Forms of corrosion

2.1 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. Uniform attack is the most common form of corrosion. It is normally characterized by a chemical or electrochemical reaction which proceeds uniformly over the entire exposed surface or over a large area. The metal becomes thinner and eventually fails.

Fig (2-1) , uniform corrosion

Cast irons and steels corrode uniformly when exposed to open atmospheres, soils and natural waters, leading to the rusty appearance. The fig (2-1) 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 galvanize but the surface of the bolt below was completely rusted (just like the nut).

2.1.1 Mechanisms

The anodic reaction in the corrosion process is always the oxidation reaction:

M = M⁺ + e^- eq (2-1)

In acidic environments, i.e., pH < 7,  the cathodic process is mainly the reduction of hydrogen ions:

2H⁺ + 2e = H₂ eq (2-2)

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^- eq (2-3)

Fig (2-2) Real uniform corrosion [6]

With uniform distribution of cathodic reactants over the entire exposed metal surface, reactions (2-2) and/or (2-3) 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.

2.1.2 Prevention or Remedial Action

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 (SA/ICCP) and Anodic Protection
• selection of a more corrosion resistant alloy (i.e. higher alloy content or more inert alloy)
• utilize coatings to act as a barrier between metal and environment.
• modify the environment or add chemical inhibitors to reduce corrosion rate.
• replace with corrosion resistant non-metallic material.

2.2 Galvanic

Accelerated corrosion which can occur when dissimilar metals are in electrical contact in the presence of an electrolyte (i.e. conductive solution). An example of this corrosion phenomenon is increased rate of corrosion of steel in seawater when in contact with copper alloys. Galvanic attack can be uniform in nature or localized at the junction between the alloys depending on service conditions. Galvanic corrosion can be particularly severe under conditions where protective corrosion films do not form or where they are removed by conditions of erosion corrosion.

Fig (2-3) Galvanic corrosion [6]

_* three conditions must be present:

1. Electrochemically dissimilar metals must be present;

2. These metals must be in electrical contact; and

3. The metals must be exposed to an electrolyte.

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.

2.2.1 Mechanism

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.

Fig (2-4) Real example of Galvanic corrosion [6]

2.2.2 Important factors in galvanic corrosion

– relative areas of anode and cathode

– difference in potential between anode and cathode

– effect of anodic polarization on anode (some may passivate)

e.g .

1: Cu-containing precipitates in aluminum alloys initiate pitting corrosion

2: Fe and Cu impurities in commercial zinc cause a large increase  corrosion rate compared to pure zinc

2.2.3 Prevention or Remedial Action

• selection of alloys which are similar in electrochemical behavior and/or alloy content.
• area ratio of more actively corroding material (anode) should be large relative to the more inert material(cathode).
• use coatings to limit cathode area.
• insulate dissimilar metals.
• use of effective inhibitor.
• 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.

2.3 crevice

Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level This form of attack is generally associated with the presence of small volumes of stagnant solution in occluded interstices, beneath deposits and seals, or in crevices, e.g. at nuts and rivet heads. Deposits of sand, dust, scale and corrosion products can all create zones where the liquid can only be renewed with great difficulty. This is also the case for flexible, porous or fibrous seals (wood, plastic, rubber, cements, asbestos, cloth,etc.).
Crevice corrosion is encountered particularly in metals and alloys which owe their resistance to the stability of a passive film, since these films are unstable in the presence of high concentrations of Cl^- and H⁺ ions.

Fig (2-5) Crevice corrosion [6]

The basic mechanism underlying crevice corrosion in passivatable alloys exposed to aerated chloride-rich media is gradual acidification of the solution inside the crevice, leading to the appearance of highly aggressive local Conditions that destroy the passivity. .
in an interstice, convection in the liquid is strongly impeded and the dissolved oxygen is locally rapidly exhausted. A few seconds are sufficient to create a “differential aeration cell” between the small deaerated interstice and the aerated remainder of the surface. However, “galvanic” corrosion between these two zones remains inactive. .
As dissolution of the metal M continues, an excess of Mn⁺ ions is created in the crevice, which can only be compensated by electro migration of the Cl^- ions (more numerous in a chloride-rich medium and more mobile than OH^- ions). Most metallic chlorides hydrolyses, and this is particularly true for the elements in stainless steels and aluminum alloys. The acidity in the crevice increases (pH 1-3) as well as the Cl^- ion concentration (up to several times the mean value in the solution). The dissolution reaction in the crevice is then promoted and the oxygen reduction reaction becomes localized on the external surfaces close to the crevice. This “autocatalytic” process accelerates rapidly, even if several days or weeks were necessary to get it under way.

