WHAT IS CORROSION AND WHY IS CORROSION TESTING FOR SOILS IMPORTANT?
Metallic elements such as iron, copper, zinc and nickel occur naturally in the form of oxides, sulfides and carbonates. The conversion from ore to metal separates the metallic element from the oxides, etc. which requires the input of large amounts of energy. The resulting metal or alloy is in a high-energy state and under the right conditions it will attempt to return to its more natural, lower-energy, state by combining with oxides, sulfides, and carbonates. This process is called corrosion.
High liability can arise due to the corrosive actions of soil. The Federal Highway Administration has estimated that the total direct cost of corrosion in the United States was determined to be $279 billion per year, which is 3.2 percent of the U.S. gross domestic product (GDP). This study indicated that major contributions to this value are from corrosion occurring on or in the ground. These include drinking water and sewer systems, highway bridges, and in gas and liquid transmission pipelines. Corrosion can be a problem for both metallic and concrete structures in contact with the ground. If corrosion is not considered, the service life of the project may be severely overestimated and public safety may be at risk.
SO WHAT ARE THESE TESTS ANYWAY AND WHY ARE THEY SIGNIFICANT?
There are six primary parameters to evaluate the corrosion potential of a soil. These are resistivity, pH, sulfate, chloride, redox potential, and sulfide. The following is a quick overview of what these parameters are and why they are important when evaluating the corrosion potential of soils.
Resistivity has historically been used as a broad indicator of soil corrosivity. Soil resistivity is a measure of how easy it is for electrons to flow in the soil. The flow of electrons is essential in most types of corrosion reactions. Other factors being equal, corrosion reactions will proceed more easily when the resistance to electron flow is lower and proceed more slowly in soils with a high resistivity. Soil resistivity is affected by both the amount of dissolved solids (salts) in the soil, as well as, the moisture content of the soil. The more dissolved solids present in the soil, the lower the resistivity will tend to be. The resistivity of a dry soil will tend to be very high. As the moisture content increases, the resistivity will drop. As the soil approaches saturation the resistivity will reach a minimum. Further increases in moisture content will result in an increase in resistivity as the dissolved solids begin to be diluted by the water. The most conservative measure of resistivity is the 100% saturated resistivity (or minimum resistivity). Soil resistivity is by no means the only parameter affecting the risk of corrosion damage. A high soil resistivity alone will not guarantee absence of serious corrosion.
pH is a measure of how acidic (pH < 7) or alkaline (pH > 7) the soil environment is. Soils usually have a pH range of 5-8. In this range, pH is generally not considered to be the dominant variable affecting corrosion rates. More acidic soils obviously represent a serious corrosion risk to common construction materials such as steel, cast iron and zinc coatings. Alkaline soils tend to have high sodium, potassium, magnesium and calcium contents. The latter two elements tend to form calcareous deposits on buried structures with protective properties against corrosion. pH, coupled with the oxidation conditions in the soil environment, can dramatically affect the nature of microbiological activity that can have a large impact on corrosion rates.
Sulfate (SO42-) is a naturally occurring form of sulfur. In California, soils can be high in sulfate and can be laced with gypsum (Calcium Sulfate) veins. If you see a white powdery substance in the soil that does not react with hydrochloric acid, chances are it is gypsum or another form of a sulfate salt.
Compared to the corrosive effect of chloride ion levels, sulfates are generally considered to be more benign in their corrosive action towards metallic materials. However, the presence of sulfates can pose a major risk for metallic materials because the sulfates can readily be converted to highly corrosive sulfides by anaerobic sulfate reducing bacteria. On the other hand, sulfates can attack concrete and chemically change the binding compounds causing expansion, cracking, and loss of strength. If you have ever seen a white powdery substance on a concrete surface you have probably seen evidence of sulfate attack. In reinforced concrete structures, sulfate attack may expose the rebar to corrosion by other compounds such as chloride or sulfide. Concrete weakened by sulfate attack will begin to irreversibly lose its strength and may be at greater risk of failure from seismic or other loading. In severe conditions, sulfate attack can decrease concrete's lifespan from 150 years to 15 years or less. Knowing the sulfate levels in the soil is an important variable when deciding what type of cement to use for a project and how it should be mixed.
Chloride ions are generally harmful, as they participate directly in the electrochemical reactions that take place during the corrosion process. Chloride can also destroy the stable layers of protection that can naturally form on the surfaces of some metals, exposing the unprotected metal to further corrosion. In reinforced concrete structures, chloride can migrate through the concrete causing the rebar to begin corroding and swelling, subsequently causing the surrounding concrete to crack and break apart. The presence of chloride also tends to decrease the soil resistivity. Chloride may be found naturally in soils derived from marine deposits and contact with brackish groundwater or from external sources such as de-icing salts applied to roadways. Knowing the levels of chloride will help with the proper design of the concrete mix and in determining the amount of concrete cover over rebar and other steel reinforcement.
REDOX POTENTIAL -
The redox potential (or Oxidation-Reduction Potential) essentially is a measure of how reduced or oxidized the soil environment is. Reduced conditions (low redox potential, less than about 100 mv) indicate that there is little or no free oxygen available. Oxidized conditions (high redox potential, greater than about 100 mv) indicate that there is free oxygen available. Typically the oxygen concentration decreases with increasing depth of soil and with increasing moisture content. The redox potential is significant because it, in part, determines the stability of metallic structures in the soil. For example, iron pipe buried in an anaerobic soil (low redox potential) will tend to not rust because the soil will not contain any free oxygen, which is needed for the iron to rust. On the other hand, the combination of anaerobic conditions and sulfur in the form of sulfate or sulfide can lead to corrosion. Under these conditions, soil microbes can convert the sulfides into sulfuric acid. The redox potential will also greatly affect the types of microbes that predominate in the soil, and thus, the types of microbially induced corrosion that occurs.
The in-situ redox potential of a soil is subject to change due to sampling. Ideally, redox samples should be collected in such a way as to minimize contact between the soil and the air. For example, collect a full brass liner of soil then quickly seal it with caps and tape. Because the redox potential can be affected by microbial activity it is best to keep the sample in a cooler with ice until it is delivered to our lab.
The presence of sulfide indicates reducing conditions in the soil. Sulfides are a reduced form of sulfur and can be created by sulfur reducing bacteria under anaerobic conditions. Sulfides can chemically react with metals and degrade their strength. They can also be involved in the generation of sulfuric acid that will attack both metal and concrete. Hydrogen sulfide is one form of sulfide that can be present in soil. If the soil smells like rotten eggs you know that hydrogen sulfide is present. Sulfides are readily oxidized to sulfate by microbes in the presence of oxygen. Because of this, it is best to follow that sample handling procedures mentioned for redox potential. Definitely avoid storing sulfide samples lose in plastic bags and get them to us as soon as you can.