Cast iron anodes

Impressed-current CP uses an outside power supply such as a rectifier to control the voltage between the pipe and an anode (cast iron, graphite, platinum clad, mixed metal oxide) in such a manner that the pipe becomes the cathode in the circuit and corrosion is mitigated. 

Piebaked anodes aie produced by molding petroleum coke and coal tar pitch binder into blocks typically 70 cm x 125 cm x 50 cm, and baking to 1000—1200°C. Petroleum coke is used because of its low impurity (ash) content. The more noble impurities, such as iron and siUcon, deposit in the aluminum whereas less noble ones such as calcium and magnesium, accumulate as fluorides in the bath. Coal-based coke could be used, but extensive and expensive prepurification would be required. Steel stubs seated in the anode using cast iron support the anodes (via anode rods) in the electrolyte and conduct electric current into the anodes (Fig. 3). Electrical resistivity of prebaked anodes ranges from 5-6 Hm anode current density ranges from 0.65 to 1.3 A/crn. ... 

The first anode installation for the cathodic protection of gas pipelines in New Orleans consisted of a 5-m-long horizontal cast-iron tube. Later old tramway lines were used. Since in downtown New Orleans there was no suitable place to install impressed current anodes and to avoid detrimental effects on other pipelines, Kuhn recommended the use of deep anodes which were first installed in 1952 at a depth... 

The assessment for nonalloyed ferrous materials (e.g., mild steel, cast iron) can also be applied generally to hot-dipped galvanized steel. Surface films of corrosion products act favorably in limiting corrosion of the zinc. This strongly retards the development of anodic areas. Surface film formation can also be assessed from the sum of rating numbers [3, 14]. 

Galvanic anodes of cast iron were already in use in 1824 for protecting the copper cladding on wooden ships (see Section 1.3). Even today iron anodes are still used for objects with a relatively positive protection potential, especially if only a small reduction in potential is desired, e.g., by the presence of limiting values U" (see Section 2.4). In such cases, anodes of pure iron (Armco iron) are mostly used. The most important data are shown in Table 6-1. 

It is interesting that the first large-scale application of cathodic protection by Davy was directed at protecting copper rather than steel. It is also a measure of Davy s grasp of the topic that he was able to consider the use of two techniques of cathodic protection, viz. sacrificial anodes and impressed current, and two types of sacrificial anode, viz. zinc and cast iron. 

Skold and Larson" in studies of the corrosion of steel and cast iron in natural water found that a linear relationship existed between potential and the applied anodic and cathodic current densities, providing the values of the latter were low. However, the recognition of the importance of these observations is due to Stern and his co-workerswho used the term linear polarisation to describe the linearity of the rj — i curve in the region of E o , the corrosion potential. The slope of this linear curve, AE — AJ or Af - A/, is termed the polarisation resistance, / p, since it has dimensions of ohms, and this term is synonymous with linear polarisation in... 

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-... 

Selective form of iron corrosion, primarily in gray cast iron but also less commonly in nodular cast iron, whereby the (anodic) iron matrix converts to iron oxide while the (cathodic) graphite remains intact. The casting retains it shape but loses all strength and can be cut with a knife. 

The chemical reactivity of metallic Mg has been utilized in several ways. It is employed in the reduction step in the manufacture of Ti, in the deoxidation and desulfurization of steels and in the nodularization of cast iron. It has also been used for the preparation of photoengraving plates, in dry batteries, and as a sacrificial anode for cathodic protection of other metals. 

End plates and ports to be PVDF-lined ductile cast iron. Gaskets to be Viton for agent and Viton GF for energetics. Electrodes to be platinized titanium, pinhole-free, to a thickness of 5 micrometers for the anodes and 2.5 micrometers for the cathodes. 

Tantalum metal is prepared from potassium fluotantalate or tantalum pen-toxide produced from the ore concentrate by solvent extraction or fractional crystallization as described. The metal is produced industrially by Balkes electrolysis process. Fused potassium fluotantalate is electrolyzed at 900°C in a cast iron pot. While the latter serves as a cathode, a graphite rod is used as the anode. A small amount of tantalum oxide is added to the melt. The unreduced potassium fluotantalate is separated from the tantalum metal produced by leaching with water. Impurities are removed from the metal by acid wash. 

Acid Coolers. Cast iron trombone coolers, once the industry standard (101), are considered obsolete. In 1970, anodically passivated stainless steel shell and tube acid coolers became commercially available. Because these proved to have significant maintenance savings and other advantages, this type of cooler became widely used. Anodic passivation uses an impressed voltage from an external electrical power source to reduce metal corrosion. 

Another method of providing protective current is to use the heads or water boxes as sacrificial anodes. These water boxes, when made of heavy steel or of cast iron, provide galvanic protection to the tube ends and tube sheet as they corrode. 

Corresponding to each anode cell are four cast iron cathodes KK, about 8 feet long, which are connected as shown in Fig. 9. A continuous production of acid takes place during electrolysis, and no harmful effect is exerted upon the nitrifying bacteria, which continue their work of converting nitrogenous matter into calcium nitrate without interruption. Stakes of tarred wood g... 

The whole of the cell rests on four cast iron frames insulated from the floor by porcelain insulators. Between the insulators and the cell there are set screws by which the required tilt of the electrolyzer can be set and its position fixed. Brackets are fitted to frames to support the denuder trough. On the frames between the denuder and the electrolyzer small stands are placed which have small insulators carrying the copper or aluminium bus bars. Between the cathode and the anode bus bar is a current switch which is controlled from the floor. This is for switching the bath off the circuit during repair. 

The properties of the interface metal/solution. Cast iron corrodes because of exposure of its graphite to the surface (graphitic corrosion), which is cathodic to both low-alloy and mild steels. The trim of a valve must always maintain dimensional accuracy and be free of pitting and hence it should stay cathodic to the valve body. Hence, in aggressive media, valve bodies are frequently chosen of steel rather than cast iron. Because of increased anodic polarization, low-alloy steel (Cr and Ni as noble components) is cathodic to normal steel in most natural media. Accordingly, steel bolts and nuts coupled to underground mild steel pipes, or a weld rod used for steel plates on the hull of a ship, should always be of a low-nickel, low chromium steel or from a similar composition to that of the steel pipe.7... 

Figure 13.13 Equivalent circuit for a) the total impedance of the cast iron-water interface b) the anodic impedance and c) the interfacial impedance of the microporous layer.

Thus, the anodic surface corresponds to the end of the macropores, whereas the cathodic reaction occurs at the end of the micropores, which are located at the walls of the macropores. It should be noted that this physical-electrical model describes the behavior of cast iron at any time of immersion. 

The current passes between a carbon anode and a cast-iron plate. Flowing through the molten ore, the current causes oxygen from the oxide to combine with carbon in the anode while aluminum metal collects at the iron plate. 

The electric current causes copper from the anode on the right to be drawn into the solution while copper from the solution is deposited on the cathode, which in this case is the platinum strip on the left. Thus the crude copper is transferred to the platinum cathode just as aluminum is deposited on a cast-iron cathode. 

Single metals and alloys such as wrought iron, bronze and lead will corrode even though they are not joined to a different metal or alloy. In reality, anodes and cathodes are set up on the surface of the metal. This will be due to the different phases present in the alloy, e.g. iron (ferrite) will be the anode and graphite the cathode in a cast iron, impurities such as sulfides present in wrought iron and variation in the copper/zinc ratio in adjacent grains in a brass alloy. The reactions taking place on the anode and cathode will be the same except the anode may be a different metal if the object was not iron. With a copper artefact, for example, the anode reaction will be ... 

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