9.2 Control of corrosion and choice of materials 9.2 Control of corrosion and choice of materials. also be corroded if their potential is reduced to below the discharge potential of

507VOLUME V / INSTRUMENTS

9.2.1 Preventing corrosion

BasicsWith reference to the general outline of the corrosion

process (see Chapter 9.1), it should be remembered that tostop corrosion or keep it below a given limit, it is possible tointervene on the four partial processes which form part of thiselectrochemical mechanism. The operations relating to theseprocesses are summarized below.

Anodic reactionThe main operations involve:

• Eliminating the driving force by selecting a material whichis more noble than the cathodic process (Ea�Ec). Forexample, noble metals such as gold or platinum in aeratedenvironments or copper can be used in a non oxidizing acidenvironment. Noble materials can also be used in the formof a deposit or coating (such as plating, for instance).

• Eliminating the driving force by bringing the potential ofthe material into the immunity zone (E�Eeq) withcathodic protection.

• Slowing down the anodic reaction by means ofpassivation phenomena which bring the material intoconditions of passivity (the formation of protective andstable oxides as in the case of stainless steels and titaniumin aerated environments).

• Slowing down the anodic reaction by bringing thematerial into conditions of passivity with the applicationof anodic protection.

• Slowing down the anodic reaction with passivatinginhibitors added to the environment, which encourage theformation of passivation films.

• Eliminating the macrocouple currents of localizedcorrosion and reinstating passivity with cathodicprotection for perfect passivity.

Cathodic reactionThe main operations involve:

• In the case of carbon steels, eliminating the driving forceby removing oxygen and maintaining a pH equal to orhigher than neutrality (in the absence of H2S).

• Increasing the overvoltages of the cathodic process in thepresence of oxygen. The physical or chemical removal of

oxygen (with degassification or an oxygen scavenger,respectively) is always preferable. However, it is alsopossible to encourage the formation of calcareousdeposits by adding phosphates and phosphonates whichreduce the quantity of oxygen able to reach the surface ofthe metal.

• Increasing the overvoltages of the hydrogen releaseprocess in acid solutions using filming inhibitors.

• Decreasing the overvoltages of the cathodic process ofhydrogen release in acid environments by cathodicalloying in the case of Ti, Cr and Ta alloys. In thesespecific instances, increasing the velocity of the cathodicprocess brings the material into conditions of passivity.

EnvironmentCorrosion can be slowed, but not stopped completely, by

increasing ohmic resistance, for example by using insulatingcoatings (thick paints and plastic coatings).

The metallic materialIt is usually impossible to intervene directly on the metal

to block or slow down its progression towards corrosionexcept in the specific instance of corrosion by electricalinterference, as in the case of pipelines running alongside thetracks of electrical transportation systems such as train andunderground lines. The introduction of insulating joints onthe pipeline reduces or even eliminates the interferencecurrent circulating inside it.

Noble and practically noble materialsThe corrosion process is prevented if the available driving

force is zero or negative. This can be achieved by choosing amaterial whose practical nobility is higher than the cathodicprocess. If the corrosive environment is a non aerated acidsolution, in which the cathodic oxygen reduction processtherefore does not occur, the potential of the cathodic processof hydrogen ion reduction is given by Ec(H��H2)��0.059 pH.Metals with a potential Ea�E°�(0.059/z)log[Mz�] higherthan the potential of the cathodic process (Ea�Ec(H��H2)) arenot subject to corrosion. Examining the electrochemicalpotential scale, it becomes apparent that the metals immunefrom corrosion in acid environments are Cu, Ag, Hg, Pt andAu. Under some specific circumstances, these metals may

9.2

Control of corrosionand choice of materials

also be corroded if their potential is reduced to below thedischarge potential of hydrogen, for example through theformation of complexes. This is the typical case of copperalloys, which undergo corrosion in ammoniac environmentsdue to the formation of the copper-ammonia complex.

In oxygenated solutions, the most noble and thereforethermodynamically favoured cathodic process is oxygenreduction, which has a potential of Ec(O2�OH�)��1.23�0.059pH. In this case, Ag and Hg are resistant to corrosion whenthe pH is close to or higher than neutrality and Pt in acidsolutions with a pH above 1, whilst Au is always immune.

It is clear that, as far as the cathodic process is concerned,the choice of noble materials is extremely limited andinapplicable in practice. For industrial applications, a metal oralloy can be chosen which has a high so-called practicalnobility, resulting from passivation processes. In this way,numerous metals with an extremely negative electrochemicalpotential (such as Cr, Al, Ti and others) are covered with aprotective film, making the material virtually immune tocorrosion and bringing the free corrosion potential to valuestypical of noble metals. In fact, the corrosion process isextremely slow, usually below 0.1 mm/yr and thereforecompatible with the duration generally required. Stainlesssteels, nickel and titanium alloys can operate in aeratedsolutions. In these cases it is important to be aware of theconditions which may destroy the passivity film, for examplethe presence of chlorides, since the corrosion process will betriggered wherever the film is broken. Localized forms ofcorrosion such as pitting and crevices in stainless steels willoccur (Bianchi and Mazza, 1989; Pedeferri, 2007).

Coatings and paintsAn effective and widely used method for preventing

corrosion is the application of a coating to insulate thesurface of the metal from the environment. The choice isextremely broad since these coatings may be organic (paints),inorganic (conversion layers, enamels) or metallic.

Metallic coatingsThese are one of the most commonly used prevention

methods and consist of a layer of a corrosion-resistant metalor metal alloy. In addition to corrosion resistance, these layersmust have other properties such as mechanical resistance,hardness, resistance to wear, electrical, optical and thermalproperties and, last but not least, an aesthetically pleasingappearance.

From the point of view of corrosion, imperfections in thecontinuity and uniformity of the surface are extremelyimportant. Indeed, if the coating is continuous and nonporous, the protection of the base metal is complete, whereasthe presence of imperfections results in the formation ofgalvanic couples, consisting of the coating and uncoveredareas of the base metal; depending on whether the coatingfunctions as an anode or cathode with respect to the baseitself, it may offer the base metal protection of cathodic typeor, on the other hand, stimulate attack.

With respect to steel, cathodic coatings, such as those ofcopper, nickel, silver, lead, chromium, and in someenvironments tin, accelerate corrosion; coatings of zinc, andin some environments tin, act as anodes. Due precisely to thisanodic behaviour, discontinuities and porosities in the coatingare not intrinsically dangerous since, in this case, it is themetal used as a coating which takes on an anodic function

and is thus exposed to attack. In the case of galvanized steelin contact with sufficiently hard water, the alkalinityproduced at the cathodic surface leads to the separation ofcarbonates, covering the sheet with calcareous deposits andthus sealing any pores or defects.

Imperfections may form during the preparation andapplication of the coating (surfaces not adequately pre-treatedor incorrect operating conditions, for example in the case ofgalvanic deposits).

Conversion layersThese are obtained on the surface of some metals, mainly

steels, galvanized steels and aluminium, following chemicalor electrochemical reactions. The most widely usedconversion processes are phosphatization, chromation andanodic oxidation. Phosphatization involves immersingarticles made of steel or galvanized steel in acid solutions ofZn and Mn phosphates which form a layer of adherent andprotective phosphates on the surface of the metal. These havea mildly anticorrosive effect and form a primer coat forpainting cycles (for example on domestic appliances andmetal shelving). Chromation has been used to finishaluminium, galvanized steel, cadmium and magnesium, bothas an anticorrosive treatment and as a pre-treatment inpainting cycles (for example, the Al alloys of aircraft). Theanticorrosive action of chromation is due to the presence ofhexavalent chromium, which is an ideal passivator, ready toact whenever the paint layer is damaged. However, thetoxicity of hexavalent chromium is leading to this treatmentbeing banned (at least in Europe). The anodic oxidation ofaluminium and other metals is an electrolytic process carriedout in order to thicken the naturally present film of oxide soas to either improve resistance to corrosion and abrasion andthe object’s aesthetic properties or to obtain an oxide filmwith special dielectric properties. A typical example ofanodic oxidation is that carried out to protect aluminiumwindow and door frames, which for this reason are describedas anodized. Anodic oxidation also serves as a pre-treatmentto anchor cycles of varnish or paint to the metal.

Cementitious coatingsGiven their alkalinity (pH of about 13), cementitious

coatings offer perfect protection for carbon steel (Bertolini etal., 2004). This is one of the main reasons for the success ofreinforced concrete structures, but it also explains whycementitious coatings are so widely used for the internalprotection of water pipes (applied by centrifuge on steel orcast-iron pipes) or for the outer coating of subsea pipelines(also acting as ballast) or buried structures, such as theoutside of oil well casings.

