Tribology International 44 (2011) 610–616

Contents lists available at ScienceDirect

Tribology International

0301-67

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/triboint

Effect of nitrogen on the corrosion–erosion synergism in an austeniticstainless steel

Diana Lopez a, Neusa Alonso Falleiros b, Andre Paulo Tschiptschin b,n

a Mechanical and Electrical Engineering School, National University of Colombia, Medellın, Colombiab Metallurgical and Materials Engineering Department, University of S ~ao Paulo, Brazil

a r t i c l e i n f o

Article history:

Received 15 June 2010

Received in revised form

20 December 2010

Accepted 29 December 2010Available online 13 January 2011

Keywords:

Corrosion–erosion synergism

High temperature gas nitriding

Austenitic stainless steels

9X& 2011 Elsevier Ltd.

016/j.triboint.2010.12.013

esponding author. Tel.: +55 11 30915656; fa

ail address: antschip@usp.br (A. Paulo Tschip

Open access under the El

a b s t r a c t

High temperature gas nitrided AISI 304L austenitic stainless steel containing 0.55 wt% N in solid

solution, was corrosion, erosion and corrosion–erosion tested in a jet-like device, using slurry

composed of 3.5% NaCl and quartz particles. Scanning electron microscopy analysis of the damaged

surfaces, mass loss measurements and electrochemical test results were used to understand the effect

of nitrogen on the degradation mechanisms. Increasing the nitrogen content improved the corrosion,

erosion and corrosion–erosion resistance of the AISI 304L austenitic stainless steel. Smoother wear

mark contours observed on the nitrided surfaces indicate a positive effect of nitrogen on the reduction

of the corrosion–erosion synergism.

& 2011 Elsevier Ltd. Open access under the Elsevier OA license.

1. Introduction

Synergism between corrosion and wear has a great impact onthe reduction of performance and service life of a wide range ofmechanical systems where stainless steels are used. Damagemechanisms are complex and it has been shown that thisinteraction, even with a slight presence of corrosion or wear,can generate significant increases in mass losses in comparison tothe case when each phenomenon works alone [1–3]. Synergismcan be assessed using Eq. (1)

T ¼ KeoþKcoþDKeþDKc ð1Þ

where T (mass loss obtained in a corrosion–erosion test) iscalculated as the sum of Keo (mass loss obtained in an erosiontest without the influence of corrosive agents and measured witha scale), Kco (mass loss obtained through polarization measure-ments using Faraday�s equation in a test performed in the sameelectrolyte but without particles), DKe (mass loss due to erosionmodified by corrosion) and DKc (mass loss due to corrosionmodified by erosion), the synergism S (Eq. (2)) being

S¼DKeþDKc ð2Þ

Other relations (Eqs. (3)–(5)) based on previous equations canbe useful in the evaluation of wear regimes [3]

T ¼ KeþKc ð3Þ

x: +55 11 30915243.

tschin).

sevier OA license.

Ke¼ KeoþDKe ð4Þ

Kc¼ KcoþDKc ð5Þ

where the wear regimes are limited by the Ke/Kc ratio

�

Ke/Kco0,1 Corrosion. � 0,1oKe/Kco1 Corrosion modified by erosion. � 1oKe/Kco10 Erosion modified by corrosion. � Ke/Kc410 Erosion.

Stainless steels are very prone to be damaged by synergisticeffects of corrosion and erosion, since its corrosion resistancedepends on the passive layer integrity, which can be easilybroken-down under erosion conditions. Several authors [4–8]have shown that the increase of the nitrogen content of austeniticstainless steels leads to an interesting and unusual combination ofproperties, granting better corrosion resistance, higher strengthand maintaining at the same time toughness and ductility. Somestudies have reported the beneficial effect of increasing nitrogencontents on the erosion and cavitation erosion resistance ofduplex stainless steels [9,10]. This result is attributed to the effectof nitrogen in lowering the stacking fault energy (SFE), leading toan increase in plasticity and work hardening rate. It has also beenreported that nitrogen additions increase the corrosion–erosionresistance of martensitic stainless steels [11]. The mechanismseems to be related to the increase of hardness and reduction ofcorrosion of the metallic matrix [12]. However, few studies havebeen done on the effect of increasing nitrogen contents on thecorrosion–erosion synergism in austenitic stainless steels.

