Hot-Dip Galvanizing Performance in Atmosphere

The corrosion rate of zinc is directly influenced by atmospheric conditions. Certain factors that specifically affect the corrosion of zinc include: temperature, humidity, rainfall, sulfur dioxide (pollution) concentration in the air, and air salinity. None of these factors can be singled out as the main contributor to zinc corrosion, but they all play a role in determining the corrosion protection hot-dip galvanized (zinc) coatings can provide in certain environments.

When galvanizing is exposed to the natural wet and dry cycles of the atmosphere, it develops a series of zinc byproducts on the surface called the patina. The patina is stable and non-reactive unless exposed to aggressive chlorides or sulfides. The patina is a key component in the longevity of the hot-dip galvanized coating in the atmosphere.

For corrosion classification purposes, atmospheres are generally divided into five groups:

Industrial Atmospheres

These environments are generally the most aggressive in terms of corrosion. Air emissions may contain some sulfides and phosphates that cause the most rapid consumption of the zinc coating. Automobiles, trucks and industrial plant exhaust are examples of these emission sources. Most city or urban area atmospheres are classified as industrial.

Suburban Atmospheres

These atmospheres are generally less corrosive than industrial areas and, as the term suggests, are found in the largely residential, perimeter communities of urban or city areas with little or no heavy industry

Temperate Marine Atmospheres

The galvanized coating’s time to first maintenance in marine environments is influenced by proximity to the coastline and prevailing wind direction and intensity. In marine air, chlorides from sea spray can react with the normally protective zinc corrosion products to form soluble zinc chlorides. When these chlorides are washed away, fresh zinc is exposed to corrosion. Temperate marine atmospheres are more corrosive than suburban atmospheres.

Tropical Marine Atmospheres

These environments are similar to temperate marine atmospheres except they are found in warmer climates. Tropical marine climates tend to be somewhat less corrosive than temperate marine climates, because many tropical areas are found relatively far removed from moderately industrial areas.

Rural Atmospheres

Rural environments are usually the least aggressive of the five atmospheric types. This is primarily due to the relatively low level of sulfur and other emissions found in such environments

Independent and industry testing of galvanized steel samples over decades in industrial, urban, rural, and marine environments, with varying degrees of chlorides, sulfides and other corrosive elements, has yielded performance data for galvanized steel in real world applications. For results, refer to the Time to First Maintenance chart.

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