Corrosion-resistance Corrosive degradation products

Figure 9 shows a schematic process of biodiesel production by the two-step supercritical methanol method. Several advantages have been attributed to the two-step reaction method. At temperature of 270°C, a common type of 316 stainless steel can fulfill the requirements of good corrosion resistance and cover the reaction condition (5). Energy requirements may be less because mild reaction conditions for hydrolysis and methyl esterification are employed, whereas high-temperature treatment causes operational and equipment problems with, in some cases, the formation of undesirable degradation products. In addition, a reaction temperature of 270°C is commonly used in industries, so such a reaction condition is applicable for commercial applications. 

The corrosion resistance of lithium electrodes in contact with aprotic organic solvents is due to a particular protective film forming on the electrode surface when it first comes in contact witfi tfie solvent, preventing further interaction of the metal with the solvent. This film thus leads to a certain passivation of lithium, which, however, has the special feature of being efiective only while no current passes through the external circuit. The passive film does not prevent any of the current flow associated with the basic current-generating electrode reaction. The film contains insoluble lithium compounds (oxide, chloride) and products of solvent degradation. Its detailed chemical composition and physicochemical properties depend on the composition of the electrolyte solution and on the various impurity levels in this solution. 

Combustion products can affect sensitive electronic equipment. For example, hydrogen chloride (HCI) is formed by the combustion of PVC cables. Corrosion due to combusted PVC cable can be a substantial problem. This may result in increased contact resistance of electronic components. Condensed acids may result in the formation of electrolytic cells on surfaces. Certain wire and cable insulation, particularly silicone rubber, can be degraded on exposure to HCI. A methodology for classifying contamination levels and ease of restoration is presented in the SFPE Handbook... 

Heat transfer may be degraded in time by corrosion, deposits of reaction products, organic growths, etc. These effects are accounted for quantitatively by fouling resistances, l/hf. They are listed separately in Tables 8.4 and 8.6, but the listed values of coefficients include these resistances. For instance, with a clean surface the first listed value of V in Table 8.4 would correspond to a clean value of U = 1/(1/12 — 0.04) = 23.1. How long a clean value could be... 

Thermal stability refers to the ability of the oil components, both base oil and additive, to resist degradation or "cracking" due to the effects of heat. The products of thermal decomposition may be corrosive and form a deposit. Corrosion of engine parts (of non-ferrous metals) occurs during the winter months for the most part under low temperature driving conditions. Water vapor may condense in the crankcase, and rusting (of ferrous metals) may occur if the motor oil does not have adequate rust inhibiting properties (Anon, 1958). 

At high concentrations, corrosion-resistant reactors and an effective acid recovery process are needed, raising the cost of the intermediate glucose. Dilute acid treatments minimize these problems, but a number of kinetic models indicate that the maximum conversion of cellulose to glucose under these conditions is 65 to 70 percent because subsequent degradation reactions of the glucose to HMF and lev-ulinic acid take place. The modem biorefinery is learning to exploit this reaction manifold, because these decomposition products can be manufactured as the primary product of polysaccharide hydrolysis (see below). 

Corrosion, the degradation of a material s properties or mass over time because of environmental effects, is a costly reality that effects every industry. A study issued by the Federal Highway Administration (FHWA) in 2002 conservatively estimates the annual direct cost of corrosion in all U.S. industry sectors at US 276 billion. Costs associated with corrosion include cathodic/anodic protection coatings inhibitors corrosion-resistant alloys and materials and maintenance, repair, and depreciation of equipment. Indirect costs, such as lost productivity, environmental or product contamination, planning and design, and lost opportunities, can easily outpace direct costs by factors of two or more. 

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