Corrosive attack

This type of corrosive attack occurs when dissimilar metals (i.e., with a different are in direct electrical... 

Weldments subjected to corrosive attack over a period of time may crack adjacent to the weld seams if the residual stresses are not removed. Gas—tungsten arc welding and gas—metal arc welding ate recommended for joining magnesium, the former for thinner materials and the latter for thicker materials. Maintaining a protective atmosphere is a critical issue in welding these alloys. 

Dry chlorine has a great affinity for absorbing moisture, and wet chlorine is extremely corrosive, attacking most common materials except HasteUoy C, titanium, and tantalum. These metals are protected from attack by the acids formed by chlorine hydrolysis because of surface oxide films on the metal. Tantalum is the preferred constmction material for service with wet and dry chlorine. Wet chlorine gas is handled under pressure using fiberglass-reinforced plastics. Rubber-lined steel is suitable for wet chlorine gas handling up to 100°C. At low pressures and low temperatures PVC, chlorinated PVC, and reinforced polyester resins are also used. Polytetrafluoroethylene (PTFE), poly(vinyhdene fluoride) (PVDE), and... 

Some tests indicate that magnesium alloys are resistant to loam sod. However, in the presence of chlorides, corrosive attack may be serious particularly if galvanic couples are present as a result of coupling to iron stmctures. 

The anodes can be made of graphite which tolerates high current densities without passivation, but are subject to gradual corrosive attack causing a... 

Corrosion attack on the polymer is influenced by permeation rate, as weU as internal stresses or fatigue, that distorts or fractures the resin glass fiber... 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibiUty for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibiUty depends on pH. The susceptibiUty temperature increases paraboHcaHy from 65°C as pH is increased from 2ero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equiUbrium potential into the passive region where a protective film is formed and maintained. 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

Union Carbide has developed Amine Guard, which essentially eliminates corrosion in amine systems (32—35). It permits the use of substantially higher amine concentrations and greater carbon dioxide pick-up rates without corrosive attack. This results in an energy requirement comparable to that of the carbonate process and allows the use of smaller equipment for a specific C02-removal appHcation thereby reducing the capital cost. 

Successful operation of the gaseous diffusion process requires a special, fine-pored diffusion barrier, mechanically rehable and chemically resistant to corrosive attack by the process gas. For an effective separating barrier, the diameter of the pores must approach the range of the mean free path of the gas molecules, and in order to keep the total barrier area required as small as possible, the number of pores per unit area must be large. Seals are needed on the compressors to prevent both the escape of process gas and the inflow of harm fill impurities. Some of the problems of cascade operation are discussed in Reference 16. 

Fretting Corrosion This attack occurs when metals shde over each other and cause mechanical damage to one or both. In such a case, frictional heat oxidizes the metal and this oxide then wears away or the mechanical removal of protective oxides results in exposure of fresh surface for corrosive attack. Fretting corrosion is minimized by using harder materials, minimiziug friction (via lubrication), or designing equipment so that no relative movement of parts takes place. 

Wastage was caused by long-term crevice corrosion. Attack was much more severe beneath the baffle than elsewhere. Subsequent investigation revealed severe damage at many baffles. 

Wastage was caused by exposure to oleic acid and short-chain organic acids in the rolling oil. Fatty acids break down to form shorter-chain acids in service. However, oleic acid, of and by itself, is fairly corrosive. Attack due to oleic acid can be reduced substantially using appropriate chemical inhibition. 

It is often found that wet corrosion attacks metals selectively as well as, or instead of, uniformly, and this can lead to component failure much more rapidly and insidiously than one might infer from average corrosion rates (Fig. 23.7). Stress and corrosion... 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio = 1) and polarization (f/jq = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

In the tidal zone and the spray zone (known as the splash zone), cathodic protection is generally not very effective. Here thick coatings or sheathing with corrosion-resistance materials (e.g., based on NiCu) are necessary to prevent corrosion attack [4]. The coatings are severely mechanically stressed and must be so formed that repair is possible even under spray conditions. Their stability against cathodic polarization (see Section 17.2), marine growths, UV rays and seawater must be ensured [4,5]. 

The horizontal surfaces should be coated because there is residual water in the ballast and there are water-oil mixtures in the crude oil tanks when ships travel empty and these can cause severe corrosion attack. In the lower part of the tank, up to about 1.5 m from the base, a combination of coating and cathodic protection with special anodes is chosen. Basically the anodes could take over the exclusive protection in this area, but with empty ballast tanks containing residual water or empty crude oil tanks with aggressive oil-water mixtures containing sulfur compounds, they do not prevent corrosion. 

The information in Sections 2.2, 2.4 and 3.3 is relevant for protection criteria. Investigations [43] with steel-concrete test bodies have shown that even in unfavorable conditions with aerated large-area cathodes and small-area damp anodes in Cl -rich alkaline environments, or in decalcified (neutral) surroundings with additions of CU at test potentials of (/f.y.cuso4 = -0.75 and -0.85 V, cell formation is suppressed. After the experiments had proceeded for 6 months, the demounted specimens showed no recognizable corrosive attack. 

A diagnosis of possible damage should be made before beginning repairs with other construction measures [48,49]. There should be a checklist [48] of the important corrosion parameters and the types of corrosion effects to be expected. Of special importance are investigations of the quality of the concrete (strength, type of cement, water/cement ratio, cement content), the depth of carbonization, concentration profile of chloride ions, moisture distribution, and the situation regarding cracks and displacements. The extent of corrosion attack is determined visually. Later the likelihood of corrosion can be assessed using the above data. 

The uncoated blade showed an 0.005 inch corrosion attack over 50% of the airfoil concave face, with about 0.010 inch penetration at the base of the airfoil. Examination of the coated blade revealed no visual evidence of attack, except for one small roughened spot on the leading edge about 1 inch up from the platform, and a second spot in the middle of the convex side about 1 inch down from the tip. 

Corrosion Probiems in Service. The wide variety of corrosive factors that are present in refining and petrochemical processing (e.g., acids, caustics, chlorides, sulfides, sulfates) give rise to a wide variety of corrosive attacks, some of which act ... 

The L-foot fm covers the tube more or less completely to protect the base tube against corrosive attack, but still leaves a potential corrosive site at the base of the fin adjacent to the preceding fin. The double L-foot is intended to provide complete coverage of the tube, where corrosion would otherwise be a problem. Where corrosion is troublesome, soldered or galvanized tubes may offer a solution. The dimensions of fmned tubes are results of past experience in the design of air cooled heat exchangers. Tube diameters range from about 1.905 cm (0.75 in.) to 5.08 cm (2.0 in,). 

Because the plates are made of thin pressed metal, materials resistant to corrosive attack can be easily selected. Plates are standard and mass-produced,. Specific applications are dealt with by changing plate arrangements. Stainless steels, monel, titanium, aluminum bronze, and other exotic metals... 

Figure 7-8H. Special metal disk holder for polymer systems using a smooth disk surface to reduce polymer adherence, and a smooth annuleir sealing area. Usually thick to avoid need for vacuum support and to allow for corrosion attack. By permission, Fike Metal Products Co. Div., Fike Corporation, Inc.

Corrosion Theory 1259. Forms of Corrosion Attack 1268. Factors Influencing Corrosion Rate 1292. Corrodents in Drilling Fluids 1300. Corrosion Monitoring and Equipment Inspections 1312. Corrosion Control 1323. Recommended Practices 1340. 

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