Corrosion indications

Microstructural examinations revealed graphitic corrosion (see Chap. 17) of the metal surfaces. The evidence of graphitic corrosion indicates one of the following ... 

By taking the ratio of the standard deviation of the potential signal to that of the coupling current signal, a parameter with the units of resistance (or Q-cm2 when corrected for area) can be calculated. This ratio has been termed the noise resistance, Rn, by Eden and Rothwell (57) and has been found to correlate to the polarization resistance, Rp, to be discussed in Chapter 4. Localized corrosion indices have also been proposed (58), though none have been sufficiently correlated with other measurements to allow generalizations to be made. 

The influence of processing techniques for grapefruit juice on the rate of can corrosion was studied by Bakal and Mannheim (1966). They found that the dissolved tin content can serve as a corrosion indicator. In Israel the maximum prescribed limit for tin content of canned food is 250 ppm. Deaeration of the juice significantly lowers tin dissolution. In a study of the in-can shelf life of tomato paste, Vander Merwe and Knock (1968) found that, depending on maturity and variety, 1 g of tomato paste stored at 22°C could corrode tin at rates ranging from 9 x 1CT6 g/month to 68 x 10 6 g/month. The useful shelf life could vary from 24 months to as few as 3 months. Up to 95 percent of the variation could be... 

Sulfur is measured on the basis of both quantity and potential corrosivity. The quantitative measurements can be made by means of a combustion bomb (ASTM D-129, IP 61). The measurement of potential corrosivity can be determined by means of a copper strip procedure (ASTM D-13, IP 154). The quantitative determination is an indication of the corrosive tendencies of the fuel combustion products, whereas the potential corrosivity indicates the extent of corrosion to be anticipated from the unbumed fuel, particularly in the fuel injection system. 

Paralinear corrosion (related to dissolution of corrosion product) does not occur for all aluminum alloys in water at all high temperatures. In Figure 13 are plotted data for an alloy (Al, 1% Ni, 0.1% Ti) corroded in water at 350°C (10). The corrosion rate was low and constant, as shown better in other figures in the same publication. For some specimens in the figure 1/3 or 2/3 of the corrosion product was removed mechanically after the first exposure period. There was no discernible effect on subsequent corrosion, indicating that control of corrosion probably resided close to the metal-oxide interface. Similar experiments for the alloys and temperatures where paralinear behavior occurs showed that removing some of the product caused an increase in subsequent corrosion rate. 

Corrosivity indicator tests normally used to evaluate BA or BS are pH, electrical resistivity, soluble chloride content, and soluble sulfate content. Materials are judged to be noncorrosive if the pH exceeds 5.5, the electrical resistivity is... 

Cyclic Potenttodynamic Polarization Method Electrochemical studies of pitting corrosion indicate that pitting occurs within or above a critical potential or potential range. The electrochemical methods involved in pitting corrosion studies are... 

This experiment, detailed on the Practical Chemistry website, can be set up in pairs or as a whole class and, by using corrosion indicator, the results can be seen within a lesson, wwwnuffieldfoundadpn.org/practi ... 

Regardless of its function as an alternate corrosion indicator of Ico , the LPR serves as a useful standalone probe of the chemical component of CMP. Since the faradaic activity of a metal surface proportionally decreases with increasing values of Rp, it is usually possible to establish a direct correlation between the MRRs and Rp as long as the mechanism of material removal is dictated by electrochemical mechanisms. The quality of this correlation can then be used as an empirical measure of the chemical component of CMP. An example of this analysis is shown in Figure 3.10, where the polish rates of TaN measured by CMP in different solutions (Janjam et al., 2010a) are... 

The assembly used for the corrosivity measurements consists of preweighed 1100 aluminum wire fastened to and tightly wound on threaded lengths of steel, copper, and nylon. Figure 3 shows a completed assembly. The details of preparation are given in ASTM G 116. The data derived fiom the individual wire-hoh combinations are used to develop corrosion indices the aluminum-nylon results reflect the general atmospheric corrosion (ACI), the aluminum-steel the marine index (MCI), and the aluminum-copper the industrial index (ICI). 

The solubility of elements in freshwater is limited and the solubility of calcium and magnesium carbonates are of particular importance in freshwaters. The solubility of carbonates is inversely proportional to the temperature of the water. In other words, as the water temperature increases, calcium and magnesium carbonates become less soluble. If the solubility decreases sufficiently, carbonates will precipitate and form a scale on the surfaces of the system. This scale can provide a protective barrier to prevent corrosion of the metallic elements in a system. Excessive scale deposits can interfere with water flow and heat transfer. The quality of the scale is dependent on the quantity of calcium that can precipitate as well as water flow and the chloride and sulfate content of the water. The tendency of water to precipitate a carbonate scale is estimated from corrosion indices such as the Langelier Saturation Index (LSI) and Caldwell-Lawrence calculations [6-8] which use calcium, alkalinity, total dissolved solids, temperature and pH properties of the water. Other indices, such as the Ryznar Index... 

TABLE 3—Atmospheric corrosion indices calculated from a modified Legault-Ledrie equation for an industrial atmosphere" (36 heats of each steel). 

The corrosion resistance of ferritic steels has been extensively studied. The following expressions summarize the effects of different alloying elements on the resistance of ferritic steels exposed to boiling corrosive solutions during slow strain tests. The stress corrosion indices (SCls) in each environment integrate the beneficial (-) or deleterious (-I-) effect of the alloying elements (in %) when the steels are in contact with such a caustic environment. In boiling 4M NaNOa at pH 2 the stress corrosion index is... 

Corrosion indicators should therefore specify the limits in the performance that can be achieved with a given material, both in terms of lifetime and acceptable stress conditions, as well as the extent to which electrochemical theory and experience are sufficient to support such a prediction. 

Dissimilar metals in intimate contact or connected by conductive path, such as water, condensation, or electrolyte, should be applied only when the functional design renders this unavoidable. If the use of dissimilar metals is necessary, an attempt to select metals which form compatible couples or groups should be made. The Galvanic Corrosion Indicator published by the International Nickel Company Ltd. can be useM. Table 9.1 shows examples of other environments with different indicators. 

Clad metals may be subject to galvanic corrosion along exposed edges, if the metals are far apart, according to a galvanic corrosion indicator (e.g. copper/aluminum clad to aluminum see Figure 9.8). 

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