Plastic paints and coatingsThese consist of a thin film of organic nature about 0.02 to

3 mm thick, applied to the surface of the metal with a solventdesiccation process, hot extrusion or chemical reticulation(Munger, 1999). The first two types of coating usethermoplastic polymers dissolved in a solvent or softened byheating; the third type uses thermoset polymers with in situreticulation. Solvent-based paints, by their very nature, create aporous film since the evaporation of the solvent inevitablyleaves pores in the paint film. For this reason, the filmsobtained with this type of paint are always permeable to waterand oxygen and therefore unable to prevent the corrosion

MATERIALS

508 ENCYCLOPAEDIA OF HYDROCARBONS

process by means of a barrier effect; therefore, they require aprimer coat with active pigments capable of actively blockingthe corrosion process. The other two types of coating, bycontrast, are always able to prevent water and oxygen reachingthe metal by means of the barrier effect and therefore do notrequire a primer coat with anticorrosive properties.

The anticorrosive primer coat consists of active pigmentsof anodic or cathodic type. Anodic pigments are compoundswhich bring steel into passivation conditions; they includephosphates, chromates and minium (Pb3O4). Chromates andminium are anodic passivators (see below), able to supply thecathodic process which keeps iron in a passive state(however, it should be remembered that these pigments have,in actual fact, been abolished due to their toxic andcarcinogenic effects). Cathodic pigments consist of a metalliczinc powder which acts in two ways: it helps to strengthen thebarrier effect and provides cathodic protection for steel. Themetallic zinc powder (above 90% in weight) is dispersed intwo types of binder/solvent: an inorganic binder, consisting ofsodium silicate in a methanol solvent or an organic binder,consisting of bicomponent and thermoset epoxy resins.

The application of paints involves so-called paintingcycles which include: preparation of the surface, primercoat, intermediate coat and finishing coat. The preparationof the surface involves the removal of oxides, incrustationsand dirt until the metal is shining white. Good surfacepreparation is essential if the painting cycle is to besuccessful. The degree of surface preparation is classified bySSPC and Sa (Munger, 1999). Classic painting cycles foroffshore structures are reported in Table 1.

The paints and coatings used in combination withcathodic protection must have good resistance to the alkalineconditions produced by cathodic protection and to theso-called cathodic disbonding caused by the release ofhydrogen.

Environmental controlCorrosion prevention can be carried out by controlling or

correcting the composition of the environment.

pH correctionThe most widely used pH correction methods include:

the alkalination of boiler waters to encourage the formation

of a magnetite film; varying the pH to modify the ability ofwater to form encrustations; the addition of alkalinesubstances to avoid acid condensates. With reference to thelatter treatment, it should be remembered that the presenceof acid substances such as CO2 and HCl, which in the dryvapour state do not cause corrosion problems, does rendercondensates highly aggressive. In these cases, the control ofacidity involves the use of neutralizers such as ammonia orvarious amines injected into the vapour stream, for examplebefore they enter the condensers at the head of crudedistillation columns, or added to the feed water of steamgenerators.

Oxygen controlRemoving oxygen from neutral or alkaline waters

reduces and in practice stops the corrosion of carbon steel.This is done with physical methods by gas stripping or avacuum treatment to reach residual oxygen contents of 0.015ppm (sufficient for boiler feed waters at low or mediumpressure) or with chemical methods often coupled withphysical methods to bring the oxygen content to a few ppb.The substances most frequently used for this purpose aresodium sulphite or hydrogensulphite and hydrazine whichact following the reactions: Na2SO3�1/2O2

��Na2SO4 and

N2H4�O2��2H2O�N2, respectively. Hydrazine may

partially decompose to form ammonia according to thereaction 3N2H4

��4NH3�N2; this produces a beneficial

alkalinity which encourages the separation of protectivefilms of magnetite on steel. Another advantage of hydrazinecompared to sulphite is that does not cause an increase in thesalinity of the water. However, hydrazine reacts with oxygenat an appreciable velocity only at temperatures above 140°C.Moreover, problems of toxicity make using this substanceinadvisable.

Corrosion inhibitorsInhibitors are substances which, when added in small

quantities to aggressive environments, can slow down or stopcorrosion processes. The substances which possess theseproperties are extremely numerous and of very differentnatures. The mechanism according to which they operate isin many cases still unclear. Generally speaking, they work bymodifying the surface layer of the metallic material to be

CONTROL OF CORROSION AND CHOICE OF MATERIALS


Table 1. Painting cycles for offshore structures (Munger, 1999)

Surface preparation Primer layer Intermediate layer Finishing layer Total thickness

Sa3 or SSPC 5 –High thickness epoxy

without solventHigh thickness epoxy

without solvent 1,000 mm

Sa2.5 or SSPC 10Inorganic zinc-richprimer max. 75 mm

Epoxy without solvent Epoxy without solvent 250 mm (minimum)

Sa2.5 or SSPC 10Inorganic zinc richprimer max. 75 mm

Epoxy without solvent Urethane or epoxy-urethane 250 mm (minimum)

ST3Organic zinc-richprimer (125 mm)

Epoxy without solvent Epoxy without solvent 300 mm (minimum)

Galvanized steel(degreasing�roughening)

– Modified vinyl or epoxy Epoxy-polyamide 180 mm (minimum)

protected following adsorption or reaction processes whichlead to the separation of products on the surface to beprotected. The consequent increase in reaction resistancesometimes results from the inhibition of the cathodic process(for example by increasing the overvoltage of hydrogen orpreventing oxygen from reaching the metal surface) and/orthe anodic process; in other cases it results from theestablishment of passive conditions. Table 2 shows theinhibitors usually employed in some environments. The vastmajority of inhibitors are basically used: in neutral orslightly alkaline environments, consisting mainly of naturalor industrial cooling waters or in any case of solutions in thepH range 5 to 9; in acid environments (pickling, aciddisincrustation treatments); in crude oil extraction andrefining processes.

The use of inhibitors, though an excellent and well-triedmethod for preventing corrosion, may have significantundesirable side-effects such as the contamination ofproducts, environmental pollution (chromates, phosphates,nitrites), damage to parts other than those protected. Forexample, some inhibitors which protect one part of a facilitymay turn out to be aggressive towards a different part or mayprevent corrosion but simultaneously make the material morebrittle. Various inhibitors employed in the past (some still inuse today) such as chromates and nitrites, are also toxic.

Classifying inhibitorsInhibitors can be classified: according to their chemical

nature (organic, inorganic inhibitors, etc.); their use(inhibitors for boiler feed waters, for pickling,disincrustation, packing) and their conditions of use(inhibitors in a solution or in the vapour phase); according tothe mechanism with which they work, illustrated in Fig. 1(cathodic inhibitors, oxidizing or non oxidizing anodic

inhibitors and mixed or multiple action inhibitors). Cathodicinhibitors cause an increase in the cathodic overvoltage andthus a decrease in the corrosion potential; anodic inhibitors,by contrast, cause an increase in the anodic overvoltage andtherefore an increase in the corrosion potential. Where aninhibitor acts both on the anodic and cathodic processes, theinhibition effect may not be accompanied by significantvariations in potential.

Cathodic inhibitors. In acid solutions, where thecathodic reaction is the release of hydrogen, all substanceswhich increase its overvoltage act as inhibitors. Thesubstances which behave in this way are those containingelements of the fifth and sixth groups of the periodic tablesuch as N, P, As, S and O, with pairs of free electrons(inorganic compounds such as the salts of arsenic, antimony,bismuth, sulphur, halogenhydric ions) or organic substanceswith double or triple bonds and pairs of free electrons,typically amines.

Anodic inhibitors. Inhibitors of this type work mainly byincreasing the overvoltage of the anodic process andencouraging the formation of surface films. These inhibitorscan be subdivided into non oxidizing inhibitors andpassivating inhibitors. Non oxidizing inhibitors modify theanodic property, encouraging the passivation of activematerials and improving the passivity of active-passivematerials. Among these inhibitors we could mention soda,borates, phosphates, polyphosphates, silicates and benzoates.These work only in the presence of oxidizing species presentin the environment or purposely added to bring the material’spotential to values within the passivity interval.

