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INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY

Department of Metallurgical Engineering and Materials Science

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S Y N O P S I S

of the Ph.D. thesis titled

MICROSTRUCTURAL AND THE CONSEQUENT LOCALIZED CORROSION RESISTANCE BEHAVIOUR OF HTHAZ OF UNS S32760 SDSS TIG WELDS PREPARED USING NITROGEN

ADDITIONS TO ARGON SHIELDING GAS

Proposed to be submitted in partial fulfillment of the degree of

DOCTOR OF PHILOSOPHY

of the

INDIAN INSTITUTE OF TECHNOLOGY, BOMBAY

By

Rahul Khandagale

(Roll No. 99411401)

Supervisors:

Prof. R. Raman and

Prof. I. Samajdar

July 2006

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Figure 1: A schematic indicating the involvement of HTHAZ via FZ in the process of exchange of nitrogen species due to the dynamic interaction between arc plasma/shielding atmosphere and highly convecting molten weld pool (FZ) during arc welding

Second generation duplex stainless steel contains nitrogen as a crucial alloying

element specifically added to enhance reaustenitization of the HTHAZ in the limited time

available as the weld cools. In the absence of nitrogen a highly ferritized [greater than

80% ferrite] can be obtained in the consequent loss in the expected physico – chemical

properties1. During arc welding there is an inevitable dynamic interaction between the

molten weld pool/arc atmosphere, molten weld pool/inert gas atmosphere and arc

atmosphere/inert gas interfaces leading to exchange of the metallurgically active nitrogen

species. The above leads to loss/gain of nitrogen from the metal depending upon the

partial pressure of nitrogen [diatomic] in the shielding gas and the monotonic nitrogen in

the arc atmosphere. The proposed model is shown in Figure 1.

The nitrogen plays two contradictory roles during the evolution of HTHAZ

microstructure i] Encouragement of reaustenitization which is beneficial ii] Formation of

1

FZ HTHAZ

LOSS GAIN(LOSS if PN2 in shielding gas mixture is below a critical value)

PN2PN

N

Cr2N which is harmful. Based on the CCT curve for the above two reaction2 it is seen that

beyond a particular level of nitrogen in the HTHAZ reaustenitization will dominate

leading to efficient trapping of nitrogen as it gets rejected from the ferrite phase, in which

the solubility of nitrogen is very low as compared to that in austenite. This leads to non

linearity of the effect of nitrogen contain in the HTHAZ on the formation of Cr 2N. This

has been confirmed by several weld simulation study on super-duplex stainless steels3, 4, 5

in the absence of adequate reaustenitization there is also a good possibility of forming

sigma phase in the HTHAZ in SDSS which contain higher levels of Cr and Mo as

alloying elements. Formation of both the above deleterious phases can lead to loss of

localized corrosion resistance.

The success of duplex stainless steel as a material with excellent pitting and SCC

resistance relies on i] Maintaining nearly 50 : 50 ferrite (): austenite () balanced

structure ii] Prevention of deleterious precipitates and phases forming during processing

in ferrite. The deviation from optimum performance can also occur due to loss of phase

balance either towards ferrite direction or austenite direction. The former occur when

effective nitrogen level is below critical level and latter when the level is above a

threshold value.

The above clearly suggests that while welding second geration DSS it is necessary

to maintain adequate nitrogen activity in the arc atmosphere so that an optimum property

of HTHAZ is obtained. Note that, in the case of fusion zone the above problem can be

offset by choosing a suitable filler metal [Ni rich].

The main objective of the present work is to determine the amount of nitrogen

that need to be added to the argon shielding gas while welding super-duplex stainless

steels [SDSS] using range of heat input normally required for these materials. Although it

is known that nitrogen needs to be added while welding second generation DSS there has

been no reported systematic investigation on the effect of nitrogen additions to the

shielding gas from the localized corrosion properties of the HTHAZ formed. Most of the

reported work on the effect of nitrogen on the response of DSS and SDSS to thermal

cycle have been limited to weld thermal simulation studies. However due to practical

limitations these studies do not properly capture the events which actually occur during

2

arc welding. This is especially true for those materials in which there is a possibility of

metallurgical interaction between the arc and the weld pool.

