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WHAT IS CORROSION?

Corrosion is the chemical or electrochemical reaction between a metal and its environment.It can also be regarded as the degradation of a material due to a reaction with itsenvironment. Degradation implies deterioration of the physical properties of the material.The material can be metals, polymers (plastics, rubbers etc), ceramics (concrete, bricks etc)or composites-mechanical mixtures of two or more materials with different properties.Because metals are the most used type of structural materials this lecture will be devoted tothe corrosion of metals.

CONSEQUENCES OF CORROSION

The consequences of corrosion are varied and the effects of these on the safe, reliable andefficient operation of the equipment or structures are often more serious than the simplelossof a mass of metal. Failures of various kinds and the need for expensive replacementsmay occur even though the amount of metal destroyed is quite small. Some of the majorharmful effects of corrosion can be summarized as follows:

Hazards or injuries to people arising from structural failure or breakdown (e.g.bridges,cars,aircraft)

Reduced value of goods due to deterioration of appearance Reduction of metal thickness leading to loss of mechanical strength and structural failure

or breakdown.

Mechanical damage to valves, pumps etc or blockage of pipes by solid corrosionproducts.

Loss of technically important surface properties of a metallic component. These couldinclude frictional and bearing properties, ease of flow over a pipe surface, electricalconductivity of contacts, surface reflectivity or heat transfer across a surface.

Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantitiesof heavy metals are released by corrosion)

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CHEMISTRY OF CORROSION

Firstly, we shall review our understanding of the nature of Matter (i.e. Atoms, Ions,molecules)

Atoms: All matter is made up of atoms composed of protons, neutrons and electrons. The

center or nucleus of the atom is composed of positively charged protons and neutrons. Theoutside of the atom has negatively charged electrons in various orbits. This is shownschematically in the figure below

All the atoms of an element have the same number of protons and electrons; hence theatom is electrically stable.

The number of protons in the atom is called the atomic number ( Np)while the sum of theprotons and neutrons in the atom gives the atomic mass (A) of the element. Every elementis represented by a chemical symbol. For example, H stands for Hydrogen, C for carbon Nafor Sodium, Fe for Iron

A typical atom is represented chemically in this form:

Where the subscript is the atomic number and superscript the atomic mass.

Ions:

Ions are formed when atoms or group of atoms lose or gain electrons. Metals lose some oftheir electrons to form positively charged ions, e.g. Fe+2, Al+3, Cu+2, etc

Non metals gain electrons and form negatively charged ions e.g. Cl-, O-2, OH-, etc

Molecules:

Compounds are groups of metal s and nonmetals that form distinct chemicals. Most of usare familiar with the formula H2O, which indicates that each water molecule is made up twohydrogen atoms and one oxygen atom. Many molecules are formed by sharing electronsbetween adjacent atoms. A water molecule has adjacent hydrogen and oxygen atomssharing some of their electrons.

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ELECTROCHEMICAL CELLS

Oxidation and Reduction:

Metals are elements that tend to lose electrons when they are involved in chemicalreactions, and nonmetals are those element s that tend to gain electrons. Sometimes theseelements form ions, charged elements or groups of elements. Metallic ions, because theyare formed from atoms that have lost electrons, are positively charged (the nucleus isunchanged). When an atom or ions loses electrons it is said to have been oxidized. Sooxidation could be define as the loss of electron(s)

A common oxidation reaction in corrosion is the oxidation of neutral sodium atoms to

positively charged iron ions:

Na Na+ + e- or Na - e- Na+(1)

The electrons lost from a metal must go somewhere, and they usually end up on anonmetallic atom forming a negatively charged nonmetallic ion. The atom that gained theelectron(s) is said to have been reduced. Hence, reduction is the gain of electron(s)

Cl + e- Cl-(2)

CAUSES OF CORROSION

Corrosion occurs by an electrochemical process. The Phenomenon is similar to that whichtakes place when a carbon- zinc dry cell generates a direct current. Basically, an anode(negative electrode).a cathode (positive electrode) an electrolyte (environment) and a circuitconnecting the anode and the cathode are required for corrosion to occur .Dissolution ofmetal occurs at the anode where the corrosion current enters the electrolyte and flows tothe cathode. Equation (1) shows what happens at the anode. Examination of that reactionreveals that oxidation occurs at the anode. Electrons lost at the anode flow through themetallic circuit to the cathode and permit a cathodic reaction to occur. The most commonand important electrochemical reactions in the corrosion of iron are thus