D. Lopez et al. / Tribology International 44 (2011) 610–616 611

The aim of this work is to study the effect of increasingnitrogen contents, obtained through high temperature gas nitrid-ing, on the corrosion–erosion resistance of an AISI 304L austeniticstainless steel.

2. Experimental procedure

2.1. Materials

Specimens of an AISI 304L (18.1Cr–8.3Ni–2.0Mn–0.95Si–0.03C wt%) austenitic stainless steel were high temperature gasnitrided (HTGN) in high purity (N2+Ar) at 1473 K under 0.15 MPaN2 partial pressures. The treatment condition was held for 6 h andafter that, the samples were direct quenched into water. Forcomparison purposes, a set of as-received samples were solutiontreated under Ar atmosphere at 1373 K during 1 h and cooledinto water.

2.2. Microstructure and hardness

The microstructure of the samples was examined by opticaland scanning electron microscopy (OM and SEM) and X-raydiffraction (XRD). Nitrogen content of the nitrided specimenswas measured using wavelength dispersive spectrometry (WDS)[13]. Hardness profiles at the transverse section of the nitridedspecimens were measured in a Zwick microhardness tester usinga 100 g load.

2.3. Corrosion–erosion tests

Corrosion–erosion tests were carried out and electrochemi-cally monitored using a modified electrochemical cell connectedto a Princeton Applied Research PAR 273 potentiostat. Fig. 1shows a representation of the setup. The modified cell uses avariable speed peristaltic pump to control the electrolyte injec-tion rate and drives the slurry through a circuit during the erosiontests. A nozzle with 3.5�10�2 m in diameter created a sub-merged jet with mean velocity of 4 m/s that impact the surfacewith an angle of 901.

The corrosion tests were carried out under the influence of ajet of 3.5% NaCl solution while the corrosion–erosion test used thesame electrolyte plus 10 wt% of round quartz particles with sizesbetween 300 and 500 mm. The impact angle and velocity were thesame of corrosion tests. The pH of the electrolyte was 5.6,measured using a Digimed DM22 pH meter. The temperature of

Fig. 1. Schematics of the corrosion–erosion testing apparatus.

the solution was controlled in all the tests and was held between294 and 299 K.

The tests were done in an aerated condition and due toagitation the solution can be supposed as oxygen saturated. Allthe specimens were mechanically ground, using SiC emery paperup to 600 grit, just before the test, to standardize the surfacefinish conditions. Subsequently they were cleaned and rinsed inan ultrasonic bath. An area of 1.3�10�4 m2 was exposed to theelectrolyte. An Ag/AgCl reference electrode and a Pt bar counterelectrode were used. Scanning was initiated after a 5 min immer-sion time, starting 100 V below the corrosion potential andending when the current density was at least 1�10�3 A. Thetests were carried out with a scanning rate of 1 mV/s.

In a second series of experiments, mass loss measurementswere taken under erosion and corrosion–erosion conditions forboth, the lean nitrogen samples and the HTGN samples containing0.55 wt% N in the surface. For the erosion tests, the slurry wascomposed by distilled water and quartz particles while forcorrosion–erosion conditions, 3.5% NaCl solution plus 10% ofquartz particles were used. The impact parameters were notchanged. Mass losses were measured every 20 min up to the firsthour and every hour up to 8 h of testing. A scale with a resolutionof 0.00001 g was used. The reproducibility of the erosion–corrosion and of the erosion tests was analyzed using statisticaltests and analysis of variance (ANOVA).

2.4. Synergism

The T parameter (mass loss obtained in a corrosion–erosiontest) was assessed from the results of the corrosion–erosion testsand Keo was obtained considering the hypothesis that corrosion isnegligible when testing stainless steels with distilled water [3,14].On the other hand, Kco was obtained from polarization data andthe procedure described by ASTM G119 standard [15] and Deanusing the passive current density [16].

