Classic Corrosion Photographs

Abstract:

This article contains Corrosion Photographs and brief description of Wet H2S Cracking, Thermal Cycling, Crevice Corrosion, Cavitation Attack, Microbiologically Induced Corrosion, Pitting, Stress Corrosion Cracking and others.

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Wet H₂S Cracking

Wet H₂S cracking can occur in susceptible steels exposed to aqueous environments containing hydrogen sulfide. It is a form of hydrogen-related cracking and can have two distinct morphologies: The first type is commonly referred to as Hydrogen Induced Cracking (HIC) and can occur where little or no applied or residual tensile stress exists. It is manifested as blisters or blister cracks oriented parallel to the plate surface.

The second type produces an array of blister cracks linked in the through thickness direction by transgranular, cleavage cracks. The latter type of cracking is referred to as Stress Oriented Hydrogen Induced Cracking (SOHIC). SOHIC can have a greater effect of serviceability than HIC since it effectively reduces load carrying capabilities to a greater degree.

Thermal Cycling in

Hydrogen Reformer Tubes

A high temperature tensile/rupture failures can result from cumulative thermal cycles in reformer tubes. These conditions promote high tensile stresses in the axial direction which lead to the formation of highly branched networks of cracks. These cracks are generally oriented in the through thickness direction and link up with time resulting in premature tube failures.

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.

Cavitation Attack

Example of cavitation attack in 440C martemsitic stainless steel with hardness about 50HRc. Application was a piston in a hydropower rock drill, life not known. Image aquired on ISI SX-30 microscope, scale bar is 10 microns.

photo courtesy of: T J Carter
B.Tech., M.I.M., C.Eng.
Advanced Engineering and Testing Services
CSIR, Auckland Park, South Africa.

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 corrosion was 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 indicates 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 13CrStainless 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.

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