Passive potential range

Anodic Protection On the reverse anodic scan there will be a low current region (LCB) in the passive range. The passive potential range of the LCB is generally much narrower than the passive region seen on a forward slow scan. In anodic protection (AP) work the midpoint of the LCB potential is the preferred design range. This factor was verified for sulfuric acid in our laboratory and field studies. 

Alloy Acid concentration Temp. ( C) Critical current density ( crit.. Am-2) Current density to maintain passivity (ip a.< Corrosion rate (mm y ) Unprotected protected Passive potential range (V)... 

Fig. 1. Polarization curve of metals with active, passive and (a) transpassive potential range including oxygen evolution (b) passive potential range going directly to oxygen evolution (c) continuing passivity for valve metals to very positive potentials. Pitting between critical pitting lim and inhibition potential fsj in the presence of aggressive anions and inhibitors.

Passivation potential — Figure 1. Polarization curves of three metals in 0.5 M H2SO4 with active dissolution, a passive potential range, and transpassive dissolution and/or oxygen evolution at positive potentials Ep(Cr) = -0.2 V, -Ep(Fe) FP(Ni) = 0.6 V [i]... 

Passivation of iron under critical conditions is predicted. Hematite (Fe203) may still be the main corrosion product in the neutral water pH (pH 7.2) region, but the passivation potential range is narrower and shifts to negative potentials, compared with regions on the diagram for ambient conditions. For chromium, no solid chromium oxide stable species is predicted within the stable region of neutral water. This indicates chromium oxidation without any passivation oxide film formation. 

As we saw in the foregoing section, pitting corrosion of passive metals occurs beyond the critical pitting potential, plt. In order to protect passive metals from pitting corrosion, therefore, it is advisable to hold the corrosion potential as far less positive from plt as possible in the passive potential range. The presence of p-type oxides, however, makes Econ more positive and hence enhances the breakout of pitting corrosion. In the same way, metals corroding in the active state will accelerate their corrosion rates when their electrode potential is made more positive (more anodic) by the presence of p-type oxides. 

Fig. 5.45 Schematic polarization curves fortype 304 stainless steel in aerated 1 N H2S04. L (low) and H (high) distinguish the effects that minor composition variables can have on the position of the active peak current density (icrit) in the stainless steel polarization curve. Estimated corrosion potentials and corrosion current densities are shown. In particular, note that corrosion can occur in the active or passive potential range depending on the position of icrit-...

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

The effect of acid concentration on polarization of active-passive metals is shown in Fig. 4.8. Higher hydrogen ion concentration increases the critical anodic current density and decreases the passive potential range. Severe corrosion conditions present at higher acidity also increase current densities and corrosion rates at all potentials. Figure 4.9 presents the data for iron passivation in phosphoric acid/phosphate buffer solutions of... 

The passive film thickness at time t can be predicted using eq. (6.1), provided that the passive current density (ip) remains constant in the passive potential range, E [Pg.185]

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