Finally, it is necessary to point out that passive current densityvalues under corrosion and corrosion–erosion conditions weretransformed into mass loss data using Faraday�s equation toquantify synergism. However, the use of this equation is veryspecific and considers uniform dissolution of one metallic ele-ment or elements in an alloy. This procedure requires knowingthe chemical composition of the alloy, the atomic weight ofthe elements and the valence number of each element in thechemical dissolution process. Establishing valence numbers forthe dissolution of a stainless steel is not a simple process and thedissolution of the different elements of the alloy is not homo-geneous. In this work, Pourbaix�s diagrams were used to select thevalence numbers, using potentials and pH values obtainedexperimentally [17].

3. Results

3.1. Microstructure and hardness

Fig. 2 shows the hardness profile of the HTGN AISI 304L steel.The nitrogen content at the surface was 0.55 wt% (304LN) and thenitrided case was 1.5 mm thick. The microstructure of thesamples was composed solely by austenitic grains containingrecrystallization twins (Fig. 3). No precipitates were detected bySEM analysis indicating that all nitrogen is in solid solution inaustenite. The heat treatment causes a noticeable increase of thegrain size, reported in Table 1, together with the hardness takenon the surface of the samples.

Fig. 2. Hardness profile of a high temperature gas nitrided AISI 304L stainless steel

sample.

Fig. 3. Austenite grains: (a) solution-annealed condition, (b) HTGN with 0.55 wt%

at the surface. Electrolytic etching with oxalic acid. 50� .

Table 1Nitrogen content and hardness at the surface of samples and grain size after the

heat treatments.

Material Nitrogen content

at the surface (wt%)

Hardness at the

surface (HV0.1)Grain size (mm)

304L solubilized 0.02 178710 189.8730.5

304N 0.55 260715 341.6793.1

Table 2Electrochemical parameters for AISI 304L stainless steel.

Condition Material Corrosion potential

(Ag/AgCl—V)

Passive current

density (A/cm2)

Corrosion under liquid

impingement

304L �0.30870.042 3.77�10�6

304N �0.19470.078 3.01�10�6

Corrosion–erosion304L �0.50870.025 2.43�10�4

304N �0.51670.023 1.09�10�4

D. Lopez et al. / Tribology International 44 (2011) 610–616612

3.2. Electrochemical tests

The results of electrochemical tests are summarized in Table 2.It is observed that the impact of the particles has a marked effecton the passive current density, increasing it two orders ofmagnitude. This result is in agreement with other studies aboutcorrosion–erosion of stainless steels [14,18,19]. It can also beobserved that nitrogen reduces the passive current density underboth testing conditions: liquid impingement and corrosion–erosion, but the reduction is more pronounced in the corrosion–erosion condition.

3.3. Mass loss measurements

Fig. 4 shows the accumulated mass loss measurements in theerosion and corrosion–erosion conditions. After a running-inperiod, the accumulated mass loss is directly proportional to thetesting time, the results being within a confidence level of 95%. Itcan be easily observed that in both types of specimens, solution-annealed and 0.55 wt% N, the corrosion–erosion condition is moresevere, and that nitrogen improves the erosion and corrosion–erosion resistance of the steel. Fig. 4 shows the confidenceintervals within a confidence level of 95% for accumulated massloss. It is observed that the effect of nitrogen on the degradationmechanisms is significant. According to the statistical analysis ofthese results, the most significant influence of nitrogen is pre-sented after a running-in period. Nevertheless, when the resultsare analyzed without considering the type of test, the effect ofnitrogen is validated and it is independent of time during therunning-in period. On the other hand, the confidence intervalsshow that the erosion test presents less variability than theerosion–corrosion test.

