Brittle passivation films

After cooling and by virtue of their shape, these hillocks form singularities where stress concentrations will encourage cracking and then flaking of any brittle thin passivation film after it has been deposited on the aluminum (Figure 5). 

Yet, for systems A and C, the measured fracture energies remain low compared with the critical fracture energy of the bulk aluminum 10 J Moreover, we do not observe islands of passivation material on the A1 fracture surface and, inversely, we do not observe A1 on debonded surfaces of the passivation films. This suggests that the loss of interfacial adhesion is close to a brittle fracture process despite the influence of plasticity of the A1 substrate and crack blunting at the interface. This sort of brittle mode of interfacial failure, including plastic flow in a ductile material (the substrate), has been observed or discussed for a sapphire/Au interface. ... 

Cracks are another type of defect that could be the location of pit initiation. Cracks can form in a brittle oxide film due to plastic deformation of the underlying metal. For example, the brittle oxide film formed by anodizing aluminum crack quite easily. Some researchers have proposed that the strong electric field present in passive films produces an electrostrictive force sufficiently strong to break up a passive film. Experimental evidence to sustain this hypothesis is lacking, however. 

The slip dissolution model assumes that plastic deformation at the crack tip is responsible for the activation. But other mechanisms can have the same effect. Tensile stress at the crack tip could, for example, break a brittle tarnish film or passive oxide film, thereby exposing the base metal to the electrolyte. Selective dissolution of alloy components at the crack tip could locally weaken the metal matrix and thus permit... 

Waltman et al. [191] have attempted to polymerize electrochemically indole (Fig. 6) and its substituted derivatives. Indole, 5-cyanoindole, and 5-indolecarboxylic acid produce thick, brittle films with conductivities around 10" S cm". Thin, passivating films are formed when monomers of 4-, 6-, or 7-methylindole and 5-bromo-, 5-chloro-, or 5-fluoroindole are used. Although the structures of the polymers are not well known, infrared spectra seem to indicate the formation of N-N linkages in the polymers, which would contribute to the low observed conductivities [192]. 

Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

This example of aluminium illustrates the importance of the protective him, and films that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective film is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a significant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also afiect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective film rather than to an increase in hardness. 

Role of PI chip coat film can be explained as follows. In a case of PSG passivation alone, cracks are generated in the PSG film by the mold stress because of the brittleness of PSG film, and A1 electrode corrodes by the moisture penetrating through these cracks. On the other hand, for a package with PI chip coat, mold stress is absorbed by this PI film and the passivation crack is prevented. 

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