Interpretation of Cyclic Potentiodynamic Polarization Test

ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2018, Vol. 54, No. 5, pp. 976–989. © Pleiades Publishing, Ltd., 2018.

INVESTIGATION METHODS FOR PHYSICOCHEMICAL SYSTEMS

Interpretation of Cyclic Potentiodynamic Polarization Test Results for Study of Corrosion Behavior of Metals: A Review1

S. Esmailzadeha, M. Aliofkhazraeia, *, **, and H. Sarlakb

aDepartment of Materials Engineering, Tarbiat Modares University, Tehran, P.O. Box: 14115-143 IranbDepartment of Materials Engineering, Isfahan University of Technology, Isfahan, 84156-83111 Iran

*e-mail: [email protected]**e-mail: [email protected]

Received June 22, 2017

Abstract—The cyclic potentiodynamic polarization technique is a method for evaluating the susceptibility ofa metal to localized corrosion such as pitting and crevice corrosion. This paper provides the information toconduct the cyclic polarization test correctly and to help performing the interpretation of the polarizationscan properly. The effect of critical parameters including solution resistivity, scan rate, point of scan reversal,aggressive ions, corrosion inhibitors, metastable pits, metallurgical variables, temperature, dissolution gases,pH, immersion duration and surface roughness on the cyclic polarization curve and results interpretation arediscussed. Then a number of cyclic potentiodynamic polarization curves for common metals and alloys inprevalent environments are given.

Keywords: Cyclic potentiodynamic polarization, Corrosion, Corrosion inhibitor, Localized corrosion, Pit-ting corrosion, Protective surface film, Scan rateDOI: 10.1134/S207020511805026X

1. INTRODUCTIONCorrosion as a natural phenomenon is the material

destructive reaction with its surrounding environment.During the past years, it has caused irreparable dam-ages for human life [1]. Human safety, economic costsand conservation of materials have been the mostimportant reasons to control corrosion [1, 2]. The useof corrosion inhibitors [3–6], cathodic protection [7–9], different types of coatings on the surface [10–13],alloying elements and additives [11, 14] are a numberof methods for controlling corrosion. In addition tothese methods, passivity is a property of some metalsor alloys, which are thermodynamically unstable, andreact inherently with environment (water or oxygen)and form a stable passive oxide film on the materialsurface. The metal surface is protected against theatmosphere by the passive layer [15]. Thickness of pas-sive films varies in a wide range. For metals such as(Fe, Cr, Co, Ni, Mo) and their alloys it is tens to hun-dreds of angstroms while the thickness of passive filmfor the other metals (Zn, Cd, Mg, Cu, Pb) could be inmicro meter range. In the case of aluminum, thethickness of oxide film changes from nano meter size(air form film) to much thicker (anodized film) [2].The protective oxide film could be destroyed by elec-trochemical and mechanical methods. For example,

putting the passivized metal in a solution containingthe aggressive ions is a common degrading condition.Localized corrosion (pitting corrosion, crevice corro-sion and stress corrosion cracking) can occur bybreakdown of the passive film in the presence ofaggressive ions. Electrochemical investigation of pas-sivity is used to evaluate the materials resistance tolocalized corrosion. Studying the material polariza-tion treatment by polarization techniques speciallycyclic potentiodynamic polarization (CPDP) tech-nique is a suitable way for investigation of the begin-ning of passivity, breakdown of oxide film, susceptibil-ity to repassivation and calculation of the rate of pit-ting corrosion due to the vast range of scanningpotential [2, 16, 17]. Although the CPDP test is a rapidand reasonable method, it is not possible to interpretall kinds of CPDP curves. Therefore, the aim of thisreview is to study the CPDP technique and moreaccurate interpretation of its results in order to predictmaterials corrosion behavior appropriately.

2. CYCLIC POTENTIODYNAMIC POLARIZATION

CPDP test was introduced for the first time in the1960s. It is widely used to determine resistance tolocalized corrosion or degradation rate in a short time[18, 19]. Thus this technique is applicable as a method1 The article was translated by the authors.

976

INTERPRETATION OF CYCLIC POTENTIODYNAMIC 977

Fig. 1. Schematic illustration of, (a) CPDP curve of a susceptible material to pitting corrosion, (b) potential-time curve duringCPDP test.

(a)

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2H2O O2+4H++4e

EfEcorr

Log (Current Density)

(b)

Time

for prediction of localized corrosion also beneficial foralloys that are passivized spontaneously and under-went localized corrosion [19].

General shape of CPDP curve is as follows; afterpassing through the region of active corrosion, thecurrent density decreases to a critical potential, calledthe “Flade potential” or “primary passivation poten-tial”. This decrease is due to the formation of the pas-sive layer on metal surface.

The passive current density is the current density inthe passive region. With further increase in the poten-tial in the passive region, a rapid rise in the anodic cur-rent can be detected. This rise is due to either the evo-lution of oxygen by the decomposition of water orbreaking the passive film and localized corrosion. Ifthe increase of current density is due to the decompo-sition of water and evolution of oxygen gas, the regionis called “transpassive region” (Fig. 1a) [2].