_* Crevice corrosion is initiated by changes in local chemistry within the crevice:

a. Depletion of inhibitor in the crevice

b. Depletion of oxygen in the crevice

c. A shift to acid conditions in the crevice

d. Build-up of aggressive ion species (e.g. chloride) in the crevice

2.3.1 MECHANISM

. Autocatalytic process are three stage :

2.3.1.1 Stage one of a crevice formation ( Induction )

At time zero, the oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere

Fig (2-6) Stage one of a crevice formation

2.3.1.2 Stage two of a crevice formation (Restricted Convection)

Because of the difficult access caused by the crevice geometry, oxygen consumed by normal uniform corrosion is very soon depleted in the crevice. The corrosion reactions now specialize in the crevice (anodic) and on the open surface (cathodic)

Fig (2-7) Stage two of a crevice formation (Restricted Convection)

2.3.1.3Stage three of a crevice formation(Obstruction and Electromigration)

In stage three of the crevice development a few more accelerating factors fully develop:

The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products. The pH in a crevice can reach very acidic values, sometimes equivalent to pure acids.

The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. See, for example, how the corrosion of steel is affected as a function of water pH.

Fig (2-8) Stage three of a crevice formation(Obstruction and Electromigration)

The corrosion products seal even further the crevice environment.

The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in the environment, such as chlorides and sulfates, that can be corrosive in their own right.

2.3.2 Prevention

Crevice corrosion can be designed out of the system

• Use welded butt joints instead of riveted or bolted joints in new equipment
• Eliminate crevices in existing lap joints by continuous welding or soldering
• Use solid, non-absorbent gaskets such as Teflon.
• Use higher alloys for increased resistance to crevice corrosion
• design installations to enable complete draining (no corners or stagnant zones)

2.4 Pitting

Pitting: Pitting Corrosion is the localized corrosion of a metal surface confined to a point or small area, that takes the form of cavities. Pitting is one of the most damaging forms of corrosion

Pitting factor: ratio of the depth of the deepest pit resulting from corrosion divided by the average penetration as calculated from weight loss.

Fig(2-9) Morphology of pitting

Pitting corrosion forms on passive metals and alloys like stainless steel 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.

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).

2.4.1 Characteristics of Pitting Corrosion

1- the alloy is passive

– pitting requires a passive external surface that can provide a high potential to cause the current to flow into the pit; if the external surface is active, this driving force is not available

– thus carbon steel will only pit if the solution tends to passivate it (e.g. alkaline solutions), it won’t pit if it is corroding generally (e.g. neutral salt solutions)

2-passivity broken down locally, usually by chloride

– the cause of the initiation of pitting corrosion is still not entirely clear, but it involves a very small pit nucleus that grows over periods of the order of seconds

3-pits become more stable as they become larger

– for very small pits the acidity will be neutralized by diffusion into the bulk solution very easily as the pits get larger the diffusion distances increase, and it gets harder for the acidity to diffuse away

4- small pits are often stabilized by a film of oxide or metal that partially

covers the entrance

– this allows pits that would not otherwise be stable to continue growing (these are known as meta stable pits)

5- pitting becomes more likely as the potential becomes more positive

– this provides a greater driving force for the corrosion process, and helps to stabilize themetastable pits

2.4.2 Mechanisms

For a homogeneous environment, pitting IS caused by the MATERIAL that may contain inclusions (MnS to pit initiation ) .

Fig (2-10) Real pitting corrosion [6]

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

2.4.3 Prevention or Remedial Action

* Pitting corrosion can be prevented through:

• Proper selection of materials with known resistance to the service environment
• Control pH, chloride concentration and temperature
• Cathodic protection and/or Anodic Protection
• increase velocity of media and/or remove deposits of solids from exposed metal surface
• use of effective chemical inhibitor to enhance resistance to localized attack.
• Deaeration of aerated environments to reduce localized corrosion through elimination of oxygen concentration cell mechanism.

2.5 Stress-corrosion cracking (SCC)

Stress-corrosion cracking (SCC) is a cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Stress-corrosion cracking may occur in combination with hydrogen embrittlement.

Fig(2-11) stress corrosion cracking [6]

The image of stress corrosion I see Is that of a huge unwanted tree Against whose trunk we chop and chop, But which outgrows the chips that drop ;

And from each gash made in its bark A new branch grows to make more dark The shade of ignorance around its base, Where scientists toil with puzzled face.

2.5.1 Mechanisms

Stress corrosion cracking results from the conjoint action of three components:

(1) a susceptible material;

(2) a specific chemical species (environment) and

(3) tensile stress.

For example, copper and its alloys are susceptible to ammonia compounds, mild steels are susceptible to alkalis and stainless steels are susceptible to chlorides.

There is no unified mechanism for stress corrosion cracking in the literature. Various models have been proposed which include the following:

· Adsorption model: specific chemical species adsorbs on the crack surface and lowers the fracture stress.

· Film rupture model: stress ruptures the passive film locally and sets up an active-passive cell. Newly formed passive film is ruptured again under stress and the cycle continues until failure.

· Pre-existing active path model : Pre-existing path such as grain boundaries where intermetallics and compounds are formed.

