Pitting potentials of aluminum

H. Bohni and H.H. Uhlig, Environmental Factors Affecting the Critical Pitting Potential of Aluminum, J. Electrochem. Soc., Vol 116, 1969, p 906-910... 

Figure 7.55 Effect of Cr concentration on pitting potential of aluminum at 25 °C. Also indicated is the pitting potential in IM KBr and in IM KI [29].

For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl ) is the most frequently encountered in service. The effect of chloride ion concentration on the pitting potential of aluminum 1199 (99.99-i-% Al) is shown in Fig. 6. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential. In the absence of dissolved oxygen or other cathodic reactant, aluminum will not corrode by pitting because it is not polar-... 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

Overall, these results indicate that chromates inhibit corrosion by elevating the pitting potential on aluminum with respect to the corrosion potential, which decreases the probability for the formation of stable pits. In general a chromate chloride concentration ratio in excess of 0.1 is necessary to observe significant anodic inhibition. 

The same publication reports on nonlinear dependence between pitting potential of ion-implanted binary surface alloys of aluminum and zero points of oxides of implanting metals. 

Fig. 7.42 Pitting potential of 99.99 wt% aluminum in several halide environments. All environments are pH = 11 except as indicated for pH = 6. Redrawn from Ref 60...

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. 

Figure 3-4. Relationship between the activity of chloride ions and the pitting potential for aluminum in NaCl solution.

Although lithium is highly reactive, addition of up to 3% Li to aluminum shifts the pitting potential of the solid solution otdy slightly in the anode cUrection in 3.5% NaQ solution (Ref 25). In an extensive corrosion investigation of several binary and ternary aluminum-lithium alloys, modifications to the microstructure that promote formation of the 5 phase (AlLi) were found to reduce the corrosion resistance of the alloy in 3.5%... 

I.L. Muller and J.R. Gaivele, Pitting Potentials of High Purity Binary Aluminum Alloys, Part I Al-Cu Alloys, Corros. ScL, Vol 17, 1977, p 179 Part II Al-Mg and Al-2ii Alloys, Corros. ScL, Vol 17,1977, p995... 

HP. DeJong and J.HM. Martens, Investigation of the Pitting Potential of Rapidly Solidified Aluminum-lithium Alloys, Aluminum, Vol 61 (No. 6), 1985, p4l6... 

DeKiibes aprocedureto determine the repassivation potential of aluminum alloy 3003- HI4 as a measure of relative susceptibility to pitting corrosion by conducting agalvanostatic polarization See also ASTM G100 summarized later in this table. 

The system aluminum/water belongs to group II where represents the pitting potential and lies between -0.8 and -1.0 V according to the material and the medium [22,23,36,39,42]. Since alkali ions are necessary as opposite ions to the OH ions in alkalization, the resistance increases with a decrease in alkali ion concentration (see Fig. 2-11). In principle, however, active aluminum cannot be protected cathodically [see the explanation of Eq. (2-56)]. 

The linear dependence of the pitting potential on ionic radius is likely a reflection of the similarly linear relationship between the latter and the free energy of formation of aluminum halides.108 It is reasonable to assume that the energy of adsorption of a halide on the oxide is also related to the latter. Hence, one could postulate that the potential at which active dissolution takes place is the potential at which the energy of adsorption overcomes the energy of coulombic repulsion so that the anions get adsorbed. 

Fig. 12.57. Correlation between pitting potential and pzc. (Reprinted from J. O M. Bockris and J. Kang, The Pro-tectivity of Aluminum and its Alloys with Transition Metals, J. Solid State Elec-trochem. 1 28,1997, with permission from Springer-Verlag.)...

Mo are single phase, supersaturated solid solutions having an fee structure very similar to that of pure Al. Broad reflection indicative of an amorphous phase appears in deposits containing more than 6.5 atom% Mo. As the Mo content of the deposits is increased, the amount of fee phase in the alloy decreases whereas that of the amorphous phase increases. When the Mo content is more than 10 atom%, the deposits are completely amorphous. As the Mo atom has a smaller lattice volume than Al, the lattice parameter for the deposits decreases with increasing Mo content. Potentiodynamic anodic polarization experiments in deaerated aqueous NaCl revealed that increasing the Mo content for the Al-Mo alloy increases the pitting potential. It appears that the Al-Mo deposits show better corrosion resistance than most other aluminum-transition metal alloys prepared from chloroaluminate ionic liquids. 

Pitting potential increased with increase in chromium contents >20 wt%, and molybdenum of 2-6 wt%. Recent results, applying microelectrochemical techniques, confirmed that even in the superaustenitic stainless steels molybdenum strongly improves the repassivation behavior but has no influence on pit initiation.27 The corrosion resistance of aluminum alloys is totally dependent on metallurgical factors.52, (Frankel)5... 

Work at Sandia National Laboratory has looked into the fundamentals of corrosion. Macroscopic results of experiments into the pitting of aluminum wire when exposed to sodium chloride solution indicate that the pitting potential is not a thermodynamic value but rather the potential associated with the kinetics of oxide breakdown. As a result, as a device becomes increasingly small, the probability of oxide breakdown will likewise decrease. At nanoscale a device made of this material would be highly unlikely to undergo oxide breakdown, and such a device would be expected to exhibit stability for long periods of time. 

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