NEW METHODS OF QUANTITATIVE ANALYSIS OF LOCALIZEDCORROSION USING SCANNING ELECTROCHEMICAL PROBES

K. R. TRETHEWEY, D. A. SARGEANT, D. .T. MARSH and S HAINESRoyal Naval Engineering CollegeManadonPlymouthPL5 3AQ, UK

ABSTRACT. A new commercial(v-tlesigned instrument using the Scanning Reference ElectrodeTechnique (SRET) has enabled the measurement of localized current densities in the vicinity ofpits in a stainless steel in natural seawater. The instrumentation is described in detail, togetherwith the novel techniques used to calibrate the system and convert variations in local voltage,resulting from the ion flux close to surfaces, into localized cumnt densities. Novel appliedpotentiodynamic pitting scans have been obtained for localized areas immediately adjacent toaccurate(v defined rexions of electrode sUifaces. New techniques such as this is essential ifimproved models ur localized corrosion ure to be develuped.

1. Introduction

Variations in electrochemical potential across metal surfaces in aqueous solutionwhich lead to localized attack have been demonstrat~d many times. The earliestreports were from the laboratories of Evans [1-3], whilst other contemporary workwas carried out by Jaenicke[4, 5]. These descriptions involved macroscopic bimetalliccouples of zinc and steel. Equipotential and ion flux lines were determined in theadjacent electrolyte by means of manually moveable reference electrodes incombination with Luggin capillaries: silver/silver chloride electrodes were used insodium chloride electrolytes and mercury/mercury sulphate electrodes in sulphatesolutions.Using the principles thus demonstrated, various attempts were made to construct

dedicated apparatus to measure localized activity over samples which were notnecessarily macroscopic bimetallic couples. The progress of these developments wasreviewed periodically [6, 7]. In 1974 a device called a "corrodescope", was used toexamine pitting of mild steel in ferric chloride solution [8]. Another device called a"scanning microprobe potentiometer", based on an earlier design by Johnston [9],was described by Gainer in 1979 [7]. This was used to examine pitting of mild steelin very dilute sodium chloride solution and, after comparison of the electrochemicalbehaviour of tin, antimony and platinum as the material in the microprobe,described a preference for tin.Almost all the studies were limited to the technology available at the time and

involved painstaking measurement and recording methods. Comparatively

417

K. R. Trethewey and P. R. Roberge (eds.), Modelling Aqueous Corrosion, 417-442.© 1994 British Crown.

418

unsophisticated mechanical scanning mechanisms were adopted with either anoscilloscope [7, 8, 10, 11] or chart recorder [12, 13] for data display. Recent majoradvances in computing and microelectronics have opened the way for new, moresophisticated designs of dedicated instrument, whether semi-automatic and usingmicroprocessors for digital data acquisition [14, 15] or others whollymicrocomputer-based [16-19J.Perhaps the most successful experimentalist has been Isaacs whose name,

Scanning Reference Electrode Technique, SRET, was applied to an apparatus whichhe used to study pitting and intergranular corrosion in stainless steels [11). Isaacs hasalso been closely associated with other innovative designs, notably the ScanningVibrating Electrode Technique, SVET, in which the probe is mounted on abiomorph piezoelectric reed which vibrates the tip normal to the electrode at acharacteristic frequency [20). SVET has been used to investigate the initiation ofstress corrosion cracking [21], surface heterogeneities [22], investigation ofprecipitation in aged duplex stainless steels [23] and galvanic corrosion (24). Anothervariation of the technique has been used in localized measurements ofElectrochemical Impedance Spectra (LEIS) [10, 13, 25, 26].In 1989, Bard and co-workers [27-29] described precisely a technique which they

named Scanning Electrochemical Microscopy (SECM). In this, a d.c. coupledscanning tip generates signals from currents carried by redox processes at tip 'andsubstrate. By this time, a device known as the Scanning Tunnelling Microscope [30]had been described. Though similar to SRET and SECM because of its use of a tipto scan over a substrate surface and in the methods of moving the tip, it isfundamentally different in principle of application and range of applications,depending upon a flow of non-Faradaic tunnelling current between tip and substrate.Ultra-microelectrodes have now been used in many electrochemical applicationswithparticular advantages in studies of localized corrosion [31, 32].At the Royal Naval Engineering College and Uniscan Instruments Ltd., a new

SRET instrument has been developed to a commercial standard with advanced datacollection and analysis facilities. In conjunction with a new method of calibration forion flux in the immediate vicinity of regions of local activity, the instrument has beenused by us in applications which demonstrate considerable advantages over previousdesigns (33). One of the greatest problems facing corrosion engineers is lifeprediction in situations of localized corrosion. Traditional electrochemical techniquesinvolving potential/current density measurements over bulk specimens have beenpoor in this respect and modelling of localized corrosion has been made verydifficult. Techniques which improve the quantitative understanding of localizedcorrosion are thus extremely valuable for the development of new models oflocalized corrosion (34). This paper describes the novel features of the design andillustrates the power of the device with our measurements of localized pitting currentdensities and potential profiles.

