Immunity region

Figure 28-11 Genetic and physical map of the X phage genome. After Szybalski. See Honigman et al.255 for a more detailed diagram of the immunity region. The gene for the lambda repressor is labeled C[.

The following regions are clearly outHned in F. 2.16 (i) immunity, in which the metal is considered to be immune from corrosion attack (ii) corrosion region, in which the metal corrodes and forms soluble species and (iii) passive region, in which the metal is coated with oxide or hydroxide. By decreasing the potential (cathodic protection), the metal can move from the active corrosion region to the immunity region. Zinc, because of its equiHbrium potential, is used as a sacrificial anode to protect the iron from corrosion. 

For stainless steels in neutral NaCl solutions there is no aetive potential range like that shown in Fig. 5.3. The alloys are passive all the way down from the region of localized corrosion to the immune region. 

The main principle of CP is to impress an external current on the material, which forces the electrode potential down to the immune region, or, for protection against localized corrosion, below a protection potential. In other words, the material is... 

Immunity region In the conditions of potential and pH of that region a metal is considered to be totally immune from corrosion attack and safe to use. Cathodic protection may be used to bring the potential of a metal closer to that region by forcing a cathodic shift, as shown for aluminum by the domain specified in Fig. 4.14 (or -1.0 to 1.2 V vs. CCSRE). 

The presence of a relatively large immunity region in Fig. 4.15 and Fig. 4.16, where corrosion products are solid and possibly protective, indicates that iron may corrode much less under these potential/ pFl conditions. 

There are three basic ways by which corrosion is prevented (a) by physical isolation of the metal from the oxidant, b) by reducing the rate of the cathodic reaction using any of the methods known in electrodics and (c) by maintaining the metal at a potential in the immunity region of the Pourbaix diagram. 

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