s11 experimental corrosion diagrams of metals

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Experimental corrosion diagrams of metals

O. V. Kurov*

A simple experimental method is suggested to characterise the electrode reactions of metals in

an electrolytic solution in order to plot the actual corrosion state of metals in the EpH plane. The

present approach is based on measurement of the composition (in particular the pH) of the near

surface layer and not on the bulk solution by application of a partition cell and simultaneous

measurement of the current and application of a new approach to separate the anodic

(dissolution) and cathodic currents. The resistance of the metal to corrosion is deduced from the

actual dissolution current. A corrosion diagram of several metals and of the FeCr binary alloy

system is constructed to illustrate the potential applications of the method.

Keywords: Polarisation diagrams, PotentialpH diagrams, Passivation, Polarisation, Corrosion potential, Electrochemical cell

Introduction

Pourbaix paved the way for the thermodynamic calcula-

tions of metal corrosion diagrams.1 He also provided

simplified versions of corrosion diagrams for engineering

application. These simplified diagrams delineate the

boundaries of the potential E for three corrosion states:

immunity, passivation (a layer of oxide is formed on the

surface of the metal; the protective capacity of the layer

remained undefined) and corrosion; the rate of metal

corrosion was not determined. However, the corrosion

diagrams proved unsatisfying for practical application, as

the predictions derived from the diagrams do not conform

with actual behaviour of the corresponding metals and

solutions. Evans2 attributed this failure to conform with

the experiment to certain kinetic features of the electrode

reactions. In particular, it is understood that the predicted

existence of oxide does not guarantee a stable passivation

layer.

The corrosion behaviour of a metal in a solution is

characterised by potentiodynamic polarisation, mea-

surement of galvanic current or spectroscopy of the

electrochemical impedance (e.g. Ref. 3). It is already

well recognised that all the important information about

the electrode reactions should be obtained by conduct-

ing measurements in the layer of solution immediatelyadjacent to the electrode. Electrode reactions remove

necessary components from the solution and discharge

their products in the nearest layer of the solution. Hence,

the composition of the layer beside the electrode will

vary in strict accordance with the electrode reactions.

Since this concentration layer is situated between the

surface of the metal and the remainder of the solution, it

may be termed the buffer layer. Under stationary

conditions (after prolonged exposure), the buffer layer

will have a certain composition. When the external

polarisation changes, the electrode reactions may change

and the composition of the buffer layer will change

accordingly; however, the rest of the solution will remain

practically unaffected.

A variety of approaches have been adopted to study the

buffer layer. Scanning reference electrode was frequent-

ly used to directly measure pH and surface potential

distribution near the metal surface in bulk and in a thin

layer of electrolyte (Refs. 4 and 5 and references therein).

Turnbull6 reviewed the methods and results of measure-

ments in confined conditions of corrosion with limited

exchange of the solution, which are encountered in pit-

tings, cavities, crevices or cracks where the entire volumeof the separated solution is in essence the buffer layer. Healso describes the use of a two-compartment cell to

investigate the pH in the solution in the vicinity of the

metal surface (at a distance of ,0?5 mm) in a crevice

using an Sb microelectrode in a glass capillary tube.

Iridium and tungsten electrodes have also been used to

measure local pH.4 More recently, flowing electrolyte

techniques have been devised. A scanning droplet cell was

used to measure potentials and impedance at the level of

grain boundaries.7 A scanning microflow cell was used to

maintain a stream of electrolyte on a metal surface and

harvest the drained electrolyte for spectroscopic analysis.8

It allowed following the evolution of potential, current

and metal dissolution with the time of the corrosionprocess.

In order to approximate the actual composition of the

solution with which the metal interacts under stable

conditions, we suggest to employ a partition cell that

makes it easy to measure and analyse the buffer layer,

does not call for special equipment, allows the use ofstandard electrodes and produces stable results (Fig. 1).

Kuhn and Chan9 emphasised the importance of

measuring the pH near the surface during electrochemi-

cal reactions. Thus, in the following, we choose the pH

as the key characteristic of the buffer layer since most

electrode reactions affect the solution pH. To obtain

information about both the electrode reactions that takeplace on the metal surface and the corrosive state of the

metal, the procedure is as follows:

17/56 Bialik St., Beer Sheva, Israel

*Corresponding author, email [email protected]

2013 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 30 January 2012; accepted 5 June 2012DOI 10.1179/1743278212Y.0000000041 Corrosion Engineering, Science and Technology 2013 VOL 48 NO 1 55


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i use a partition electrochemical cell containing

the metal and the solution as model of the buffer

layer of the solution

ii apply the selected electrode potentials (from an

external source of potentials) in a potentiostatic

regimen of polarisation

iii after a certain exposure time at each potential,

measure the pH of the solution in the working

chamber or determine the concentration of

certain ions by chemical analytic methods

iv using the results of the measurements, construct

a polarisation diagram for E versus pH (or E

versus the concentration C of certain ions).