(1)

Observation of the transpassive region occurs atdifferent potentials for various materials. For instanceE for copper in 1 N H2SO4 is about 1800 mV (SHE)and for stainless steel 304 in 1N H2SO4 is 1200 mV(SHE) approximately. For Some materials such astitanium, the range of passivation is very wide. Thisbehavior is due to the high ohmic resistance of protec-tive film (TiO2). However, this film is unstable in thepresence of chloride ions and it may be destroyed [20].The reason of increase in anodic current density at thepotential below the oxygen evolution potential is theinitiation of pitting and occurrence of localized corro-sion (Epit). In order to compare the resistance of differ-ent samples, the current density at which scanning

2

+2 2

O

2H O = O + 4H + 4e ,

1.228 0.0591 pH ( 25 C, 1 atm)E = T = P

−

− =�

PROTECTION OF METALS AND PHYSICAL CHEMISTR

direction is reversed should be constant in variousexperiments. According to the ASTM standards, thiscurrent density is called as threshold current density(it) and is about 5 mA/cm2 or 2 times (decades) morethan the passive current density. The vertex potential isthe potential at which the scan direction is reversed(Eν). The direction of scanning potential is reversedtoward potentials that are more active (Fig. 1b) [16, 21,22]. The test is performed by the defined instructionsby ASTM standards in this field [2].

In order to assess the corrosion susceptibility ofsmall implant devices and localized corrosion suscep-tibility of iron, nickel, or cobalt-based alloys CPDPmeasurements should be carried out according to thedefined ASTM standards (F2129, G61) [21, 23].Implant devices made of metals with high corrosionresistance are used instead of organs and damaged tis-sues in body. Thus, it is needed to determine their cor-rosion behavior and susceptibility to localized corro-sion. The following section describes the general pro-cedure of the test that is the same in two standards, butfor more information about preparation of the speci-mens and more details of conducting the test such ascorrosive solution, pH and temperature, it should bereferred to the references [21] and [23]. After thepreparation of the test specimen according to the stan-dards, for stabilizing the rest potential Er (the potentialof working electrode to the reference electrode underthe open circuit condition (OCP)), the specimen isimmersed in a prepared electrolyte (according to thestandard) Before that, it should be purged sufficiently(minimum of 1h) with a gas for removing oxygen(according to the standards). The specimen isimmersed until the OCP variation rate is less than3 mV/min. Then the potential scan starts at the OCPand moves in the more noble direction in a cyclic path

Y OF SURFACES Vol. 54 No. 5 2018

978 ESMAILZADEH et al.

Fig. 2. Schematic illustration of CPDP tests for three different conditions, (a) exhibiting protection potential, (b) with oxygenevolution, (c) without protection potential.

(a)

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Epit

Ecorr


(b)

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Ef

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Ecorr


(c)

Hystersis Loop

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Ef

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Ecorr


Anodic to CathodicTransition

with a slow scan rate (0.6 V/h ≈ 1.66 mV/s (±5%)between the working electrode and auxiliary elec-trode. Finally, the scanning potential direction isreversed to the starting point. Along with the continu-ous scanning potential, the current response is moni-tored. According to ASTM F2129 standard, there arethree different states: the scanning potential continuesuntil the hysteresis loop is completed indicating repas-sivation (Fig. 2a) or oxygen evolution occurs and theanodic to cathodic transition potential is reached(Fig. 2b) or the hysteresis loop is not completed andcorrosion potential is reached (Fig. 2c). According toASTM G61 standard, the reverse scan continues untilthe hysteresis loop is completed or the corrosionpotential is reached [21, 23].

The interpretation of the CPDP scan is difficult.The extracted parameters from the cyclic curves arenot constant for each material. They are empiricalparameters and change in different experimental con-ditions. The parameters used to interpret the CPDPcurves include: pitting potential, repassivation or pro-tection potential, potential of anodic to cathodic tran-sition, hysteresis and active passive transition (anodicnose). The first three potentials are based on the dif-ference of corrosion potential [19]. In the CPDP test,relative position of pitting potential and repassivationpotential or protective potential with respect to thecorrosion potential are the most important parametersfor evaluating the pitting corrosion behavior [24].

2.1. Pitting Potential

In the anodic polarization scan, scanning startsfrom corrosion potential after reaching the steadystate. Before reaching the potential of oxygen evolu-tion, a rapid increase in the current density may occur.Two reasons have been mentioned for this increase. Ifthe surface of oxide is not perfect, for example, somedefects exist on the film; the passive film will not bestable over the passive region. At a potential below the

PROTECTION OF METALS AND PHYSICAL

oxygen evolution potential, the surface defects will beactive and begin to propagate which can increase thecurrent density.

The other reason for the increase in the currentdensity earlier than the oxygen evolution could be thebreakdown of the oxide film and occurrence of the pit-ting corrosion in the presence of aggressive ions. In theabsence of aggressive ions, the passive film will be sta-ble over the electrode potential of O2 evolution.

The potential at which the current density increasesrapidly for either of two above reasons is given differ-ent names: critical pitting corrosion, pitting corrosion,breakdown potential or rupture potential [25–27].Also in some studies, the numerical values of pittingcorrosion resistance, Rpit, have been calculated fromthe equation: Rpit = |Ecorr – Epit| corresponding logiversus E plot of Fig. 3 [28–30].

For some materials, the pitting potential coincideswith the corrosion potential, which occurs when thereis an oxide film on the material surface prior to thepolarization. Due to the intersection of cathodicbranch with the transpassive region of anodic branch,the value of pitting potential is the same as corrosionpotential. The curve shown in Fig. 3 is related to theCPDP of Al alloy after 24 h immersion in 3.5 wt %NaCl. Due to the high activity of Al and formation ofan air-form oxide layer on its surface, the increase inthe current density at corrosion potential shows thatthe values of the corrosion potential and pitting poten-tial are the same [24, 31, 32].