3.4. Surface evaluation

Fig. 5 shows the appearance of the eroded surface of a solution-annealed sample after an erosion test. A typical symmetric W

shaped topography, characteristic of normal impact was observedas shown in Fig. 6. The depth of the deepest valley is 170 mm for thesolution-annealed sample and 102 mm for the HTGN sample. Thisdepth is less than the thickness of the nitrided layer (250 mm)

Fig. 4. Accumulated mass losses as a function of exposure time.

Fig. 5. Optical microscopy image of the eroded surface of a solution-annealed

sample tested after 8 h of erosion wear test.

Fig. 6. Two-dimensional profile of the eroded surface of a solution-annealed AISI

304L sample after 8 h of erosion wear test.

Fig. 7. SEM images of the transverse section of a solution-annealed sample tested

under erosion–corrosion. Deformation bands are observed.

Fig. 8. Debris obtained from an eroded surface of a solution-annealed sample.

D. Lopez et al. / Tribology International 44 (2011) 610–616 613

having a constant nitrogen content around 0.55%, granting that thewear process is occurring in a homogeneous layer of high nitrogensteel. This consideration is very important since if this condition isnot achieved, the wear and corrosion mechanisms would signifi-cantly differ.

Fig. 7 shows a SEM image of a transverse section taken from a304L solution-annealed sample tested under erosion conditions,

showing a work-hardened layer of approximately 10 mm thick.Fig. 8 shows debris obtained after an erosion test of a solution-annealed sample. It is possible to observe the plate shape of thedebris, typical of normal impact and showing a forging withrepetitive impact wear mechanism. The debris particles aremagnetic, suggesting that a deformation induced martensitictransformation occurred as a result of the impact of the erosiveparticles. This transformation was confirmed through X-raydiffraction analysis using grazing angle, as shown in Fig. 9.

3.5. Corrosion–erosion synergism

Table 3 shows the parameters obtained from mass loss ratemeasurements after 8 h testing (T and Keo) and electrochemicalpolarization (Kco). These values were used to calculate thecorrosion–erosion synergism S for both surface conditions, solu-tion-annealed and high nitrogen stainless steel. Keo, Kco and S arealso described in terms of the percentage of the total mass lossrate T.

The results show that the nitrided condition has betterresistance to erosion and corrosion–erosion when compared tosolution-annealed condition. The addition of 0.55% N decreasedthe total mass loss by 8% and the erosion mass loss by 5%.

D. Lopez et al. / Tribology International 44 (2011) 610–616614

Synergism between corrosion and wear was greater in the AISI304L specimens than in the nitrided ones, being reduced by 23%by the addition of nitrogen. This result can be associated to thebetter corrosion resistance of the nitrided AISI 304L steel due tothe formation of a more protective passive layer. The corrosionterm was reduced by 62%, due to nitrogen alloying.

Fig. 10 shows the distribution of the erosion, corrosion andcorrosion–erosion synergism terms for the solution-annealed andthe 0.55 wt% N nitrided specimens. One can see that despite thecorrosion term being very small (�10�2) it has an importanteffect on the synergism term. The distribution for erosion, corro-sion and synergism terms for the nitrided samples is not verydifferent, being the synergism term 12.15%.

Table 4 shows the corrosion modified by erosion DKc, erosionmodified by corrosion DKe and total contribution of the erosionand corrosion terms, Ke and Kc, calculated using Eqs. (4) and (5).Distribution of synergism terms DKc and DKe is shown in Fig. 11.KeXKc ratio is bigger than 10 for both specimens, solution-annealed and nitrided. This result indicates that degradation ofthe surfaces is controlled by erosion in both cases.

Fig. 11 shows that nitrogen addition has a marked beneficialeffect on the corrosion modified by erosion, DKc. This termrepresents 17% of the synergism corrosion–erosion, while in thespecimen without nitrogen, the effect of erosion on corrosion is40%. This effect is illustrated in Fig. 12, which shows thetopography of a solution-annealed and a nitrided sample after acorrosion–erosion test. Both surfaces revealed extensive plasticdeformation, lips, craters and extruded material. However, thesurface of the solution-annealed sample shows more irregularcontours on deformed lips, indicating a corrosion process mod-ified by erosion (Fig. 12a and c). On the other hand, nitridedsurface shows cleaner contours demonstrating a beneficial effectof nitrogen addition on the corrosion–erosion synergism (Fig. 12band c). Further tests are needed to address different flow velo-cities and geometries.

