Mechanism of Passivation

17.2.1 Mechanism of Passivation

The theory of passivators has already been discussed in part, and should be referred to again, in Section 6.3 . Passivators in contact with a metal surface act as depolarizers, initiating high current densities that exceed i critical for passivation

at residual anodic areas. Only those ions can serve as passivators that have both an oxidizing capacity in the thermodynamic sense (noble oxidation – reduction potential) and that are readily reduced (shallow cathodic polarization curve), as

illustrated in Fig. 17.1 . Hence, SO 2 4 − or ClO − 4 ions are not passivators for iron, because they are not readily reduced, nor are NO − 3 ions compared to NO − 2 , because nitrates are reduced less rapidly than are nitrites, with the former reduc- ing too sluggishly to achieve the required high value of i critical . The extent of chemical reduction on initial contact of a passivator with metal, according to this viewpoint, must be at least chemically equivalent to the amount of passive fi lm

Figure 17.1. Polarization curves that show effect of passivator concentration on corrosion of iron. An oxidizing substance that reduces sluggishly does not induce passivity (dotted cathodic polarization curve) (schematic).

PASSIVATORS

formed as a result of such reduction. For the passive fi lm on iron, as has been discussed earlier, this is on the order of 0.01 C/cm 2 (100 C/m 2 ) of apparent surface area. The total quantity of equivalents of chemically reduced chromate is found to be of this order, and it is probably also the same for other passivators acting on iron. The amount of chromate reduced in the passivation process was arrived at from measurements [1] of residual radioactivity of a washed iron surface after

exposure to a chromate solution containing radioactive 51 Cr. The following reac- tion applies, assuming that all reduced chromate (or dichromate) remains on the

metal surface as adsorbed Cr 3+ or as hydrated Cr 2 O 3 :

=− 406 kJ Residual radioactivity accounts for 3

Cr O 2 2 7 − + 8 H + + Fe → 2 Cr 3 surface + + 4 HOOO 2 + 2 ⋅ ads on Fe

× 10 −7 eq or 0.015 C passive - fi lm substance/cm 2 ). The equation assumes an adsorbed passive - fi lm structure, but the same reasoning applies whatever the structure. Reduction of passivator continues at a low rate after passivity is achieved, equivalent in the absence of dissolved oxygen to the value of i passive < 0.3 μ A/cm [ 2 ( < 3 mA/m 2 )] based on observed corrosion rates of iron in chromate solutions. Iron oxide and chromate reduction products slowly accumulate. The rate of reduction increases with factors that increase i passive , such as higher H + activity,

× 10 16 Cr atoms/cm 2 (1.5

higher temperatures, and the presence of Cl − . It is found, in practice, that less chromate is consumed as exposure time continues, consistent with i passive that also decreases with time.

For optimum inhibition, the concentration of passivator must exceed a certain critical value. Below this concentration, passivators behave as active depolarizers and increase the corrosion rate at localized areas, such as pits. Lower concentrations of passivator correspond to more active values of the oxidation –

reduction potential, and eventually the cathodic polarization curve intersects the anodic curve in the active region instead of in the passive region alone (Fig.

17.1 ). The critical concentration for CrO 2 − , NO − , MoO 2 − , or WO 2 − is about 10 −3 4 2 4 4 to

10 −4 M [2 – 5] . A concentration of 10 −3 M Na 2 CrO 4 is equivalent to 0.016% or 160 ppm. Chloride ions and elevated temperatures increase i critical as well as i passive , thereby raising the critical passivator concentration to higher values as well. At

70 – 90 ° C, for example, the critical concentration of CrO 2 4 − and NO − 2 is about

10 −2 M [4, 5] . Should the passivator concentration fall below the critical value in stagnant areas (e.g., at threads of a pipe or in crevices), the active potential at such areas in galvanic contact with passive areas elsewhere of noble potential promotes localized corrosion, or pitting, at the active areas through the action of passive – active cells. For this reason, it is important to maintain the concentration of passivators above the critical value at all portions of the inhibited system by stirring, rapid fl ow rates, and avoiding crevices and surface fi lms of grease and dirt. Since consumption of passivators increases in the presence of chloride and sulfate ions, it is also essential to maintain as low a concentration of these ions as possible.

306 INHIBITORS AND PASSIVATORS

Some substances indirectly facilitate passivation of iron (and probably of some other metals as well) by making conditions more favorable for adsorption of oxygen. In this category are alkaline compounds (e.g., NaOH, Na 3 PO 4 , Na 2 O · n SiO 2 , Na 2 B 4 O 7 ). These are all nonoxidizing substances requiring dis- solved oxygen in order to inhibit corrosion; hence, oxygen is properly considered the passivating substance. The mechanism of passivation is similar to that

described in Sections 7.2.1.3 and 7.2.3 . High concentrations of OH − displace H adsorbed on the metal surface, thereby decreasing the probability of a reaction between dissolved O 2 and adsorbed H. The excess oxygen is then available to adsorb instead, producing passivity. In addition to passive fi lms of this kind, protection is supplemented by diffusion - barrier fi lms of iron silicate, iron phos- phate, and so on.

