Erosioncorrosion of duplex stainless steel under Kuwaitmarine condition

Essam Hussaina, A. Husainb*aKuwait College of Technological Studies, Automotive and Marine Engineering Department

P.O. Box 23167, Safat 13092, KuwaitbKuwait Institute for Scientific Research, Department of Building and Energy Technologies,

P.O. Box 24885, Safat 13109, KuwaitTel. 965 483 6100 Ext. 4522; Fax 965 484 5763; email: xhusain@yahoo.com, ematouq@hotmail.com

Received 22 November 2004; accepted 21 February 2005

Abstract

Erosioncorrosion of stable passive metals like Duplex Stainless Steels (DSS) proceeds by the repeated

removal and repair of the oxide film. When sand particles are entrained it can be a problem in marine pumps,

piping systems, heat exchangers, and units handling seawater in the Arabian Gulf for use as part of desalination

plants. More specifically, sand particles in the presence of seawater enhanced the erosion corrosion problem. If

the passive film structure is stable, it will reform spontaneously when it becomes damaged. The erosion-

corrosion study is concerned with the application of a jet impingement apparatus to study the behavior of

DSS steel of Cr/Ni/Mo/N grade in simulated Arabian Gulf seawater. An interference color imaging technique

(ICI) for thickness measurement of the oxide film passivity, pitting, and transpassivity of the surface of DSS has

been developed under the effect of well defined hydrodynamic condition and potentiodynamic polarization

technique. Electrochemical imaging technique of surface corrosion potential mapping (SCM) was also intro-

duced to measure the rate of repair of the passive film combined poteniodynamic polarization under flowing

conditions, both with and without the addition of sand particles. The main aim is to obtain a better under-

standing of the electrochemistry of DSS alloys in marine erosioncorrosion condition and to examine the

viability of SCM and ICI to be used as a diagnostic tool for DSS material evaluation. The results indicated that

both techniques gave optimum imaging analysis of DSS steel. Water jet impingement showed higher current

densities at the ferrite dissolution potential and austenite pitting potential during the introduction of sand

particles.

Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 2226 May 2005.

European Desalination Society.

0011-9164/05/$ See front matter 2005 Elsevier B.V. All rights reserved

*Corresponding author.

Desalination 183 (2005) 227234

doi:10.1016/j.desal.2005.02.051

Keywords: Erosioncorrosion; Duplex stainless steel (DSS); Interference color imaging technique (ICI); Surface

corrosion potential mapping (SCM); Potentiodynamic

1. Introduction

Over the years, improvements in alloydesign were made for stainless steel. Thishas been satisfied by the development ofDSS alloys. They were chosen to contain amixture of austenitic and ferritic phases,usually in approximately equal proportions.For example, the Cr/Ni balance was adjusted,normally with the addition of ca. 0.10.17nitrogen as an austenitiser. This effect vastlyimproved weldability with high corrosionresistance [1,2]. Current interest in high alloysteels has centered on chloride media such ashandling chlorinated seawater in desalinationplants. Standard test and several studies haveusually employed acid with oxidizing condi-tion according to ASTM G48 [3]. In the pre-sent work, the extent to which sand erosioncorrosion was likely to pose a problem in thismedia has been studied using interferencecolor imaging technique (ICI) and surfacecorrosion potential mapping (SCM). Thisstudy is part of a larger experimental investi-gation on the effect of Kuwait Gulf seawateron different grade of DSS steels.

The aim of the study described in thispaper was to measure the effects of flowingseawater containing sand particles on thethickness of the passive oxide films developedon the surface of duplex stainless steel.The developed oxide film was assessed fromits optical interference images by ICI. Thetechnique has been correlated with surfaceelectrochemical imaging SCM techniquedeveloped earlier by Husain [4].

According to Evans [5] interference colorsoccurs between light reflected from the sur-face of the oxide film and light retrieved fromreflection at the oxide/metal interface when

their paths differ by an odd number of halfwavelengths. Owing to the high refractiveindex of the oxide film, the light inside thefilm is almost normal to the surface for awide range of angles of incidence. The ima-ging colors were reproduced under stagnantcondition and whilst under the effect of thedifferent hydrodynamic conditions beneaththe jet apparatus.