Fig. 9. Grazing angle XRD spectra in a solution-annealed sample before and after

an erosion test.

Table 3Mass loss rates under corrosion–erosion, erosion and corrosion conditions.

Material Mass loss rate under

corrosion–erosion condition T

(g/cm2 min)

Mass loss rate und

erosion condition

(g/cm2 min)

304L 6.06�10�5 5.18�10�5 (85.46

304LN 5.59�10�5 4.91�10�5 (87.84

4. Discussion

The corrosion mass loss rates, obtained for both materials, thesolution-annealed and HTGN steel, are very low in comparisonwith the erosive mass loss rates. However their effect on thesynergism is noticeable. Part of the particle’s impact energy isspent in plastic deforming the surface of the specimen, creatinglips, breaking the passive layer in some regions and increasing thesurface area of the material exposed to the corrosive environ-ment. Corrosion processes have more available material to attack,leaving lips more vulnerable to be removed by impacts ofparticles, establishing a cooperating mechanism between erosionand corrosion that increase considerably the total mass loss rate.

It has been shown that alloying with nitrogen has a solid-solution strengthening effect in the bulk material and changesthe dislocation arrangement under plastic deformation [20–24].These modifications have important effects on the mechanicalproperties of stainless steels. However, the results obtained in thiswork show that the effect on the erosion resistance of austeniticstainless steel is not as noticeable as shown in other researchworks with martensitic and duplex stainless steels [25,26].Accordingly, nitrogen addition did not change the mechanism ofsynergy, remaining erosion as the dominant mechanism of sur-face degradation. However the erosion mass loss rate of thenitrided samples decreased by 5% in comparison with the solu-tion-annealed samples.

er

Keo

Mass loss rate under

corrosion condition Kco

(g/cm2 min)

Synergism S (g/cm2 min)

%) 1.67�10�8 (0.03%) 8.79�10�6 (14.51%)

%) 6.41�10�9 (0.01%) 6.79�10�6 (12.15%)

Fig. 10. Synergism between erosion and corrosion in AISI 304L stainless steel in

the (a) solution-annealed condition and (b) nitrided condition.

Table 4

Synergism, DKc, DKe, Ke, Kc and erosion–corrosion ratio Ke/Kc in AISI 304L stainless steel in the solution-annealed condition and nitrided condition.

Material S (g/cm2 min) DKc (g/cm2 min) DKe (g/cm2 min) Ke (g/cm2 min) Kc (g/cm2 min) Ke/Kc

304L 8.79�10�6 3.56�10�6 (40.4%) 5.23�10�6 (59.6%) 5.70�10�5 3.57�10�6 15.9

304LN 6.79�10�6 1.14�10�6 (16.7%) 5.66�10�6 (83.3%) 5.47�10�5 1.14�10�6 47.8

Fig. 11. Distribution of synergism terms in AISI 304L stainless steel in the

(a) solution-annealed condition and (b) nitrided condition.

D. Lopez et al. / Tribology International 44 (2011) 610–616 615

On the other hand, the corrosion resistance of the austeniticsteel is clearly increased by alloying with nitrogen, which has acritical role on the reduction of the synergism. The importance ofthe DKe and DKc terms changes with the addition of nitrogen,passing from DKe¼59,6% and DKc¼40,4% for the solution-annealed samples to DKe¼83,3% and DKc¼16,7% for the nitridedsamples. Nitrogen decreases the DKc term, which describes theeffect of erosion on corrosion. This result shows that the effect ofnitrogen is more accentuated on the increase of corrosion resis-tance of the austenitic steel.

