Forms of Corrosion

Forms of Corrosion

Valuable information for solving corrosion problems can often be obtained through careful observation of the corroded test specimens or failed equipment.

In fact, corrosion is classified by the way in which it displays. Its various forms are grouped based on the appearance of the corroded metal with corrosion forms identified by visual observation. In most cases the naked eye is sufficient, but sometimes magnification is helpful or required.

Corrosion’s many forms are discussed in terms of their characteristics, mechanisms, and preventive measures at the jump links below.

Group 1:
Corrosion determined by visual observation

Group 2:
Corrosion needing additional means of examination

Related: Microbiologically Influenced Corrosion

While not technically a form of corrosion, microbes or bacteria on material surfaces can speed the deterioration process causing microbiologically influenced corrosion.

cavitation

Cavitation Erosion

Cavitation occurs when a fluid's operational pressure drops below its vapor pressure causing gas pockets and bubbles to form and collapse. This can occur in what can be a rather explosive and dramatic fashion. In fact, this can produce steam at the suction of a pump in a matter of minutes. When a process fluid is supposed to be water in the 20-35°C range, this is entirely unacceptable. Additionally, this condition can form an airlock, which prevents any incoming fluid from offering cooling effects, further exacerbating the problem. The locations where this is most likely to occur, include:

  • At the suction of a pump, especially if operating near the net positive suction head required ((NPSHR)
  • At the discharge of a valve or regulator, especially when operating in a near-closed position
  • At other geometry-affected flow areas such as pipe elbows and expansions
  • By processes incurring sudden expansion, which can lead to dramatic pressure drops

This form of corrosion will eat out the volutes and impellers of centrifugal pumps with ultrapure water as the fluid. It will eat valve seats. It will contribute to other forms of erosion corrosion, such as found in elbows and tees. Cavitation should be designed out by reducing hydrodynamic pressure gradients and designing to avoid pressure drops below the vapor pressure of the liquid and air ingress. The use of resilient coatings and cathodic protection can also be considered as supplementary control methods.


monel

Crevice Corrosion

Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded coating, threads, lap joints and clamps. Crevice corrosion is initiated by changes in local chemistry within the crevice:  

  1. Depletion of inhibitor in the crevice
  2. Depletion of oxygen in the crevice
  3. A shift to acid conditions in the crevice
  4. Build-up of aggressive ion species (e.g. chloride) in the crevice
crevice

As oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between crevice (microenvironment) and the external surface (bulk environment). The chronology of the aggravating factors leading to a full blown crevice is illustrated here.

-Initially, oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere.

crevice3

The cathodic oxygen reduction reaction cannot be sustained in the crevice area, giving it an anodic character in the concentration cell. This anodic imbalance can lead to the creation of highly corrosive micro-environmental conditions in the crevice, conducive to further metal dissolution. This results in the formation of an acidic micro-environment, together with a high chloride ion concentration.

-Metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products.

pillow corrosion

All forms of concentration cell corrosion can be very aggressive, and all result from environmental differences at the surface of a metal. Even the most benign atmospheric environments can become extremely aggressive as illustrated in this example of aircraft corrosion (Image courtesy Mike Dahlager). This advanced form of crevice corrosion is called 'pillowing'. 

The most common form is oxygen differential cell corrosion. This occurs because moisture has a lower oxygen content when it lies in a crevice than when it lies on a surface. The lower oxygen content in the crevice forms an anode at the metal surface. The metal surface in contact with the portion of the moisture film exposed to air forms a cathode.

A special form of crevice in which the aggressive chemistry build-up occurs under a protective film that has been breached is called "filiform corrosion." A severe form of crevice corrosion can occur under insulation.


Corrosion Fatigue

Corrosion fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated.  If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time.

In a corrosive environment the stress level at which it could be assumed a material has infinite life is lowered or removed completely. Contrary to a pure mechanical fatigue, there is no fatigue limit load in corrosion-assisted fatigue.

