sulphate solution

sulphate solution

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Results in Physics 11 (2018) 570–576

Contents lists available at ScienceDirect

Results in Physics journal homepage: www.elsevier.com/locate/rinp

Effect of heat treatment processes on the localized corrosion resistance of austenitic stainless steel type 301 in chloride/sulphate solution

T



Roland Tolulope Lotoa, , Cleophas Akintoye Lotoa,b, Idehai Ohijeagbona,c a

Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa c Department of Mechanical Engineering, University of Ilorin, Kwara State, Nigeria b

A R T I C LE I N FO

A B S T R A C T

Keywords: Corrosion Pitting Chloride Steel

The effect of quenching and annealing heat treatment on the localized corrosion resistance of 301 austenitic steel in 2 M H2SO4/0.75%–2.25% NaCl was studied by potentiodynamic polarization, open circuit potential measurement and optical microscopy analysis. The corrosion rate of the quenched, annealed and untreated steel increased with increase in NaCl concentration. The quenched steel had the lowest corrosion rate values, followed by the annealed steel. Variation in Cl− ion concentration had no significant effect on the pitting corrosion resistance and passivation behavior of the quenched steel, though increase in current density at breakdown potential was observed at higher Cl− ion concentration. The untreated steel experienced significant reduction and collapse of it passive film after 1.5% NaCl. Delayed passivation occurred on the annealed steel following metastable pitting lead to short passivation range. Open circuit potential measurement showed large cathodic shift of the corrosion potential for the quenched steel, compared to the anodic shift for the untreated steel. Optical microscopic images showed a deteriorated morphology and the presence of different phases for the untreated 301SS. Intergranular cracks were observed on the annealed and quenched 301SS while corrosion pits was observed on the annealed 301SS.

Introduction Austenitic stainless steels represent about 60% of the world’s total stainless steel production [1]. They display properties such as good corrosion resistance, high strength, toughness, good ductility and weldability [2]. These steels are extensively applied in food and beverage manufacturing industries, chemical processing plants, bulk storage etc. due to their good corrosion resistance in mildly corrosive conditions. 301 grade of the austenitic steel have a nominal weight composition of about 17% Cr and 7% nickel. They are employed as aircraft structural parts, roof drainage products and a wide variety of industrial applications. Its corrosion resistance is appreciable, but in many applications it is less than that of type 304 stainless steel. The corrosion resistance of 301 steel is due to the formation of a thin, invisible and passive oxide layer on the surface, composed of the chemical combination of iron and chromium oxide [3]. Breakage or collapse of the protective film due to flaws, inclusions, aggressive anions, potential scanning under induced current etc. leads to localized corrosion reactions on the steel surface which eventually penetrates to the alloy substrate. The most common of this corrosion type on stainless steels are pitting and intergranular corrosion. Intergranular corrosion occurs



at the crystallites of metal alloys due to grain boundary depletion of corrosion resisting constituents especially chromium. In some cases it occurs by the segregation of impurities at the grain boundary areas [4]. Pitting corrosion is a severe form of localized corrosion resulting in the formation and progression of microscopic holes on the steel surface which penetrates inwardly within the alloy, leading to premature failure of equipment, structures and devices [5–8]. Previous research on intergranular and pitting corrosion shows that they are a catastrophic form of corrosion, and new insight to their mechanism, propagation and control is of utmost importance [9–16]. Heat treatment processing of austenitic steels causes some significant alterations in the microstructure, grain dimension, crystallographic orientation and phase transformations. All these do play a major role in the corrosion resistance of the alloy. Chen et al [17] and Isfahany et al [18] studied the effect of heat treatment on microstructure, mechanical and corrosion properties of 316L and 420 steel. They both observed that heat treatment strongly influences the corrosion resistance of stainless steel due to changes in the contents of σ and δ phases, precipitation of M7C3 and secondary hardening. Research by Pezzato et al [19] showed that precipitation of secondary phases on duplex steel after isothermal aging affect the corrosion resistance of the alloys. According to Tukur et al

Corresponding author.

https://doi.org/10.1016/j.rinp.2018.09.056 Received 29 August 2018; Received in revised form 26 September 2018; Accepted 30 September 2018 Available online 05 October 2018 2211-3797/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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Corrosion rate, CR (mm/y) was determined from the following mathematical relationship;

