Different Types of Corrosion
Content
Different Types of Corrosion
- Recognition, Mechanisms & Prevention
Crevice Corrosion Recognition What is crevice corrosion? Crevice Corrosion Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and nonmetallic material. Outside the gap or without the gap, both metals are resistant to corrosion. The damage caused by crevice corrosion is normally confined to one metal at localized area within or close to the joining surfaces. In this photo, a type 316 stainless steel tube and tube sheet from a heat exchanger in a seawater reverse osmosis (SWRO) desalination plant suffered crevice corrosion due to the presence of crevice (gap) between the tube and tube sheet.
Mechanisms What causes crevice corrosion? Crevice corrosion is initiated by a difference in concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell (different (differential ial aeration cell in the case of oxygen). Outside of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower. Chlorides concentrate inside the crevice (the anode), worsening the situation. Ferrous ions form ferric chloride and attack the stainless steel rapidly. The pH and the oxygen content are lower in the crevice than in the bulk water solution, just as they are inside a pit. The pH inside the crevice may be as low as 2 in a neutral solution. Once a crevice has formed, the propagation mechanism mechanism for crevice corrosion is the same as for pitting corrosion. The major factors influencing crevice corrosion are: metal-to-metal, tal, metal-to-non-metal crevice type: metal-to-me crevice geometry: gap size, depth, surface roughness material: alloy composition (e.g. Cr, Mo), structure environment: pH, temperature, halide ions, oxygen
A material's resistance to crevice corrosion is usually evaluated and ranked using the critical crevice temperature (CCT) in accordance with the ASTM Standard G4803: Standard Test Methods for Pitting and Crevice Corrosion of Stainless Steels and Alloys by Use of FeCl3. The critical crevice temperature is the minimum temperature (°C) to produce crevice attack and CCT is usually lower than the critical pitting temperature (CPT).
Prevention How to prevent crevice corrosion? Crevice corrosion can be designed out of the system Use welded butt joints instead of riveted of riveted or bolted joints in new equipment Eliminate crevices in existing lap joints by continuous welding or soldering Avoid creating stagnant conditions and ensure complete drainage in vessels non-absorbent ent gaskets such as Teflon. Use solid, non-absorb Use higher alloys (ASTM G48) for increased resistance to crevice corrosion
For more details More details on crevice corrosion are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion Testing and Monitoring Made Easy: -Techniques and Applications ((3) 3) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days) Corrosion Control and Prevention in Seawater Desalination Plants (1 day)
Uniform Corrosion (General Corrosion) Recognition What is uniform corrosion? Uniform corrosion or general corrosion, as sometimes called, is
defined as a type of corrosion attack (deterioration) that is more or less uniformly distributed over the entire exposed surface of a metal (see illustration below). Uniform corrosion also refers to the corrosion that proceeds at approximately the same rate over the exposed metal surface. Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance. The photo on the right showed uniform corrosion (rusting) of a pair of steel nuts used to fasten a galvanized steel clamp on a street lamp post. In sharp contrast, the galvanized steel clamp did not show any signs of corrosion but its surface was discolored by the rust. It is also interesting to note that the surface of the top bolt looked like galvanized but the surface of the bolt below was completely rusted (just like the nut).
The photo (above, left) shows a steel coupon corroded (rusted) uniformly over its entire surface after immersion in oxygen aerated water while the same batch of coupons exp exposed osed to deaerated water (above, right) retained their metallic appearance with no visible corrosion (rust). In natural environment, oxygen is the primary cause of uniform corrosion of steels and other metals and alloys.
