1

Corrosion resistance performance of newly developed Cobalt-free Maraging Steel

over Conventional Maraging Steel in Acidic media

Asiful H. Seikh a, Hossam Halfa

b, and El-Sayed M. Sherif

a,*

a

Centre of Excellence for Research in Engineering Materials, Advanced Manufacturing Institute, King

Saud University, P.O. Box - 800, Riyadh 11421, Saudi Arabia. b Steel Technology Department, Central Metallurgical R&D Institute (CMRDI), Helwan, Egypt.

*Corresponding Author. Tel.:+966 533203238; Fax-+966 1 4670199. E-mail: esherif@ksu.edu.sa

Abstract:

The corrosion resistance behaviour of newly developed M23 & M29 grades cobalt free maraging

steel over conventional C250 maraging steel was investigated at ambient temperature in 1M H2SO4

solution using linear polarization and electrochemical impedance spectroscopy (EIS) techniques. To

reveal the corrosion resistance performance some significant characterization parameters from linear

polarization and EIS curves were analyzed and compared. The results show that the corrosion resistance

of the M23 & M29 cobalt free maraging steel is more than the conventional C250 maraging steel in acid

solutions at ambient temperature.

Keywords: Maraging steels; Polarization; Electrochemical Impedance Spectroscopy; Corrosion

Resistance

1. Introduction

Over the past 40 years, a generic class of ultra-high strength maraging steels has been developed

mainly for many tooling applications including missile and rocket motor cases, aircraft, aerospace, wind

tunnel models, landing gear components, high performance shafting, gears, fasteners, and nuclear and gas

turbine applications[1-4]. Maraging refers to the aging of martensite, a martensite that is easily obtained at

normal cooling rates due to the high nickel content [5]. The alloy is a low carbon steel that classically

contains about 18 wt% Ni, substantial amounts of Co and Mo, together with small additions of Ti.

However, depending on the demands dedicated by the application, the composition of the material can be

modified [6]. Because of the low carbon content, maraging steels have good machinability [7]. They have

higher modulus of elasticity, lower thermal expansion coefficient, high strength, moderate toughness, and

good weldability as compared with conventional alloy steels [8]. Another important property is its high

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thermal conductivity, which reduces surface temperature during thermal loading and lowers thermal

stresses. The ultra-high strength of maraging steels is due to precipitation, usually of intermetallic

compounds, during the aging process [9-12]. Recent studies revealed that strengthening of 18 wt.% Ni

(18Ni) maraging steels results from the combined presence of Ni3Ti and Fe2Mo [11] or Fe7Mo6 [12]

precipitates. The formation of Ni3Ti takes place rapidly due to the fast diffusion of Ti atoms [11, 12].

High nickel maraging steels are very sensitive to two phenomena which deteriorate their mechanical

properties due to the austenitic reversion phenomenon and overaging problem. Recent studies carried out

by Vanderwalker [13] indicated that the finest particles Ni3Ti nucleates more easily and faster than Ni3Mo

particles. These results raised the question of the possibility of partially or completely substitution of Mo

with Ti which in turn reduces the cost of maraging steels and may overcome the problem of overaging.

On the other hand, the high contents of strategic alloying elements such as Ni, Co and Mo make these

steels expensive. While, in these types of steel, the primary strengthening effect comes from the

combination of nickel and molybdenum. Cobalt is used for increasing the transformation temperature and

decreasing the solubility of molybdenum, which in turn enhances the formation of molybdenum

intermetallic precipitation (Ni3Mo, Fe Mo and Fe7Mo6). Due to the sharp increase in the cobalt price, the

development of a family of cobalt free maraging steel is promoted. However, eliminating cobalt to reduce

the production cost of maraging steel, leads to retarding the formation of intermetallic precipitates which

in turn reflects negatively on the behaviour of these steels and acceleration of the austenitic reversion

phenomenon. Titanium can be used as the primary strengthening element replacing cobalt in steels.

Furthermore, to overcome the problem of retained austenite, it is supposed that nickel content can be

reduced to 12% [14].