In the present work studies on localised corrosion resistance of the HTHAZ

formed during TIG welding of UNS S32760 super-duplex stainless steels (12.5mm thick

plates in solution annealed followed by water quenched condition) have been carried out.

Bead-in-groove welds were prepared using the recommended Ni rich (9%Ni, 2.5mm

diameter) filler wire. To vary the effective nitrogen activity in the HTHAZ controlled

amount (0 to 5 vol. %) of nitrogen additions to the argon shielding gas was made. To

vary the weld cooling rate and hence the HTHAZ cooling rate a range of heat inputs was

selected (0.33 to 1.21 kJ/mm). The above heat input corresponded to the HTHAZ cooling

rate to, as represented by t12/8 , 0.37 to 4.9 s which is wide enough range to represent the

weld cooling rates normally encountered. The HTHAZ of different welds were subjected

to different characterization techniques for the purpose of evaluating the evolved

microstructure with respect to γ:δ phase balance and presence of deleterious phases such

as and Cr2N and the consequent localised corrosion behaviour.

This thesis consists of eight chapters.

Chapter 1 briefly introduces the present project and states the objective and the

scope of work.

Chapter 2 presents the detailed and critical literature survey related to the

evolution, the metallurgy, mechanical properties and corrosion characteristics of wrought

duplex stainless steel in general and super duplex stainless steel in particular. Aspects

related to formation of various deleterious precipitates and phases such as sigma [],

Cr2N, secondary austenite (’), chi (), R, pi (), carbides etc. have been particularly

discussed in greater detail6. A separate section, related to welding metallurgy of these

materials to bring about the effect of the weld thermal cycle on the metallurgy of

evolution of microstructure of HAZ with particular reference to high temperature heat

affected zone [HTHAZ] which is the zone of interest in the present work, conclude this

chapter3,4,5.

Chapter 3 is related to the important topic of the interaction of the metallurgically

active nitrogen with the molten weld pool during arc welding of DSS. The crucial point

of increased solubility of nitrogen in the molten metal under arc much above that

3

expected from Severt’s law considerations has been discussed. The chapter concludes

with the effect of welding parameters on the nitrogen pick up [or loss] by the molten weld

metal during arc welding.

Chapter 4 presents the available information on the effect of nitrogen, a strong

austenite stabilizer but can also encouraged formation of Cr2N precipitates in the ferrite

phase [in which the solubility of nitrogen is extremely low as compared to austenite

phase] on the evolution of HTHAZ microstructure. Conditions favouring ferrite rich

unbalanced structure with the attendant deleterious precipitates and phases and those

favouring nearly balanced precipitate free balanced structure have been discussed. The

importance of suitable combinations weld heat input [which affects the weld cooling rate

as represented by t12/8] and the nitrogen content of the material to ensure acceptable

HTHAZ microstructure has been brought out.

Various electrochemical techniques for evaluating localized corrosion resistance

of passive metal/alloys such as cyclic pitting scan, electrochemical reactivation potentio-

dynamic test[EPR,DLEPR],rapid scratching electrode ,galvanostatic, electrochemical

noise analysis [ECNA] and the novel micro-electrochemical Ecorr corrosion potential

noise probing technique [a new technique under development in this laboratory]11

have been describe in chapter 5. a discussion on the rescaled range analysis7 [R/S][Hurst

analysis], a mathematical tool for appropriately analyzing the essentially non linear Ecorr

vs time noise data has also been included in this chapter.

Various experimental procedures adopted related to preparation of welds,

metallurgical characterization of the HTHAZ of the welds produced by optical and

electron microscopy, preparation of appropriate test specimens for different

electrochemical corrosion tests to evaluate the localized corrosion resistance of the

HTHAZ of the welds have been presented in chapter 6. The chapter also includes brief

description of various experimental set ups used for corrosion studies. The results

obtained from each of the above experimental procedure are also included in this chapter.

For the purpose of convenience results obtained have been presented in the form of

tables, figures, micrographs etc. immediately after each type of experiment. Some

representative results are shown in Figures 1 to 5 and Tables 1 to 3.