2. Experimental methods

2.1. Erosion corrosion ICI and SCM tests

A close flow loop for the jet impingementapparatus was designed with test specimenmounted directly beneath the orifice of thejet nozzle as illustrated in Fig. 1a. The loopcontained approximately 3 l of artificial sea-water and capable of producing a velocity of8.5 ms1 at the 5 mm diameter orifice. A 1.5-lcapacity glass cell contained the samples,together with a platinum counter electrodeand standard calomel reference electrode.The seawater temperature was controlled at24C using an electrical heater and thermostattogether with a water-cooled heat exchanger.The sand particles were introduced to theflow and were 250300 m in diameter,varying in shape from rounded to angular.Corrosion experiments were performed on atest specimen consisting of cylindrical electro-des machined with the dimensions shown inFig. 1b.

An electrical connection was attached toeach electrode and it was then arranged con-centrically and mounted in epoxy resin. Thesurfaces were ground and polished down to1 m diamond paste and then degreased andultrasonically cleaned and air dried. The

228 E. Hussain, A. Husain / Desalination 183 (2005) 227234

electrode surface was polarized by holding itat a potential of 400, 700 or 900 mV (SCE)for six hours. Meanwhile, the specimen filmthickness and surface topography was ana-lyzed with an interference color imaging tech-nique (ICI) and surface corrosion potentialmapping (SCM). The SCM technique hasbeen used for the measurement of surfaceequipotential lines emerging on the specimensurface using the same set of sample underpotentiodynamic polariztion. These scannedsurface of equipotential lines represent partialanodic and cathodic electrochemical current.Details about the experimental setup of SCMtechnique is given in reference [4]. The speci-men was then tested at the same potentialunder flowing seawater with the addition ofsand particles.

It was assumed that the passive currentdensity measured during erosioncorrosionof the duplex stainless steel in seawater con-taining sand particles was due to disruptionand repair of the oxide film. The total kineticenergy of particles impacting the surface hasbeen considered. Therefore, with 3 g sandand an orifice velocity of 8.5 ms1, the

passive current density at 0 mV (SCE) was7.5 Acm2. The mean impact crater wasmeasured by scanning electron microscopyto be 11.8 m in diameter. It follows thatfor a total number of particle impacts, N, of2400 per second, the mean number of impactson each point of the surface, n, was 0.013 persecond. Therefore, the mean interval betweenthese impacts would have been in the order of76 seconds.

2.2. Materials Specification

The tests were performed on duplex stain-less steel (UNS31803) with the compositionshown in Table 1.

2.3. Hydrodynamic conditions

The hydrodynamic flow characteristic dueto jet impingement on a flat plate was estab-lished according to that proposed by Efird[6]. The shear stress on the DSS surface ofthe sample, w, was calculated from the fol-lowing formula

w 0:179wU0Re 0:182 rr0

1

where: w is the density of the fluid, U0 is thejet velocity, Re is the Reynolds number at theorifice and;

Re 2r0U0

2

r is the radial distance from the centre of thejet, r0 is the orifice radius and is the kine-matics viscosity.

Table 1

Composition of Duplex Stainless Steel Material

Cr Ni Mo N C Si Mn P S

21.9 5.4 3.1 0.18 0.02 0.49 1.5 0.025 0.003

Fig. 1. Schematic diagram of the test set up (a) closed

loop impingement apparatus and (b) the cylindrical

electrodes specimens.

E. Hussain, A. Husain / Desalination 183 (2005) 227234 229

2.4. Interference Color Imaging Technique(ICI) for film thickness measurement

Topographical images in terms of interfer-ence colors of the specimens surface weretaken at a range of radial distances and com-pared with those in the Michel-Levy chart [7].The path difference was identified for eachcolor, enabling the film thickness to be calcu-lated for each region using Eq. 3. The valuesof refractive index for each wavelength wereassumed to be those for oxides on iron, asshown in Table 2.

