Corrosion

Prevention and

Control

Corrosion Prevention and Control

Significance and Purpose

Electrochemical Nature of Aqueous Corrosion

Corrosion Rate Determinates

Galvanic and Concentration Cell Corrosion

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Corrosion

• Corrosion can be defined as the degradation of a material due to a reaction with its environment.

• Degradation implies deterioration of physical properties of the material. This can be a weakening of the material due to a loss of cross-sectional area, it can be the shattering of a metal due to hydrogen embrittlement, or it can be the cracking of a polymer due to sunlight exposure.

• Materials can be metals, polymers (plastics, rubbers, etc.), ceramics (concrete, brick, etc.) or composites-mechanical mixtures of two or more materials with different properties.

• Most corrosion of metals is electrochemical in nature.

Significance and Purpose

• With a knowledge of the types of and an

understanding of the mechanisms and causes of

corrosion and degradation, it is possible to take

measures to prevent them from occurring.

• For example:

1. we may change the nature of the

environment,

2. select a material that is relatively

nonreactive,

3. protect the material from appreciable

deterioration.

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Electrochemical Nature of Aqueous Corrosion

For metallic materials, the corrosion process is normally

electrochemical, that is, a chemical reaction in which there is

transfer of electrons from one chemical species to another.

Metal atoms characteristically lose or give up electrons in

what is called an oxidation reaction. For example, the

hypothetical metal M that has a valence of n (or n valence

electrons) may experience oxidation according to the reaction

M Mn+ + ne−

in which M becomes an n positively charged ion and in the

process loses its n valence electrons; is used to symbolize an

electron. Examples in which metals oxidize are

Fe Fe2+ + 2e−

Al Al3+ + 3e−

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The site at which oxidation takes place is called the anode; oxidation is sometimes called an anodic reaction. The electrons generated from each metal atom that is oxidized must be transferred to and become a part of another chemical species in what is termed a reduction reaction. For example, some metals undergo corrosion in acid solutions, which have a high concentration of hydrogen (H) ions; the H ions are reduced as follows:

2H+ + 2e− H2

and hydrogen gas (H2) is evolved. Other reduction reactions are possible, depending on the nature of the solution to which the metal is exposed. For an acid solution having dissolved oxygen, reduction according to

O2 + 4H+ + 4e− 2H2O

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will probably occur. Or, for a neutral or basic aqueous

solution in which oxygen is also dissolved,

O2 + 2H2O + 4e− 4(OH −)

Any metal ions present in the solution may also be

reduced; for ions that can exist in more than one valence

state (multivalent ions), reduction may occur by

Mn+ + e− M(n-1)+

in which the metal ion decreases its valence state by

accepting an electron. Or a metal may be totally reduced

from an ionic to a neutral metallic state according to

Mn+ + ne− M

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The location at which reduction occurs is called the

cathode. Furthermore, it is possible for two or of the

reduction reactions above to occur simultaneously.

An overall electrochemical reaction must consist of at

least one oxidation and one reduction reaction, and will be

the sum of them; often the individual oxidation and

reduction reactions are termed half-reactions. There can

be no net electrical charge accumulation from the

electrons and ions; that is, the total rate of oxidation must

equal the total rate of reduction, or all electrons generated

through oxidation must be consumed by reduction.

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For example, consider zinc metal immersed in an acid solution containing H+ ions. At some region on the metal surface, zinc will experience oxidation or corrosion as illustrated in Figure, and according to the reaction

Zn Zn2+ + 2e−

Since zinc is a metal, and therefore a good electrical conductor, these electrons may be transferred to an adjacent region at which the H+ ions are reduced according to

2H+ + 2e− H2(gas)If no other oxidation or reduction reactions occur, the total electrochemical reaction is just the sum of reactions

Zn Zn2+ + 2e−

2H+ + 2e− H2(gas)

Zn + 2H+ Zn2+ + H2(gas)

The electrochemical reactionsassociated with the corrosion of zinc in an acidsolution.

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Another example is the oxidation or rusting of iron in water, which contains dissolved oxygen. This process occurs in two steps; in the first, Fe is oxidized to Fe2+

[as Fe(OH)2],

Fe + O2 Fe2+ + 2OH − Fe(OH)2

and, in the second stage, to Fe3+ [as Fe(OH)3]

according to

2Fe(OH) 2 + O2

+ H 2O 2Fe(OH)3

The compound Fe(OH)3+ is the all too familiar rust. As a

consequence of oxidation, the metal ions may either go

into the corroding solution as ions (Zinc Reaction) or

they may form an insoluble compound with non-metallic

elements as in reaction Fe3+ [as Fe(OH)3].

