News > Corrosion Resistance

Corrosion Resistance

2/4/2011
Cast Iron has, for hundreds of years, been the preferred piping material throughout the world for drain, waste, and vent plumbing applications and water distribution. Gray iron can be cast in the form of pipe at low cost and has excellent strength properties. Unique corrosion resistance characteristics make cast iron soil pipe ideally suited for plumbing applications.
 

Cast Iron has, for hundreds of years, been the preferred piping material throughout the world for drain, waste, and vent plumbing applications and water distribution. Gray iron can be cast in the form of pipe at low cost and has excellent strength properties. Unique corrosion resistance characteristics make cast iron soil pipe ideally suited for plumbing applications.

Cast iron and steel corrode; however, because of the free graphite content of cast iron (3% - 4% by weight or about 10% by volume), an insoluble graphitic layer of corrosion products is left behind in the process of corrosion. These corrosion products are very dense, adherent, have considerable strength, and form a barrier against further corrosion. Because of the absence of free graphite in steel, the corrosion products have little or no strength or adherence and flake off as they are formed, thus presenting fresh surfaces for further corrosion. In tests of severely corroded cast iron pipe, the graphitic corrosion products have withstood pressures of several hundred pounds per square inch although corrosion had actually penetrated the pipe wall.

The majority of soils throughout the world are non corrosive to cast iron. More than 329 water and gas utilities in the United States have cast iron distribution mains with continuous service records of more than 100 years. Nine have mains more than 150 years old. Over 95 percent of all cast iron pipe that has ever been installed in underground service in the United States is still in use.

The corrosion of metals underground is an electrochemical phenomenon of two main types: galvanic and electrolytic.

Galvanic corrosion is self-generating and occurs on the surface of a metal exposed to an electrolyte (such as moist, salt-laden soil). The action is similar to that which occurs in a wet, or dry, cell battery. Differences in electrical potential between locations on the surface of the metal (pipe) in contact with such soil may occur for a variety of reasons, including the joining of different metals (iron and copper or brass for example). Potential differences also may be due to the characteristics of the soil in contact with the pipe surface, e.g., pH, soluble salt, oxygen and moisture content, soil resistivity, temperature and presence of certain bacteria. Any one or a combination of these factors may cause a small amount of electrical current to flow through the soil between areas on the pipe or metal surface. Where this current discharges into the soil from such an area, metal is removed from the pipe surface and corrosion occurs.

Electrolytic corrosion occurs when direct current from outside sources enters and then leaves an underground metal surface to return to its source through the soil; metal is removed and in this process and corrosion occurs.

Over 95 percent of the soils in the United States are non corrosive to cast iron. Those few soils that are somewhat corrosive to cast iron include natural soils containing high concentrations of decomposing organic matter (swamps, peat bogs, etc.) alkalis, or salt (tidal marshes). Man-made corrosive soils result from the discharge of various mining and other industrial and municipal wastes into refuse dumps or landfills.

The National Bureau of Standards and the Cast Iron Pipe Research Association (now known as the Ductile Iron Pipe Research Association, DIPRA) have studied underground corrosion of cast iron pipe for many years. As a result of these studies, a procedure has been developed for determining the need for any special corrosion protection and a simple and inexpensive method of providing such protection by means of a loose wrap of polyethylene film. The information contained in American National Standard A21.5, American Society of Testing and Materials A674, A74 and A888, and American Water Works Association Specification C 105 provide installation instructions and an appendix that details a 10 point scale to determine whether the soils are potentially corrosive to cast iron. Information on this Standard is available by emailing the Cast Iron Soil Pipe Institute and its member companies.

Hubless pipe and fittings are equally resistant to corrosion to hub and spigot pipe and fittings. Because the 300 series of nickel-chromium stainless steel is even more resistant to corrosion than cast iron, the stainless steel housings on No-Hub couplings used to join hubless cast iron soil pipe and fittings require no more special protection against corrosion than the pipe itself. Roughly 1 1/2 Billion No-Hub couplings installed during the past 35 years in North America attest to the durability of these couplings.

Internal corrosion of cast iron soil pipe and fittings can be caused by strong acids or other aggressive reagents with a pH of 4.3 or lower if allowed to remain in contact cast iron pipe for an extended period of time without sufficient dilution to raise the pH above 4.3. If the run of piping into which the acidic waste is discharged has sufficient upstream flow of non-acidic waste, the resulting rinsing action tends to raise the pH of the combined waste to a level which will not corrode cast iron. However, by avoiding low pH discharges altogether, one can limit or eliminate internal corrosion problems, assuring the building owner and occupants many years of trouble free service.

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