Passivation, metals processing

This occurs in strongly acid or strongly alkaline solutions, but there are specific exceptions. Thus in concentrated nitric acid the metal is passive and the kinetics of the process are controlled by ionic transport through the... 

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. 

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... 

Theberge SM, Luther GW, Rozan TF, Rickard DT (2000) Evidence for aqueous clusters as intermediates during copper sulfide formation. Abstr Am Chem Soc 220 353 Thompson DN, Sayer RL, Noah KS (2000) Sawdust-supported passive bioremediation of western United States acid rock drainage in engineered wetland systems. Minerals Metall Process 17 96-104 Tolman RC (1966) Consideration of Gibbs theory of surface tension. J Chem Phys 16 758-774 Tolman RC (1949) The effect of droplet size on surface tension. J Chem Phys 17 333-337 Tomino H, Kusaka I, Nishioka K, Takai T (1991) Interfacial tension for small nuclei in binary nucleatioa J Crystal Growth 113 633-636... 

The RH in most indoor environments is usually not above 70 percent and, thus, the CRH of most common metals is seldom exceeded. The time-of-wetness will be quite small. The corrosion rate is likely to be comparable to the outdoor rate (at similar contaminant levels) when the surfaces are dry. Such rates are insignificant compared to the wet rates for most metals (18). In many cases, the anions associated with deposited substances may play the dominant role in surface processes (24). The concentrations of sulfate, nitrate, and chloride, which accumulate on these surfaces, are likely to increase continuously. After 10 years exposure, total anion concentrations of five to ten /ng/cm can be expected in urban environments. These anions, especially chloride, are well known to dramatically affect the corrosion rates of many metals in aqueous solutions. This acceleration is often a result of solubilization of the surface metal oxide through complexation of the metal by the anions. Chloride, in particular, can dramatically lower the RH above which a moisture film is present on the surface, since chloride salts often have low CRHs (e.g., zinc chloride - < 10 percent calcium chloride - 30 percent and aluminum chloride - 40 percent). The combination of the low CRHs of chloride salts and the well documented ability of dissolved chloride to break down metal oxide passivation set chloride apart from the other common anions in ability to corrode indoor metal surfaces. Some nitrate salts also have moderately low CRHs (e.g., zinc nitrate -38 percent calcium nitrate - 49 percent aluminum nitrate - 60 percent). 

Halide ions, according to the adsorption theory of passivity, tend to break down passivity by competing with the passivator for adsorption sites on the metal surface. Should a halide ion find a vacant site and closely approach the surface, hydration and dissolution of metal ions are favored, and the anodic reaction can proceed with low activation energy, in contrast to the high activation energy required when a passivator is adsorbed. The anode reaction, if it persists, is confined to localized areas where the competitive process first succeeds, because surrounding metal immediately becomes cathode of an electrolytic cell, and is protected by flow of current from further anode activity, a process called cathodic protection. This attack at specific sites leads to corrosion pitting typical of metals otherwise passive that are actually corroded by their environment. 

Other microorganisms promote corrosion of iron and its alloys through dissimilatory iron reduction reactions that lead to the dissolution of protective iron oxide/hy dr oxide films on the metal surface. Passive layers are either lost or replaced by less stable films that allow further corrosion. Obuekwe and coworkers [60] evaluated corrosion of mild steel under conditions of simultaneous production of ferrous and sulfide ions by an iron-reducing bacterium. They reported extensive pitting when both processes were active. When only sulfide was produced, initial corrosion... 

This type of diagram indicates the possible electrochemical process on a metal surface if the potential and the pH of the electrochemical systems are known or estimated. In fact, corrosion rates can not be determined from a Pourbaix diagrams. The diagram includes regions identified as corrosion where a metal oxidizes, passive region where a metal is protected by a stable oxide film being adhered on the metal surface, and immunity where corrosion or passivation are suppressed. 

Cluster Tools. Many cleanroom processes can be combined into what is termed acluster tool. These are discrete functional modules linked to a central robot that allows the semiconductor device manufacturer to purchase one process tool that may perform dry etching, metallization and passivation in one piece of equipment. By 1995, cluster tools are expected to make up greater than 60% of the semiconductor thin-film processing equipment market. P ]... 

Fig. 10.25 Schematic log i - diagrams illustrating the principle of cathodic and anodic protection for a metal exhibiting passivity. The cathodic process (C) may be reduction of H2O or O2. The zones of electrode potential for protection are indicated.

Transition metals, such as Fe, Cr, Ni and Ti, demonstrate an active-passive behavior in aqueous solutions. Such metals are called active-passive metals. The above metals exhibit S-shaped polarization curves which are characteristic of such metals. Consider, for instance, the case of 18-8 stainless steel placed in an aqueous solution of H2SO4. If the electrode potential is increased then the current density rises to a maximum, with the accompanying dissolution of the metal taking place in the active state. The current density associated with the dissolution process indicates the magnitude of corrosion. At a certain potential, the current density is drastically reduced as the metal becomes passivated because of the formation of a thick protective film. Iron shows passivity... 

---