Anodic reaction (corrosion)

Fe Fe2+ + 2e-

The cathodic reaction that usually occurs in deaerated acids is

2H+ + 2e- H2

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In alkaline (basic) and neutral aerated solutions, the predominant cathodic reaction is

O2 + 2H2O + 4e- 4(OH)

In this latter case corrosion is usually accompanied by the formation of solid corrosiondebris from the reaction between the anodic and cathodic products

Fe2+ + 2OH- Fe(OH)2 , iron (II) hydroxide

Pure iron (II) hydroxide is white but the material initially produced by corrosion is normallya greenish colour due to partial oxidation in air.

4Fe (OH)2 + 2 H2O + O2 4Fe (OH)3 , hydrated iron (III) oxide

Further hydration and oxidation reactions can occur and the reddish rust that eventuallyforms is a complex mixture. The insoluble corrosion product is formed by a secondarychemical reaction.

ELECTRODE POTENTIALS

When a metal is immersed in an aqueous solution containing a definite concentration ofions, C, there is one electric potential(voltage) at which equilibrium can exist between themetal and the solution. The greater the negative potential, the greater is the tendency of ametal to dissolve.

The electrode potential is given by the Nernst Equation:

E = E0 + 0.0592/n LogC

Where

E0 =Standard Electrode Potential of the metal

C = Concentration in kmol of ions

N = valency of the ion in the equation

Now for a solution containing 10-3 mol of Fe+2 per liter;

E = -0.44 +(0.0592/2) Log10-3

E = -0.44 + (0.0296) (-3)

E = -0.529V

Nernst Equation is also given in natural logarithm thus

E = E0 RT/nF Loge C

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Where

R =Universal gas constant (8.314KJ/Kg mol.K)

T = Absolute temperature in K

F = Faraday, which is the quantity of electricity that will liberate ordeposit one

mole of a substance during electrolysis.

The standard electrode potential,E0 of metals are arrange in their order of increasingpotential to form Galvanic, electrochemical or electromotive series. The ability of metals toresist corrosion is to some extent dependent upon their position in the electrochemicalseries. The farther two metals are separated from one another in the series, the morepowerful is the electric current produced by their contact in the presence of an electrolyte.Also the more rapidly the lower metal is attacked, the more will the higher metal beprotected.

FORMS OF CORROSION

1. General CorrosionThis is 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. On a polished surface, this

type of corrosion is first seen as a general dulling of the surface and, if allowed to continue,

the surface becomes rough and possibly frosted in appearance. The discoloration or generaldulling of metal created by its exposure to elevated temperatures is not to be considered as

uniform etch corrosion. The use of chemical-resistant protective coatings or more resistant

materials will control these problems.

While this is the most common form of corrosion, it is generally of little engineering

significance, because structures will normally become unsightly and attract maintenance

long before they become structurally affected. The facilities shown in the picture below

show how this corrosion can progress if control measures are not taken.

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2. Galvanic CorrosionGalvanic 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 buildup of corrosion at the joint between the

dissimilar metals. For example, when aluminum alloys or magnesium alloys are in contact

with steel (carbon steel or stainless steel), galvanic corrosion can occur and accelerate the

corrosion of the aluminum or magnesium.

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This can be seen on the photo above where the aluminum helicopter blade has corroded

near where it was in contact with a steel counterbalance.

Galvanic Series In Sea Water

Noble

(least active)

Platinum

GoldGraphite

Silver

18-8-3 Stainless steel, type 316 (passive)

18-8 Stainless steel, type 304 (passive)

Titanium

13 percent chromium stainless steel, type 410 (passive)

7NI-33Cu alloy

75NI-16Cr-7Fe alloy (passive)

Nickel (passive)

Silver solder

M-Bronze

G-Bronze

70-30 cupro-nickel

Silicon bronze

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Copper

Red brass

Aluminum bronze

Admiralty brass

Yellow brass

76NI-16Cr-7Fe alloy (active)

Nickel (active)

Naval brass

Manganese bronze

Muntz metal

Tin

Lead

18-8-3 Stainless steel, type 316 (active)18-8 Stainless steel, type 304 (active)

13 percent chromium stainless steel, type 410 (active)

Cast iron

Mild steel

Aluminum 2024

Cadmium

Alclad

Aluminum 6053

Galvanized steel

Zinc

Magnesium alloys

Magnesium

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 themetals 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.

