Corrosion Mechanisms
The following are some common corrosion and other material failure mechanisms mentioned in this website with simple explanations and definitions.
Corrosion
Corrosion is the deterioration or degradation of a material
due to the reaction of the material with its environment.
The deterioration can be in the form of thinning of the material cross section,
development of cracks, or changes to material properties such as reduced ductility
(as in the case of hydrogen embrittlement). There are many forms of corrosion.
Some forms typically encountered are: uniform corrosion, galvanic corrosion,
pitting corrosion, crevice corrosion, flow accelerated corrosion, stress corrosion, hydrogen embrittlement,
corrosion fatigue, and microbiological corrosion
Uniform Corrosion
Dissolution of a metal which occurs at an approximately uniform rate over the entire surface or large fraction of the surface. The disolution process is typically electrochemical in nature. Uniform corrosion is sometimes referred to as general corrosion. The material is thinned fairly uniformly and a layer of oxides or rust may be present on the surface. A commonly observed example of uniform corrosion is the creation of surface rust on steel exposed to air and moisture.
Galvanic Corrosion
Localized corrosion caused by the interaction between two dissimilar metals.
The two metals must be in an electrolyte and be in electrical contact with each other.
The less corrosion resistant metal is the anode. The more corrosion resistant metal is the cathode.
As the corrosion proceeds, electrochemical reactions occur and electrons are transferred from the anode to the cathode.
The rate of corrosion is increased for the anode in comparison to the rate of corrosion of similar metals not in electrical contact and the corrosion of the cathode will decrease or stop.
An example of galvanic corrosion is found in a standard "D-cell" flashlight battery. In the battery, the outer casing is zinc (the anode) with a carbon electrode (the cathode) at the center. In between is an ammonium chloride paste (the electrolyte). When an electric circuit is completed ( the flashlight is lit), electrons are transferred from the zinc to the carbon and the zinc casing corrodes. Older used batteries would eventually corrode through the casing and the corrosive fluid would leak out.
Cathodic protection of carbon steel piping utilizes the galvanic corrosion process. Sacrificial anodes of magnesium or aluminum are electrically attached to the piping. As the anodes corrode, an electric current is transmitted to the carbon steel pipe and greatly reduces or stops the corrosion of the pipe.
Pitting Corrosion
Pitting corrosion is an extremely localized form of corrosion characterized by small holes or cavities randomly located on otherwise intact surfaces. Pitting occurs as a result of the development of a localized environment which is electrochemically different than the surroundings. This may be due to small surface imperfections such as a scratch in a protective oxide film or a small hole in a protective coating. Pitting in aqueous environments tends to occur in stagnant regions. Chemical contaminants on the surface of a material can also produce pitting.
Crevice Corrosion
Crevice corrosion is a localized form of corrosion that occurs the stagnant regions formed by crevices in aqueous environments. To function as a crevice corrosion site, an opening must be wide enough to permit liquid entry but sufficiently narrow to maintain stagnant zone. Thus crevice corrosion usually occurs in openings a few thousands of an inch or less in width. Depletion of oxygen in the crevice or changes in the concentration of other fluid constituents causes the environment of the crevice becomes electrochemically different than the bulk fluid. This creates an electrochemical potential, which provides the driving force for corrosion to occur.
If chlorides are present in the solution, an autocatalytic reaction is created between the negatively charged chloride ions and the positively charged metal ions from the metal dissolving in the crevice. The chloride ions increase the rate of transport of the metal ions from the crevice and greatly increase the overall rate of the corrosion reaction.
Crevice corrosion can be minimized or eliminated by careful attention to design to avoid crevice geometries, use of welded instead of bolted or riveted joints, avoiding stagnant conditions, and by keeping oxygen and chloride concentrations low in the fluid.
Flow Accelerated Corrosion
Often abbreviate FAC. Highly localized thinning of a carbon steel pipe wall or component as a result of the chemical dissolution of the protective oxide film and underlying base metal in a flowing water environment. Factors which affect FAC are material composition, water pH and oxygen concentration, temperature, pipe geometry and fluid velocity, and steam quality.
The most significant variable is the composition of the carbon steel in terms of the trace concentrations of chromium, molybdenum, and copper. FAC can essentially be eliminated by increasing the concentration of chromium above 1%.
The chemistry of the water flowing in the pipe has the next largest effect. Increasing the pH and oxygen concentrations reduce the rate of FAC. At pH of 9.3 or greater, the rate of FAC is essentially negligible.
