evaluation of fatigue damage for duplex s

8/6/2019 Evaluation of Fatigue Damage for Duplex s

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International Journal of Fatigue 25 (2003) 11891194

www.elsevier.com/locate/ijfatigue

Evaluation of fatigue damage for duplex stainless steels inaggressive environments by means of an electrochemical fatigue

sensor (EFS)

A. Girones a, A. Mateo a,, L. Llanes a, M. Anglada a, J. DeLuccia b, C. Laird b

a Department de Ciencia dels Materials i Enginyeria Metal.lurgica, Universitat Politecnica de Catalunya, Barcelona, Spainb Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, USA

Abstract

The EFS (Electrochemical Fatigue Sensor) is a device that is based on a study of electrochemicalmechanical interactions,providing information about the status of fatigue damage in a material, and promises to become a useful non-destructive, nichetesting tool. The EFS has been shown to function for several commercial alloys, as well as in different environments. The presentwork is to confirm the feasibility of applying the EFS for assessing fatigue damage in duplex stainless steels. A second goal is toinvestigate the possibility of using an aggressive environment as an electrolyte within the EFS device. In doing so, corrosion-fatiguetesting was conducted in EFS solutions at three different pH values: 8.4, 6 and 2. The results show that the fatigue life is longerat a pH value of 6. Such an unexpected environmental effect is discussed and related to the dissolution of the surface roughnessgenerated by cyclic deformation, together with a higher repassivation rate. 2003 Elsevier Ltd. All rights reserved.

Keywords: Electrochemical fatigue sensor (EFS); Duplex stainless steels (DSSs); Corrosion-fatigue

1. Introduction

The EFS (Electrochemical Fatigue Sensor) is a devicedeveloped by researchers at the University of Pennsyl-vania (UPenn), which is based on studies of electro-chemicalmechanical interactions. Previous studies oncorrosion-fatigue employing a three-electrode cell andpotentiostatic control of the corrosion process have dem-onstrated that, when a structure undergoes fatigue load-ing, the transient electrochemical current changes withthe applied stress and with fatigue microplasticity, whichis different for tension and compression, allowing adirect evaluation of the fatigue damage [14].

The transient current measured (also referred to asEFS signal) consists of several parts: a DC componentreflecting the general electrochemical reaction enforcedby the applied voltage, and fluctuating components cor-responding to the mechanical reaction of the specimen

Corresponding author. Tel.: +34-93-401-1089; fax: +34-93-401-

6706.

E-mail address: [email protected] (A. Mateo).

0142-1123/$ - see front matter 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0142-1123(03)00119-1

[1,4]. One of these components reflects an interactionbetween the elastic strain of the specimen and the electri-cal double layer, and generally exhibits the same fre-quency, although not necessarily the same phase, as thestress cycles. Another component has twice such a fre-quency and is associated with cyclic microplasticity. Fre-quency doubling is caused by the electrochemical reac-tion to fresh surfaces produced by the irreversiblesurface effects produced by microplasticity, generated inboth tensile and compression reversals. Fast Fouriertransform (FFT) analysis and treatment of the transientcurrent can thus provide information about the status ofthe fatigue damage in the structure under inspection,reflecting changes in strain localization and respondingactively to the presence of cracks even at the initiationstage. The EFS is also sensitive to the plasticity associa-ted with crack growth, showing a specific crack peakwithin the EFS signal and providing a measure of thecrack size and growth rate.

The materials considered during the first stages of theEFS project were alloys of steel, aluminium and titanium[4]. Preliminary investigation of the EFS studied itsfeasibility as a practical tool for non-destructive evalu-

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1190 A. Girones et al. / International Journal of Fatigue 25 (2003) 11891194

ation of fatigue damage in commercial structures. Hence,

it is important to extend its application to other alloys,

such as stainless steels and of particular interest in the

present work, duplex stainless steels.

