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Atmospheric Corrosion of Galvanized Steel

ENVIRONMENT:

Atmospheric - Marine

MATERIAL:

Hot Dipped Galvanized Steel Antenna

SERVICE TIME:

Less than 1 year

FAILURE:

Atmospheric Corrosion

 

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.

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.

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.

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.

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.

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.

 
Figure 1. Cross-section of galvanized steel coating (200X Original Magnification)

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