CHARACTERISTICS OF PASSIVATION AND THE FLADE POTENTIAL

6.2 CHARACTERISTICS OF PASSIVATION AND THE FLADE POTENTIAL

Suppose iron is made anode in 1 N H 2 SO 4 and is arranged so that, as the potential

CHAR AC TERISTICS OF PASSIVATION AND THE FL ADE POTENTIAL

required to maintain the prevailing potential with respect to a reference elec- trode. In the laboratory, this is usually accomplished using a potentiostat, which controls the potential between the “ working ” electrode and the reference elec- trode by automatically adjusting the current between the “ working ” electrode and an inert electrode, usually platinum, that is called the “ auxiliary ” or “ counter ” electrode. The resulting polarization curve, shown in Fig. 6.1 [5] , is called a potentiostatic polarization curve.

Galvanostatic polarization is an alternative to potentiostatic polarization. In galvanostatic polarization measurements, the current between working and counter electrodes is controlled, and the potential between working and refer- ence electrodes is automatically adjusted to the value required to maintain the current. Galvanostatic measurements can be made using, for example, the circuit shown in Fig. 5.3 ( a ) in Section 5.3 . The galvanostatic polarization curve for iron

in 1 N H 2 SO 4 is shown in Fig. 6.2 . The potentiostatic polarization curve provides considerably more informa- tion about passivity than does the galvanostatic curve. Referring to Fig. 6.1 , iron is active at small current densities and corrodes anodically as Fe 2+ in accord with Faraday ’ s law. As current increases, a partially insulating fi lm forms over the electrode surface, composed probably of FeSO 4 . At a critical current density, i critical , of about 0.2 A/cm 2 (higher on stirring or on lowering pH of the environment), the current suddenly drops to a value orders of magnitude lower, called the passive current density, i passive . At this point, the thick insulating fi lm dissolves, being replaced by a much thinner fi lm, and iron becomes passive. The value of i passive decreases with time, diminishing to about

86 PASSIVIT Y

Figure 6.2. Galvanostatic anodic polarization curve for iron in 1 N H 2 SO 4 .

7 μ A/cm 2 in 1 N H 2 SO 4 . In other electrolytes, i passive may be higher or lower than in 1 N H 2 SO 4 . The true critical current density for achieving passivity of iron in absence of an insulating reaction - product layer is estimated to be about 10 –

20 A/cm 2 , as shown by short - time current – pulse measurements. On further gradual change of the potential, the current density remains at the above low value, and the corrosion product is now Fe 3+ . At about 1.2 V, the equilibrium oxygen electrode potential is reached, but oxygen is not evolved appreciably until the potential exceeds the equilibrium value by several tenths of a volt (oxygen overpotential). The increased current densities in the region

labeled “ transpassive ” represent O 2 evolution plus Fe 3+ formation. When the applied potential is removed, passivity decays within a short time in the manner shown in Fig. 6.3 . The potential fi rst changes quickly to a value still noble on the hydrogen scale, and then it changes slowly for a matter of seconds to several minutes. Finally, it decays rapidly to the normal active poten- tial of iron. The noble potential arrived at just before rapid decay to the active value was found by Flade [6] to be more noble the more acid the solution in

which passivity decayed. This characteristic potential, φ F , was later called the Flade potential, and Franck established it to be a linear function of pH [7] . His measurements in acid media, combined with later data by others, provide the relation at 25 ° C:

φ F ( volts, S.H.E.) =+ . 0 63 0 059 − . pH (6.1) This reproducible Flade potential and its 0.059 pH dependence is characteristic

of the passive fi lm on iron. A similar potential – pH relation is found for the passive fi lm on chromium, for Cr – Fe alloys, * and for nickel, for which the stan-

* When activated cathodically, the Flade potential of chromium and stainless steels follows the relation n (0.059 pH), where n may be as high as 2. For self - activation, n is 1 [8] .

CHAR AC TERISTICS OF PASSIVATION AND THE FL ADE POTENTIAL

Figure 6.3. Decay of passivity of iron in 1 N H 2 SO 4 showing Flade potential, ϕ F .

dard Flade potentials (pH = 0) are less noble than for iron, in accord with more stable passivity.

Stability of passivity is related to the Flade potential, assuming the following schematic reaction to take place during anodic passivation:

MHO + →⋅+ OM 2 H + + 2 2 − e (6.2) where − φ F is the potential for the reaction, and O · M refers to oxygen in the

passive fi lm on metal M whatever the passive fi lm composition and structure may

be. The amount of oxygen assumed to be combined with M has no effect on present considerations. It follows, as observed, that

φ = φ + log H ( + ) F 2 F = φ F − . 0 059 pH (6.3)

2 The positive value of φ F for iron (0.63 V) indicates considerable tendency for

the passive fi lm to decay [reverse reaction of (6.2) ], whereas an observed negative value of φ F = −0 2 .V for chromium indicates conditions more favorable to passive - fi lm formation and, hence, greater stability of passivity. The value of φ F for nickel is 0.2 V. For chromium – iron alloys, values range from 0.63 V for pure iron to increasingly negative values as chromium is alloyed, changing most rapidly in the range 10 – 15% chromium and reaching about − 0.1 V at 25% chromium.

The corresponding φ F values are shown in Fig. 6.4 .

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Figure 6.4. Standard Flade potentials for chromium – iron alloys and chromium [7 – 9] .

The potential ( P in Fig. 6.1 ) at which passivity of iron initiates (passivating potential) approximates, but is not the same as, the Flade potential because of IR drop through the insulating layer fi rst formed and because the pH of the electrolyte at the base of pores in this layer differs from that in the bulk of solution (concentration polarization). These effects are absent on decay of passivity.