Corrosion graphite

Graphitic Corrosion Graphitic corrosion usually involves gray cast iron in which metalhc iron is converted into corrosion products, leaving a residue of intact graphite mixed with iron-corrosion products and other insoluble constituents of cast iron. 

See Chap. 5, Oxygen Corrosion Chap. 6, Biologically Influenced Corrosion and Chap. 17, Graphitic Corrosion. ... 

Differences in alloy carbon concentration, heat treatment, and mechanical forming usually produce only small differences in corrosion rate in a pH range of 4—10. It is less certain how corrosion rates vary at high and low pH due to these factors. Cast irons containing graphite particles may experience a unique form of attack called graphitic corrosion (see Chap. 17, Graphitic Corrosion ). 

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

Changing the pump metallurgy to a more corrosion- and cavitation-resistant material, such as stainless steel, is a potential solution to this type of problem. Note, however, that all other cast iron pump components that have sustained graphitic corrosion should be replaced to avoid the possibility of galvanic corrosion (see Chap. 16) between retained graphitically corroded cast iron components and new components. 

Another form of microstructural galvanic corrosion, graphitic corrosion, is unique to gray and nodular cast irons. It may be encountered in cast iron pumps and other cast iron components. It is a homogeneous form of galvanic corrosion, not requiring connection to a different metal. 

Graphitic corrosion has two distinct features that are useful in distinguishing it from other forms of corrosion. First, it affects an unusually limited number of metals the only metals commonly affected are gray cast iron and nodular cast iron. Second, metal that has experienced graphitic corrosion may retain its original appearance and dimensions. Consequently, graphitic corrosion frequently escapes detection. 

The basic mechanisms involved in graphitic corrosion are familiar and easily understood. Hence, remedial and preventive measures are relatively simple to implement. Although commonly categorized as a form of dealloying, graphitic corrosion has much in common with galvanic corrosion. 

Graphitic corrosion is a slow corrosion process, typically requiring many years to effect significant damage. Complete penetration of thick cross sections has, however, occurred in as little as 2 years in adverse environments. On the other hand, cast iron components can be found in use in Europe after 160 years of service. Although graphitic corrosion causes a substantial reduction in mechanical strength, it is well known that corroded cast iron, when sufficiently supported, may remain serviceable when internal pressure is low and shock loads are not applied. 

Note two cautions First, the graphitic corrosion should not be used interchangeably with the term graphitization. Graphitization, a... 

Figure 17 Mechanistic correlation between dry cell and graphitic corrosion.

The occurrence of graphitic corrosion is not location specific, other than that it may occur wherever gray or nodular cast iron is exposed to sufficiently aggressive aqueous environments. This includes, and is common to, subterranean cast iron pipe, especially in moist soil (Case History 17.1). Cast iron pump impellers and casings are also frequent targets of graphitic corrosion (Case Histories 17.2 through 17.5). 

On subterranean pipeline, look for graphitic corrosion on the very bottom of the line where it rests on the backfill. When graphitic corrosion occurs under these conditions, the affected region may be a narrow zone running along the pipe bottom over some distance (Case History 17.1). 

Control of graphitic corrosion can be effected by gaining control of the critical factors that govern it. 

Note also that graphitic corrosion may occur preferentially in poorly accessible areas, such as the bottom of pipelines. Trouble-free service of cast iron components does not necessarily indicate that all is well, since components suffering severe graphitic corrosion may continue to operate until an inadvertent or intentional (e.g., pressuretesting) shock load is applied. At this point massive, catastrophic failures can occur. 

Figure 17.9 Pump impeller that has suffered erosion associated with graphitic corrosion.

Figure 17.10 shows metal loss on the throat of the pump housing. External pump housing surfaces were also affected (Fig. 17.11). Note the large tubercles. (Tubercles are knoblike mounds of corrosion products. They typically have a hard, black outer shell enclosing porous reddish-brown or black iron oxides) (see Chap. 3, Tuberculation ). The metal surface beneath these tubercles had sustained graphitic corrosion, in some cases to a depth of Vi in. (0.6 cm) (Fig. 17.12).

The pump has experienced graphitic corrosion. Figures 17.10, 17.12, and 17.14 illustrate typical appearances of graphitically corroded cast iron. In addition, cavitation damage (see Chap. 12) has produced severe metal loss in specific areas (see Fig. 17.13). The soft, friable corrosion products produced by graphitic corrosion are susceptible to cavitation damage at relatively low levels of cavitation intensity. 

Figure 17.11 External surface of a pump housing showing tubercles capping sites of graphitic corrosion.

Figure 17.14 Cross section through the pump housing wall. Note the black and brown graphitic corrosion-product layer near the center of the photo.

Graphitic corrosion of the cast iron produced a soft, mechanically weak corrosion product that is highly susceptible to cavitation damage, even at relatively low cavitation intensities. The black coating on the impeller surface is visual evidence of graphitic corrosion. The spongelike surface contours are typical of cavitation damage (see Chap. 12). 

Figure 17.15 Section of a cast iron pump impeller that has suffered graphitic corrosion followed by cavitation damage.

Failures of this type occurred every 6 to 9 years. The well was operated intermittently, since several available wells are operated on a rotating basis. It is probable that most of the graphitic corrosion occurred during idle times actual metal loss occurred during operation of the pump. 

Figure 17.18 Close-up of the perforation in Fig. 17.16 viewed from the external surface. Graphitic corrosion of the external surface permitted erosion by turbulent water escaping through the perforation.

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