Maraging steels have developed as alternative materials to conventional quenched and tempered

steels for advanced technologies such as aerospace, nuclear and gas turbine applications. Several

industries used acid solutions for cleaning, pickling, descaling, acidizing processes by which steels come

in contact with acids. The corrosion of maraging steels in acidic solutions results mainly during the acid

cleaning, where the scales and corrosion products that form on the steel surface and cause a negative

effect on the performance of the steel equipment. A search of the literature reveals only a few reports on

the corrosion studies of 18 Ni 250 grade maraging steel, which is entirely in martensitic phase. Bellanger

and Rameau[15] have studied the effect of slightly acidic pH with or without chloride ions in radioactive

water on the corrosion of maraging steel and have reported that corrosion behavior of maraging steel at

the corrosion potential depends on pH, and intermediates remaining on the maraging steel surface in the

active region favoring the passivity. The effect of carbonate ions in a slightly alkaline medium on the

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corrosion of maraging steel was studied by Bellanger [16]. Maraging steels were found to be less

susceptible to hydrogen embrittlement than common high-strength steels owing to significantly low

diffusion of hydrogen in them [17]. Poornima et al. [4] have studied the corrosion behavior of 18 Ni 250

grade maraging steel in a phosphoric acid medium and reported that the corrosion rate of the annealed

sample is less than that of the aged sample. Similar observations also have been reported for the corrosion

of 18 Ni 250 grade maraging steel in sulfuric acid medium [18].

In this work, the corrosion behavior of two grades of cobalt free as well as conventional maraging steel

welded pipe steel in 1M H2SO4 solutions was investigated using linear polarization and electrochemical

impedance spectroscopy techniques.

2. Experimental Precedures

2.1. Materials preparation

With the objective of this study, a new grade of maraging steel (free cobalt low nickel maraging

steel) has been produced by electro-slag remelted (ESR) under calcium fluoride (CaF2) based slags.

Method and condition of production was published elsewhere [14]. In this work, new grade of maraging

steels comparable to the conventional C250 (18Ni 250 maraging steel) was studied. Table 1 shows the

chemical composition of investigated steel.

Table 1: Chemical compositions for the Maraging steel, C-250, M23 & M-29 grads.

Steel

Grade C Cr Co Mo Ni Ti W Al Fe

C-250 0.03 7.5 4.8 18 0.4 0.4 Balance

M-23 0.0325 0.004 1.76 11.16 0.641 1.11 0.102 Balance

M-29 0.077 0.0153 1.62 10.67 0.511 1.01 0.059 Balance

The samples of two grades M-23 & M-29 cobalt free maraging steel and conventional C-250

maraging steel were cut into coupons of dimension 1 × 1 × 0.5 cm used as the test material. The coupons

were degreased with acetone, air dried and embedded into two-component epoxy resin and mounted in a

glass holder. A copper wire was soldered to the rear side of the coupon as an electrical connection. Prior

to each experimental run, the exposed surface of the electrode (of area 1 cm2) was wet polished with

silicon carbide abrasive papers up to 1000 grit, rinsed with ethanol and finally dried in air. This was used

as the working electrode during the electrochemical test.

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2.2 Equipments

The metallurgical structures of two grades M-23 & M-29 cobalt free maraging steel and

conventional C-250 maraging steel were observed by metallographic microscope. The electrochemical

experiments were performed by Autolab Potentiostat (PGSTAT20 computer controlled) operated by the

general purpose electrochemical software (GPES) version 4.9.

2.3 Optical microscopy

One of the surfaces of the each metallographic specimens M23, M29 and C250 were grounded

mechanically on the silicon carbide abrasive papers sequentially on 60, 120, 240, 320, 400, 600 grit

silicon carbide papers and polished on a Sylvet cloth using coarse and fine Geosyn- Grade I slurry of

Al2O3. Specimens were cleaned, washed by water and then by alcohol and dried. All the polished

specimens were etched using 2 Nital solution (2 HNO3 in Methanol). The etched specimens were

tested one by one using an optical microscope. The photographs of the microstructure were taken with

help of a Camera fitted with a microscope.

2.4. Solution preparation.

Experiments were done in stagnant 1M H2SO4 solution. The solution was prepared from analytical

grade reagents and distilled water. While the electrolyte solutions were in equilibrium with the

atmosphere, all experiments were carried out under thermostatic conditions 25oC (±0.1

oC).

