CRITICAL PITTING POTENTIAL

6.6 CRITICAL PITTING POTENTIAL

When iron is anodically polarized potentiostatically in 1 N H 2 SO 4 to which sodium chloride is added ( > 3 × 10 −4 moles/liter), the apparent transpassive region is shifted to lower potentials. However, instead of oxygen being evolved, the metal corrodes locally with formation of visible pits. Similarly, if 18 – 8 stainless steel is anodically polarized in 0.1 N NaCl within the lower passive region, the alloy

remains passive, analogous to its behavior in Na 2 SO 4 solution, no matter how long the time. But above a critical potential, called the critical pitting potential (CPP), there is a rapid increase in the current, accompanied by random formation

of pits (Fig. 6.8 ). The CPP for 18 – 8 moves toward less noble values as the Cl − concentration increases, and it moves toward more noble values with increasing pH and with lower temperatures [25] . The CPP is also moved in the noble direc-

tion by adding extraneous salts to the NaCl solution — for example, Na 2 SO 4 , NaNO 3 , and NaClO 4 (Fig. 6.8 ). If suffi cient extraneous salt is added to shift the CPP to a value more noble than the prevailing corrosion potential, pitting does not occur at all on exposure of the stainless steel to the aqueous salt mixture. The added salts under these conditions become effective inhibitors. Addition of

3% NaNO 3 to 10% FeCl 3 , for example, has prevented pitting of 18 – 8 stainless steel or any appreciable weight loss for a period of more than 25 years, whereas in the absence of NaNO 3 the alloy is observed to corrode by pitting within hours [26] . The 3% NaNO 3 addition moves the CPP to a value more noble than the open - circuit cathode potential or the reversible potential for Fe 3+

− + e → Fe 2+ .

Figure 6.8. Potentiostatic polarization curves for 18 – 8 stainless steel in 0.1 M NaCl showing increasingly noble values of the critical pitting potential with additions of Na 2 SO 4 , 25 ° C.

PASSIVIT Y

T A B L E 6.1. Critical Pitting Potentials in 0.1 N NaCl at 25 ° C

Potential (V, s . h . e .)

Reference

[29] Ni

Al a − 0.37

0.28 [30] Zr

0.46 [31] 18 – 8 stainless steel b 0.26 [30]

30% Cr – Fe 0.62 [30] 12% Cr – Fe

> 1.0 (1 N NaCl)

≈ 1.0 (1 N NaCl, 200 ° C)

a φ critical (V, s . h . e .) = − 0.124 log (Cl − ) − 0.504, where (Cl − ) is activity of Cl − . b φ critical (V, s . h . e .) =

− 0.088 log (Cl − ) + 0.168.

Along the same lines, Leckie [27] polarized 18 – 8 stainless steel at a constant 0.1 V below the CPP in 0.1 N NaCl for 14 weeks without observable pitting.

CPPs of several metals in 0.1 N NaCl are listed in Table 6.1 , values of which were derived from anodic polarization curves. Most of the data were obtained by allowing 5 minutes or more at a given potential and observing whether the resultant current increases or decreases with time. The CPP is the most noble potential for which the current decreases or remains constant; it is usually con- fi rmed by holding the potential at the critical value for 12 hours or more and observing absence of pits under a low - power microscope.

CPPs can be used to compare the relative susceptibility of different alloys to pitting corrosion; however, because CPPs are the result of short - term experi- ments, they should be used with caution in predicting immunity to pitting corro- sion in long - term service. Pitting corrosion has been reported in long - term exposure at potentials below the CPP [28] .

It is observed that an increase in the chromium content of stainless steels — and, to a lesser extent, an increase of nickel content — shifts the critical potential to more noble values, corresponding to increased resistance to pitting [30, 31] . The noble critical potentials for chromium and titanium [more noble than the

oxygen electrode potential in air (0.8 V) in accord with O 2 (0.2 atm) + 4H + (10 −7 ) + 4e − → 2H 2 O] indicate that these metals are not expected to undergo pitting corrosion in aerated saline media at normal temperatures. At elevated tempera- tures and high Cl − concentrations, however, the critical potentials become more

active, so that pitting of titanium, for example, is observed in concentrated hot CaCl 2 solution despite its immunity to pitting in seawater. The critical potential has been explained, from one point of view, as that value needed to build up an electrostatic fi eld within the passive or oxide fi lm suffi cient to induce Cl − penetration to the metal surface [34] . Other anions may also penetrate the oxide, depending on size and charge, contaminating the oxide and making it a better ionic conductor favoring oxide growth. Eventually, either

CRITIC AL PIT TING TEMPER ATURE

the oxide is undermined by condensation of migrating vacancies, or cations of the oxide undergo dissolution at the electrolyte interface; in both cases, pitting results. The induction period preceding pitting is related to the time required for

the supposed penetration of Cl − through the oxide fi lm.

Alternatively, the critical potential is explained in terms of competitive adsorption of Cl − with oxygen of the passive fi lm (adsorption theory). The metal has typically greater affi nity for oxygen than for Cl − , but, as the potential is made more noble, the concentration of Cl − ions at the metal surface increases, eventu- ally reaching a value that allows Cl − to displace adsorbed oxygen. The observed induction period is the time required for successful competitive adsorption at favored sites of the metal surface, as well as time for penetration of the passive

fi lm. Adsorbed Cl − , compared to adsorbed oxygen, results in lower anodic over- voltage for metal dissolution, which accounts for a higher rate of corrosion at any site where the exchange has taken place. Extraneous anions, such as NO 3 − or SO 2 4 − , which do not break down the passive fi lm or cause pitting, compete with Cl − for sites on the passive surface, making it necessary to shift the potential to

a still more noble value in order to increase Cl − concentration suffi cient for suc- cessful exchange with adsorbed oxygen. * Below the CPP, Cl − cannot displace adsorbed oxygen so long as the passive fi lm remains intact; hence, pitting is predicted not to occur. Should passivity break down because of factors other than those described [e.g., reduced oxygen or depolarizer concentration at a crevice (crevice corrosion), or cathodic polar- ization of local shielded areas], pitting could then initiate independent of whether the overall prevailing potential is above or below the critical value. But under conditions of uniform passivity for the entire metal surface, application of cathodic protection to avoid pitting corrosion need only shift the potential of the metal below the critical value. This is in contrast to the usual procedure of cathodic protection, which requires polarization of a metal to the much more active open - circuit anode potential.

The relation between minimum anion activity necessary to inhibit pitting of

18 – 8 stainless steel in a solution of given Cl − activity follows the relation log (Cl − ) = k log (anion) + const. The same relation applies to inhibition of pitting of alu- minum. The equation can be derived assuming that ions adsorb competitively in accord with the Freundlich adsorption isotherm [26] .