Effect of laser nitridation on the electrochemical corrosion behaviour of 316 stainless steel

Effect of laser nitridation on the electrochemical corrosion behaviour of 316 stainless steel

Thin Solid Films, 176 METALLURGICAL (1989)197-206 AND PROTECTIVE 197 COATINGS EFFECT OF LASER NITRIDATION ON THE ELECTROCHEMICAL CORROSION BEHAVI...

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Thin Solid Films, 176 METALLURGICAL

(1989)197-206

AND PROTECTIVE

197

COATINGS

EFFECT OF LASER NITRIDATION ON THE ELECTROCHEMICAL CORROSION BEHAVIOUR OF 316 STAINLESS STEEL VANITA

NAIK,

Department

S. M. CHAUDHARI,

of Physics, University

S.M.

KANETKAR

of Poona 411407

AND S. B. OGALE

(India)

A. P. B. SINHA National Chemical Laboratory,

Poona 411-007

(India)

C. K. GUPTA Bhabha Atomic Research Centre. Trombay, Bombay 4oo-085 (India) (Received

January

25,1989;

accepted

March

30,1989)

Nitridation of 316 stainless steel is achieved by using pulsed-laser-induced reactive quenching at the interface between the steel sample and a thin liquid nitrogen overlayer. The electrochemical corrosion behaviour of the virgin sample, the as-nitrided sample, and the nitrided and annealed sample is studied by the potentiokinetic polarization technique. It is shown that the nitrided and annealed sample has a considerably higher corrosion resistance than does the virgin sample. The state of the sample surface after the different treatments is characterized by small-angle X-ray diffraction, scanning electron microscopy and conversion electron Miissbauer spectroscopy.

1.

INTRODUCTION

The corrosion resistance of a material is an important consideration in deciding its applicability in specific situations of interest. This is especially so where the material is exposed to extreme environmental conditions’. Among metallic systems, stainless steel is an alloy which is considerably resistant to corrosion under various conditions and is therefore used in a wide range of industrial environments’. Depending on the application, however, the composition of steel has to be varied and correspondingly its corrosion resistance also varies. Thus a need is felt to decouple the issue of corrosion protection from that of property control. As corrosion is essentially a surface-related and surface-mediated phenomenon, the decoupling can be achieved most conveniently by surface modification. Previous studies have shown that this can be done either by depositing corrosion-resistant layers or by synthesizing corrosion-resistant surface compounds in the form of carbides or nitrides. Nitridation is known to be a useful process3 in corrosion control of iron and its alloy, and it can be achieved by various means, including treatment under nitrogen plasma and nitrogen ion implantation4. Recently, we have shown that surface modification can also be achieved by processing the solid-liquid interface by pulsed laser5*6,and nitridation by this method is considerably faster and more efficient than by other processes currently in use’*‘. In our process the sample to be nitrided is pulsed-laser treated in either liquid nitrogen or liquid ammonia. The 004~6090/89/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

198

V. NAIK t’t d.

laser energy is absorbed by the metal surface, and creates a very high temperature at the liquid-solid interface, which leads to high-temperature and high-pressure chemical reactions over a time-scale of the order of a few tens of nanoseconds. Depending on the laser energy density, nitridation can be achieved by this process for depths of a few hundred to a few thousand angstroms. In this paper, we examine the applicability of this new process to control corrosion of 316 stainless steel. 2.

EXPERIMENTAL

DETAILS

High-purity 316 stainless steel foils were used. The samples (of 1 cm2 area) were polished using emery paper (up to 4/O) and then degreased in acetone. Some of these samples were laser nitrided, whereas others were only laser irradiated in air by pulsed ruby laser (2 = 693.4 nm). In this study the irradiations were performed at an energy of 8 J cm - 2 under liquid nitrogen. Also, all the laser-treated samples were annealed at 250°C for 2 h in vacuum. The as-received, laser nitrided and laser nitrided and annealed samples were subjected to _electrochemical measurements’. Low-angle X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the surface properties of the samples before and after the different treatments. Low-angle XRD was carried out on a Rigaku (Japan) machine, using Seeman-Bohlin geometry. Miissbauer spectra of the samples were recorded in conversion electron mode by using a conventional constant-acceleration Mossbauer spectrometer and 50 mCi 57Co-Rh Mossbauer source. Conversion electrons were detected by incorporating each of the samples in a small gas flow proportional counter through which a mixture of helium and 4% ethanol was allowed to flow continuously”. The conversion electron Miissbauer spectra were recorded on a multichannel analyzer in the multiscaling mode and were least-squares fitted by the MOSFIT computer program”. 3.

