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Corrosion, aggressiveness and scaling assessment

Corrosion is most commonly the conversion of a metal to its oxide and consequent loss of strength and function. The most familiar form of corrosion is rust, which is a mixture of oxides of iron. Other metals can also become corroded, but rust is the biggest problem, both because iron is the commonest construction metal and because of the way iron rusts: Iron oxides take up more room than iron, and so rust expands, cracking structures.

Aggressiveness is the propensity of a soil or water to dissolve cement structures. This is mostly a function of total salt load, the sulphate level and the acidity.

Scaling is the deposition of insoluble salts, oxides and hydroxides from water on surfaces. Inside pipes, this can block flow. Common scaling substances include calcium carbonate, calcium sulphate, magnesium hydroxide and iron oxides.

The durability of concrete, steel and other metal structures is influenced by the soil and water environment. The corrosion or scaling potential of soil and water poses important limitations on how structures are built.

Causes of corrosion and scaling

The corrosiveness of a soil or wet environment towards metals and its aggressiveness towards concrete structures can be estimated by a series of chemical and physical tests and an assessment of the physical environment. For example, the intertidal zone with its very high salinity due to chlorides and high oxygen levels is highly corrosive towards steel, but the same high salinity deep in a soil profile with no oxygen is much less corrosive.

The major factor in the corrosion of metals is the electrical conductivity (EC) of the surrounding soil solution or water, which is due mostly to dissolved salts. The EC can be greater in sandy than in clayey soils, and is greater in salt water than in fresh water.

The two things that affect concrete structures the most are acidity and sulphate, both of which chemically eat away at the cement matrix.

The tests thus involve the measurement of pH, salinity, sulphate, chloride resistivity and soil texture.

The major factor in the potential for scaling is temperature: The lower the temperature, the less dissolved chemicals water can hold, and the more solids are deposited on surfaces. Scaling is particularly a problem where water naturally contains a high concentration of dissolved salts: commonly called hard water.

Lab testing

We use a suite of test packages to assess the corrosion potential of soil, water and other liquids on concrete, steel and other metals. We developed these tests to complement or augment the structural and physical assessments performed by geotechnical laboratories that do not usually conduct such specialist chemical methods.

By offering a suite of tests that cover a range of corrosive influences, we can cover as many circumstances as possible in which corrosion and scaling can cause threats to built structures. These considerations are becoming increasingly important as development in areas of saline soils, acid sulphate soils, marine and coastal soils, and industrially contaminated soils occurs.

The suite of six test packages is based on the standards listed in the references below. These test packages allow us to provide sound interpretation and guidance on the severity of corrosion and scaling threats and to suggest appropriate management techniques.

Taking samples

For water, we require a minimum volume of 1 L. Water should be stored at 4 °C while in transit to the laboratory.

Soil testing requires a minimum sample size of 1.5 kg. Collect samples that are representative of the materials to which the structures are or will be exposed. Keep them at field moisture content in sealed plastic bags for transport to the laboratory.

Sampling techniques are specific to a site and project. Please call if you need more information about sampling or the tests available.

Measurement and interpretation of results

In the next issue we will go into depth about the measurement and interpretation of results, and measures you can take to protect structures from salinity.

References

AS2159: 1995. Piling – Design & Installation.

DIN 4030. 1991. Assessment of Water, Soil & Gases for Their Aggressiveness to Concrete, Parts I and II.

TN 37. 1980. Durability of Concrete Structures. CCAA.

TN 57. 1989. Durable Concrete Structures. CCAA.

TN 59. 1998. Cements – Properties & Characteristics, CCAA.

AS1289.4.4.1: 1997. Soil Chemical Tests – Determination of the Electrical Resistivity of a Soil – Methods for Sands and Granular Materials.

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