Corroding metals

Moist iodine vapor rapidly corrodes metals, including most stainless steels. The initial process is the formation of corrosion centers where small amounts of metal iodide are formed which deHquesce, and the corrosion then takes place electrochemically (41,42). Only titanium and molybdenum steels are unattacked by iodine (42,43). The corrosion of molten iodine has also been studied. 

In addition to films that originate at least in part in the corroding metal, there are others that originate in the corrosive solution. These include various salts, such as carbonates and sulfates, which may be precipitated from heated solutions, and insoluble compounds, such as beer stone, which form on metal surfaces in contac t with certain specific products. In addition, there are films of oil and grease that may protect a material from direct contact with corrosive substances. Such oil films may be apphed intentionally or may occur naturally, as in the case of metals submerged in sewage or equipment used for the processing of oily substances. 

In EIS, a potential is applied across a corroding metal in solution, causing current to flow The amount of current depends upon the corrosion reaction on the metal surface and the flow of ions in solution. If the potential is apphed as a sine wave, it will cause harmonics of the current output. The relationship between the apphed potential and current output is the impedance, which is analogous to resistance in a DC circiiit. 

As a result of the concentration of acidic species, such as chloride and sulfate, material scraped from the inside of tubercles is virtually always acidic when mixed with water. Acidity varies not only from tubercle to tubercle but also from place to place in a given tubercle. Acidity is greatest near the corroded metal surface. The size of the fluid-filled cavity can indicate acidity. The larger the cavity, the more acidic the internal environment. 

The depressions and plugs of corroded metal were caused by dezincification. The hole shown in Fig. 13.10A was caused when one such plug blew out of the wall due to the pressure difference between internal and external surfaces. 

It is interesting to note that the iron is not removed but is transformed in situ to iron oxide. What is even more remarkable is that the volume change anticipated when iron is converted to iron oxide does not occur. Consequently, surface contours of the corroded metal remain intact, even to the point of retaining fine detail such as machining marks. 

Affected metal is converted to a very soft material that can be dislodged with a sharp metal probe or cut with a knife. Graphitically corroded metal frequently has an oily, shppery feel, especially after surface deposits are scraped away. Because of its graphite content, chunks of corroded metal can be used as writing implements in the absence of a pencil (Fig. 17.4). Graphitically corroded metal will produce a dull thud rather than a metallic sound when struck with a hammer. 

Figure 17.6 Black, graphitically corroded metal at the surface surrounding bright, unaffected metal.

It is important to note that graphitically corroded metal may be overlooked in a simple visual inspection. Appropriate use of a sharp probe or hammer should be helpful in the identification. 

Sodium and potassium are restricted because they react with sulfur at elevated temperatures to corrode metals by hot corrosion or sulfurization. The hot-corrision mechanism is not fully understood however, it can be discussed in general terms. It is believed that the deposition of alkali sulfates (Na2S04) on the blade reduces the protective oxide layer. Corrosion results from the continual forming and removing of the oxide layer. Also, oxidation of the blades occurs when liquid vanadium is deposited on the blade. Fortunately, lead is not encountered very often. Its presence is primarily from contamination by leaded fuel or as a result of some refinery practice. Presently, there is no fuel treatment to counteract the presence of lead. 

CoiTosion prevention is achieved by correct choice of material of construction, by physical means (e.g. paints or metallic, porcelain, plastic or enamel linings or coatings) or by chemical means (e.g. alloying or coating). Some metals, e.g. aluminium, are rendered passive by the formation of an inert protective film. Alternatively a metal to be protected may be linked electrically to a more easily corroded metal, e.g. magnesium, to serve as a sacrificial anode. 

Chemical Reactivity - Reactivity with Water. Dissolves and forms a weak solution if nitric acid. The reaction is not hazardous Reactivity with Common Materials May corrode metals in presence of moisture Stability During Transport Stable Neutralizing Agents for Acids and Caustics Flush with water Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Angreifbarkeit, /. attackability. aogreifen, v.t. attack, act on, affect corrode (metals) lay hold of fatigue undertake, aogrenzen, v.t. border on, adjoin. — v.i. border (on). — angrenzend, p.a. adjacent, adjoining, contiguous. 

Other properties of interest are carbon residue, sediment, and acidity or neutralization number. These measure respectively the tendency of a fuel to foul combustors with soot deposits, to foul filters with dirt and rust, and to corrode metal equipment. Cetane number measures the ability of a fuel to ignite spontaneously under high temperature and pressure, and it only applies to fuel used in Diesel engines. Typical properties ol fuels in the kerosene boiling range are given in Table 1. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

Transfer of charge through the solution and corroding metal. 

Graphic estimation of the corrosion rate and corrosion potential of a metal immersed in a corrosive high-conductivity electrolyte, from the intersection of the polarisation curves for the appropriate anodic and cathodic reactions, has been proposed and explained by several authorities. These polarisation curves can be further used to illustrate the effect of imposing additional anodic or cathodic potentials on to a corroding metal (see also Sections 1.4 and 10.1). 

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