One of the mechanisms by which the corrosion resistance ofthe steel is increased could be related to the production ofammonia during the dissolution of the steel surface, increasingthe pH level at the surface and promoting repassivation of thedamaged layer [25,26]. However, other mechanisms have beenproposed such as strengthening of the passive layer by formationof oxynitrides films and nitrates and nitrides formation that act aslocal corrosion inhibitors [27,28]. Some studies have reported thatdetachment of the passive layer could be favored by coalescenceof vacancies, left by migration of metallic atoms [29]. If thesevacancies are filled by nitrogen atoms, an additional mechanismof structural reinforcement could be acting in the improvement ofcorrosion resistance by nitrogen additions. Some studies havereported nitrogen detection in the metal–passive layer interface[30–33], supporting this hypothesis.

Fig. 12. SEM images of the eroded surfaces of AISI 304L samples in the

(a) solution-annealed and (b) HTGN condition after corrosion–erosion testing.

10000� . (c) Schematic drawing representing the effect of nitrogen on the

erosion–corrosion mass removal mechanism.

5. Conclusions

0.55 wt% N in solid solution in austenite improves thecorrosion–erosion resistance of the AISI 304L austenitic stainlesssteel and reduces erosion in 5%, corrosion in 63% and synergism in23% when tested in slurry composed by quartz particles and 3.5%NaCl. However further tests are needed to address different flowvelocities and geometries, such as an elbow.

Nitrogen addition decreases the passive current density duringpolarization tests of austenitic stainless steel samples testedunder liquid impingement corrosion and corrosion–erosion.

D. Lopez et al. / Tribology International 44 (2011) 610–616616

Nitrogen alloying increased the repassivation ability of thepassive layer at the surface, reducing the corrosion of lips andmarks left by impact of slurry particles.

The effect of nitrogen on the corrosion–erosion resistance ofaustenitic stainless steel is more accentuated through the increase ofits corrosion resistance, illustrated by the decrease of the DKc term,which describes the effect of erosion on corrosion.

Acknowledgments

The authors want to acknowledge CNPq—Conselho Nacionalde Desenvolvimento Cientıfico e Tecnologico. Project nos.475956/2003-6 and 490316/2004-2.

References

[1] Mischler S. Triboelectrochemical techniques and interpretation methods intribocorrosion: a comparative evaluation. Tribology International 2008;41:573–83.

[2] He DD, Jiang XX, Li SZ, Guan HR. - Erosion-corrosion of stainless steels inaqueous slurries—a quantitative estimation of synergistic effects. Corrosion2005;61:30–6.

[3] Zhou S, Stack MM, Newman RC. Characterization of synergistic effectsbetween erosion and corrosion in an aqueous environment using electro-chemical techniques 1996;52:934–46Corrosion 1996;52:934–46.

[4] Berns H. Manufacture and application of high nitrogen steels. ISIJ Interan-tional 1996;36:909–14.

[5] Gavriljuk VG, Berns H. High nitrogen steels: structure, properties, manufac-ture, applications. Springer-Verlag; 1999.

[6] Hanninen H. Applications and performance of high nitrogen steels. In: AkdutN, de Cooman BC, Foct J, editors. Proceedings of HNS 2004, Belgium, 2004, p.371–80.

[7] Simmons JW. Strain Hardening and plastic flow properties of nitrogen-alloyed Fe–17Cr–(8-10)Mn–5Ni austenitic stainless steels. Acta Materialia1997;45(1997):2467–75.

[8] Speidel MO. Applications and services, in high nitrogen steels and stainlessSteels: manufacturing. In: Mudali K, Raj B, editors. Properties and applica-tions. ASM International; 2004. p. 266.

[9] Berns H, Kuhl A. Reduction in wear of sewage pump through solutionnitriding. Wear 2004;256:16–20.

[10] Garzon CM, Thomas H, Santos JF, Tschiptschin AP. Cavitation erosionresistance of a high temperature gas nitrided duplex stainless steel insubstitute ocean water. Wear 2005;259:145–53.

[11] Mesa DH, Toro A, Tschiptschin AP. The effect of testing temperature oncorrosion–erosion resistance of martensitic stainless steels. Wear 2003;255:139–45.