Passivators. These inhibitors exert a dual action: on theone hand they modify the anodic property whilst on theother they act as cathodic reagents, providing a sufficientlynoble cathodic process to bring the material into conditions

MATERIALS


Table 2. Inhibitors typically used in some industry sectors (Pedeferri, 2007)

Environment Example of inhibitors used

WatersPotable cooling water Deposition of CaCO3, silicates, polyphosphates, salts of zinc chromates, nitrites

(3-400 ppm), calcium polyphosphates (15-37 ppm), silicates (20-40 ppm)

Cooling circuits in vehicles Borax, sodium phosphate, mercaptobenzothiazole, benzotriazole

Condensed steam Neutralizers: ammonia, morpholine, cyclohexamine, benzylamine, long-chainaliphatic amines

Brines and sea water Brines in cooling facilities: chromates (2,000-3,000 ppm); brackish water: sodium nitrite (3-10%); boiling brines with chromates and phosphates (50-100 ppm)

Acid pickling solutionsSulphuric acid Phenylthiourea, ortho-toluene thiourea, mercaptans, sulphides (0.003-0.01%)

Hydrogen chloride Pyridine, quinoline, various amines, phenylthiourea, dibenzyl sulphoxides

Oil industryExtraction Various amines

Refining Imidazoline and derivatives

Concrete(contaminated by chlorides) Calcium nitrite, organic inhibitors

of passivity. Among inhibitors of this type we could mentionchromates (in the process of elimination in many countriesbecause they are carcinogenic), nitrites, permanganate,molybdates, tungstates etc., in addition, of course, to oxygen.The correct functioning of these inhibitors can only beguaranteed if they are present in quantities above a thresholdvalue; they may even accelerate corrosion when theirconcentration is insufficient. For this reason, these inhibitorsare considered unsafe.

Inhibitor efficiencyThe efficiency of inhibitors depends on their nature and

concentration, on the nature of the metallic material and itssurface state, the temperature and the factors influencingtheir supply to the metal surface (agitation, the shape of thestructure to be protected, the presence of cracks, etc). Theefficiency of an inhibitor can be defined as follows:

[1]

where x is the efficiency (percentage), icorr is the corrosionrate of the metal in the absence of the inhibitor and icorr-inhibis the corrosion rate of the metal in the presence of theinhibitor.

In order to function, inhibitors must be present inconcentrations above the efficiency concentration; theseconcentrations are generally lower for oxidizing inhibitors(roughly 10�3 mol/L) and higher for other types, which mustoften also act as pH correctors. The efficiency concentrationdepends on surface conditions (smooth clean surfaces needfar lower concentrations than those which are porous orcovered with corrosion products or scales) and thecomposition of the environment (the concentration ofinhibitors should be increased in the presence of specieswhich tend to block their action). The relationship betweenthe concentration of the aggressive species and the minimumconcentration of anodic inhibitor is given by an equationwhich takes account of the competitive adsorption processbetween the inhibitor and the aggressive species:

[2] logCi�KlogCa�constant

where Ci is the minimum concentration of inhibitor, Ca is theconcentration of the aggressive ion (such as chlorides), K is

a parameter linked to the valencies of the respective ions.The logCa�logCi ratio above which localized corrosionoccurs is described as critical for the aggressive species inquestion.

Fluid dynamic conditions are important because theyinfluence the mass transfer rate of the inhibitor from thesolution to the metal according to Fick’s law, valid in theturbulent regime:

[3] J �V 0.8DC

where J is the mass transfer rate, V is the flow rate and DC isthe difference in concentration of the species dissolved insolution between the metal surface and the solution itself. Ifthe solution is in motion, the inhibitor reaches the metalsurface more easily and its concentration more easilyexceeds the minimum concentration needed to protect themetal. For this reason, stagnant conditions usually require ahigher concentration of inhibitor than those in motion.

Nature of the metallic material. Certain inhibitorswork only on specific metallic materials. For example,benzotriazole and mercaptobenzothiazole are specificinhibitors for copper and its alloys, as are benzoates forsteels, fluorides for magnesium, silicates for iron, zincand magnesium, polyphosphates for steel and zinc etc.Only chromates in neutral or slightly alkalineenvironments have a fairly general inhibiting action.Table 3 reports the behaviour of the main inhibitors. Inthe case of appliances made of different materials,mixtures of inhibitors are used.

Environmental conditions. The inhibitor may have adetrimental effect if it is used under unsuitableenvironmental conditions, for instance in an incorrect pHrange or at locally insufficient concentrations as is the casefor passivators. For example, the pH value below whichsome inhibitors become ineffective is roughly 5 for nitrites,6 for benzoates and 7.2 for phosphates; the chromates have abroader use range but are most effective within the pH range8-8.5. As far as the influence of temperature is concerned,the general rule according to which the efficiencyconcentration of an inhibitor increases in line withtemperature holds true. Some inhibitors lose their efficiencyat a given critical temperature, such as polyphosphates above80°C. Table 4 shows typical corrosion inhibitors.

ξ(%) =−

100i i

icorr corr-inhib

corr



inhibitor

inhibitor

anodic inhibitor

E

A

B A

BEcor,A

Ecor,B

logjlogjcor,Alogjcor,B

cathodic inhibitor

E

Ecor,B

Ecor,A

logjlogjcor,Alogjcor,B

Fig. 1. Mechanisms by which inhibitors operate.

MATERIALS


Table 3. Efficiency of inhibitors used in neutral solutions (Pedeferri, 2007)

Metal Chromates Nitrites Benzoates Borates Phosphates Silicates Tannin

Low carbon steel Efficient Efficient Efficient Efficient EfficientFairly

efficientFairly

efficient

Cast iron Efficient Efficient Inefficient Variable EfficientFairly

efficientFairly

efficient

Zinc and its alloys Efficient Inefficient Inefficient Efficient –Fairly

efficientFairly

efficient

Copper and itsalloys

EfficientPartiallyefficient

Partiallyefficient

Efficient EfficientFairly

efficientFairly

efficient

Aluminium and its alloys

EfficientPartiallyefficient

Partiallyefficient

Variable VariableFairly

efficientFairly

efficient

Lead and its alloysfor welding

– Aggressive Efficient – –Fairly

efficientFairly

efficient

Table 4. Typical corrosion inhibitors (Pedeferri, 2007)

Metal Environment Inhibitor

Steel

Citric acidDilute sulphuric acid

Concentrated phosphoric acidBrines containing oxygen

Blends of ethylene glycol and waterSodium chloride 0.05%

Brines containing sulphidesWater

Hydrocarbons and water

Cadmium saltsAromatic amines

Dodecylamine 0.01-0.5%Methyl-, ethyl- or propyl-dithiocarbamates 0.001-3%

Trisodium phosphate 0.025%; alkaline phosphates or boratesSodium nitrite 0.2%

FormaldehydeBenzoic acid

Sodium nitrite

Aluminium

Hydrogen chloride 1%

Nitric acid 10%Phosphoric acid 20%

Concentrated sulphuric acidChlorinated water

Sea waterSodium carbonate 1%

Sodium sulphideBlends of ethylene glycol and waterChloridized aromatic hydrocarbons

Commercial ethanol

a-phenyl acridine, b-naphthoquinone, acridine, thiourea or 2-phenylQuinoline 0.003 M

Hexamethylene tetramine 0.1%Sodium chromate 0.5%Sodium chromate 5%Amyl stearate 0.3%

Sodium silicate 0.2%Sodium metasilicate 1%

Sodium nitrite or sodium molybdateNitrochlorobenzene 0.1-2%

Carbonates, lactates, acetates or borates alkalines 0.03%

Cadmium (on steel) Blends of ethylene glycol and water Sodium fluophosphate 1%

MagnesiumAlcohols

TrichloroethyleneWater

Alkaline sulphidesFormamide 0.05%

Potassium bichromate 1%

Lead Neutral solutions Sodium benzoate

Copper and brassDilute sulphuric acid

Blends of ethylene glycol and waterNeutral solutions

Benzyl thiocyanateAlkaline borates or phosphates, mercaptobenzothiazole or benzothiazole

Mercaptobenzothiazole or benzothiazole, 0.2-0.3%

Tin (on steel)Alkaline soaps

Sodium chloride 0.05%Sodium nitrite 0.1%Sodium nitrite 0.2%

TitaniumHydrogen chloride

Sulphuric acidOxidizing agentsOxidizing agents

Zinc (on steel) Distilled water Blends of calcium and zinc metaphosphates 15 ppm

9.2.2 Electrical protection(cathodic and anodic)

So-called electrical protection is based on two principles: athermodynamic principle involving the elimination of thedriving force (cathodic protection for thermodynamicimmunity or quasi-immunity) and a kinetic principle whichinvolves creating and maintaining conditions of passivity(cathodic protection for passivity and anodic protection;Lazzari and Pedeferri, 2006).

