THEORIES OF PASSIVITY

6.5 THEORIES OF PASSIVITY

There are two commonly expressed points of view regarding the composition and structure of the passive fi lm. The fi rst holds that the passive fi lm (Defi nition

1 or 2) is always a diffusion - barrier layer of reaction products — for example, metal oxide or other compound that separates metal from its environment and that decreases the reaction rate. This theory is sometimes referred to as the oxide - fi lm theory.

The second holds that metals passive by Defi nition 1 are covered by a che- misorbed fi lm — for example, of oxygen. Such a layer displaces the normally adsorbed H 2 O molecules and decreases the anodic dissolution rate involving hydration of metal ions. Expressed another way, adsorbed oxygen decreases the exchange current density (increases anodic overvoltage) corresponding to the overall reaction M → M z+

− + ze . Even less than a monolayer on the surface is observed to have a passivating effect [17] ; hence, it is suggested that the fi lm

cannot act primarily as a diffusion - barrier layer. This second point of view is called the adsorption theory of passivity.

There is no question on either viewpoint that a diffusion - barrier fi lm is the basis of passivity of many metals that are passive only by Defi nition 2. Examples of protective fi lms that isolate the metal from its environment are (a) a visible

lead sulfate fi lm on lead immersed in H 2 SO 4 and (b) an iron fl uoride fi lm on steel immersed in aqueous HF.

THEORIES OF PASSIVIT Y

For metals that are passive by Defi nition 1, based on marked anodic polar- ization, the fi lms are usually invisible, about 2 to 3 nm thick. Metals and alloys in this category have been the source of extended debate and discussion on the mechanism of passivity over the past 150 years. If the surface is abraded, local high temperatures generated at the surface produce a detectable oxide, but this is not the passive fi lm.

Low - energy electron - diffraction (LEED) techniques are used to detect adsorbed fi lms, including those responsible for passivity. Surface analytical tech- niques that can be used to study fi lms on metals include Auger electron spec- troscopy (AES), X - ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), and others [18, 19] . Using these advanced techniques, evi- dence has been developed to show that the passive fi lm formed on iron in borate

solution is an anhydrous, crystalline spinel with a “ γ - Fe 2 O 3 /Fe 3 O 4 - like ” structure [19] . The adsorption theory derives support from the fact that most of the metals that are passive by Defi nition 1 are transition metals of the periodic table; that is, they contain electron vacancies or uncoupled electrons in the d shells of the atom. The uncoupled electrons account for strong bond formation with compo-

nents of the environment, especially O 2 , which also contains uncoupled electrons (hence its slight paramagnetic susceptibility) resulting in electron - pair or cova- lent bonding supplementary to ionic bonding. Furthermore, transition metals have high heats of sublimation compared to nontransition metals, a property that favors adsorption of the environment because metal atoms tend to remain in their lattice, whereas oxide formation requires metal atoms to leave their lattice. The characteristic high energies for adsorption of oxygen on transition metals correspond to chemical - bond formation; hence, such fi lms are called chemisorbed in contrast to lower - energy fi lms, which are called physically adsorbed . On the nontransition metals, such as copper and zinc, oxides tend to form immediately, and any chemisorbed fi lms on the metal surface are short - lived. On transition metals, the initially chemisorbed oxygen is generally more stable thermodynami- cally than is the metal oxide [20] . This situation reverses for multilayer adsorbed oxygen fi lms (lower energy of bonding to the metal) that revert in time to metal oxides. But such oxides are less important in accounting for passivity than the chemisorbed fi lms that form initially and continue to form on metal exposed at pores in the oxide.

