BEHAVIOR OF PASSIVATORS *

6.3 BEHAVIOR OF PASSIVATORS *

The same Flade potential is obtained whether iron is passivated by concentrated nitric acid or is anodically polarized in sulfuric acid, indicating that the passive fi lm is essentially the same in both instances. In fact, when iron is passivated by

immersion in solutions of passivators, whether chromates ( CrO 2 4 − ), nitrites ( NO 2 − ), molybdates ( MoO 2 4 − ), tungstates ( WO 4 2 − ), ferrates ( FeO 4 2 − ), or pertechne- tates ( TcO 4 − ), the corresponding Flade potentials are close to values obtained

* See also Chapter 17 .

BEHAVIOR OF PASSIVATORS

otherwise [10, 11] . Hence, it can be concluded that the passive fi lm on iron is essentially the same regardless of the passivation process. The amount of passive -

fi lm substance, as determined by coulometric and other measurements, is approx- imately 0.01 C/cm 2 in all cases. Passivation proceeds by an electrochemical mechanism, with some exceptions discussed later. The standard Flade potential of iron passivated by chromates ( φ F = 0 54 . V ) is less noble than for iron passivated by HNO 3 φ ( F = 0 63 . V ). One explanation [11] is that chromates adsorb on the passive fi lm more strongly than do nitrates, thereby reducing the overall free energy of the system and increasing the stability of the passive fi lm. Other passivators presumably adsorb similarly, but with dif- fering energies of adsorption.

Passivators are reduced at cathodic areas at a current density equivalent to

a true current density at anodic areas equaling or exceeding i critical (10 – 20 A/cm 2 ) for passivation of iron. The passivator is reduced over a large cathodic area of the metal surface to an extent not less than that necessary to form a chemically equivalent passive fi lm at small residual anodic areas. The small passive areas, in turn, adsorb passivator, thereby becoming noble to adjoining passive or non- passive areas, causing passivity to spread. When the passive fi lm is complete, it acts as cathode over its entirety, and further reduction of passivator proceeds at a much slower rate, equivalent to the rate of continuous passive fi lm break- down, or to i passive . Since breakdown is accelerated by presence of chlorides

and by elevated temperatures, consumption of passivator is also increased correspondingly.

Passivators as a group are inorganic oxidizing agents that have the unique property of reacting only slowly when in direct contact with iron, but they are reduced more rapidly by cathodic currents. For this reason, they can adsorb fi rst on the metal surface, with each site of adsorption adding to the cathodic area. The higher the concentration of passivator, the more readily it adsorbs, and the smaller the residual anodic areas become; this situation obviously favors increased anodic polarization and ultimately passivation. It requires about 0.5 – 2 h for the

passive fi lm to form completely when iron is immersed in 0.1% K 2 CrO 4 , with the shorter time being characteristic of aerated solution while the longer time is typical of deaerated solution [12, 13] .

6.3.1 Passivation of Iron by HNO 3

In nitric acid, the cathodic depolarizer (passivator) is nitrous acid, HNO 2 . This must form fi rst in suffi cient quantity by an initial rapid reaction of iron with HNO 3 . As nitrous acid accumulates, anodic current densities increase, eventually reaching i critical . Passivity is then achieved, and the corrosion rate falls to the com- paratively low value of about 2 gmd [14] .

If urea is added to concentrated HNO 3 , passivation is interrupted because urea reacts with nitrous acid in accord with

( NH 22 ) CO + 2 HNO 2 → 2 N 2 + CO 2 + 3 HO 2 (6.4)

PASSIVIT Y

thereby decreasing the nitrous acid concentration. The rate of reaction is, nevertheless, suffi ciently below that of HNO 2 production so that passivity can still occur, although periodic breakdown and formation of the passive fi lm usually result.

Hydrogen peroxide added to concentrated nitric acid also causes periodic breakdown and formation of passivity, probably by oxidizing HNO 2 to HNO 3 [14] . The peroxide, of itself, is not as effi cient a cathodic depolarizer as HNO 2 , and hence the passive fi lm in its presence can repair itself only when the moment- ary surface concentration of HNO 2 formed by reaction of iron with HNO 3 is suffi ciently high. After passivity is achieved, the surface concentration of HNO 2 is diminished by reaction with peroxide below that needed to maintain passivity, whereupon the cycle is repeated.

If iron is fi rst immersed in dilute chromate solution for several minutes, it remains passive in concentrated nitric acid without the initial reaction to form HNO 2 . The passive fi lm is preformed by chromate, and nitrous acid is no longer necessary as depolarizer to reach i critical , but is needed only in concentration suf- fi cient to maintain the fi lm already present.