Failure Analysis Case History No. 002

Industry: Commercial
Specimen Description: Water heater inlet pipe
Material: Galvanized carbon steel
Environment: potable water
Background:
Galvanized carbon steel nipples in residential water heaters were failing after less than two years service. The nipples were downstream of, and attached to, copper flexible hoses with electrically isolated, galvanized couplings.
Time in Service: < 2 years.
Findings:
Laboratory failure analysis of hot and cold water nipples from a water heater showed that the nipples experienced corrosion under I.D. tuburcles. Analytical analysis showed that the water chemistry was classified as slightly corrosive, based on the Langlier Saturation Index, with a calculated LSI = -0.32. Energy dispersive spectrometry (EDS) chemical analysis of the tubercles showed that they contained copper, as well as iron, zinc, chlorine and silicon. The hot water nipples experienced less under-deposit corrosion than did the cold water nipples. This was due to the increased scaling tendency of the higher temperature water.

The galvanized nipples experienced underdeposit corrosion at low velocity areas due to a buildup of deposits. The oxygenated water and copper deposition from corrosion of the upstream copper hose contributed to the corrosion.

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