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Prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand alone data. This is specifically referenced in ASTM B117, Standard Practice for Operating Salt Spray (Fog) Apparatus which also states correlation and extrapolation should be considered only in cases where appropriate corroborating long-term atmospheric exposures have been conducted:

3.2 Prediction of performance in natural environments has seldom been correlated with salt spray results when used as standalone data.

The constant wetting of the zinc coating by this test method does not allow naturally occurring wet-dry cycles to take place and these cycles are necessary for zinc corrosion products, the zinc patina of zinc-oxide, zinc-hydroxide, and zinc carbonate to form.  This patina is in part what makes galvanized steel durable for many decades without maintenance.  Because the patina is unable to develop during testing, salt spray tests attack the wrong material (zinc metal instead of the self-protecting zinc patina), and do not corroborate the long-term corrosion data from hot-dip galvanized steel samples that have been collected worldwide over time, even if provided with a passivation treatment such as chromating or phosphating. 

Since the 1920’s, hot-dip galvanized steel samples have been placed in thousands of different environments and the corrosion rate has been measured.  Five environments are generally recognized (rural, urban, industrial, temperate marine, and tropical marine) and the corrosion rate in each has been plotted on a graph to provide an estimated durability for hot-dip galvanized steel.

Time to first maintenance chart

To assist in predicting the life of hot-dip galvanized coatings in atmospheric environments, the AGA Time to First Maintenance Charts can be used. For soil environments, consult the Service Life of Galvanized Steel Articles in Soil Applications chart. If the corrosion rate and time to first maintenance for a specific project site is desired, a more accurate estimate may be obtained by inputting data such as average precipitation, average salinity, and average sulfur dioxide for the local area around the project site into an online model called the Zinc Coating Life Predictor.

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