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Electrical isolation flanges like those shown above are used to prevent galvanic corrosion.

Insulating gaskets, usually polymers, are inserted between the flanges, and insulating

sleeves and washers isolate the bolted connections.

3. Concentration Cell CorrosionConcentration 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:

1. metal ion concentration cells2. oxygen concentration cells, and3. active-passive cells.

MetalIonConcentrationCells

http://corrosion.ksc.nasa.gov/images/gal2.jpg

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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 concentrationbetween 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 (usually an oxide) for corrosion

protection; e.g., austenitic corrosion-resistant steel, can be corroded by active-passive cells.

The corrosive action usually starts as an oxygen concentration cell; e.g., salt deposits on the

metal surface in the presence of water containing oxygen can create the oxygen 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 andby application of protective coatings

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4. Pitting CorrosionPitting corrosion is a randomly occurring, highly localized form of attack on a metal surface,characterized by the fact that the depth of penetration is much greater than d diameter ofthe area affected. Pitting is one of the most destructive forms of corrosion, yet itsmechanism is not completely understood. Steel and galvanized steel pipes and storagetanks are susceptible to pitting corrosion.

Methods that can be used to control pitting include maintaining clean surfaces, application

of a protective coating, and use of inhibitors or cathodic protection for immersion service.

Molybdenum additions to stainless steel (e.g. in 316 stainless) are intended to reduce

pitting corrosion.

The rust bubbles or tubercules on the cast iron above indicate that pitting is occurring.

Researchers have found that the environment inside the rust bubbles is almost always

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higher in chlorides and lower in pH (more acidic) than the overall external environment. This

leads to concentrated attack inside the pits.

Similar changes in environment occur inside crevices, stress corrosion cracks, and corrosionfatigue cracks. All of these forms of corrosion are sometimes included in the term "occluded

cell corrosion."

Pitting corrosion can lead to unexpected catastrophic system failure. The split tubing above

left was caused by pitting corrosion of stainless steel. A typical pit on this tubing is shown

above right.

Sometimes pitting corrosion can be quite small on the surface and very large below the

surface. The figure below left shows this effect, which is common on stainless steels and

other film-protected metals. The pitting shown below right (white arrow) led to the stresscorrosion fracture shown by the black arrows.

http://corrosion.ksc.nasa.gov/images/pit4.jpghttp://corrosion.ksc.nasa.gov/images/pit3.jpghttp://corrosion.ksc.nasa.gov/images/pit2.jpghttp://corrosion.ksc.nasa.gov/images/pit4.jpghttp://corrosion.ksc.nasa.gov/images/pit3.jpghttp://corrosion.ksc.nasa.gov/images/pit2.jpghttp://corrosion.ksc.nasa.gov/images/pit4.jpghttp://corrosion.ksc.nasa.gov/images/pit3.jpghttp://corrosion.ksc.nasa.gov/images/pit2.jpg

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5. Crevice CorrosionCrevice corrosion refers to the localized attack on a metal surface at, or immediatelyadjacent to the gap or crevice between two joining surfaces. The gap or crevice can beformed between two metals or a non metallic material. Outside the gap or without the gap,both metals are resistant to corrosion. The damage is normally confined to one metal atlocalized area within or close to the joiningsurfaces

The photo at the right is a stainless steel tubeand tube sheetfrom a heat exchanger in a desalination plantthat has suffered crevice corrosion due to thepresence of crevice(gap) between

the tube and tube sheet.

Crevice is initiated by a difference inconcentration of some chemical constituents,usually oxygen, which sets up anelectrochemical concentration. Outside of thecrevice(cathode),the oxygen content and pHare higher- but chlorides are lower. The chlorides concentrate inside the crevice (anode),worsening the situation. Crevices can be designed out of the system by weldingcontinuously around a lap joint, the use of solid, non absorbent gasket such as Teflon etc.