Mechanisms What causes uniform corrosion? The anodic reaction in the corrosion process is always the oxidation reaction: +
M = M + e (1) In acidic environments, i.e., pH<7, the cathodic process is mainly the reduction of hydrogen ions: +
2H + 2e = H2 (2) In alkaline or neutral environment, i.e., pH=7 or pH>7, reduction of dissolved oxygen is the predominant cathodic process that causes uniform corrosion: -
O2 + 2H2O + 4e = 4OH (3) With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions (2) and/or (3) take place in a "uniform" manner and there is no preferential site or location for cathodic or anodic reaction. The cathodes and anodes are located randomly and alternating with time. The end result is a more or less uniform loss of dimension.
Prevention How to prevent uniform corrosion? Uniform corrosion or general corrosion can be prevented through a number of methods:
Use thicker materials for corrosion allowance Use paints or metallic coatings such as plating, galvanizing or anodizing
Use Corrosion inhibitors or modifying the environment Cathodic protection (Sacrificial Anode or Impressed Current -ICCP) and Anodic Protection
For more details More details on Uniform Corrosion or General Corrosion are included in the following corrosion courses which you can take as in-house training courses, courses, course-on-demand, course-on-demand, online courses or distance learning courses: c ourses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion and Monitoring Made Easy: -Techniques and Applications ((3) 3) Corrosion,Testing Metallurgy, Failure Analysis and Prevention (3 days)
Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
Galvanic Corrosion, Bimetallic Corrosion or Dissimilar Metal Corrosion Recognition What is galvanic corrosion? Galvanic corrosion or "Bimetallic Corrosion" or "Dissimilar Metal Corrosion", as sometimes called, is defined as the accelerated corrosion of a metal because of an electrical contact (including physical contact) with a more noble metal or nonmetallic conductor (the cathode) in a corrosive electrolyte. The less corrosion resistant or the "active" member of the couple experiences accelerated corrosion while the more corrosion resistant or the "noble" member of the couple experiences reduced corrosion due to the "cathodic protection" effect. The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi-metallic joint, the degree of accelerated attack is reduced. In this photo, a 5-mm thick aluminum alloy plate is physically (and hence, electrically) connected to a 10-mm thick mild steel structural support. Galvanic corrosion occurred on the aluminium plate along the joint with the mild steel. Perforation of aluminum plate occurred within 2 years due to the huge acceleration factor in galvanic corrosion. c orrosion.
Mechanisms What causes galvanic corrosion? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. When the corrosion potentials of various metals and alloys are measured in a common electrolyte (e.g. natural seawater) and are listed in an orderly manner (descending or ascending) in a tabular form, a Galvanic Series is created. It should be emphasized that the corrosion potentials must be measured for all metals and alloys in the same electrolyte under the same environmental conditions (temperature, pH, flow rate etc.), otherwise, the potentials are not comparable. The potential difference (i.e., the voltage) between two dissimilar metals is the driving force for the destructive attack on the active metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte will also affect the degree of attack. The cathode to anode area ratio is directly proportional to the acceleration factor.
Prevention How to prevent galvanic corrosion? Galvanic corrosion can be prevented through a number of methods: Select metals/alloys as close together as possible in the galvanic series. Avoid unfavorable area effect of a small anode and large cathode.
Insulate dissimilar metals wherever practical Apply coatings with caution. Paint the cathode (or both) and keep the coatings in good repair on the anode. Avoid threaded joints for materials far apart in the galvanic series.
For more details More details on Galvanic Corrosion or Bimetallic Corrosion are included in the following corrosion courses which you can take as in-house training courses, courses, course-on-demand, course-on-demand, online courses or distance learning courses: c ourses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion Testing and Monitoring Made Easy: -Techniques and Applications ((3) 3) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
Dealloying, Selective Leaching & Graphitic Corrosion Recognition What is dealloying? Dealloying is the selective corrosion of one or more components of a solid solution alloy. It is also called parting, selective leaching or selective attack. Common dealloying examples are decarburizati decarburization, on, decobaltification, denickelification, dezincification, and graphitic corrosion. Decarburization is the selective loss of carbon from the surface layer of a carbon-containing alloy due to reaction with one or more chemical substances in a medium that contacts the surface. Decobaltification is selective leaching of cobalt from cobalt-base alloys, such as Stellite, or from cemented carbides. Denickelification is the leaching of after nickelextended from nickel-containing commonly observed in selective copper-nickel alloys service in freshalloys. water.Most Dezincification is the selective leaching of zinc from zinc-containing alloys. Most commonly found in copper-zinc alloys containing less than 85% copper after extended service in water containing dissolved oxygen. of gray cast iron in which the metallic constituents Graphitic corrosion is the deterioration of gray are selectively leached or converted to corrosion products leaving the graphite intact. Graphitic corrosion should not be confused with another term graphitization, which is used to describe the formation of graphite of graphite in iron or steel, usually from decomposition of iron carbide at elevated temperatures.