RESULTS

AND DISCUSSION

The potentiokinetic polarization curves for the corrosion of the untreated and laser-treated samples of 316 stainless steel in 0.1 N H,SO, solution are shown in Fig. 1. Comparison of the result for the laser nitrided and annealed sample with those for the laser nitrided and as-received samples shows that the critical current density has been reduced significantly in the case of the nitrided and annealed sample. For the as-nitrided sample, the current density is seen to increase differently from that of the as-received sample, but at a higher polarization voltage its corrosion behaviour becomes almost the same as that of the as-received sample (Fig. 1). It can be seen from this figure that in the nitrided and annealed sample the current density is only 7uAcmm2 compared with 1 mA cm -’ in the as-received and laser-nitrided cases. This shows that annealing is essential for improvement of corrosion resistance. Low-angle XRD was carried out to investigate the structural changes induced by laser nitridation and by subsequent annealing. The results are shown in Figs. 2 and 3. The low-angle XRD patterns were recorded in all cases by keeping the grazing angles at 1” and 3”. The pattern for the as-received sample (Fig. 2) can be analysed in

LASER NITRIDATION

w

OF

I I -As

2000

M 9

3 16 STAINLESS

199

STEEL

recewed - Laser nttrlded

1600-

- Laser

------____

____--10

CURRENT

Fig. 1. Potentiokinetic Fig. 2. Low-angle

nhded

100 DENSITY

polarization

XRD pattern

1000

(p~/cm2)--

-

28

(DEGREE1

~

curves for 316 stainless steel.

of as-received

316 stainless steel.

TABLE I AS-RECEIVED

3 16 STAINLESS

Peak index

d values

Main possible assignments

Other possible assignments

41)

2.047

Fe-Cr [330], Fe,-Mo Fe-M0 [420], [410]

B(1) C(l) D(1)

1.795 1.920 1.075

Cr-Fe-Mo-Ni, Fe-Cr-Mo [212], Fe [lOOI, nFe,O, NiFe [400], FeMo [318] Fe0 [006], FeCr [532] Fe0 [440], Fe [220]

STEEL

[200]

FeC

terms of the existence of different phases. Table I summarizes the different possible assignments. The expected phases can be attributed to Cr-Fe-Mo-Ni, Fe-Cr-Mo, Fe and Fe20,. Also, the NiFe, FeMo phases could be present. The XRD pattern of the laser-nitrided sample (Fig. 3(a)) clearly shows that there is formation of nitride phases on the metallic surface. The phases which occur are mainly Fe,N, Cr-Fe-MoNi, Fe,N, Fe,N, Fe,N-Fe,N and FeNiN (Table II). Also, there is a prominent contribution from a sulphide, NiO,,,Cr,.,,O,,,,S. As the average sulphur concentration in the steel is only 0.03%, it appears that during laser nitridation the metal melts; sulphur becomes segregated on the surface of the metal during resolidification, and thus enhances its surface layer concentration. From the low-angle XRD results taken at higher values of the angle of incidence (Fig. 3(b)) it was observed that this sulphide phase does not exist in the bulk of the metal; this supports the segregation possibility. The oxygen which is a part of the sulphide matrix is expected to be incorporated during laser treatment under liquid nitrogen, where oxygen is known to be present in dissolved state. The precipitation of the sulphide phase can be explained as follows. Because of the high temperature involved in laser irradiation, there is formation of a melt in the steel. The presence of non-metallic constituents can impart an ionic character and high electrical conductivity to this melt. In such a case the rate of charge transfer

V. NAIK

200

et cd.

reactions are generally fast. The formation of the protective oxide depends on the solubility equilibrium for the oxide in the melt: M”+ +n/2 02- = MO,,, In oxyanion melts such as sulphates, the oxide ion concentration is controlled by the acidAbase equilibrium for the anion, e.g. so

4

2+ = so,+o2-

In such oxyanion melts the anion can also provide the oxidizing species, e.g. SO,+2e-