[12] Toro A, Sinatora A, Tanaka DK, Tschiptschin AP. Corrosion–erosion of nitrogenbearing martensitic stainless steels in seawater-quartz slurry. Wear2001;251:431–8.

[13] Toro A, Tschiptschin AP. Chemical characterization of a high nitrogenstainless steel by optimized electron probe microanalysis. Scripta Materialia2010;63:803–6.

[14] Madsen BW. Measurement of erosion–corrosion synergism with a slurrywear test apparatus. Wear 1988;123:127–42.

[15] ASTM Annual Book of Standards, 3.02, Standard practice G 119, ASTM.[16] Dean SW. Calculation of alloy equivalent weight. Materials Performance

1987;26:51–2.[17] Pourbaix M. Atlas of electrochemical equilibria in aqueous solutions. Perga-

mon; 1974.[18] Neville A, Hu X. Assessment of electrochemical response from high alloy

stainless steels during slurry impingement and single impacts to improveunderstanding of erosion–corrosion. British Corrosion Journal 2002;37:43–7.

[19] Lu BT, Luo JL. Synergism of electrochemical and mechanical factors inerosion–corrosion. Journal of Physics Chemistry B 2006;110:4217–31.

[20] Gavriljuk VG, Berns H, Escher C, Glavatskaya NI, Sozinov A, Petrov YN. Grainboundary strengthening in austenitic nitrogen steels. Materials Science andEngineering A 1999;271:14–21.

[21] Milliner P, Solenthaler C, Uggowitzer P, Speidel MO. On the effect of nitrogenon the dislocation structure of austenitic stainless steel. Materials Science

and Engineering 1993;A164:164–9.[22] Petrov YN, Gavriljuk VG, Berns H, Schmalt F. Surface structure of stainless

and Hadfield steel after impact wear. Wear 2006;260:687–91.[23] Garzon CM, Tschiptschin AP. New high temperature gas nitriding cycle that

enhances the wear resistance of duplex stainless steels. Journal of MaterialsScience 2004;39:7101–5.

[24] Tschiptschin AP. ‘‘Powder metallurgy aspects of high nitrogen stainlesssteels. In: Mudali K, Raj B, editors. High nitrogen steels and stainless steels:manufacturingproperties and applications. ASM International; 2004. p. 300.

ISBN 0-87170-793-4.[25] Lopez D. Estudo do desgaste erosivo–corrosivo de ac-os inoxidaveis de alto

nitrogenio em meio lamacento. PhD thesis, Univeristy of Sao Paulo, Brazil,2007 [In Portuguese].

[26] Mudali UK, Raj B. High nitrogen steels and stainless steel: manufacturingproperties and applications. In: Mudali K, Raj B, editors. ASM International;2004. p. 300. ISBN 0-87170-793-4.

[27] Jargelius-Pettersson RFA. Electrochemical investigation of the influence ofnitrogen alloying on pitting corrosion of austenitic stainless steels. Corrosion

Science 1999;30:1639–64.[28] Levey PR, Van Bennekom A. A mechanistic study of the effects of nitrogen on

the corrosion properties of stainless steels. Corrosion 1995;51:911–21.[29] Ryan M. Peering below the surface. Nature materials 2004;3:663–4.[30] Clayton CR, Halada GP, Kearns JR. Passivity of high-nitrogen stainless alloys:

the role of metal oxyanions and salt films. Materials Science and EngineeringA 1995;198:135–44.

[31] Olefjord I, Wegrelius L. The influence of nitrogen on the passivation ofstainless steels. Corrosion Science 1996;38:1203–20.

[32] Willenbruch RD, Clayton CR, Oversluizen M, Kim D, Lu Y. An XPS andelectrochemical study of the influence of molybdenum and nitrogen on the

passivity of austenitic stainless steel. Corrosion Science 1990;31:179–90.[33] Lei MK, Zhu XM. Role of nitrogen in pitting corrosion resistance of a high-

nitrogen face-centered-cubic phase formed on austenitic stainless steel.Journal of the Electrochemical Society 2005;152:291–5.