i Graphitic corrosion of a gray cast iron valve
Mechanisms What causes dealloying? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. Modern alloys contain a number of different alloying elements that exhibit different corrosion potentials. The potential difference between the alloying elements is the driving force for the preferential attack on the more "active" element in the alloy. In the case of dezincification of brass, zinc is preferentially leached out of the copper-zinc alloy, leaving behind a copper-rich surface layer that is porous and brittle.
Prevention How to prevent dealloying? Dealloying, selective leaching and graphitic corrosion can be prevented through the following methods: Select metals/alloys that are more resistant to dealloying. For example, inhibited brass is more resistant to dezincification than alpha brass, ductile iron is more resistant to graphitic corrosion than gray cast iron. Control the environment to minimize the selective leaching Use sacrificial anode cathodic protection or impressed current cathodic protection
For more details More details on dealloying, selective leaching and graphitic corrosion are included in the following corrosion short courses which you can take as in-house training courses, courseon-demand, online courses or distance learning courses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days)
Pitting Corrosion Recognition What is pitting corrosion? Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting corrosion is one of the most damaging forms of corrosion. Pitting factor is the ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. The following photo shows pitting corrosion of a SAF2304 duplex stainless steel after exposure to 3.5% NaCl solution.
Pitting corrosion is usually found on passive metals and alloys such aluminium alloys, stainless steels and stainless alloys when the ultra-thin passive film (oxide film) is chemically or mechanically damaged and does not immediately re-passivate. The resulting pits can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of a metal.
ASTM-G46 has a standard visual chart for rating of pitting corrosion.
The shape of pitting of pitting corrosion can only be identified through metallography where a pitted sample is cross-sectioned and the pit shape, the pit size, and the pit depth of penetration can be determined.
Mechanisms What causes pitting corrosion? For a defect-free "perfect" material, pitting corrosion is caused by the ENVIRONMENT (chemistry) that may contain aggressive chemical species such as chloride. Chloride is particularly damaging to the passive film (oxide) so pitting can initiate at oxide breaks. The environment may also set up a differential aeration cell (a water droplet on the surface of a steel, for example) and pitting can initiate at the anodic site (centre of the water droplet). For a homogeneous environment, pitting IS caused by the MATERIAL that may contain inclusions (MnS is the major culprit for the initiation of pitting in steels) or defects. In most cases, both the environment and the material contribute to pit initiation. The ENVIRONMENT (chemistry) and the MATERIAL (metallurgy) factors determine whether an existing pit can be repassivated or not. Sufficient aeration (supply of oxygen to the reaction site) may enhance the formation of oxide at the pitting site and thus repassivate or heal the damaged passive film (oxide) - the pit is repassivated and no pitting occurs. An existing pit can also be repassivated if the material contains sufficient amount of alloying elements such as Cr, Mo, Ti, W, N, etc.. These elements, particularly Mo, can significantly enhance the enrichment of Cr in the oxide and thus heals or repassivates the pit. More details on the alloying effects can be found in the technical paper on on "Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail". A material's resistance to pitting corrosion is usually evaluated and ranked using the critical pitting temperature (CPT) in accordance with the ASTM Standard G48-03: Standard Test Methods for Pitting and Crevice Corrosion of Stainless Steels and Alloys by Use of FeCl3. The critical pitting temperature is the minimum temperature (°C) to produce pitting corrosion and CPT is usually higher than the critical crevice temperature (CPT (CPT). ).
