text: Corrosion of cast-iron-pipe can occur on both the internal and external surfaces.  In electro-chemical corrosion, internal anodes develop where bare iron is exposed to aggressive waters, promoting iron to move into solution.  The iron combines with various components in the water, forming a tubercle on the pipe interior.  This process of tuberculation can eventually cause significant restrictions in cross-sectional area within the pipe.  Since the tubercles are irregularly shaped, buildup of bacterial growths on the surface are likely.  As more iron moves into solution, the result is a loss of pipe structure over time potentially affecting pipe integrity.  In storm and sanitary sewer systems, the creation of acidic gases (such as hydrogen sulfide) by microbial action can further corrode internal pipe walls but is most pronounced on the inside ‘ceiling’ of the pipe.
  Starting in 1922, internal cement linings were introduced to act as a barrier to minimize internal corrosion.  In 1929, the American Standard Association (ASA) Sectional Committee issued a tentative standard for cement-mortar linings, which was published in 1932.  In 1939, American Standard A21.4 was published, which defined a Type I cement coating on the inside of waterline pipe to be used.  When the standard was updated in 1953, the Type I cement was replaced with Type II, since it was believed that Type II was a more closely controlled product.  The standard was further revised in 1964, which included the use of either Type I or Type II cement, and making two different mortar thicknesses available.