Passivation crack, prevention

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

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. 

PI chip coat is effective for the prevention of passivation crack, A1 slide and package crack of semiconductor devices. Low thermal eiqpansion PI can be regarded as the most suitable material for this application. 

This is the case for magnesium and calcium electrodes whose cations are bivalent. The surface films formed on such metals in a wide variety of polar aprotic systems cannot transport the bivalent cations. Such electrodes are blocked for the metal deposition [28-30], However, anodic processes may occur via the breakdown and repair mechanism. Due to the positive electric field, which is the driving force for the anodic processes, the film may be broken and cracked, allowing metal dissolution. Continuous metal dissolution creates an unstable situation in the metal-film and metal-solution interfaces and prevents the formation of stable passivating films. Thus, once the surface films are broken and a continuous electrical field is applied, continuous metal dissolution may take place at a relatively low overpotential (compared with the high overpotential required for the initial breakdown of the surface films). Typical examples are calcium dissolution processes in several polar aprotic systems [31]. 

Singbeil and Garner (10) showed that the use of anodic protection can prevent stress-corrosion cracking in the pressure vessel steels exposed to alkaline solutions used in digesters in the pulp and paper industry, as shown in Fig. 17. The 200 mV anodic polarization placed the material above the active-passive transition where cracking had been shown to occur (10). 

Fluid displacive resin-embedding of wet sediment is a passive procedure, samples are never physically dry and the fabric is supported by fluid throughout. Chemical dehydration prevents the cracking (common in vacuum dried sediment), and the technique requires no specialized drying, or resin-impregnating equipment. Water-saturated samples are more successfully embedded using fluid replacement than those which are partially dry. 

The LSP mechanism proposes that SCC results from the effect of the structure ahead of the crack tip [61]. This mechanism assumes that a galvanic corrosion between active sites (weakened passive site) and surroimding passive surfaces produces large anodic currents at the rupture site. Repassivation of the active sites is prevented by the presence of weakened passive films on the surface. It has been su ested that the weakened passive film... 

Chromium (Cr). Chromium increases the overall corrosion resistance of steel. Stainless steels contain in excess of 12% by weight of chromium. The corrosion resistance increases with increase in chromium content. The presence of chromium leads to the formation of a regenerative passive protective layer of chromium oxide that prevents further corrosion of steel. Chromium also contributes to increasing the hardenability of steel. It is a ferrite stabilizer, which means it promotes the formation of ferrite. Ferrite is resistant to the propagation of cracks. Presence of chromium increases the resistance of steel to pitting attacks. 

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

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