Corrosion resistance performance of newly developed...
Transcript of Corrosion resistance performance of newly developed...
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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: [email protected]
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
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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.
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