Erosion Corrosion

DESCRIPTION

Erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative motion of the environment and the metal surface. It is characterized by surface features with a directional pattern which are a direct result of the flowing media. Erosion corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys). Alloys which form a surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly accelerates. Other factors such as turbulence, cavitation, impingement or galvanic effects can add to the severity of attack.

Prevention or Remedial Action

• selection of alloys with greater corrosion resistance and/or higher strength.
• re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the environment.
• reduction in the corrosive severity of the environment.
• use of corrosion resistant and/or abrasion resistant coatings.
• cathodic protection.

Standard Test Methods

• ASTM G-32 - method of vibratory cavitation erosion testing.
• ASTM G-73 - practice for liquid impingement erosion testing
• ASTM G-75 - test method for slurry abrasivity by miller number.
• ASTM G-76 - practice for conducting erosion tests by solid particle impingement using gas jet.
• NACE TM0170 - method of conducting controlled velocity laboratory corrosion tests.
• NACE TM0286 - cooling water test units incorporating heat transfer surfaces.

Evaluation of Erosion Corrosion

Many specialized tests have been utilized to evaluate erosion corrosion. Typically, the nature of the attack from erosion corrosion and/or velocity accelerated corrosion can be vary specific to the geometry and exposure conditions. Therefore, the results of tests and the test/service conditions must always be careful examined. The most commonly utilized methods are spinning cylinder and disk apparatus since they are relatively easy to set-up and they produce conditions that are easily evaluated. However, they do not always give conditions that represent those in actual service. Recently, great use of jet impingement and actual pipe flow cells have been utilized which can more accurately simulate conditions of turbulent flow and multiphase environments. These tests should be conducted to produce carefully quantified conditions of wall shear stress that match those in the intended service. The wall shear stress is a measure of the mechanical action produced on the surface of the material by the flowing media and most directly relates to the damage or removal of normally protective corrosion products and inhibitor films.