Cathodic protectionCathodic protection involves sending a current to the

surface of the metal (cathodic current) using a galvanic anodeor an impressed current system (Fig. 2). The current lowersthe metal’s potential and reduces the corrosion rate. If thecurrent (or rather the current density) is sufficient to bring themetal below its equilibrium potential, given by Nernst’sequation, conditions of thermodynamic immunity are created(E�Eeq; Eeq�E°�(RT/zF)ln[Mz�] from it can be deducedthat Eeq�E°�0.354/z, assuming a concentration of metalions of 10�6 mol/L) and the corrosion rate is zero. If thepotential is between that of free corrosion and that ofimmunity (Ecorr�E�Eeq), the corrosion rate is lowered butnot eliminated: a decrease in potential of 100-150 mV leadsto a reduction in the corrosion rate of one order of magnitudecompared to the rate of free corrosion. For example, a buriedsteel pipeline may have a free corrosion potential of �0.55 VCSE (Copper Sulphate Electrode) and a mean corrosion rateof 30-100 mm/yr; lowering the potential by 200 mV reducesthe corrosion rate by over one order of magnitude and thus tobelow 10 mm/yr. These conditions are described asthermodynamic quasi-immunity. The protection potentials for immunity and quasi-immunity of metals are shown inTables 5 and 6.

As far as materials with active-passive behaviouroperating under conditions of passivity are concerned (suchas stainless steels and the iron in concrete), two remedies canbe used to prevent or block localized corrosion caused bychlorides: cathodic protection for immunity, applicable to allmaterials, and cathodic protection for perfect passivity, whichhas the aim of maintaining the material in a passive state. Inthe latter case, applicable only to materials with active-passive behaviour, the current sent to the structure must lowerthe potential below the repassivation potential, in other wordsto within the passivity interval described as perfect passivity.It is important to distinguish between two different situations:

the material is passive (because the chlorides have notreached a critical concentration) or localized pitting corrosionhas already set in. In the former case, it is possible to preventthe onset of pitting even after the critical chlorideconcentration has been reached by bringing the metal’spotential below the pitting potential. This particular form ofpreventive cathodic protection is known as cathodicprevention. On the other hand, if pitting has begun, it is nolonger sufficient to bring the potential below the pitting



I I� �

metallic conductor

metallicstructure

galvanicanode

A

metallicstructure

anode

DC feeder

B

Fig. 2. Scheme showing the functioning of cathodic protection: A, with galvanic anodes; B, impressed current.

Table 5. Theoretical protection potential for immunityof some metals (Lazzari and Pedeferri, 2006)

Metals

Protection potential for immunity(V SHE at 25°C)

pH�0 pH�7 pH�14

SilverCopperLeadIronAluminium

+0.44�0.14�0.31�0.62�1.6

�0.44�0.14�0.31�0.62�1.6

�0.32�0.38�0.74�0.92�1.9

SHE, Standard Hydrogen Electrode

Table 6. Protection potential for quasi-immunityand passivity used in the earth and in sea water

(Lazzari and Pedeferri, 2006)

Metallicmaterials

Earth Sea water

(V CSE) (V SCE) (V Zn)

Carbon steels:– normal conditions– anaerobic

conditions– in concreteCopper and its alloys

Lead

ZincAluminiumStainless steels

�0.85

�0.95�0.75

�0.45 to�0.60

�0.50 to�0.65�1.00�0.8�0.40

�0.80

�0.90�0.70�0.50

�0.45 to�0.60�1.10�0.9�0.50

�0.25

�0.15�0.35�0.55

�0.60

0�0.15�0.55

SCE, Saturated Calomel Electrode

potential; it must be brought below the repassivationpotential.

To create conditions of protection, adequate protection ofcurrent density must reach the surface of the metal. Table 7reports the values for the protection of current densities in theprincipal environments. Protection of current density Jprotdepends on the velocity Ic with which, at the protectionpotential, cathodic processes are produced on the structure’ssurface; like Ic, it therefore depends on environmentalconditions. For example, if the corrosion process takes place, asis the case in the earth and in water, under oxygen diffusioncontrol conditions, protection of current density depends on allthose conditions which determine the limit velocity of oxygensupply: a) the concentration of dissolved oxygen; b) the state ofagitation in the environment; c) the temperature; d) the presenceof deposits or coatings. In sea water, for example, depending onvariations in agitation, temperature and oxygen concentration,protection of current density may pass from the 20 mA/m2

required to protect structures immersed in mud on the sea floorto the 1 A/m2 and above needed to protect uncovered parts ofships close to the propellers, in other words under conditions ofmaximum agitation and oxygenation. A beneficial effect, foundboth in the earth and in natural waters, is provided byprecipitation, as a consequence of local alkalinization processes,carbonates and corrosion products on the metal surface, whichreduce the exposed surface of the metal.

Natural environments and all those in which the cathodicreaction is oxygen reduction are particularly well-suited tothe application of cathodic protection because the protectioncurrent is identical to the diffusion limit current of oxygen,which is never particularly high. Other environments, for theopposite reasons, are not well-suited to the application of thismethod; examples include acid environments, where thecathodic reaction is the release of hydrogen and theprotection currents are two or three orders of magnitudehigher than the diffusion limit current for oxygen.

Cathodic and anodic reactionsThe nature of cathodic processes depends on the environmentin which the structure to be protected is found. In natural

environments (the earth, waters) and in concrete, the maincathodic reaction is oxygen reduction:

[4] 1/2O2�H2O �2e��2OH� (cathodic process)

When the potential is lower than the equilibrium potential forthe release of hydrogen (Eeq,H), the reaction is accompaniedby the following process:

[5] 2H2O �2e��H2�2OH�

Both reactions lead to an increase in pH on the surfaces onwhich they occur.

Galvanic anodes. The anodic reaction consists of thedissolution of the metals forming the anode (Zn, Mg, Al); thismay be followed by hydrolysis phenomena with theseparation of hydroxides in the solid phase and theconsequent acidification of the environment which plays animportant role in avoiding phenomena of anode passivation,especially when aluminium anodes are used. In the case ofthe soluble anodes used in impressed current systems, such asscrap iron, the anodic reaction is the dissolution of the metal.

Inert anodes. In impressed current systems, where inertor insoluble anodes are used, the anodic reaction generallyentails the release of oxygen:

[6] 2H2O��O2 � 4H� � 4e�

In the presence of chlorides, this reaction is accompanied bythat which releases chlorine, becoming predominant in thecase of sea water:

[7] 2Cl��Cl2�2e�

The release of oxygen causes acidification around the anode;this may significantly increase the aggressiveness of theenvironment with respect to the anodic material.

Insulating coatingsIf an insulating coating is present, the protection current

may fall to a tiny percentage of that required by the baremetal, since the current exchange takes place on areas notcovered by the coating, where pores, defects or damage to thecoating are present. This can be expressed by the formula:

[8] jR� j0(1 �x)

where j0 is the protection of current density of the bare metal,jR is the protection of current density of the coated structure,x is the efficiency of the coating defined as the unit fractionof the coated surface. The efficiency of a coating varies overtime; for example, for buried pipes which are not subjected tomaintenance it may fall to 90% after 10-20 years. This agingprocess is further accelerated for structures in contact withsea water; for example, on the hull of a well-coated ship (fivelayers of paint) the efficiency of the coating may pass from99.9 to 99% in the first year and subsequently fall below98%, even if maintenance work is carried out.

Calcareous depositsMetallic structures in sea water which are protected

cathodically are covered by a protective layer consistingmainly of calcium carbonate and magnesium hydroxide,commonly known as a calcareous deposit. Its formation,especially on bare structures, is providential, since it reducesthe protection current by an order of magnitude. Thecalcareous deposit exerts a dual protective effect: it forms abarrier which limits the diffusion of oxygen and it keeps the

MATERIALS


Table 7. Approximate values for protectioncurrent densities in natural environments


EnvironmentProtection

current density (mA/m2)

Bare metal

Neutral aerated earthDamp earthEarth (hot pipelines)Dry concrete (in air)Water-saturated concreteFresh waterHot waterSea waterChemical environments (acidic)

20-1505-2030-605-150.2-2

30-16050-16050-550

50-1,500

Coated metal

EarthSea water

0.01-10.1-10

pH on the metal surface alkaline and thus tends to passivateit. The mechanism governing the formation of calcareousdeposits can be broken down into the following broadphases: alkalinization of the cathodic zone as a result ofcathodic reactions (oxygen reduction and hydrogen release);shift in the carbon equilibrium and precipitation ofcarbonates; precipitation of hydroxides. When protection isinterrupted, the calcareous deposit begins to dissolve andgradually disappears over time, following an inversesequence to that of formation.

