Aluminum pitting potential

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)]. 

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]. 

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. 

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. 

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... 

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. 

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. 

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...

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

Cathodically protect at a potential below the critical pitting potential. An impressed current can be used or in good conducting media (e.g., seawater), stainless steel can be coupled to an approximately equal or greater area of zinc, iron, or aluminum [44], Austenitic stainless steels used to weld mild-steel plates, or 18-8 propellers on steel ships, do not pit. 

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].

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. 

Mica particles were cast in various aluminum alloys [87, 88]. In 3.5 wt % NaCl solutions, the presence of mica particles depressed pitting potentials by approximately 20-30 mV, in comparison to the monolithic matrix Eilloys, suggesting that the presence of mica particles may slightly weaken the passive aluminum film. Corrosion behavior was also affected by the precipitation of secondary phases. In some cases, precipitates were preferentially attacked. Pits around and away from mica particles, interfacial corrosion of the mica-matrix interface, and exfoliation of mica particles were also observed. 

Rapid dissolution of alloys in chloride-containing solution is discussed next. When chloride ions are adsorbed on the interface of passive layer and a sulfuric solution, metastable ion complexes gradually form from the anions of a passive layer. These metastable ion complexes enable the anions to dissolve. Once the ion complexes that are on the passive layer/solution interface dissolve into the sulfuric solution, the inner ion complexes of the passive layer move to the passive layer/solution interface in order to correlate with the applied potential. The inability of the anions to form oxide implies the continuous formation of metastable ion complexes and dissolution of ions. Since A1 easily forms [A1(S04 )]+ with (S04)2-, and A1(0H)S04 with (804)2- and (OH)-, respectively [36], these metastable ion complexes combine with Ch and dissolve afterwards. Therefore, pitting easily occurs on the surface of aluminum alloys. Next, the aluminiferous passive layer and non-aluminiferous passive layer are compared. Fig. 10 shows the pitting potential (Epit) of the alloys and SS 304 in different solutions. The value of Epit for C-0 is almost independent of chloride concentration. The value of Epit for C-0.25 decreases abruptly for a chloride concentration exceeding 0.50 M. This value is close to that of SS 304. The values of Epit, for C-0.25, C-0.50,... 

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