The Effect of Water PH on Rates of Erosion in Carbon Steel

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The Effect of Water PH on Rates of Erosion in Carbon Steel
Carbon steel corrodes easily, especially in acidic environments. (Photos.com/Photos.com/Getty Images)

The erosion of carbon steel occurs via a chemical reaction. Depending on the conditions, erosion occurs in air, soil, and in water. pH has the greatest influence on the erosion of carbon steel. Carbon steel is an alkaline metal and favours similar conditions. Thus, exposure to lower pH levels increases the rate of erosion.

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Carbon Steel

Globally, around 85 per cent of annual steel production consists of carbon steel. As an engineering material it is used extensively in construction projects ranging from pipelines, fossil and nuclear fuel plants, mining equipment and transportation. In spite of such heavy industrial reliance on carbon steel, it is quite susceptible to erosion, also referred to as corrosion. The economic consequences of widespread erosion of carbon steel warrants investment into its reduction.

pH

The pH meter ranges from 1 to 14, with 1 being the most acidic, 14 being most alkaline, and 7 being neutral. In solutions, acidic environments are more positively charged, as they contain positively charged hydrogen ions. Alkaline solutions however, are more negatively charged. In chemistry, solutions of this nature are called electrolytes.

Example of Erosion

In solutions where several hydrogen ions are present, carbon steel loses electrons and becomes oxidised. The now positively charged carbon steel reacts with water molecules to form the unstable ferrous hydroxide on its surface, which then reacts continuously with other water molecules to eventually form ferric oxide, otherwise known as rust.

Rate of Erosion

At 6.5 to neutral pH, erosion rates for carbon steel are significantly close to 10 mils per year (1 mil = 1 thousandth of an inch). The rate decreases gradually as pH levels rise, and falls from 8 mils per year to near zero between pH 8.5 and 9.5. This is because carbon steel is also alkaline, and favours alkaline conditions.

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