Corrosion fatigue and fretting are both in this class. Much lower failure stresses and much shorter failure times can occur in a corrosive environment compared to the situation where the alternating stress is in a non-corrosive environment.

The fatigue fracture is brittle and the cracks are most often transgranular, as in stress-corrosion cracking, but not branched. The corrosive environment can cause a faster crack growth and/or crack growth at a lower tension level than in dry air. Even relatively mild corrosive atmospheres can reduce the fatigue strength of aluminum structures considerably, down to 75 to 25% of the fatigue strength in dry air. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment. Control of corrosion fatigue can be accomplished by either lowering the cyclic stresses or by various corrosion control measures. See checklist.

Protection Possibilities Checklist

  • Minimize or eliminate cyclic stresses
  • Reduce stress concentration or redistribute stress (balance strength and stress throughout the component)
  • Select the correct shape of critical sections
  • Provide against rapid changes of loading, temperature or pressure
  • Avoid internal stress
  • Avoid fluttering and vibration-producing or vibration-transmitting design
  • Increase natural frequency for reduction of resonance corrosion fatigue
  • Limit corrosion factor in the corrosion-fatigue process (more resistant material / less corrosive environment)

Dealloying (Selective Leaching)

Dealloying or selective leaching refers to the selective removal of one element from an alloy by corrosion processes. A common example is the dezincification of unstabilized brass, whereby a weakened, porous copper structure is produced. The selective removal of zinc can proceed in a uniform manner or on a localized (plug-type) scale. It is difficult to rationalize dezincification in terms of preferential Zn dissolution out of the brass lattice structure. Rather, it is believed that brass dissolves with Zn remaining in solution and Cu replating out of the solution. Graphitic corrosion of gray cast iron, whereby a brittle graphite skeleton remains following preferential iron dissolution is a further example of selective leaching. The term "graphitization" is commonly used to identify this form of corrosion but is not recommended because of its use in metallurgy for the decomposition of carbide to graphite.

During cast iron graphitic corrosion, the porous graphite network -- that makes up 4-5% of the total mass of the alloy -- is impregnated with insoluble corrosion products. As a result, the cast iron retains its appearance and shape but is weaker structurally. Testing and identification of graphitic corrosion is accomplished by scraping through the surface with a knife to reveal the crumbling of the iron beneath. Where extensive graphitic corrosion occurs, usually the only solution is replacement of the damaged element.


Environmental Cracking

Environmental cracking refers to a corrosion cracking caused by a combination of conditions that can specifically result in one of the following forms of corrosion damage:

Stress Corrosion Cracking (SCC)

Corrosion Fatigue

Hydrogen Embrittlement

Stresses that cause environmental cracking arise from residual cold work, welding, grinding, thermal treatment, or may be externally applied during service and, to be effective, must be tensile (as opposed to compressive).

Stress definition or stress variables

  • Mean stress
  • Maximum stress
  • Minimum stress
  • Constant load/constant strain
  • Strain rate
  • Plane stress/plane strain
  • Modes I, II, or III
  • Biaxial
  • Cyclic frequency
  • Wave shape

Stress origin

  • Intentional
  • Residual
  • Shearing, punching, cutting
  • Bending, crimping, riveting
  • Welding
  • Machining
  • Grinding

Produced by reacted products

  • Applied
  • Quenching
  • Thermal cycling
  • Thermal expansion
  • Vibration
  • Rotation
  • Bolting
  • Dead load
  • Pressure

The cracks form and propagate approximately at right angles to the direction of the tensile stresses at stress levels much lower than those required to fracture the material in the absence of the corrosive environment. As cracking penetrates further into the material, it eventually reduces the supporting cross section of the material to the point of structural failure from overload. SCC occurs in metals exposed to an environment where, if the stress was not present or was at much lower levels, there would be no damage. If the structure, subject to the same stresses, were in a different environment (noncorrosive for that material), there would be no failure. Examples of SCC in the nuclear industry are cracks in stainless steel piping systems and stainless steel valve stems.