2.5. Linear polarization and EIS measurements

Electrochemical experiments were performed in a conventional three-electrode cell in which an

Ag/AgCl was the reference electrode, platinum foil was the counter electrode and the maraging steels

were the working electrode (WE). All potentials quoted in this paper were referred to the Ag/AgCl. The

area of the WE exposed to the solution was 1 cm2. Corrosion tests were carried out by using an Autolab

Potentiostat (PGSTAT20 computer controlled) operated by the general purpose electrochemical software

(GPES) version 4.9. The free corrosion potential (versus Ag/AgCl) was followed after immersing the

working electrode in the test solution until the potential stabilized within ±1mV. This was followed by

electrochemical impedance spectroscopy test; EIS data were recorded for the steel specimens at corrosion

potentials (Ecorr) after 10 min, 60 min, 120 min and 24 h of immersion in the test solution. The frequency

was scanned from 100 KHz to 100 mHz, with an ac wave of ± 5 mV peak-to-peak overlaid on a dc bias

potential to get Nyquist and bode plots. The best equivalent circuit of Nyquist plots was calculated by fit

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and simulation method. The linear potentiodynamic polarization curves were obtained by scanning the

potential in the forward direction from −0.1 to 0.1V against Ag/AgCl at a scan rate of 1 mV/s.

All the electrochemical experiments were recorded after immersion of the electrode in the test

solution at a temperature of (25±1)0C. Fresh solution and fresh steel samples were used after each weep.

For each experimental condition, two to three measurements were performed to ensure the reliability and

reproducibility of the data.

3. Results and Discussion

3.1. Metallographic studies of the Maraging steel

Fig.1 shows the microstructure of different heats of maraging steel under investigation produced

by electro-slag re-melting after optimum aging conditions. Also this figure shows the microstructure of

standard steel C250 (18Ni250) after full heat treatment. Microstructure of steel produced by both IF and

ESR comprises martensite + retained austenite. By comparing between microstructure of investigated

steels we found that, the structure of ESR are very finer, well distributed and free from segregation or

band structure than IF steels.

Typical optical micrographs of the C250 maraging steel produced by vacuum arc remelting and

aged under optimum condition are shown in first row in Fig.1. The microstructure, in general appeared

lamellar in morphology. The prior-austenite grain boundaries could not be resolved easily. The bright

patchy regions, shown by arrows in the micrograph, correspond to regions having considerable volume

fraction of reverted austenite. The presence of inter lath austenite, though not fully resolved, is also

indicated in the microstructure.

The optical micrographs of the maraging steels produced by ESR in the aged conditions are shown

in Fig.1. The microstructure in the aged condition essentially consisted of packets of martensite, within

prior-austenite grains. The austenite grains, which had transformed into packets of martensite, could still

be recognized due to the preferential etching along their boundaries and also due to the fact that the

martensite packets within an austenite grain did not extend beyond the respective prior-austenite grain

boundary.

The martensite substructure could not be observed because of the narrowness of the martensite

laths. During aging of the steels under investigation the well-known precipitation reactions lead to

hardening. It is generally believed that initial precipitation in cobalt free molybdenum containing

maraging steel at 480°C occurs as Ni3Mo, which on prolonged aging is replaced by either Fe2Mo or the σ

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phase [19]. Since the alloy additionally contains titanium as a supplemental hardener, the precipitation of

Ni3Ti has also been reported; alternatively, it has been suggested that part of the titanium may be present

in the molybdenum precipitate, i.e. as Ni3(Mo, Ti) [20, 21]. The substructure of lathe martensite consists

predominantly of a high density of tangles dislocations within laths [19].

Steel

Grade

Heat treatment conditions

Annealed Aged

C250

M23

M29

Fig. 1: Microstructure of investigated steels

Page: 7

3.2. Potentiodynamic polarization measurements

The polarization curves for maraging steel specimens (M23, M29 & C250) in 1M H2SO4 solution

at room temperature are shown in Fig. 2. Polarization parameters obtained from the curves are as shown

in Table 3. These parameters include the values of corrosion current densities (Icorr), corrosion potential

(Ecorr), cathodic Tafel slope (bc), anodic Tafel slope (ba) and polarization resistance (Rp). The corrosion

current density, icorr, was determined graphically for all by extrapolating the cathodic and anodic Tafel

slopes to the Ecorr (versus Ag/AgCl). From the slope analysis of the linear polarization curves in the

vicinity of corrosion potential, the values of polarization resistance (Rp) in acid solution were obtained.

Fig. 2: Potentiodynamic polarization curves for maraging steel samples in 1M H2SO4. [BLUE-C-

250, RED-M23, GREEN-M29].

Table 2: Potentiodynamic polarization parameters obtained for the Maraging specimens in 1M H2SO4 at

room temperature.

