Passive Film on Aluminum

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

L. Tomcsanyi, K. Varga, I. Bartik, G. Horanyi, and E. Maleczki, Electrochemical Study of the Pitting Corrosion of Aluminum and Its Alloys II, Study of the Interaction of Chloride Ions with a Passive Film on Aluminum and Initiation of Corrosion, Electrochim. Acta, Vol 34, 1989, p 855-859... 

Lopez et al. [160] applied the technique of Fermi level shift monitoring to characterize the add base properties of passive films on aluminum. The decreasing trend of relative basicity was found to be boehmite > thermal oxide > NaOH-degreased surface > silicate containing detergent-degreased surface > phosphoric add anodic film. The... 

Upon exposure to air, aluminum forms a chemically inert AI2O3 oxide film that is a rapidly forming self-healing film. Therefore, the passive film on aluminum, as well as the corrosion product layer, is a main barrier and leads to a resistant material in natural environments. 

The passive film on aluminum also consists of an inner layer of AI2O3 and an outer layer of AlOOH. 

A galvanic cell is formed when two metals differing in potential are joined together. For instance, if copper is joined to aluminum, aluminum would corrode because it has a more negative potential (—1.66 V) than copper (-f0.521 V). Copper being less active becomes the cathode and aluminum becomes the anode. But if iron is joined to aluminum, the iron corrodes (in seawater), due to the passive film on aluminum which causes it to behave like a nobler metal than iron (but not nobler than copper). The formation of such galvanic cells often leads to the corrosion of underground buried structures. A steel plate with copper rivets... 

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... 

Bonhoeffer, Vetter, and others (63) have made extensive studies on iron which indicate that the passive film is composed of one or more oxides of iron. Young (64). Vermilyea (65) and Johansen et at, (57) have shown that the Mott-Cabrera concepts are applicable for the thin films on Ta, Ti, Hf, and Hb. Petrocelli (58) has shown evidence that the dissolution of aluminum In sulfuric acid takes place through a thin film and that the process appears to follow the Motr-Cabret a theory. Stern (66) reports data indicating that the kinetic. for the anodic oxidation of stainless steel are similar to those for aluminum apd tantalum (67). Pryor (68) has recently reviewed the work on passive films on iron and suggests a single passive film of y contains non-uniform defect concentra-... 

Fig. 8 Adsorption of (a, b) sulfate and (c, d) chloride on the passive film of aluminum in 0.1 M NaCl04 solution at different pH and open circuit potential, (a) Sulfate and (c) chloride surface concentration versus time at different pH...

Other metals that have favorable reversible Flade potentials and form passive film on their surfaces include titanium, silicon, aluminum, tantalum, and niobium. Naturally formed aluminum oxide protects the underlying aluminum metal at pH between 4 and 8. Titanium possesses very high oxidizing potentials and is used to manufacture anodes for cathodic protection systems for the chlorine-alkafi process (production of hydrogen, chlorine, and sodium hydroxide) and many other appfications. 

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). 

The research on corrosion, started in this institute in the 1950s, continued successfully further. The intergranular corrosion of steels was measured by an electrochemical potentiodynamic reactivation method [310-312]. Since the 1960s, the passivity of brass was further studied, the rates of corrosion were measured by polarization resistance, the effect of deformation on anodic dissolution of steels was followed, and the surface roughness of metals was measured other subjects of research were, e.g., the behavior of passive films on steel, the effect of compositirai and motion of electrolyte on corrosion of passivated aluminum, the cathodic protection of passive metals against corrosion, the anodes for cathodic protection of steels, etc.[313-316]. Measurements of polarization resistance in the system iron—concentrated sulfuric acid or boiling nitric acid, of corrosion and matter... 

More recently, Vertes and coworkers have investigated the spectral properties of the passive film on tin in borate buffer medium (pH 8.4) over a wide potential range. A typical voltammetric curve for Sn in this specific electrolyte is shown in Figure 23. At -0.9 V, the transmission Mossbauer spectrum of a " Sn-enriched tin film, electrodeposited on an aluminum substrate (curve A, Figure 24), was found to have clear contributions due to )8-Sn, 8 = 2.5, and Sn02 or Sn(OH)4, 8 = 0.03 (Table XI). The small absorption peak at about 4.2 mm s was attributed to the high-velocity component of a... 

A critical issue is the stabiUty of the hydride electrode in the cell environment. A number of hydride formulations have been developed. Table 5 shows hydride materials that are now the focus of attention. Most of these are Misch metal hydrides containing additions of cobalt, aluminum, or manganese. The hydrides are prepared by making melts of the formulations and then grinding to fine powers. The electrodes are prepared by pasting and or pressing the powders into metal screens or felt. The additives are reported to retard the formation of passive oxide films on the hydrides. 

In most corrosion processes passivity is desirable because the rate of electrode dissolution is significantly reduced. The rate of aluminum corrosion in fresh water is relatively low because of the adherent oxide film that forms on the metal surface. A thicker film can be formed on the surface by subjecting it to an anodic current in a process known as anodizing. In most electrochemical conversion processes passive films reduce the reaction rate and are, therefore, undesirable. 

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