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Corrosion Rate DeterminatesThe half-cell potentials listed in the table are thermodynamic parameters that relate to systems at equilibrium .

For example, from the figures, it was tacitly assumed that there was no current flow through the

external circuit. Real corroding systems are not at equilibrium; there will be a flow of electrons from

anode to cathode (corresponding to the short-circuiting of the electrochemical cells in Figures 17.2

and 17.3) ,which means that the half-cell potential parameters (Table 17.1) cannot be applied. Furthermore,

these half-cell potentials represent the magnitude of a driving force, or the tendency for the

occurrence of the particular half-cell reaction. However, it should be noted that although these

potentials may be used to determine spontaneous reaction directions, they provide no information as

to corrosion rates. That is, even though a ΔV potential computed for a specific corrosion situation

using Equation

is a relatively large positive number, the reaction may occur at only an insignificantly slow rate. From

an engineering perspective, we are interested in predicting the rates at which systems corrode;

requires the utilization of other parameters, the corrosion rates.

ΔV

The corrosion rate, or the rate of material removal as a

consequence of the chemical action, is an important corrosion

parameter. This may be expressed as the penetration rate (CPR),

or the thickness loss of material per unit of time. The formula for

this calculation is

CPRwhere:

W = the weight loss after exposure

t = time of exposure

ρ = density

A = represents exposed specimen area

k = constant (magnitude depends on the system of units

used)

CPR = mils per year (mpy) or millimeters per year (mm/yr)

CPR (mm/yr)

where 1 mil = 0.001

in.

• W = mg

• ρ = gm/cm3

• A = cm2

• t = hrs.

• k = 87.6

CPR (mpy)

where 1 mil = 0.001

in.

• W = mg

• ρ = gm/cm3

• A = in2

• t = hrs.

• k = 534

For most applications a corrosion penetration rate less than about 20 mpy (0.50 mm/yr) is acceptable.

Inasmuch as there is an electric current associated with electrochemical corrosion reactions, we can also express corrosion rate in terms of this current,

where:i = current density (current per\ unit surface area of material corroding)

r = rate (mol/m2-s)n = number of electrons associated with the ionization of each metal atom, and is 96,500 C/mol.

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Polarization:• the displacement of each electrode potential from its

equilibrium value• the magnitude of this displacement is the overvoltage,

normally represented by the symbol 𝜼• Overvoltage is expressed in terms of plus or minus volts (or

millivolts) relative to the equilibrium potential.

For example, suppose that a zinc electrode has a potential of

-0.621V after it has been connected to the platinum

electrode. Its equilibrium potential is -0.763V; therefore,

There are two types of polarization—activation and

concentration—which control the rate of electrochemical

reactions.

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Prediction of Corrosion Rates

Activation Polarization:

• All electrochemical reactions consist of a sequence of steps

that occur in series at the interface between the metal

electrode and the electrolyte solution. • it refers to the condition wherein the reaction rate is

controlled by the one step in the series that occurs at the

slowest rate. • the term “activation” is applied to this type of polarization

because an activation energy barrier is associated with this

slowest, rate-limiting step.

To illustrate, let us consider the reduction of hydrogen ions to

form bubbles of hydrogen gas on the surface of a zinc electrode

(Figure 17.6). It is conceivable that this reaction could proceed

by the following step sequence:

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1. Adsorption of H+ ions from the solution onto the zinc surface2. Electron transfer from the zinc to form a hydrogen atom

H+ + e− H

3. Combining of two hydrogen atoms to form a molecule of hydrogen,

2H H2

4. The coalescence of many hydrogen molecules to form a bubbleThe slowest of these steps determines the rate of the overall reaction.

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Concentration Polarization:

• Exists when the reaction rate is limited by diffusion in the

solution. • For example, consider the hydrogen evolution reduction

reaction. When the reaction rate is low and/or the

concentration of H+ is high, there is always an adequate supply

of hydrogen ions available in the solution at the region near

the electrode interface (Figure 17.8a). On the other hand, at

high rates and/or low H+ concentrations , a depletion zone

may be formed in the vicinity of the interface, inasmuch as

the H+ ions are not replenished at a rate sufficient to keep up

with the reaction (Figure 17.8b). Thus, diffusion of H+ to the

interface is rate controlling, and the system is said to be

concentration polarized.

