Metallurgical Evaluation of Corroded Piping - Pit Seen at Center

 

Chemical, Metallurgical and SEM Analysis Used to Determine Causes of Corrosion

We performed failure analysis to determine the causes of corroded irrigation piping – that were possibly related to issues with self-draining gaskets and/or non-conformance to specification.

The investigation involved:

  • Chemical analysis to identify the alloy
  • Stereo microscopy of corroded ID surfaces
  • Cross section examination to determine alloy microstructure and mode of corrosion
  • Scanning electron microscopy

The chemical analysis discovered the primary composition was copper (Cu) , Iron (Fe), Magnesium (Mg),
Manganese (Mn), Silicon (Si), and Zinc (Zn). The presence of 0.84 weight percent manganese indicates that the alloy is a 3000 series aluminum alloy. These aluminum alloys can be work hardened, but not heat treated. The composition of the alloy most closely matches type 3004, although the alloy is somewhat below specification in manganese and somewhat above the specification in iron.

Subsequent stereo microscopy discovered corrosion pits, and scanning electron microscopy (SEM) discovered chloride pitting – a common occurrence as the chloride ion is known to facilitate breakdown of the aluminum oxide film.

In order to eliminate this mechanism promoting accelerated corrosion attack, one of our recommendations was to reduce the chlorine/chloride in the irrigation fluid system.

 


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Related Services

Materials & Metallurgical Consulting
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Resources

Tom O’Donnell, PE
Introduction to Corrosion
Links to Engineering Resources

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