The adsorption theory emphasizes that the observed Flade potential of passive iron is too noble by about 0.6 V to be explained by any of the known iron oxides in equilibrium with iron. Observed values of the Flade potential are con- sistent with a chemisorbed fi lm of oxygen on the surface of iron, the correspond- ing potential of which is calculated (see Problem 2 in Chapter 6 ) using the observed heat and estimated entropy of adsorption of oxygen on iron in accord with the reaction [21]

OO 2 ⋅ ads on Fe + 6 H + + 6 e − → 3 2 H O Fe surface +

calc = . 0 57 V (6.7)

94 PASSIVIT Y

The measured chemical equivalents of passive - fi lm substance (about 0.01 C/cm 2 ) correspond (roughness factor = 4) to one atomic layer of oxygen atoms (r = 0.07 nm) over which one layer of oxygen molecules ( r

= 0.12 nm) is chemisorbed; hence, the adsorbed passive fi lm may be represented above by O 2 ·O ads on Fe . The value of the Flade potential, as cited in Eq. (6.1) , corresponds to φ obs = 0 63 . V , which is in reasonable agreement with the calculated value, 0.57 V. The agreement is still better if account is taken of adsorbed water displaced from the metal surface by the passive fi lm during the passivation reaction, this displace- ment presumably involving a greater free energy change than the adsorption of water on the passive fi lm itself [8] .

Further confi rming evidence that the passive fi lm on iron contains oxygen in

a higher - energy state than corresponds to any iron oxide is obtained from the

ability of the passive fi lm to oxidize chromite, CrO − , to chromate, CrO 2 2 − 4 , in NaOH solution [14] . This oxidation does not occur with active iron. The maximum oxidizing capacity corresponds to 0.012 C/cm 2 of passive - fi lm substance in accord with the reaction

OO 2 ⋅ ads on Fe + Fe OH + − + CrO 2 − + HO 2 → Fe OH) ( 3 + CrO 2 4 − (6.8)

=− 138 000 , J Calculations show that the same reaction will not go for chromium or the stain-

less steels, on which the passive fi lm is more stable. On breakdown of passivity, 0.01 C/cm 2 of adsorbed oxygen on iron reacts with underlying metal in accord with

OO 2 ⋅ ads on Fe + 2 Fe + 3 HO 2 → 2 Fe OH) ( 3 or Fe O 2 3 ⋅ 3 HO 2 (6.9)

A layer of Fe 2 O 3 (mol. wt. = 159.7; d = 5.12 g/cm 3 ) is formed equal to a minimum of (0.01

× 159.7)/(6 × 96,500 × 5.12) = 5.4 nm thick (based on apparent area). The hydrated oxide would be thicker. This value compares in magnitude with mea- sured values for the thickness of the decomposed passive fi lm (2.5 – 10 nm). It is the decomposed passive fi lm that is presumably isolated in experiments designed to remove the passive fi lm from iron.

According to the adsorption theory, passivity of chromium and the stainless steels, because of their pronounced affi nity for oxygen, can occur by direct che- misorption of oxygen from the air or from aqueous solutions, and the equivalents of oxygen so adsorbed were found [22] to be of the same order of magnitude as the equivalents of passive fi lm formed on iron when passivated either anodically, by concentrated nitric acid, or by exposure to chromates. Similarly, oxygen in air can adsorb directly on iron and passivate it in aerated alkaline solutions, or also in near - neutral solutions if the partial pressure of oxygen is increased suffi ciently.

THEORIES OF PASSIVIT Y

6.5.1 More Stable Passive Films with Time

Flade observed that the longer iron remains in concentrated nitric acid, the longer the passive fi lm remains stable when the iron is subsequently immersed in sulfuric acid [6] . In other words, the fi lm is stabilized by continued exposure to the passivating environment. Similarly, Frankenthal [17] noted that the fi lm on 24% Cr – Fe thickened and became more resistant to cathodic reduction when

the alloy was passivated for longer times in 1 N H 2 SO 4 at potentials noble to the passivating potential ( P in Fig. 6.1 ), although less than monolayer quantities of oxygen (measured coulometrically) suffi ce to passivate this alloy in this solution. The observed stabilizing effect is likely the result of positively charged metal ions entering the adsorbed layers of negatively charged oxygen ion and mole- cules, with the coexisting opposite charges tending to stabilize the adsorbed fi lm. Low - energy electron - diffraction data for nickel single crystals [23] . indicate that the fi rst - formed adsorbed fi lm consists of a regular array of oxygen and nickel ions located in the same approximate plane of the surface. This initial adsorbed layer is found to be more stable thermally than the oxide, NiO. At increasing