Power plants, with large networks of carbon steel piping, are susceptible to FAC. Numerous pipe ruptures have occurred at power plants - a partial list is shown in the following table. Note more nuclear plants are on the list due to reporting requirements with the Nuclear Regulatory Commission (NRC). Pipe ruptures and leaks have occurred at fossil power plant that are suspected to have been caused by FAC and not been reported.
Plant
Oconee Unit 3
Oyster Creek
Navajo Unit 3
Browns Ferry Unit 1
Oconee Unit 2
Calvert Cliffs Unit 1
Trojan
Connecticut Yankee
Surry Unit 2
Hatch Unit 2
Ginna
ANO Unit 2
S.M. de Garona
Loviisa Unit 1
Millstone Unit 3
Millstone Unit 2
Almaraz Unit 1
Sequoyah Unit 2
Loviisa Unit 2
Pleasant Prairie Unit 1
Millstone Unit 2
Green Bay
Fort Calhoun
Callaway
Mihama Unit 3
V.C. Summers
Iatan Unit 1
Year
1976
1978
1982
1982
1983
1984
1985
1985
1986
1986
1986
1989
1989
1990
1990
1991
1991
1993
1993
1995
1995
1996
1997
1999
2004
2004
2007
System
Extraction Steam
Feedwater
Feedwater
Moisture Separator Reheater Drain
Reheater Drain
Cold Reheat
Heater Drain
Feedwater
Condensate
Extraction Steam
Moisture Separator Reheater Drain
Extraction Steam
Feedwater
Feedwater
Moisture Separator Reheater Drain
Reheater Drain
Extraction Steam
Extraction Steam
Feedwater
Feedwater
Heater Drain
Feedwater
Extraction Steam
Reheater Drain
Condensate
Heater Drain
Attemperator Spray
Plant Type
Nuclear
Nuclear
Fossil
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Nuclear
Fossil
Nuclear
Fossil
Nuclear
Nuclear
Nuclear
Nuclear
Fossil
Fatalities / Injuries
1 Injured
4 Killed
2 Killed
2 Killed
5 Killed 6 Injured
2 Killed
Stress Corrosion
Often abbreviated SCC for stress corrosion cracking. The development of a crack or network of cracks in a material due to the simultaneous presence of a sustained tensile stress and a corrosive environment. Susceptibility to this form of corrosion is dependent on the specific material, stress level, and corrosive environment. Materials that are immune to stress corrosion in one environment may readily crack in a different environment. Similarly, cracking may not occur at low stress levels in a certain material, but may occur at high stress levels in the same material in the same environment.
Stress corrosion may have a long incubation period. That is there may be a long time before cracks develop. During this period, there may be little or no detectable indications of corrosion. Once cracks form, the propagation of the cracks may be rapid. Failure of the component may occur in a relatively short time after the onset of cracks. This makes stress corrosion particularly difficult to detect by non-destructive techniques.
Examples of material / environment combinations that can result in stress corrosion are copper alloys in ammonia environments, certain aluminum alloys in sea water environments, and carbon steels in caustic environments. An unusual example which has had a huge economic impact was the development of stress corrosion cracks in nuclear steam generator tubes. In this case, the corrosive environment was extremely high purity water.
The photomicrographs below show stereo and optical microscope images of a cross section of a bolt which failed from stress corrosion. Note the network of branching cracks. Branching cracks are frequently present with stress corrosion. Also note that the cracks initiated at the thread roots, which are the highest stress areas, and that there is little or no exterior corrosion or thinning of the bolt cross section.
Stereo Microscope image (left) and Optical Microscope Image (right) of cross section of fractured bolt. The cross section was etched with Nital to reveal a tempered martensite structure.
Branching cracks emanating from high stress regions such as the root of threads is indicative of stress corrosion.
Selective Leaching
Also known as dealloying, a form of corrosion where a less corrosion resistant constituent of an alloy selectively corrodes and is removed from the material. The remaining structure is a porous matrix with little structural strength.
Selective leaching commonly occurs in brasses, which are alloys of copper and zinc. The zinc is less corrosion resistant than the copper and selectively corrodes. A porous matrix of copper is left that has little strength.
Cast iron pipes are also subject to selective leaching.
Here the iron selectively corrodes, leaving a porous matrix of graphite and iron oxides.
This process is commonly referred to as graphitic corrosion or graphitization. The pipe becomes very brittle and susceptible to fracture.
Cast iron pipes have been used extensively for both water and natural gas distribution. As the infrastructure ages, this form of corrosion has become an increasing problem.
Fatigue
Fatigue is the progressive damage of materials caused by the application of a cyclic load. Fatigue can result in the formation of cracks and in the ultimate failure of the material. The cyclic load resulting in the failure may be much less than the single application load required to fail the material.
