Mesa Corrosion of Steel Tubing
Mesa corrosion is one of the common types of corrosion experienced in service involving exposure of carbon or low alloy steels to flowing wet carbon dioxide conditions at slightly elevated temperatures. An iron carbonate surface scale will often form in this type of environment which can be protective rendering a very low corrosion. However, under the surface shear forces produced by flowing media, this scale can become damaged or removed and exposure fresh metal to corrosion. This localized attack produces mesa-like features by corroding away the active regions and leaving the passive regions relatively free of corrosion resulting the surface profile reminiscent of the mesas produced in rock by wind and water erosion.
Velocity Accelerated Corrosion of Cupro-Nickel Tubing
The corrosion features on the inside of this heat exchanger tube were produced by high temperature, flowing seawater. The alloy was 90-10 cupro-nickel which has limited flow resistance in seawater of around 9 to 12 ft/sec (3 to 4 m/sec) depending on many factors. In this case, the flow rate was in this range which produced the accelerated corrosion on the outside portion of the bend. "Horseshoe" or "U" features are characteristic of this type of attack which are oriented with the closed end of the features pointing in the direction of flow.
Carburization of Modified HK-40 Furnace Tubing
Massive Carburization occurred in service due to upset conditions in an ethylene furnace. Conditions that caused the failure included excessive temperature in combination with high carbon activity in the process. The attack resulted in the formation extensive surface carburization and severe grain boundary carbides which progressed through the material from the I.D. surface. These changes in microstructure resulted in severe embrittlement of the tubing and premature failure.