Microbiologically Influenced Corrosion (MIC) of Indoor Sprinkler System Galvanized Steel Pipe

We tested a galvanized steel pipe from an indoor sprinkler system to determine the cause of pinhole corrosion. We used high-resolution 3D optical microscopy with our Keyence 7000N and laser-induced breakdown spectroscopy (LIBS) for elemental microanalysis to find that microbiologically influenced corrosion (MIC) caused the corrosion.

Steel Pipe Corrosion Images and Findings:

Indoor Sprinkler System Galvanized Steel Pipe

Indoor sprinkler system galvanized steel pipe with pinhole corrosion caused by microbiologically influenced corrosion (MIC)

Sectioned Pipe Showing Interior Corrosion

Interior corrosion of galvanized steel pipe of indoor sprinkler system.
Sectioned steel pipe of indoor sprinkler system showing interior corrosion in relation to the pinhole.

Interior of the Pipe with Pinhole Corrosion

Interior of galvanized steel pipe with pinhole corrosion
Interior of galvanized steel pipe with pinhole corrosion caused by MIC

MIC Pit on the Inside Wall of the Pipe

  • The deepest part of the pit is the black vertically oriented area on the right side of the image below.
  • A hazy translucent line along the bottom edge suggests the presence of organic material.
  • However, the outside wall pit was more likely the cause of this corrosion pit.
MIC pit on inside wall of steel pipe of sprinkler system
The MIC pit on the inside wall of the sprinkler system steel pipe.

Exterior Wall MIC Pit

  • The pit is above the red scale bar, facing upward.
  • The pit interior is dark brown, with wide lips of bare steel.
  • The light material on the right is the galvanized coating; outside the pit, the white materials are primarily zinc oxide.
  • The white, somewhat translucent biofilm material is present near the edges of the pit.
Pit in exterior wall of the galvanized steel pipe of the sprinkler system
The exterior wall MIC pit of the sprinkler system galvanized steel pipe.

LIBS Analysis Results for White Film at the Pit Edge

  • LIBS analysis showed the white edge material to be organic.
  • Previous LIBS analysis on the reddish-brown material in the pit bottom identified it as iron oxide and iron hydroxide corrosion products.
LIBS analysis results for film at the pit edge show it to be organic
LIBS analysis results for the white film at the pit edge show it to be organic

Biofilm Analysis with LIBS

  • LIBS analysis of the biofilm showed the laser pulse destruction of the fragile film.
  • The weakest pulse strength was used, yet it blew away the fragile organic material.
Biofilm analyzed with LIBS was destroyed by laser pulses

Smaller Pit Analysis

  • A second smaller pit, which had not erupted through the pipe, was also tested and showed much biofilm on the inner wall.

Conclusion:

Anderson Materials Evaluation concluded that pitting corrosion of the indoor sprinkler system galvanized steel pipe was caused by Microbiologically Influenced Corrosion (MIC).

For more examples of our corrosion analysis investigations, click Here.

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