2.2. Protective PotentialIn the CPDP curve, for investigating the material

resistance to localized corrosion after increasing thecurrent density at pitting corrosion, the scanningdirection changes. Then with the potential reductiontoward the negative potentials, the current density inthe reverse scan will be higher than the current densityin the forward scan (positive hysteresis). The scanning

CHEMISTRY OF SURFACES Vol. 54 No. 5 2018


Fig. 3. Schematic illustration of CPDP of Al in 3.5 wt %NaCl solution.

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Fig. 4. Schematic illustration of CPDP curve and corro-sion parameters. The arrows show the direction of polar-ization.

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Pitting initiatesand propagates

Only prior pittingpropagate

Hystersis Loop

Anodic to CathodicTransition

Pitting will notinitiate or propagate

Epit

Erep

EfEcorr

continues until the reverse curve crosses the forwardpolarization curve. This intersection point is namedprotection potential and it is introduced as a potential,at which the anodic current density reaches the lowestvalue on the reverse polarization scan [33, 34]. Repas-sivation potential is the potential at which the growthrate of pits is stopped. The amount of hysteresis or inthe other words, the difference between Epit – Erp indi-cates the amount of localized corrosion. If the currentdensity in the reverse curve is more than the currentdensity in the forward scanning curve, it will be indic-ative of pitting corrosion. Actually pitting is the majorform of corrosion in this condition [29, 35]. The resis-tance of material to localized corrosion is evaluated onthe basis of the Erep measurement to the Ecorr. If Ereplies in the more noble values than the Ecorr the propa-gation of active pits is diminished or stopped. There-fore, at the potentials between the protection potentialand corrosion potential, the passive film is stable andno pits will initiate or grow. Also in this region crevicecorrosion and crack initiation and propagation willnot take place. This region is called perfect passivity.At the potential between the pitting potential (Epit) andprotection potential (Erp), only old pits propagate andno new pits nucleate (Fig. 4). If Ecorr lies between Epitand Erep, the repassivation of pits will not take placecompletely and preformed pits continue to grow andpropagate (Fig. 2c) [2, 19].

Pitting potential is the minimum potential at whichthe material tends to the pitting corrosion. Above the pit-ting potential, new pits will initiate and develop [29].

The difference between the Epit and Erp and also thearea of the hysteresis loop indicate the probability ofpitting corrosion. The more difference between the


Epit and Erp and the larger area of positive loop demon-strate the probability of low pitting corrosion resis-tance [2, 26, 36].

2.3. Hysteresis

The occurrence of hysteresis in the CPDP curve iswhen the forward curve is not overlaid with the reversescanning curve. The difference between forward andreverse current density at the same potential demon-strates the size of hysteresis. The more differencebetween the current densities is the result of disruptionof the surface passivity at high potentials. Thus the big-ger size of hysteresis loop means more passive film dis-ruption, following with more difficulty for restoringthe damaged passive film [37]. There are two types ofhysteresis at the more positive potentials: negative hys-teresis happens when the degree of surface passivationis greater at more noble potentials, which causes thecurrent densities in the reverse scan to be lower thanthe current densities in the same potentials of forwardscan (Fig. 2b). Positive hysteresis is related to thedecrease of the passivity due to the localized corrosion(pitting and crevice corrosion) that causes the increaseof the current density in reverse scan, in comparison tothe current density in the forward scan at the samepotential (Fig. 4). In the positive hysteresis, the direc-tion of the current density changes toward the lowercurrent densities. Therefore, the slow decrease of cur-rent density in the reverse scan in positive hysteresis isindicative of the difficulty in surface repassivation orstopping the growth of pits [2]. Sometimes in differentliteratures, the definition of the type of hysteresis isswitched but in this paper, the meaning of positive and



Fig. 5. Schematic illustration of CPDP curve with steppotential called pit transition potential (Eptp).

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Fig. 6. Schematic illustration of CPDP curve with positivehysteresis.

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Hystersis Loop

Epit

Ecorr

negative hysteresis is according to the above descrip-tions.

The absence of hysteresis loop during the potentialscan (the forward curve coincides with the reversecurve) means that the localized corrosion does notoccur but it could be a sign of an active surface andgeneral corrosion [37, 38]. Sometimes there is apotential step in the positive hysteresis loop during thereverse scan as shown in Fig. 5. It was proposed thatthe potential step is called pit transition potential (Eptp)at which the repassivation of small pits is completedbut deeper pits require a further drop of potential torepassivation [24, 39]. In the positive hysteresis, at thepoint of reversal potential, two types of behaviors canbe seen. Either the anodic current density is decreasedor increased as shown in Figs. 4 and 6 respectively.Decrease of the current density immediately afterreversing scan is due to the reduce of the pits growthrate which stops at repassivation potential while thehigher current is concerned to increase of the growthrate of pits even after reverse of scan [28, 40].