Prevention How to prevent pitting corrosion? Pitting corrosion can be prevented through: Proper selection of materials of materials with known resistance to the service environment Control pH, chloride concentration and temperatur temperature e Cathodic protection and/or Anodic Protection
Use higher alloys (ASTM G48) for increased resistance to pitting corrosion
For more details More details on pitting corrosion are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion Testing and Monitoring Made Easy: Techniques and Applications ((3 3 days) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days) Corrosion in Fire Protection Systems (FPS): Detection, Mitigation and Prevention (1 day) Microbiologically Influenced Corrosion (MIC): Recognition, Mitigation and Prevention (1 day) Corrosion Control and Prevention in Seawater Desalination Plants (1 day)
Intergranular Intergr anular Corrosion (Cracking) (C racking) Recognition What is intergranular corrosion? Intergranular corrosion is sometimes also called "intercrystalline corrosion" or "interdendritic corrosion". In the presence of tensile stress, cracking may occur along grain boundaries and this type of corrosion is frequently called "intergranular stress corrosion cracking (IGSCC)" or simply "intergranular corrosion cracking". "Intergranular" or 'intercrystalline" means between grains or crystals. As the name suggests, this is a form of corrosive attack that progresses preferentially along interdendritic paths (the grain boundaries). Positive identification of this type of corrosion usually requires microstructure examination under a microscopy although sometimes it is visually recognizable as in the case of weld decay.
The photos above show the microstructure of a type 304 stainless steel. The figure on the left is the normalized microstructure and the one on the right is the " sensitized sensitized"" structure and is susceptible to intergranular corrosion or intergranular stress corrosion cracking.
Mechanisms What causes intergranular corrosion? This type of attack results from local differences in composition, composition, such as coring commonly encountered in alloy alloy castings. Grain boundary precipitation, notably notably chromium carbides in stainless steels, is a well well recognized and accepted mechanism of intergranular corrosion. The precipitation of chromium carbides consumed the alloying element element - chromium from a narrow band along the grain grain boundary and this makes the zone anodic to the the
unaffected grains. The chromium depleted zone becomes the preferential path for corrosion attack or crack c rack propagation if under tensile stress. Intermetallics segregation at grain boundaries in aluminum alloys also causes intergranular corrosion but with a different name - "exfoliation".
Prevention How to prevent intergranular corrosion? Intergranular corrosion can be prevented through: Use low carbon (e.g. 304L, 316L) grade of stainless steels Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347). Titanium and niobium are strong carbide- formers. They react with the carbon to form the corresponding carbides thereby preventing chromium depletion. Use post-weld heat treatment.
For more details More details on intergranular corrosion are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion Testing and Monitoring Made Easy: -Techniques and Applications ((3) 3) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
Intergranular Intergr anular Corrosion: Weld Decay Recognition What is weld decay? Weld decay is a form of intergranular corrosion, usually of stainless steels or certain nickel-base alloys, that occurs as the result of sensitization in the heataffected zone during the welding operation. The corrosive attack is restricted to the heat affected zone (HAZ) (HAZ).. Positive identification of this type of corrosion usually requires microstructure examination under a microscopy although sometimes it is possible to visually recognize weld decay if parallel lines are already formed in the heat affected zone along the weld (see photo below).
Mechanisms What causes weld decay? As in the case of intergranular corrosion, grain boundary boundary precipitation, notably chromium carbides in in stainless steels, is a well recognized and accepted accepted mechanism of weld decay. In this case, the the precipitation of chromium carbides is induced by by the welding operation when the heat affected affected zone (HAZ) experiences a particular temperature temperature o o range (550 C~850 C). The precipitation of chromium carbides consumed the alloying alloying element - chromium from a narrow band along the grain boundary and this makes the zone anodic to the unaffected grains. The chromium depleted zone becomes the preferential path for corrosion attack or crack propagation if under tensile stress.
