types of corrosion- hani aziz ameen

8/6/2019 Types of Corrosion- Hani Aziz Ameen

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Dr. Hani Aziz Ameen Types of Corrosion

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Types of Corrosion

Asst. Prof. Dr. Hani Aziz Ameen

Technical College - BaghdadDies and Tools Engineering Department

E-mail:[email protected]

1. Atmospheric Corrosion

Atmospheric corrosion is defined as the corrosion or degradation of

material exposed to the air and its pollutants rather than immersed in a liquid.

This has been identified as one of the oldest forms of corrosion and has been

reported to account for more failures in terms of cost and tonnage than any

other single environment. Many researchers classify atmospheric corrosion

under categories of dry, damp, and wet, thus emphasizing the different

mechanisms of attack under increasing humidity or moisture.

Corrosively of the atmosphere to metals varies greatly from one

geographic location to another, depending on such weather factors as wind

direction, precipitation and temperature changes, amount and type of urban

and industrial pollutants and proximity to natural bodies of water. Service life

may also be affected by the design of the structure if weather conditions cause

repeated moisture condensation in unsealed crevices or in channels with no

provision for drainage. [1]

2- Uniform Corrosion

Uniform corrosion 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.[2]
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With aluminum, this type of corrosion is observed especially in highly

acidic or alkaline media, in which the solubility of the natural oxide film is

high .

The dissolution rate of the film is greater than its rate of formation;

however, the ratio of both rates can change over time.[3]

General corrosion or uniform corrosion occurs in the solutions where

pH is either very high or very low or at high potentials in electrolytes with

high chloride concentrations. In acidic (low pH) or alkaline (high pH)

solutions, the aluminum oxide is unstable and thus nonprotective. [1]

3- Galvanic Corrosion

Economically, galvanic corrosion creates the largest number of

corrosion problems for aluminum alloys. Galvanic corrosion, is also known as

a dissimilar metal corrosion, occurs when aluminum is electrically connected

to a more noble metal, and both are in contact with the same electrolyte.[4]

Fig.(1) Galvanic Reaction [5]

When two dissimilar metals are in contact, the corrosion rate of the

more active metal(more negative Ecorr) is accelerated, while the corrosion

rate of the noble metal(less negative Ecorr) is reduced.

The higher the difference in Ecorr, the more severe is galvanic

corrosion.


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Galvanic Series: a list of measured corrosion potentials (Ecorr) of a

number of metals and alloys in a given electrolyte.

Prevention of Galvanic Corrosion eliminate electrical contact b/w dissimilar

metals (use gaskets, washers, o-rings, ext)

If electrical contact can not be avoided it is preferable to:

select dissimilar metals that are close in the galvanic series

design for a small Ac/Aaarea ratio

give thickness allowance for the more active metal

coat the cathode to reduce Ac/Aa [2]

4- Crevice Corrosion

Crevice corrosion requires the presence of a crevice a salt water

environment oxygen (Fig.( 2)). The crevice can result from the over lap of

two parts or a gap between a bolt and a structure . When aluminum is wetted

with the salt water and water enters the crevice, little happens initially. Over

time inside the crevice oxygen is consumed due the dissolution and

precipitation of aluminum.

Fig. (2): Crevice Corrosion[1]

Crevice corrosion can occur in a saltwater environment if the crevice

become deaerated and the oxygen reduction reaction outside of the crevice


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mouth under these conditions. The crevice becomes more acidic and

corrosion occurs at an increasing rate.[1]

Crevice corrosion initiated by changes in local chemistry within the

crevice:

a. depletion of inhibitor in the crevice isb. depletion of oxygen in the crevicec. a shift to acid conditions in the crevice d. build-up of aggressive ion species (e.g. chloride) in the crevice[6]

5- Intergranular corrosion

Intergranular (intercystalline) corrosion is selective of grain boundaries

or closely adjacent reaction without appreciable attack of the grains

themselves. Intergranular corrosion is a generic term that includes several

variations associated with different metallic structures and thermomechanical

treatment intergranular corrosion is caused by potential differences between

the grain boundary region and the adjacent grain bodies.Salt water exposure can cause intergranular corrosion (IGC) in some

aluminum alloys. Dix explained IGC of Al-copper (Cu) alloys in 1940.

During aging at elevated temperatures (200C), precipitation of discrete

particles occurs, with more advanced precipitation at the grain boundaries

than in the grain matrix. The grain boundaries are surrounded by narrow

zones of Al that etch smoothly. These zones become more pure, with a more

active corrosion potential (solution potential) in aerated salt water [4].


