Corrosion of graphite

Investigation of Mechanisms of Electrochemical Corrosion of Graphite Electrodes... 

In contrast with AGR-reactors, the high temperature reactors (HTR) currently in development, which are also gas-cooled graphite-moderated reactors, helium is the cooling gas, thereby avoiding the troublesome corrosion of graphite by carbon dioxide, according to the equation ... 

Voltammetry data of graphite electrodes in molten NaN03/KN03 at 240-350°C indicate an anodic reaction involving 2 ion and NO that proceeds via an oxide group on the graphite surface. The corrosion of graphite was related to the formation of a NO3 intermediate [84, 85]. 

F E Sloan and J B Talbot, Corrosion of graphite fibre reinforced composites 1 galvanic coupling damage . Corrosion 1992 48(10) 830-838. Corrosion of graphite fibre reinforced composites 2 anodic polarization damage . Corrosion 1992 48(12) 1020-1026. 

However, the very strong acidic conditions increase the oxygen potential and additionally oxygen formation is kinetically hindered at usual anode materials, i.e., it needs a high charge transfer over-potential. Therefore - in contrast to chlor-aUcali electrolysis - no oxygen is detectable in the anode gas and no corrosion of graphite anodes occurs (service life 10 years [4]). 

Pb(CH3)4 can be applied to extract aluminium and alkylaluminium alkoxide impurities from organoaluminium complexes used in the electrolytic preparation of tetraorganolead compounds [177]. Pb(CH3)4 was claimed to prevent corrosion of graphite by CO2 used as fluid coolant in nuclear reactors [178], and to stabilize halogenated aryl compounds, employed as dielectric, insulating, or cooling agents [179]. 

W. J. IIallictt et al., Dynamic Corrosion of Graphite by Liquid Bismuth, USAEC R( poi t XAA-SR-188, North American Aviation, Inc., Sept. 22, 1952. 

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

The principal disadvantage of the acid process is the higher capital cost involved mainly because of more processing steps and the corrosivity of hot, concentrated phosphoric acid which requires a reactor built from dense graphite. 

The resistance of graphite to thermal shock, its stabiUty at high temperatures, and its resistance to corrosion permit its use as self-supporting vessels to contain reactions at elevated temperatures (800—1700°C), eg, self-supporting reaction vessels for the direct chlorination of metal and alkaline-earth oxides. The vulnerabiUty of cemented joints in these appHcations requires close tolerance ( 0.10 mm) machining, a feat easily accompHshed on graphite with conventional metal machining equipment. 

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

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

Similarly, graphitically corroded cast iron (see Chap. 17) can assume a potential approximately equivalent to graphite, thus inducing galvanic corrosion of components of steel, uncorroded cast iron, and copper-based alloys. Hence, special precautions must be exercised when dealing with graphitically corroded pump impellers and pump casings (see Cautions in Chap. 17). 

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). 

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

Graphitically corroded cast irons may induce galvanic corrosion of metals to which they are coupled due to the nobility of the iron oxide and graphite surface. For example, cast iron or cast steel replacement pump impellers may corrode rapidly due to the galvanic couple established with the graphitically corroded cast iron pump casing. In this or similar situations, the entire affected component should be replaced. If just one part is replaced, it should be with a material that will resist galvanic corrosion, such as austenitic cast iron. 

Most of the surface is covered with a black corrosion product that is thicker in relatively low-flow areas near the hub. This layer of soft corrosion product can be shaved from corroded surfaces. Microstructural examinations revealed flakes of graphite embedded in iron oxide near the surfaces. 

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.

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). 

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