Types of metallic corrosion

Uniform attack on a metal results in uniform corrosion. This is exploited in the processing and finishing of metals (Chapter 15). However, metallic structures are rarely homogeneous and surfaces are rough corrosion occurs, preferentially within fissures in the surface (crevice corrosion), making corrosion faster. 

In general, metals or alloys that are used are covered with oxide or hydroxide films. Formation of cracks and fissures can destroy the passivation. The depth of crevices increases rapidly because it is only there that the metal is not covered with a protective layer of oxide/hydroxide (see Fig. 16.8). The result is an increase in surface roughness and possible problems due to reduction in mechanical strength. 

In the case of scratches in paint films that cover the metal the principle is the same (Fig. 16.9). There is oxide formation at the scratch site, the corrosion continuing, therefore, in a direction parallel to the metal surface. 

Many other types of corrosion can occur. An important example is bimetallic corrosion which occurs at the junction between two different 

Another form of corrosion is due in part to the mechanical forces applied to metals, stress corrosion. When the corrosion reaction occurs with hydrogen evolution, hydrogen atoms, owing to their small size, can enter the metallic lattice and thus reduce the strength of the interatomic bonds. This is known as hydrogen embrittlement. If afterwards we apply a mechanical stress to the metal there is a greater possibility that it will rupture. Corrosion fatigue can have similar effects. This has been held responsible for some aeroplane crashes. 

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Before examining in detail the theories of aqueous corrosion processes and the bases for making quantitative calculations of corrosion rates, it will be useful to develop qualitatively the major phenomena involved. The following sections review several general types of metal/corrosive-environment combinations, the chemical reactions involved, idealized mechanisms for the transfer of metal ions to the environment, and the electrochemical processes occurring at the interface between the metal and the aqueous environment. 

Biomaterials. Just as stem designs have evolved in an effort to develop an optimal combination of specifications, so have the types of metals and alloys employed in the constmction of total joint implants. Pure metals are usually too soft to be used in prosthesis. Therefore, alloys which exhibit improved characteristics of fatigue strength, tensile strength, ductihty, modulus of elasticity, hardness, resistance to corrosion, and biocompatibiUty are used. 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Hence, the hot-dip compounds, or greases smeared cold, are better for assemblies with non-metallic parts masked if necessary. Solvent-containing protectives therefore find greater application in the protection of simple parts or components. The available means of application, the nature of any additional packaging and the economics and scale of the protective treatment are further factors that influence the choice of type of temporary corrosion preventive. 

Deposits of sand, dirt or permeable corrosion products on the metal surface (a type of crevice corrosion that is referred to as deposit attack). 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

Exposure to external atmospheres The rate of corrosion will depend mainly on the type of metal or alloy, rainfall, temperature, degree of atmospheric pollution, and the angle and extent of exposure to the prevailing wind and rain. 

Nature of the metal surface Clean, smooth, metal surfaces usually require a lower concentration of inhibitor for protection than do rough or dirty surfaces. Relative figures for minimum concentrations of benzoate, chromate and nitrite necessary to inhibit the corrosion of mild steel with various types of surface finish have been given in a recent laboratory studyThese results show that benzoate effectiveness is particularly susceptible to surface preparation. It is unwise, therefore, to apply results obtained in laboratory studies with one type of metal surface preparation to other surfaces in practical conditions. The presence of oil, grease or corrosion products on metal surfaces will also affect the concentration of inhibitor required with the... 

The primary difference between these types of metal wastage and oxygen corrosion is that these are all indirect forms of attack, induced by surface shielding (areas of metal surface under deposits or foulants, or cracks and gaps in the metal that are close to a shielding surface). 

A type of metal wastage caused by the mechanical action abrasion of the metal surface by high velocity steam, air bubbles or solid particles. Often part of a larger erosion-corrosion process. 

Depending on the binder and type of metal cast, the pH of spent foundry sand12 can vary from approximately 4 to 8. As such, it has been reported that some spent foundry sands can be corrosive to metals.14 Spent foundry sand must be monitored to assess the need to establish controls for potential phenol discharges.91516... 

In a process environment, the probes must neither corrode nor display catalytic or inhibitory properties. Quinn et al. report that about 30 reactions were run and five different types of metal probes were utilized with a highly corrosive reaction mixture before a custom built PEEK (polyether ether ketone) probe yielded... 

This mix of metals may be good for maximizing heat-transfer coefficients and specific applications, but it makes the heat exchanger susceptible to many types of metal wastage and corrosion fatigue failure if vigilance and good water treatment are not practiced. 

The initial surface composition of boiler tubing, prior to its installation will have an important impact on the amount and type of activated corrosion products in an aqueous reactor coolant. Consequently, the type of thermal pre-treatment the tubing undergoes, for example, for mechanical stress release,will affect the surface oxide film, and ultimately, the corrosion behavior. This particular work has been directed toward characterization of surface oxide films which form on Inconel 600 (nominal composition 77% Ni, 16% Cr, 7% Fe, — a tradename of Inco Metals Ltd., Toronto Canada) and Incoloy 800 (nominal composition 31% Ni, 19% Cr, 48% Fe 2% other, — a tradename of Inco Metals Ltd., Toronto, Canada) heated to temperatures of 500-600°C for periods of up to 1 minute in flowing argon. These are conditions equivalent to those experi enced by CANDU(CANadian Deuterium Uranium)ractor boiler hairpins during in situ stress relief. 

In such reactions as the tarnishing of silver in air, the oxidation of aluminum in air, or attack of lead in sulfate-containing environments, thin, tightly adherent protective films are formed, and the metal surface remains smooth. It should be mentioned that underground corrosion is frequently observed as localized corrosion. Oxidation, sulfidation, carburization, hydrogen effects, and hot corrosion can be considered as types of general corrosion.20... 

Figure 6.20 Two types of crevice corrosion oxygen concentration cell and metal ion concentration cell774...

Filiform Corrosion. This consists of the filamentary corrosion occuring on metallic surfaces and is a special type of crevice corrosion, sometimes called underfilm corrosion. It is frequently observed under the painted body of some used cars. It appears as a blister... 

A common form of corrosion met with this type of metal is pitting corrosion. Pitting arises when local cells are formed. This may be due to the presence of impurities in the metal such as sulphides, silicates, or noble metals such as copper. These areas will form local cathodes, adjacent weak spots in the oxide film will break down, and the underlying metal corrodes locally. Species within the aqueous medium may also induce pitting chloride ions are particularly aggressive in this respect [21]. 

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