• Crevice Corrosion- is produced at the region of contact of metals with metals or metals with nonmetals. It may occur at washers, under barnacles, at sand grains, under applied protective films, and at pockets formed by threaded joints.

• Pitting Corrosion- is localized corrosion that occurs at microscopic defects on a metal surface. The pits are often found underneath surface deposits caused by corrosion product accumulation.

• Uniform Corrosion- also called general corrosion. The surface effect produced by most direct chemical attacks (e.g., as by an acid) is a uniform etching of the metal.

• Stress Corrosion Cracking- is caused by the simultaneous effects of tensile stress and a specific corrosive environment. Stresses may be due to applied loads, residual stresses from the manufacturing process, or a combination of both.

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Forms of Corrosion

• Erosion Corrosion- is the result of a combination of an aggressive chemical

environment and high fluid-surface velocities.• Microbial Corrosion

- is corrosion that is caused by the presence and activities of microbes. This corrosion can take many forms and can be controlled by biocides or by conventional corrosion control methods.

• Corrosion Fatigue- is a special case of stress corrosion caused by the combined effects of cyclic stress and corrosion. No metal is immune from some reduction of its resistance to cyclic stressing if the metal is in a corrosive environment.

• Galvanic Corrosion

• Concentration Cell Corrosion

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• Galvanic corrosion is an electrochemical action of two

dissimilar metals in the presence of an electrolyte and an

electron conductive path. It occurs when dissimilar metals are

in contact.

• It is recognizable by the presence of a build-up of corrosion at

the joint between the dissimilar metals.

• For example, steel screws corrode when in contact with brass

in a marine environment; or if copper and steel tubing are

joined in a domestic water heater, the steel will corrode in the

vicinity of the junction. Depending on the nature of the

solution, one or more of the reduction reactions,

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Galvanic Corrosion

Anodic (most active)• The natural differences in metal potentials

produce galvanic differences, such as the galvanic series in sea water. If electrical contact is made between any two of these materials in the presence of an electrolyte, current must flow between them. The farther apart the metals are in the galvanic series, the greater the galvanic corrosion effect or rate will be. Metals or alloys at the upper end are noble while those at the lower end are active. The more active metal is the anode or the one that will corrode.

• Control of galvanic corrosion is achieved by using metals closer to each other in the galvanic series or by electrically isolating metals from each other. Cathodic protection can also be used to control galvanic corrosion effects.

• The scuba tank photo suffered galvanic corrosion when the

brass valve and the steel tank were wetted by condensation. • Electrical isolation flanges like those shown are used to

prevent galvanic corrosion. Insulating gaskets, usually

polymers, are inserted between the flanges, and insulating

sleeves and washers isolate the bolted connections.• The photo below shows the effects of galvanic corrosion,

caused by a stainless steel screw causing galvanic corrosion

of aluminum. The picture shows the corrosion resulting from

only six months exposure at the Atmospheric Test Site.

Concentration cell corrosion occurs when two or more

areas of a metal surface are in contact with different

concentrations of the same solution. There are three

general types of concentration cell corrosion:

• Metal Ion Concentration Cells

• Oxygen Concentration Cells

• Active-Passive Cells.

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Concentration Cell Corrosion

Metal Ion Concentration Cells• In the presence of water, a high concentration

of metal ions will exist under faying surfaces and a low concentration of metal ions will exist adjacent to the crevice created by the faying surfaces. An electrical potential will exist between the two points.

• The area of the metal in contact with the low concentration of metal ions will be cathodic and will be protected, and the area of metal in contact with the high metal ion concentration will be anodic and corroded.

• This condition can be eliminated by sealing the faying surfaces in a manner to exclude moisture. Proper protective coating application with inorganic zinc primers is also effective in reducing faying surface corrosion.

Oxygen Concentration Cells• A water solution in contact with the metal surface

will normally contain dissolved oxygen. An oxygen cell can develop at any point where the oxygen in the air is not allowed to diffuse uniformly into the solution, thereby creating a difference in oxygen concentration between two points.

• Typical locations of oxygen concentration cells are under either metallic or nonmetallic deposits (dirt) on the metal surface and under faying surfaces such as riveted lap joints.

• Oxygen cells can also develop under gaskets, wood, rubber, plastic tape, and other materials in contact with the metal surface.

• Corrosion will occur at the area of low-oxygen concentration (anode). The severity of corrosion due to these conditions can be minimized by sealing, maintaining surfaces clean, and avoiding the use of material that permits wicking of moisture between faying surfaces.

