Electrochemical Study of UNS S32550 Super Duplex Stainless Steel Corrosion in Turbulent Seawater Using the Rotating Cylinder Electrode

CORROSION SCIENCE SECTION

CORROSION—Vol. 60, No. 6 5610010-9312/04/000091/$5.00+$0.50/0

© 2004, NACE International

Electrochemical Study of UNS S32550 SuperDuplex Stainless Steel Corrosion in TurbulentSeawater Using the Rotating Cylinder Electrode

G. Kear,‡,* B.D. Barker,** and F.C. Walsh***

ABSTRACT

The cathodic and anodic characteristics of freshly polishedand pre-reduced UNS S32550 (ASTM A479) super duplexstainless steel in a filtered and conductivity-adjusted seawa-ter have been investigated under controlled flow conditions.A rotating cylinder electrode was used together with bothsteady and non-steady-state voltammetry and a potentialstep current transient technique to investigate the electrodereactions in the fully characterized electrolyte. Both oxygenreduction and hydrogen evolution were highly irreversibleand the material exhibited excellent passivation and re-passivation kinetics. Relative corrosion rates were derivedand the corrosion mechanism of the alloy was found to becompletely independent of the mass-transfer effects, whichcan contribute to flow-induced corrosion.

KEY WORDS: duplex stainless steel, flow-induced corrosion,mass transfer, oxygen reduction, polarization, rotating cylin-der electrode, transpassivity

Submitted for publication March 2003; in revised form, Decem-ber 2003.

‡ Corresponding author. E-mail: [email protected].* Erosion-Corrosion Group, Division of Mechanical Engineering,

The University of Queensland, Brisbane QLD 4072, Australia.Present address: Branz Ltd., Materials Section, Science andEngineering Services, Private Bag 50 908, Porirua, New Zealand.

** Applied Electrochemistry Group, Centre for Chemistry, Univer-sity of Portsmouth PO1 2DT, United Kingdom.

*** ElectroChemical Engineering Group, School of Engineering Sci-ences, Room 1001/1002, Building 30, University ofSouthampton, Highfield, Southampton SO17 1BJ, United King-dom.

† Trade name.(1) UNS numbers are listed in Metals and Alloys in the Unified Num-

bering System, published by the Society of Automotive Engineers(SAE International) and cosponsored by ASTM International.

INTRODUCTION

The Ferralium 255† and SD40† alloys (conformingto UNS S32550[1]) can be thought of as second-generation duplex stainless steels (SS) where nitro-gen is used as an austenite stabilizer (replacing somenickel from first-generation grades of duplex SS).1

The addition of nitrogen has been claimed to improvetensile properties, pitting, and crevice corrosionresistance after welding by reducing the detrimentalinfluence of heat-affected zones (a problem withearlier grades) by stabilizing the austenitic phase athigher temperatures.1-2 The elemental composition ofthe UNS S32550 alloy is given in Table 1.

The 255 grade, with 25% wt/wt chromium wasfirst developed in the late 1970s for use in chemicalplants and the offshore oil and gas industry.3-5 TheSD40 grade, which is the subject of this work, wasproduced later; it has an austenite-ferrite ratio of ap-proximately 50-50 and meets the pitting resistanceequivalent number (PREN) of 40, which appears aspart of some project specifications for 25% Cr superduplex SS.6 Within chloride media, where its pittingcorrosion resistance has been predicted to be high,7-10

the material has been aimed at the manufactureof fasteners, flanges, pumps, and values, where flow-induced corrosion can be a potential problem.

Although there are several publications dealingwith UNS S32550 pitting resistance and metallurgy,the rates of electrochemical charge transfer and reac-tant/product mass transfer at this alloy in neutralchloride media have not been examined. Using an-


562 CORROSION—JUNE 2004

odic linear sweep voltammetry (LSV) in 0.5 mol dm–3

sodium chloride (NaCl), Ashary and Tucker have pro-duced anodic polarization curves which did not showthe initial passivation peak typical for lower alloyedstainless and carbon steels.11 This indicates that thealloy was self-passivating under the imposed condi-tions.12-13 From the anodic polarization data, a highresistance to pitting and crevice corrosion was pre-dicted. This conclusion is in agreement with the workof Radford, where the UNS S32550 alloy showed ex-cellent single metal and galvanic corrosion resistancein seawater exposure tests.14

Further anodic polarization studies and expo-sure trials in neutral chloride electrolytes by Garfias-Mesias and Sykes and Ben Salah-Rousset, et al., alsohave produced favorable passivation and pitting re-sistance behavior of a similar order to that observedby Ashary and Tucker.10,15

Few kinetic studies are available that consideroxygen (O2) reduction at SS in near-neutral solutionscontaining chloride. None have dealt with the UNSS32550 alloy. Muralidharan, et al., and Babic andMetikos-Hukovic, however, have studied the electro-chemical reduction of O2 on Type 316 (UNS S31600)and Type 304 (UNS S30400) SS, respectively, in0.5 mol dm–3 NaCl solution.16-17 Under conditions offull mass-transfer control, an overall complete four-electron reduction to hydroxide ions predominated:

O H O e OH2 22 4 4+ + →– – (1)

In both studies, reaction orders with respect to O2 ofapproximately 1 were determined, relating to a valueof m = 1 in Equation (2):

r r

j k zF OO Om

2 2 2= [ ] (2)

Here, r

jO2,

r

kO2, z, and F are the cathodic current den-

sity and potential dependent rate constant for pureO2 reduction, the stoichiometric number of electronsexchanged, and Faraday’s constant. In both cases,Tafel slopes, which were dependent on potentialsweep direction and solution pH, ranged from–0.125 V decade–1 to –0.130 V decade–1 and –0.115 Vdecade–1 to –180 V decade–1 for the UNS S31600 andUNS S30400 alloys, respectively.

In the only paper to deal with the reduction of O2

on a duplex SS, Gojkovic, et al., examined O2 and hy-drogen peroxide (H2O2) reduction on a 22.5% wt/wtchromium alloy using a rotating disc electrode (RDE)

in a 0.5-mol dm–3 NaCl electrolyte.12 Again, first-orderkinetics were assumed with respect to O2, and Tafelslopes measured at both pre-reduced and “pre-corroded” surfaces were approximately –0.150 V de-cade–1. Irreproducibility of the LSV curves, however,did not allow the extraction of detailed, charge-transfer-controlled data. As with all the works quotedhere, a clearly defined limiting current density wavewas observed for O2 reduction.

Of particular interest for the UNS S32550 alloyexamined in this work was the relative contributionof mass transfer to the corrosion mechanism. Flow-induced corrosion can result from either of thefollowing:

—direct mass transfer of electrochemical prod-ucts or reactants

—removal or thinning of passive layers (eitherthrough chemical dissolution or physical effects)

Any material, which has a reduced corrosion ratewhen passive, has the potential to exhibit an in-crease in the observed corrosion rate if such layersare attenuated or removed. The potential of such anoccurrence will depend both on the nature of thechemical system and on the fluid flow characteristics.

Due to the lack of electrochemical data mea-sured for this alloy under conditions of controlledflow, a detailed kinetic study was initiated in ourlaboratories to quantify both the cathodic and anodicbehavior in seawater. Absolute quantification of theeffects of seawater fluid flow was required prior toapplication of the material to service. A fully charac-terized, filtered seawater was chosen as the electro-lyte of interest and the rotating cylinder electrode(RCE) was taken as a tool to impose conditions ofcontrolled, turbulent fluid flow and mass-transferrates. This simple flow geometry is currently usedextensively in corrosion testing where mass transferand/or mechanical stresses induced by the fluid cancontribute to corrosion mechanisms. For brevity, themechanics of RCE fluid flow and the theory of con-vective-diffusion are not covered here, but furtherdetails can be found in the large number of RCEreviews and technical publications available in theliterature.18-24

EXPERIMENTAL PROCEDURES

Working ElectrodeThe rod used to produce the active section of the

RCE (diameter 1.99 cm, length 1.60 cm, and area10.048 cm2) met UNS S32550 (ASTM A479). High-

TABLE 1Alloy Percentage Elemental Composition (wt/wt) as given by UNS S32550 (ASTM A479)

Fe C Si Mn P S Cr Ni Mo Cu N

Bal. 0.04 1.0 1.50 0.04 0.03 24 to 27 4.5 to 6.5 2.9 to 3.9 1.5 to 2.5 0.10 to 0.25


CORROSION—Vol. 60, No. 6 563

density polyethylene ([C2H2]n) rod was used as inertsheathing material and electronic insulation. TheRCE was designed (Figure 1) for use with a PineInstruments Company, model AFMSRX† analyticalrotator fitted with the MSRX Arbor–ACMDI1906C†

rotator arm (1/4 UNF connecting thread). The elec-trode surface was degreased in ethanol (C2H5OH) andmechanically polished on micro-polishing cloth priorto each individual polarization and corrosion poten-tial measurement.

The mechanical polishing procedure, which wasdevised to give maximum reproducibility, consisted ofa double, 3-min polish with a 0.3-µm α-alumina(Al2O3) slurry. The electrode was initially polished tothis grade using silicon carbide (SiC) paper (< P 400)and Al2O3 particles (<5 µm). The initial polishes werefollowed by 1-min “cleaning” sequences with adouble-distilled water-soaked polishing cloth to re-move adhered alumina particles. The cleaning se-quence was repeated three times. Washing of boththe electrode and polishing cloth with a jet of double-distilled water interrupted each step in the overallpolishing procedure. Polishing materials were sup-plied by Buehler†. Pre-reduced surfaces were ob-tained by galvanostatic reduction in the deaeratedelectrolyte with an applied current density of–0.5 mA cm–2 for 200 s.