419

2. Materials used in corrosion experiments

The corrosion test material used in the experiments described here was a martensiticstainless steel, FV448 of composition 10.6% Cr; 1.11% Mn; 0.75% Ni; 0.38% Si;0.28% Nb; 0.28% Mo; 0.14% V; 0.1% C; trace P, Co, N; rem Fe. A specimen ofbar with outside diameter 12 mm, 37.5 mm circumference, was polished to a 2 JLmdiamond paste finish, cleaned, degreased and rotated at 150 rpm. The electrolytewas fresh, aerated natural seawater obtained from Plymouth Sound with a consistentconductivity of 50 mS and pH of 8.2.

Figure 1 - The commercial version of the SRET equipment.

3. Design of a modern instrument

3.1. GENERAL DESCRIPTION

The latest commercial quality SRET instrument, Figure 1, is illustratedschematically in Figure 2. It consists of a conventional three electrode corrosion cellwith reference (Saturated Calomel Electrode, SCE) and auxiliary electrodesconnected to a potentiostat controlling the d.c. polarization of the specimen, orworking electrode. A cylindrical specimen (in the form of either sealed tube or bar)

420

immersed in the electrolyte is configured as the working electrode and is rotated atspeeds of 5 - 250 rpm by a computer-controlled, indirect drive stepper-motor. Amagnetic triggering device mounted on the motor shaft provides synchronised pulsesfor the data collection programme and allows the precise identification of positionon the surface of the specimen.

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., ..

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-

Figure 2 - Schematic diagram of the SRET equipment.

A significant feature of the design is the commercially manufactured SRET probewhich consists of a pair of platinum electrodes made from wire of diameter 0.2 mm.The probes have electrochemically sharpened tips of approximate radius l#Lm and,apart from the tips, have as little platinum exposed to the electrolyte as possible:protruding platinum is coated with an insulating layer once the body of the probehas been fabricated. One tip is closer to the specimen surface than the other and theseparation distance is a few mm. The front probe samples the electric field createdby the ion flux close (10-20 #Lm) to the surface of the specimen, whilst the rear probesamples the noise in the bulk electrolyte. The output from the platinum electrodesis taken to an a.c. coupled differential amplifier before being digitised into a formthat the computer can use and display. For a description of the electrochemistry, the

Figure 3a - The SRETdifferential platinum probe

with sharp platinumelectrode tips at bottom left· c.

C1 CI

421

Figure 3b - Schematicdiagram ofprobe

construction.

..n

Cgpper Noise Shield (CNS) -Extertor po¥-' mouWlftg -_ ..... (f'Pl

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422

reader is referred to the work of Bard [27-29). The control unit, as well asmonitoring the detected corrosion signal, trigger pulses, potentiostat voltage, andcorrosion current, houses the power supplies and stepper motor control cards. Thewhole equipment is PC computer controlled and takes full advantage of the latestdevelopments both in hardware, for optimum data acquisition, handling and storage,and software for high resolution colour displays and versatile, modern DOS andWINDOWS-based user interfaces.

3.2. PROBE MATERIALS AND DEVELOPMENT OF THE DESIGN

The most significant factor determining system sensitivity is probe design. There aremany factors which contribute to the response of the probe such as geometry, tiplength and profile, material and degree of activation. Considerable effort has beenexpended at RNEC to optimise these factors so that the present design generatesextremely sensitive, stable data with high resolution. In the past, probes used toinvestigate localized corrosion activity have been made of metal/metal oxide ormodified versions of the Saturated Calomel Reference Electrode. The Calomelprobes used in conjunction with Luggin capillaries were reported as unsuitable foruse in dilute electrolytes because of the leakage of ions from the comparativelyconcentrated electrolyte in the Luggin tube. Almost all the probes described in theliterature have been configured for connection to single-ended electronics and arestrongly susceptible to electrochemical and radiated noise pick-up. The tin-filledglass capillary probe, first recommended by Gainer [7) was successful in the earlydevelopment of this instrument [35] but difficulties in conversion of the design to anintegral differential configuration limited its application.In 1988, Ford [36) demonstrated the advantages of using a differential probe

