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Stop, in the name of rust: Chemical corrosion inhibitors

May 25, 2016

Chemical corrosion inhibitors are added to substances such as gas or potable water to decrease a fluid’s corrosivity, which is a measure of the aggressiveness of a fluid at corroding other materials.

Over time, the inside of a pipe can corrode and deposit chemicals into the fluid passing through it.

Over time and without proper treatment, the fluid passing through a pipe can cause the pipe to corrode. With the use of corrosion inhibitors, however, the corrosivity of the fluid is significantly reduced.

Chemical corrosion inhibitors, the use of which dates back to the early 19th century, are added to substances such as gas or potable water to decrease the fluid’s corrosivity. Corrosivity is a measure of the aggressiveness of fluid at corroding other materials, such as metal pipes. The effectiveness of such chemical inhibitors depends on the composition, quantity, and flow regime of the fluid in which it is being used. Ideally, a chemical corrosion inhibitor is compatible with the environment and is cost effective in its utilization.

Corrosion inhibitors are classified as follows:

  • Inorganic
    • Anodic
      • Nitrates
      • Phosphates
      • Hydroxides
      • Silicates
    • Cathodic
      • Ions of magnesium
      • Ions of zinc
      • Ions of nickel
  • Organic
    • Mixed
      • Amines
      • Urea
      • Aldehydes
      • Ascorbic acid
      • Caffeine

Typically, a chemical corrosion inhibitor works by forming a passivation layer, which is a thin layer of film that causes the material being protected to become less mutable to the corrosive characteristics of the fluid passing through it. There are two ways in which the inhibitor forms a protective layer. The first involves chemical adsorption on the surface metal. With this mechanism, the protective layer is formed either through the properties of the inhibitor itself or through the combination between the inhibitor ions and the metallic surface that it is protecting. In the second way, the chemical inhibitor will lead a formation of a passivation layer by oxide protection of the base metal.

Example Usage

An example of the use of chemical corrosion inhibitors is in water treatment. Sodium carbonate is usually added to water to counteract the corrosive characteristics of acidic additives used to treat the water. Furthermore, fluorosilicates, which is used to fluoridate potable water, increase the amount of silicates in the water. The extra silicates then act as anodic corrosion inhibitors by creating a passivation layer on the pipe surface.

Any Downsides?

A downside to such prevalent use of chemical corrosion inhibitors is its cost. According to NACE (National Association of Corrosion Engineers) International, the total consumption of corrosion inhibitors in the United States alone has doubled from around $600 million in 1982 to nearly $1.1 billion in 1998. And according to a 2002 federal study initiated by NACE, found the annual cost of corrosion inhibitors to be $276 billion.

Flint, Michigan

The Flint water crisis, which started in April of 2014, is a direct result of the high cost of chemical corrosion inhibitors. At a time when the annual cost of corrosion inhibitors was rising, the city of Flint (located in Michigan) decided to switch its water source from the Detroit Water and Sewerage Department to the Flint River in order to reduce the cost of drinking water. However, while the water from the Detroit Water Department was already treated, the surface water of the Flint River was not. Flint city officials who made this change had failed to utilize chemical corrosion inhibitors in treating the Flint River water, which led lead from old, rusty pipes to leach into the water supply, seriously affecting the health of people, especially children, in Flint.

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