6. StressCorrosionCrackingStress corrosion cracking (SCC) 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.

http://corrosion.ksc.nasa.gov/images/pit5.jpghttp://corrosion.ksc.nasa.gov/images/pit1.jpghttp://corrosion.ksc.nasa.gov/images/pit5.jpghttp://corrosion.ksc.nasa.gov/images/pit1.jpghttp://corrosion.ksc.nasa.gov/images/pit5.jpghttp://corrosion.ksc.nasa.gov/images/pit1.jpg

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Cross sections of SCC frequently show branched cracks. This river branching pattern is

unique to SCC and is used in failure analysis to identify when this form of corrosion has

occurred.

The photo below shows SCC of an insulated stainless-steel condensate line. Water wetted the

insulation and caused chlorides to leach from the insulation onto the hot metal surface. This is

a common problem on steam and condensate lines. Control is by maintaining the jackets

around the lines so that moisture doesn't enter the insulation or is quickly drained off.

http://corrosion.ksc.nasa.gov/images/scc2.jpghttp://corrosion.ksc.nasa.gov/images/scc1.jpghttp://corrosion.ksc.nasa.gov/images/scc2.jpghttp://corrosion.ksc.nasa.gov/images/scc1.jpg

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7. Erosion Corrosion

corrosion is the result of a combination of an aggressive chemical environment and high

fluid-surface velocities. This can be the result of fast fluid flow past a stationary object, such

as the case with the oil-field

check valve shown on the left below, or it can result from the quick motion of an object in a

stationary fluid, such as happens when a ship's propeller churns the ocean.

Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the

surfaces from many other forms of corrosion.

Erosion corrosion can be controlled by the use of harder alloys (including flame-sprayed or

welded hard facings) or by using a more corrosion resistant alloy. Alterations in fluid

velocity and changes in flow patterns can also reduce the effects of erosion corrosion.

Erosion corrosion is often the result of the wearing away of a protective scale or coating on

the metal surface. The oil field production tubing shown above on the right corroded when

the pressure on the well became low enough to cause multiphase fluid flow. The impact of

collapsing gas bubbles caused the damage at joints where the tubing was connected and

turbulence was greater.

Many people assume that erosion corrosion is associated with turbulent flow. This is true,

because all practical piping systems require turbulent flow-the fluid would not flow fast

enough if lamellar (non turbulent) flow were maintained. Most, if not all, erosion corrosion

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can be attributed to multiphase fluid flow. The check valve on the left above failed due to

sand and other particles in an otherwise noncorrosive fluid. The tubing on the right failed

due to the pressure differences caused when gas bubbles collapsed against the pipe wall

and destroyed the protective mineral scale that was limiting corrosion.

8. IntergranularCorrosionIntergranular corrosion is an attack on or adjacent to the grain boundaries of a metal or

alloy. A highly magnified cross section of most commercial alloys will show its granular

structure. This structure consists of quantities of individual grains, and each of these tiny

grains has a clearly defined boundary that chemically differs from the metal within the grain

center. Heat treatment of stainless steels and aluminum alloys accentuates this problem.

The picture above shows a stainless steel which corroded in the heat affected zone a short

distance from the weld. This is typical of Intergranular corrosion in austenitic stainless steels.

This corrosion can be eliminated by using stabilized stainless steels (321 or 347) or by using

low-carbon stainless grades (304L or 3I6L).

9. DealloyingDealloying is a process in which one element is preferentially removed from an alloy. Thisoccurs without appreciable change in size or shape of the component; but the affected areabecomes weak, brittle and porous. The two most important examples of dealloying are thepreferential removal of zinc from copper-Zinc alloys(dezincification),and the preferentialremoval of iron from gray-cast iron (graphite corrosion).Graphite corrosion sometimesoccurs on underground cast iron water mains and leads to splitting of the pipe when thewater pressure is suddenly increased.

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10.Microbial CorrosionThis is a bio-corrosion arising from activities of sulphate reducing bacteria (SRB) in line pipesteels. SRB aided corrosion affects the internal surface of line pipe steels (Note O2 is

absent).There are two ways in which micro-organisms are involved in corrosion processes.

Firstly, by virtue of their growth and metabolism, they can introduce into an otherwiseinnocuous system, chemical entities such as acids,alkaline,sulphides and other aggressiveions which will render the environment corrosive .Secondly, their presence could providethe structure with a concentration cell with areas being anodic compound to the rest.