configuration over the conventional single-ended system used by Isaacs and Vyas[11]. It was originally proposed because of the Common Mode Noise Rejection(CMNR) benefits offered by modern op/amp signal conditioning but we have foundthat it can affect the spatial resolution in a complex way. To overcome this and toprovide a basis for quantification, the calibration routine based on a goldpoint-in-space (PIS) specimen was developed [33). Work is continuing in ourlaboratories on an a.c. calibration process which uses the test sample and does notrequire a separate PIS specimen.The probe, Figure 3a, provides excellent noise reduction properties and good

immunity to radiated electromagnetic interference. This differential platinum unitwith an electrochemically profiled tip of about 2 JLm width can be readily optimisedin the laboratory to suit a particular set of environmental conditions. Otherconfigurations are available for different specimen geometries and specialapplications requiring particular immunity from aggressive environments.The latest probe design is shown incross-section in Figure 3b: both front and rearprobes extend from the probe tip to the electronic connector, with a copper noiseshield (CNS) to provide immunity to electromagnetic radiation. The probe body is

423

of injection moulded polymer with conical geometry at the tip. The whole assemblyis coupled to the electronics via a three pin connector waterproof to IP65 (DIN40050). Figure 4 illustrates the probe outputs from four different probes of gold andplatinum, indicating how, for given experimental conditions, the probe output canbe maximised by achieving the correct sensitization of the probe material.

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Figure 4 . Comparison ofprobe output from gold and platinum probes in different sentizations andidentical experimental conditions.

3.3. PROBE TIP PROFILING

Besides such parameters as electrolyte resistance, electrochemical parameters of theprobe material, and physical dimensions such as the ratio of exposed probe lengthto height above the surface, the main parameters controlling the spatial resolutionof the SRET system are the dimensions and profile of the probe tips. These tipparameters are optimised after much experimentation by electrochemical polishingprinciples similar to those used for transmission electron microscopy specimenpreparation. An etching solution is used consisting of 60% (v/v) saturated calciumchloride, 4% concentrated hydrochloric acid solution, 5% glycerol, 31% distilledwater, prepared at room temperature. The probe is mounted in a small automaticdevice incorporating a motor and cam arrangement. This allows the probe tip tooscillate vertically in and out of the solution with 10 V a.c, applied between it anda platinum cathode, also in the solution. The etching process continues until the tipno longer makes contact with the solution thus self-limiting the process and

424

achieving a very sharp tip. Alternatively, immersion in fresh etchant solution for aperiod of up to 30 min with periodic inspection can be used. The speed of oscillationand the displacement are adjustable to optimise the tip profile. A typical probe tipis about 2-3 ~m in diameter, thinned over a length of 0.5 mm. Ideally, the probe tipis conical, but most tips have slightly convex sides and a few are concave.Considerable effort has been expended to develop procedures which apply optimalcoatings to the platinum exposed beyond the encapsulant such that only the actualtips of the platinum are exposed to the electrolyte. The final procedure, platinising,is carried out in a solution of platinic chloride (3 g) and lead acetate (0.2 g) in 100ml distilled water. An applied potential of up to 1-2 V a.c. is necessary to form athin, sooty deposit in a few seconds.

3.4. 'SHAPE FACTOR' OF THE PROBE OUTPUT SIGNAL

Spatial resolution and probe sensitivity are related in a complex way to thesignal-to-noise ratio and ion selectivity produced by the probe response to changesin ion flux density. It is therefore necessary to describe differences in probe responseduring experimentation. The approach adopted involves describing the responsecurve by a parameter called the 'shape factor'.Communications engineers often use a parameter called a '0' factor, where 0 is

the ratio of the inductive reactance at resonance to the circuit resistance. Theresponses of filters with upper and lower frequency limits are sometimes describedby the 0 factor method where 0 is the ratio of the centre frequency to thebandwidth. Unfortunately, 0 can be affected by the point at which the bandwidthis specified. After considerable work, it has been decided to define the SRET proberesponse shape factor as the ratio of the maximum peak height to the full width athalf maximum (FWHM). Figure 4 includes the calculated shape factors for outputsfrom four different probe materials and sensitizations.

3.5. SYSTEM SENSITIVITY: MINIMUM DETECTABLE SIGNAL (MDS)

Most of the experimentation carried out at RNEC has involved natural seawater.However, the sensitivity of the SRET equipment is considerably enhanced in moredilute electrolytes. Figure 5 is a log-log plot of probe output versus electrolyteconductivity in aqueous solutions of sulphuric acid, hydrochloric acid, iron (III)chloride and sodium chloride of varying concentrations. A PIS specimen wasimmersed in various electrolytes and a constant current of 0.1 rnA applied for avariety of electrolytes, all at 20°C. When these results are translated into currentdensities by the methods described above, it is found that current densities of theorder of 10-7 A cm-2 can be detected in the very low conductivity electrolytes. It isalso found that as the probe output increases with decreasing conductivity, theresolution of the probe, as described by the shape factor, is not adversely affected.Additionally, the signal-to-noise ratio for the different electrolytes remains constant.