An everyday example of fatigue is the repeated bending of a paper clip until failure occurs.
Fatigue is affected by many factors including the number of load cycles, the applied load in each cycle, surface roughness, temperature, stress concentrations, and microscopic discontinuities and defects in the material.
Corrosion fatigue is the damage of a material caused by the application of a cyclic load in a corrosive environment. The presence of a corrosive environment can greatly exacerbate the damage of the cyclic load and accelerate the rate of crack propagation once a crack has formed.
Once a fatigue crack is formed, the crack may propagate with each new load cycle. This incremental propagation of the crack can leave telltale marks on the fracture surface called striation. The SEM image below shows an example of striations on the fracture surface of a medium carbon steel bolt subjected to fatigue.
SEM Image of fatigue striations on the fracture surface of medium carbon steel bolt
subjected to an alternating stress amplitude of 29 ksi for 108006 cycles to failure.
Fretting Corrosion
Fretting corrosion is corrosion caused by or accelerated by the relative motion of two surfaces in contact in the presence of a corrosive environment. The relative motion of the surfaces results in wear of the surface and removal of the protective oxide film from the metal surface. This exposes fresh material to the corrosive environment, which then corrodes. As the process continues, the removed oxides and metallic debris further exacerbate the surface wear. The rate of degradation of the surface is far greater than that caused by wear or corrosion alone.
The relative motion of the surfaces may be quite small. Motions of a thousandth of an inch or less can cause fretting corrosion. Vibrations are a common source of motion for fretting corrosion.
Hydrogen Embrittlement
Hydrogen embrittlement is the degradation of the strength and fracture toughness of a material due to the presence of excessive concentrations of hydrogen within the material.
The presence of hydrogen in the metal can cause grain boundary decohesion and the development of intergranular cracks.
Intergranular cracks in a material containing excessive amounts of hydrogen may not form until the material is loaded in tension in service. This can lead to a phenomenon known as delayed fracture.
Once a material is loaded In tension in service, the hydrogen migrates and concentrates in high stress regions. When sufficient concentrations are reached, grain boundary decohesion occurs and intergranular cracks are formed. The tip of the crack becomes the new high stress region and the process repeated itself, allowing the crack to propagate and reducing the intact cross-sectional area. Over a relatively short period of time, which may be on the order of hours to weeks depending on the material, stress, and hydrogen concentration, the cracked area reaches a critical size. At that point, the remaining intact cross-sectional area can no longer carry the applied load and the part fails.
The source of the hydrogen can be the manufacturing process as well as the service environment. Hydrogen can be introduced into the material during manufacturing by a number of processes. Possible sources of hydrogen include poorly controlled furnace atmospheres, improperly selected furnace atmospheres, acid cleaning, electroplating, and the welding process. In environmentally induced hydrogen embrittlement, the source of the hydrogen is a corrosion process, with the morphology of the failure very similar to stress corrosion.
Microbiological corrosion
Microbiological corrosion is corrosion caused by or exacerbated by the presence of microscopic life forms such as bacteria. The microorganisms attach themselves to a surface and create a specialized form of crevice corrosion. A corrosive environment is created as a result of the life cycle processes of the organism or from the decay of the organism when it dies. Biocides and selection of resistant materials can be used to control microbiological corrosion.
Intergranular Attack
Intergranular attack or intergranular corrosion is corrosion that occurs preferentially at or adjacent to grain boundaries in a metal alloy. The corrosion occurs as a result of compositional differences at or adjacent to the material grain boundaries – essentially galvanic corrosion on a microscopic scale. These compositional differences may be caused by enrichment or depletion of alloy elements or by impurity segregation.
Intergranular corrosion of sensitized stainless steel is an example of corrosion caused by the depletion of an alloy element adjacent to the grain boundary.
When stainless steel is heated to a temperature between approximately 500°C and 800°C, chromium carbide (Cr23C6) precipitates at the grain boundaries if the carbon content is 0.02% or greater. This results in a chromium depleted zone adjacent to the grain boundaries. When this region is exposed to a corrosive medium, the chromium depleted zone rapidly corrodes with little or no corrosion of the remainder of the metal grains. The result is a network of fissures or cracks along the grain boundaries through the material.
The temperatures that cause sensitization can be readily achieved in areas adjacent to welds. These areas are referred to as the heat affected zone of the weld. Sensitization of stainless steel can be avoided by using low carbon versions of the alloy. Sensitization can be repaired by heat treating the material after welding to redistribute the chromium and eliminate the depleted zone.