2.4. Anodic to Cathodic Transition PotentialIt is the potential at which the anodic current den-

sity varies to the cathodic current density. At thereverse curve, the drastic decrease of the corrosioncurrent density at a potential called active-passivetransition potential (anodic nose) that is more noblethan Ecorr occurs for alloys that are susceptible to pas-sive and restore the damaged oxide film (Fig. 4) orthe alloys that are not susceptible to pitting corrosion(Fig. 2b). For these two groups of the alloys, the dif-ference between the anodic to cathodic transitionpotential and Ecorr are used to determine the per-


sistence of the passive film. According to the place ofanodic to cathodic transition potential relative to theEcorr, the passive layer stability is evaluated. If in thereverse scan, the anodic to cathodic transition poten-tial is more noble than Ecorr, the passive layer will notbe stable at Ecorr (Fig. 4), while the passivity will per-sist, if Ecorr gets more noble than the anodic tocathodic transition potential [19, 37]. For example,the corrosion behavior of AZ91D and Mg–1.5Zn–0.6Zr by CPDP curves after immersion in 5 wt %NaCl solution saturated with Mg(OH)2 at 25°C for30 min has shown that the breakdown potential or pit-ting potential is obvious in the anodic polarizationcurve of Mg–1.5Zn–0.6Zr whereas the point ofincreasing in the current density (Ebd) is not obviousfor AZ91D. It can be said that the existence of a denseand protective corrosion film on the surface of Mg–1.5Zn–0.6Zr alloy is the reason of it. According to theCPDP curves of alloys, for Mg–1.5Zn–0.6Zr theanodic to cathodic transition potential on the reversescan is higher (more positive) than the corrosionpotential on the forward curve but for the AZ91D it isopposite, anodic to cathodic transition potential islower (more negative) than the Ecorr on the forwardscanning curve. Therefore, for Mg–1.5Zn–0.6Zr, thepotential of the corroded areas is more positive thanthe uncorroded area, leading them to act as thecathodic areas and pitting corrosion is stopped whilefor AZ91D the corroded areas as the anodic areas con-tinue to corrode. Finally the result of this behavior isthe corrosion spread to the depth and formation ofdeep pits for AZ91D but the corrosion spread to thewidth and stopping the growth of pits for Mg–1.5Zn–0.6Zr alloy [41].



3. EFFECTIVE PARAMETERSON CPDP CURVE

In the following, the effect of critical parameters oncorrosion behavior and their effects on the interpreta-tion of results have been brought.

3.1. Solution ResistivityIn an electrochemical cell, the arrangement of

electrodes in the cell must be in a way that voltage dropbetween the working electrode and reference electrodegets minimum value. The drop voltage is usuallyresulted from the solution resistance, film resistanceon the surface and electrical resistance of electrodesand leads. If the drop voltage is significant, it willcause disruptions including: difference between theeffective potential (scan rate) on an alloy surface andapplied potential (scan rate) by potentiostat. Byincreasing the current density, the created deviation involtage or scan rate increases. The increase of the solu-tion resistance and following the increased voltagedrop affects the polarization scan by the change ofpotentials and current densities. One of the effectiveparameters on the voltage drop is electrolyte rotationrate. For example, the CPDP of steel in the acetonecyanohydrins showed that by increase the f luid rota-tion rate, the primary passivation potential increases.It means that in higher rotation rate and higher voltagedrop, the more noble potential for initiation of passiv-ation is needed [19, 37].

3.2. Scan RateScan rate is the value of potential that changes per

unit of time. If the surface defined as a simple resistor(polarization resistance) and a capacitor (double layercapacity) in parallel, the polarization scan rate shouldbe slow enough. Because in low scan rate the relation-ship between current and voltage only reveals the cor-rosion process at the interface of material surface withthe electrolyte for each potential. Otherwise the cur-rent obtained from the polarization scan, not onlyshows the value of current at the corrosion process but,it includes the charge of surface capacitor. Therefore,if the scan rate is not small enough, the current densityobtained from CPDP curve will be greater than thecurrent density derived from the corrosion reactions.The methodology for choosing the lowest value of thescan rate has been described previously [37]. It hasbeen suggested that the maximum permissible scanrate at which the capacity is not considered is very lowfor alloys susceptible to passivity [19, 37]. Also scanrate is an effective parameter on the value of pittingpotential because pit nucleation and its propagationare dependent on the amount of time at each potential[2]. For example in the CPDP curve of Al alloy in3.5 wt % NaCl solution at a scanning rate of 1 mV/s atroom temperature the pitting potential is absent. Inother words, at scanning rate of 1 mV/s, the potential


gap of the alloy is small and undistinguishable. There-fore, the test should be carried out at a relative lowscan rate (less than 0.1 mV/s) to appear the pittingpotential in the curve [18]. The change of scan ratefrom 0.2 mV/s to 5 mV/s did not show notable influ-ence on the pitting potential of AA5083 aluminumalloy, but the protective potential of the alloy shifted tomore negative potentials. The effect of scan rate is pre-sented by a linear relationship between the protectionpotential and scan rate,

(2)

where V is the scanning rate [42].

3.3. Point of Scan Reversal

The image of polarization scan and the measure-ment of the repassivation potential are depended onthe current density or potential at which the scan isreversed. Scanning to high positive potentials causesthe surface to be exposed to more vulnerability. Theprocedure used for polarization scan and choice ofmaximum current density (potential) in the forwardscan, must be in accordance to the practical condi-tions. The following example has been brought for bet-ter understanding the effect of point of scan reversal.Incoloy 825 (UNS NO8825) showed two differentcorrosion behaviors in an atmosphere with low pH 1–2. By changing the reverse point in CPDP scan, theappearance of curve changed. At the high potential ofscan reversal a positive hysteresis loop was observedthat means the specimen has experienced the localizedcorrosion. Furthermore, being the repassivationpotential more active than the corrosion potential wasindicative of non-restorable of the passive surface. Butthere was no evidence of localized corrosion in prac-tice. In the second CPDP test, the point of reversalwas chosen lower than the prior. In this condition anegative hysteresis loop was generated. It means thatthe localized corrosion does not occur on the surfaceof the specimen and it confirmed the observations.The reason of this behavior is because of an electro-chemical transformation CrIII–CrVI at the potentialsabove the 0.8–1.0 V (SCE). Thus in practical condi-tions, as long as the specimen is not exposed to thepotentials above this rang, the passive film will not bedamaged [37]. The point of reversal scan is effectivenot only on the repassivation potential and on behav-ior of the reverse scan but its influence has beenobserved on the corrosion parameters on the forwardscan. The CPDP scan of gold in deaerated 0.6 M NaClchanged with the increase of the point of scan reversal(vortex potential). The changes included the decreaseof corrosion potential on the forward scan and rise incorrosion potential (anodic to cathodic transition) onthe reverse scan. The change of species activity such asgold-oxygen complex has been proposed as the proba-ble reasons [43]. Two parameters ΔE = Eν – Ecorr and