425

It has been concluded from these results that the shape factor is affected by theprobe geometry and its position relative to the specimen. This has given goodinformation for optimisation of the probe manufacturing technique for maximumresponse. It also means that further improvements to the probe tip profiles willresult in even better performance. In summary, spatial resolution, sensitivity andsystem noise are all improved by optimising the geometry, using fine tip diametersand by use of coated shanks of the exposed platinum probe tips. The best systemperformance is achieved when the front and rear probes have equal active areasexposed to the electrolyte.

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Figure 5 - A log-log plot ofprobe output versus electrolyte conductivity in aqueous solutions ofhydrogen sulphate. hydrogen chloride, iron (1Il) chloride and sodium chloride of varying

COlicentrations.

3.6. THE POINT-IN-SPACE EXPERIMENT: CURRENT DENSITYMEASUREMENTS AND SYSTEM CALIBRATION

Quantifying the degree of localized corrosion in terms of current density, as opposedto simply measuring the amplitude of the electric field adjacent to the source ofactivity, has been a major goal of the SRET development programme. To achievethis a reliable calibration routine has been devised which takes into account theprevailing electrochemical conditions and other variables affecting the level of SREToutput. It uses an active 'point-in-space' (PIS) principle by which known current isdriven through an electrode of known surface area. The signal measured at theSRET probe can then be used as calibration factor for the current density sensedin solution provided that identical experimental conditions prevail during the

426

corrosion test. Since the calibration process is fundamental to the quantitativeaspects of the SRET instrument, significant space will be devoted in this paper todiscussion and validation of the techniques used. Having validated the techniques,the act of calibration in anyone experiment is comparatively straightforward.Ultimately, it may be possible to incorporate most of the actions into the instrument;for the present, however, it is necessary to carry them out manually.

known, appliedcurrent

10micron ..j

t(gold) PISspecimen

insulation

rotation

ion flux

•

to differentialamplifier

front probe:samples ion flux

rear probe:samples noise

Figure 6 - Diagrammatic representation (not to scale) of the point-in-space calibration experiment.

The PIS specimen used, Figure 6, was a 0.2 mm diameter gold wire mounted ininsulating sleeving such that only the flat cross-sectional area of 3.140 x 10-4 cm2 wasexposed to the electrolyte. Five experiments, (a)-(e), were conducted.

(a) To establish the relation between the detected signal level and thespecimen-to-probe distance, the specimen was immersed in natural seawater androtated at 100 rpm. A constant current of 2.0 rnA was applied between the PISspecimen and an auxiliary carbon electrode of diameter 5 mm, immersed to a depthof 100 mm and located at a distance of 200 mm behind the probe, colinearly withthe PIS and probe as the PIS passed the probe. The distance of the scanning probefrom the PIS was varied and the detected signal was recorded. The results are shownin Figure 7a. For distances up to 500 j£m, the relationship was linear and of the form

y (mV) = -0.052x (jLm) + 43.

At an operating distance of 20 j£m, a current of 2.0 rnA applied between the PISspecimen and an auxiliary carbon electrode produced a probe output of 40 mV atunit amplifier gain. (It should be remembered that these figures are

427

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Figure 7 - SRET calibration curves: a. (Top) probe output versus probe tip-to-specimen distance.Linear behaviour is taken only over the distance 0 to 500 1JI1l.

b. (Bottom) probe output versus PIS current density.

428

probe-dependent and re-calibration is required each time the probe is changed ordifferent experimental conditions are used.)

(b) To test the relationship between probe output and PIS current density a secondcalibration experiment was conducted with the system gain set to 500X in order tosimulate typical operating conditions. The results, Figure 7b, showed that therelationship between probe output and applied current density was also linear overthe range tested.

(c) To validate the method of calibration, an area scan of the PIS was recorded andthe net fluxes over a defined area of a plane cutting the electrochemical current pathwas compared with bulk measurements. Figure 8a shows the area map resultingfrom the PIS calibration experiment, whilst 8b shows the three-dimensional plot ofthe data. The contour map in Figure 8a is taken directly from the instrument (colouris actually used) and shows the actual potentials measured by the SRET probeacross the plane measuring 10.3 mm x 7.7 mm perpendicular to the ion fluxemanating from the PIS specimen. The potential contours are stepped according toa user-determined scale of magnitudes and colours. Each pixel in the map isobtained from a sensed potential over a region of space 20 JLm square. Because ofthe great size of the data files involved, these were reduced for the 3-d plots byaveraging over ten measurements. Thus, the flux emanating from the PIS specimenis translated into a matrix of signals, measured in millivolts, each millivolt signalrepresenting the current flux over an area 200 JLm x 200 JLm. If we assume that thesum of all these (positive and negative) signals is equivalent to the total fluxemanating from the PIS specimen, then the calibration can be achieved by summingall the potentials. A value of 4206 mV was obtained and made equivalent to the bulkcurrent applied to the PIS of 17.5 JLA.It is noticeable that the flux occupies a larger area than that of the PIS electrode