Corrosion Parameters

Materials ba

mV/decade

bc

mV/decade

Ecorr

mV Icorr µA Rp Ω

C-250 38.07 25.74 -338 620.35 10.75

M23 37.31 24.41 -324 206.10 31.09

M29 37.86 21.87 -332 172.55 34.89

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3.3. Electrochemical impedance spectroscopy (EIS) measurements

The corrosion response of for maraging steel specimens in 1M H2SO4 solution has been

investigated using Electrochemical Impedance Spectroscopy at room temperature and Nyquist plots

represents in the Fig. 3a. It is seen from this figure that the shapes of Nyquist plots for the C-250, M23 as

well as M 29 specimens’ are similar at each concentration, with one depressed semicircle indicating that

the corrosion mechanism is similar for all regions. As can be seen from those figures, the Nyquist plots do

not yield perfect semicircles as expected from the theory of EIS. The deviation from ideal semicircle was

generally attributed to the frequency dispersion [22] as well as to the inhomogenities of the surface and

mass transport resistant [23]. The capacitance loop intersects the real axis at higher and lower frequencies.

At high frequency end the intercept corresponds to the solution resistance (Rs) and at lower frequency end

corresponds to the sum of Rs and charge transfer resistance (Rct). The difference between the two values

gives Rct. The value of Rct is a measure of electron transfer across the exposed area of the metal surface

and it is inversely proportional to rate of corrosion [24].

Impedance behaviour can be well explained by pure electric models that could verify and enable

to calculate numerical values corresponding to the physical and chemical properties of electrochemical

system under examination [25]. The simple equivalent circuit that fit to many electrochemical systems

consisting of a parallel combination of a double layer capacitance (Cdl) and the charge transfer resistance

(Rct) corresponding to the corrosion reaction at metal/electrolyte interface and the solution resistance (Rs)

between the working and reference electrode [26, 27]. To reduce the effects due to surface irregularities of

metal, constant phase element (CPE) is introduced into the circuit instead of a pure double layer

capacitance which gives more accurate fit [28]. The impedance of CPE can be expressed as =1/ 0

( ) , where Y0 is the magnitude of CPE, n is the exponent (phase shift), ω is the angular frequency and j

is the imaginary unit. CPE may be resistance, capacitance and inductance depending upon the values of n

[26]. In all experiments the observed value of n ranges between 0.8 and 1.0, suggesting the capacitive

response of CPEs. The EIS parameters such as Rct, Rs and CPEdl for both the solutions are listed in Table

3 indicate that the values of Rct are less for C-250 than that of M-23 & M-29 in each immersion time.

Bode plots shown in Fig. 3b indicates the presence of one time constant, corresponding to one depressed

semicircle that obtained in case of Nyquist plots.

The corrosion resistance calculated from EIS results shows the same trend as those obtained from

polarization measurements. The difference in corrosion resistance of the two methods may be due to the

different surface status of the electrode in two measurements [29].

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( a ) (b)

Fig. 3: EIS [Nyquist plot] (a) and Bode Plot (b) curves for maraging steel samples in 1M H2SO4, [BLUE-

C-250, RED-M23, GREEN-M29].

Table 3: Electrochemical Impedance parameters obtained for the Maraging specimens in 1 M H2SO4 at

room temperature.

Materials Rs Ω CPE mMho n Rct Ω

C-250 1 0.993 0.936 12

M23 1 0.956 0.847 30

M29 1.2 1.22 0.762 32.2

3.4. Effect of microstructure/grain size on corrosion of Maraging steels

Comparing the polarization curves and data it is found that, polarization curve of C-250 maraging

steel shifted towards more anodic (positive) region and Icorr values of C-250 increases compared to M 23

and M 29 maraging steel as shown in Fig. 2. The percentage of retained austenite significantly influences

the corrosion of Maraging steels; limiting its usefulness as a high strength material. Further deterioration

in the corrosion properties of maraging steels is obtained by micro-segregation of retained austenite in

localized area i.e. the solute segregation to the existing areas that causes galvanic corrosion.

The amount of retained austenite formed after solution treatment and after aging at optimum

condition for maraging steel produced by vacuum melting (C250) and others produced from different

heats of ESR were studied using X-ray diffraction. For C250 steel, after solid solution annealing treatment

at optimum condition, with no refrigeration treatment, about 10 ± 0.5% retained austenite was detected.

On the other hand, maraging produced by electro-slag remelting after full heat treatment contain about 1+

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5% retained austenite and X-ray diffraction results confirmed that there is a complete martensite

transformation after solution-treatment for investigated steels. For different steels under investigation

austenite contents were detected by X-ray diffraction method as shown in Table 4.