Many of the applications in which duplex steels areused involve cyclic loading under normal operating con-

ditions; for instance, tubing for undersea service [5].Although in recent years there has been sustained

activity towards improving characterisation and under-

standing of the mechanical and microstructural aspectsassociated with the cyclic deformation of DSSs [610],a more complete knowledge is still required in order to

make the design of components and structures more

reliable, taking the maximum benefit of the propertiesof these materials.

Another interesting point to develop is the feasibilityof evaluation by the EFS technique of the damage of a

metallic structure subjected to corrosion fatigue pro-

cesses. For a more accurate evaluation of the damage,

the EFS should be able to work under the same chemical

or electrochemical conditions experienced by the

material in service. Thus, it is necessary to determine the

feasibility of using, as an electrolyte, the same chemical

solution as the one in contact with the material during

its service.Following the above ideas, a first aim of this work is

to assess the suitability of the EFS for duplex stainless

steels and, once that is confirmed, a second goal is tostudy the functionality of using an aggressive environ-

ment (EFS solutions acidified with concentrated HCl topH = 6 and pH = 2) as an electrolyte within the EFS

device, as well as to determine the mechanisms of cor-rosion-fatigue damage in these environments.

2. Experimental procedure

The material used is a superduplex stainless steel UNS

S32750 (commercial designation SAF 2507). It was sup-

plied in the annealed condition and in the form of round

bars of 20 mm in diameter by Sandvik AB, Sweden.

Table 1 gives its chemical composition in weight percentas well as the volume fractions of the constitutive

phases. The grains in both phases are highly elongated

in the rolling direction (this direction corresponds to the

loading axis in all the specimens tested) with an equival-

ent grain diameter of 7 m for the ferritic grains and 10

m for the austenitic ones (Fig. 1).Cylindrical specimens were machined from the round

Table 1

Chemical composition in weight percent and volume fractions of UNS S32750

Cr Ni Mo Mn Si C N /g(%)

UNS S32750 25.0 7.0 3.79 0.40 0.34 0.01 0.26 46/54

Fig. 1. Microstructure of duplex stainless steel type UNS S32750

(rolling direction).

bars. Specimens were mechanically polished with emery

paper with increasingly finer grit down to 5 m, andfinally they were rinsed and cleaned in an ultrasoundbath in acetone and ethanol.

Fatigue tests were carried out at room temperature in

a MTS servo-hydraulic testing machine (MTS 810 Test-

Star II System) under constant amplitude sinusoidal loadcontrol with a stress ratio of R = 1 and at a frequency

of 1 Hz. The stress amplitudes studied were chosen from

the three-stage cyclic stressstrain curve previouslyobtained for this material [11], in which three different

modes of plastic strain accommodation may be described

as a function of the stress or strain amplitude. Two stress

amplitudes were selected for study: a) stress amplitudeof / 2 = 540 MPa, within stage III of the referredcurve and corresponding to a ferritic-like behaviour, i.e.

ferrite is the phase that accommodates the main part of

the plastic deformation, and b) stress amplitude of/ 2 = 500 MPa, belonging to stage II or mixed

austenitic/ferritic-like behaviour, i.e. both phases

develop a similar plastic activity.

Corrosion fatigue tests were performed using a cor-

rosion cell (Fig. 2) filled with 50 ml of an electrolyteinitially free of hydrochloric acid. The electrochemicaldevice consisted of a three-electrode system: a platinum

mesh working as the counter-electrode, a Calomel elec-

trode (reference electrode) and the specimen itself as the

working electrode. A potentiostat was used to control

and measure the electrochemical processes occurring

within the corrosion cell. The shoulders of the specimenswere covered by Teflon caps and Parafilm for insulation




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1191A. Girones et al. / International Journal of Fatigue 25 (2003) 11891194

Fig. 2. Scheme of the corrosion cell with the three-electrode system.

purposes and all the junctions of the corrosion cell were

sealed with wax.