surface, indicating that there is a 'splaying out' of flux lines from the surface into theelectrolyte. The currently adopted probe geometry and material gives the maximumsignal for any given set of system parameters and approximates to a conicalgeometry. The specimen is normally actively polarised against a graphite rod counterelectrode. It might be expected that lines of flux between working and counterelectrode are approximately parallel, the SRET probe sampling the flux across aplane perpendicular to this flux. However, even though the sample-probe distanceis small (10-20 JLm) and the specimen geometry for a comparatively large diameterapproximates to a planar surface from which the ion flux is emitted, the flux divergesin the electrolyte. Thus the flux detected by the probe occupies a greater area thanthe active area on the specimen and produces an apparently greater feature than isactually present. Obviously, this affects the resolution of closely-spaced features onthe surface by the instrument: both resolution and signal size are improved byreducing the specimen-probe distance and this emphasises the need for preciselocation of the probe tip at a given distance from the surface being scanned. This

429

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Figure 8 - a, (Top) Two-dimensional area scan, dimensions in microns, obtained directly from thesoftware during the calibmtion experiment (c) with the PIS specimen This is actually a colour

photogmph with the colour graduation scale in millivolts visible at the bottom. b, (Bottom) Threedimensional graph constructed from the data in Figure 8a showing the distribution of the signal overthe area scanned. (Note that it is not representative of a physical pit.) The vertical calibmtion (z-axis)

is in millivolts, whilst the x- and y- axes calibmtion steps are each about 200 microns.

430

effect is also dependent upon the experimental conditions, notably, electrolyteconductivity, and the surface finish of the specimen.

(d) A cylindrical specimen of FV448 was mounted in cold-setting resin in such a wayas to expose a rectangular area approximately 5 mm x 2 mm. At time zero, an areascan was initiated of the whole of the exposed metal using an amplifier gain of 1000,the same as had been used in the calibration experiment. The specimen wassubjected to an applied potentiostatic voltage of 0 V SeE, rotating at 100 rpm innatural seawater. During the time of the scan, 305 seconds, the net current flow tothe working electrode (specimen) was measured. At the end of the scan, pits hadbeen created on the specimen, the areas ofwhich were determined by image analysistechniques.

Figure Y - Scanning electron micrograph of the specimen used in calibration experiment (d) aftercompletion of the area scan.

Figure 9 is an electron micrograph of the FV448 surface after completion of thearea scan. In this case, it is apparent that pitting activity was focused at creviceswhere the specimen joined the mounting material. This is a common problem insuch experiments, hut does not invalidate the concept of the experiment. Figure lOashows the area map corresponding to the surface shown in Figure 9 and Figure lObshows the equivalent three-dimensional graph of the activity.

431

j surface

Figure 10 - a. (Top) Two-dimensional area scan, dimensions in microns, obtained directly from thesoftware during the calibration experiment (d) with the FV448 specimen. b. (Bottom) Three

dimensional graph constructed from the data in Figure lOa showing the distribution of the signal overthe area scanned. The vertical calibration (z-axis) is in millivolts, whilst the x- and y- axes calibration

steps are each about 160 microns.

432

The next phase of the procedure involves the creation of a matrix of millivoltagemeasurements for the pitted surface. The same summation that was used in (c) gavea total of 686940 mV which can be converted

total current = 686940/4206 x 17.5 /LA = 2.86 rnA.

This compared to the actual bulk current applied to the pitting surface of 1.8 rnA.

(e) In order to provide additional support for the result in (d) an alternativevalidation procedure was used. The maximum probe output measured from the PISspecimen was 57 mY. This was taken to represent the total current density of thePIS specimen: 17.5/LA over the PIS area of 0.0314 mm2, Le. 0.557 rnA mm2.

TABLE 1Results from calibration validation experiment (e)

Total current from the six pits =

Pit number

1

2

3

4

5

6

Current density/mA cm,2(from probeoutput)

953.8

1147.5

747.1

712.0

1144.1

576.6

Area of Pit/10,4 cm2

1.139

1.112

0.611

2.149

1.697

2.172

Pit Current/mA

0.1086

0.1276

0.0456

0.1459

0.1942

0.1252

0.7471

Six pits were identified on the specimen, Figure 9, and correlated with thescanned image, Figure lOa. The maximum probe outputs were converted to currentdensities for each of the six pits and the areas of each pit calculated by imageanalysis. This allowed calculation of the local currents from each pit which, whensummed gave the total current. Table 1 lists the results obtained. The current fromthe six pits was found to be 0.747 rnA which compared to 1.8 rnA from the bulkspecimen.