Comparison of the applications of cathodic protectionThe most widespread applications of cathodic protection

concern structures placed in natural environments. Galvanicanodes are used in environments with high conductivity,such as sea water, and may even be appropriate inenvironments with low conductivity where small currents areneeded as in the earth and for the cathodic protection ofreinforced concrete. Impressed current systems are requiredin resistive environments, such as the earth and concrete, andare preferable for the protection of large structures. Asignificant benefit is that the system has considerableoperating flexibility since it is possible to vary and regulatethe current supplied. The advantages and limitations of thesetwo applications are shown in Table 8.

Cathodic protection with galvanic anodesThis type of protection was first applied by Humphrey

Davy in 1824 to protect copper using iron and zinc anodes,in other words less noble materials. For structures in carbonsteel, numerous materials meet this condition, as can be seenby examining the electrochemical series of the elements;however, practical applications involve the use of alloysbased on aluminium, magnesium and zinc.

Two parameters characterize an anode: the potential andthe current capacity. The potential of the anode determines

the driving force and thus the minimum number of anodesneeded to obtain protection; the current capacity, in otherwords the charge supplied per unit of weight, determines theconsumption of the anodes and therefore the mass needed toensure that protection continues over time. The currentcapacity or its inverse, theoretical consumption, are obtainedfrom Faraday’s laws: Dm�echemq/F where Dm is the masspassing into solution, echem is the electrochemical equivalentgiven by the atomic weight divided by the charge, q is thecharge circulated, F is Faraday’s constant, equivalent to96,485 C mol�1. Expressing consumption in kg/A�yr givesDm�0.33echem. The theoretical capacity is generallyexpressed in A�h/kg. Table 9 shows the consumption andtheoretical capacity of Al, Zn and Mg.

The current supplied by a galvanic anode is given byOhm’s law (I�DE/R) where DE is the driving force, in otherwords the energy available to overcome ohmic drops,obtained by subtracting the overvoltages at the electrodes (haand hc) from the system’s electromotive force, and R is thetotal resistance, R�Ra�Rcable, where Rcable is the resistanceof the metallic circuit, generally negligible, and Ra is theohmic resistance of the electrolyte localized at the anode.The latter is calculated using the empirical formulaereported in cathodic protection manuals (Eni-Agip, 1994;Pedeferri, 2007; NACE, 2002). For elongated anodes, themost widely used formula is Dwight’s formulaRa�r[ln(8L/d)�1]/2pL, where r is the resistivity of theenvironment, L the length of the anode and d the equivalentdiameter. For supported compact anodes, resistance is givenby the empirical formula Ra�r/(a�b), where a and b are thedimensions of the anode (length and width).

The driving force DE is given by the difference betweenEa, the practical potential of the anode, also known as thedriving potential, and Ec, the protection potential. Forexample, for steel structures the driving force DE has a valueof 250 mV for zinc anodes, 300 mV for aluminium anodes



Table 8. Comparison between galvanic anode systems and impressed current systems (Lazzari and Pedeferri, 2006)

Galvanic anode system Impressed current system

advantages

• Does not require a generator• Does not require current regulation• Easy to install• Does not cause problems of interference• The number of anodes may be increased after

start-up• No maintenance costs• Makes it possible to obtain a good distribution

of uniform current• The installation of anodes is inexpensive if carried

out during construction or start-up• No problems of use permits for areas around

the structures

• Can be designed for a broad range of voltages and currents

• Every anode or groundbed can deliver a high current• A single installation can protect very large surface

areas• Voltage and current may be varied• Can be used in environments with high resistivity• Efficient for the protection of bare or poorly coated

structures

limitations

• Modest driving force• Modest current delivery• Installation may be expensive if carried out after

start-up• Bare or poorly coated structures require large

numbers of anodes• Inefficient in environments with high resistivity

• Causes problems of interference• Subject to generator breakdown and vandalism• Requires periodic inspections and maintenance• Requires a current generator• Entails operating costs• Superprotection conditions may arise with damage

to coatings or embrittlement by hydrogen• Cables and cable-anode connections subject

to breakage

and 800 mV for magnesium anodes. Table 10 shows the typesof anodes which should be used as a function of theenvironment’s resistivity.

For applications in the earth, a backfill is used to createoptimum conditions for the anode to function, preventing itspassivation (for example, zinc anodes may become passive inthe presence of sulphides, aluminium anodes in allenvironments which do not contain chlorides) and keepingresistivity around the anode low. A typical composition is amixture of chalk, bentonite and sodium sulphate in a ratio of70:20:10 (in weight).

From an economic point of view, aluminium alloys arepreferable; the cost of producing an identical charge, bearingin mind practical consumption and the average cost per unitmass, is 100 for Al alloys, 300 for Zn alloys and 750 for Mgalloys.

Calculating the number of anodes. For a structure to beprotected, not only is it necessary to establish the type ofanodic material but also the number and mass of the anodesand their distribution. There is no single solution to thisproblem: the number of anodes may vary depending on thetype of anode chosen, the duration of protection and the typeof structure. The solution to be sought is the cheapest.

The number of anodes cannot be calculated by simplydividing the total mass of the anodes (calculated on the basisof the required duration) by the mass of a single anodechosen from a catalogue, without checking the deliverycapacity. In this case, the total anodic mass will be sufficientfor the required duration of protection but may be unable todeliver the current needed for protection, which does notdepend on mass but on the anode’s surface area. In otherwords, the mass/surface area ratio of the anodes must beoptimized to ensure both protection and its duration.

The general procedure used to calculate the number ofanodes involves calculating the total protection current(multiplying the total surface area to be protected by theprotection of current density) and calculating the total massof the anodes (multiplying the total protection current byanodic consumption by duration). After choosing an anode(mass and dimensions) the delivery capacity is checked bothduring the initial phases and at the end of the project. If theoutcome of this check is not satisfactory, the calculation isrepeated with a different anode.

Impressed current systemsIn impressed current systems, the current is supplied by

an external direct current generator, through a groundbedable to deliver a current to the environment.

Anodic materials. The exchange of current between thegroundbed and the environment takes place through ananodic reaction which depends on the anodic material and theenvironment. For example, in the case of carbon steel anodes,the anodic reaction is the dissolution of iron with theconsumption of the anode; for so-called insoluble anodesmade of platinated titanium, graphite etc., the reactions mayrelease oxygen or chloride depending on the environment andthe supply current density. Table 11 shows the commercialcompositions, consumption and working conditions of themain anodic materials.

Delivery voltage. Since a current generator is used, thedesign must above all take into account a calculation of theminimum delivery voltage which, on the basis of electricaland electrochemical considerations, is identical to the so-called cell voltage, given by Vmin�y*�IRtot where I is thetotal protection current circulating in the electrical circuit andis a project datum; Rtot is the total resistance of the circuit(calculated or set); y* represents the sum of thethermodynamic and kinetic contributions of electrodereactions, negligible if soluble iron anodes are used but equalto 2-3 V in the case of inert anodes.

Types of groundbed. In sea water and inside appliances,anodes are used without backfill, in other words exposing theanodic material directly to the environment. The anodes maybe of different shapes depending on specific requirements,especially for appliances; in these cases the design of theanode structure is conditioned not only by current supplyrequirements but also by considerations of a mechanicalnature, such as how they are fixed to the structure.

In the earth, on the other hand, the sizing of thegroundbed is determined mainly by the need to obtain lowanodic resistance, usually lower than 2 W; this is due in partto the limitations imposed, for safety reasons, on the feedvoltage, which must not normally exceed 50 V. To reach thisobjective, backfills consisting of coal dust are used toincrease the effective size of the groundbed and thus decreaseits resistance. In practice, three types of groundbed are used

MATERIALS


Table 9. Physical and electrochemical propertiesof magnesium, zinc and aluminium


Mg Zn Al

Atomic weightSpecific mass at 20°C (g/cm3)Equivalent weight

24.321.7412.16

65.387.1432.69

26.972.708.99

Theoretical consumption– kg/A�yr– dm3/A�yr

3.982.3

10.691.5

2.941.1

Theoretical capacity A�h/g*E° potential (V SHE)

2,200�2,363

820�0.762

2,980�1.662

*, the relation linking theoretical capacity to theoretical consumption is:consumption (kg/A�y)�theoretical capacity (A�h/kg)�8,760

Table 10. Galvanic anodes for natural watersand earths as resistivity varies

Anodic materialResistivity (W�m)

Waters Earth

Aluminium Up to 1.5 Not used

Zinc Up to 5With backfill

up to 15

Magnesium Above 5 Not used

Magnesium (�1.5 V CSE)with backfill Up to 40

Magnesium (�1.7 V CSE)with backfill Up to 40-60

in the earth: horizontal, shallow vertical and deep vertical. Asingle groundbed may contain one or more anodes,depending on two requirements: duration and resistance.