The electrolyte used in this investigation, referred to

as the EFS solution, presents the following chemicalcomposition: 0.3 M H3BO3, 0.075 M Na2B4O710H2Oand 0.06 M Na2MoO42H2O [4]. In order to assess theEFS device for the detection and analysis of the EFSsignal in aggressive environments, the starting EFS sol-

ution with an initial pH of 8.4 was acidified with concen-trated HCl to pH = 6 and to pH = 2. A minimum of two

specimens were tested for each experimental condition.

Corrosion fatigue tests were performed under a con-

stant potential within the passive region. From potent-

iodynamic polarization curves obtained in a previousstudy [12], a potential value of+0.2 V was selected. Thespecimen was kept in the electrolyte at this constant

working potential for 2 h before starting the test. Thus

the specimen at rest was maintained at a surface state of

benign passivity, i.e. exhibiting minimal electrochemi-

cal current.

Measurements of both the stressstrain and the elec-trochemical responses, obtained for both specimens

tested for each condition of pH value and stress ampli-

tude, were fed to a data acquisition system using Lab-View software, for data presentation and signal pro-

cessing. Further analysis of the EFS signal was possible

by means ofsoftware developed and improved by UPenn

researchers [4].

Finally, scanning electron microscopy (SEM) examin-

ation of the external surfaces and fracture surfaces of thesamples fatigued under stress amplitude control up to

fracture was conducted.

3. Results and discussion

Primarily, tests were performed to assess the EFS

technique (i.e. EFS solution at pH = 8.4) for DSSs. EFS

Fig. 3. EFS waveform obtained at 97.2% of fatigue life for UNS

S32750 tested at s/ 2 = 500 MPa, R = 1, 1 Hz and EFS solution

(pH = 8.4).

waveform (i.e. current v. time) of the superduplex steel

cycled at s/ 2 = 500 MPa is shown in Fig. 3. Superim-

posed on the diagram is a dimensionless waveform ofthe stress cycle, indicating the EFS responsestress

relationship. For most of the fatigue life, the waveformis out of phase, but rather similar, to that of the stress

cycles. The EFS signal obtained for the specimens tested

at s/ 2 = 540 MPa presents a similar waveform and

current density.

Crack existence could be detected because of an

additional peak (crack peak) that appears at the start of

the tensile reversal in the EFS signal (Fig. 3). As thecrack grows, the crack peak amplitude increases, provid-

ing a measurement of crack size and growth rate (Fig.

4). This additional peak is induced by the increase in the

area of fresh metal in contact with the electrolytic sol-

ution during crack propagation and thus, a higher current

density is registered. During the compression reversal,

the crack peak does not appear in the EFS waveform as

crack closure takes place, and thus this extra area is nolonger exposed to the environment until the next ten-

sile reversal.

By Fourier analysis, the EFS signal was analyzed into

its components, i.e. base, elastic and plastic currentdensities. These current densities are also suitable for

crack detection, but so far it is easier to detect the

additional peak than crack initiation from individual

Fig. 4. EFS waveforms obtained at different values of fatigue life

showing crack peak evolution for UNS S32750 tested at s/ 2 =

500 MPa, R = 1, 1 Hz and EFS solution (pH = 8.4).



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Fig. 5. Total, plastic and elastic current densities for the UNS S32750

tested at s/ 2 = 500 MPa, 1 Hz and EFS solution (pH = 8.4).

Fig. 6. EFS waveforms at different values of fatigue life for the UNS

S32750 tested at s/ 2 = 500 MPa using EFS solution at pH = 6.

cycles. The appearance of a fatal crack corresponds to

an exponential increase of the current density (Fig. 5).The determination of the number of cycles at which this

exponential increase takes place gives a measure of theonset of rapid cracking.

From the above results it can be concluded that the

EFS technique can be applied to duplex stainless steels,

and may be used as a non-destructive tool for fatiguedamage evaluation.