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3.7. DISCUSSION OF CALIBRATION TECHNIQUES

The correlation shown in experiment (d) is taken as a clear vindication of thecalibration process. Similarly, in experiment (e) the total current supplied to thespecimen of 1.8 mA compared well with 0.747 mA calculated from the SRET probelocal measurements of the six pits examined. The correlation of bulk currentssupplied to a working electrode with those measured by SRET local measurementsafter calibration is very good considering the possible difficulties. In the conditionsused in calibration experiment (c) it is clear that some flux is being detected by theSRET probe even at the extremities of the area measurement. Thus, the currentindicated by the potential summing over the matrix of measurements is lower thanit should have been. Offset against this is the fact that in experiment (d) the activityover the FV448 specimen was high at the edges of the specimen and activity in thisexperiment too was well beyond the metallic area and even beyond the resin areaincluded in the scanned region. Thus, although the current calculated by the matrixsumming method was greater than the applied current (2.86 mA compared to 1.8mA respectively), this is undoubtedly due to flux in solution beyond the area ofmeasurement. This may be either positive or negative and so a calculated currentgreater or less than the applied current is equally likely. This effect is irrelevant tothe support of the calibration procedure. Once the PIS signal has been quantifiedin terms of the probe output, the SRET instrument is clearly capable of displayinglocalized current densities over regions of activity to within a factor of 2 to 3,provided that:

(a) The specimen-to-probe distances are the same in both cases;(b) The areas of activity of PIS and local anodes are comparable; and(c) All other experimental conditions are constant.

Similarly, in experiment (e) the correlation is good. The current obtained by localmeasurement was less than half that measured by the ammeter: inevitably, there arepits which are beyond the resolution of the SRET probe and which contribute to thedifference observed. This experiment was less precise than experiment (d) and wasused only to provide additional support for it.The variable sensitivity of micro-electrodes to ions in solution is well established

in relation to the electrochemistry of ion selective electrodes. The use of a PISspecimen of the same material as that under test was tried but not adopted as astandard technique because of the difficulties associated with the dissolution processon the tiny areas involved: inevitably, the PIS specimen suffered extremely rapiddissolution and in an irregular, localized manner which interfered with thecalibration process. For a time, gold was chosen as the material for both scanningprobe and PIS specimen because of its extremely good electrochemical stability andcorrosion resistance, and because experiments with platinum initially gave unstablebehaviour, possibly because of inexperience in preparation of the electrodes and the

434

acquisition ofproperly electrochemically activated surfaces. Once the manufacturingtechniques had been perfected, platinised platinum became the preferred materialfor probes because of a greater sensitivity and ease of manufacture. Platinum givesa more robust performance whilst gold is soft and easily damaged. Gold, however,remains a suitable choice for PIS specimens.Under any conditions, net current density is the sum of current densities of all

oxidation and reduction current densities. Particularly under applied polarisationconditions in which a specimen actively corrodes, the net current density isdominated by the oxidation current density, often involving conversion of metalatoms to oxidised species [37]. Thus it could be argued that the determination of theSystem Calibration Factor (SCF), that is, the factor relating the measured probepotential to the net ion flux at the point of measurement can indeed be used toprovide a reliable, quantitative measure of corrosion current density.

3.8. EFFECT OF SPECIMEN ROTATION SPEED

The effect of electrolyte velocity, i.e. specimen rotation speed, is a subject worthyof discussion. A specimen was immersed in natural seawater at ambient temperatureand rotated at speeds in the range 0 to 0.8 m sol. The variation of probe signal withspecimen rotation speed, Figure 11, is a result of increased probe sensitivity as itcuts ion flux lines at an increasing rate: no signal is detected when the specimen isstationary. Apart from a small change in the velocity range 0 - 0.1 m S-l, the effectis negligible. In a typical experiment with rotation speeds of up to 250 rpm andspecimen diameters of 30 mm maximum, bulk electrolyte velocity is a maximum of0.4 m sol. This velocity is not sufficiently great to induce turbulent hydrodynamic flowin the boundary layer and hence no evidence of electrolyte flow is apparent in theresults: potential contour maps around pits are circularly symmetric about the centreof the pit when there is no interfering effect due to adjacent corrosion sites. (Figure8a is a good example of this, although its aspect ratio from the software is notcorrect.) In a very few experiments, some asymmetry of the potential map wasascribed to velocity effects. A consequence of this is that the rotation of thespecimen plays no role in determining the solution chemistry within pits, other thanin providing oxygen saturation of the bulk electrolyte in the hydrodynamic layers.One real effect of velocity occurs when corrosion product over a repassivating pitgrows to a sufficient height above the surface of the specimen to enter thehydrodynamic layer and be swept away, thus probably resulting in re-activation ofthe corrosion process. Otherwise, the use of this instrument to study effects at highervelocities and under turbulent flow regimes is not compatible with the precisionstepper motors used to control the specimen and probe.

