Corrosion morphology

Virtually all metallurgies can be attacked by corrosive bacteria. Cases of titanium corrosion are, however, rare. Copper alloys are not immune to bacterial attack however, corrosion morphologies on copper alloys are not well defined. Tubercles on carbon steel and common cast irons sometimes contain sulfate-reducing and acid-producing bacteria. Potentially corrosive anaerobic bacteria are often present beneath... 

Corrosion morphology must be consistent with biological attack. 

Corrosion morphologies. Sulfate-reducing bacteria frequently cause intense localized attack (Figs. 6.2 through 6.7). Discrete hemispherical depressions form on most alloys, including stainless steels, aluminum. Carpenter 20, and carbon steels. Few cases occur on titanium. Copper alloy attack is not well defined. 

Failure due to corrosion may also involve a combination of two corrosion modes. In analyzing a failure due to corrosion it is necessary to identify the corrosion mode tentatively by visual and metallographic methods, the corrosive agents, corrosion morphology, and to make a definitive identification of the corrosion product or scale. 

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

Chemical behavior under process conditions. Will the process fluids chemically attack the separation equipment (corrosion, morphological changes such as swelling, etc.) and/or react themselves (polymerize, oxidize, etc.) This is a very important consideration as it affects the lifetime and rehability of the process. 

According to X-ray diffraction and light optical investigations, the anodic dissolution of -brass results in a e — y — a phase transformation with porous product phases [22, 23]. As revealed by a more detailed investigation of the corrosion morphology, the extent of this transformation depends on the overpotential of the zinc dissolution reaction [24]. At a low overpotential of Eh = —0.75 V, the only product phase is y. Similar to the scheme of... 

Corrosion Morphologies—Protective Layers Formed at High Temperature... 

Corrosion Morphologies—Phase/Phase Morphologies and Microstructural Defects... 

Song et al. (1999b) have suggested that the primary a and eutectic a phases, which have different aluminum content, have different electrochemical behavior. Both the primary and eutectic a phases can form galvanic corrosion cells with the / phase as illustrated in Fig. 4-25. There are, therefore, two kinds of corrosion morphology ... 

Actual exposure (Song et al., 1999b) of die-cast AZ91D in NaCl (Fig. 4-26) illustrates both kinds of corrosion morphology. 

Figure 4-26. Typical corrosion morphology of die-ca.st AZ91D after a few minutes in 1 m NaCl (pH 11) (Songetal., 1999b).

Lyon, Thompson, and Johnson [56] point out that the high sodium chloride content of the salt spray test can resnlt in corrosion morphologies and behaviors that are not representative of natnral conditions. Harrison has pointed out that the test is inappropriate for use on zinc—galvanized snbstrates or primers with zinc phosphate pigments, for example — becanse, in the constant wetness of the salt spray test, zinc undergoes a corrosion mechanism that it wonld not nndergo in real service [57]. This is a well-known and well-docnmented phenomenon and is discnssed in depth in chapter 7. 

Corrosion morphologies of AZ91E after 4 hour immersion in 5% NaCI (based on Song, 2005a). 

The corrosion morphology of Mg-Gd-Y alloy after the immersion time... 

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