Graphite galvanic corrosion

G lv nic Corrosion. Galvanic corrosion is an electrochemical process with four fundamental requirements (/) an anode (magnesium), 2) a cathode (steel, brass, or graphite component), (J) direct anode to cathode electrical contact, and (4) an electrolyte bridge at the anode and cathode interface, eg, salt water bridging the adjacent surfaces of steel and magnesium components. If any one of these is lacking, the process does not occur (133,134). 

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. 

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. 

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 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. 

Exposure of the metal to an environment that is sufficiently aggressive to generate a potential difference capable of driving a galvanic corrosion reaction between the graphite and the iron... 

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. 

As noted previously, dealloying also affects pre-boiler components manufactured of cast iron, such as FW pumps and valves. These components may, under certain (long-term) circumstances, suffer a selective form of preferential leaching galvanic corrosion. Specifically, graphitic corrosion may take place when the (anodic) iron matrix con-... 

By impressing upon the system a d-c current of a magnitude that opposes the current generated by the galvanic corrosion cell. In this case a piece of metal e.g., scrap iron or graphite) is used as the anode. 

Carbon itself has been successfully used as a biomaterial. Carbon based fibers used in composites are known to be inert in aqueous (even seawater) environments, however they do not have a track record in the biomaterials setting. In vitro studies by Kovacs [1993] disclose substantial electrochemical activity of carbon fiber composites in an aqueous environment. If such composites are placed near a metaUic implant, galvanic corrosion is a possibility. Composite materials with a polymer matrix absorb water when placed in a hydrated environment such as the body. Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature towards lower values [Delasi and Whiteside, 1978], hence a reduction in stiffness and an increase in mechanical damping. Water immersion of a graphite epoxy... 

Fig. 6. 27 SEM analysis of anode coating top surfaces after the 20 h galvanic corrosion test (a) Ni8o-Al2o (b) Nigs-Ahs (c) Nigo-Aho (d) Ni/graphite coupled to Nigs-Alos [80].

E6.2. Predict whether or not galvanic corrosion will cause the following alloys to be subjected to leaching (i) carbon and carbon steel alloys in an oxidizing atmosphere, (ii) steel rivets in aluminum drain gutters, (iii) copper-nickel alloy in refinery condenser tubes, (iii) graphite fiber-reinforced aluminum composites, (iv) brass in water, (v) iron-chromium alloys, and (vi) carbon steel pipe in contact with the weld to stainless steel pipe. 

Several aluminium alloys show very good corrosion resistance in various atmospheres. Some pitting occurs, but the pits remain small. Maximum depth seldom exceeds 0.5 mm during 6-20 years of exposure it is usually in the order of 0.1 mm. Some alloys may, however, be attacked by intergranular corrosion or exfoliation corrosion (see Section 7.7). Extensive galvanic corrosion may occur on aluminium in contact with copper, mild steel (in marine atmosphere) and graphite, less in contact with stainless steel, while aluminium is compatible with zinc [8.2]. 

Graphite, which is an electrical conductor, can serve as an eflBicient cathode during galvanic corrosion in Gr/Al MMCs. Czyrklis [28] and others [29-32] have found that the... 

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