ElectrolyteNatural seawater was collected from a short-

term holding tank at the University of PortsmouthInstitute of Marine Sciences. The seawater in thetank was pumped from Langstone Harbour, Hamp-shire, United Kingdom and has a daily turnover. Theseawater was vacuum-filtered down to a 0.2-µm poresize membrane filter to remove suspended solids,microorganisms, and the majority of biologicalspores. The resulting electrolyte was subsequentlyreferred to as “filtered seawater.” The electrolyticconductivity (κ) of the filtered seawater was adjustedto 50.8 ± 0.1 mS cm–1 at 25 ± 0.2°C using a Metler-Toledo MPC 227† conductivity/pH meter. This valueof κ (corrected to 25°C) was taken from a long-termstudy of Langstone Harbour seawater.14 The filteredand conductivity-adjusted seawater (pH 8.05 ± 0.15)was stored in the absence of light at 3°C. Conductiv-ity adjustment of the filtered seawater ensured thatall physical parameters of the seawater remainedconstant throughout the period of the investiga-tion.25-26 Mean values of the physical and chemicalquantities of the filtered seawater including chlorideion concentration, salinity, and dissolved O2 aregiven in Tables 2 through 4.

Directly measured values of bicarbonate alkalin-ity and chloride ion concentration were measured viapotentiometric titration.27 Mean literature values ofthese parameters, along with a value of pH, also aregiven from a general review of tabulated data.25-26,28-30

All literature concentrations were converted from gkg–1 through a standard seawater density26 at 25°Cand 35‰ of 1.0234 g cm–3. With reference to Table 3,salinity (S) was measured in triplicate with a Profi-Line LF 197, WTW Measurement Systems, Inc.†

saliniometer, via conductivity25 measurements, andpotentiometric titration27 against AgNO3. Kinematicviscosity (ν) was determined with a B-type Ostwald/U-tube viscometer and dissolved O2 concentrationswere estimated with a Jenway 3420† dissolved oxy-gen meter. Literature values of O2 concentration alsowere calculated using the Weiss relationship and theTruesdale, et al., relationship assuming that whenκ = 50.8 mS cm–1, S = 33.4 ± 0.1‰.26,31 Partial pres-sures were calculated assuming Henry’s law constantfor in water at 25°C to equal 2.51 × 1010 K atm–1

(3.34 × 1012 K Pa–1).32 The literature values of relativedensity and kinematic viscosity were calculated fromtabulated data.25

General ProcedureAll electrochemical measurements were made at

25 ± 0.2°C with an Eco Chemie Autolab potentiostat(PGSTAT20† computer controlled) using the GeneralPurpose, Electrochemical Software† (GPES), version4.5. The electrochemical cell incorporated a thermo-statically controlled glass water jacket, a platinumgauze counter electrode, and a Radiometer Analytical

FIGURE 1. Sectional view of the RCE design.



A/S, REF 401,† saturated calomel electrode (SCE)used in conjunction with a Luggin-Haber capillary.The internal, wetted dimensions of the cell were 6 cmdiameter and 10 cm height, to give an approximateelectrolyte volume of 280 cm3. The resulting RCEradius/cell wall radius gap ratio of 0.33 is predictedto inhibit the formation of Taylor-Couette flow at lowrotation rates.33-34 No compensation was made forelectrolyte resistance. The electrolyte was aerated for5 min prior to the commencement of measurementwith a gas diffuser connected to an air pump. A sec-ond diffuser located at the surface of the electrolytealso was used to aerate during long-term procedures.The second diffuser had no observable influence overmass transfer either to or from the RCE. Deaerationwas achieved by purging with 4.0 spot (99.99%)nitrogen for 10 min. This time scale was adequateto remove oxygen to the purity level of the nitrogenused. Measurements in deaerated electrolyte weremade in conjunction with a nitrogen gas blanket toprevent air ingress. Data were collected over a rangefrom 100 rpm to 4,000 rpm (peripheral velocities, U,from 10 cm s–1 to 419 cm s–1). Table 5 lists the fullrange of values. Unless stated to the contrary, all po-tentials are stated with respect to the SCE. All of theexperimental data used in this work for the quantita-

tive derivation of rate constants and relative corro-sion rates were produced under steady-state condi-tions using either potential step or linear polarizationresistance (LPR) techniques.

RESULTS AND DISCUSSION

Corrosion PotentialCorrosion potential measurements were made for

both freshly polished and pre-reduced surfaces as afunction of electrode rotation rate. Mean transients(calculated from three repeated measurements) ateach rotation rate are given in Figures 2 and 3. Over-all, the reproducibility of the corrosion potential tran-sients was poor. For the polished and pre-reducedsurfaces, standard deviations (95% confidence limits)from the mean stabilized potential values were foundto be largely independent of rotation rate and variedbetween 14 mV and 61 mV, and 1 mV and 26 mV,respectively. The relatively large range of the stan-dard deviation for each surface was of a greater mag-nitude than the variation in the mean corrosionpotential, and it must be assumed, therefore, thatthe corrosion potential of the alloy has a value inde-pendent of mass-transfer conditions. It may be thecase that some error in the measurement of potential

TABLE 2Total Chloride and Bicarbonate Alkalinity Determined for the Filtered Seawater

