Aluminum passive film formation

Let us assume that the total surface of an electrode is in an active state, which supports dissolution, prior to anodization. The application of a constant anodic current density may now lead to formation of a passive film at certain spots of the surface. This increases the local current density across the remaining unpassivated regions. If a certain value of current density or bias exists at which dissolution occurs continuously without passivation the passivated regions will grow until this value is reached at the unpassivated spots. These remaining spots now become pore tips. This is a hypothetical scenario that illustrates how the initial, homogeneously unpassivated electrode develops pore nucleation sites. Passive film formation is crucial for pore nucleation and pore growth in metal electrodes like aluminum [Wi3, He7], but it is not relevant for the formation of PS. 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

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). 

Passivity — An active metal is one that undergoes oxidation (-> corrosion) when exposed to electrolyte containing an oxidant such as O2 or H+, common examples being iron, aluminum, and their alloys. The metal becomes passive (i.e., exhibits passivity) if it resists corrosion under conditions in which the bare metal should react significantly. This behavior is due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. Such films often form spontaneously from the metal itself and from components of the environment (e.g., oxygen or water) or may be formed by an anodization process in which the anodic current is supplied by a power supply (see -> passivation). For example, A1 forms a passive oxide film by the reaction... 

Finally, several film-formation mechanism can be active in parallel for a given inhibitor/metal system. As an example, radiotracer experiments by Canes and coworkers on the accumulation of phosphate on aluminum are reproduced in Fig. 10 [42, 43]. After a first rapid increase in phosphate surface concentration, which was assigned to phosphate adsorption and (partial) incorporation into the passive film, a slower accumulation is observed, which can continue up to several days and which was explained by precipitation... 

Compact films, in particular oxide films, form a barrier between the metal and its environment, thus protecting the metal. The corrosion of iron in a dry atmosphere stops after the formation of an oxide film that is only a few nanometers thick. On certain metals, compact films also form in humid or liquid environments. These are referred to as passivefilms. They are present on stainless steel in aqueous environments, on aluminum exposed to humid air, and on carbon steel in alkaline surroundings, to cite a few examples. The rate of corrosion of metals coated with a passive film usually is very low. 

It is welt known that aluminum corrodes in both acid and alkaline solutions, but not in neutral ones (pH = 4 to 8) because of aluminum oxide formation whkh serves as a passivation film Ideally the pH of the water extracted solution of encapsulation material is controlled in this region. But when halide ions like Q and Br exist in... 

In summary for anodic corrosion, water molecules permeate through the encapsulation material to reach the chip surface. Delamination between the lead frame and encapsulation material must accelerate moisture penetration into devices. Delamination between chips and encapsulation material allows water film formation. If halides, such as chloride ion are present in the water film, the corrosion of aluminum is accelerated. Corrosion takes place mainly on aluminum of the bonding pad, since the chip is usually coated with passivation film. But when passivation defects exist, moisture penetrates through them and aluminum corrosion of the wiring conductor occurs. Then improvements in adhesion, and lowering molding stress and chloride content are important. 

An oxide layer is readily formed on many metals when they are made anodic in aqueous solutions. In the case of aluminum, this process is called anodization. It is also referred to as a passive film which reduces the corrosion rate. Such passive films can be thin, from 0.01 pm, and fragile and easily broken. Thus, when steel is immersed in nitric acid or chromic acid and then washed, the steel does not immediately tarnish nor will it displace copper from aqueous CUSO4. The steel has become passive due to the formation of an adhering oxide film which can be readily destroyed by HCl which forms the strong acid FeCU-. 

In fact, the compound Zr COz (OH)e or ZnCOz 3Zn OH)2 is zinc carbonate or white mst or wet-storage stain (porous). Atmospheric corrosion of aluminum is due to a passive oxide film formation instead of a porous layer. The gray/black-color film may form as follows... 

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). 

Here is a solvated ion, e is an electrom, and n represents the ion state of charge. The electrons, liberated by the oxidation, must flow through the material M to be consumed in an appropriate cathodic reaction. Beyond a solubility limit, precipitates of hydroxide or hydrated oxide are formed, and this surface film can provide a barrier to further dissolution. In fact, there are two film formation mechanisms the dissolution-precipitation mechattism addressed before and also the solid-state oxidation proeess M + H2O MO + 2H+ + 2e. Some films are termed passive, for stainless steels or aluminum alloys, for instance. These films can play an important role in environment-sensitive erack mitiation and fracture. Under thermodynamic equilibrium conditions, the film stability may be inferred from E =y(pH) diagrams, where E is the electrical potential related to the chemical free energy G by G = -nEF, and F is Faraday s ntrmber. At eqirilibrium, one can define the electrode potential (related to AG) and the eurrent density 1(1 ... 

The effect of film formation on the tendency of the metal to passivate solution cannot be predicted by emf series. For instance, titanium and aluminum are more negative than iron. However, in certain environments they form a film which makes their potential less active than iron. The effect of film formation on the tendency of the metals to corrode is kinetic and cannot be predicted by the thermodynamic emf series. 

It is a form of localized corrosion of a metal surface where small areas corrode preferentially leading to the formation of cavities or pits, and the bulk of the surface remains unattacked. Metals which form passive films, such as aluminum and steels, are more susceptible to this form of corrosion. It is the most insidious form of corrosion. It causes failure by penetration with only a small percent weight-loss of the entire structure. It is a major type of failure in chemical processing industry. The destructive nature of pitting is illustrated by the fact that usually the entire system must be replaced. 

The most important condition is that the metal must be in a passive state for pitting to occur. Passive state means the presence of a film on a metal surface. Steel and aluminum have a tendency to become passive, however, metals which become passive by film formation have a high resistance to uniform corrosion. The process of pitting destroys this protective film at certain sites resulting in the loss of passivity and initiation of pits on the metal surface. It may be recalled that passivity is a phenomenon which leads to a loss of chemical reactivity. Metals, such as iron, chromium, nickel, titanium, aluminum and also copper, tend to become passive in certain environments. 

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