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Under the nominal anaerobic conditions employed in these experiments we would expect the CR to be low and within the range 0.1 to 10 μm yr–1 according to recent

reviews [21, 22], with the most reliable measurements yielding values ≤ 0.1 μm yr–1 on Fe3O4-covered surfaces [23, 24]. Our rate is at the upper end of this range and remains unchanged throughout the pre-transition period. This high steady-state rate provides convincing evidence that complete passivation of the steel is not achieved during the pre-oxidation stage.

At the low levels of O2 present in this experiment these high rates cannot be sustained at such a low ECORR by O2 reduction. Since the HCO3–/CO32– content of the solution is high (0.2 M) the most likely cathodic reactant is the proton supplied by HCO3– dissociation. It is possible that this reaction occurs on the electrically-conducting Fe3O4

layer shown to be present by Raman spectroscopy along with the siderite deposit. The fact that Fe3O4 can catalyze H+ reduction has been claimed by King et al. [25]. As observed for O2 reduction [26-28], catalysis could be achieved by the use of FeII/FeIII

adjacent sites on the surface in an electron donor/acceptor relay process.

However, since the major transition appears only to occur when traces of

dissolved O2 are present in solution, it is reasonable to conclude that its presence is a key influence on film properties over the pre-transition period (5 to 26 days). Although our Raman analyses cannot be considered comprehensive, there is no evidence for the formation of the green rust phases indicative of the conversion of FeIIoxides/hydroxides to FeIIIoxides/hydroxides by reaction with dissolved O2 [5, 29], confirming that the continuous purging maintained very low O2 levels.

A possibility is that O2 diffuses into faults in the preformed deposit leading to a localized formation of Fe3+ by oxidation of the Fe2+ produced by the slow, but on-going, corrosion process,

4Fe2+ + O2 + 2H2O → 4Fe3+ + 4OH– (3.6)

The rate of this reaction will be determined by the balance between the stabilization of Fe2+by complexation with carbonate/bicarbonate (Fe(HCO3)+, Fe(CO3)22–, etc. [30-33]) and catalysis of Fe3+ production by Cl– [29, 34]. At the pH used in our experiments the solubility of Fe3+ is almost at a minimum and would be expected to produce insoluble FeIII deposits in the absence of local pH changes. Despite the production of OH– in reaction (3.6), the subsequent production of protons via the hydrolysis reaction

Fe3+ + xH2O → Fe(OH)x(3–x)+ + xH+ (3.7)

would be expected to be overwhelming and to lead to acidification providing the surface area to solution volume ratio within the location is sufficiently large. Such a process is consistent with the EIS data which shows that the accelerating increase in ECORR as the major transition is approached and is accompanied by a pore opening process and the onset of a diffusion-controlled process.

Additional processes which could contribute to pore-opening could be the coupling of anodic metal dissolution at the base of the acidified locations and the reductive dissolution of Fe3O4 [35-38],

Fe3O4 + 8H+ + 2e–→ 3Fe2+ + 4H2O (3.8)

and the catalysis of oxide dissolution by reaction with Fe2+ [39],

Fe3O4 + 8H+ + 2Fe2+→ 3Fe2+ + 2Fe3+ + 4H2O (3.9)

Reaction (3.9) is feasible in the pre-transition period as the potential range (period 2 in Figure 3.5) is consistent with the results observed during rotating-ring disc electrode (RRDE) experiments in HCO3–/CO32– solutions [40].

The major transition in ECORR could then be attributed to the increased polarization of the anodic metal dissolution reaction supported by the reduction of

protons on Fe3O4 surfaces outside the acidified location. The accelerating release of Fe2+ would stimulate reaction (3.8) which would, at least temporarily, maintain the

acidification and the pore-opening process by producing Fe3+ and hence H+ (via Reaction (3.6)).

Once pore-opening is achieved, the separation of anodes and cathodes in this manner could be stabilized by a number of features. Diffusion of HCO3–/CO32– into the pores could buffer the pH to higher values. Since Fe2+ forms stable, soluble complexes with HCO3–/CO32–, anodic dissolution would be maintained, while the higher pH, coupled to a more positive ECORR, would stabilize Fe3O4 against further reductive dissolution. Stratmann et al. have demonstrated that a pH < 6 is required for the reductive dissolution of Fe3O4 [41]. In addition, pore closure by re-growth of an

Fe(OH)2/Fe3O4 layer could be prevented by the accumulation of Cl– within the pore in the manner recently described by Marcus et al. [42]. Figure 3.13 schematically summarizes the film transition process proposed.

Thus, the more positive ECORR and significantly higher CR could be sustained as a pitting process supported by H+ reduction on Fe3O4 surfaces outside the active location. The erratic ECORR observed immediately after the transition could reflect Fe(OH)2/FeCO3 deposition within the pores. Comparison of the post-transition ECORR values to

voltammograms recorded in concentrated carbonate solutions [26, 27, 40] shows it to be in the active-passive transition region, when a combination of active and passive states would be anticipated. In addition, RRDE studies confirm that both Fe2+ and Fe3+ can be released in this potential region, and under potentiostatic conditions, cathodic currents are recorded at potentials only slightly cathodic of the post-transition ECORR [3, 22].

FIGURE 3.13: Schematic illustrating the film transition process, within an acidified pore, assuming separation of anode (iron dissolution) and cathode (bicarbonate reduction).

These observations confirm that the coexistence of separate anodic and cathodic sites is feasible, and it is worthwhile noting that the ECORR achieved is within the range able to support high pH stress corrosion cracking (SCC), –680 to –710 mV, in

concentrated HCO3–/CO32– solutions [25]. Local separation of anodic and cathodic sites, therefore, could play a role in the initiation or early-stage growth of high-pH SCC cracks.