Flow-accelerated corrosion
Encyclopedia
Flow-accelerated corrosion (FAC), also known as flow-assisted corrosion, is a corrosion
mechanism in which a normally protective oxide layer on a metal surface dissolves in a fast flowing water. The underlying metal corrodes to re-create the oxide, and thus the metal loss continues.
By definition, the rate of FAC depends on the flow velocity. FAC often affects carbon steel piping carrying ultra-pure, deoxygenated water or wet steam. Stainless steel
does not suffer from FAC. FAC of carbon steel halts in the presence of small amount of oxygen dissolved in water. FAC rates rapidly decrease with increasing water pH
.
FAC has to be distinguished from erosion corrosion
because the fundamental mechanisms for the two corrosion modes are different. FAC does not involve impingement of particles, bubbles, or cavitation
which cause the mechanical (often crater-like) wear on the surface. By contrast to mechanical erosion, FAC involves dissolution of normally poorly-soluble oxide by combined electrochemical, water chemistry and mass-transfer phenomena. Nevertheless, the terms FAC and erosion are sometimes used interchangeably because the actual mechanism may, in some cases, be unclear.
FAC was the cause of several high-profile accidents in power plants, for example, a rupture of a high-pressure condensate line in Virginia Power's Surry nuclear plant in 1986.
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
mechanism in which a normally protective oxide layer on a metal surface dissolves in a fast flowing water. The underlying metal corrodes to re-create the oxide, and thus the metal loss continues.
By definition, the rate of FAC depends on the flow velocity. FAC often affects carbon steel piping carrying ultra-pure, deoxygenated water or wet steam. Stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
does not suffer from FAC. FAC of carbon steel halts in the presence of small amount of oxygen dissolved in water. FAC rates rapidly decrease with increasing water pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
.
FAC has to be distinguished from erosion corrosion
Erosion corrosion
Erosion corrosion is a degradation of material surface due to mechanical action, often by impinging liquid, abrasion by a slurry, particles suspended in fast flowing liquid or gas, bubbles or droplets, cavitation, etc...
because the fundamental mechanisms for the two corrosion modes are different. FAC does not involve impingement of particles, bubbles, or cavitation
Cavitation
Cavitation is the formation and then immediate implosion of cavities in a liquidi.e. small liquid-free zones that are the consequence of forces acting upon the liquid...
which cause the mechanical (often crater-like) wear on the surface. By contrast to mechanical erosion, FAC involves dissolution of normally poorly-soluble oxide by combined electrochemical, water chemistry and mass-transfer phenomena. Nevertheless, the terms FAC and erosion are sometimes used interchangeably because the actual mechanism may, in some cases, be unclear.
FAC was the cause of several high-profile accidents in power plants, for example, a rupture of a high-pressure condensate line in Virginia Power's Surry nuclear plant in 1986.
Further reading
- "Flow Accelerated Corrosion is Still With Us...,http://mmengineering.com/pdf%20files/Conduit%20-%20Summer%202008.pdf" 2008 By Dave Daniels, M&M Engineering
- "Flow Accelerated Corrosion Evaluation, A Case Study http://mmengineering.com/pdf%20files/Conduit%20-%20Summer%202008.pdf" 2008 By Jon McFarlen, M&M Engineering