ASTM G51: Measuring pH of Soil for Use in Corrosion Testing

What the ASTM G51 test measures

ASTM G51 measures the pH of soil for corrosion evaluation. A soil's acidity or alkalinity strongly affects how fast buried steel, ductile iron, galvanized coatings, and the reinforcement in buried concrete corrode. The test is one of a standard group, with soil resistivity, chlorides, and sulfates, used to classify a site's corrosivity and to decide on coatings, cathodic protection, and material selection for pipelines, piles, culverts, and mechanically stabilized earth wall reinforcement.

How the ASTM G51 test works, step by step

The method is written for field or laboratory use on a soil at its natural moisture. A pH electrode, calibrated against buffer solutions, is placed in direct contact with the soil, or with the soil brought to a saturated paste with distilled water, and the reading is taken once it stabilizes. Temperature is recorded because the electrode response and buffer values shift with it. Several readings across the sample or site are averaged.

What the numbers mean

Most soils fall between pH 5 and 8.5. Below about 5.5 the soil is considered aggressive to steel and zinc; above 9 it may attack aluminum and some coatings. MSE wall specifications commonly require backfill pH between 5 and 10 for steel reinforcement, and many pipeline and culvert selection guides step to heavier coatings or alternative materials outside pH 6 to 8.

What can throw the result off

An uncalibrated or drifting electrode, soil that has dried and oxidized since sampling, and contamination of the sample with cement, lime, or fertilizer are the typical problems. Samples for pH should be sealed at their field moisture and tested promptly.

Common questions about ASTM G51

How we help with ASTM G51 testing

We run the corrosivity package, pH, resistivity, chlorides, and sulfates, on MSE wall backfill, pipeline routes, and foundation soils, and report against the project specification limits.