Corrosion

Written by Jerry Ratzlaff on . Posted in Cathodic Protection

Corrosion is the thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.  Examples of non metallic corrosion include the dissolution of ceramic materials or the discoloration and weakening of polymers by the sun's ultraviolet light.

  • Abbreviated as CRSN

The corrosion resistance of a pipe or a metal is the ability of the material to resist the corrosive effects of its environment.  Internal corrosion is caused by the reaction of the fluid in a pipe to the pipe proper.  External corrosive forces would be the reaction of the pipe to the soil or air.  By isolating the flow line from its corrosive environment by lining the pipe or wrapping the pipe, the corrosive effects can be mitigated.

Oil Field Corrosion

Corrosion in the oil field is caused by carbon dioxide, hydrogen sulfide or organic acids dissolved in the produced fluids.  

Variables affecting Corrosion

Temperature

Like many chemical reactions, the rate of corrosion generally increases when the temperature increases.  As a rule of thumb, the reaction rate doubles for every ten degrees Centigrade increase in temperature.

Pressure

The main concern with pressure and its affect on corrosion is that under lower pressures, dissolved corrosive gases can break out of the solution which may increase the corrosivity.  

Velocity

Usually velocity's contribution to erosion is that under very low flows, localized corrosion (pitting) is likely.  With casing gasses in CVR systems, corrosion  will likely present on the bottom of the pipe as condensate precipitates out of the casing gas as it cools.  It moves slowly relative to the gas in the line.  

With very high velocities, the corrosion can be present in corrosion-erosion.  

Forms of Corrosion

  • General Corrosion
  • Galvanic Corrosion - Caused by dissimilar metals
  • Pitting - Localized on metal surface
  • Crevice - Portion of the surface is isolated from the environment
  • Intergranular - Corrosion at the grain boundaries
  • Selective Leaching - Only one metal in an alloy is attacked
  • Stress Corrosion Cracking
  • Corrosion Erosion - Velocity aggravated corrosion
  • Corrosion Fatigue - Corrosion on cyclic loads
  • Cavitation - Corrosion gas bubble formation & immediate collapse