4

Figure 2: Low (25X) and high (250X) magnification optical micrographs respectively showing the composite zone [(a),(b)] and HTHAZ [(a’),(b’)] of welds prepared with 0 vol.% nitrogen additions to the argon shielding gas and t12/8 of 0.41 s [(a),(a’)] and 3.0 vol.% nitrogen additions to the argon shielding gas and t12/8 of 0.37 s [(b),(b’)]{Figure 2 (a): weld 30(W30),(b): weld 48(W48)}.[Note the significant amount of phase formation in the ferrite phase in the case of weld prepared with 0 vol.% nitrogen(a’).Cr2N although present can not be revealed by optical microscopy.]

5

500m

WZ

HAZ

(a)

(b)

50m

(a’)

(b’)WZ

HAZ

Base metal

HTHAZ

HTHAZ

Figure 3: Ecorr vs time noise data obtained with the novel micro-electrochemical

corrosion potential noise probing (90 µm diameter probe) technique (under

development in this laboratory) on the welds with defective HTHAZ [W30](a) and

defect free HTHAZ [W48](b).[The above curves corresponds to the same welds

shown in Figure 2. Note the active and negative going trend of the Ecorr vs t noise

signal obtained from the defective weld (a).]

Figure 4: (a) E vs t response anodic galvanostatic current density of 10µamp/cm2 in 0.85

M NaCl solution at 500C for good weld [W 48] and bad weld [W30].The response of as

received material [AR] and thermally aged [HT] [8500C+3hrs.] to this test have been

included in the Figure (b) E vs t response anodic galvanostatic current density of

10µamp/cm2 in 0.1 M Na2O3 solution at room temperature of 250C for good weld [W 48],

6

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0 200 400 600 800 1000

Time(Sec)

Eco

rr(v

) S

CE

-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

0.01

0 200 400 600 800 1000

Time (Sec)

Eco

rr (

V)

SC

E

HTHAZ

FZ/HAZ

FZ

HTHAZ FZ/HAZ

FZ

(a) (b)

-0.2

0

0.2

0.4

0.6

0.8

0 100 200 300 400 500 600

Time (Sec)

E (

V,S

CE

)

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100 120 140 160 180 200

Time (sec)

E(V

,SC

E)

HT

AR

W9

W1

W33

W30 W48HT

W30

AR W48

(a) (b)

bad weld [W30] and several other welds [W33:Weld No.33, t12/8=0.47s(0.5%nirogen),

W1:Weld No.1, t12/8=1.33s(0%nitrogen),W9: Weld No.9, t12/8=1.74s(1%nitrogen),

W30:Weld No.30, t12/8=0.41s(0%nitrogen), W48:Weld No.48,

t12/8=0.67s(3%nitrogen)] with different t12/8 / vol.% nitrogen additions combinations

[see Table 2]. The response of as received material and thermally aged [8500C+3hrs.] to

this test have been included in the figure.

[the galvanostatic test is based on the suggestions made by Newman et al 9,10 The test

in chloride media [Figure 4 (a)] is a clear indicator of good and bad passive material

which has been confirmed with other electrochemical tests [see Table 1] Figure 4 (b)

shows the presence of Cr2N [assuming the hypothesis put forward by Newman et

al10] shows the non linear behaviour of Cr2N formation with respect to vol.%

nitrogen additions to the argon shielding gas and combinations.]

Figure 5: Dependence of vol.% ferrite [estimated by image analyzer] in the HTHAZ on

vol.% nitrogen additions to the argon shielding gas and HNet [as represented by t12/8 ].

[ the Figure clearly brings out the complex interdependence of the reaustenitization

process which depends on the effective nitrogen available in the HTHAZ [which in

7

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6

Vol.% nitrogen in the shielding gas

% f

erri

te

1.33 sec

1.60 sec

1.992 sec

3.4 sec

4.92 sec

3.4sec

1.60sec

4.92sec

1.992sec1.33 sec

t12/8

turn depends on vol.% nitrogen additions to the argon shielding gas and HNet latter

of which decides the weld pool size and weld pool/arc/inert atmosphere interface

area] and t12/8 which depends only on the HNet for a given plate thickness and if

preheat temperature if any. This result also brings about an important fact of

inadequacy of results obtained in weld simulation studies on materials which are

capable of metallurgically interacting with the arc during welding.]

