ANODIC PROTECTION AND TRANSPASSIVITY

6.4 ANODIC PROTECTION AND TRANSPASSIVITY

The electrochemical nature of the passivation process is the basis on which anodic polarization leads to passivation. The anodic polarization that is required for passivation can be achieved by applying current or by increasing either the cathodic area or the cathodic reaction rate. High - carbon steels, for example —

which contain areas of cementite (Fe 3 C) that act as cathodes — are more readily passivated by concentrated nitric acid than is pure iron. For this reason, nitrating mixtures of sulfuric and nitric acids are best stored or shipped in drums of steel of the highest carbon content consistent with required mechanical properties. Similarly, stainless steels that may lose passivity in dilute sulfuric acid retain their corrosion resistance if alloyed with small amounts of more noble constituents of low hydrogen overpotential, or of low overpotential for cathodic reduction of dissolved oxygen, such as Pd, Pt, or Cu [15] . The polarization diagram correspond- ing to increased passivity by low overpotential cathodes is shown in Fig. 6.5 , and the corresponding improved corrosion resistance to sulfuric acid is shown in Fig. 6.6 .

Because titanium, unlike 18 – 8 stainless steel, has a low critical current density for passivity in chlorides as well as in sulfates, passivity in boiling 10% HCl is made possible by alloying titanium with 0.1% Pd or Pt [16] . Pure titanium, on the other hand, corrodes in the same acid at very high rates (see Fig. 25.2 , Section

25.3 ). Based on the same principle, the corrosion resistance of metals and alloys with polarization curves having active – passive transitions can be greatly improved by an impressed anodic current initially equal to or greater than the critical current for passivity. The potential of the metal moves into the passive region

ANODIC PROTEC TION AND TR ANSPASSIVIT Y

Figure 6.5. Polarization diagram for metal that is either active or passive, depending on overvoltage of cathodic areas (differing cathodic reaction rates).

Figure 6.6. Corrosion rates in sulfuric acid of 18 – 8 stainless steel alloyed with copper or pal- ladium, 360 - h test, 20 ° C [15] .

(Fig. 6.1 ), so that the current density and accompanying corrosion rate corre- spond to the low value of i passive . This process is called anodic protection (see

Section 13.9 ) because the current fl ow 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 Defi nition

1, Section 6.1 ).

92 PASSIVIT Y

If the cathodic polarization curves of Fig. 6.5 intersect the anodic curve at a still more noble potential, within the transpassive region, the corrosion rate of, for example, stainless steel, is greatly increased over the corrosion rate at less noble potentials within the passive region, and the corrosion products become

Cr O 2 2 − 7 and Fe 3+ . Transpassivity occurs not only with stainless steels, but also with chromium, for which the potential for the reaction

Cr O 2 2 7 − + 14 H + + 12 e − → 2 Cr + 7 HO 2 = . 0 30 V (6.5) is less noble than the potential for the oxygen evolution reaction

e 2 − + 4 + 4 → 2 HO 2 = . 1 23 V (6.6) Appreciable corrosion in the transpassive region does not occur for iron in sul-

furic acid (oxygen evolution is the primary reaction), but increased corrosion does occur in alkaline solutions that favor formation of ferrate, FeO 2 4 − . Transpas- sivity accounts for an observed increase of corrosion rate with time for 18 – 8 stainless steels in boiling concentrated nitric acid, in which corrosion products

accumulate, in particular Cr O 2 2 − 7 , and move the corrosion potential into the transpassive region.