06.12.18   Robert C. Rabeler, PE | More by this Author


Does the pH of soil always affect soil corrosion? How do you know when pH is important? When should you consider measuring pH and what should you take into account when evaluating the measurement results? For answers to these burning questions, and more, keep reading!

The pH value is a measure of the hydrogen ion concentration (actually the logarithm of the reciprocal of the hydrogen-ion concentration) in the soil. A pH value of 7 is neutral. Values lower than 7 are acidic and values higher than 7 are basic (or alkaline).

The pH of soil is dependent on the amount of rainfall, soil drainage, type of vegetation, and the nature of the soil. For example, alkaline soils include soils with an abundance of calcium carbonate (calcareous soils) and soils with high amounts of sodium (alkali soils). Acidic soils include highly organic soils, and soils where soluble salts are stripped out due to heavy rainfall.

Soil pH can be determined by either colorimetic analysis or electometric analysis. Colorimetic measurement is simple and easily performed in the field. A sample of soil is mixed with water and a chemical, and the pH is determined by comparing the resulting color change with a color chart based on pH values. However, determination can sometimes be difficult because mixing the soil with water can turn the solution turbid resulting in interference with determination of the pH. Electrometic analysis equipment can be used both in the laboratory as well as in the field. In the lab, the soil is normally air-dried, mixed with water, and then tested. The lab results generally correspond well with field measurements, except for some poorly aerated soils that have high concentrations of soluble sulfur (in which case the lab results show lower pH values). Also realize that the pH value of soil can change over time with variations in climate and other factors.

So what does pH have to do with soil corrosion?
That depends on the type of buried metal. For typical steel/iron/cast iron, when the pH is less than about 4, protective oxide films tend to dissolve and corrosion rates increase. When the pH is between about 4 and 10, soil corrosion is independent of pH. When the pH is greater than 10, the metal becomes passive and corrosion rates decrease with increasing pH. For example, placing reinforcing steel in concrete (sound concrete pH is normally around 11) provides good protection of the steel. However, deicing salts added to concrete reduce the pH resulting in corrosion attack (also due to attack by the chloride ion) on the reinforcing steel causing pop-outs and potholes.

Copper is relatively unaffected by pH since hydrogen ions are generally not part of the corrosion process for copper. On average, copper corrodes at a rate of about 1/6 that of iron.

Zinc, which is used for galvanizing steel to make the steel last longer, is an amphoteric metal. That is, zinc is corroded with both high (greater than 12) and low (below about 5 to 6) pH environments. Zinc tends to corrode in the same soils as iron does, however at a slower rate.

Aluminum is also an amphoteric metal (corroding at both high and low pH values). When the pH values are between about 5 and 8.5, the pH has little impact on soil corrosivity. However, the corrosion rate increases rapidly when the pH is outside of that range.

Lead is also an amphoteric metal. When the pH values are between about 4 and 10, the pH has little impact on soil corrosivity. However, the corrosion rate increases rapidly when the pH is outside of that range.

In conclusion, when soil corrosivity is a concern, measuring soil pH makes sense. However, to properly analyze and evaluate the results, the type of metal buried in the soil must be taken into account.

If you have questions about soil conditions, contact Bob Rabeler, PE.

To learn more about underground corrosion, check out SME’s earlier blogs on galvanic corrosion and why metals placed below the ground corrode.

TAGS: Geotechnical

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