· Embrittlement model: Hydrogen embrittlement is a major mechanism of SCC for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.

2.5.2 Prevention

Stress corrosion cracking can be prevented through :

• Control of stress level (residual or load) and hardness.
• Avoid the chemical species that causes SCC.
• Use of materials known not to crack in the specified environment.
• Control temperature and or potential

2.6 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 “interranular stress corrosion cracking (IGSCC)” or simply “intergranular corrosion cracking“.

Fig (2-12) intergranular corrosion[6]

“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 bourdaries). 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” structure and is susceptible to intergranular corrosion or intergranular stress corrosion cracking.

2.6.1 Mechanisms

This type of attack results from local differences in composition, such as coring commonly encountered in alloy castings. Grain boundary precipitation, notably chromium carbides in stainless steels, is a well recognized and accepted mechanism of intergranular corrosion. The precipitation of chromium carbides consumed the 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.

Fig (2-13) intergranular corrosion[6]

Intermetallics segregation at grain boundaries in aluminum alloys also causes intergranular corrosion but with a different name - “exfoliation”.

2.6.2 Prevention

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.

2.7 selective leaching

*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 decarburization, decobaltification , denickelification, dezincification, and graphitic corrosion or graphitization.

*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 selective leaching of nickel from nickel-containing alloys. Most commonly observed in copper-nickel alloys after extended service in fresh water.

*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.

*Graphitic corrosion is the deterioration of gray cast iron in which the metallic constituents are selectively leached or converted to corrosion products leaving the graphite intact. It is sometimes also referred to as graphitization.

A common example is the dezincification of unstabilized brass, whereby a weakened, porous copper structure is produced. as shown in Fig(2-14)

Fig (2-14) forms of dezincification [6]

he 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. Graphitization of gray cast iron, whereby a brittle graphite skeleton remains following preferential iron dissolution is a further example of selective leaching.

2.7.1 Mechanisms

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 is the driving force for the preferential attack on the more “active” element in the alloy.

Fig (2-15) dezincification [6]

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

2.7.2 Prevention

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 that alpha brass, ductile iron is more resistant to graphitic corrosion than gray cast iron.
• Control the environment to minimize the selective leaching

2.8 Erosion Corrosion

Erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative motion of the environment and the metal surface.as shown in Fig(2-16) .It is characterized by surface features with a directional pattern which are a direct result of the flowing media. Erosion corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys).

Fig(2-16) Erosion corrosion [6]

Alloys which form a surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly accelerates. Other factors such as turbulence, cavitation, impingement or galvanic effects can add to the severity of attack.

2.8.1 Mechanism

There are several mechanisms described by the conjoint action of flow and corrosion that result in flow-influenced corrosion:

Mass transport-control: Mass transport-controlled corrosion implies that the rate of corrosion is dependent on the convective mass transfer processes at the metal/fluid interface. When steel is exposed to oxygenated water, the initial corrosion rate will be closely related to the convective flux of dissolved oxygen towards the surface, and later by the oxygen diffusion through the iron oxide layer. Corrosion by mass transport will often be streamlined and smooth.

Fig (2-17) Real of erosion corrosion [6]

Phase transport-control: Phase transport-controlled corrosion suggests that the wetting of the metal surface by a corrosive phase is flow dependent. This may occur because one liquid phase separates from another or because a second phase forms from a liquid. An example of the second mechanism is the formation of discrete bubbles or a vapor phase from boiler water in horizontal or inclined tubes in high heat-flux areas under low flow conditions. The corroded sites will frequently display rough, irregular surfaces and be coated with or contain thick, porous corrosion deposits.

Erosion-corrosion: Erosion-corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical processes. It is often accepted that a critical fluid velocity must be exceeded for a given material. The mechanical damage by the impacting fluid imposes disruptive shear stresses or pressure variations on the material surface and/or the protective surface film. Erosion-corrosion may be enhanced by particles (solids or gas bubbles) and impacted by multi-phase flows. The morphology of surfaces affected by erosion-corrosion may be in the form of shallow pits or horseshoes or other local phenomena related to the flow direction.

Cavitation : Cavitation sometimes is considered a special case of erosion-corrosion and is caused by the formation and collapse of vapor bubbles in a liquid near a metal surface. Cavitation removes protective surface scales by the implosion of gas bubbles in a fluid. Calculations have shown that the implosions produce shock waves with pressures approaching 60 ksi. The subsequent corrosion attack is the result of hydro-mechanical effects from liquids in regions of low pressure where flow velocity changes, disruptions, or alterations in flow direction have occurred. Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on the surface

2.8.2 Prevention or Remedial Action

• selection of alloys with greater corrosion resistance and/or higher strength.
• re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the environment.
• reduction in the corrosive severity of the environment.
• use of corrosion resistant and/or abrasion resistant coatings.
• cathodic protection.