Calculation scheme. Sizing has the aim of determiningthe type and dimensions of the groundbeds and thecharacteristics of the power source. Often, the technicalsolution adopted is imposed by external conditions: forexample, as far as the type of groundbed is concerned, inurban areas it is preferable to use deep vertical groundbedswhich do not require land use permits. The generalcalculation procedure involves first calculating the protectioncurrent (with the attenuation method when dealing withpipelines or as a product of the total surface area to beprotected and the protection of current density); this isfollowed by the choice of the groundbed and the calculationof total resistance Rtot as the sum of the resistance to earth Ra(anodic resistance, for example using Dwight’s formula) andthe resistances of the conductors (anodic and cathodic). Italso involves the calculation of the minimum feed voltageV�IRtot�y*, where y* is the thermodynamic contributionand I is the maximum required protection current, and finallya check on the anodic supply and duration of the anodes.

Anodic protectionThis is applied exclusively to materials with active-

passive behaviour and involves sending a current from themetal to the solution (anodic current) using a cathode with animpressed current system which keeps the potential constant(potentiostatic conditions). The current causes an increase in

the metal’s potential and an initial increase in the corrosionrate until the passivation current density is exceeded, thematerial is passivated and the anodic current falls to the valueof passivity current density. Under these conditions thematerial is passive and remains so as long as the potential iskept within the passivity interval. Typical applications foranodic protection are the protection of carbon steel insulphuric acid and ammonia solutions, of stainless steels insulphuric acid, mixtures of sulphuric and nitric acid,phosphoric acid, soda and of titanium in hydrogen chloridesolutions. Table 12 shows the operating conditions for anodicprotection in some environments. Unlike cathodic protection,anodic protection involves the use of a special power sourceknown as a potentiostat, able to keep the metal’s potential at aconstant level. Another peculiarity is the high initial currentdensity required to create passivity and the passivity currentdensity which is far lower, by as much as a few orders ofmagnitude. This makes it necessary for the power source tosupply a high initial voltage, unnecessary during normaloperation. To avoid the system being excessively oversized,the partial and progressive passivation technique is oftenadopted, for example during the gradual filling of tanks.

9.2.3 Metallic materials

Structure and propertiesMetals are solids composed of atoms arranged in a

uniform way within the crystal lattice which repeats one



Table 11. Main anodic materials for impressed current cathodic protection and their use(Lazzari and Pedeferri, 2006)

Anodic material Description and compositionUse and anodic current density (A/m2)

Earth Fresh water Sea water Marine mud

Iron Steel or cast iron 5 NR NR NR

GraphiteGraphite impregnated with oil, resins

or wax2.5/10* 2.5 20 2.5

Iron-siliconCast iron with 14% Si, 0.75% Mn,

0.95% C10* 10 NR NR

Iron-silicon-chromiumCast iron as above with the addition

of 4.5% Cr10* 10 15 NR

Lead-silver Pb with 2% Ag NR NR 32-65 NR

Lead-silver-antimony Pb, 1% Ag, 6% Sb NR NR 50-200 NR

Magnetite Non stoicheiometric Fe3O4 20 20 60 20

Platinum on TiDeposit of platinum or its alloys,

on titanium 100* 150 500 NR

Platinum on NbPlatinum-niobium deposit

as base metal100* 150 500 NR

Anodes with plasticsupport

Loaded polymersor coated with conductive layer

0.5 0.5 NR NR

MMO activatedtitanium

Deposit of mixed oxides of noblemetals on titanium as a base metal

50/100* 150 600 100

NR, Not Recommended; *, with a backfill of carbon coke

elementary unit. The bond strengths which hold the atomstogether inside the crystal lattice form the metallic bond,characterized by a fairly high bond energy (from 100 to 800kJ/mol) and by the presence of moving electrons which givemetals excellent electrical and thermal conductivity. Thecrystal lattice of metals may be: Body-Centred Cubic (BCC),Face-Centred Cubic (FCC) and HExagonal Compact (HEC;Fig. 3). Metals consist of numerous crystal grains, each ofwhich is an ordered lattice, separated by the grain boundaries.The type of crystal lattice and the bond strength (which risesas the melting temperature increases) determine the metal’smechanical properties, such as its tensile strength, ductilityand toughness. In general, the tensile strength follows thesequence BCC�HEC�FCC, whilst ductility follows almostthe inverse order FCC�BCC�HEC. As well as by the crystalstructure, mechanical properties are heavily influenced by theability to block the movement of the linear defects presentinside the crystal lattice, known as dislocations, so that ametal is ductile when the dislocations can move freely (e.g.metals with an FCC structure such as copper, aluminium andaustenitic stainless steels), whilst it becomes brittle when thedislocations are blocked (e.g. metals with a BCC structure inthe presence of precipitates, as in tempered steels).Resistance is improved through so-called reinforcementmechanisms which have the fact that they block thedislocations in common. The main mechanisms are based on:a) the size of the crystal grain; b) solid solutions; c)precipitates dispersed in the lattice; d ) cold working.Decreasing the size of the crystal grain leads to an increase inthe grain boundaries which act as an effective obstacle to themovement of the dislocations. The increase in mechanicalresistance is inversely proportional to the square root of the

size of the grain (Hall-Petch law). Solid solutionstrengthening is obtained by replacing some atoms in thecrystal lattice with other metals. Three situations are possible:the substituent atom has the same size as the substitutedatom, or is larger or smaller. In the first case, the mechanicalproperties remain identical, whilst in the other two cases theyimprove since the crystal lattice is distorted, forming anobstacle to the movement of the dislocations. The precipitates

MATERIALS


FCCFace-Centered

Cubic

HECHExagonal

Compact

BCCBody-Centered

Cubic

Fig. 3. Types of crystal lattice in metals.

Table 12. Operating variables for the anodic protection of different materials under different environmental conditions(Riggs and Locke, 1981; Lazzari and Pedeferri, 2006)

Material EnvironmentTemperature

(°C)Critical Cd

(mA/m2)Passivity Cd

(mA/m2)

AISI 304

H3PO4 (115%)H3PO4H3PO4

HNO3 (80%)HNO3

H2SO4 (67%)H2SO4

NaOH (50%)

24821772482248225

0.150.365025120

5,100469

1.51.5

22,000310

3,100930

2,9004.4

AISI 316

H3PO4 (75-80%)H3PO4H3PO4

H2SO4 (67%)H2SO4H2SO4

104121135246693

–––

5,00040,000110,000

140,000350,000440,000

100300900

C steel

OleumH2SO4 (96%)

H2SO4H2SO4

25274993

1.1–––

44�106

1.1�104

1.2�105

1.1�106

20 alloy H2SO4 (50%) 120 – 1�104

Titanium H2SO4 (40%) 60 200 200

CD, Current Density

obtained with heat treatments represent excellent obstaclesto the movement of the dislocations, especially when theyare very fine and dispersed. Cold-working involves coldplastic deformation, which produces a considerable increasein the number of dislocations present in the grains, whichtherefore hinder one another. Generally speaking, theimprovement in tensile strength traction obtained withreinforcement mechanisms is accompanied by a decrease inductility and toughness.

Mechanical properties are determined by mechanicaltests: a) the tensile test to determine the tensile strength(yield strength and breaking strength), ductility and theelasticity module; b) the hardness test to measure hardness,which is correlated with the breaking strength; c) theresilience or impact test to measure resistance to impactwhich is correlated with toughness; d ) the mechanicalfracture test to determine the fracture toughness.

Carbon steels and low alloy steelsSteels are iron-carbon alloys with the addition of other

elements in concentrations below 5% which have a BCCstructure. Carbon steels essentially consist of Fe-C (the carboncontent is below 0.8%) with the presence of Si, Al and Mn. Anincrease in mechanical resistance is obtained both byincreasing the carbon and manganese content and with heattreatments (annealing, normalizing, quench and tempering).These have the aim of creating a fine microstructure offerritic-pearlitic or bainitic type which gives steel excellentmechanical properties and good toughness.

Low alloy steels, in addition to C, Si, Al and Mn, alsocontain other elements such as Cr, Ni, Mo in concentrationsbelow 5% and extremely small amounts of V, Nb and W;with suitable heat treatments, these elements give steelproperties of excellent mechanical resistance associated withexcellent toughness.

