Cast Iron Soil Pipe & Fittings Handbook, Chapter 1, Cast Iron Soil Pipe History, Uses and Performance

Cast Iron Soil Pipe & Fittings Handbook

Chapter 1 - Cast Iron Soil Pipe History, Uses and Performance

Requirements For A Safe And Durable Drain, Waste and Vent System

The satisfactory performance of a piping system used for drain, waste, vent and sewer plumbing requires that the material possess the following important characteristics:

Durability exceeding expected life of the building
Resistance to corrosion
Noncombustible and does not contribute to the spreading of flames
Resistance to abrasion
Ability to withstand temperature extremes
Ability to withstand traffic and trench loads
Low coefficient of expansion/contraction
Joints which resist infiltration and exfiltration
Strength and rigidity
Resistance to noise transmission

Cast Iron Soil Pipe and Fittings Meet or Exceed All These Requirements

Tests of cast iron soil pipe for these properties reveal its superior characteristics as a material for all drain, waste, vent, and sewer piping.

Corrosion Resistance

Cast iron has, for hundreds of years, been the premier piping material throughout the world for drain, waste, and vent plumbing applications and water distribution. Cast iron can be cast in the form of pipe or fittings at low cost and has excellent strength properties. Unique corrosion resistance characteristics make it ideally suited for plumbing applications. Cast iron, because of the presence of free graphite, when exposed to corrosion leaves behind an insoluble layer of corrosion products that provide somewhat of a barrier against additional corrosion.

"Cast Iron is a generic term that identifies a large family of ferrous alloys. Cast irons are primarily alloys of iron that contain more than 2% carbon and 1% or more silicon. Low raw material costs and relative ease of manufacture make cast irons the least expensive of the engineering metals. Cast irons can be cast into intricate shapes because of their excellent fluidity. Because of the excellent properties obtainable with these low-cost engineering materials, cast irons find wide application in environments that demand good corrosion resistance. Services in which cast irons are used for their excellent corrosion resistance include water and soils." (ASM Handbook; Corrosion of Cast Irons, Vol. 13; p. 567)

The majority of soils throughout the world are non-corrosive to cast iron. More than 410 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 performing its intended function.

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 occurring in a wet or dry cell battery. Differences in electrical potential between areas 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). Potential differences may also 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 the presence of certain bacteria. Any one of 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 structure such as pipe. At the point where current leaves the metal surface to return to its source through the soil, metal is removed 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 the natural soils containing high concentration 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) have studied the underground corrosion of cast iron pipe for many years. As a result of these studies, a procedure has been developed for deter-mining the need for any special corrosion protection and a simple and inexpensive method of pro-viding such protection by means of a loose wrap of polyethylene film. This information for the correct use of polyethylene is contained in ANSI/AWWA C105/A21.5. Also, ASTM A 674 pro-vides installation instructions and an appendix which details a 10 point scale to determine if the soils are potentially corrosive to cast iron. Information on this Standard is available from the Cast Iron Soil Pipe Institute and its member Companies.

Since the 300 series of nickel-chromium stainless steel is even more resistant to corrosion than cast iron, the stainless steel No-Hub couplings used to join hubless cast iron soil pipe require no more special protection against corrosion than the pipe itself. Over 1 billion No-Hub couplings installed since 1961 in the United States attest to the durability of these couplings.

Expansive Soils

Some dense clay soils expand and shrink when subjected to wetting and drying conditions. In dry periods, cracks form and when wet conditions return, the soil absorbs moisture and expands. If this condition is present it is recommended that the trench be excavated to greater than normal depth and select backfill materials be used to provide for protection from this movement.

Resistance to Abrasion

Cast iron soil pipe is highly resistant to abrasion from sand, gravel, glass particles, garbage disposal residue, dishwasher discharge, and debris being carried in suspension, both at low and high velocities, or washed along the lower portion of the sewer or drain. This characteristic has been very well documented by examinations of existing soil pipe.