435

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>~ 15:;a.:;o..e10a.

5

o-t----+----+----f-----+---t----+----+------io 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

specimen rotation speed I m s-1

Figure JJ - The effect on probe output of specimen rota/ion speed

4. Application of SRET to quantitative localized corrosion

4.1. LOCALIZED POTENTIODYNAMIC PITTING SCAN

The cylindrical FV448 steel specimen was scanned in natural seawater at 0.1 mV S·1scan rate from -500 to +500 mV SCE using a digital ramp generator and currentsmeasured with a meter. The probe potential was measured on a digital voltmeterand an oscilloscope was used to observe the system noise level. In this case, theamplifier input to the probe was d.c. (rather than a.c.) coupled and care was takento correct any offset voltage appearing at the output. The potentiodynamicpolarization curve and the probe output plots were recorded directly on an X-Yplotter.Figure 12 is a conventional pitting scan obtained manually by monitoring the bulk

(;urrent density Oowing from the specimen. Although the loop is somewhat smallerthan that usually obtained from this material, the pitting loop is clearly visible witha maximum current density of 0.45 rnA cm-2• (The decision to reverse the scan atthis potential was an arbitrary one based upon an instrumental limitation whichwould have resulted in off-scale measurements.) The standard pitting scan, Figure12, generated from current densities of the bulk specimen is quite normal andexhibits behaviour typical of a material which sustains active pits at potentials above-100 mV SCE. When the scan direction is reversed, the pitting activity quicklydiminishes.

436

Figure 13 shows the pitting activity along just one circumference of the specimen.At the point of reversal of the scan (0 mV SCE) the activity at position 34,500micron on the circumference is over 27 rnA cm-2• Two very active pits labelled C andD are observed to grow and repassivate in accordance with the applied potential.The current density is about 12 rnA cm-2 over pit D and over Pit C is about 27 rnAcm-2. The latter value compares with 0.45 rnA cm-2 measured from the bulkspecimen, an increase of 60 fold. We have previously reported [33] localized pittingcurrent densities of > 250 rnA cm-2 compared to bulk current densities of 0.85 rnAcm-2 in Type 304 stainless steel.A particularly interesting feature of Figure 13 is that when the pitting activity is

great in one area (represented by negative current densities) other parts of thespecimen show significantly positive current densities. Whilst it is recognised that thespecimen is being actively polarised to anodic potentials, as measured in the normalway, such measurements of positive current density must be taken as indicative ofion flux in solution in the cathodic sense. It is a contentious argument whether theseregions can be described as local cathodes [33], but it is undisputable that theseareas suffer no visible corrosion, whilst those with negative current densities suffersevere pitting. The results highlight yet again the problems associated with theinterpretation on a local scale of results obtained from measurements of the bulkspecimen such as traditional pitting scans.The data corresponding to Pits C and D in Figure 13 were replotted in the form

of traditional pitting scans and are shown in Figure 14. To the best of our knowledgesuch localized pitting scans are a novel feature of our experiments. A significantdifference between the FV448 behaviour reported here and that for a type 304stainless steel reported previously [33\ is that the 304 was much slower to repassivatecompared to the FV448. This capability of the new instrument to directly measurerepassivation rates is a particularly strong feature which has not yet been used toadvantage in our work.

4.2. PIT LIFE HISTORY

An experiment was carried out on a different specimen to examine the pitdevelopment at a single applied potential, -70 mV SCE. Figure 15 shows a pittinglife history of a single circumference of a specimen, held potentiostatically at -70 mVSCE. Two pits A and B are observed to develop at locations on the circumferencecorresponding to about 26000 microns and ooסס3 microns respectively. The pittingintensity clearly varies with time for each of the pits. After 230 seconds the activityover pit B is very low compared to pit A, whilst they are of comparable activity after50 seconds.Data for localized current densities associated with pitting are scarce. Original

work by Newman and co-workers [38] using artificial pits (electrochemical studieson specimens equivalent to our PIS specimens) has suggested that current densityat pit initiation is typically 5 A cm-2, whilst during propagation it is in the region of

-0.05 o 0.05 0.1 0.15culTent density !rnA cm-2

0.2 0.25 0.3 0.35 0.4 0.45

437

0.5

-100

W

11l>~ -150..~s&.

