Corrosion, forms

Magnesium Designing Around Corrosion," Form No. 141-396-R87, The Dow Chemical Co., Midland, Mich., 1987. 

The second approach, changing the environment, is a widely used, practical method of preventing corrosion. In aqueous systems, there are three ways to effect a change in environment to inhibit corrosion (/) form a protective film of calcium carbonate on the metal surface using the natural calcium and alkalinity in the water, (2) remove the corrosive oxygen from the water, either by mechanical or chemical deaeration, and (3) add corrosion inhibitors. 

Impingement and erosion-corrosion forms of attack will usually be intensified by the presence of solid particles in the fluid. Variations of the jet test have been proposed to take this effect into account. ... 

In any manner, even though some of the classical corrosion forms can be also found in archaeological finds, fhey are frequently configured in a very complex form—either from a morphological, structural, or chemical viewpoint. Moreover, corrosion products bear a large piece of information about the artifact s life and should not be inadvertently removed, contrarily to modern metals, which would simply be etched. It is a big challenge to conserve such objects. On the one hand, it... 

Figure 16.16 is a representation of Fig. 16.1 a in terms of the type of reaction it shows, as a function of pH, the zones where corrosion by dissolution occurs, where initial corrosion forms insoluble oxides on the surface that impede further reaction (passive zone) and the region where the metal is stable (immune zone). 

This equation shows that with other conditions equal, the thickness of the tube has very little influence on the transmission of heat, and with a copper tube K in. thick, the amount of heat transmitted would be 66,300 B.t.u. For an iron tube He in. thick, and A = 35, Q would be 61,812 B.t.u. The smaU theoretical advantage of copper over iron is also true in actual practice, except that usually iron tubes are more easily corroded than copper tubes, and the corrosion forms an insulating film on the iron which greatly reduces the heat transmission. For practical purposes iron of the same thickness will transmit about 80 per cent of the heat transmitted by copper. 

Corrosion can take place uniformly over a rubbed surface or selectively at surface inclusions, grain boundaries and between dissimilar materials, etc. For the former case, a problem results only if corrosion is excessive. Limited corrosion is necessary for anti-wear and extreme pressure protection. Selective corrosion can weaken surface structures and initiate fatigue. Moreover, abrasion is implicated in the promotion of corrosive wear since abrasive particles remove soft protective, corrosively formed, reaction films. 

It is interesting to see that bismuth ions, Bi3+ or BiO+, inhibit the corrosion of metallic iron, zinc, and cobalt in perchloric acid solution. The bismuth ions are reduced in the cathodic reaction of metal corrosion forming metallic precipitates of bismuth on the corroding metal surface. 

HAZARD RISK Not combustible, but extremely irritating if involved in fire evolves heat when combined with water corrosive forms hydrogen gas upon contact with metals heating emits highly corrosive fumes NFPA Code not available. 

V. Chaker, (Ed.), Corrosion Forms and Control for Infrastructure, NACE International, Houston, Tex., 1992. 

N.S. Berke, M.C. Hicks, Estimating the life cycle of reinforced concrete decks and marine piles using laboratory diffusion and corrosion data, in V. Chaker (Ed.), Corrosion Forms and Control for Infrastructure ASTM STP2137, ASTM International, Philadelphia, PA, 1992, pp. 207-230. 

These three assumptions lead to uniform (general) corrosion. But this is only one of several corrosion forms that occur under different conditions. The other forms of corrosion depend on the deviations from the mentioned assumptions. Such deviations... 

These conditions will cause various deviations in the geometry and appearance of the attack compared with uniform corrosion, and it is convenient to classify corrosion just after the appearance of the corroded surface, as done by Fontana and Greene [7.1] (possibly with supplementary information about important service conditions). The advantage of such a classification is that a corrosion failure can be identified as a certain corrosion form by visual inspection, either by the naked eye or possibly by a magnifying glass or microscope. Sinee each form of corrosion has its characteristic... 

The relative frequency of occurrence of various corrosion forms depends on the type of industry and environment. One example from the chemical industry is shown in Table 7.1. In the heading we notice the high proportion of corrosion failures. The percentages of crevice corrosion and galvanic ( ) corrosion are surprisingly low. 

Thermogalvanic corrosion is prevented by appropriate design and measures to avoid uneven heating/cooling and forming of hot spots. For heat-insulated pipes and equipment is important that the insulation is continuous. The corrosion form may under certain conditions be prevented by cathodic protection or coatings. 

If there is a crevice on a component made of a material liable to crevice corrosion, and this component is connected to a more noble material with free surfaces, crevice corrosion may be intensified strongly. Such a case is a couple of an aluminium component (with a crevice) and a steel plate in water containing some chloride. The corrosion form can be called galvanic crevice corrosion. The crevice corrosion rate will be particularly high if the more noble metal acts as an efficient cathode in the given environment. The explanation is the same as for ordinary galvanic corrosion. 

Reasonably, the corrosion form is typical at relatively high velocities between the material surface and flie fluid, and it is particularly intensive in cases of two-phase or multiphase flow, i.e. hquid-gas and liquid-solid particle flow. Components often liable to erosion corrosion are propellers, pumps, turbine parts, valves, heat exchanger tubes, nozzles, bends, and equipment exposed to liquid sputter or jets. Most sensitive materials are those normally protected by corrosion products with inferior strength and adhesion to flie substrate, e.g. lead, copper and its alloys, steel, and under some conditions aluminium/aluminium alloys. Stainless steel, titanium... 

In some eases, the wear effects are more abrasive than erosive. Erosion is characterized by impingement, which implies a finite angle of impaet relative to the material surfaee. If, on the other hand, wearing particles are mowed in eontact with and parallel to the material surface, we have a case of abrasion, and a more eorreet name of the corresponding corrosion form would be abrasion eorrosion. This form will reasonably also comprise cases where corrosion products between two components are removed by relative movement of the components. 

Leaving cavitation corrosion and fretting aside as separate corrosion forms (see Sections 7.10 and 7.11), erosion and abrasion corrosion can be divided into three types, a), b) and c), as described below. The first two types are erosion corrosion, while type c) is to be considered as abrasion corrosion. The three types may overlap each other and partly occur simultaneously in the same system. 

Macroscopically, the crack surfaces may look brittle and diseoloured, dull or darkened by oxide layers. The initiation site may be discovered as a pit (formed by pitting or deposit corrosion) or, e.g. as a defect caused by forming or machining. On parts in service it may, however, be difficult to discover cracks, because they are narrow and filled or covered by corrosion products. As shown in the examples in Tables 7.1 and 7.2, SCC is a common and widespread corrosion form in the process industry. 

A tank made of stainless AISI304 steel (18-8 CrNi) contains stagnant seawater of 15 C. In die course of few months, corrosion has penetrated a 4 mm thick plate in the bottom. State cause, corrosion form and how the conditions can be improved. 

In the steel dealt with in Exercises 10 and 11, cracks may arise even though there is no significant stress. What does this indicate about the composition and treatment of the material How can we prove which corrosion form has acted and that the necessary conditions for this corrosion form have existed ... 

The choice between these possibilities is usually based upon economic considerations, but in many cases aspects such as appearance, environment and safety must also be taken care of Two or more of the five principles are commonly used at the same time. It is important to decide upon corrosion prevention at the design stage. In addition to the sections in the present chapter, reference is made to recommended protection methods under the treatment of various corrosion forms in Chapter 7. 

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