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Fig. (3) Intergranular Corrosion In A Recrystallized Wrought Structure.[7]

In some alloy systems, IGC is a result of galvanic corrosion between

anodic grain- boundary precipitates and the depleted zones, rather than

between the matrix and the depleted zone.

In 2xxx series alloys, it is a narrow band on either side of the boundary

that is depleted in copper, in 5xxx series alloys; it is the anodic constituent

Mg2Al3 when that constituent forms a continuous path along a grain boundary

in copper free 7xxx series alloys. It is generally considered to be anodic zinc

and magnesium bearing constituents on the grain boundary. The 6xxx series

alloys generally resist this type of corrosion, although slight intergranular

attack has been observed in a aggressive environments [7] as shown in

Fig.(4).

Fig.(4) Interganular Corrosion [8]


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6- Exfoliation corrosion

Exfoliation is yet another 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 [9].

Fig.(5) Exfoliation Corrosion In An Aluminum Alloy[10]

Mechanisms Exfoliation is a special type of intergranular corrosion that

occurs on the elongated grain boundaries by heavy deformation during hot or

cold rolling and where no recrystallization has occurred. The corrosion

product formed 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. Exfoliation is characteristic for the 2000(Al Cu), 5000 (Al


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Mg), and 7000 (Al Zn Mg) series alloys which grain boundary

precipitation or depleted grain boundary regions.

Exfoliation corrosion (as shown in figure(2-6)) can be prevented

through:

the use of coatings selecting a more exfoliation resistant aluminum alloy using heat treatment to control precipitate distribution[ 9]

Fig. (6) Exfoliation of AlAlloys[11]

7- Erosion Corrosion

Erosion Corrosion of aluminum occurs in high velocity water and is

similar to jet impingement corrosion. Erosion Corrosion of aluminum is

very slow in pure water, but is accelerated at pH > 9, especially with high

carbonate and high silica content of the water. Aluminum is very stable in

neutral water; however it will corrode in either acidic or alkaline waters.


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To prevent erosion corrosion, one may change the water chemistry or

reduce the velocity of the water, or for the water chemistry, the pH must be

below 9, and the carbonate and the silica levels must be reduced [1].

8- Stress Corrosion Cracking (SCC)

Stress corrosion cracking (SCC) (as shown in Fig.(7)) is the bane of

aluminum alloys. SCC requires three simultaneous conditions, first a

susceptible alloy, second a humid or water environment, and third a tensile

stress which will open the crock and enable crack propagation. SCC can occur

in two modes intergranular stress corrosion cracking (lGSCC) which is the

more common form, or transgranular SCC (TGSCC). In TGSCC, the crack

follows the grain boundaries. In transgranular stress corrosion cracking

(TGSCC), the cracks cut through the grains and are oblivion to the grain

boundaries.

The general trend to use higher strength alloys peaked in 1950 with

alloy 7178T651 used on the Boeing 707, then the industry charged to using

lower strength alloys. The yield strength of the upper wing skin did not

exceed the 1950 level until the Boeing 777 in the 1990s. The reason behind

selecting the lower strength alloys for the Boeing 747 and the L.1011 was the

aircraft designers chose an alloy with better SCC resistance rather than the

higher yield strength .[1]

Fig.(7 ) Crack Initiation From The Pit Root At Weld Pool [12]


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9- Corrosion Fatigue

Corrosion fatigue (as shown in Fig.(8)) can occur when an aluminum

structure is repeatedly stressed at low stress levels in corrosive environment.

A fatigue crack can initiate and propagate under the influence of the crack-

opening stress and the environment. Similar striations may sometimes be

found on corrosion fatigued samples , but often the subsequent crevice

corrosion in the narrow fatigue crack dissolves them Fatigue strengths of

aluminum alloys are lower in such corrosive environments as seawater and

other salt solutions than in air , especially when evaluated by low stress long

duration tests . Like SCC of aluminum alloys, corrosion fatigue requires the

presence of water. In contrast to SCC, however, Corrosion fatigue is not

appreciably affected by test direction, because the fracture that results from

this type at attack is predominantly Transgranular. [4]

(a) (b)Fig.(8) (A) Corrosion Fatigue Curve In Different Environments

(B) Appearance Of Fatigue Crack

10- Filiform Corrosion

Filifrom corrosion (as shown in Fig.(9)) (also known as worm track

corrosion) is a cosmetic problem for painted aluminum. Pinholes or defects

in the paint from scratches or stone bruises can be the initiation site where


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corrosion begins with salt water pitting. The mechanism of filiform corrosion

(as shown in Fig.(10)) requires chlorides for initiation and both high humidity

and chlorides for the propagation of the track.