Source of Value Bicarbonate Alkalnity (mol dm–3) [Cl–]/mol dm–3

Directly measured 0.0027 ± 0.0010 0.550 ± 0.013Literature (surface seawater at 35‰ and 25°C) 0.0024 0.558 – 0.559

TABLE 3Physical Parameters of the Filtered Seawater(A)

Source of Value pH κ (mS cm–1) S/‰ (g kg–1) Relative Density ν (×102)/cm2 s–1

Directly measured 8.0 ± 0.2 50.8 ± 0.2 33.4 ± 0.2(B),(C) 1.026 ± 0.050 1.07 ± 0.0134.4 ± 0.9(D)

Literature 7.9 ± 0.5 — — 1.028 ± 0.001 0.93 ± 0.01

(A) Literature values calculated assuming conductivity, κ, to equal 50.8 mS cm–1.(B) Direct readings from a commercial saliniometer.(C) Indirect determination calculated from conductivity measurements.(D) Conductivity measurements.

TABLE 4Values of Dissolved Oxygen Concentration in the Filtered Seawater at 25 ± 0.2°C

Source of Value ppm mol dm–3 atm Pa

Directly measured 6.90 ± 0.30 2.16 ± 0.01 × 10–4 0.167 17,093Literature 6.80(Α) 2.13(A) 0.167(A) 16,885(A)

(for κ = 50.8 ± 0.1 mS cm–1) 6.66(B) 2.08(B) 0.163(B) 16,489(B)

(A) Calculated using the Weiss relationship.(B) Calculated using the Truesdale, et al., relationship.

Concentration Partial Pressure



at the pre-reduced metal surface may have occurreddue to surface charging of hydrogen induced by thecathodic polarization.

The UNS S32550 stabilized corrosion potentials,however, appeared to be dependent on surface condi-tion with values of –330 ± 40 mV and –450 ± 40 mVfor the polished and pre-reduced surfaces. As will beseen later, the values for freshly polished surfaceswere more noble due to the presence of a thermaloxide film produced by the polishing sequence ratherthan a relatively base passive film grown at the pre-reduced surface during the potential equilibrationperiod. To maintain maximum reproducibility in allthe polarization procedures to follow, measurementswere initiated only after a potential stabilizationperiod of 2,000 s at the specified rotation rate.

Cathodic PolarizationNon-steady-state LSV in deaerated filtered sea-

water was used in the analysis of species on thefreshly polished surfaces. As Figure 4 shows, single,broad peaks were observed, the peak current densi-ties (jp) of which varied linearly with potential sweeprate. The linear dependence of jp on potential sweeprate would appear to indicate that the reduction pro-cess involved a surface film or surface-adsorbed spe-cies.35 The peak was not observed at any pre-reducedsurface. The broad potential range over which thepeak occurred is probably due to a significant ohmicpotential drop produced as a direct result of the highcurrents involved with reactions occurring at a large

electrode surface area covered with a film of limitedionic and electronic conductivity.

Assuming surface films formed on SS are oxides,Birks and Meier reported an approximate analysis ofthe composition of a thermal oxide formed at a15%Cr SS to be a layer of continuous mixed iron-chromium oxides (FeO·Cr2O3 and FeO·[Fe,Cr]2O3)overlaid with hematite (Fe2O3).

36 At lower Cr concen-trations, however, a thermal film was said to be

TABLE 5Values of Applied RCE Rotation Rate and Peripheral Velocity

rpm 100 200 300 400 600 800 1,000 1,200 1,400 2,000 3,000 4,000cm s–1 10 20 31 41 62 83 104 124 145 209 314 419

FIGURE 2. Typical mean corrosion potential transients for both freshlypolished and pre-reduced surfaces as a function of RCE rotationrate.

FIGURE 3. Stabilized corrosion potentials for freshly polished andpre-reduced surfaces as a function of RCE peripheral velocity. Theerror bars represent the standard deviation from the mean at 95%confidence limits (mean calculated from three repeated measure-ments at each velocity).

FIGURE 4. Non-steady-state linear sweep voltammetry at freshlypolished surfaces in static, deaerated, filtered seawater at potentialsweep rates of 10, 20, 30, 40, 60, and 80 mV s–1.



based primarily on iron oxides with discrete andisolated areas of the FeO·Cr2O3-mixed oxide. Underthermodynamically ideal conditions, the limitingmole fraction for film formation can be calculated togive an approximate estimation of the concentrationof chromium at the surface of the electrode on whichpure chromium(III) oxide (Cr2O3) will form on a Fe-Cralloy.13 Taking two distinct films of ferrous oxide (FeO)and Cr2O3 at the surface of either one or both of theaustenitic and ferrite phases, a limiting mole fractionof ≅0.58 Cr will be required to produce a pure filmof Cr2O3 at the electrode/thermal oxide film interface(based on a change in free energies of formationfor FeO ≅ 25527°C and Cr2O3 Cr ≅ 98627°C kJ mol–1 ofoxide).13

The value of 0.58 is considerably higher than themole fraction of 0.24% to 0.27% chromium initiallypresent at the alloy surface. It is unlikely that therewould have been a 31% to 34% decrease in iron atthe surface of the alloy during the mechanical polish-ing procedure. It is probable, therefore, that thesingle peak produced at the UNS S32550 alloy (24%to 27% Cr) in Figure 4 is due to the reduction of sur-face species, which can be approximated roughly byFeO[Fe,Cr]2O3 (but not pure Cr2O3). An attempt atgalvanostatic, electrometric reduction of the freshlypolished surface was performed in deaerated electro-lyte. As a result of the very low thickness of the oxidelayering, no potential arrests were observed.

