I will be using some steel in a below-grade application. If I hot-dip galvanize the steel, how long will it last?

As is the case with any corrosion protection system, the two most common questions about hot-dip galvanizing are in reference to the life of the coating and the costs associated with using it. Fortunately, the American Galvanizers Association has developed a tool that allows specifiers to quickly reference a visual guide approximating the corrosion protection performance of hot-dip galvanized steel in soil conditions known as the Service Life of Galvanized Steel Articles in Soil Applications chart.  The visual aid estimates the amount of corrosion protection provided by a hot-dip galvanized coating in soil applications given the initial thickness of the coating and varying soil conditions. The soil chart features four separate graphs, each representing different soil conditions. Determining which chart to use proves to be the most difficult step in the process. Once chosen, each graph can quickly estimate the service life of hot-dip galvanized steel articles in that soil type.

Determine the Appropriate Chart: Classifying the Soil With the determination of only two environmental factors of a soil sample, a specifier can choose which of the four graphs in the Service Life of Galvanized Steel Articles in Soil Applications chart to use. The determination of a third factor is not necessary but increases the accuracy of the prediction. Laboratory testing of soil samples in the area of a project established the corrosivity of the soil and which graph should be used for a service life estimation. Yes or No answers in the flowchart below determine which graph represents the soil for the project in question. Below the flowchart are methods for finding each of the values needed.

Chloride Concentration
Chloride ions are the most corrosive ions to metal found in soil. Their presence may be due to a soils proximity to a coastal environment or from outside sources like de-icing salts. A chloride test such as ASTM D512 Standard Test Methods for Chloride Ion in Water of a water extract from the soil sample reports the amount of chlorides present in the soils moisture. To translate the measured concentration of the extract into parts per million of the soil as a whole, the following  calculation is used:

Moisture Content Soils with higher moisture contents are generally more corrosive to galvanized steel than drier soils. The weigh-bake-weigh method of ASTM D2216 Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass reports the moisture content of a soil sample as a percentage.

pH Soils can be relatively described as being acidic, neutral, or alkaline. Soils in neutral ranges (pH equal to or near 7) form more hospitable environments for galvanized steel with lower observed corrosion rates. ASTM G51 Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing outlines mixing a soil sample with distilled water and measuring the pH of the mixed sample. This test determines the pH level to be used with the Service Life of Galvanized Steel Articles in Soil Applications chart.

Coating thickness Once the appropriate chart is selected for the soil in question, an estimation of service life can be found once the thickness of the galvanized coating is known.  Coating thickness measurements with a magnetic or electronic thickness gauge should be taken in a variety of places over the entire surface area of the galvanized article to obtain an average coating thickness value. If the steel is not yet galvanized, the minimum coating thickness that will be applied to the article can either be found in ASTM A123 Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products, ASTM A153 Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, or ASTM A767 Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement.

Reading the Chart All four charts show the relation between the thickness of the galvanized coating in mils (x-axis) to the estimated service life of the articles (y-axis) in the soil conditions previously determined. However, for each coating thickness, there will be a range of estimated service lives. The red line in each chart represents the minimum service life observed in Corrpros study for the National Corrugated Steel Pipe Association in conjunction with the American Iron and Steel Institute. The blue line represents the average, and the green line represents the longest service life observed in the study. Each line in the charts is accompanied by the additional information of pH and moisture levels. This is why all three pieces of information are recommended to get the most accurate information from the charts.

Background The primary source for information in the development of the Service Life of Galvanized Steel Articles in Soil Applications was the Condition and Corrosion Survey on Corrugated Steel Storm Sewer and Culver Pipe: Final Report prepared by Corrpro Companies for the National Corrugated Steel Pipe Association in cooperation with the American Iron and Steel Institute. Service life for the purpose of this chart is defined as the time to full consumption of the coating plus 25%. This is an appropriate service life for hot-dip galvanized structural items in soil applications. At that time, the steel will need to be inspected for corrosion and the possibility of repair.