To interpret the changes in the pH of the solution, the ex-

perimental EpH diagrams and the theoretical Pourbaixsdiagrams for the corresponding pure metals are super-

imposed. Using Pourbaix diagrams, it is necessary to

take into account the potentials of the metal and the

corresponding real concentrations of the buffer layer of

the solution with which the metal interacts and not the

whole initial solution. The superposition revealed near or

complete matching between the boundaries of phase

transitions on the Pourbaix diagrams and the kinks in the

EpH curves.10 The coincidence of the lines helps to

interpret the E versus pH curve, namely, to attribute the

form of the line to the corresponding electrode reaction.

We thus constructed diagrams of the corrosion regions of

the type of Pourbaix diagrams except for the newdiagrams being constructed according to the composition

of the buffer layer for several metals in a range of pH

values.11 These diagrams show the corrosion states of themetals, but they are still lacking an essential character-istic: a measure of the resistance to corrosion and of thepotency of passivation.

Therefore, in addition to the measurement of the EpH relation, it is necessary to measure also the metaldissolution current at each polarisation potential inorder to elucidate the actual corrosive state of the metal.

Thus, while passivation is determined by the thermo-dynamic stability of the metal oxide in the approach ofPourbaix, we determine the passivation by the actual

vanishing of the dissolution current. Only vanishing ofthe current indicates the existence and the stability of theoxide.

The initial dissolution potentials and the corrosionpotentials of many metals lie in the zone of cathodicdepolarisation in the EI diagram. The cathodic linerepresents the sum (cathodic and anodic) current; there-fore, it is necessary to separate from the line of anodic(dissolution) current. In the present work, the cathodiclines of the current in the cathodeanode transition zonewere studied for a number of metals to enable the

separation of the dissolution current and the identifica-tion of the electrode reactions. Then, the approach wasextended to the CrFe alloys in a 3%NaCl solution, and acorrosion diagram was constructed for the FeCr binarysystem.

Materials and methodsThe experiments were conducted in a compartmentalisedelectrochemical cell (Fig. 1) simulating the buffer layer ofthe solution. Polarisation diagrams in terms ofEIand EpH for the buffer solution under synchronous measure-ment ofIand pH were constructed for a number of metals(Pt, Zn, Cu, alloys 2024 and 304, Fe, Cr and a series of

alloys in the FeCr system). Pt was chosen to represent aninsoluble metal. To ensure a nearly stationary state of thebuffer solution and of the current in the cell, we performedall measurements following a prolonged exposure (4 h) tothe potential. The stationary metal corrosion potentials

Ecor were determined after 24 h of residence in thesolution. All the reported potentials are expressed relativeto the hydrogen standard electrode.

The volume of cell in which the tested electrode wasplaced measured 12 cm3. The samples were in the form ofcylinders 8 mm in diameter; the Pt sample was in the formof a cylindrical spiral. The corrosive solution consisted of3%NaCl, pH 6?5. The area of sample immersed in thesolution was 12 cm2.

The cell used to model the buffer layer of the solutionrepresents an approximation to the real state of the bufferlayer and introduces a certain measure of error. Theprincipal error is the jump of the ion concentration in thesolution on the two sides of the partition and, correspond-ingly, the formation of a liquid boundary potential. Thisresults in the real potential of the metal deviating from thepotential applied by the potentiostat. This deviation ismost significant for large concentration jumps. For aneutral chloride solution and a negative polarisationof 22?0 V applied to Fe (i.e. maximum concentrationdifferences), computation by Hendersons method12 givesa potential drop of jd50?1 V at the diaphragm for the

maximum concentration. For small currents and forpotentials close to the potential of a standard hydrogenelectrode in a neutral solution, the correspondingly jd

1 Compartmentalised electrochemical cell; partition is

kept open under conditions of operation and closed

during measurements

Kurov Experimental corrosion diagrams of metals

56 Corrosion Engineering, Science and Technology 2013 VOL 48 NO 1


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tend to 0. In this study, we examined minimal currents atthe stage of cathodicanodic transition, and therefore, any

errors introduced by the diaphragm were minimal.