Prevention How to prevent weld decay? Weld decay can be prevented through: Use low carbon (e.g. 304L, 316L) grade of stainless steels Use stabilized grades alloyed with titanium (for example type 321) or niobium (for example type 347). Titanium and niobium are strong carbide- formers. They react
with the carbon to form the corresponding carbides thereby preventing chromium depletion. Use post-weld heat treatment (PWHT).
For more details More details on weld decay and weldment corrosion are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion Testing and Monitoring Made Easy: -Techniques and Applications ((3) 3) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
Intergranular Corrosion: Knife Intergranular Knife-Line Attack (KLA) Recognition What is knife-line attack? Knife-Line Attack (KLA) is a form of intergranular corrosion of an alloy, usually stabilized stainless steel, along a line adjoining or in contact with a weld after heating into the sensitization temperature range. The corrosive attack is restricted to extremely narrow line adjoining the fusion line. Attack appears razor-sharp (and hence the name of "knife-line" attack). It is possible to visually recognize knife-line attack if the lines are already formed the along the weld.
Mechanisms What causes knife-line attack? For stabilized stainless steels and alloys, carbon is bonded with stabilizers (Ti or Nb) and no weld decay occurs in the heat affected zone during welding. In the event of a subsequent heat treatment or welding, however, precipitation of chromium carbide is possible and this leaves the narrow band adjacent to the fusion line susceptible to intergranular corrosion.
Prevention How to prevent knife-line attack? Knife-Line Attack can be prevented through: o Heat treatment - heating the weld to 1065 C to re-stabilize the material.
For more details
More details on knife-line attack and weldment corrosion are included in the following corrosion courses which you can take as in-house training courses, courses, course-on-demand, course-on-demand, online courses or distance learning courses: c ourses: Corrosion and Its Prevention (5-day module) Corrosion and Its Prevention (2-day module) Corrosion Testing and Monitoring Made Easy: -Techniques and Applications ((3) 3) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days) Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
Intergranular Corrosion: Exfoliation Corrosion Intergranular Recognition What is exfoliation? Exfoliation is a special form of intergranular corrosion that proceeds laterally from the sites of initiation along planes parallel to the surface, generally at grain boundaries, forming corrosion products that force metal away from the body of the material, giving rise to a layered appearance. Exfoliation is sometimes described as lamellar, layer, or stratified corrosion. In this type of corrosion, attack proceeds along selective subsurface paths parallel to the surface. It is possible to visually recognize this type of corrosion if the grain boundary attack is severe otherwise microstructure examination under a microscope is needed. Exfoliation corrosion in an aluminum alloy exposed to tropical marine environment. Also note the paint failures caused by corrosion of aluminum at the coating/aluminium interface.
Mechanisms What causes exfoliation? Exfoliation is a special type of intergranular corrosion that occurs on the elongated grain boundaries. The corrosion product that forms has a greater volume than the volume of the parent metal. The increased volume forces the layers apart, and causes the metal to exfoliate or delaminate. Aluminum alloys are particularly susceptible to this type of corrosion.
Prevention How to prevent exfoliation corrosion? Exfoliation corrosion can be prevented through: the use of coatings selecting a more exfoliation resistant aluminium alloy using heat treatment to control precipitate distribution.
For more details More details on exfoliation corrosion are included in the following corrosion courses which you can take as in-house training courses, course-on-demand, online courses or distance learning courses: Corrosion Corrosion and and Its Its Prevention Prevention (5-day (2-day module) module) Corrosion, Metallurgy, Failure Analysis and Prevention (3 days) Marine Corrosion, Causes and Prevention (2 days) Materials Selection and Corrosion (2 days)