Steady-state LSV was used to qualitatively studyO2 reduction and hydrogen (H2) evolution (reductionof water). Figure 5 shows two families of curves pro-duced at both the freshly polished and pre-reducedsurfaces at rotation rates of 100, 200, 800, and1,400 rpm. A linear y-axis is used to best show theoverall response of O2 mass transfer on electroderotation rate. From these plots, it is clear that boththe O2 reduction and hydrogen evolution reactionsare extremely irreversible. The influence of O2 masstransfer only becomes significant at potentials morenegative than –1.0 V (beyond which a region of mixedcharge-transfer and mass-transfer-controlled O2 re-duction is observed). Limiting mass transfer of O2,however, does not solely influence the measuredcurrent density prior to the exponential increasein the rate of charge-transfer-controlled hydrogenevolution (which is significant at potentials morenegative than –1.2 V). Consequently, a limiting cur-rent density “plateau” for O2 reduction is not ob-served. The absence of a limiting current regionindicates a rate of O2 reduction much slower thanthat quoted at the Introduction for both the austen-itic and duplex SS.

The cathodic polarization behavior of the metalsin the mixed charge and mass-transfer-controlledcurrent region was investigated in a more quantita-tive and reproducible manner using a potential step,current transient technique with hydrodynamic (ro-tation rate) steps.37 Typical trends in potential andelectrode rotation rate used to study O2 reduction inthe potential region of mixed controlled cathodic cur-rent are represented schematically in Figure 6. Aninitial potential (E) was applied and the rotation ratewas varied in increments (the time scale of whichwas adjusted to ensure a steady-state current re-sponse). The rotation rate then was lowered to theoriginal level; simultaneously, the potential wasstepped to a new and more negative value. The se-quence was repeated over the whole range of re-quired overpotentials (η).

The Koutecky-Levich approach38 to the analysisof mixed charge and mass-transfer-controlled datacan be used to accurately extract a current response,

FIGURE 5. Steady-state cathodic linear sweep voltammetry at bothfreshly polished and pre-reduced surfaces at a potential scan rateof 1.0 mV s–1.

FIGURE 6. Schematic potential and rotation rate transients, whichdescribe the potential step current transient with hydrodynamic stepconditions.



which is independent of species transport within theelectrolyte phase. A typical series of Koutecky-Levichplots are given in Figure 7 for pure O2 reduction onthe pre-reduced surface. All slopes are corrected forbackground current (resulting from hydrogen evolu-tion and the reduction of surface films) and plottedat equal increments of η = 50 mV throughout theregion of mixed-controlled current response. Abovea critical peripheral velocity, each surface conditionexhibited a rotation rate dependence (a change inslope) on the reduction mechanism. An apparentchange in mechanism from an irreversible reactionrate to a more reversible reaction rate was observedin each case at 600 rpm to 800 rpm (U ≅ 63 cm s–1 to84 cm s–1).

Extrapolation of the Koutecky-Levich plots toan infinite rate of mass transfer38 for each conditionproduced apparent “Tafel slopes” describing purecharge-transfer control of the reactions (Figure 8). Ineach case, the intercepts were derived from extrapo-lation of the currents measured at high electroderotation rates. High slope values for pure O2 reduc-tion were measured for both the freshly polished andpre-reduced UNS S32550 surfaces (–0.21 ± 0.01 Vdecade–1 and –0.27 ± 0.01 V decade–1, respectively).The large negative values are typical of slopes mea-sured at blocked surfaces and are the result of resis-tance provided by the surface film formed at UNSS32550. Moreover, these values are not an artefact ofthe Koutecky-Levich approach to current extraction.Similar slopes (–0.241 ± 0.010 mV decade–1 [freshlypolished] and –0.198 ± 0.011 mV decade–1 [pre-re-duced]) also were measured directly for the same sys-tem using the results of the cathodic linear sweepvoltammetry.

From the previously reviewed work, the rate con-trolling process in the charge-transfer mechanism ofO2 reduction can be assumed to be controlled by theexchange of a single electron (Reaction [3]):

O e O2 21+ →– – (3)

The rate-determining step can be the addition of thefirst electron to the surface adsorbed O2 molecule toform the peroxyl ion.39

The cathodic potential-dependent rate constantfor O2 reduction reaction was derived using Equation(2) and used in the assessment of the pure charge-transfer-controlled kinetics of the O2 reduction reac-tion. From the approximate parallel behavior of theKoutecky-Levich slopes,17 the order of reaction withrespect to O2 in each case was taken as 1 and thenumber of electrons exchanged in the rate controllingmechanism was taken as unity. These assumptionsare in agreement with the previously quoted works ofGojkovic, et al., Muralidharan, et al., and Babic andMetikos-Hukovic.12,16-17 Figure 9 describes the expo-

FIGURE 7. Koutecky-Levich plots for pure oxygen reduction at pre-reduced surfaces.