Steels are classified on the basis of their chemicalcomposition or their mechanical or use characteristics. TheEuropean classification system (UNI-CEN) grades steels onthe basis of their mechanical properties using the symbolFeXXX where XXX is the breaking strength in MPa (forexample, Fe510 is a carbon steel with a breaking strength of510 MPa); or on the basis of chemical composition using thesymbol CXX for carbon steels (XX�100�% C; for example,C20 is a carbon steel with a carbon content of 0.2%). Lowalloy steels are graded using the symbol XX-elements-Y(where the carbon content C is XX/100 and the content ofother elements is given in decreasing percentages byY/constant, where this constant is: 4 for Ni, Cr, Mn, Si, Co,W; 10 for Al, Mo, Nb, Ti, Cu, V; 100 for N, P, S; 1,000 for B(for example, 40NiCrMo4 is a low alloy steel containing0.4% C, 1% Ni and less than 1% Cr and Mo); 30CrAlMo510is a low alloy steel containing 0.3% C, 1.25% Cr, 1% Al andless than 1% Mo).

The AISI (American Iron and Steel Institute)classification makes use of a 4-figure symbol NNXX whereXX�100�% C. The number NN represents the type ofsteel; for example, carbon steels are designated by the series10XX, 11XX (with S being specified), 12XX (with P beingspecified), 13XX (steels with an Mn content of 1.6-1.9%).Low alloy steels are: 40XX C steels with Mo (Mo 0.2-0.3%); 41XX C steels with Cr-Mo (Cr 0.8-1.2%;Mo 0.15-0.25%); 43XX C steels with Ni-Cr-Mo (Ni 1.6-2.0%; Cr 0.8-1.2%; Mo 0.15-0.25%).

The API (American Petroleum Institute) classificationdesignates steels according to use. For example, steels usedfor pipelines are designated by the symbol API 5L XYYwhere YY is the yield strength in kpsi, for example, API 5LX42. Steels used for oil wells are designated with the nameAPI grade symbol XX where XX is the yield strength inkpsi, for example API L80. The ASTM (American Societyfor Testing and Materials) and ASME (American Society ofMechanical Engineers) classifications and the unifiedclassification UNS (ASTM E527, 1983) are also used.

An important property of low alloy carbon steels isweldability, in other words the ease with which they can besubjected to welding processes without side effects such asthe formation of cracks and variations in microstructure.Weldability is given by the Carbon Equivalent (CE),determined by the composition of the steel using theformula:

[9]

where the symbols indicate the percentage content of theelements. To ensure good weldability in a steel, the CE mustbe lower than 0.45.

Alloyed steels and stainless steelsAlloyed steels are iron alloys with the addition of other

elements in percentages above 5%. The structure of alloyedsteels depends on their composition and may be either BCCor FCC. Stainless steels are alloyed steels with a minimumCr content of 12%. This content is the minimum needed togive steel the property of stainlessness, in other words thecapacity to cover itself with an adherent and protectiveoxide. The addition of other elements (such as Ni) makes itpossible to improve other properties such as mechanicalresistance and ductility. The structure and type of stainlesssteel obtained, depending on its composition, are specifiedby the Schaeffler diagram in Fig. 4. Ferritic and martensiticsteels have excellent mechanical properties but low ductility;austenitic steels have excellent ductility. Duplex stainlesssteels have a mixed austenitic-ferritic structure and have thebest mechanical properties and corrosion resistance.

Stainless steels are classified on the basis of theirchemical composition or structure. The Europeanclassification system grades stainless steels on the basis of

CE CMn Cr + Mo + V Ni + Cu= + + +6 5 15



5

10

15

20

25

2010 30

50%AISI 304

martensite

austenite

ferrite

duplex

Fig. 4. Schaeffler diagram for the definition of the microstructure of stainless steels depending on composition.

chemical composition using the symbol: X (100�% C) CrNi(%%) (for example X10CrNi18-8 is an austenitic stainlesssteel 18-8 containing 0.1% carbon, 18% chromium and 8%nickel). The AISI classification is based on the structure ofthe steel and uses a symbol with three numbers. The 200series comprises austenitic steels with Mn; the 300 seriescomprises austenitic steels, for example AISI 304 (18%Cr-8% Ni) and AISI 316 (18% Cr-10% Ni-3% Mo) the bestknown austenitic stainless steels; the 400 series comprisesthe ferritic and martensitic stainless steels; the P-H(Precipitation Hardening) series comprises those stainlesssteels which have been hardened with a heat precipitationtreatment.

Metallic materials for high temperaturesThe resistance to hot oxidation of steels increases with

the chromium content. Table 13 summarizes the grades ofcarbon steel, low alloy and alloyed steels resistant to hotoxidation.

Breaking mechanisms of metallic materialsThe choice of materials must take into consideration the

breaking mechanisms, the potential for undertaking nondestructive checks, monitoring and inspections and theireffects on the safety and reliability of the facility.

ThinningThis type of decay is characteristic of the corrosion

processes which are triggered when even slightly acidicliquids come into contact with metal surfaces lackingprotective oxides. The most common example is carbonsteels on which the film of iron oxide does not ensuresufficient protection given its porosity and mechanicalinstability.

Thinning is infrequent in active-passive materials suchas stainless steels which, when used correctly, are protectedby a film of chromium oxide which is chemically resistant,lacking porosity and mechanically fairly stable. More or lesssevere thinning problems occur at high temperatures, when

the breaking temperatures of the oxide scale formed whenhot are exceeded.

Also subject to thinning are steels exposed to the ashesproduced by the combustion of coal and hydrocarbons with ahigh sulphur, sodium and vanadium content, when thetemperature of the metal exceeds 500°C (for example, steamsuperheating pipes, furnace pipes, hooks and their supports).Severe attack is also possible on high alloy steels, such as the 20 alloy (25Cr-20Ni) which thins at a rate of 0.7 mm/yr.Type 625 Ni alloys and the alloy 50Cr-50Ni are resistant (0.1 mm/yr and 0.05 mm/yr respectively).

Thinning occurs alongside the erosion-corrosion orabrasion phenomena which emerge in the presence offluids (liquids or gases) under high velocity conditions,worsened by significant quantities of solids. The APIRP14E standard specifies the critical velocity for erosion-corrosion (in the absence of suspended abrasive solids)for different classes of materials; it is proportional to aconstant C characteristic of each metal (40 for copper, 60for copper-nickel 70-30, 120 for carbon steels, 500 forstainless steels).

The corrosion rate for the different types of thinning isoften acceptable (for example 1 mm/yr) and remainsconstant over time as long as the operating conditions arekept constant. This makes it possible both to predict howcorrosion will progress and to monitor it by measuring thethickness in those areas of the facility thought to beimportant. In other words, unless operating conditions vary,thinning does not lead to sudden or unpredictable criticalsituations.

PerforationsThese are typical of localized corrosion attacks which

occur when the conditions for macrocouple corrosionemerge:• On active materials: galvanic corrosion (very small

anodic area and very large cathodic area), corrosion bydifferential aeration (or beneath a deposit), corrosion byelectrical interference (stray currents).

• On active-passive materials: pitting corrosion, crevicecorrosion.

Pitting corrosion takes on special importance for stainlesssteels in the presence of chlorides. It is important to payparticular attention to non operating conditions which maycause unforeseen attack. A typical example is hydraulictesting carried out using water with a limited chloridecontent when the water is not completely discharged (oftendue to negligence); this may lead to stagnant conditions andconcentration of the chlorides, the worst conditions for theonset of pitting.

Fracture of the materialsFractures may occur for purely mechanical reasons,

especially in the case of brittle fractures in steels attemperatures below the ductile-brittle transition temperatureor due to fatigue. More frequently it occurs due to stresscorrosion (SCC, Stress Corrosion Cracking, see Chapter 9.1)and the effects of intergranular corrosion, for example insensitized stainless steels. In these cases the fractures occurcatastrophically and without warning. It is thereforenecessary to choose the right material for the operatingconditions, the environment and variations in its compositionover time.

MATERIALS


Table 13. Formation temperature of scaleson steels used for high temperature applications

Scale formation temperatures in hot air(�3 mm/yr)

Materials Temperature (°C)

C steel (0.1% C) 480

5Cr-0.5 Mo 620

7 Cr-0.5 Mo 650

9 Cr-1.0 Mo 680

12 Cr (AISI 410) 760

27 Cr (AISI 446) 1,030

AISI 304, 321, 347 (18 Cr – 8 Ni) 900

AISI 316 (18 Cr-10Ni-3 Mo) 900

AISI 310 (25 Cr-20 Ni) 1,150

Choosing metallic materials for wells and petrochemical plants

Steels for high temperaturesFor furnace pipes (temperatures up to 650°C) low alloy

steels containing Cr-Mo are used. Cr improves resistance tocreep due to the formation of dispersed carbides whosegrowth at temperatures above 500°C is prevented by thepresence of Mo, reducing its susceptibility to embrittlement(known as temper brittleness). The main types of steel aresummarized in Table 14. Steels used at high temperatures aresubject to two phenomena: creep and hot oxidation. Design isbased on behaviour in relation to creep using the Larson-Miller parameter of the steel adopted. The size of the crystalgrains influences creep resistance: the larger the grain size,the better the resistance to creep (whilst the mechanicalproperties and toughness worsen). However, the effect of thedispersion of precipitates (carbides, nitrides, intermetalliccompounds) prevails with respect to grain size.