p –0.921 – 0.013 ,E = V



Fig. 7. Schematic illustration of CPDP scan of Al alloy in3.5 wt % NaCl at 1 mV/s.

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Ecorr

Fig. 8. Schematic illustration of CPDP scan of Al alloy in0.1 m/L NO2SO4 + 10 mmol/L NaCl at 1mV/s.

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Epit

ΔE' = Eν – have been used for comparing the sta-bility of the protective oxide layer in different condi-tions, that Eν is the point of scan reversal (vortexpotential) and Ecorr and are the corrosion poten-tials in the forward scan and reverse scan respectively.The greater amount of ΔE and ΔE ' indicate of morecorrosion resistance [44].

3.4. Effect of Aggressive Ions

The presence of some ions such as chloride ions inthe electrolyte connected with a material has anaggressive effect on durability of a passive film formedon the material surface. The chloride ions cause thematerial suffering from the localized corrosion by gen-erating pits on the surface. The critical pitting poten-tial reduces (shift to the active direction), as the chlo-ride ions concentration increases in the solution andconversely, as the solution containing chloride ions ismore dilute the value of the pitting potential will befound at the high (more noble) potentials. For 304stainless steel and aluminum, a linear relationship hasbeen found between the chloride ion concentrationand pitting potential [2, 45]. In addition, the change ofbehavior of polarization scan for high chromium castirons (HCCLs) has been completely obvious by addingof chloride ions to the 0.01 M NaOH. It includes, for-mation of negative hysteresis in the absence of Cl–, theobservation of metastable pits in the SEM images afteradding 100 mg/L and occurrence of pitting corrosionby creation of stable and irreparable pits in the pres-ence of 500 mg/L and 1g/L [46].

In some cases the presence of chloride ions in thesolution only decreases the stability of passive film by

corr'E

corr'E


lowering the breakdown potential and does not causepitting corrosion [43]. In addition to Cl– ion, F– ionsand Br– decrease the pitting corrosion resistance byshifting the pitting potential or protective potential tothe more active direction. For example, Epit (Erep) ofα-brass alloy has a linear relationship with the loga-rithmic concentration of F– ions [47, 48]. One of theeffects of aggressive ions is prevention from theappearance of localized corrosion parameters such asEpit in the CPDP curve of a material. Figure 7 shows aschematic figure of the CPDP curve of Al alloy in3.5 wt % NaCl at scanning rate of 1 mV/s. As it can beseen, although the pitting potential and rapid increasein current density are absent in the curve, in thereverse curve the anodic to cathodic point is moreactive than the open circuit potential. Thus fordescribing the corrosion behavior of the passive film,it is required to increase the potential gap. In order toincrease the potential gap and appear the pittingpotential, a solution containing trace chloride ionsshould be replaced instead of 3.5 wt % NaCl solutionFig. 8 [18].

To evaluate the aggressive effect of chloride ions oncorrosion behavior of materials, the CPDP curves of304 stainless steel and a duplex stainless steel in twosolutions, without and with chloride ions, for compar-ison have been exhibited in Figs. 9 and 10 respectively[49, 50].

3.5. Effect of Corrosion InhibitorOne of the methods to prevent the localized corro-

sion is the addition of corrosion inhibitors to the elec-trolyte. Corrosion inhibitors (anodic and cathodic)increase the material resistance against corrosion by



Fig. 9. Schematic illustration of CPDP scan of 304 stainless steel in (a) 3.5 wt % NaCl, (b) 1N H2SO4.

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(a) (b)

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Fig. 10. Schematic illustration of CPDP scan of a duplex stainless steel in (a) 0.1 M H2SO4, (b) 0.1M H2SO4 + 0.1M NaCl.

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different mechanisms. Therefore, it is expected thatthe appearance of polarization scan will be under theinfluence of corrosion inhibitors. In general, it can besaid that the addition of corrosion inhibitors increasethe critical pitting potential [34, 51–54]. For exampleNa2SO4 as corrosion inhibitor in a 0.1 M NaCl solu-tion increases the pitting potential of type 304 stainlesssteel [2]. 5-(3-Aminophenyl)-tetrazole as corrosioninhibitor in 3.5 wt % NaCl changes the CPDP curve ofMg/Mn alloy by decreasing in cathodic current den-sity, anodic current density and following that thedecrease of the corrosion current density, also the shiftof Ecorr and Eprot toward the negative potentials and


positive potentials respectively [55]. In some cases, thecorrosion inhibition effect of corrosion inhibitors onlocalized corrosion is when its concentration reachesto sufficient values. For example, the higher concen-trations of nitrite ( ) ions than 250 ppm couldinhibit the localized corrosion of 2205 duplex stainlesssteel in acetic acid solution [48].