Chapter 7 is the penultimate chapter of the present thesis. Discussion of the

various results related to HTHAZ of different welds prepared namely degree of

ferritization, formation of sigma, Cr2N and localized corrosion resistance is presented in

this chapter. Before analyzing the actual results basis for expected variation in different

microstructural features of the HTHAZ and its consequent localized corrosion resistance

in response to the experimental input of HNet [hence t12/8]8 combinations has been

formulated. Due recognition is given to the fact that the kinetics of evolution of HTHAZ

microstructure is far different from that associated with CCT diagram determination and

weld thermal simulation study the latter of which has limitation with respect to maximum

temperature, heating and cooling rates that can be simulated.

The final chapter 8 of the thesis presents conclusions related to a] the localized

corrosion resistance of HTHAZ of the various welds depending upon the combination of

the heat input HNet [which determines the t12/8] and vol.% nitrogen addition to the argon

shielding gas [which together with HNet decides the effective nitrogen activity in the

HTHAZ] combination. b] The evolution of HTHAZ microstructure with respect to alpha

gamma balance and presence of deleterious precipitates such as sigma and Cr2N. c] The

satisfactory performance of the novel micro-electrochemical corrosion potential noise

probing technique [under development in this laboratory] for evaluating local localized

corrosion resistance [the exposed area limited to 90 micrometer diameter] of passive

surfaces and encouraging results using galvanostatic technique suggested by Newman9, 10.

The chapter also includes practical suggestions levels of nitrogen additions to be made to

argon shielding gas while TIG welding of UNS S32760 super duplex stainless steels

using a range of heat input normally used. While making this suggestion it is kept in

8

mind that not only has good localized corrosion resistance but also has reasonably

balanced microstructure with respect to alpha (δ) and gamma (γ).

Table 1: Basis for evaluation of HTHAZ of different welds using various electrochemical tests

Sr.

No.Corrosion Test Basis for evaluation of localized corrosion resistance

1 DLEPR(2M H2SO4 + 0.01M KSCN + 0.5M NaCl, RT)

Based on ir / if ratio. Acceptable : < 10 X 10-3

Unacceptable : >10 X 10-3

2 Rapid scratching electrode (3.5% NaCl, RT)

Based on repassivation tendency Erepass =Emax – (Ecorr+ 50mV)

Acceptable : >800 mV Unacceptable : < 800 mV

3 Galvanostatic (0.85 M NaCl, 500C )9

Based on E vs Time curve Acceptable: rapidly attaining noble potential

without significant noise towards active potential. Unacceptable: noisy and tendency to attain active

potential.

4 Galvanostatic (0.1M Na2CO3,

RT )10 Based on E vs Time curve Has been used for precipitate phase identification

only because transpassive dissolution in mild alkaline solution is not stable because of formation of stable Fe+++ oxide or oxyhydroxide.

Cr2N starts dissolving at lower potential (hence earlier) than χ than .

5 Micro-electrochemical corrosion potential noise probing (3.5% NaCl, RT)[Under development in this laboratory]11

Based on Hurst exponent (H) along with whether the Ecorr vs t tends to go +ve or –ve.

Acceptable: H > 0.8 with +ve going tendency.Unacceptable : H > 0.6 with –ve going tendency

9

t12/8

(s)

0 vol.% nitrogen 0.5 vol.% nitrogen 1.0 vol.% nitrogen 3.0 vol.% nitrogen 5.0 vol.% nitrogenIr/If

X 103

Hurst (H)

Gal-NaCl

Ir/If

X 103

Hurst (H)

Gal-NaCl

Ir/If

X 103

Hurst (H)

Gal-NaCl

Ir/If

X 103

Hurst (H)

Gal-NaCl

Ir/If

X 103

Hurst (H)