Carbide instability. Carbides play an important role inblocking the viscous creep of dislocations, preventing cross-slip and climb. However, they may be subject to phenomenaof spheroidization and graphitization which decrease theireffects. Fine grain killed steels are subject to spheroidizationat temperatures above 485°C. The addition of Cr and Mostabilizes the carbides.

Steels which have operated at around 450°C may presentbrittle behaviour at ambient temperature (tempered creep).The addition of Mo reduces this risk.

Ferritic stainless steels containing �13% Cr may becomebrittle if they operate for long periods at temperatures above480°C due to sigma phase precipitation (BCC structure withCr between 40 and 60%). Austenitic stainless steels are alsosubject to the formation of the sigma phase which rendersthem brittle. AISI 309 is particularly susceptible whilst 304 isvirtually immune. Long-term operation at temperaturesbetween 550 and 870°C causes the sensitization of austeniticstainless steels with the precipitation of carbides at the grainboundary (and subsequent intergranular corrosion).

Overheating. This may lead to cracks due to localizedcreep, modifications in microstructure and acceleratedoxidation. The first two mechanisms lead to plasticdeformation with blistering and thinning whilst oxidationcauses generalized thinning.

Hydrogen attackHydrogen atmospheres at temperatures over 200°C and

pressures above 7 bar cause hydrogen damage with theformation of blisters and decarburation of the steel(worsening its mechanical properties). Cr-Mo steels areresistant to hydrogen damage (carbide stability). Resistanceconditions are verified using Nelson curves (Fig. 5). Theformation of blisters and cracks is due to the production ofmethane as the hydrogen reacts with (free) carbon.

In the presence of acid and hydrogen sulphide (H2S)attack, the atomic hydrogen penetrates the crystal lattice ofiron, causing:• Step Wise Cracking (SWC) or blisters. This phenomenon,

often known as HIC (Hydrogen Induced Cracking),occurs in C-Mn steels even in the absence of tensilestresses.

• Hydrogen embrittlement in susceptible materials (highresistance steels) in the presence of tensile stresses abovea critical threshold.

SCC in soda and alkaline environmentsCarbon steel suffers from SCC according to the anodic

dissolution mechanism in strongly alkaline environments(pH�12) when stress is close to the yield limit and the



tem

pera

ture

(°C

)

700

200

300

400

500

600

partial pressure pH2 (bar)

carbon steel

0.5Mo

10050

core surface

-

-

-

-

-

Fig. 5. Nelson curves for low alloy steels in a hydrogen atmosphere (Graver, 1985).

Table 14. Steels used in petrochemical plants

Designation Cr (%) Mo (%)ASTM A182

MouldedASTM A335

PipelinesASTM A387

Sheets

0.5Cr-0.5 Mo 0.5-0.8 0.45-0.65 F1 T1 Grade 2

1Cr-0.5 Mo 0.8-1.25 0.45-0.65 F12 T12 12

1.25Cr-0.5 Mo 1.00-1.50 0.45-0.65 F11 T11 11

2.25Cr-1 Mo 1.90-2.60 0.8-1.1 F22 T22 22

3Cr-1 Mo 2.65-3.35 0.8-1.1 F3 T3 21

5Cr-0.5 Mo 4.00-6.00 0.45-0.65 F5 T5 5

7Cr-0.5 Mo 6.00-8.00 0.45-0.65 F7 T7 –

9Cr-1 Mo 2.65-3.35 0.9-1.1 F9 T9 –

temperature is above 50°C. Even stainless steels, Monel andNi alloys are vulnerable to SCC in soda at high temperaturesif they are subjected to high stress. The choice of materials isbased on use maps, like that shown in Fig. 6.

S/H2S corrosionThe steels used in petroleum refining processes are

subjected to severe corrosion attacks at temperatures over260°C when the sulphur content (like elementary sulphur, inH2S and in mercaptans) is high. The severity of the attackdepends on the sulphur content, the temperature and thepresence of gaseous hydrogen.

At temperatures above 150°C, mixtures of hydrogen andsulphur compounds lead to the formation of H2S, becomingsignificant above 260°C and peaking at 370°C. C steel haslow resistance but the addition of Cr improves resistance toattack: e.g., at 370°C, for H2S partial pressures higher than 1 bar, carbon steel has a corrosion rate of 3 mm/yr; 5Cr-0.5Moof 2 mm/yr; 7Cr-0.5Mo of 1 mm/yr; 9Cr-0.5Mo of 0.5 mm/yr; 12Cr and 18Cr of 0.1 mm/yr. Resistance tocorrosion depends on the formation of a protective sulphide,which becomes more effective as the Cr content increases.

H2/H2S corrosionIn H2/H2S atmospheres, Cr-Mo steels cannot withstand

temperatures above 315°C. It is necessary to use ‘calorized’austenitic stainless steels, the surface of which has beenenriched with Al, of type AISI 304 (18Cr-8Ni), 316(18Cr-10Ni) or superaustenitic steel (20Cr-32Ni) which havecorrosion rates of less than 0.25 mm/yr at 500°C.

Corrosion by naphthenic acidsWhen organic acids are present in crude oil, especially

those with a naphthenic structure, carbon and low alloysteels are subject to corrosion at temperatures between 200

and 400°C with a peak at 275°C (Corrosion […], 1998). Thecorrosion attack is often associated with erosion-corrosionphenomena due to high turbulence. The corrosionmechanism entails the formation of iron complexes with theorganic acids.

Carbon steel performs well at temperatures below220°C; at higher temperatures resistant materials like castiron and Cr-Mo steels with a chromium content of up to 12%are used. In the case of severe attack it is necessary to usestainless steels of types AISI 316 309 and 310; AISI 304 andits derivatives offer lower resistance. Monel Inconel andHastelloy B are suitable, but attention must be paid to thepresence of S and its organic compounds.

Bibliography

API (American Petroleum Institute) (1991) Design and installation ofoffshore production platform piping system, API RP14-E.

Bennet L.H. et al. (1978) Economic effects of metallic corrosion inthe United States, Washington (D.C.), NBS, 511, 1-3.

EFC (European Federation of Corrosion) (2002) Guidance on materialsrequirements for corrosion resistant alloys exposed to H2S in oiland gas production, London, The Institute of Materials, 17.

References

ASTM (American Society for Testing and Materials) (1983) Standardpractice for numbering metals and alloys (UNS), 10 May, ASTME527.

Bertolini L. et al. (2004) Corrosion of steel in concrete. Prevention,diagnosis, repair, Weinheim, John Wiley-VCH.

Bianchi G., Mazza F. (1989) Corrosione e protezione dei metalli,Milano, Masson Italia.

Corrosion in the oil refining industry. Proceedings of the NationalAssociation of Corrosion Engineers conference (1998), Phoenix(AZ), 17-18 September.

Eni-Agip (1994) Costo della corrosione in Agip, Rapporto interno Eni,Divisione Agip, Unità CORM.

Graver D.L. (edited by) (1985) Corrosion data survey, Houston (TX),NACE.

Lazzari L., Pedeferri P. (2006) Cathodic protection, Milano,Polipress.

Munger C.G. (1999) Corrosion prevention by protective coatings,Houston (TX), NACE, Item 37507.

NACE (National Association of Corrosion Engineers) (2002) Cost ofcorrosion study unveiled. Supplement to materials performance,Houston (TX), NACE, 1-12.

Pedeferri P. (2007) Corrosione e protezione dei materiali metallici,Milano, Polipress, 2v.

Riggs O.L., Locke C.E. (1981) Anodic protection. Theory and practicein the prevention of corrosion, New York, Plenum Press.

Luciano Lazzari

Dipartimento di Chimica, Materiali e Ingegneria chimica ‘Giulio Natta’

Politecnico di MilanoMilano, Italy

MATERIALS


tem

pera

ture

(°C

)

50

100

150

NaOH (%)

carbon steels

stainless steelsand nickel alloys

250 50

Fig. 6. Isocorrosion maps for the selection of materials in alkaline solutions (Graver, 1985).

Documents

9.2 Control of corrosion and choice of materials 9.2 Control of corrosion and choice of materials. also be corroded if their potential is reduced to below the discharge potential of