3.6. Metastable Pits

When CPDP scan is done at electrode potentialsbelow the pitting potential the curve often containssome fluctuations. The reason of this transient is due

2NO−



to metastable pits which nucleate and grow but theirgrowth is stopped quickly and repassivized. Althoughboth of the stable pit and metastable pit cause theincrease in the anodic current density, it takes place formetastable pit in a limited time due to the insufficientdevelopment of the concentrated acidic chloride solu-tion within the metastable pits. In relation to therepassivation mechanism of metastable pits, it is sug-gested that the presence of salt films prevents thetransfer of cations out and further growth of the prop-agation pits [19, 37]. The observation of these f luctu-ations in anodic branch of the CPDP curves has beenreported in literatures [46, 56, 57].

3.7. Effect of Metallurgical Variables

Metallurgical variables have various effects on thepassive film stability such as heterogeneity at grainboundaries, disorder in the passive film by impurityatoms or inclusions. For example, sulfide inclusions(MnS) in the material microstructure act as sites forpit nucleation. Observation the positive hysteresisloop for a crystalline alloy (ASM1651) and an amor-phous alloy (C-22) indicate the occurrence of local-ized corrosion. The values of the breakdown potentialsand the repassivation potentials for the ASM1651 havebeen nobler than the C-22 alloy. Therefore, the crys-talline alloy has the higher corrosion resistance thanthe amorphous alloy [58].

3.8. Effect of Temperature

Temperature changes have influences on the cor-rosion rate. Therefore, it should be effective on theCPDP parameters. For some materials, there is a crit-ical pitting temperature (CPT). CPT is the minimumtemperature at which stable pits start to grow and fol-lowing that, the corrosion pitting occurs. A highercritical pitting temperature belongs to the alloys withhigher pitting corrosion resistance [2]. It is observedthat there is a breakdown potential transition fromhigh potentials to low potentials by the increase intemperature. At temperature lower than the criticalpitting potential temperature, the meta stable pits gen-erate and the increase in current density is not due tothe pitting but at temperatures above the CPT, the sta-ble pits generate and pitting occurs [59]. For types 304and 316 stainless steels in a chloride solution, theincrease in the temperature decreases the pittingpotential. The effect of temperature on the anodicbranch of CPDP curve for cold rolled steel in the pres-ence of Cl– ions has been observed while the changesof cathodic branch were not visible [60]. In addition tothe change of corrosion parameters on the anodicbranch, the decrease of the pits numbers and theincrease in pit size and its depth has been reported for304 stainless steel in the range of temperatures 20–50°C [56].


3.9. Effect of Dissolved Gases in the Electrolyte

The atmosphere, at which a specimen is tested, isvery effective on its behavior during the CPDP test.The dissolution of different gases such as H2, O2 andCO2 influences the corrosion behavior, formation ofthe passive film and pitting corrosion. Destructiveeffects of hydrogen on high alloyed stainless steelinclude the decrease of Ecorr, Erep, Epit and the stability ofthe protective passive film [61]. The CPDP scan of car-bon steel is the same with and without oxygen gas, so ithas a little influence on the corrosion behavior of car-bon steel. But the presence of CO2 gas changes the car-bon steel behavior from passive to active-passive [62].

3.10. Effect of pH

Chromium-nitrogen steel and chromium-nickelsteel with chemical compositions of Cr23N1.2 andCr18Ni9 respectively have shown three different cor-rosion behaviors in 3.5 wt % NaCl solution with therange of pH 1–12. In the pH 1–2, the electrolyte is anacidic solution so steel has an active corrosion. By theincrease in pH, the active corrosion behavior of twoalloys become active-passive and breakdown of thepassive film is observed due to pitting corrosion. Thesize of passive region that means the durability of pas-sivation, increasing in basic solution with pH 12.0 andoccurring the transpassivity in high positive potentials[63]. The typical CPDP curves for galvanized steel insimulated rust layer solution containing 0.6 M NaClwith pH of 7.0, 10.0 and 13.0 has shown differentbehaviors of the specimen. At pH 7.0 the increase inrate of anodic current density was high and then itbecame slow with the increase in the potential and thespecimen did not corrode locally. At pH 10.0 thereverse anodic curve showed the transition fromunpassivized to passivized corrosion behavior and aclassical passive region was observed at potentialsmore than the Epit = –0.56 V vs. SCE. At pH 13.0 thecurrent density in the reverse anodic curve increasedwhich was indicative of the formation of corrosionproducts [60]. Al based alloy in acidic, neutral andalkaline chloride solutions have shown the same pit-ting potential while the values of corrosion potentialand cathodic current density have changed.

3.11. Effect of Immersion Duration

One of the effective parameters on the CPDP testis the exposure time of alloy in the corrosive electro-lytes. The increase in the immersion duration ofmonel-400 (a nickel based alloy) causes the negativeshift of Ecorr and Eprot. In other words, it causes themore agglomeration of chloride ions inside the pits. Italso leads to pits growth and the decrease of the pittingcorrosion resistance [64]. The rise of immersion dura-tion of Mg in naturally aerated stagnant seawater and3.5 wt % NaCl from 60 min to 6 days decreases the



corrosion current density. Thus the further exposuretime of Mg in contact with the aggressive ions reducesthe uniform corrosion whereas it increases the pittingcorrosion [65].