The propagation depends on where and how the alloy is used. The

filament must be initiated by Chlorides, and then it proceeds by a mechanism

similar to crevice corrosion. The head is acidic, high in chlorides and

deaeratied and is the anodic site. Oxygen and water vapor diffuse through the

filiform tail, and drive the cathodic reaction [3].

Fig.(9) Filiform Corrosion [13]

Fig.(10) Mechanism Of Filiform Corrosion[3]


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Filiform corrosion can be prevented by sealing defects with point or

wax, and keeping the relative humidity low.

Filiform corrosion occurs with all types of paints: acrylic lacquers,

epoxy-polyamides, epoxy-amines and polyurethanes, and whatever the classic

mode of application, whether with liquid paint or electrostatic powdering. It

does not occur under sealed coatings such as electricians tape.[3]

11- Microbiological Induced Corrosion

Microbiological Induced Corrosion (MIC) applies to a corrosive

situation which is caused aggravated the biological organisms. A classic case

of MIC is the growth of fungus at the waterfuel interface in aluminum aircraft

fuel tanks. The fungus consumes the high octane fuel, and excretes an acid

which attacks and pits the aluminum fuel tank and causes leaking. The

solution for this problem is to control the fuel quality and prevent water from

entering or remaining in the fuel tanks. If fuel quality control is not feasible,

then fungicides are sometimes added to the aircraft fuel.[1]

As already identified, MIC operates as an individual nodule covering a

pit. The development of this process occurs in three phases, which are

follows:

attachment of microbes.

growth of nodule and initial pit.

mature pit and nodule.[12]

Fig.(11 ) Shows Multiple Nodules .[14]


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One nodule is broken open, showing black corrosion products inside

the nodule. Pits are found under most large nodules.[14]

The phase recognition of desirable site ( metallurgical feature desirable

to bacteria) as shown in Fig.(12 a ). Phase two- colony formation and

development of crevice corrosion as shown in Fig.(12 b ) and phase three

nodule as formed over nature pit as shown in Fig.(12 c ).

(a) (b) (c)

Fig.(12) Microbiological Induced Corrosion

12- Pitting Corrosion of Aluminum Alloys

Pitting corrosion (as shown in Fig(13)) is defined as localized

accelerated dissolution of metals that occurs as a result of a breakdown of the

protective passive film on the metal/ alloy surface. In an aggressive

environment, typically containing halide ions, pits initiate and grow in an

autocatalytic manner, where the local environment within the pits becomes

more aggressive because of decrease in pH and increase in chloride

concentration, which further accelerates. The pit growth usually takes a

variety of shapes [15].


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Fig.(13) Mechanism of Pitting Corrosion of Aluminum [3]

Pit shapes can be simply divided into isotropic and anisotropic groups.

Shapes are isotropic, while those in Fig.(14), are isotropic and are called

microstructural orientated pitting. The variation in pit shape could mainly

depend on the microstructure of metals or alloys such as alloy composition

and aspect ratio of grains. Even though there are some differences in pitting

corrosion between stainless steels and Al alloys, e.g., hydrogen bubbles form

at the active pit surface in Al alloys; both materials basically share a similar

mechanism.

In general, pitting corrosion involves three stages:

1) pitting initiation

2) metastable pitting

3) pitting growth.


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Fig.(14) Pit Shapes [15]

12.1 Pit Initiation

As mentioned above, aggressive anions such as chloride are believed to

cause passive film breakdown. However, the exact mechanism of the passivefilm breakdown is still unclear. A number of models have been proposed to

explain passive film breakdown or pit initiation [15].

Three main models are as shown in Fig.(15):

1) adsorption mechanism

2) penetration mechanism and

3) film breaking mechanism.

These models have been reviewed in depth in the literature [16].

The adsorption theory emphasizes the importance of adsorption of

aggressive anions like chloride ions.

