH solutions. It is still much larger than the dissolution current density on a passivated surface in KOH (a dissolution rate of 2-10 A/s is equivalent to a current density of several milliamperes per square centimeter). In alkaline solutions, the dissolution rate of silicon oxide is less than 0.01 A/s (see Chapter 4), which is several orders of magnitude smaller than the etch rates of a (111) surface. Thus, it is unlikely that the silicon surface of any orientation is covered by Si02 during etching. 

Fig. 6.1), so that the current density and accompanying corrosion rate correspond to the low value of (passive- This process is called anodic protection (see Section 13.9) because the current flow is in the direction opposite to that which is used in cathodic protection. Whereas cathodic protection can, in principle, be applied to both passive and nonpassive metals, anodic protection is applicable only to metals that can be passivated when anodically polarized (see Definition 1, Section 6.1). 

The second holds that metals passive by Definition 1 are covered by a chemisorbed film—for example, of oxygen. Such a layer displaces the normally adsorbed H2O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M -1- ze. Even less than a monolayer on the surface is... 

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