Active-Passive Cells• Metals that depend on a tightly adhering passive film for corrosion protection can be corroded

by active-passive cells. • The corrosive action usually starts as an oxygen concentration cell. If the passive film is broken

beneath the salt deposit, the active metal beneath the film will be exposed to corrosive attack. An electrical potential will develop between the large area of the cathode (passive film) and the small area of the anode (active metal). Rapid pitting of the active metal will result.

• This type of corrosion can be avoided by frequent cleaning and by application of protective coatings

• There are a number of means of controlling corrosion.

The choice of a means of corrosion control depends on

economics, safety requirements, and a number of

technical considerations.

• Design

• Materials Selection

• Protective Coatings

• Inhibitors and Other Means of Environmental Alteration

• Corrosion Allowances

• Cathodic Protection

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Corrosion Control

Design

• Engineering design is a complicated process that includes

design for purpose, manufacturability, inspection, and

maintenance. One of the considerations often overlooked

in designing manufactured products is drainage.

• The corrosion of the automobile side panel above could

have been minimized by providing drainage to allow any

water and debris to fall off of the car instead of collecting

and causing corrosion from the far side of the panel.

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Materials Selection

• Carbon SteelMost large metal structures are made from carbon steel-the world's most useful structural material. Carbon steel is inexpensive, readily available in a variety of forms, and can be machined, welded, and formed into many shapes.

This large statue by Pablo Picasso in front of the Chicago city hall is made from a special form of carbon steel known as weathering steel. Weathering steel does not need painting in many boldly exposed environments. Unfortunately, weathering steel has been misused in many circumstances where it could not drain and form a protective rust film. This has given the alloy a mixed reputation in the construction industry.

• Stainless SteelThe stainless steel body on this sports car is one example of how stainless steels can be used. The stainless steel is virtually immune to corrosion in this application-at least in comparison to the corrosion that would be experienced by conventional carbon steel or aluminum auto bodies.Stainless steels are a common alternative to carbon steels. There are many kinds of stainless steels, but the most common austenitic stainless steels. These austenitic stainless steels are frequently immune to general corrosion, but they may experience pitting and crevice corrosion and undergo stress corrosion cracking in some environments.

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Materials Selection

• AluminumAluminum alloys are widely used in aerospace applications where their favorable strength-to-weight ratios make them the structural metal of choice. They can have excellent atmospheric corrosion capabilities. Unfortunately, the protective properties of the aluminum oxide films that form on these alloys can break down locally and allow extensive corrosion. • Copper AlloysBrasses and bronzes are commonly used piping materials, and they are also used for valves and fittings. They are subject to stress corrosion cracking in the presence of ammonia compounds. They also suffer from dealloying and can cause galvanic corrosion when coupled with steel and other structural metals. Most copper alloys are relatively soft and subject to erosion corrosion.

The dezincification shown could have been controlled by using inhibited brasses.• TitaniumTitanium is one of the more common metals in nature, but its limited use means that small-scale production operations result in a relatively expensive metal.

There are two general types of titanium alloys-aerospace alloys and corrosion resistant alloys. The crevice corrosion of an aerospace alloy flange in a saltwater application is a classic example of how titanium gets misused.

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Protective Coatings

• Protective coatings are the most commonly used

method of corrosion control.

• Protective coatings can be metallic, such as the

galvanized steel shown, or they can be applied as a

liquid "paint."

• The air conditioner on the left is starting to show rust

stains due to problems with protective coating.

• The same types of problems are starting to appear on

the aluminum airplane wing shown.

Inhibitors and Other Means of Environmental

Alteration

• Corrosion inhibitors are chemicals that are added to

controlled environments to reduce the corrosivity of

these environments.

• Examples of corrosion inhibitors include the chemicals

added to automobile antifreezes to make them less

corrosive

Corrosion Allowances• Engineering designers must consider how much metal is necessary to

withstand the anticipated load for a given application. Since they can make mistakes, the use of the structure can change, or the structure can be misused, they usually are required to over design the structure by a safety factor that can vary from 20% to over 300%. Once the necessary mechanical load safety factor has been considered, it becomes necessary to consider whether or not a corrosion allowance is necessary to keep the structure safe if it does corrode.

• The picture shows extra steel added to the bottom of an offshore oil production platform. The one inch of extra steel was added as a corrosion allowance.

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Cathodic Protection

• Cathodic protection is an electrical means of corrosion control.

Cathodic protection can be applied using sacrificial (galvanic) anodes or

by means of more complicated impressed current systems.

• This fishing boat has sacrificial zinc anodes welded to the hull to slow

down corrosion.

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