When using the acidified EFS solutions, the EFS wav-eforms obtained (Figs. 6 and 7) show that the EFS signal

could be registered and analyzed properly in both EFS

solutions (pH = 6 and 2) without interference that couldaffect the analysis and processing of the EFS signal.

Fig. 7. EFS waveforms at different values of fatigue life for the UNS

S32750 tested at s/ 2 = 500 MPa using EFS solution at pH = 2.

Thus, a change in the chemical composition and pH of

the electrolyte does not seem to affect the acquisition

and analysis of the EFS signal generated by the DSS.

When comparing the EFS signals for the different EFS

solutions, it is observed that for the same value of fatiguelife, the current density of the EFS signal in the solution

at pH = 2 is higher than the values obtained for testsperformed with the EFS solution (pH = 8.4) and/or EFS

solution at pH = 6. This fact can be explained in terms

of solution aggressiveness. Thus, this stronger environ-mental effect of the EFS solution is attributed to the

higher concentration of Cl ions as well as the lower

pH, which consequently provoke more electrochemical

damage to the protective passive film, which is mani-fested as a higher value of current density. The high con-

centration of Cl ions induces a weaker surface passivelayer, and during cyclic loading the persistent slip bands

(PSBs) generated will rupture the passive film, exposingthe unprotected material to the aggressive environment.

Thus, the more vulnerable the film, the stronger is theeffect of the environment, as lower PSB disturbances

will be necessary to induce passive film breakdown, andtherefore more premature crack nucleation will take

place. From studying the influence of the pH, it has beenproved that pH directly influences the repassivation kin-etics, i.e. passive film reconstruction generally is fasterfor lower values of pH [13] and therefore the unprotected

material remains in contact with the corrosive solution

for a shorter period of time. On the other hand, it must

be taken into account that at low pH, H2-embrittlement

processes can be active, contributing to enhanced crack

propagation rates [1314].Another important point observable within the EFS

waveforms was the double-character of the additional

peak (crack peak) observed in all tests performed with

the EFS solution at pH = 2 (Fig. 8). Meanwhile, the

crack peak for the solutions at pH = 8.4 and pH = 6 did

not present this shape (Figs. 3 and 6). This double peak

can be attributed to the propagation of several cracks

simultaneously, since their individual peaks would

superimpose giving an integrated peak. Thus, the EFSwould give a complex summation of the combined dam-

Fig. 8. EFS signal obtained at 97.5% of fatigue life for UNS S32750

tested at s/ 2 = 500 MPa using EFS solution at pH = 2.



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age. The presence of more than one crack propagating

inside the material is due to the higher aggressiveness

of the solution with pH = 2, which produces a higher

dissolution rate of the material from a larger number of

nucleation sites than in the specimens tested for the othersolutions. This fact has been proved from fracture sur-

face analysis by SEM. Fractographs of the specimenstested at pH = 2 show a minimum of two crack fronts.

Moreover, the presence of brittle, transgranular facets in

the ferritic phase may be attributed to hydrogenembrittlement (Fig. 9) whereas only fatigue striations

can be seen for specimens tested at pH = 8.4 and 6.

When comparing the influence of the corrosiveenvironment compositions on the fatigue life, important

life reductions take place for the most acid solution.

Within this framework, it is seen that a stronger fatiguelife reduction occurs at the higher stress amplitude of

540 MPa. This may be attributed to a larger contribution

of the ferritic phase to plastic strain accommodation.

Although at s/ 2 = 500 MPa, the ferritic phase is also

plastically active, it contributes less to plastic strain

accommodation, and thus the detrimental effect of H2 isless important as the mechanisms for the introduction of

this species to the bulk are less active [13].

