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Failure Analysis Case Histories
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Atmospheric Corrosion of Galvanized Steel
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ENVIRONMENT:
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Atmospheric - Marine |
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MATERIAL: |
Hot Dipped Galvanized Steel Antenna |
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SERVICE TIME:
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Less than 1 year |
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FAILURE:
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Atmospheric Corrosion |
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The antenna consisted of two hot-dipped galvanized
screens connected together with a galvanized steel bracket which also supported
an aluminum dipole. The antenna was intended for a mildly corrosive marine
environment.
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The antenna consisted of two hot-dipped galvanized
screens connected together with a galvanized steel bracket which also supported
an aluminum dipole. The antenna was intended for a mildly corrosive marine
environment.
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Visual examination revealed excessive corrosion
(red rust) covering 90% of the bracket surface. The aluminum dipole was speckled
with spots of white powder. Both screens were uniformly covered with light green
to yellow deposit. After scrapping off deposits on the screens, widely scattered
rust colored stains were observed on the galvanized surface.
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A SEM (scanning electron microscope) with EDS
(energy dispersive x-ray spectroscopy) was used to identify the elemental
components of the deposits from the screens. The analysis showed high levels of
chlorides and silicon. Also present in unexpected amounts were sulfur, aluminum,
potassium, calcium and magnesium. The high chloride content is expected due to
the marine environment. The sulfur content is an indication of exposure to
possible industrial pollutants, i.e., acid rain. The silicon, aluminum,
potassium , calcium, and magnesium are most likely due to airborne particles. An
analysis of the white deposits on the aluminum dipole also revealed high levels
of chlorides and sulfur.
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A cross section of a rod from the screen revealed
the galvanizing to be intact over most of the sample (Figure 1). Occasional pitting of the
galvanized coating was observed. Some of these pits penetrated into the steel
substrate. |
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Failure was due to a combined effect of corrosion and the build-up of particulates on the screen. The
atmosphere that this antenna was exposed to would not be solely considered
a marine environment. The levels of sulfur and particulate matter present indicate
a possible industrial environment was also present. Therefore, galvanized
steel and aluminum components would be expect to corrode. |