Stress cells can exist in a single piece of metal where a portion of the metal's microstructure possesses more stored strain energy than the rest of the metal. Metal atoms are at their lowest strain energy state when situated in a regular crystal array.

Deviations from this lowest-strain state follow:

  • Gain boundaries
  • High localized stress
  • Cold worked

Grain boundaries: By definition, metal atoms situated along grain boundaries are not located in a regular crystal array (i.e. a grain). Their increased strain energy translates into an electrode potential that is anodic to the metal in the grains proper. Thus, corrosion can selectively occur along grain boundaries.

High localized stress: Regions within a metal subject to a high local stress will contain metal atoms at a higher strain energy state. As a result, high-stress regions will be anodic to low-stress regions and can corrode selectively. For example, bolts under load are subject to more corrosion than similar bolts that are unloaded. A good rule of thumb is to select fasteners that are cathodic (i.e. higher on the Electrochemical Series) to the metal being fastened in order to prevent fastener corrosion.

Cold worked: Regions within a metal subjected to cold-work contain a higher concentration of dislocations, and as a result will be anodic to non-cold-worked regions. Thus, cold-worked sections of a metal will corrode faster. For example, nails that are bent will often corrode at the bend, or at their head where they were worked by the hammer.


High hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion
High hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion.

Erosion

Erosion corrosion is an acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface. The increased turbulence caused by pitting on the internal surfaces of a tube can result in rapidly increasing erosion rates and eventually a leak. Erosion corrosion can also be aggravated by faulty workmanship. For example, burrs left at cut tube ends can upset smooth water flow, cause localized turbulence and high flow velocities, resulting in erosion corrosion. A combination of erosion and corrosion can lead to extremely high pitting rates. In offshore well systems, the process industry in which components come into contact with sand-bearing liquids, is a problem.

Materials selection plays an important role in minimizing erosion corrosion damage. Caution is in order when predicting erosion corrosion behavior on the basis of hardness. High hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion. Design features are also particularly important.

It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters are useful in this context. Rough surfaces are generally undesirable. Designs creating turbulence, flow restrictions, and obstructions are undesirable. Abrupt changes in flow direction should be avoided. Tank inlet pipes should be directed away from the tank walls, towards the center. Welded and flanged pipe sections should always be carefully aligned. Impingement plates of baffles designed to bear the brunt of the damage should be easily replaceable.

The thickness of vulnerable areas should be increased. Replaceable ferrules, with a tapered end, can be inserted into the inlet side of heat exchanger tubes to prevent damage to the actual tubes. Several environmental modifications can be implemented to minimize the risk of erosion corrosion. Abrasive particles in fluids can be removed by filtration or settling, while water traps can be used in steam and compressed air systems to decrease the risk of impingement by droplets. De-aeration and corrosion inhibitors are additional measures that can be taken. Cathodic protection and the application of protective coating may also reduce the rate of attack.


Exfoliation

Exfoliation corrosion is a particular form of intergranular corrosion associated with high-strength aluminum alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage.

Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. The damage often initiates at end grains encountered in machined edges, holes, or grooves and can subsequently progress through an entire section.

The image of the “Exfoliation of an aircraft component” shows how corrosion separates into distinct layers that have expanded to occupy a much larger area than the original, uncorroded part. The structural integrity of this part disappeared long ago.

Notice how the corrosion separates into distinct layers which have expanded to occupy a much larger area than the original, uncorroded part. Obviously, the structural integrity of this part disappeared long ago. (courtesy ) Mike Dahlager
Exfoliation of an aircraft component (Image courtesy of Mike Dahlager).

Fillform Corrosion

Fillform is a special form of crevice corrosion in which the aggressive chemistry build-up occurs under a protective film that has been breached. Filiform corrosion normally starts at small, sometimes microscopic, defects in the coating. Lacquers and "quick-dry" paints are most susceptible to the problem. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Where a coating is required, it should exhibit low water vapor transmission characteristics and excellent adhesion. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality.