It is noted that longer fatigue lives are obtained atpH = 6 solution with respect to the benign solution (EFS

solution pH = 8.4). Previously, it was proved that fatigue

life values in air are quite similar to those obtained for

this benign EFS solution [9,12]. When studying the

effect of the pH = 6 electrolyte, shorter fatigue lives

would be expected due to the stronger aggressiveness of

this solution compared to the benign one. A significantfactor may be the anodic dissolution of the PSBs gener-ated during the fatigue process. If PSBs are electro-

chemically dissolved, the surface roughness is reduced

and thus fatigue nucleation is delayed. From Table 2, it

can be observed that this effect is clearly more important

at lower stress amplitudes. This fact is directly connected

Fig. 9. SEM fractograph of UNS S32750 tested in EFS solution at

pH = 2.

with the PSBs height. For higher amplitudes, the PSBs

formed during each loading reversal are higher, and thus

the PSB dissolution is not efficient.SEM examinations of the lateral surfaces from the

samples tested with EFS solution at pH = 6, shows ahigher density of PSBs in the austenitic phase than in

air. Furthermore, their height appears to be smaller, afact that would corroborate the higher rate of PSBs dis-

solution at this pH. On the other hand, a high density of

small cracks, mainly nucleated in the PSBs, are observedin lateral surfaces of samples tested at pH = 2, evidence

that is in agreement with the occurrence of H2-embrittle-

ment, and probably, the enhancement of localized strain.

The capability of the EFS to detect crack propagation

has been used to determine the percentage of fatigue life

in which long crack propagation takes place, and theresults have been compared (Table 3) and related to the

corrosion-fatigue mechanisms that influence fatigue life.From this table, it can be observed that rather similar

results have been obtained for the highest stress ampli-

tude. At high stress amplitudes, the strong strain incom-

patibilities between neighbouring grains leads to an

enhanced crack nucleation process, in such a way, that

at these amplitudes short crack growth plays a major role

within the whole fatigue process. On this basis, cracknucleation and early growth takes place at shorter frac-

tions of fatigue lives.

4. Conclusions

From the fatigue tests conducted using the EFSdevice, with three different pH solutions, the followingis concluded:

The EFS can be an important tool for non-destructive

analysis and evaluation of fatigue damage in commer-

cial alloys, as well as in the laboratory, its applica-

bility being demonstrated for DSSs.

Aggressive environments do not influence the acqui-sition and analysis of the EFS signal from the stain-

less steel studied. Thus, the EFS may work properlyin aggressive environments on resistant alloys.

Double crack peaks in the tensile part of the EFS

waveform correspond to multicrack propagation, a

phenomenon especially important in aggressive

environments where H2-embrittlement takes place.

Longer fatigue lives at pH = 6 solution could bemainly attributed to anodic dissolution of surface

roughness generated during the fatigue process.

Acknowledgements

The financial support of Spanish MCYT (GrantMAT99-0781) and Departament dUniversitats, Recerca



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Table 2

Fatigue-corrosion results obtained under stress control at s/ 2 = 500 MPa and s/ 2 = 540 MPa

Electrolyte Fatigue life (cycles) Fatigue life reduction/increase

s/ 2 = 500 MPa s/ 2 = 540 MPa s/ 2 = 500 MPa s/ 2 = 540 MPa

EFS pH = 8.4 86 200 42 400 EFS + HCl pH = 6 145 100 43 900 +83.1% +4.5%

EFS + HCl pH = 2 59 500 21 500 30.8% 48.8%

Table 3

Percentages of fatigue life at which crack propagation is detected by

means of EFS

Electrolyte Crack propagation (% Nf)

s/ 2 = 500 MPa s/2 = 540 MPa

EFS pH = 8.4 88.8 76.7

EFS + HCl pH = 6 93.6 81.5

EFS + HCl pH = 2 89.6 81.5

i Societat de la Informacio de la Generalitat de Catalu-

nya (ACI 2000-26) is acknowledged. The authors

warmly thank Sandvik AB (Sweden) for supplying thestudied duplex material. Also, one of the authors (A. G.)

wants to thank Ministerio de Educacion y Ciencia, for

the FPI grant which allowed her to realise a stage at the

University of Pennsylvania.

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evaluation of fatigue damage for duplex s

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