Results and discussion

General observations in cathodic regionEpH and EI polarisation diagrams in the potentialinterval from 21?8 to z2?2 V corresponding to theinterval of potentials in Pourbaix diagrams wereconstructed based on the results of measurements in apartitioned cell for the selected metals in a neutralsolution of 3%NaCl. We start with the characteristicfeatures of the cathodic region of negative potentials. At

high currents, a linear relation between current andpotential prevails (Fig. 2a). At the potential E521?5 to

21?6, a transition occurs to a low current region, I51 to103 mA cm22. Plotting the data in this region in alogarithmic scale (Fig. 2b) reveals that all the studiedmetals form straight parallel lines indicating that thecurrent vary exponentially with the potential. Thecommon behaviour of all the metals indicates that asimilar surface reaction (hydrogen reduction) withidentical solution including the buffer layer takes placewithout an active participation of the metal surface. At

higher potentials, each line deviates from a straight linein the EI diagram at a characteristic potential listed in

Table 1. The straight current line segment of platinumreaches E520?3 V. Since Pt is insoluble, we find that

the exponential law of hydrogen depolarisation holds atleast up to E520?3 V. The divergence of the lines of thedifferent metals reflects beginning of metal dissolution.This is also confirmed by the EpH diagrams: atelectrode potentials E521?8, the pH of buffer solutionwas maintained at ,12 for all the metals, making

evident the active discharge of hydrogen ions (Figs. 3d,4b, 5b, 6b, 7b and 8b, d, f, h, j, l, n). The EpHpolarisation diagram of the buffer layer shows adeflection in the direction of lower pH values near the

potential where the current lines begin to deviate fromthe straight line (Figs. 3a, 4a, 5a, 6a, 7a and 8a, c, e, g, i,

k, m). This is due to the sum of two processes: theformation of a hydroxide of the dissolved metal and thegradual cease of the reduction in hydrogen (hydrogendepolarisation).

Non-passivating metalsThe beginnings of metal dissolution potentials arepractically identical with the potentials marking the startof dissolution of the respective metal on the Pourbaixdiagrams at a solution pH of 12. The magnitude of thedivergence of the current line for a given metal from thestraight Pt line may be used to obtain an estimate ofthe metal dissolution current. Figure 3 shows schematiccurves for a non-passivating metal (Fig. 3a and b) and a

passivating metal (Fig. 3c and d). The dotted lines in thediagram indicate the calculated metal dissolution currentlines. The dissolution starts at point A, where thecathodic current line diverges from the initial straightline. The dissolution current in the region of cathodicpolarisation at the potential E1 is given by BF5E1C, andthe cathodic current is E1F. The calculated dissolutioncurrent can be verified by the current at the corrosionpotential Ecor on the EI diagram. By the potential ofcorrosion Ecor the condition of equality of the anodicand cathodic currents must be fulfilled. In Fig. 3b thiscondition is satisfied at the intersection of the straight lineAD and the dotted line at a potential Ecor - this is point D(with actual measurements - close to D). So that D is the

control point.

Passivating metalsA distinguishing feature in the cathodic zone of the EpH diagrams of passivating metals is the presence of adouble kink corresponding to cathodic depolarisationbefore and after the formation of the passivating film(Fig. 3c).

In the beginning of segment AP on the EI line(Fig. 3d), the dissolution current is calculated as fornon-passivating metals. At point P, the passivating filmis fully formed, the metal dissolution nearly ceases andthe cathodic depolarisation declines sharply.

It is found that the passivation current is approxi-mately equal to the minimal current at the line of activeanodic solution. (Fig. 3d).

a full diagrams with linear scale; b cathodic lines in low current region with logarithmic scale

2 EI polarisation diagrams; potentials are reported relative to hydrogen standard electrodes

Table 1 Potentials where metal current lines begin todeviate from straight line

Metal Cr Zn Fe Ni Cu Pt

E/V 21.3 21.3 20.8 20.8 20.5 20.3


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Next, the low current cathodic region and the anodic

regions will be examined for two non-passivating metals

and two passivating metals in a 3%NaCl solution in

comparison with Pt. This comparison allows the identi-

fication of the dissolution current of the reactive metals.

ExamplesZn

Figure 4 shows EpH and Elog I polarisation dia-

grams of Zn and Pt for comparison. The point of

divergence of the cathodic line of current from the initial

straight line corresponds to the potential of the start of

metal dissolution (E521?3 V). The metal dissolution

current line (dotted line in Fig. 4b) up to its meeting with

the anodic current is constructed using the difference

between the cathodic line of current and the initial

straight line.