FIGURE 8. Comparison of Tafel slopes derived using the Koutecky-Levich equation for pure oxygen reduction at freshly polished andpre-reduced surfaces.

FIGURE 9. Plots describing the potential-dependent rate constantfor pure oxygen reduction as a function of negative overpotentialand surface condition.



nential response of the resulting potential-dependentrate constants for pure O2 reduction as a function ofmean overpotential.

The values of the rate constant for O2 reductionat the pre-reduced surface varied between approxi-mately 0.004 cm s–1 and 0.14 cm s–1 over overpoten-tials of –325 mV to –725 mV and 0.004 cm s–1 to0.16 cm s–1 for the polished surface and –450 V to–850 V for the polished surface. When the reactionrates are considered as a function of overpotential,the rate of O2 reduction is clearly greater at thepre-reduced surface than at the polished surfacesupporting a thermal oxide film. The relationshipbetween the rate of O2 reduction and applied poten-tial in each case can be described accurately bythe general exponential relationship shown inEquation (4):

r r

k kzFE

RTO oC

2= exp

(– )α (4)

where R and T are the molar gas constant and theabsolute temperature, respectively, and E is theelectrode potential. If the value of the cathodiccharge-transfer coefficient, αC, is assumed to equal0.5, the value of the potential independent, electro-chemical rate constant (

r

ko) for O2 reduction at thefreshly polished surface will be 2.00 × 10–7 cm s–1.The equivalent value for the pre-reduced surface in-dicated an order of magnitude increase in reactionrate at 3.00 × 10–6 cm s–1 relative to the value for afreshly polished surface. This data again gives fur-ther indication of the presence of two fundamentallydifferent species or thicknesses of surface filmpresent at the UNS S32550 alloy, which influencethe rate of cathodic charge transfer to a peculiarmagnitude.

Anodic DissolutionThe electrodissolution of the UNS S32550 alloy

freshly polished and pre-reduced surfaces was exam-ined initially via reverse LSV. The potential of theelectrode in each case was linearly swept from thestabilized corrosion potential to 1.2 VSCE. The poten-tial sweep then was reversed until a potential of ap-proximately –0.85 V was achieved. Voltammogramswere measured at a potential sweep rate of 1.0 mV s–1

for controlled RCE rotation rates in the range from200 rpm to 1,400 rpm. A series of typical plots mea-sured at 300, 800, 1,000, and 1,400 rpm are given inFigure 10 for freshly polished surfaces. The overall,large-scale current response of the plots was identi-cal between the two types of surface conditions. Aswill be seen, however, considerable variation was ob-served at low positive overpotentials.

No initial passivation peak was observed foreither surface condition, which indicated that theUNS S32550 alloy is self-passivating in each case.13

A relatively small but reproducible peak (jp = 5.8 ×10–3 mA cm–2) was always observed at 0.68 V on theinitial anodic sweep. This peak, given the identityPeak (a), was followed by a rapid increase in currentdensity at ≈0.98 V. At more positive potentials, thecurrent appeared to be influenced heavily by electro-lyte mass transfer where an apparent limiting cur-rent (indicated by broken line in Figure 10) wasalways observed. For example, at 200 rpm and1,400 rpm, the magnitude of the limiting currentincreased with rotation rate from 0.026 mA cm–2 to0.121 mA cm–2 for freshly polished surfaces and0.042 mA cm–2 to 0.165 mA cm–2 at pre-reduced sur-faces. The reverse scans universally exhibited zerohysteresis, indicating excellent re-passivation kinet-ics. A new corrosion potential was observed at 0.28 Vimmediately prior to a small, reproducible reductionpeak at 0.10 V (jp = –2.3 × 10–3 mA cm–2). This ca-thodic peak was given the identity of Peak (b). At stillmore negative potentials of –0.60 V to –0.500 V, alarge, irreproducible peak (jp = –0.05 mA cm–2 to–0.11 mA cm–2) was measured in each case.

West has shown that, thermodynamically, addi-tions of chromium to levels of 12% (wt/wt) and aboveconfers properties to SS very similar to that of chro-mium itself.40 In terms of regions of immunity, pas-sivity, and corrosion, the E-pH diagram for Fe-12Crin dilute chloride solution (equivalent to that of sea-water) was shown to be very similar to that of purechromium. More detailed E-pH diagrams for chro-mium in 3.6 mol dm–3 NaCl solution at 25°C (Figure11) have been produced by Macdonald, et al.41 Allow-ing for the standard hydrogen electrode (SHE) to SCEconversion, the double arrow on Figure 11 indicatesthe limits of the potentials examined in this work viathe reverse LSV technique.