Gal-NaCl

AR 1.3 - - - - - - - - - - - - - -HT 100 - - - - - - - - - - - - - -

0.365 15.0 - - - - - - - - 2.65 0.6+ - - - -0.387 - - - 24.2 0.7- - - - - - - - - - -0.411 30.6 0.7- Bad -- - - - - - - - - - - -0.435 - - - - - - 60 0.8+ Bad - - - -- - -0.459 - - - - 0.6- Bad - - - - - - 4.69 1.0+ Good0.592 20.0 0.8- - - -- - - - - - - - - - -0.680 - - - 1.22 0.6- - 23 - - 0.26 0.9+ Good - - -0.711 20.5 0.6- - - - - 12.9 0.7- - - - - 1.63 1.0+ -1.054 - - - 18.2 0.7- - 11.1 0.7- - - - - 2.79 1.0+ Good1.170 - - - - - - - - - 0.82 0.6- - - - -1.289 - - - - - - 8 - - - - - - - -1.334 23.2 0.7- - - - - - - - - - - - - -1.377 27.6 - - - - - - - - 4.52 0.8+ - - - -1.420 - - - 21.4 0.5+ - - - - 3.92 - - 1.29 1.0+ Good1.600 24.3 - Bad - - - - - - - - - - - -1.644 - - - - - - - - - 5.5 0.6+ - 1.96 1.0+ -1.742 - - - 20.7 0.8- - 0.54 0.7- Bad - - - - - -1.941 - - - 14.2 0.8+ Good - - - - - - - - -1.992 11.3 - - - - - - - - - - - - - -2.041 - - - - - - 1.05 0.8+ Good - - - - - -2.598 - - - - - - - - - - - - 3.04 0.7+ -2.778 - - - - - - - - - 1.96 0.6+ Good - - -3.360 - - - 12.2 0.7- Bad - - - - - - 1.66 1.0+ Good3.428 21.5 0.7- Bad - - - - - - - - - - - -3.635 - - - - - - 0.85 0.8+ Good - - - - - -4.368 - - - - - - - - - 0.12 0.5- Good - - -4.445 - - - 25.9 0.8- Bad - - - - - -4.921 24.4 - - - - - 0.48 0.8- - - - - - - -Table 2: Consolidated localized corrosion resistance test results of HTHAZ of various welds [42 in all] [AR: As received UNS S32760, HT: Thermally aged UNS S32760 at 8500C +3hours, Gal-Nacl: Galvanostatic studies in 0.85 M NaCl]

10

[Results of pitting scan have not been included]

11

It is seen from the result [Table 2] that welds prepared with 0, 0.5 and 1 vol. %

nitrogen addition to the argon shielding gas have been associated with poor localized corrosion

of the HTHAZ. This can be ascribed to the formation of sigma phase and Cr2N which precipitate

ferrite phase leading to Cr/Mo depleted regions. The non linear behaviour of formation of Cr 2N

with respect to the input parameters namely vol.% nitrogen additions to the argon shielding gas

and HNet [which determines the t12/8 ] as well as weld pool /arc/atmosphere interface area and

interaction time has also been observed. Limited TEM studies have been done on the HTHAZ

with respect to Cr2N formation. The extent of Cr2N formation has been assessed on the basis of

galvanostatic studies in 0.1 M Na2CO3 has recommended by Newman et al9, 10. It is seen from the

result that the HTHAZ of the welds prepared with 3.0 and 5.0 vol. % nitrogen additions to the

shielding gas too contain Cr2N. However the HTHAZ of these weld shave shown good localized

corrosion resistance. This means that Cr2N by itself need not lead to drop in localized corrosion

resistance. Hence the loss of localized corrosion resistance of HTHAZ should be ascribed mainly

to formation of sigma phase with Cr2N formation enhancing this effect. This has been the case

welds prepared with none or zero addition of nitrogen in the argon shielding gas and low heat

input.

It is thus advisable to keep the nitrogen additions to around 3.0 vol. % in welding

UNS S32760 super duplex stainless steels [SDSS]. Also, to ensure adequate effective nitrogen

activity in the HTHAZ higher values of nitrogen addition [but still around 3.0 vol. %] should be

used with higher heat inputs. Too higher value of nitrogen additions will lead to the problem of

loss of phase balance in the HTHAZ towards austenite side that should be avoided. Finally

reference to be made to the Table 3 which gives conditions for obtaining welds acceptable both

with respect to localized corrosion resistance of HTHAZ and its phase balance while welding

UNS S32760 SDSS.

Before concluding it is interesting to note that the novel micro-electrochemical corrosion

potential noise probing technique [under development in this laboratory]11 appears to be quiet

efficient in assessing local localized corrosion resistance of the passive materials. Galvanostatic

technique suggested by Newman et al 9, 10 appear to be very promising and can found a good

economic alternative to the normally employed electrochemical techniques involving

potentiostats.