Table 4: Retained austenite measurements by X-ray for investigated steel.

Steel No. Process Net austenite, % by X-ray

Annealed

Aged

C250 Vacuum 10 13

M23 ESR3 1.5

1.9

M29 ESR1 1.1 1.3

It is clear from Table 4 and Fig. 1 that, the amount of retained austenite in investigated steels not

depend mainly on chemical composition of investigated steels but also on the production condition.

Increasing the amount of alloying elements i.e. Co, Mo, Cr, and Ti is accompanied by increasing the

tendency to form retained austenite. ESR has advantages that it has low local solidification time (LST)

than conventional casting method. The cooling rates estimated using the Rosenthal theory [30, 31] is of

the order of 1000Cs

-1 while the local cooling rate in vacuum ingots is 5–25

oCs

-1. This is an important

difference of the ESR process compared to the vacuum melting technique and leads furthermore to very

fine and well distributed microstructures compared to vacuum melting steels as shown in Fig. 1.

It is evident from the Nyquist plots that the impedance response of C-250 specimens showed a

marked difference with that of M 23 & M 29 samples. The smallest capacitive loop in high frequency

range is noticed in C-250 steel for each immersion time than M-23 & M29 as shown in Fig. 3a, which

reveals the highest corrosion rate for C-250 steel. The total impedance value of C-250 seen in Bode

profile [Fig. 3b] is the lowest one among the three cases.

4. Conclusions

The electrochemical behavior of the C-250, M-23 and M-29 maraging steel was investigated at

ambient temperature in 1M H2SO4 solution using linear polarization and electrochemical impedance

spectroscopy (EIS) techniques. From polarization data it is found that, for Ecorr (versus Ag/AgCl) values

of C-250 shifted towards more anodic (positive) region and Icorr values of C-250 increases compared to M-

23 and M-29 indicates that corrosion resistance of C-250 is less than that of M-23 and M-29. The

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corrosion resistance, calculated from EIS results, shows the same trend as those obtained from

polarization measurements.

Acknowledgement

This project was supported by NSTIP strategic technologies program number (11-ADV1853-02) in the

Kingdom of Saudi Arabia.

Reference

1. Razek, J, Klein, IE, Yahalom, J, ‘‘Structure and Corrosion Resistance of Oxides Grown on Maraging

Steel in Steam at Elevated Temperatures.’’ Appl. Surf. Sci., 108 159–167 (1997)

2. Ohue, Y, Matsumoto, K, ‘‘Sliding–Rolling Contact Fatigue and Wear of Maraging Steel Roller with

Ion-Nitriding and Fine Particle Shot-Peening.’’ Wear, 263 782–789 (2007)

3. Wang, W, Yan, W, Duan, Q, Shan, Y, Zhang, Z, Yang, K, ‘‘Study on Fatigue Property of a New 2.8

GPa Grade Maraging Steel.’’ Mater. Sci. Eng. A, 527 3057–3063 (2010)

4. Poornima, T, Nayak, J, Shetty, AN, ‘‘Corrosion of Aged and Annealed 18 Ni 250 Grade Maraging

Steel in Phosphoric Acid Medium.’’ Int. J. Electrochem. Sci., 5 56–71 (2010)

5. Decker, R.F. and Floreen, S. "Intern. Symp. on Maraging Steels Recent Development and

Applications". Pheonix, Arizona, 1 (1998) 1-88.

6. Stiller, K, Danoix, F, Bostel, A, ‘‘Investigation of Precipitation in a New Maraging Stainless Steel.’’

Appl. Surf. Sci., 94 326–333 (1996)

7. Klobcar, D, Tusek, J, Taljat, B, Kosec, L, Pleterski, M, ‘‘Aging of Maraging Steel Welds During

Aluminum Alloy Die Casting.’’ Mater. Sci., 44 515–522 (2008)

8. Grum, J, Slabe, JM, ‘‘Effect of Laser-Remelting of Surface Cracks on Microstructure and Residual

Stresses in 12Ni Maraging Steel.’’ Appl. Surf. Sci., 252 4486–4492 (2006)

9. Decker, R.F. and Eash, J.T. and Goldman, A.J.: 18% Nickel Maraging Steel, ASM Trans. Quart., 55

(1962) 58.