= S02+02-

Further reduction steps in sulphate melts can lead to the formation of the metal sulphide. If this takes place in the interfacial region between the metal and the oxide via laser-generated pore holes it can eventually cause mechanical failure in the protective oxide layer and accelerate the corrosion process. This is probably one of the reasons for the observed higher corrosion rate in the laser-treated sample. Sedricks has reported the role of sulphide inclusions in corrosion of stainless steels.12 The low-angle XRD results obtained for laser nitrided and annealed 316 stainless steel (Fig.4(a)) show that the contribution of the surface sulphide is remarkably reduced after annealing. This is seen for the case where the angle of incidence was kept at l”, whereas for the same sample, at an angle of incidence of 3” (Fig.4(b)) it is observed that this sulphide phase is completely absent. Also, the pattern seems to be dominated by the nitride phases (Table III). Nitride formation is known to improve corrosion resistance 4. This is a possible reason for a significant enhancement in corrosion resistance of the nitrided sample upon annealing. The low-angle XRD pattern of the sample treated in air (Fig. 5(a)) shows the characteristics of the as-received sample itself, except for line-narrowing, which can be attributed to grain growth as a result of laser annealing. Comparison of these data (Table IV) with the result for the laser-nitrided sample (Fig. 3(a)) shows that treatment in air and under liquid nitrogen lead to characteristically and significantly distinct results. Upon annealing, the air-treated sample is seen to lead to some precipitation of sulphide phase (see Fig. 5(b)). It thus appears that sulphur segregation and formation of a sulphide layer is an intrinsic characteristic of the radiation-processed 3 16 stainless steel. To understand the microstructural features of the untreated and treated steel samples, we characterized the samples by conversion electron Mossbauer spectroscopy (CEMS). The CEM spectrum of the virgin sample is shown in Fig. 6(a) and it can be computer fitted with magnetic and non-magnetic spectral contributions’3. The non-magnetic contribution (singlet: isomer shift (I.S.) = -O.O73mms-‘), is clearly the major one, as is the case with most industrial steels. The spectrum corresponding to the as-nitrided sample is shown in Fig. 6(b) and it is comprised of only a singlet with an I.S. value of 0.027mms - ‘. This value is not significantly different from that obtained for the singlet in the case of the virgin sample. To understand this result the information obtained from X-ray studies is useful. From Fig. 3 it should be noted that the main peak in the as-nitrided sample, namely, F(2a),

LASER NITRIDATION

-

OF

3 16 STAINLESS

28 (DEGREE)

201

STEEL

__

316 stainless steel in liquid nitrogen at an energy density Fig. 3. Low-angle XRD patterns of laser-treated of 6 JUT-*. Pattern (a) corresponds to grazing angle TV= 1”; (b) corresponds to a = 3”.

TABLE I1 316 STAINLESSSTEEL(LASERTREATEDINLIQUIDNITROGEN) Peak index

d values

Main possible assignments

WW FW

3.736 2.965

Fe,N[ 1001 Ni O.&rO.~~%llS

Wa)

2.408

Wa) A(24

2.244 2.062

W4 I(24 WV F(2b)

1.774 1.575 3.736 3.026

G(2b) W’b) A(W

2.414 2.271 2.085

JCW

1.913

U-W

WV KW CW4

1.865 1.802 1.600 1.278

D(2b)

1.088

WI, CrA

Other possible assignments

CrS,.,, P201

Cr-Fe-Mo-Ni, Fe,N [020,210] Fe-Cr-Mo [112,410] Fe,N [loll, Fe,N-Fe,N [loll, Cr-Fe-Mo-Ni Fe,N [121], CrFe-Mo-Ni Fe,N-Fe,N [102] Fe,N [ 1001 ~oi~~Z~$$% ::, [220] Cr-Fe-Mo-Ni, Fe,N [020,210] Fe-Cr-Mo-Ni [112,410] Fe,N [loll, Fe,N-Fe,N [loll, Cr-Fe-Mo-Ni Cr-Fe-Mo-Ni, Fe-Cr-Mo [212] Cr-Fe-Mo-Ni, Fe-NiN [002] Fe,N (1211, Cr-Fe-Mo-Ni Fe,N-Fe,N [102] Fe,N [221,300], FeNiN (2101 Fe,N-Fe,N [004], Fe,N [222], Fe,NiN [222]

Fe-Cr [002] F&r [330]

FeCr [002] FeCr [330] FeCr, FeMo [222] FeCr [231], FeNi [lOO]

FeCrMo, FeMo [609], Fe-Ni [44O] Fe,Mo [312], Fe-Ni [311]

202

V.