The crawling under paint corrosion called 'filiform'
The crawling under paint corrosion called 'filiform'.

fretting

Fretting Corrosion

Fretting corrosion refers to corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration. Pits or grooves and oxide debris characterize this damage, typically found in machinery, bolted assemblies, and ball or roller bearings. Contact surfaces exposed to vibration during transportation are exposed to the risk of fretting corrosion.

Damage, as seen in this image (Courtesy Mike Dahlager), can occur at the interface of two highly loaded surfaces, which are not designed to move against each other. The most common type of fretting is caused by vibration. The protective film on the metal surfaces is removed by the rubbing action and exposes fresh, active metal to the corrosive action of the atmosphere. 


Galvanic corrosion inside horizontal stabilizer in the Statue of Liberty. Stainless screw with cadmium plated steel lock washer.
Galvanic corrosion inside horizontal stabilizer in the Statue of Liberty. Stainless screw with cadmium plated steel lock washer.

Galvanic Corrosion

Galvanic corrosion (also called ' dissimilar metal corrosion' or wrongly 'electrolysis') refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.

Either (or both) metal in the couple may or may not corrode by itself (themselves). When contact with a dissimilar metal is made, however, the self corrosion rates will change: corrosion of the anode will accelerate corrosion of the cathode will decelerate or even stop. Galvanic coupling is the foundation of many corrosion monitoring techniques.

The driving force for corrosion is a potential difference between the different materials. The bimetallic driving force was discovered in the late part of the eighteenth century by Luigi Galvani in a series of experiments with the exposed muscles and nerves of a frog that contracted when connected to a bimetallic conductor. The principle was later put into a practical application by Alessandro Volta who built, in 1800, the first electrical cell, or battery: a series of metal disks of two kinds, separated by cardboard disks soaked with acid or salt solutions. This is the basis of all modern wet-cell batteries and it was a tremendously important scientific discovery because it was the first method found for the generation of a sustained electrical current.

The principle was also engineered into the useful protection of metallic structures by Sir Humphry Davy and Michael Faraday in the early part of the nineteenth century. The sacrificial corrosion of one metal such as zinc, magnesium, or aluminum is a widespread method of cathodically protecting metallic structures.

In a bimetallic couple, the less noble material will become the anode of this corrosion cell and tend to corrode at an accelerated rate, compared with the uncoupled condition. The more noble material will act as the cathode in the corrosion cell. Galvanic corrosion can be one of the most common forms of corrosion as well as one of the most destructive.

The relative nobility of a material can be predicted by measuring its corrosion potential. The well-known galvanic series lists the relative nobility of certain materials in sea water. A small anode/cathode area ratio is highly undesirable. In this case, the galvanic current is concentrated onto a small anodic area. Rapid thickness loss of the dissolving anode tends to occur under these conditions. Galvanic corrosion problems should be solved by designing to avoid these problems in the first place. Galvanic corrosion cells can be set up on the macroscopic level or on the microscopic level. On the microstructural level, different phases or other microstructural features can be subject to galvanic currents.


Hydrogen Embrittlement

This type of deterioration can be linked to corrosion and corrosion-control processes. It involves the ingress of hydrogen into a component, an event that can seriously reduce the ductility and load-bearing capacity as well as cause cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials. Hydrogen embrittlement occurs in a number of forms but the common features are an applied tensile stress and hydrogen dissolved in the metal. Examples of hydrogen embrittlement are cracking of weldments or hardened steels when exposed to conditions that inject hydrogen into the component. Presently, this phenomenon is not completely understood and hydrogen embrittlement detection, in particular, seems to be one of the most difficult aspects of the problem. Hydrogen embrittlement does not affect all metallic materials equally. The most vulnerable are high-strength steels, titanium alloys, and aluminum alloys.