The straight line of cathodic current at the stationary

corrosion potential (Ecor520?81 V) gives the value of

the hydrogen depolarisation current at the corrosionpotential. The anodic dissolution (broken) line intersects

the straight line, indicating the same magnitude of the

anodic current. This confirms that the hydrogen de-polarisation proceeds practically unhindered even whendissolution of the metal takes place and that theZn(OH)2 formed during such dissolution does not con-stitute an appreciable obstacle to either metal dissolu-

tion or hydrogen depolarisation; therefore, the surface

of the Zn is not passivated.Cu

The types of EpH and EI diagrams obtained for Cu

(Fig. 5) confirm that it is a non-passivating metal.Dissolution begins at E520?5 V. In the potential interval

20?5 to 20?3 V, the metal dissolution current is cal-culated according to the difference between the cathodicline and the initial straight line; at higher potentials, themetal dissolution line joins the anodic line.

Al alloy 2024

Judging by the characteristic form of the EpHpolarisation diagram (Fig. 6) of the buffer layer, alloy

2024 may be classed among the passivating metals.According to the EI diagram, dissolution of the metalbegins at E522 V. A comparative study of the two

a, b non-passivating metals; c, d passivating metals

3 Schematic EpH and EI polarisation diagrams




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4 EpH and EI polarisation diagrams for Zn

5 EpH and EI polarisation diagrams for Cu

6 EpH and EI polarisation diagrams for Al alloy 2024




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diagrams indicates that the metal is passive starting from

potentials E521?4 V and higher. The stationary corro-sion potential Ecor520?49 V lies dangerously close tothe zone of active metal dissolution (i.e. the reserve ofpassivity is not sufficiently large to afford protection).

Stainless steel 304

The types of EpH and Elog I diagrams obtained foralloy 304 (Fig. 7) correspond to a passive metal. Thecorrosion behaviour of this alloy is similar to that of Cr(Fig. 8m and n), except that the current passivationinterval is smaller for this alloy (E520?8 to z0?2 V)than for Cr (E521?0 to z0?8 V). The broken line onthe Elog Idiagram shows the metal dissolution current.The stationary corrosion potential Ecor5z0?055 V lies

in the zone of passive current quite far from the zone ofactive dissolution.

FeCr binary system

The present approach to characterise the corrosion ofmetals has been extended to a systematic examination ofthe evolution of the electrochemical behaviour in alloysystems. Comparing the behaviour at different composi-tions may be important from both basic science andengineering selection of materials. The most interestingsystem to study is the FeCr binary system due to thepassivation properties that the Cr imparts on steels. Webegin with the behaviour of the pure elements.

Fe

As shown by the Elog Idiagram for Fe (Fig. 8a and b),beginning of Fe dissolution takes place at E520?8 V.This is also confirmed by the EpH diagram of thebuffer layer: starting from the potential E520?8 V, thepH is seen to decline due to formation of Fe(OH) 2 inthe solution. The measured stationary corrosion poten-tial Ecor52410 mV.

The anomalous deflection of the current line and thepH line on the Elog Iand EpH diagrams, at potentialsE from 20. 5 to 20?4 V, is probably due to the changein valence of the dissolved ions, namely, Fe2zRFe3z.On the theoretical Pourbaix diagram for Fe,1 this is

where the boundary between the formation of Fe(OH)2and of Fe(OH)3 lies. The calculated metal dissolutionline (dotted line in Fig. 8b) changes smoothly into the

anodic current line. The difference between the dissolu-

tion current line and the straight line of the cathodiccurrent at the stationary corrosion potential demon-strates that hydrogen depolarisation proceeds even afterdissolution of the metal starts and that the Fe(OH)2formed during iron dissolution does not obstruct eithermetal dissolution or hydrogen depolarisation.

Cr

The EpH polarisation diagram for Cr (Fig. 8m and n)shows three kinks. The first kink above E521?3 V isdue to the sum of gradual cease of the hydrogendepolarisation and dissolution of metal that formschromium hydroxide. The second kink represents thecease of the hydrogen depolarisation. The third kink at

1 V is due to the breakup of the passivation and thebeginning of active metal dissolution that inducesacidification of the buffer layer. The stationary corro-

sion potential Ecor5z0?2 V lies in the zone of passivemetal dissolution.

