Crevice Corrosion of Stainless Steel - Crevice corrosion is the localize corrosive attack that occurs as a result of the occluded cell that forms under a crevice on the metal surface. On this stainless steel test coupon from an ASTM G48 Method B ferric chloride test, the crevice was a non-metallic block. Note that the test coupon was not attacked by corrosion except in the middle of the coupon where it was in contact with the block and on the edges were a rubber band used to hold the block in place also came in contact with the metal surface.
Microbiologically Induced Corrosion in a Sour Gas Pipeline - The highly localized corrosion shown in the figure is typical of that resulting from microbial action. One of the features of this type of attack are the elongated pits which tunnel into the specimen often in an irregular manner. The pit was one of several located near the gas/water interface. The pipeline was left for a prolonged period in a shut-in (static) condition which promoted the growth of bacteria and highly localized corrosive attack. Sulfate reducing bacteria were suspected due to the combination of sulfate species in the water and anaerobic conditions. The corrosion was mitigated by a closer control of operating conditions and chemical treatment.
Pitting in Aluminum - The localized pitting corrosionwas produced in aluminum floats on a storage tank roof. The exposure conditions involved hydrocarbon fluids following an initial hydrotest. The pitting occurred in the absence of chlorides at a near neutral pH where aluminum would be expected to exhibit good resistance to corrosion. Sulfur corrosion products were found in the pits and sulfate reducing bacteria were suspected resulting from prolonged exposure to hydrotest water.
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.
Stress Corrosion Cracking of Stainless Steel - The example shown indicates many intersecting, branched cracks with a transgranular propagation mode. These are typical of stress corrosion cracking (SCC) in austenitic stainless steel. In this case, however, the alloy was reported to be resistant to SCC in the NaCl brine service environment. The location of cracking was limited to a region covered by an elastomeric sleeve. Under the sleeve, evidence of severe general and pitting corrosion were found and evidence of sulfur-containing corrosion products. Analysis of the elastomer indicate that it was not the correct grade and chemical degradation had occurred in service to produce organic acids and sulfur compounds. This local environment resulted in enhanced localized susceptibility of the material to pitting corrosion and SCC.
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.
Internal Coating Failure in a Pipeline - The I.D. coating failure was characterized by the formation of blisters in a field applied epoxy coating. Upon closer examination, the blisters were found to formed within the coating resin as a result of foaming during the coating operation. This is contrasted from the more common case where blistering is initiated on the metal surface by contamination or inadequate preparation. In this case, water was adsorbed from the I.D. environment by the epoxy resin and collected in the voids within the epoxy resin resulting in the formation of blisters within the coating.
Oxidative Degradation of Carbon-Carbon Composites - Carbon-carbon fiber composites are advanced materials with excellent strength to weight performance characteristics. However, at high temperature in the presence of aerated atmospheres, the carbonaceous fiber material will be susceptible to oxidation resulting in both mass loss and a reduction in strength. Rapid thermal flucuations common in thin components can work synergistically with the environmental degradation to produce accelerated attack. Various glasseous, multilayer ceramic coatings are often used to provide resistance to this form of environmental attack which must have both chemical resistance and controlled thermal expansion properties.
Localized Corrosion in 13Cr Stainless Steel - In many industrial process environments, stainless steels provide for long term serviceability by maintaining a passive oxide layer on the metal service. The sample in the figure shows signs of local area corrosion from loss of passivity in an aqueous brine containing dissolved carbon dioxide and hydrogen sulfide. The passive regions still show the signs of the original machining marks whereas the active regions show a black corrosion product and extensive pitting. In environments with little or no hydrogen sulfide, this material would be expected to exhibit excellent corrosion resistance. However, the localized breakdown in the protective passive layer in this case is the direct result of relatively high hydrogen sulfide and chloride concentrations in the simulated service environment.