Repassivation Rate

Film is locally removed by dissolution, surface shear stress or particle/bubble impact but it can repassivate. Erosion corrosion rate is a function of the frequency of film removal, bare metal dissolution rate and subsequent repassivation rate. 

Repassivation processes have become an important subject in stress corrosion studies and also in other forms of corrosion, e.g., pitting corrosion and corrosion fatigue. A range of scratching and straining electrode techniques have been employed. While it is not possible to go into detail, the results have to be examined in relation to the techniques employed, e.g., has repassivation started before the scratching or straining has stopped It is important also to know whether the current measured under potentiostatic conditions is a complete anode current or the difference between an anode current and a cathode current (most commonly due to H" " ion reduction). Typical repassivation rates correspond to an equation (2 1 ) of the type ... 

Measurement of the electrochemical current noise is aimed at correlating the observed current fluctuations with breakdown and repair events that might lead to the formation of stable growing pits [53, 54], In view of this mechanistic interpretation, the application of statistical methods to the occurrence of current spikes and the observed probability of pit formation lead to a stochastic model for pit nucleation. The evaluation of current spikes in the time and frequency domain yields parameters such as the intensity of the stochastic process X and the repassivation rate r [53]. They depend on parameters such as the potential, state of the passive layer, and concentration of aggressive anions. 

Table lc.37 Repassivation rates in artificial saliva versus standard calomel electrode (Ref. 43)... 

The amount of metal dissolved per cycle depends on the electrochemical parameters of the system, such as the passivation current density and the repassivation rate. On the other hand, the rate of deformation determines the frequency of activation. In Figure 11.42, crack propagation rates are plotted as a function of the repassivation... 

Contain elements that enable fast repassivation rate at scratches and flaws on multiple occasions. 

Because of its high resistance to general corrosion in acidic sulfate solutions, high resistance to wear, and high repassivation rate, Ultimet alloy was the highest ranked material for fabrication of conductor rolls in elec-trogalvanizing plants (Rebak, 1998). 

SCC data are presented as velocity versus static stress parameter such as K, yet it is the dynamic plasticity at the crack tip that actually features the newly formed oxide film [67,113], This problem was considered by Vermilyea [24], who showed that under a static stress the corrosion process could advance the crack tip into a region that had reached an equilibrium distribution of plastic strain (e ), and produce a strain transient (Ae I that could fracture a newly formed oxide (Fig. 23). Later Sieradzki [44] showed that the depth of transient corrosion required in Vermilyea s original model was um-easonably large ( tm), confirming that static slip-dissolution models can apply only to intergranular SCC, where there is a directional active path with a very low repassivation rate [33]. It is not clear how the shielding of the crack... 

The formation of passive film is most important, because SCC takes place commonly in alloys that are covered by a highly protective film, such as aluminum or steel alloys. Under a tensile strain, the slip plane breaks the protective film as shown in Fig. 4.45a, a small part of the film undergoes dissolution as shown in (b) and later repassivation takes place as in (c). As pointed out earlier, if repassivation occurs too rapidly, corrosion attack would be too small and the crack would propagate slowly. On the other hand, if repassivation occurs very slowly, excessive metal dissolution occurs on the crack tip and sides. This widens and blunts the crack tip, and the crack grovvTh is arrested. The greatest damage is caused by moderate repassivation rates. 

Makar et al. [113] showed that (i) increasing A1 from 1 to 9% increased the repassivation rate of Mg alloys, attribnted to the snperior passivation of Al, and (ii) Al extended the pH range over which Mg alloys form a protective film. By analysing the Mg(OH)2 snrface film formed in aqneous environments using X-ray and electron diffraction, Fairman and Bray [106] proposed that two AP ions replace three Mg ions in the tetrahedral Mg(OH)2 lattice, resulting in vacant lattice sites. The resulting film was thicker and was considered more protective. This was used to explain the perceived increased general corrosion resistance of Mg-Al alloys. 

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