12

Sr. No.HNet range

(kJ/mm)

0 vol.% N2 0.5 vol.% N2 1 vol.% N2 3 vol.% N2 5 vol.%N2

LCR BM LCR BM LCR BM LCR BM LCR BM

1 0.30-0.40 N.A N.A N.A N.A N.A MOD A A A A

2 0.41-0.50 N.A N.A N.A N.A N.A A A A A A

3 0.51-0.65 N.A N.A N.A N.A MOD N.A A A A N.A

4 0.66-0.80 MOD N.A N.A N.A A MOD A A A N.A

5 0.80-1.21 N.A N.A N.A N.A A MOD A A A N.A

Table 3: Acceptable HNet range and vol. % nitrogen additions to the argon shielding gas while TIG welding UNS S32760

(12.5mm thick, without preheat)

( LCR: Localised corrosion resistance of HTHAZ ; BM : Balanced microstructure ; N.A : Not Acceptable ;

MOD : Moderate ; A : Acceptable.)

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REFERENCES

1. J. O. Nilsson, “Super duplex stainless steels-Overview”, Material Science and Technology,

vol.8, pp. 685-700 (1992).

2. B.Joseffsson, J.O.Nilsson, and A.Wilson, “Phase transformation in duplex steels and the

relation between continuous cooling and isothermal heat treatment”, Proc.Conf. “Duplex

stainless steels’91”, p.67, Les Ulis, France, Les Editions de Physique (1991).

3. A.J.Ramirez, J.C.Lippold, S.D.Brandi, Metallurgical and Materials transactions A, vol.34A,

p.1575 (2003).

4. H. Hoffmeister and G.Lothongkum, “Quantitative effects of nitrogen contents and cooling

cycles on δ γ transformation, chromium nitride precipitation and pitting corrosion resistance

after weld simulation of duplex stainless steels.” Vol.2, paper 55, Jacques Charles, Welding in

the World, vol.36, pp.43-54(1995).

5. E.I. Kivineva and N.E.Hannerz: “The properties of Gleeble simulated heat affected zone of

SAF 2205 and SAF 2507 duplex stainless steels.” .Vol.2 paper 7, Proc. Conf. Duplex 94,

Glasgow, 1994.

6. J.O Nilsson and P.Liu, Mater. Sci. Technol., 7,(9), pp.853-862 (1991

7. J.Feder, “Fractals”, Plenum, New York, 1988.

8. Bjorn E. S. Lindblom, Berthold Lundqvist and Nils Erick Hannerz, Scandinavian Journal of

Metallurgy, 20, pp.305-315 (1991).

9. M.A. Domninguez-Aguilar, R.C.Newman, Corrosion Science (2005) (article in press).

10. M.A. Domninguez-Aguilar, R.C.Newman, Corrosion Science (2005) (article in press).

11. R.Raman, DRDO sponsored project on “The development of micro-electrochemical corrosion

potential noise probing technique for evaluating local localized corrosion resistance of passive

materials in aqueous solution” (June2003 to August 2006). Being carried out in the Department

of Metallurgical Engg. and Material Science, I.I.T. Bombay (India).

14

Publication resulting from the Ph.D work:

1) Ramesh P , R.Khandagale, R.Dalvi, Depashree Nage, R.Raman “Repassivation Studies by Rapid Scratch Technique on Super Duplex Stainless Steel, Steel Welds in non-deaerated 3.5% NaCl Solution”, published in NACE India, conference held in Goa(India), from 28 th – 30th November 2002.

2) R.Raman, R.Khandagale, “Development of micro electrochemical Ecorr noise probing system to study degradation or enhancement of localized corrosion resistance of passive materials.”, published in NACE India, conference held in Goa (India), from 28th – 30th November 2002.

3) R.Raman, R.Khandagale, “GTAW of UNS S32760 Super Duplex Stainless Steel using Nitrogen additions in the shielding gas – microsturctural and localized corrosion resistance aspects of fusion zone / heat affected interface”, one day seminar on ‘Advance in Welded Fabrication’, held in BARC, Mumbai (India)on 18” January 2003.

4) R.Raman, P.Raju, P.J.Antony, R.Khandagale , “Analysis of corrosion potential noise by R/S analysis and wavelet transform methods” International conference on corrosion (CORCON2003) in Mumbai(India), Dec. 1 to 4, 2003.

………………………………….

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