oxygen pressures, several adsorbed layers, probably consisting of O 2 , form on top of the fi rst layer and result in an amorphous fi lm. Additional metal ions in time succeed in entering such a fi lm, particularly in the noble potential range, becoming a mobile species within the adsorbed oxygen layer. Protons from the aqueous environment are also incorporated. Okamoto and Shibata [24] , for

example, showed that the passive fi lm on 18 – 8 stainless steel contains H 2 O. Eventually, the stoichiometric oxide is nucleated at favorable sites on the metal surface, and such nuclei then grow laterally to form a uniform oxide fi lm; but the adsorbed (passive) fi lm remains intact at pores in the oxide. A schematic structure of the fi rst - formed adsorbed passive fi lm is shown in Fig. 6.7 [20] . The

initial passive fi lm grows into a multilayer adsorbed M · O · H structure, which can

Figure 6.7. Schematic structure of initial passive fi lms containing less or more than mono- layer amounts of adsorbed oxygen, along with schematic structure of a thicker passive fi lm containing additional metal ions and protons in nonstoichiometric amounts.

96 PASSIVIT Y

be considered to be an amorphous nonstoichiometric oxide. It differs markedly in protective properties from the stoichiometric oxide into which it may eventu- ally convert.

6.5.2 Action of Chloride Ions and Passive – Active Cells

Chloride ions — and, to a lesser extent, other halogen ions — break down passivity or prevent its formation in iron, chromium, nickel, cobalt, and the stainless steels. From the perspective of the oxide - fi lm theory, Cl − penetrates the oxide fi lm through pores or defects easier than do other ions, such as SO 2 4 − . Alternatively, Cl − may colloidally disperse the oxide fi lm and increase its permeability. On the other hand, according to the adsorption theory, Cl − adsorbs on the metal surface in competition with dissolved O 2 or OH − . Once in contact with the metal surface, Cl − favors hydration of metal ions and increases the ease with which metal ions enter into solution, opposite to the effect of adsorbed oxygen, which decreases the rate of metal dissolution. In other words, adsorbed chloride ions increase the exchange current (decrease overvoltage) for anodic dissolution of the above - mentioned metals over the value prevailing when oxygen covers the surface. The effect is so pronounced that iron and the stainless steels are not readily passivated anodically in solutions containing an appreciable concentra-

tion of Cl − . Instead, the metal continues to dissolve at high rates in both the active and passive potential ranges. Breakdown of passivity by Cl − occurs locally rather than generally, with the preferred surface sites being determined perhaps by small variations in the passive - fi lm structure and thickness. Minute anodes of active metal are formed surrounded by large cathodic areas of passive metal. The potential difference between such areas is 0.5 V or more, and the resulting cell is called a passive –

active cell . High current densities at the anode cause high rates of metal penetra- tion, accompanied by cathodic protection of the metal area immediately surrounding the anode. This fi xes the anode in place and results in pitting cor- rosion. Also, the greater the current fl ow and cathodic protection at any pit, the less likely it is that another pit will initiate nearby; hence, the observed number of deep pits per unit area is usually less than that of smaller shallow pits. It is evident, because of the possibility of passive – active cell formation, that deep pitting is much more common with passive metals than with nonpassive metals.

Halogen ions have less effect on the anodic behavior of titanium, tantalum, molybdenum, tungsten, and zirconium. Passivity of these metals may continue in media of high chloride concentration, in contrast to the behavior of iron, chro- mium, and Fe – Cr alloys, which lose passivity. This behavior is sometimes explained by formation of insoluble protective Ti, Ta, Mo, etc., basic chloride fi lms. However, the true situation is probably related to the high affi nity of these

metals for oxygen, making it more diffi cult for Cl − to displace oxygen of the passive fi lm, in accord with the noble critical potentials of these metals above which pitting is initiated, if pitting occurs at all.

CRITIC AL PIT TING POTENTIAL