Pipes gray cast iron

Gray iron is resistant to concentrated acids (nitric, sulfuric, phosphoric) as well as to some alkahne and caustic solutions. Caustic fusion pots are usually made from gray cast iron with low silicon content cast-iron valves, pumps, and piping are common in sulfuric acid plants. 

Sampie Specifications 10 in. (25 cm) outside diameter, gray cast iron pipe... 

FIGURE 21.7 Graphitization of a gray cast iron pipe, caused by long-term service in slightly acidic water. Copyright by Marcel Dekker, Inc. reprinted with permission [5]. 

Conventional Process for Production of Naphthalenedisulfonk Acid. The standard process for sulfonating aromatic hydrocarbons uses sulfuric acid and oleum as reactants. Under these conditions, the reaction vessels can be made out of gray cast iron, and steel can be used for piping and fittings. 

Obtain a general idea of the condition of gray cast iron pipe, especially when severely graphitized, by soimding the pipe with a hammer. A dull thud suggests extensive graphitization. 

Graphite flakes are cathodic to iron, and corrosion is localized to iron which starts leaching (becomes a porous mass) and leaves a rich residue of graphite flakes. Graphitization is observed on gray cast iron water pipes. 

Typical failures of cast iron pipes are characterized by leaks in pipe wall along the graphite flake/ferrite interface. The ratio of mechanical failure between the cast iron and ductile iron pipes is 2.5 to 1.0. Following is a comparison of pitting depths of ductile and gray cast iron pipes in aggressive soils (Table 9.18)[34j ... 

Table 9.18 Average maximum pit depth for ductile and gray cast iron pipes in aggressive soils [34]...

It is recommended to use ductile (nodular) cast iron instead of gray cast iron (brittle). Ductile iron pipe with a cement mortar lining has given excellent performance. 

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). 

Figure 17.5 Brittle fracture through pipe. The gray material is graphitically corroded cast iron. Unaffected pipe wall metal is orange from normal rust.

Piping systems for MAPP Gas must be made of steel. [5] Schedule 40 pipe is suitable if joints are welded. Schedule 80 pipe is recommended for threaded joints. In no case shall threaded joints be welded after assembly. Fittings may be rolled, forged, or cast steel, malleable iron, or modular iron. Gray or white cast iron fittings must not be used. 

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