Path difference 2t 2n 12

3

Where n = 1, 2, 3 etc; t, thickness of theoxide film; , wavelength most stronglyabsorbed in the incident light; , refractiveindex of the oxide at this wavelength.

Accordingly, when light is reflected at aninterface between media of different opticaldensity a phase change can occur, which isequivalent to an additional film thickness C.Consequently, the maximum interferencewould occur at film thicknesses of t = /4C,t = 3/4C, t = 5/4C etc. However, fora transparent oxide film on a metal substrateit is usual to assume that no phase changetakes place and that C = 0. The refractiveindex of the film on stainless steel was as

assumed by Evans. However, refractiveindex is dependant on wavelength and a cor-rection can be made. The values shown inTable 2, used in the present work, are for amixed iron oxide where;

1:35 18:8 104

24

3. Results and Discussion

Samples of water/sand were collectedfrom the loop in order to measure the quan-tity of sand that was suspended in the flowand to allow for any particle that had settledout. From these measurements, the rates ofparticle impacts were calculated. For theaddition of 3 g of sand, the rates were 2400and 1400 impacts/second at velocities of 8.5and 7.9 ms1, respectively. It follows thatthe corresponding average times betweenparticle impacts over the electrode surfacewere 0.42 and 0.71 m sec. The effect of add-ing sand was to erode the oxide film, parti-cularly in the stagnation region beneath theorifice.

The developed interference colors remain-ing after exposure at velocities of 8.5 and7.9 ms1 are shown in Figs. 24, respec-tively. The original film was thin in the high

Table 2

Complementary colors displayed by destructive interference [7]

Wavelength (nm) Color Absorbed Complementary Color Observed Refractive Index ()

417.5 Violet Yellow-green 2.43457.5 Blue Yellow 2.25485 Blue-green Orange 2.15495 Green-blue Red 2.12530 Green Purple 2.02570 Yellow-green Violet 1.93577.5 Yellow Blue 1.89589 Orange Blue-green 1.90700 Red Green-blue 1.73

230 E. Hussain, A. Husain / Desalination 183 (2005) 227234

turbulence region and the sand particlesappeared to have little additional effect duethe low angle of the impacts on the surface.Similarly, no film thinning was recorded inthe low turbulence region.

Short exposure times of 1060 minutesremoved the oxide film in the stagnationregion alone (Fig. 3), whereas longer exposure(618 hr) progressively removed the oxide overwider areas of the surface (Fig. 2). In the

Fig. 2. Film thickness measurements for DSS in seawater containing 3 gm sand at 8.5 ms1 with potentials of400, 700 & 900 mV (SCE) for 618 hours.

Fig. 3. Film thickness measurements for DSS exposed to seawater at 8.5 ms-1 with potentials of 400, 700 &

900 mV (SCE) with 3 gm of sand after 618 hours.

E. Hussain, A. Husain / Desalination 183 (2005) 227234 231

stagnation region, the film was a first ordergrey-blue color for all exposure times. Thisfilm color corresponded to approximately33 m in thickness and suggests that the filmbeing removed and immediately reforming.

3.1. SCM scan and the effect of potentiodynamicpolarization

An important feature of the polarizationbehavior of the duplex stainless steel was theexistence of two discrete pitting potentials.The steel was passive at the open circuitpotential and remained so up to 400 mV(SCE). Above this potential pitting occurredin the ferrite phase, while the austeniteremained passive until a potential close to900 mV (SCE) has been reached and pittingoccurred in both phases. This has been con-firmed by the SCM mapping of the surfaceobtained at 400, 700, and 900 mV as shownin Fig. 4. Metallographic examination alsoconfirmed that in the range 400900 mV(SCE) pitting was in the ferrite alone (Fig. 5).

The largest influence of flow and sanderosion on the polarization behavior wasrecorded on the central electrode (cylindrical 1),positioned in the stagnation region directlybeneath the orifice. Fig. 3 shows the resultsfor a velocity of 8.5 ms1 both in flowingseawater alone and with the addition of 3 gof sand. Each of the polarization scans hadthe same general features as in the low tur-bulence region on Ring 3, shown in the sameFigure, and exhibited a passive range andseparate pitting potentials for the ferrite andaustenite phases.

