The corrosion of bare steel in soils varies significantly based on the type and location of the soil. The corrosion rate of steel in soil can range from less than 20 microns per year in favorable conditions, to 200 microns per year or more in very aggressive soils. Thus, highly corrosive soils will dictate the need for a reliable corrosion protection system to ensure long-term protection. In numerous types of soils, hot-dip galvanizing can provide the necessary corrosion protection to extend the life of the steel by many years.
Predicting soil corrosivity is a daunting task that begins with classification of the soil. Due to the varying physical and chemical characteristics of soil, it is extremely difficult to predict underground corrosion rates. Even in very close proximity, soil content conditions can have significant variations. In order to predict the performance of hot-dip galvanized steel in soil, you must first try to classify the soil in which the steel will be embedded. The properties of soil that have the most effect on the corrosion rate of zinc are aeration, moisture content (or time of wetness), pH, temperature, and resistivity.
Generally speaking, sandy, well-aerated soils with a neutral or slightly basic pH are likely to cause only limited corrosion of zinc, most likely below ten microns per year (0.4 mil/year). Determining whether or not galvanized steel is suitable for application in soils is very difficult.
View corrosion data for zinc/hot-dip galvanizing in soils:
Corrosion of Hot-Dip Galvanized Steel in Soils
One of the easiest ways to view soil classification is through color. Red, yellow, or brown soil indicates the presence of oxidized iron that corresponds to soil that is well-aerated. Soils that are gray indicate that it is poorly aerated and are generally more corrosive to zinc.
Particle size plays a very large role in determining the longevity of hot-dip galvanizing in a particular soil. Particle size will dictate the amount of aeration as well as the time of wetness for contacting galvanized steel. Soil particle sizes are generally divided into three categories: sand (0.07 – 2 mm), silt (0.005 – 0.07 mm), and clay (< 0.005 mm). In sandy soils, the larger particle size allows for air to enter between the particles and promote aeration of the soil. At the same time, aerated soils allow moisture that remains in the soil from rainfall or other sources to evaporate at a much faster rate than non-aerated soils of smaller particle size. This resulting aeration and shorter time of wetness correlates to a lower corrosion rate of zinc.
High levels of bacteria in the soil tend to consume any oxygen present, thus making the soil poorly aerated. Hot-dip galvanized steel will not perform as well in soils containing large amounts of organic bacteria.
Soils also have a wide range of chemical properties, as well as physical properties, that make it difficult to predict corrosion rates of zinc. Studies have shown that the pH of soil can vary from 2.6 up to 10.2. The pH has a dramatic affect on the corrosion rate of zinc. Hot-dip galvanized steel performs the best in soils that are neutral or slightly basic. Extreme corrosion rates and deep pitting are associated with soils having very low pH (highly acidic), less than 4.0.
Plentiful rainfall typically translates into soils that are more acidic and are more corrosive to zinc. Also, rainfall affects the time of wetness for the soil and the contacting galvanized parts. The longer that the galvanized steel remains wet, the higher the corrosion rate.
Temperature also plays a role in the rate at which corrosion of galvanized steel in soil occurs. One study has shown that the corrosion rate doubles in samples when the temperature was raised from 40 F – 68 F (4 C – 20 C). The lower temperature results in a higher resistivity of the soil. Some studies have shown corrosion rates are lower in soils with high resistivities, however, a good correlation between resistivity and corrosion rates does not exist.
Hot-dip galvanized steel extends the service life of steel covered by soil. Predicting how much longer the galvanized coating will extend the life of the steel is an inexact science and requires the gathering of multiple soil characteristics. To provide quick reference to coating selection on steel , the American Galvanizers Association (AGA) has produced the chart linked above. Contained in the chart is the anticipated service life of galvanized coatings based on soil pH, water concentration, and chloride concentration. This chart is applicable to all uses of hot-dip galvanized steel in soil and can be used as a guide to anticipate its service life in soil.