3.12. Effect of Fluid Velocity

Fluid velocity is one of the effective parameters onpitting resistance of some alloys. For alloys such asnickel-chromium high molybdenum alloy and tita-nium alloys, due to the stability of passive film, therate of corrosion is nil even at high velocity. For agroup of alloys such as copper based alloys, cast ironand carbon steel, the corrosion rate increases byincreasing the velocity. It is due to more availability ofdissolved oxygen at high velocity and increase in theoxygen limiting current density. For example, theapplication of magnetic stirring in deaerated 0.6 MNaCl causes the increase in the limiting current den-sity of pure gold in the cathodic region of reverse scan[66]. Another group of alloys, nickel copper alloy,nickel chromium alloy and stainless steel types 316,304 have exhibited less occurrence of pitting on thesurface at higher velocity of electrolyte. For thesealloys, the high velocity of f luid does not permit todeposit suspended solids on the surface and formationpit under the deposits [20].

3.13. Effect of Surface Roughness

Pits initiate at specific sites on the surface such assulfide inclusions and roughness. According to stud-ies, at temperatures above the critical pitting tempera-ture the available sites for pit initiation change. Theeffect of roughness for 904 stainless steel in 1 M NaClwas observed on the critical pitting temperature. Theaverage of CPT decreases with the increase in surfaceroughness. By potentiostatic technique, the biggestCPT for 904 S.S polished to 3 μm finished was 56°Cwhile for the 60 grit surface finished, it was 46°C [59].The initiation of pits was evaluated for type 301 stain-less steel with different surface roughness (range of sil-icon carbide papers was 240, 400, 800, 1000 and 1500grits). The pitting potential for 301 S.S with smoothsurface and low roughness was higher than the sampleswith rougher surfaces. It was because of the difficultyin initiation of meta stable and stable pits on thesmoother surface [67]. The effect of roughness on thepassive film of SS316VM has been investigated in twodifferent conditions. The efficiency of passive filmformed naturally, exhibited dependency to the surfaceroughness, as the pitting potential rose with thedecline of surface roughness. It means that the surfaceroughness is effective on the nucleation and propaga-tion of metastable and stable pits. But for the passivefilm formed by CPDP passivation method, the effectof roughness was observed on the both pitting corro-sion and general corrosion and it was related to thechange of roughness only in the range of higher surface


roughness. Therefore, the pitting corrosion of 316stainless steel modified by CPP method was not sensi-tive to the surface roughness [68].

4. EXPERIMENTAL POURBAIXDIAGRAMS BY CPDP CURVES

Experimental Pourbaix diagram for different alloysin the environments containing aggressive ions is con-structed by electrochemical methods such as CPDPcurves. It is one of the other applications of CPDP test[69]. Pourbaix diagram as an equilibrium diagram isused for describing the corrosion behavior of puremetals in pure water or simple aqueous solutions.There are three main domains, immunity, passivityand corrosion in the pourbaix diagram that show theequilibrium state of metal in the specified condition (Eand pH). In the region of immunity, the metal is ther-modynamically stable and is not corroded. Passivity isthe region where an oxide protective film is formed onthe metal surface and acts as a corrosion barrier andthe region where the metal undergoes general corro-sion and metal ions are thermodynamically stable iscalled corrosion region [2, 35]. Although the calcu-lated pourbaix diagram is a useful tool for determiningthe corrosion behavior of a material in different condi-tions of potential and pH, it is often available for puremetals and is not applicable for complicated industryalloys. The other limitations for pourbaix diagram arethat the plot does not consider localized corrosion inthe presence of aggressive ions in the real solution andin the passive region of pourbaix diagram, the qualityof passive film and degree of corrosion protection bythe film could not be considered [69]. These limita-tions have caused the construction of experimentalpourbaix diagrams by CPDP curves for industryalloys. For this construction, at first the CPDP is car-ried out at constant conditions and at constant con-centration of aggressive ions for example chloride ionsat different pH. Corrosion potential, f lade potential(primary passivation potential), protection potentialand pitting potential are determined from the CPDPcurve with respect to the pH of solution and trans-ferred on the E-pH diagram. Transition of the corro-sion parameters causes to establish five regimes withinthe Pourbaix diagram. Immunity, general corrosion,imperfect passivity, perfect passivity and pitting arethe equilibrium domains for the corrosion behavior ofthe alloy at different potentials and pH. Figure 11 isrelated to the construction of an experimental Pour-baix diagram by anodic polarization curves for iron atconstant concentration of Cl– and at different pH.According to the anodic polarization curves, the cor-rosion behavior of iron varies with the increase of pH.At acidic solution (pH 5) the metal shows general cor-rosion only, the metal undergoes pitting corrosion andrepassivation by the increase of pH (pH 9) and finallyat basic solution (pH 13) the metal shows a passivationbehavior in a large range of potentials with no pitting



Fig. 11. Schematic illustration of the experimental Pourbaix diagram for iron in 0.01 M Cl− (right) constructed from experimentalanodic polarization curves (left).

Pote

ntia

l


pH 5 pH 9 pH 13

1.0

0

1.0

0 9 14pH

Generalcorrosion

Pitting

Imperfectpassivity

Perfectpassivity

Immunity

E in V vs. S.H.E.

[2]. It can be seen that in the experimental Pourbaixdiagram, the immunity is the region below the corro-sion potentials, the general corrosion region is an areaat the potentials between the corrosion potential andFlade potential also the potential range between theFlade potential and protective potential is called per-fect passivity. In the region of perfect passivity, thereare no pits which initiate or grow, while in the imper-fect passivity region that exists between the protectionpotential and pitting potential, the pits that initiatedpreviously, have the chance to grow. In the pittingregion that lies at the regime above the pitting poten-tial, there is the risk of initiation and growth for pits onthe surface of passive film [69].