A competitive adsorption of chloride ions and oxygen finally may lead

to film thinning. The penetration model emphasizes the importance of anion

penetration and ion migration through the passive film. The point defect

model addresses the transport of cationic vacancies to the metal/oxide


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during the induction time prior to the onset of stable pitting. Fig. (16) shows

typical metastable pit current transients on stainless steels, in chloride solution

under an applied anodic potential. The current increases corresponding to the

growth of metastable pit followed by a sharp current decrease due to

repassivation process. Since metastable pits experience initiation, growth, and

repassivation, a better metastable pitting. Understanding of these three stages

for the metastable pit can be gained through study of metastable pitting

phenomenon was first observed in stainless steel in the early 1970s.

Frankel and coworkers used the term of metastable pitting for the first

time. Over the past 30 years, metastable pitting has been systematically

investigated by analyzing pit current density for individual metastable pits and

stochastic approaches to groups of metastable pits.

These detailed studies show that the early development of metastable

pits appears to be identical to that of metastable pits, and the probability of

metastable pitting is directly correlated to the intensity of metastable pitting

events. Metastable pits repassivate probably when the porous cover ruptures

and the pit electrolyte is diluted. In contrast to a huge amount of studies on

corrosion of stainless steels, literature on corrosion of Al or Al alloys is

limited. Pride et al. studied metastable pitting on pure Al. They found that the

number of metastable pits and the current.

Spikes increase with increasing applied potential below pitting

potential and the chloride concentration. A critical transition from metastable

pitting to stable pitting in Al has been found in their study.


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Fig.(16) Metastable Pit Transients Observed [17]

12.3 Pit Growth[16]

Above the pitting potential, stable pits grow at a rate depending on

alloy composition, local pit environment and pit bottom potential. Due to the

autocatalytic nature of pitting corrosion, the local pit environment and bottom

potential is severe enough to prevent repassivation. Pit growth can be

controlled by each or combinations of three factors mainly chargetransfer,

ohmic and mass transport. For a hemispherical pit, different rate controlling

factors would lead to specific relationships between current I, current density

i, pit radius or depth r, time t, and potential E.

. Under charge transfer control, Tafels law describes ( Ei exp ) .

. Under ohmic control, it can be derived rI and rIrIi // 2 . From

Faradays law,


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dtdri / , leading to 2/1tr and thus 2/1tI and 2/1ti . Ohms law

determines Ei .

. Under mass transport control, according to Ficks laws, rIi / , thus

2/1ti

. i is E independent.The similar i-t relationship for ohmic control and mass transport control

makes it difficult to distinguish

For a 3D sample, the non-steady state nature of pit deepening and the

problem with accurate measurements of pit current density complicate the

clear identification of the i-E relationship. In a conventional measurement of

i-E relationship, current may come from several pits with unknown activesurface areas and presumably is evenly distributed on the pits. However, the

assumption of even distribution is not possible since different pits initiated at

different potentials grow at different rates. Artificial pit electrodes, formed by

imbedding a wire in epoxy have been extensively used to study iron and

stainless steel behavior. The artificial pit electrode geometry forms a single pit

in which the whole electrode area is active, generates a natural pit

environment, and provides an ideal one-dimensional transport condition. For

Al and Al-alloys, similar to artificial pit electrodes, artificial crevice

electrodes have been used since large crevice area facilitates the escape of H2

bubbles. The results indicate that pits can grow either in the active state

without salt film precipitation or in a salt-film-covered state. The active state

is dominated by ohmic control while a salt-film-covered state is dominated by

mass transport control. Other single pit techniques include the exposure of

small area, laser irradiation of a small spot, and implantation of an activating

species at a small spot. These studies suggested different viewpoints of either

ohmic control or mass transport control. Besides the electrochemical methods,

non-electrochemical techniques have been also used. Hunkeler and Bohni

measured the time for pit to penetrate Al foils of varying thickness to

determine the pit growth rate. They found that at fixed applied potential, pit


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depth and current density i were time dependent: 2/1td and 2/1ti . Pit

growth on Al was ohmic controlled since the growth rate was correlated to the

conductivity of the electrolyte. Detailed studies of 2D pit in Al and other

types of thin films by Frankel and coworkers found that the high current

density increased linearly with potential and reached a limiting value at higher

potentials (Fig.(17)). Therefore, the pit growth at the beginning is controlled

by ohmic control and after some time controlled by the masstransport.[16]

Fig.(17) Anodic And Net Current Densities Change As A Function Of

Potential For 100 Nm Al Film In 0.1M Nacl Solution [16]

12.4 Pitting Stability

Local pit environment and chemistry are believed to be very important

for pit growth and repassivation. Among the various species present within

pits such as metal cations, metal hydroxide, Cl- and H+, acidification within

pits as a result of hydrolysis is generally recognized to be a critical factor.