Without sand additions, the current den-sity in the passive range was essentially thesame on the three cylindrical specimens.However, the addition of sand particlescaused a systematic increase in the currentdensities recorded on cylindrical 1, beneaththe orifice. This is the most pronounced effectobserved in the passive range, where anincrease of one order of magnitude occurred,but a smaller increase also took place in theferrite pitting range. The increase in passive

Fig. 4. Film thickness measurements for DSS in seawater containing 3 gm sand at 7.9 ms-1 with potentials of

400, 700 & 900 mV (SCE); for exposure of 1 hour.

232 E. Hussain, A. Husain / Desalination 183 (2005) 227234

current density with the addition of sand alsodisplaced the intersection of the anodic andcathodic polarization curves and resulted in alowering of the open circuit potential. Thefact that a discernable color was presentdirectly beneath the orifice (Figs. 2 and 3)indicates that the film was reforming rapidlybetween particle impacts and that filmremoval and repair were occurring simulta-neously as competing processes.

In flowing seawater, without sand addi-tions, the passive current density was closeto 0.8 Acm2 (Fig. 4) and the additionalcurrent measured when sand was addedrepresented that required to repair the film.

However, the typical blue-grey color of thefilm (33-m thickness) shows that there wasinsufficient time between particle impacts forit to grow to its steady state thickness. Otherresearchers have investigated the chargepassed when the passive film on stainlesssteels is mechanically damaged. In each case,a large initial current occurs and graduallydecays.

Furthermore, it was anticipated that thecurrent density would be shown to be directlyrelated to the total kinetic energy of theimpacting sand particles. Clearly, in one sec-ond a total charge of 1.5 C would pass inreforming the passive film, because of 2400

Fig. 5. SCM scan of DSS surface at (a) passivity 400 mV, (b) pitting at 700 mV and (c) transpassivity at 900 mV.

E. Hussain, A. Husain / Desalination 183 (2005) 227234 233

impacts. Therefore, the charge passed perimpact may be estimated as 6.3 1010 C.Similarly, as there is (0.013) impacts/sec oneach point of the surface, the mean chargepassed in one second on each point wouldbe in the range of 8.2 1012 C.

3.2. Interference and SCM imaging colors

Pronounced interference colors were visi-ble on the electrode surfaces after exposure toflowing seawater for six or more hours at theanodic applied potentials.

The colors differed at each potential butall displayed essentially three zones whichcorresponded to the stagnation, high turbu-lence and low turbulence regions.

The thickness was similar in the stagnationand low turbulence regions but considerablythinner in the high turbulence region. Thiseffect might be attributed to the high surfaceshear stress in that position. Similar trendwas observed at a jet velocity of 7.9 ms1.

The SCM maps in Fig. 5 clearly indicatedthe viability of the technique to be used as adiagnostic tool for DSS steel surface charac-terization. The reddish color images of thesurface indicated anodic areas which prob-ably represent ferrite dissolution whereas theblush color indicated cathodic areas and pas-sivity of the surface. The transpassive regionof the surface has been clearly defined whereferrite and austenite interlink at such highpolarization potential.

4. Conclusions

Interference color imaging analysis onDSS stainless steel cylindrical specimensshowed a distinct series of zones with distancefrom the concentric during damage andgrowth of passive oxide film. Some of thesezones were correlated very well with thickness

of the oxide layer, metallographic and SEMobservations under controlled hydrodynamiccondition. The color images also show clearlythe incubation periods before the onset of anyvisible passive oxide film on the DSS samples.The SCM observation technique as a stand-alone test does however, show the potentialdifferences which correlate with film thick-nesses and other zone or even a general visualimage of the three zones altogether specifi-cally when a wider sample is being used.Both techniques have offered considerablecontribution and strong evidence for the pro-tective performance of DSS oxide film duringerosion corrosion in simulated seawaterenvironment.

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