Aluminum passivity breakdown

On pure aluminum, Hebert and Alkire [68] proposed that passivity breakdown occurred when a critical concentration of aluminum chloride is attained in the crevice environment. Indeed, they showed convincingly (Fig. 25) that the presence of aluminum cations in the solution caused passivity breakdown for a pH and chloride concentration at which solutions containing only sodium cations did not. 

Based on an experimental work on alloy 7475 crevices and propagating cracks, Holroyd et al. [30] agree with this assumption. The critical aluminum concentration required decreases as chloride content increases [68] but low pH and high chloride content are not a prerequisite for passivity breakdown. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

Regions characterized by large anodic overpotentials. Under such conditions, complete passivation and severe oxidation of most metal surfaces occurs. A breakdown of passive oxide layers and pitting corrosion is observed for transition-metal model systems. In this section are considered also the surfaces of electropositive metals such as aluminum. 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

It has been well documented that polarity reversal is due to the formation of passive film and its breakdown. In the case of aluminum it should be noted that the passive oxide, being amphoteric, is stable in the pH range of 4—9 and the film breakdown occurs at very high chloride concentrations (Figure 7.106). 

Other Types of Passive Aluminum Oxide Films, Anof, Modified Aluminum, Alloys If aluminum is polarized to higher potentials, breakdown of the film can occur. Optical observation shows small sparks on the surface indicating local... 

It is necessary to exceed the critical anodic potential (23) bd for the electrochemical breakdown of passivation by pitting and consists of these factors (i) presence of halides at the interface (ii) induction time for the initiation of the breakdown process, leading to localized conditions that may increase the localized corrosion current density (iii) development of favorable conditions inside the pits for propagation when the local sites become immobile and localized at certain sites. Electrochemical breakdown of some metal oxides is possible in the case of copper, lead, and tin cathodically to metal while ferric oxide is reduced to the ferrous ion in aqueous solutions. Zinc and aluminum oxides are not cathodically reducible and in these cases hydrogen is reduced. The vigorous evolution of hydrogen assisted by electron conducting zinc oxide can accelerate the breakdown of passivity. 

Aylor and Moran [45], who conducted polarization experiments on Gr/Al MMCs in seawater, also h5rpothesized that diffiision of carbon into aluminum could lower the integrity of the passive film, rendering it more susceptible to breakdown. Wielage [46] reported that the pitting potentials of various squeeze-cast Gr/Al MMCs were approximately 20 mV more active than that of the pure aluminum matrix material in a chloride solution. Aluminum carbide was found at the Gr fiber-matrix interfaces. Shimizu et al. [37], however, found that pitting potentials of a squeeze-cast, short fiber Gr/6061 Al MMC had pitting potentials similar to that of monolithic 6061 Al in a chloride solution. 

SECM was also used to initiate pitting on steel and aluminum and to examine the pit growth and corrosion products (112a, 112b). In these studies, the UME tip was used to generate aggressive Cl ions in close proximity to stainless steel and aluminum surfaces. The tip and substrate current were then monitored to detect corrosion events. Fluctuations in the tip and substrate current were observed, which were indicative of the breakdown of the passive film and pit initiation on the metal. SECM and CV measuranents provided evidence that the large tip current fluctuations observed were caused by the reduction of Fe, which was released from the iron surface in the breakdown of the passive film of iron. 

A major issue, for the passivation and corrosion resistance of aluminum alloys, is the existence or not of second phase inter-metallic particles resulting from alloying with elements that have low solubility in aluminum (Rynders et al., 1994 Kowal et al., 1996). These particles are detrimental to the resistance of the passive film to breakdown (the first stage of a localized corrosion process). In contrast to stainless steels, this factor often overwhelms the beneficial alloying effects. However, it must be pointed out that alloying elements such as copper in solid solution are beneficial (Muller and Galvele, 1977). Other elements, such as chromium, molybdenum, titanium, tantalum, and niobium, seem to improve the corrosion resistance of aluminum, but their solubility is too low for them to be used in conventional alloy processes, and they require the use of rapid quenching processes or some sort of nonequilibrium surface deposition. 

Penetration of chloride ions this mechanism [first discussed by Hoar et al. (1965)] involves, following the adsorption of Cl" on the passive film surface, the entry of Cl" into the film and its transport through the passive film to the metal/oxide interface, where it causes breakdown of the passive film. The accumulation of Cl" at the interface or the formation of metal chloride may cause the film breakdown. Support of this mechanism is provided by the observation of chlorides in the inner oxide part of the passive film on nickel (Marcus and Herbelin, 1993), Fe-Cr (Yang et al., 1994), and aluminum (Natishan etal., 1997). 

Even hand-scribed, polymer-coated, bulk zinc samples showed inhomogeneous current distributions. Not surprisingly, local breakdown of passive layers was observed as the initial corrosion step. With regard to the forming of coil-coated steel, cracks induced by roll coating of painted aluminum-zinc coated steel sheets were characterized in dilute sodium sulfate solution. Anodic and cathodic sites were observed with a peak distance of about 500 pm, which could not be predicted by visual inspection. 

In contrast to the situation for aluminum, electrochemical studies on the pitting of stainless steels are fi aught with contradictions because of the effect of experimental factors. Several investigations have used breakthrough potential or breakdown potential, as a measure of pitting susceptibility. As pointed out earlier, the current increases rapidly at the breakdown potential (Fig. 4.24), this is considered as an indication of a breakdown of the passive, corrosion-resistant steel film and initiation of pitting. 

A passive film formed on the metallic surface block will suppress corrosion. They are very thin films (50 to 100 A). In aluminum, thin films of boehmite (AlOOH) and bayerite (Al(OH)3) are formed which affect the corrosion process. Thick films are liable to breakdown and accelerate corrosion. Copper and nickel also have good passive film forming properties. The stifling of anodic sites by corrosion products, such as iron... 

Lithium bis(oxalato)borate (LiBOB) shows only moderate solubility up to about 1.0 M in some organic solvents (such as blends of PC and EC). Its conductivity is about 8-9 mS cm in appropriate solvents [97] (in DME even 14.9 mS cm at ambient temperature [98]). A major advantage is its thermal stability (up to 300 °C [99]) and the passivation film on aluminum, formed by the first cycle. This passivation film protects the aluminum current collector even at higher potentials than LiPFfi does, without breakdown up to 5.75 V [97, 100]. Furthermore, LiBOB has slightly better cycHng stability at ambient temperature, which is considerably increased at temperatures up to 70 °C [97]. Another advantage is that LiBOB forms... 

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