It is probable that the anodic Peak (a) is the fullor partial conversion of the FeO·[Fe,Cr]2O3 surface

FIGURE 10. Typical reverse voltammetric plots measured at freshlypolished surfaces. The broken line indicates the approximate potentialat the limiting current.



film initially present at the SS to a new film consist-ing primarily of the now thermodynamically stableCr2O3. Both of these species can be assumed to havep-type semiconducting properties.42 On the returnscan, the new, more-noble corrosion potential canbe attributed to the corrosion potential at a pure ormixed film of Cr2O3 and the subsequent Peak (b) canbe taken as the reduction of this species. FromFigure 11, it also can be seen that in the absence ofpitting, further anodic polarization of the electrodewould lead to transpassive behavior at more positivepotentials where the soluble chromate(VI) (CrO4

2–)anion is the most thermodynamically stable speciesand a passive film of Cr2O3 will actively dissolve atthese potentials. It is likely that the mass transfer ofCrO4

2– away from the surface of the RCE is the ratedetermining step for the electrodissolution process atpotentials more positive than 0.98 V.

Diagnostic plots of log10 (limiting current density,jL) vs log10 (peripheral velocity, U) were produced todetermine the dependence of the rate of mass trans-fer on RCE peripheral velocity. The results were ap-plied to a simple power law relationship:

j k UL RCEn= × (5)

log log log10 10 10j k n UL RCE= + × (6)

From the resulting linear plots of log10 (jL) vs log10(U),velocity power values (n) were determined from theslopes as 0.69 ± 0.1 and 0.67 ± 0.1 for freshly pol-ished and pre-reduced surfaces, respectively. Bothlinear correlation coefficients (R) were acceptable ineach case at R > 0.97. The related values of the RCEpower law constant (kRCE) were obtained from theanti-log10 of y-axis intercept as 3.6 × 10–6 A cm–2 and5.7 × 10–6 A cm–2. The values of n are very close to themuch-quoted Eisenberg-Tobias-Wilke (ETW) value of0.70 for full mass-transfer control to or from theRCE.43 They also lie well within the precision quotedin the research literature for a fully smooth RCE,where values can range from 0.62 to 0.80.44 Takingvalues of n to be 0.69 and 0.67, plots analogous toLevich plots45 for the RCE were produced, which, ineach case, exhibited linearity and intercepts thatpassed through or close to the origin (Figure 12).From these criteria, the electrodissolution reactionfor the UNS S32550 alloy at potentials more positivethan 0.98 V can be taken to be under completemass-transfer control.

The relatively large and non-steady-state Peak (c)in Figure 10 is certainly the result of the remainingsurface film reduction (FeO·[Fe,Cr]2O3). The charac-teristics of Peak (c) did show some dependence on theelectrode peripheral velocity. A general decrease injp and the overall charge associated with the reduc-tion process was observed with increasing peripheral

velocity (jp ≅ –0.10 mA cm–2 and –0.04 mA cm–2 at200 rpm and 1,400 rpm [21 cm s–1 to 146 cm s–1],respectively). This could be due to either one or bothof the following:

—greater wall shear stresses at higher rotationrates, which facilitate the physical removal ofsolid electrodissolution products

—greater rates of mass transfer at higher rota-tion rates, where more rapid dissolution ofsolid electrodissolution products via CrO4

2– willreduce the existing film thickness more rapidlyat large positive overpotentials

It is more probable that the influence of dissolu-tion will dominate in this case due to the relativelylow wall shear stress applied during RCE rotation.23-24

At low positive overpotentials (η = 400 mV to 500 mV),plots of log10 current density (j) vs E were produced to

FIGURE 11. Redox potential-pH (Pourbaix) diagram for chromiumin 3.6 mol dm–3 NaCl solution at 298 K.41

FIGURE 12. Plots of limiting current vs Un for the mass-transfer-controlled electrodissolution as a function of surface preparation.



examine the polarization behavior of both freshly pol-ished and pre-reduced surfaces within the potentialwindow of –0.3 V to 0.2 V. Figure 13 shows such aseries of plots given as a function of RCE rotationrate where all rotation rates from 200 rpm to1,400 rpm are included for each surface condition.It is apparent that the currents within this potentialrange are very dependent on surface condition, butcompletely independent of electrode rotation rate. Noanodic Tafel region is observed for the pre-reducedsurface, which appears to achieve a limiting/passiva-tion current at η values immediately positive of thecorrosion potential. The equivalent currents at thefreshly polished surface were initially much lower. Asubsequent linear increase in reaction rate for thissurface was observed prior to the achievement of alimiting/passivation current of similar magnitude tothat of the pre-reduced surface. At a potential of0.300 VSCE, the passivation current density for polishedand pre-reduced surfaces were 2.6 ± 0.3 µA cm–2 and3.6 ± 0.2 µA cm–2, respectively.

