Graphite, corrosion potentials

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. 

Note the potentials of the graphite and the aluminum alloy that you determined. If these two are connected with an electrical contact, their potentials should move toward each other. Further, since the solution is relatively conductive, and assuming that the electrical lead connecting them was highly conductive, they would come to the same potential. Therefore connect the leads of the two electrodes together and connect them both to the positive (or V) lead of the voltmeter. Measure the potential of this galvanic couple relative to one of the reference electrodes and confirm that the couple potential does indeed rest somewhere in between the corrosion potentials of the two materials. 

Cast iron is initially anodic to low-alloy steels and not far different in potential from mild steel. As cast iron corrodes, however, especially if graphitic corrosion takes place, exposed graphite on the surface shifts the potential in the noble direction. After some time, therefore, depending on the environment, cast iron may achieve a potential cathodic to both low-alloy steels and mild steel. This behavior is important in designing valves, for example. The trim of valve seats must maintain dimensional accuracy and be free of pits consequently, the trim must always be chosen cathodic to the valve body making up the major internal area of the valve. For this reason, valve bodies of steel are often preferred to cast iron for aqueous media of high electrical conductivity. 

Corrosion rates differ only sligthly in the various cast iron types. Grey cast iron (GC) is eroded somewhat more than cast iron with lamellar graphite (GGL) and ductile cast iron with graphite spheres (GCG). Table 29 lists the free corrosion potentials and corrosion rates for uniform surface corrosion in the cast iron types acc. to DIN EN 1561 [87], DIN EN 1563 [88] and DIN 1694 [89]. 

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). 

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... 

Electrodes were manufactured of NGZ, NGF and TEG and were tested under comparable conditions. The relative resistivity of graphite electrode to corrosion was compared on stability of CVs during the N (stands for number ) of consecutive voltammetric cycles in a given potential region. The relative resistivity to corrosion was estimated by the number of cycles for very stable cycling (Nstabie) and for saving of electrochemical activity (Nmax). 

The active material of the negative electrode consists of metallic cadmium. Addition of iron (up to 25%), nickel, and graphite, prevents agglomeration [348]. Cadmium does not undergo corrosion, since the equilibrium potential is higher than that of hydrogen in the same solution. 

Figure 3. Carbon corrosion rate versus carbon weight loss for both conventional and graphitized KB-supported Pt catalysts. The carbon corrosion rate (in units of A/g( ) is based on the measured CO2 concentration at the exit of a 50 cnr cell using a GC, assuming 4e /( (T molecule. The carbon weight loss is obtained by integrating the measured CO2 evolution rate over time. The cell is operating on neat H2/N2 (95 °C, 80% RIIjn, and 120 kPaa, s) with potential held at 1.2 Volts versus RHE.

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