Therefore the experimental pourbaix diagram con-structed from the CPDP curves is more useful than thecalculated pourbaix diagram due to their applicationfor various alloys, determination general and localizedcorrosion of the alloys in chloride solution and evalu-ation of passivation degree for protected film.

5. THE CPDP CURVESOF SOME COMMON METALS AND ALLOYS

In this section, the behavior of localized corrosionof some more common metals and alloys (iron [70],316L [71], 420 [34] and 430 [33] stainless steels, alu-minum [72], 6061 aluminum alloy [73], magnesium[74], AZ31 [75] and AZ91D [76] magnesium alloys,brass (copper based alloy) [77], low carbon mild steel[78]) by their CPDP curves in NaCl and H2SO4 solu-tions as more applicable solutions with the CPDPcurves of titanium alloy (Ti-6Al-4V) in Ringer physi-


ological solution [79] and nitinol alloy in PBS (phos-phate buffered saline) and bile according to ASTM F2129 standard [80] for determining the localized cor-rosion of materials, has been shown in Fig. 12. Due tothe effect of different parameters on the shape ofCPDP curves and material corrosion resistance (men-tioned in the previous sections), the given curves arerelated to the given conditions and they probablychange in different conditions.

In the CPDP curve for iron in 3.5 wt % NaCl ashort active- passive region is observed. Reduction ofanodic current density at the f lade potential is an indi-cation of passivation behavior of iron, due to eitherformation of a passive layer or precipitation of corro-sion products on the surface. The positive hysteresisloop in the reverse scan, higher than Ecorr shows thatpitting corrosion is occurred at potentials more posi-tive than Ecorr but it is stopped by the repassivation ofpits on the surface [53]. The localized corrosionbehavior of 316L stainless steel in 3.5 wt % NaCl solu-tion is similar to that expressed for iron. The differenceis that the surface area of hysteresis loop for 316Lstainless steel is larger than that observed for iron. Itmeans that 316L stainless steel suffered severe pittingcorrosion [81]. As can be seen in the CPDP curvespresented for 316L, 420 and 430 stainless steel theexact value of f lade potential is not recognizable, thatis typical for steel substrates. Due to the more positivevalue of Erep than Ecorr for 420 and 430 stainless steel,they are not susceptible for repassivation of pits andstopping the localized corrosion even at potentialsbelow Ecorr [33, 34]. A notable issue in the CPDPcurves for Al and 6061 Al alloy is the same value of


PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 54 No. 5 2018


Fig. 12. Schematic illustration of CPDP curves of common metals and alloys.

Pote

ntia

l

Pote

ntia

lPo

tent

ial

Pote

ntia

lPo

tent

ial


Log (Current Density) Log (Current Density) Log (Current Density)


Iron in 3.5 wt % NaCl 316L stainless steel in 420 stainless steel in3.5 wt % NaCl 3 wt % NaCl

430 stainless steelin 3 wt % NaCl

Magnesium in

Aluminum in 3.5 wt % NaCl

6061 Al Alloy in3.5 wt % NaCl

3.5 wt % NaCl

Brass in 1 wt %

AZ31 alloy in3.5 wt % NaCl

AZ91D alloy in3.5 wt % NaCl

H2SO4

Nitinol in PBS solution Nitinol in bile solution

LC mild steelin 3.5 wt %

Ti-6Al-4V in Ringerphysiological solution


Ecorr and Epit, that is due to the intersection of cathodicbranch with the transpassive region [24]. Active disso-lution in the anodic branch of CPDP curve for Mg andits alloys exhibits that no passivation behaviorobserved in 3.5 wt % NaCl solution. Also a large pos-itive hysteresis loop in the reverse scan indicates thatMg and its alloys are susceptible to pitting corrosion[82]. In the reverse scan of potential for brass in 1 wt %H2SO4, a small positive hysteresis is observed. It is dueto the type of localized corrosion, removing moreactive zinc from brass selectively, that is usually calleddezincification [83]. In the CPDP curve for mild steel,a sharp anodic current increase at potentials aboveEcorr indicates an active corrosion behavior of surface.Also the absence of hysteresis loop in the reverse scancan be a sign of uniform corrosion [78]. Stability ofpassive behavior at higher potentials than Ecorr and thenegative hysteresis in the CPDP curve of Ti-6Al-4Vconfirm the high localized corrosion resistance of thisalloy [84]. Comparing the CPDP curves for Nitinolstents in both PBS and reconstituted bile, it is obviousthat the passivized corrosion behavior is preserved forNitinol in bile solution at high potentials so that nolocalized corrosion occurs [80].

6. CONCLUSIONIn this review, the CPDP technique as an applica-

ble and useful method to evaluate the susceptibility ofdifferent metals and alloys to localize corrosion hasbeen described. This study includes the informationthat helps us to conduct the CPDP test and predict thecorrosion behavior of the materials correctly. Some ofthis information can be summarized as follows:

(1) ASTM G61 and F2129 standards are the stan-dards to conduct the CPDP test for iron, nickel orcobalt based alloys and small implant devices respec-tively.

(2) The parameters used to interpret the CPDPcurve are including: pitting potential, repassivation orprotection potential, anodic to cathodic transitionpotential, hysteresis and active-passive transitionpotential.

(3) The difference between pitting potential andprotective potential with corrosion potential is themost important parameters for evaluating the materialpitting corrosion resistance.

(4) Scan rate, point of scan reversal, aggressive ionsand temperature are some of different effective param-eters on the corrosion behavior and the shape ofCPDP curves. Therefore the effects of these parame-ters should be determined for predicting the localizedcorrosion resistance of a material correctly.

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