-250 I:-300 I

Figure /2 - Potentiodynamic pitting scan of bulk specimen, scan rate 0./ mV S-I. AITOWS indicate scandirection.

0.000

5000 ooסס1 ..ooסס4E -5.000 dstance on circumference lmicronu

1i -10.000

! -- -270 mV (ase)

1-15.000 ---+-- -120 mV (ase)

u-- -60 mV (ase)

-20.000 -OmV

---+-- -60 mV (desc)

,,~I -- -120 mV (desc) pnc

-30.000

Figure 13 - Electrochemical activity at points on a single circumference of the specimenduring the pitting scan in Figure /2. Traces represent the current densities calculated at each

circumferential point at each applied potential shown;asc = ascending scan direction, desc = descending scan direction.

438

-5.000 0.000 5.000

current density I rnA cm-2

10.000 15.000 20.000 25.000 30.000

-so

-100

w~>~ -ISO..11$0Q.

-200

-250

-m~Figure 14 - Potentiodynamic pitting scan of local areas of specimen corresponding

to Pits C and D in Figure 13.

distance on c;:irwmference I micron

400003500030000

PitS

1500010000

----.- 230s

-----+- 25 s

--SOs

5000

4 T

I

:1,N 6E

t..c

-2E

fc:•... -4Eg..

-6

-8

-10

Figure 15 - Pit life history of a circumference of a specimen held ata potentiostatic potential of -70 m V SCE.

439

100 mA cm-2• Results from this work indicate varying maximum current densities inthe range 0.1 to 1.0 A cm-2• However, in all measurements carried out so far, thereseems to be no direct relationship between measured current density (as indicatedby the probe signal) and pit size or geometry. Thus, the current density variesthroughout the whole life of the pit and pits of similar size and shape may exhibitmarkedly different current densities depending upon the stage of the corrosionprocess. This, in turn, must be dependent upon the solution chemistry within the pit.Information about the latter is not available using this instrument at the presenttime, but may be achievable at a later stage in the development process. However,there is much scope for a thorough investigation of such behaviour using this newinstrument.

4.3. OTHER APPLICATIONS

The results reported in this work provide ample demonstration of the great scopefor application of this new instrument to a wide range of techniques. In work as yetunpublished, we have reported the investigation of localized effects in theheat-affected zone of Zeron weldments. We have also studied defects in coatingsapplied to containers in the food and drinks industries, and to razor blades. We havecarried out measurements of potential/current distributions across complex geometryspecimens in which bimetallic combinations have been used. Thus we have been ableto use the most modern instrumentation to far more easily repeat the originalpainstaking measurements of Evans and others [1-4]. The additional advantage ofthese kinds of experiments with materials such as Kelocouple, an explosion-bondedtransition joint used to join aluminium superstructures to steel hulls in ships [39], isthat no applied polarisation is necessary in order to detect high current densities. Inother experiments we have incorporated crevices to give information aboutelectrochemical activity in crevice corrosion. In a new design currently beingmanufactured, the limitation of the present instrument to scanning cylindricalspecimens will be overcome and flat specimens will become accessible to the samemeasurements.

5. Conclusions

A new commercial quality instrument utilising the Scanning Reference ElectrodeTechnique has been described which enables novel quantitative localized corrosionmeasurements to be made with ease and accuracy. Evidence in support of calibrationprocedures has been presented which explains methods for the determination oflocalized corrosion current densities. Applied potentiodynamic pitting scans oflocalized regions of a corroding surface have been generated and techniques forquantitatively monitoring the initiation, growth and repassivation of pits in real timehave been described.

440

6. Acknowledgements

This work would not have been possible without the efforts of many past studentsat RNEC, in particular the contributions to early instrumental development madeby Dr Steve Bates, Lt. Cdr Stephen Gosden, Lt Cdr Gilles Hainse, Lt Chris Ford,Lt John Corderoy, Lt David Hill, Lt Jason Strutt and Lt John Bonnar. Continuoustechnical support was provided by Mr Jim Gardner, whilst assistance with theapplications was given by Lt Cdr A Tamimi. Computing support has been providedby Lt Cdr John Keenan. The current phase of the project has received immensesupport from Mr Terry Johnson, Dr Graham Johnson and Mr John Griffiths ofUniscan Instruments Ltd. John Griffiths has worked unstintingly to blend ourworkmanlike ideas with his own imaginative designs, whilst Terry Johnson wasfar-sighted enough to take the commercial plunge and risk a great deal to effect thetransition from lab bench to showroom. The authors are extremely grateful to allthese friends and colleagues who have turned a scientific idea into a commercialreality.

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