WATCH THIS SPOT for your next NDT Myth, kindly put together for us by Council member Hannes Barnard.
Definition
Corrosion that occurs due to moisture associated with atmospheric conditions.
Which materials are affected?
Carbon
steels and low alloy steels.
It can also affect copper alloyed aluminium.
How is atmospheric corrosion recognised?
Atmospheric
corrosion forms on the outside of an object that is exposed to the atmosphere. The surface has a distinctive iron oxide (red rust)
colour. Atmospheric corrosion damage could be local or general. Localised corrosion would be in the form of a deep pit or a few corrosion
pits in isolated areas. This often happens where a breakdown of a paint coating on the object traps moisture. General corrosion could
be in the form of a pitted surface over large areas and could even be a smooth loss of thickness over an area that cannot be easily
seen without the aid of a straight edge. This normally happens when no coating of paint was present.
How can atmospheric corrosion
be detected?
Atmospheric corrosion is detected visually. The external surface is exposed to the atmosphere and a loss of material
thickness can be seen visually. A build up of corrosion product (scale) may be present. A straightedge placed over the surface may
be necessary to see the effect of smooth atmospheric corrosion and an ultrasonic thickness measurement should always be done in the
suspected area.
Areas where atmospheric corrosion are found
The main factor is the presence of moisture in the atmosphere.
Therefore, corrosion may be predicted simply by the location of the object. Marine and Industrial environments are the most corrosive.
Chemicals like salt and sulphur in the industrial atmosphere will accelerate the rate of corrosion. A moisture drift on the wind from
cooling towers and even shaded areas where the sun does not readily dry away moisture may affect objects. Areas where water stands
for longer than normal will be more corroded. Corrosion increases with temperature, however above 120?C the surface is usually too
dry for corrosion to take place.
Can atmospheric corrosion be avoided?
Design equipment to ensure proper drainage. Proper
surface preparation and a quality coating applied correctly to protect the external surfaces. Regular inspection should be done on
coated surfaces. A local repair should be done where a breakdown is noted to arrest the progress of corrosion. Excess corrosion product
build-up should be removed with care when the object is under pressure. Severe cases of excess corrosion product build-up should be
removed and inspected after pressure is reduced.
