Soil Corrosion Data
The primary source of information on the performance of galvanized steel articles in soil conditions is the Condition and Corrosion Survey on Corrugated Steel Storm Sewer and Culvert Pipe: Final Report prepared by Corrpro Companies for the National Corrugated Steel Pipe Association (NCSPA) in cooperation with the American Iron and Steel Institute (AISI).
The study examined materials from 122 US sites with conditions varying from a low pH of 4.1 to a high pH of 10.3 to create a database for a statistical model. The statistical model developed can accurately predict the average service life of hot-dip galvanized steel culvert based on the measured soil corrosion rate, which was determined by measurements of pH, resistivity, moisture content in percentages, and chloride content. The condition of existing galvanized articles was evaluated by simple pipe-to-soil potential measurements using a copper-copper sulfate reference electrode.
After the evaluation of the soil and the galvanized coatings, probability functions were used to predict the time to first perforation of the wall of the 16 gauge galvanized corrugated steel pipe. Based on previous work by Richard Stratfull of the California Department of Transportation, the predicted service life of 16 gauge corrugated steel pipe will be twice the number of years to first perforation. Therefore, the model and analysis developed by Corrpro conservatively estimate the service life of corrugated steel pipes in soil applications to be time to first perforation plus 50%.
Using the same data and statistical model developed by this study. The AGA modified the service life equation to be more appropriate for structural steel elements buried in soil. For structural galvanized articles, the service life is defined as total zinc coating consumption, plus 25% (so 75% of the base steel integrity is present at the end of the life).
The main factors that affect the corrosion rate of hot-dip galvanized steel in soils, as noted by the AGA Soil Chart are chlorides, moisture content, and pH, with resistivity playing a secondary role. The science behind those factors is based on a study conducted in the 1970's by Dr. Warren Rogers titled Mean Time to Corrosion Failure (MTCF) of Underground Storage Tanks (USTs). Using data from examinations of failed and functioning UST's, he developed a model to predict MTCF from a number of factors that were measured at the UST site. He applied this model to more than 23,000 sites which helped refine and verify the accuracy of the model.
The four variables with the most profound impact on the corrosion rate of the USTs and what observations he made on their interactions include:
- Chlorides - The presence of chloride ions causes the resistivity to be lower and makes the zinc coating more susceptible to corrosion. Along with high moisture levels in the soil, high chlorides will increase the rate of the corrosion of the zinc coating.
- Moisture Content - For hot-dip galvanized steel, the soil moisture content primarily affects the activity of the chloride ions. If the moisture content was below 17.5%, the chloride ion concentration does not significantly affect the corrosion rate of the zinc. If the moisture content was above 17.5%, the chloride ion concentration has a significant effect on the corrosion rate of zinc.
- pH - The lower the ph (< 7.0) values of soil have a higher corrosion rate on zinc coatings. If the pH is above 7.0, then the corrosion rate of the soil yields a longer service life of the zinc coating.
- Resistivity - This parameter follows the chloride ion concentration in that higher resistivity means lower chloride ion content and a lower corrosion rate of the zinc coating.
The combination of the variables identified by Dr. Rogers and the Corrpro study were both taken into account in the development of the final Service Life of Galvanized Steel Articles in Soil Applications chart.