Sources of Hydrogen

Sources of hydrogen causing embrittlement have been encountered in the making of steel, in processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen as a contaminant in the environment that is often a by-product of general corrosion. It is the latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor coolant to remove oxygen from reactor coolant systems. Hydrogen entry, the obvious pre-requisite of embrittlement, can be facilitated in a number of ways summarized below: (Defence Standard 03-30, October 2000)

  1. by some manufacturing operations such as welding, electroplating, phosphating, and pickling; if a material subject to such operations is susceptible to hydrogen embrittlement then a final, baking heat treatment to expel any hydrogen is employed.
  2. as a by-product of a corrosion reaction such as in circumstances when the hydrogen production reaction (Equation 2) acts as the cathodic reaction since some of the hydrogen produced may enter the metal in atomic form rather than be all evolved as a gas into the surrounding environment. In this situation, cracking failures can often be thought of as a type of stress corrosion cracking. If the presence of hydrogen sulfide causes entry of hydrogen into the component, the cracking phenomenon is often termed “sulphide stress cracking (SSC)”
  3. the use of cathodic protection for corrosion protection if the process is not properly controlled.

Hydrogen Embrittlement of Stainless Steel

Hydrogen diffuses along the grain boundaries and combines with the carbon, which is alloyed with the iron, to form methane gas. The methane gas is not mobile and collects in small voids along the grain boundaries where it builds up enormous pressures that initiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained at a neutral or basic pH in plants without aluminum components.

If the metal is under a high tensile stress, brittle failure can occur. At normal room temperatures, the hydrogen atoms are absorbed into the metal lattice and diffused through the grains, tending to gather at inclusions or other lattice defects. If stress induces cracking under these conditions, the path is transgranular. At high temperatures, the absorbed hydrogen tends to gather in the grain boundaries and stress-induced cracking is then intergranular. The cracking of martensitic and precipitation hardened steel alloys is believed to be a form of hydrogen stress corrosion cracking that results from the entry into the metal of a portion of the atomic hydrogen that is produced in the following corrosion reaction.

Hydrogen embrittlement is not a permanent condition. If cracking does not occur and the environmental conditions are changed so that no hydrogen is generated on the surface of the metal, the hydrogen can rediffuse from the steel, so that ductility is restored.

To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amount of residual hydrogen in steel, controlling the amount of hydrogen pickup in processing, developing alloys with improved resistance to hydrogen embrittlement, developing low or no embrittlement plating or coating processes, and restricting the amount of in-situ (in position) hydrogen introduced during the service life of a part.


Intergranular corrosion'
Intergranular corrosion of a failed aircraft component made of 7075-T6 aluminum.

Intergranular Corrosion

The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Intergranular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries) or specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.

The attack is usually related to the segregation of specific elements or the formation of a compound in the boundary. Corrosion then occurs by preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance - thus making the grain boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary and, in a severe case of grain-boundary corrosion, entire grains may be dislodged due to complete deterioration of their boundaries.

In any case, the mechanical properties of the structure will be seriously affected. A classic example is the sensitization of stainless steels or weld decay. Chromium-rich grain boundary precipitates lead to a local depletion of Cr immediately adjacent to these precipitates, leaving these areas vulnerable to corrosive attack in certain electrolytes. Reheating a welded component during multi-pass welding is a common cause of this problem. In austenitic stainless steels, titanium, or niobium can react with carbon to form carbides in the heat affected zone (HAZ) causing a specific type of intergranular corrosion known as knife-line attack. These carbides build up next to the weld bead where they cannot diffuse due to rapid cooling of the weld metal. The problem of knife-line attack can be corrected by reheating the welded metal to allow diffusion to occur.

Wrought aluminum alloys

AlGrainRev'
SL = Short longitudinal
ST = Short transverse
LT = Longitudinal transverse

Exfoliation corrosion is a further form of intergranular corrosion associated with high strength aluminum alloys. Alloys that have been extruded or otherwise worked heavily, with a microstructure of elongated, flattened grains, are particularly prone to this damage. Corrosion products building up along these grain boundaries exert pressure between the grains and the end result is a lifting or leafing effect. The damage often initiates at end grains encountered in machined edges, holes or grooves and can subsequently progress through an entire section.