-28

(DEGREE)

NAIK

et d.

~

Fig. 4. Low-angle XRD patterns of 316 stainless steel laser-treated in liquid nitrogen 250°C for 2 h. Pattern (a) corresponds to a = 1”; (b) corresponds to OL= 3”.

and annealed

TABLE III STAINLESS STEEL LASER TREATED

Peak index

IN LIQUID NITROGEN

d values

Main possible assignments

2.893

Ni o.szCrO.&?O.,,S

Cr&,CrSI.I, GW

AND ANNEALED

2.405 2.229 2.083 1.809

W4

1.272

DW GW

1.089 2.353

Mb)

2.085

WW

1.800

Wb)

1.278

WW

1.090

Other possible assignments

WI, P201

Fe,N-Fe,N [lOO] Fe,N [lOO], Cr-Fe-Mo-Ni Fe-Cr-Mo-Ni Fe&Fe,N [loll, Cr-Fe-MO-Ni Fe,N [121], CrFeMoNi Fe,N [103], Fe,N [221,300], Fe,N [023,213], CrFeMoNi, FeNiN [210] FeCrMo Fe,N-Fe,N [lOO], Fe,N [lOO], Cr-Fe-Mo-Ni Fe,N-Fe,N [loll, Cr-Fe-Mo-Ni Fe,N [121], Cr-Fe-Mo-Ni Fe,N-Fe,N [103], Fe,N [103] Fe+CrMo

has been attributed to an oxysulphide iron. Thus, it seems that removal of from the iron environment does not Also, as the oxysulphide itself is free

FeCr [O&2],Fe,Mo [l lo] Fe-Cr-Mo [002,400]

FeCr [330], FeMo [212], Fe,Mo [201] FeCr [312], NiFe [400], [200], Fe-M0 [316] FeCr [532], Fe,Mo [302] Cr-Fe-Mo-Ni, Fe-Cr-Mo Fe-Ni [220] Fe-Ni [522] FeCr [002], Fe,Mo [l lo] Fe-Cr-Mo [002,400] FeCr [330], FeMo [212], Fe,Mo [201] FeCr [312], NiFe [400,200], Fe-MO [318] FeCr [333], Fe,Mo [205] Fe-Ni

[522]

of nickel and chromium and does not contain some concentration of nickel and chromium influence the effective hyperfine interactions. of iron it does not appear in the Mijssbauer

at

LASER NITRIDATION

-c-

OF

28

3 16

STAINLESS

(DEGREE)

203

STEEL

~

Fig. 5. Low-angle XRD patterns of 316 stainless steel samples corresponding to grazing laser treated in air, and (b) laser treated in air and annealed at 250 “C for 2 h.

angle a = l”:(a)

TABLE IV STAINLESS

STEEL

LASER

TREATED

IN AIR AND

LASER

TREATED

Peak index

d Values

Main possible assignment

A&4

2.089

nFe,O,,

B(4a)

1.801

C(4a) D(4a) F(4b)

1.275 1.086 2.946

WV

2.405 2.224 2.056 1.776 1.273 1.086

D(W

Other possible assignments

Fe203 [OOG],

[email protected], WOI, F&O4 14001 nFe,O,, Fe,O, [711], Fe,O, [2112] nFe,O, Fe,O, [312_l] Ni o.52Cr0.,, OO.Il VW

Cr&CrS,.,, W4b) A(4b) B(4b) C(4b)

IN AIR AND ANNEALED

nFe,O, Fe,O, [512] nFe,O,, Fe,O, nFe,O,

-

f2201

[400]

nFe,O,

Fe,O,

[312_l]

-

spectrum. The other two main peaks in the XRD pattern of Fig. 3 have been attributed to phases of the Fe-N system, such as Fe,N, Fe,N, etc. Some of these phases should have shown a magnetic contribution. However, the absence of such a contribution indicates that these phases are in the form of small clusters, which leads to non-magnetic spectral contributions via superparamagnetic relaxations. Another possibility is that these phases are buried below the oxysulphide layer and are not revealed by CEMS, which only scans the top 0.25 urn layer. The CEMS spectrum of the nitrided and annealed sample is shown in Fig. 6(c), and it can once again be fitted only with a single line with an IS. value of - 0.043 mm s- ‘. This shows that the iron-

204

V. NAIK

cVELOCITY

(mm/set

et d.