Galvele calculated the acidification in 1D pit, based on metal dissolution,

hydrolysis, and mass transport. He found that a critical value of the product

x.i (x is pit depth and i is current density), was the critical acidification within

pits to sustain pit growth Fig.(18). This critical product can be used to explain


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the pitting potential and repassivation potential, and determine the current

density required to initiate pitting and to sustain pit growth at a defect of a

given size in passive film such as crack. Although, for some metals, other

factors like chloride concentration are more important than acidification. Thus

the critical value x.i (sometimes Ipit/rpit used) can be used as a criterion for

pitting stability.

Williams et al. correlated pit stabilization with metastable pitting. They

suggested that Ipit/rpit for metastable pits formed on steels must exceed

4 10-2

A/cm2

for stable growth.

At a higher current density during pit growth, a salt film may form on

the pit surface due to saturation of ionic species. For Al pits in chloride

solution, this salt film was considered to be aluminum chloride AlCl3 or

aluminum oxy-chlorides such as Al (OH) 2Cl and Al (OH) Cl2 according to

measured pH and possible hydrolysis processes. Upon salt film precipitation,

as described above, the pit growth is under mass transport control. A salt film

can enhance pitting stability by acting as buffer of ionic species that can

dissolve into pit to sustain a severe condition in the pit environment such as

high acid concentration.

The potential distribution in pits is considered to be another important

factor to stabilize pit growth. When the IR drop is less than a critical value, pit

growth stops due to repassivation, if the alloy undergoes active/passive

transition in the pit environment .

In fact, all of the factors above might be generalized to pit growth

current density, since a pit must maintain a minimum current density for

stabilized growth However, the critical pit current density and the effect of

environment factors need to be investigated further.[16]


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Fig.(18) Concentration of Al3+

, Al(OH)2+

, and H+

As A Function of

The Product of The Depth X and The Current Density In A Unidirectional Pit[16]

References

[1] Corrosion of Aluminum and its Alloys: form of corrosion , key to

Metals Nonferrous ,2008.

[2] FONTANA M. and GREENE N D. CORROSION ENGINEERING

2nd MC Graw- Hill,1978.

[3] Michel Jacques Corrosion of aluminum Member of the commission of

Experts with in International Chamber of commerce .Paris. France .2004

[4] Rollason E.C. Metallurgy for Engineering Edward Arnold Publishers,

4th Edition, 1973.

[5] Williams DE, stewartds and Balk will P, corrosion, science, 36,

p.1213,1994.

[6] Pierre R. ROBERGE, P Crevice corrosionModel six of 28lcorrosion:

Impact, Principles, and Practice-PHD, P Eng. 1999-2009

[7] Dr. Zuhair M. GasemME 472-062- Corrosion Engineering Uniformed


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and Galvanic Corrosion ME DEPT KFUPM Dhahran, Saudi

Arabia , 2008.

[8] T.D.Burleigh, E. Ludwiczak and R.A.PetriIntergranular corrosion Of an

AluminumMagnesium-Silicon copper alloy corrosion science vol51,NO.

1 ,January 1995.

[9] Randy K. K and Kent and Roy Baggerly Corrosion Related Failures

Intergranular corrosion AE Engineers, Inc.p777.,1998.

[10] Different Types of CorrosionRecognition, Mechanisms &Prevention

Intergranular Corrosion: Exfoliation Corrosion ,2009.

[11] http: II Kogas - Corrosion needs Microscopic study.,2003

[12] Abhay k. Tha; G Naga shiresha, k stress corrosion cracking in

aluminum alloy. AFNOR 7020T6 water tank adaptor for liwuid propulsion

system organization trivandrum G95 .022 India may 2007.

[13] Corrosion AT SEA A&M ENVIRONMENTAL TECHNOTESBOEING

Volume 5 Number 1,February, 2000.

[14] Roland J. Huggins, P.E. Article Microbiology Influenced corrosion

What its and how works,2000.

[15] Chemical & Process TechnologyPitting corrosion Mechanism &

Prevention, August 2007.

[16] K. Srinivasa Rao Pitting corrosion of heat treatable Aluminum alloys

and weld: Trans. India in St. Met, vol 57. No. 6, pp (503-610)., December

2004.

[17] Ch .9 pitting corrosion http:// Corrosion .Kaist .ac.Kr.,2001

types of corrosion- hani aziz ameen

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