CORROSION EROSION PITTING SPECIMENS
19.0. CORROSION / EROSION PITTING:
Both corrosion and erosion happen due to certain external actions on a surface. Corrosion means the destruction of materials through chemical reactions whereas erosion means the carrying away of the topsoil from the surface of the earth.
Corrosion normally happens because of chemical reactions. Erosion occurs by chemical reactions and by certain forces of nature. Corrosion also means the loss of electrons from the metals when it comes in contact with the moisture and oxygen in the atmosphere. Erosion happens because of natural forces like water and wind. Other factors such as acid rain, salt effects and oxidation of materials are also known to cause erosion.
In terms of the process, corrosion is an electro chemical process whereas erosion is a physical process. The corrosion of metals is often referred to as rusting and it is evident in the material itself. Erosion is a natural process that removes or carries away materials from one place to another. For instance, when sand is carried away from the beach or riverbanks, it is still sand even after erosion. Corrosion isn’t like that. When corrosion takes place, the material will be transformed to another chemical compound known as rust.
Various types of corrosion include galvanic, crevice, pitting, intergranular and selective leaching. Erosion also involves several different processes like weathering, transportation, and dissolution. Both corrosion and erosion can be prevented. To prevent corrosion, a protective layer is coated on the surface of the metal that constantly comes in contact with the atmosphere. Terracing the terrain or planting more trees on the surfaces where erosion is likely to happen can prevent erosion.
19.1. EVALUATION OF CORROSION/EROSION
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.
STANDARD TEST METHODS
ASTM G-32 - Method of vibratory cavitation erosion
ASTM G-73 - Practice for liquid impingement erosion testing.
ASTM G-75 - Test method for slurry abrasively 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.
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 varying
specific to the geometry and exposure conditions. Therefore, the results of
tests and the test/service conditions must always be carefully 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.