10. Kim, Y.G; Lee, C.S. and Kim, G.S. Sci. Eng., 89(1987) 17.

11. Pestov, I.V.; Maloletnev, A. Ya; Peraks, M. D.; Leonova, N. K. Metall. Term-obrab. Met., 4 (1983)

38.

12. Floreen, S, Cobalt-Free maraging Steels, H. S. Pat. No. 4443,254 April 18(1984)

13. Vanderwalker, D. M.: In Maraging Steels—Recent Developments and Applications, TMS-AIME,

Warrendale, PA, 1988, pp. 255–68.

14. Hossam Halfa, Ayman Fathy, Mohamed Kamal, Mamdouh Eissa and Kamal El-Fawahkry"

Enhancement of mechanical properties of developed Ti-containing Co-free low Ni maraging steel by

ESR", STEEL GRIPS 8 (2010) No. Product& Quality, p. 278/284.

Page: 12

15. Bellanger, G, Rameau, JJ, ‘‘Effect of Slightly Acid pH With or Without Chloride in Radioactive

Water on the Corrosion of Maraging Steel.’’ J. Nucl. Mater., 228 24–37 (1996)

16. Bellanger, G, ‘‘Effect of Carbonate in Slightly Alkaline Medium on the Corrosion of Maraging

Steel.’’ J. Nucl. Mater., 217 187–193 (1994)

17. Rezek, J, Klein, IE, Yhalom, J, ‘‘Electrochemical Properties of Protective Coatings on Maraging

Steel.’’ Corros. Sci., 39 385–397 (1997)

18. Poornima, T, Jagannatha, N, Nityananda, AN, ‘‘Studies on Corrosion of Annealed and Aged 18 Ni

250 Grade Maraging Steel in Sulphuric Acid Medium.’’ Portugal. Electrochim. Acta, 28 173–188

(2010).

19. Schmidt, M. L.A.: Edited by Richerd K. Wilson, The Materials, Metals & Materials Socity, 1988.

20. Fathy, A.; Mattar, Ti; El-Faramawy, H and Bleck. W.; steel Research, Vol. 73, No. 12, pp. 549-

556(2002)

21. Pardal, J.M.; Tavares, S.S.M.; Terra, V.F.; Da Silva, M.R. and Dos Santos, D.R.: Journal of Alloys

and Compounds, Volume 393, Issues 1-2, 3, Pages109-113, May (2005)

22. Hassan H. H., Abdelghani E. and Amin M. A. (2007) Inhibition of mild steel corrosion in

hydrochloric acid solutionby triazole derivatives: Part I. Polarization and EIS studies, Electrochimica

Acta, 52(22), 6359-6366.

23. Bommersbach P., Alemany-Dumont C., Millet J.P. and Normand B. (2005) Formation and behaviour

study of an environment-friendly corrosion inhibitor by electrochemical methods, Electrochimica

Acta, 51(6), 1076-1084.

24. Rosenfield I.L. (1981) Corrosion Inhibitors (McGraw-Hill, New York ,p.66).

25. Sathiya Priya A. R., Muralidharan V. S. and Subramania A.(2008) Development of Novel Acidizing

Inhibitors for Carbon Steel Corrosion in 15% Boiling Hydrochloric Acid. Corrosion, 64(6), 541-552.

26. Singh A. K., Shukla S. K., Singh M. and Quraishi M.A. (2011) Inhibitive effect of ceftazidime on

corrosion of mild steel in hydrochloric acid solution, Materials Chemistry and Physics, 129(1), 68-76.

27. El Azhar M., Mernari B., Traisnel M., Bentiss F.and Lagrenée M. (2001) Corrosion inhibition of mild

steel by the new class of inhibitors [2,5-bis(n-pyridyl)-1,3,4-thiadiazoles] in acidic media, Corrosion

Science, 43(12), 2229-2238.

28. Macdonald J.R., Johnson W.B. and Macdonald J.R (1987) Theory in impedance Spectroscopy, (John

Wiley & Sons, New York).

29. Raman A. and Labine P. (1986) Reviews on Corrosion Inhibitor Science and Technology,(vol. 1,

NACE, Houston, TX , 5).

30. Mittemejier, E.J.; Vangent, A. and Javanderschaaf, P., Metallurgical Transactions, Vol. 17A, p. 1441

(1932).

31. Sinha, P.; Sivakumar, D.; Babu, N.S.; Tharian, K.T. and Natarajan, A.: Steel Research, Vol. 11, p.

490 (1995).