Most instances of corrosion in the absence of oxygen have been attributed to SRB,namelydesulfovibrio and desulfotomaculum.Typical examples of such environment are waterheavily polluted with organic matter. These microbes are obligated anaerobes, that is theydo not grow in the presence of even traces of oxygen but usually grow well at pH 5 and 9

and temperatures between 25o

C and 44o

C. They utilize hydrogen or some reducingsubstance for their life process. The corrosion is often localized and is generallycharacterized by a black corrosion product with a strong smell of hydrogen sulphide (H2S).

CORROSION CONTROL MEASURES

Corrosion mitigation can be accomplished by design considerations, by employingcorrosion-resistant materials of construction, by employing cathodic protection, by usingprotective coatings and corrosion inhibitors

Design considerationsThe use of acceptable engineering practices to minimize corrosion is fundamental tocorrosion control. This is accomplished by engineering design. One of the most importantfactors in designing for corrosion control is to avoid crevices where deposits of water-soluble compounds and moisture can accumulate and are not accessible for maintenance.Any region where two surfaces are loosely joined or come into proximity also qualifies as acrevice site. Joining geometries also present various crevice corrosion problems.

Cathodic ProtectionCathodic protection is an electrical method of fighting against corrosion on metallic

structures that are exposed to electrolyte such as soils and waters. Corrosion control isachieved by forcing a defined quantity of direct current to flow from auxiliary anodes,through the electrolyte, and onto the metal structure to beprotected.Theoretically,corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open circuit potentials of theanodic sites. The entire protected structure becomes cathodic relative to the auxiliaryanodes. Therefore, corrosion of the metal structure will cease when the applied cathodiccurrent equals the corrosion current. There are two basic methods of corrosion control by

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cathodic protection. One involves the use of current that is produced when twoelectrochemically dissimilar metals or alloy are metallically connected and exposed to theelectrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protectionsystem. The other method of cathodic protection involves the use of a direct current powersource and auxiliary anodes, which is commonly referred to as an impressed current

cathodic protection system.

Galvanic anode systems employ reactive metals as auxiliary anodes that are directlyelectrically connected to the steel to be protected. The difference in natural potentialsbetween the anode and the steel, as indicated by their relative positions in the electro-chemical series, causes a positive current to flow in the electrolyte from the anode to thesteel.Thus,the whole surface of the steel becomes more negatively charged and becomesthe cathode. The metals commonly used, as sacrificial anodes are aluminuim,zinc andmagnesium. These metals are alloyed to improve the long-term performance anddissolution characteristics.

Advantages of Galvanic system

Simple to install Independent of a source of external electric power Suitable for localised protection Less liable to cause interaction on neighbouring structures i.e. interference problems

(stray currents) are rare.

Minimum maintenance requiredDisadvantages of galvanic System

Limited driving potential Lower current output Can be ineffective in high-resistivity environments

Impressed-current systems employ direct current usually provided by a rectifier attached toan AC power source. The rectifier converts alternating current to direct current. Directcurrent from the rectifier flows to the buried impressed current anode, from the anodethrough the soil electrolyte and onto the surface of the steel. The impressed current anodesused in soil are made of materials such as graphite,steel,high silicon cast iron or mixedmetal oxides on titanium

Advantages of Impressed Current

Able to supply a relatively large current capable of protecting large structures Availability of large driving potential Capability of variable current output Applicability to almost any soil resistivity

Disadvantages of Impressed current system.

Possible interference problems(stray currents)

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Loss of AC power causes loss of protection Higher maintenance and operating cost. Higher cost for small installations More frequent monitoring

Structures that are commonly protected by cathodic protection are the exterior surfaces of:

Pipelines Ships hull Storage tank bases Jetties and harbor structures Steel sheet, tubular and foundation pilings Offshore platforms, floating and subsea structures

Cathodic protection is also used to protect the internal surfaces of:

Large diameter pipelines Ships tank (product and ballast) Storage tanks (oil and water) Water circulating system Design considerations

Any Substance, when added in small quantities to a corrosive environment containingcarbon steel or an alloy that will decrease the corrosion rate is called an inhibitor. Inhibitorsfunction in various ways and are beyond the scope of this manual. Additions of solublehydroxides, chromates phosphates, silicates and carbonates are used to decrease thecorrosion rates of carbon steel and alloys in various corrosive environments. Theconcentration of a given inhibitor needed to protect a metal will depend on a number offactors such as composition of the environment,temperature,velocity of the liquid, thepresence or absence in the metal of internal or external stresses, composition of the metaland the presence of any other metal contact.

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