Electronics corrosion products

Surface films are formed by corrosion on practically all commercial metals and consist of solid corrosion products (see area II in Fig. 2-2). It is essential for the protective action of these surface films that they be sufficiently thick and homogeneous to sustain the transport of the reaction products between metal and medium. With ferrous materials and many other metals, the surface films have a considerably higher conductivity for electrons than for ions. Thus the cathodic redox reaction according to Eq. (2-9) is considerably less restricted than it is by the transport of metal ions. The location of the cathodic partial reaction is not only the interface between the metal and the medium but also the interface between the film and medium, in which the reaction product OH is formed on the surface film and raises the pH. With most metals this reduces the solubility of the surface film (i.e., the passive state is stabilized). 

Both reactions indicate that the pH at the cathode is high and at the anode low as a result of the ion migration. In principle, the aeration cell is a concentration cell of H ions, so that the anode remains free of surface films and the cathode is covered with oxide. The J U curves in Fig. 2-6 for anode and cathode are kept apart. Further oxidation of the corrosion product formed according to Eq. (4-4) occurs at a distance from the metal surface and results in a rust pustule that covers the anodic area. Figure 4-2 shows the steps in the aeration cell. The current circuit is completed on the metal side by the electron current, and on the medium side by ion migration. 

The ion S " reacts with ferrous Fe ion to form black iron sulfide FeS corrosion product. The hydrogen ions are reduced by electrons produced by anodic reaction in step 1 and form hydrogen atom H ... 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

The principal use of gold is as a very thin coating about 0-05 /xm thick for electrical and electronic applications. Because of the thinness of gold electrodeposits, porosity must be very carefully controlled since seepage of corrosion products from substrate or undercoat exposed at these pores can have serious adverse effects on both appearance and electrical properties of the composite. The porosity can vary with the thickness of the deposit (Fig. 13.1), and with the type of plating bath and with its method of operation (Fig. 13.2), and the phenomenon has been extensively studied by Clarke and many other workers. 

A third mechanism by which the structural bonds between Fe atoms in iron oxides may be weakened involves reduction of structural Fe to Fe". In natural environments, reductive dissolution is by far the most important dissolution mechanism. It is mediated both biotically and abiotically. The most important electron donors, particularly in near surface ecosystems result from metabolic oxidation of organic compounds under O2 deficient conditions. In anaerobic systems, therefore, the availability of Fe oxides i. e. the electron sink, may control the degradation of dead biomass and organic pollutants in the ground water zone (see chap. 21). Reductive dissolution is also often applied to the removal of corrosion products from piping in industrial equipment and the bleaching of kaolin. 

This parabolic law, which indicates that diffusion is rate-limiting, is of overwhelming importance for scale formation. Wagner (1933) showed that the parabolic scale constant (and hence, rate of oxidation) can be calculated using the enthalpy of formation of the corrosion product, the electrical conductivity of the protective film and the transport number of the ions and electrons in the film. 

Kassim, J. Baird,T. Fryer, J.R. (1982) Electron microscope studies of iron corrosion products in water at room temperature. Corrosion ScL 22 147—158... 

The galvanic or contact method was the precursor to electrochemical treatment [283]. In an electrolytically conducting solution, the artifact is brought into contact with a piece of a less noble metal— usually zinc or aluminium. While the metal corrodes, the electrons supplied to the object allow the reduction of the tarnish layer. Although simple, this method presents some drawbacks, like the progressive contamination of the solution by the corrosion products of the active metal and also limitations with respect to the choice of applicable solution. 

In the corrosion example, we defined a layer of material containing oxidation and corrosion products. In this domain named C, for example, two phases were specified to exist, according to some particular model of the process. One of the phases is denoted CuO, and represents an oxide coating that has formed. The second phase, is designated CuS2, and contains products of an atmospheric sulfidation process. Within each phase, different chemical species may reside, for example, Cu+ ions, elemental Cu, holes, and electrons. The list of species depends on the particular reaction mechanism. 

Figure 7.1 Archaeological textile remains preserved in iron corrosion, Macro photograph (a) with scanning electron micrograph (b) (original magnification 2000x). The iron corrosion products have formed a negative cast around the wool fibers prior to their degradation many centuries before examination. (Photo R. C. Janaway.) (See color insert following p. 178.)...

Metals are subject to electrochemical corrosion in the presence of water Metal atoms lose electrons to become positively charged metal ions that go into solution. These then react with other chemical species in the soil ground-water to form solid corrosion products (e.g., metal oxides, hydroxides, sulfates). It is these solid corrosion products that often form a colored matrix with soil particles around the corroding object (Cronyn 1990). The initial formation of the metal ions takes place at a site on the metal known as the anode, whereas the electrons produced consumed by another reaction with an electron acceptor (the cathode). Due to the electrical conductivity of metals the location of the anode and cathode can be at different locations on the metal surface. In the presence of water and oxygen the cathodic reaction is... 

Keepax, C. (1975). Scanning electron microscopy of wood replaced by iron corrosion products. /. Archaeol. Sci. 2,145-150. 

X-ray photoelectron spectroscopy (XPS), also called electron spectroscopy for chemical analysis (ESCA), is a useful measure to know chemical environment of elements in material surface. The strength of XPS is its ability to identify different chemical states. This function is useful in physics, chemistry and material science, such as oxidation/corrosion products, adsorbed species or thin-film growth processes. Analysis of insulators is possible, and XPS is also capable of semiquan-titative analysis. 

Porosity is particularly important in thin cathodic coatings, such as gold contacts on copper used in the electronics industry, because corrosion will occur on the more reactive substrate and result in either perforation of the base, or the formation of corrosion products on the surface, which in turn have a detrimental effect. 

X-ray or electron diffraction allows Identification of crystalline species by the long-used Hannawalt-Dow-ASTM-JCPDS system(4). Small particles can be removed for analysis in a small rotating specimen X-ray powder camera, or by extraction replication and selected area diffraction in a Transmission Electron Microscope (5). For those specimens where a residue of reactant or corrosion product is too adherent, the material may be removed for analysis by micro-bulldozing (with a microhardness Indentor), micro-jack hammering (with a needle attached to a small piezoelectric crystal on a pencil-like rod), and micro-boring (with a precision controlled dental drill)(5). 

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