Lamellar Corrosion

Recognized authority on all things materials vs. corrosion, Paul Dillon says the following about lamellar corrosion:

“Lamellar corrosion or exfoliation? ASM’s Glossary of Terms, Volume 13, Corrosion, p. 6 defines exfoliation as follows:

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 way from the body of the material, giving it a layered appearance. It also indicates that it is synonymous to 'lamellar corrosion'. The Encyclopedia Britannica indicates that this term suggests a composition or arrangement in the form of a thin, flat layer or scale. Nothing in the Glossary restricts the term to aluminum.

Under Materials Selection, p. 334, Greg Kobrin states that exfoliation affects primarily aluminum alloys, attack proceeding laterally from initiation sites on the surface and generally proceeding intergranularly along planes parallel to surface. Don Sprowls discusses evaluation of exfoliation on pp. 242-3, particularly as regards the ASTM tests (e.g., G34, G64, G66, G85, etc,).

Under Temper Effects, p. 295, it is stated that the structural layers are pushed apart by voluminous corrosion products.

I edited NACE Manual No.1, The Forms of Corrosion Recognition and Prevention in 1977-78 with a series of authorities on Fontana's Eight Forms of Corrosion. In that version, no mention was made of exfoliation. Dale McIntyre updated this in 1982 as Volume 1 and 2. In the latter, two case histories of exfoliation of aluminum are given.

The term seems to have been largely pre-empted by the corrosionists concerned with aluminum. However, in Vol. 13, exfoliation of 80-20 and 70-30 cupronickels is reported under Closed Feedwater Heaters (pp. 989-990), the susceptibility increasing with the nickel content. Allegedly, this problem (at its worst in peaking/cycling service) is reduced by steam or nitrogen blanketing. It has also been reported in superheaters and reheaters.

In ferrous alloys, exfoliation is characterized by excessive internal growth of oxide, which has a volume some seven times that of the steel. Excessive internal growth of oxide can elevate temperature and the exfoliated material damage turbines. Exfoliation occurs in ferritic materials when multilayer growth occurs. Stresses are induced by temperature cycles and by the difference in thermal expansion between the scale and tube. Exfoliation can also occur in austenitic stainless steels, again because of the difference in thermal expansion between the metal and the oxide.

Apparently, we must use the broader term, ‘lamellar corrosion,’ if we concede the much wider occurrence of exfoliation in the light metals. Personally, I don't care but then the Glossary should be rewritten in that wonderful new 2nd edition of Volume 13!"


Pack Rust

Pack rust is a form a localized corrosion typical of steel components that develop a crevice into an open atmospheric environment. This expression is often used in relation to bridge inspection to describe built-up members of steel bridges that are showing signs of rust packing between steel plates. (Images courtesy of Wayne A. Senick, Termarust )

Termarust'
ack Rust 2'

Pitting'
Post-examination should reveal the local cathode, since it will remain impervious to the corrosion attack.

Pitting Corrosion

Pitting corrosion is a localized form of corrosion by which cavities or "holes" are produced in the material. Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against. Corrosion products often cover the pits. A small, narrow pit with minimal overall metal loss can lead to the failure of an entire engineering system. Pitting corrosion, which, for example, is almost a common denominator of all types of localized corrosion attack, may assume different shapes. Pitting corrosion can produce pits with their mouth open (uncovered) or covered with a semi-permeable membrane of corrosion products. Pits can be either hemispherical or cup-shaped. 

Pitting is initiated by:

  1. Localized chemical or mechanical damage to the protective oxide film; water chemistry factors which can cause breakdown of a passive film are acidity, low dissolved oxygen concentrations (which tend to render a protective oxide film less stable) and high concentrations of chloride (as in seawater)
  2. Localized damage to, or poor application of, a protective coating
  3. The presence of non-uniformities in the metal structure of the component, e.g. nonmetallic inclusions

Theoretically, a local cell that leads to the initiation of a pit can be caused by an abnormal anodic site surrounded by normal surface which acts as a cathode, or by the presence of an abnormal cathodic site surrounded by a normal surface in which a pit will have disappeared due to corrosion.