The differences in anodic polarization behavior atpotentials close to the corrosion potential also can beexplained through the different types of corrosionproduct film at the surface of the SS. The freshly pol-ished surface is overlain with a thermal oxide filmwith an origin from the mechanical polishing, whichhas a comparatively high electronic and/or ionicresistance. A passive film, distinct from the thermaloxide film, with comparatively low electronic resis-tance, is predicted to have formed on the baresurface after cathodic pre-reduction. The differingconducting properties of these films have induced thedissimilar cathodic and anodic polarization behavior.At potentials immediately anodic to the corrosionpotential, it is proposed that a transformation of thethermal oxide film occurs at a rate that is directlyproportional to the magnitude of η (applied energy).

The transformation is indicated to be from a veryprotective thermal oxide to a less-protective passivefilm similar to that existing at the pre-reduced sur-face. The process can be assumed to be similar to theanodically induced transformation of an anodizedfilm (with highly protective characteristics) to passivefilm (with lesser protective characteristics). In thiscase it is not unreasonable to assume that time fortransformation is dependent on applied field (V/d)strength, where V is the potential difference across afilm of thickness, d. If this interpretation is assumed,the mean anodic “Tafel slope” of 318 mV decade–1

measured at the polished UNS S32550 surfaces can-not be due to a true dissolution reaction, but rathera solid-state conversion reaction that proceeds as afunction of potential field strength. The reactioncould be either a modification of the existing mixedFeO·[Fe,Cr]2O3 lattice structure or the oxidation ofthe components to give an inherently less-protective/more soluble film.

Linear Polarization ResistanceSince true, anodic Tafel slopes from UNS S32550

could not be extracted from either surface condition,it was not possible to apply a simple Tafel slope ex-trapolation to the mixed potential to obtain corrosionrates.46 Moreover, it was not possible to derive anaccurate proportionality constant (B) for use in theStern-Geary equation.47

jR

BRcorr

A C

p A C p

=×

× +=

β ββ β2 3. ( ) (7)

where jcorr, Rp, βA, and βC are the corrosion currentdensity, resistance polarization, and the anodic andcathodic Tafel slopes, respectively. Independent LPRmeasurements were made, however, to obtain relativerates of corrosion. In every case, polarization wasachieved at a potential scan rate of 0.167 mV s–1 andoverpotentials of ±10 mV (cathodic to anodic). Be-tween each measurement, the electrode was repol-ished, pre-reduced (if necessary), and allowed toachieve corrosion potential stabilization period at thespecific rotation rate. All measurements were per-formed in triplicate. From the LPR results given inFigure 14, the precision of the measurements wasalways lower and more significant than any apparentrotation rate-dependent trend in 1/Rp. Assumingcomplete independence of rotation rate, therefore, anoverall 1/Rp value of 0.018 ± 0.015 mA V–1 cm–2 wastaken for either surface condition. For comparativepurposes, this value is 94% to 98% lower thanequivalent values measured for nickel aluminiumbronze (BS 2874: 1986: CA 104[2]) and 90-10 copper-nickel (BS 2874: 1986: CN 102) under identical sur-face preparation and experimental conditions(measured at a RCE rotation rate of 4,800 rpm).44

FIGURE 13. Plots of log10 anodic current vs applied potential as afunction of RCE rotation rate.



Such a low rate of corrosion is to be expected if thepolarization behavior of the alloy is considered. Theextremely irreversible rates of both O2 reduction andhydrogen evolution act together with an effective,self-passivating anodic protection mechanism toprovide a mixed potential under complete charge-transfer control with a very low corrosion current.

CONCLUSIONS

❖ The UNS S32550 alloy exhibited excellent passiva-tion and re-passivation characteristics in the filteredseawater at turbulent flow velocities of up to 1.5 m s–1.In this relatively short-term exposure study, thematerial exhibited a high corrosion resistance to thefiltered seawater when in both the freshly polishedand pre-reduced states. The kinetics of oxygen re-duction and hydrogen evolution were severely irre-versible and, as with the anodic polarization behaviorat low overpotentials, the reaction mechanisms dis-played significant dependence on the surface condi-tion. The surface films found at the freshly polishedand the pre-reduced surfaces, respectively, were pro-posed to be predominantly the following:

—a thermal oxide film produced by the wet pol-ishing procedure with comparatively high elec-tronic and ionic resistance; and

—a passive film formed on the pre-reduced sur-face over the corrosion potential equilibrationperiod with comparatively low electronic resis-tance.

❖ Both cathodic and anodic polarization close to thecorrosion potential behaved independently of reac-tant/product mass-transfer rates. Thus, if the ex-perimental conditions of this study are extrapolatedto the field, the corrosion rate of UNS S32550 is pre-dicted to have little dependence on fluid velocity via amass-transfer-facilitated, flow-induced corrosionmechanism.

ACKNOWLEDGMENTS

The authors acknowledge DSTL-Farnborough,U.K. and QinetQ-Haslar, U.K. for financial contribu-tions to the research program. UNS S32550 materi-als were supplied by Meighs Ltd.

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FIGURE 14. Inverse resistance polarization vs RCE peripheralvelocity for freshly polished and pre-reduced UNS S32550 alloysurfaces in the aerated, filtered seawater.



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Documents

Electrochemical Study of UNS S32550 Super Duplex Stainless Steel Corrosion in Turbulent Seawater Using the Rotating Cylinder Electrode