Passivation of iron by molybdates and tungstates, both of which inhibit in the near - neutral pH range, requires dissolved oxygen [6] , contrary to the situa- tion for chromates and nitrites. In this case, dissolved oxygen may help create just enough additional cathodic area to ensure anodic passivation of the remain-

ing restricted anode surface at the prevailing rate of reduction of MoO 2 4 − or of WO 2 4 − , whereas in the absence of O 2 , i critical is not achieved. Sodium benzoate [6, 7] (C 6 H 5 COONa), sodium cinnamate [8] (C 6 H 5 · CH · CH · COONa), and sodium polyphosphate [9, 10] (NaPO 3 ) n (Fig. 17.2 ) are further examples of nonoxidizing compounds that effectively passivate iron in the near - neutral range, apparently through facilitating the adsorption of dissolved oxygen. As little as 5 × 10 −4 M sodium benzoate (0.007%) effectively inhibits steel in aerated distilled water [11] , but inhibition is not observed in deaerated water. The steady - state corrosion rate of iron in aerated 0.01 M (0.14%) sodium benzo- ate, pH 6.8, is only 0.001 gmd, whereas in deaerated solution the rate is 0.073 gmd. Inhibition occurs only above about pH 5.5; below this value the hydrogen evolu- tion reaction, for which benzoate ions have no inhibiting effect, presumably becomes dominant, and the passive fi lm of oxygen is destroyed.

Gatos [12] found that optimum inhibition of steel in water of pH 7.5 contain- ing 17 ppm NaCl occurred at and above 0.05% sodium benzoate or 0.2% sodium cinnamate. By using radioactive 14 C as tracer, only 0.07, 0.12, and 0.16 mono- molecular layer of benzoate [0.25 nm 2 , roughness factor 3] was found on a steel surface exposed 24 h to 0.1, 0.3, and 0.5% sodium benzoate solutions, respectively, and inhibition was observed in all the solutions. Clearly, a very small amount of benzoate on the metal surface increases adsorption of oxygen, or, alternatively, decreases reduction of oxygen at cathodic areas. This effect is specifi c to the cathodic areas of iron because iron continues to corrode in 0.5% sodium benzo- ate when coupled to gold, on which the reduction of oxygen is apparently not retarded.

The mechanism of inhibition in the case of sodium polyphosphate solutions may depend in part on the ability of polyphosphates to interfere with oxygen reduction on iron surfaces, making it easier for dissolved oxygen to adsorb and, thereby, to induce passivity. Other factors enter as well; there is, for example, evidence of protective fi lm formation of the diffusion - barrier type on cathodic

PASSIVATORS

Figure 17.2. Effect of oxygen concentration on sodium polyphosphate as a corrosion inhibi- tor of iron showing benefi cial effect of dissolved O 2 and Ca 2+ , 48 - h test, 25 ° C [9] . ( Reproduced with permission. Copyright 1955, The Electrochemical Society .)

areas, and such diffusion - barrier fi lms probably account for the observed inhibi- tion of steel exposed to as high as 2.5% NaCl solutions containing several hundred parts per million calcium polyphosphate [13] . At low concentration of dissolved oxygen, corrosion of iron is accelerated by sodium polyphosphate because of its metal - ion - complexing properties (Fig. 17.2 ). Calcium, iron, and zinc polyphos- phates are better inhibitors than the sodium compound.

In line with the theory of passivators just described, transition metals are those expected and found to be inhibited best by passivators; their anodic polar- ization curves have the shape shown in Fig. 17.1 , allowing passivity to be estab- lished and then maintained at low current densities. A lesser degree of inhibition can be obtained with the nontransition metals, such as Mg, Cu, Zn, and Pb, using, for example, chromates. Protection of these metals apparently results largely from formation of relatively thick diffusion - barrier fi lms of insoluble metal

308 INHIBITORS AND PASSIVATORS

T A B L E 17.1. Effect of Chromate Concentration, Chlorides, and Temperature on Corrosion of Mild Steel [14]

Velocity of Specimen: 0.37 m/s; 14 - day tests Na 2 Cr 2 O 7 2 · 2H O (g/liter) →

0 0.1 0.5 1.0 % NaCl

Temperature ( ° C) Corrosion Rate (mm/y)

0.441 a 0.131 a 0.007

0.178 a 0.120 a 0.062

0.014 0.034 a Pitted.

chromates mixed with oxides. There is also the possibility that adsorption of CrO 2 4 − on the metal surface contributes in some degree to the lower reaction rate by decreasing the exchange current density for the reaction M → M 2+

+ 2e − . An inhibiting mechanism similar to that for nontransition metals in contact with passivators probably also applies to steel in concentrated refrigerating brines (NaCl or CaCl 2 ) to which chromates are added as inhibitors (approximately

1.5 – 3.0 g Na 2 Cr 2 O 7 /liter adjusted with NaOH to form CrO 2 4 − ). In the presence of so large a Cl − concentration, passivity of the kind discussed under Defi nition 1 (Section 6.1 ) does not take place. The reduction in corrosion rate is not as pro- nounced as when chlorides are absent [14] (see Table 17.1 ), and any reduction that occurs apparently results from formation of a surface diffusion barrier of chromate reduction products and iron oxides. Chromates are not adequate inhib- itors for the hot concentrated brine solutions that, in the past, were sometimes mistakenly proposed as antifreeze solutions for engine cooling systems.