In the second case, post-examination should reveal the local cathode, since it will remain impervious to the corrosion attack as in the picture of an aluminum specimen shown in the image. Most cases of pitting are believed to be caused by local cathodic sites in an otherwise normal surface.

Apart from the localized loss of thickness, corrosion pits can also be harmful by acting as stress risers. Fatigue and stress corrosion cracking may initiate at the base of corrosion pits. One pit in a large system can be enough to produce the catastrophic failure of that system. An extreme example of such catastrophic failure happened recently in Mexico, where a single pit in a gasoline line running over a sewer line was enough to create great havoc to a city, killing 215 people in Guadalajara.

Some definitions:

  • Pitting: Corrosion of a metal surface, confined to a point or small area, that takes the form of cavities. 
  • Pitting Factor: Ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss. 
  • Pitting Resistance Equivalent Number (PREN): an empirical relationship to predict the pitting resistance of Naustenitic and duplex stainless steels. It is expressed as PREN = Cr + 3.3 (Mo + 0.5 W) + 16N.

Types of pitting corrosion:

Trough Pits:

Narrow, deepNarrow, deep
Shallow, wideShallow, wide
EllipticalElliptical
Vertical grain attackVertical grain attack

 


Sideway Pits:

SubsurfaceSubsurface
UndercuttingUndercutting
Horizontal grain attackHorizontal grain attack

Intergrain scc'
Intergranular SCC of an Inconel heat exchanger tube with the crack following the grain boundaries.

Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. The problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity.

Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific alloy-environment-stress combinations.

Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. The problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity.

Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific alloy-environment-stress combinations.

stress cracking'
SCC in a 316 stainless steel chemical processing piping system.

Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Macroscopically, SCC fractures have a brittle appearance. SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the damage not easily predicted. Experimental SCC data is notorious for a wide range of scatter. A disastrous failure may occur unexpectedly, with minimal overall material loss.

The first micrograph (X500) illustrates intergranular SCC of an Inconel heat exchanger tube with the crack following the grain boundaries.

The second micrograph (X300) illustrates SCC in a 316 stainless steel chemical processing piping system. Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern. (Both images courtesy of Metallurgical Technologies, Inc.

The catastrophic nature of this severe form of corrosion attack has been repeatedly illustrated in many news-worthy failures, including the swimming pool roof collapse in Uster, Switzerland and the Boeing 747 crash in Amsterdam.

One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.

Despite the extensive qualification of Inconel for specific applications, a number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620oC to 705oC, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack.


Uniform Corrosion

Uniform Corrosion

Uniform corrosion is characterized by corrosive attack roceeding evenly over the entire surface area, or a large fraction of the total area. General thinning takes place until failure. On the basis of tonnage wasted, this is the most important form of corrosion.

However, uniform corrosion is relatively easily measured and predicted, making disastrous failures relatively rare. In many cases, it is objectionable only from an appearance standpoint. As corrosion occurs uniformly over the entire surface of the metal component, it can be practically led control by cathodic protection, use of coatings or paints, or simply by specifying a corrosion allowance. In other cases, uniform corrosion adds color and appeal to to a surface. Two classics in this respect are the patina created by naturally tarnishing copper roofs and the rust hues produced on weathering steels.

The breakdown of protective coating systems on structures often leads to this form of corrosion. Dulling of a bright or polished surface, etching by acid cleaners, or oxidation (discoloration) of steel are examples of surface corrosion. Corrosion ant resist alloys and stainless steels can become tarnished or oxidized in corrosive environments. Surface corrosion can indicate a breakdown in the protective coating system, however, and should be examined closely for more advanced attack. If surface corrosion is permitted to continue, the surface may become rough and surface corrosion can lead to more serious types of corrosion.