In the interval from E521?4 t o 1?0 V, the metal

dissolution current is calculated using the differencebetween the cathodic current lines and the initial straightline. We can assume that the metal dissolution current is

equal to the minimum initial current on the anodic line.The metals and alloys examined above, i.e. Zn, Cu, Al

alloy 2024, stainless steel 304, Fe and Cr, have beenthoroughly studied in practical applications, and a great

deal is known about their corrosion properties. Inparticular, the potentials at which metal dissolutionbegins1 and their different tendencies to passivation inchloride solution are well known.13 In general, theforegoing analysis of the EpH and EI diagramsconforms with the known data.

FeCr Binary system

In Fig. 8, we constructed the EpH and EIdiagrams inthe range of small metal dissolution currents (thecathodeanode transition) for a series of alloys in thebinary system with the aim of establishing the bound-aries of passivation. The following alloys were preparedand tested: Fe, Fe8?5Cr, Fe13Cr, Fe28Cr, Fe50Cr,

Fe70Cr and Cr. To construct a general corrosiondiagram for FeCr using the EpH and EI diagramsof the individual metals, the potentials of several

7 EpH and EI polarisation diagrams for alloy 304




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a,b

Fe;c,

dFe8?

5Cralloy;e,

fFe13Cra

lloy;g,

hFe28Cralloy;i,jFe50Cralloy;k,

lFe70Cralloy;m,nCr

8

Ep

H

andE

Ipolarisationdiagrams




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8

Co

ntinued




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characteristic points are listed in Table 2: beginning and

end of hydrogen depolarisation and metal dissolution,

and beginning and end of the passivation potentials.

In constructing the corrosion diagram for the binary

system, the main emphasis was on determining the

boundaries of the states of corrosion of the alloys(Fig. 9). The line of stationary corrosion potentials Ecoron the diagram served as orientation in evaluating the

position of the boundary between passivation and

corrosion. The diagram reveals the increase in corrosion

resistance with rising content of Cr in two jumps: at

the celebrated 13Cr jump and at ,60Cr a second jump.

The alloys between 13 and 70%Cr are close to the

passivation boundary; therefore, their passivation is

relatively unstable.These diagrams have been prepared assuming stag-

nant (no flow) conditions. However, there will probably

be changes in a moving solution, for example, in altering

9 Diagram showing corrosion resistance in FeCr binary system; upper boundary of each figure indicates compositions

of five alloys that were tested (2: Fe8?5Cr alloy; 3: Fe13Cr alloy; 4: Fe28Cr alloy; 5: Fe50Cr alloy; 6: Fe70Cr

alloy)

Table 2 Corrosion and passivation potentials in FeCr binary system/V

Metal* Fe Fe8.5Cr Fe13Cr Fe28Cr Fe50Cr Fe70Cr Cr

Ecor 20.41 20.34 20.09 20.08 20.07 z0.14 z0.201 0 0 0 0 0 0 02 20.8 21.0 21.1 21.3 21.3 21.3 21.33 21.0 21.1 21.2 21.2 21.2 21.24 20.6 20.7 20.8 20.8 20.85 0 0 0.1 0.8 0.8

*1: potential at the end of hydrogen depolarisation; 2: potential at the beginning of metal dissolution; 3: potential at the beginning ofpassivation (early signs of passivation); 4: potential at the beginning of complete passivation; 5: potential at the end of passivation.




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the formation of the normal pH buffer layer and on theelectrochemical double layer. It seems that we can expectthat (i) the intensity of depolarisation will increase ateither the cathodic or the anodic, or on both reactions

and (ii) the corrosion potential of the metal will changeto accommodate the kinetic changes in the form of thepolarisation curve.

Conclusions1. During the interaction between the metal and the

electrolytic solution, a buffer layer is formed near themetal surface. The composition of the buffer layerchanges in accordance with changes in the potential ofthe polarised metal and is important for understanding

the metal corrosion process. The theoretical basis forconstructing diagrams of metal corrosion in an intervalof potentials that was provided by Pourbaixs thermo-

dynamic diagrams is adjusted here for the buffer layerwhere the actual electrode reactions take place.

2. The state of corrosion or the electrode reactions of

metals in intervals of potentials and the metal dissolu-

tion currents were established by simultaneous measure-ments of the pH of the buffer layer and the total current.

3. The pattern of the EpH curve enables todistinguish the potential intervals of hydrogen polarisa-tion, start of metal dissolution, passivation limits andtransition between oxidation states of the metal. The

pattern of the EI lines allowed to separate the anodic

and cathodic currents for several metals and alloys by

comparison with Pt. A corrosion diagram for the whole

FeCr binary system was constructed and revealed two

transitions in the resistance to corrosion.

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solutions; 1966, London, Pergamon Press.

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s11 experimental corrosion diagrams of metals

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