Corroding surfaces

The hydroxyl ions migrate inward, attracted by the positive charge that is produced by the ferrous ion generated near the corroding surface (Fig. 3.4). Other anions such as carbonate, chloride, and sulfate also concentrate beneath the shell. Carbonate may react with ferrous ions to form siderite (FeCOa) as in Reaction 3.4 (Fig. 3.7) ... 

Clostridia are anaerobic bacteria that can produce organic acids. Short-chain organic acids can be quite aggressive to steel. Clostridia are frequently found deep beneath deposit and corrosion-product accumulations near corroding surfaces and within tubercles. Increased acidity directly contributes to wastage. 

A typical microbiological analysis in a troubled carbon-steel service water system is given in Table 6.2. Table 6.3 shows a similar analysis for a cupronickel utility main condenser that showed no significant corrosion associated with sulfate reducers. When biological counts of sulfate reducers in solid materials scraped from corroded surfaces are more than about 10, significant attack is possible. Counts above 10 are common only in severely attacked systems. 

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

Passive attack beneath slime is usually general, and rusting on steel may color surfaces brown and red (Fig. 6.14). If sulfate reducers are present, pitting may be superimposed on the generally corroded surface (see Fig. 6.2). 

Most of the surface is covered with a black corrosion product that is thicker in relatively low-flow areas near the hub. This layer of soft corrosion product can be shaved from corroded surfaces. Microstructural examinations revealed flakes of graphite embedded in iron oxide near the surfaces. 

Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

Metal loss in these areas had produced a smooth surface, free of deposits and corrosion products. The rest of the internal surface was covered by a thin, uniform layer of soft, black corrosion product. The graphitically corroded surfaces of the pump casing provided soft, friable corrosion products that were relatively easily dislodged by the abrasive effects of high-velocity or turbulent water (erosion-corrosion). 

Surface modifications and surfiice roughness Cu, Mo, and Be laser mirrors atomic oxygen modified (corroded) surfaces and films, and chemically etched surfaces. 

Corrosion Potential—the potential of a corroding surface in an electrolyte relative to some reference electrode. 

With respect to general corrosion, once a surface film is formed the rate of corrosion is essentially determined by the ionic concentration gradient across the film. Consequently the corrosion rate tends to be independent of water flow rate across the corroding surface. However, under impingement conditions where the surface film is unable to form or is removed due to the shear stress created by the flow, the corrosion rate is theoretically velocity (10 dependent and is proportional to the power for laminar flow and... 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Corrosion and scale deposition are the two most costly problems in oil industries. Corrodible surfaces are found throughout production, transport, and refining equipment. The Corrosion and Scale Handbook gives an overview of corrosion problems and methods of corrosion prevention [159]. 

Experiments with low- and high-velocity conditions were performed in standard laboratory tests [539]. It was found that corrosion is governed by the flow of the reactants and the products to and from the corroding surface. Corrosion in oxygenated fluids is increased with the velocity of the fluid because... 

Under favorable environmental conditions, a chemical equilibrium is established between a corroded surface layer and its surroundings, which may lead to the preservation of the bulk of copper thus ancient objects made... 

Su-Il Pyun provide a comprehensive review of the physical and electrochemical methods used for the determination of surface fractal dimensions and of the implications of fractal geometry in the description of several important electrochemical systems, including corroding surfaces as well as porous and composite electrodes. 

Iso-UP has ester bonds only in the main chain where hydrolysis occurs, so a part of reaction products from the main chain dissolves into the solution. While the crosslink formed by styrene remains unaffected because of its stable C-C bonding. As a result, the corroded surface layer resists the diffusion of NaOH solution. This mechanism is just like an oxidation of the metal at high temperature with formation of thick, cohered oxide scale, and can be expressed by similar relation of Wagner s parabolic law as shown in Equation 2. The concept of corrosion in metals can be applied in this case too. 

Various NDE techniques are used to locate defects and flaws in the failed or similar equipment that may not be apparent during the macro visual inspection. An analysis of cracks and other damage during the initiation or progressive phases often provides more information regarding the failure mechanism(s) than the same analysis would at locations where complete failure occurred. Considerable secondary damage to a worn, fractured, or corroded surface may occur after failure. The most common methods of NDE are given below. 

Bauer et ah, 1986). It is also a significant corrosion product of Fe alloy phases on Antarctic meteorites where its formation is induced by the chloride ions coming from airborne seaspray and/or volcanic activity (Buchwald and Clarke, 1989). In these meteorites, akaganeite is located adjacent to the corroding surface and beneath a layer of goethite/spinel into which it eventually transforms. 

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