I-

Fig. 6. Room-temperature CEM spectrum of 316 stainless steel: (a) as-received,(b) nitrogen,(c) laser treated and annealed at 250°C for 2 h.

laser treated in liquid

related phases are stable upon .annealing at 250°C and the modifications in corrosion behaviour are primarily due to changes in the oxysulphide layers. The scanning electron micrograph of the virgin sample shows a morphology typical of a mechanically polished surface (Fig. 7). In the case of the as-nitrided sample (Fig. 8(a)) the metallic surface is more rough; whereas in the case of the nitrided and annealed sample (Fig. 8(b)) the surface has become relatively smooth as a result of annealing. The increased current density at lower potential values in the case of the laser-treated sample may be a result of surface roughening which leads to enhancement of the effective area contributing to corrosion. The bubbles seen in the scanning electron micrographs represent evolution of gas from the metallic surface during laser processing and subsequent annealing.

Fig. 7. Scanning

electron

micrograph

of as-received

316 stainless steel.

Fig. 8. Scanning electron micrograph of 316 stainless treated and annealed at 250 “C for 2 h.

steel: (a) laser treated

in liquid nitrogen,

(b) laser

4. CONCLUSION By using pulsed-laser-induced reactive quenching, nitridation of 316 stainless steel is achieved. The potentiokinetic polarization studies are carried out to observe the corrosion behaviour of the virgin sample, the as-nitrided sample and the nitrided and annealed sample. The nitrided and annealed sample shows considerably higher corrosion resistance than do the virgin sample and the as-nitrided sample. The possible reasons for the observed effects are discussed in the light of structural, microstructural and morphological information obtained from XRD, CEMS and SEM results. ACKNOWLEDGMENT

Financial support for this project under the Indo-U.S. collaboration programme is gratefully acknowledged. Thanks are also due to Dr. (Mrs.) A. K. Mitra for carrying out the SEM. REFERENCES H. H. Uhlig, in Corrosion And Corrosion Control: An Introduction To Corrosion Science And Engineering, Wiley, New York, 1963. G. Wranglen, in An Introduction to Corrosion and Protection of Metals, Institute for Metallskydd, Stockholm, 1972. K. Umeda, Y. Kawashimo, M. Nakasone, S. Harada and A. Tasaki, Jpn. J. Appl. Phys., 23 (1984) 1576. Y. S. Dorik, V. G. Bhide, S. M. Kanetkar, S. V. Ghaisas, S. M. Chaudhari and S. B. Ogale, J. Appl. Phys., 56 (1984) 2566. P. P. Patil, D. M. Phase, S. A. Kulkarni, S. V. Ghaisas, S. K. Kulkarni, S. M. Kanetkar and S. B. Ogale, Phys. Rev. Letf., 58 (1987) 238. S. B. Ogale, P. P. Patil, D. M. Phase, Y. V. Bhandarkar, S. K. Kulkarni, Smita Kulkarni, S. V. Ghaisas, S. M. Kanetkar, V. G. Bhide and Supratik Guha, Phys. Rev., B, 36 (1987) 8237. P. P. Patil, Ph. D. Thesis, Poona University, 1988. A. Morsako, K. Takahashi, M. Matsumoto and M. Naoe, J. Appf. Phys., 63 (1988) 3230.

206

9 10 11 12 13

V. NAIK

et cd.

S. M. Kanetkar, Y. S. Dorik, S. M. Chaudhari, S. V. Ghaisas, S. B. Ogale and V. G. Bhide, Thin Solid Films, 136 (1986) 45. S. B. Ogale, S. V. Ghaisas, S. M. Kanetkar and V. G. Bhide, Proc. Indian Nurn. Sci. Acad., 51 A (1985)211. E. Kreber, MOSFIT program, Universitat des Saarlandes, Saarbriicken; adopted by S. K. Date, National Chemical Laboratory, Pune, 1978. A. J. Sedricks, Int. Met. Rev. (C. B.J, 28 (1983) 295. G. P. Huffman, in R. L. Cohen (ed.), Applications of MCssbauer Spectroscopy, Vol. 11, Academic Press, New York, 1980, p. 189.