[20] exposure of 304 steel to elevated temperature causes sensitization of the steel leading to carbide precipitations at grain boundaries. This has deleterious effects on the resistance of the steel to intergranular corrosion. Wilmar et al [21] and Tan et al [22] studied the effect of heat treatment on the localized corrosion reactions of 17Cr-6Mn-5Ni-1.5Cu austenitic steel and duplex stainless steels SAF2507. Observation showed formation of Mn enrichment promoted preferential adsorption of chlorides on the austenitic steel while annealing heat treatment increased the pitting resistance of the duplex alloy due to variation of pitting resistance equivalent number of ferrite and austenite phase. Annealing heat treatment process results in larger grain sizes because it allows the grains to grow due to orientation of the molecules which fits into the natural grain structure. During this process chromium carbides precipitates mostly along the grain boundaries, consequentially leading to chromium depleted areas and very significant loss of corrosion resistance [23]. Quenching of austenitic stainless steels causes the dissolution of carbide preventing their re-precipitation and allows chromium enrichment in the substrate alloy matrix. This dissolution increases the retained austenitic phases in the steel’s microstructure resulting in increased corrosion resistance [24]. The influence of heat input can be both detrimental and/or beneficial on the steel’s microstructural properties. This research aims to study the effect of quenching and slow cooling heat treatment process from 1000 °C on the localized corrosion resistance of type 301 austenitic stainless steel in dilute acid chloride solution.

CR =

D is the density in (g/cm ); Eq is the metal alloy equivalent weight (g). 0.00327 is the constant for corrosion rate [26]. Open circuit potential measurement (OCP) was performed at 0.05 V/s step potential for 1500 s to study the thermodynamic stability of the alloys at rest potentials [27]. Micro analytical images of the 301 steel surface configurations were studied before and after electrochemical degradation with Omax trinocular metallurgical microscope using ToupCam analytical software. Result and discussion Potentiodynamic polarization studies The potentiodynamic polarization plots of untreated, annealed and quenched 301SS in 2 M H2SO4 at 0.75%–2.25% NaCl are shown from Fig. 1(a) to Fig. 3. Table 2 depicts the data obtained from the polarization test of 301SS samples. The results generally show that the corrosion resistance of the three 301SS samples decreased with increase in chloride concentration resulting in anodic dissolution of the alloy surface. The anodic dissolution is responsible for the production metal cations (M+) in electrolyte, leading to diffusion of Cl− anions. Cl− intensifies the metal ions’ diffusion through the passive layer. Subsequently, the resulting metal chloride reacts with H2O as follows [28]:

301 austenitic stainless steel (301SS) obtained from McMaster University, Hamilton, Ontario, Canada is the steel alloy studied. The steel was analyzed at the Materials Characterization Laboratory, Department of Mechanical Engineering, Covenant, Ogun State, Nigeria with the chemical (wt. %) compositions result shown in Table 1. The 301SS samples were machined to samples with exposed surface area dimensions of 0.8 cm2, 0.91 cm2 and 0.91 cm2 for the untreated, annealed and quenched samples. Annealing and quenching heat treatment of the 301SS samples was performed in a muffle furnace after heating the steel to 1000 °C and held for 30 min. The annealed 301SS was cool slowly in air while the quenched 301SS was rapidly cooled in distilled water to achieve the required metallurgical structure. The temperature was maintained with a regulator at an accuracy ± 10 °C linked with a thermocouple (K-Type) to achieve the required temperature. They subsequently underwent metallographic preparation using abrasive silicon carbide papers with grits of 60, 120, 220, 320, 600, 800 and 1000. Polishing was done with diamond liquid paste to 6 µm before washing with distilled water and acetone for electrochemical tests. Recrystallized sodium chloride purchased from Titan Biotech, India was formulated in proportional concentrations of 0.75%, 1%, 1.25%, 1.5%, 1.75%, 2% and 2.25% in 200 mL of 2 M H2SO4 solution, prepared from analar grade of H2SO4 acid (98%) with distilled water. Electrochemical test were performed on untreated, annealed and quenched 301SS at 37◦C ambient temperature with a three electrode cell (platinum counter electrodes, Ag/AgCl reference electrodes and resin embedded 301SS electrodes) containing 200 mL of the acid media and connected to Digi-Ivy 2311 electrochemical workstation. Potentiodynamic polarization plots obtained at scan rate of 0.0015 V/s from −0.75 V and +1.5 V set potentials. The corrosion current density, Cd (A/cm2) and corrosion potential, Cp (V) values were obtained from the polarization plots through Tafel extrapolation method [25].

M+ Cl− + H2 O→ MOH + H+ + Cl−

Si

Ni

Cr

Mn

P

N

C

Fe

% Composition (301SS)

0

1

8

16

2

0.045

0.1

0.15

72.7

(2)

The cathodic reaction occurs on the alloys surface according to the equation below:

2H2 O→ O2 + 4H+ + 4e−

(3)

Observation of Table 2 shows the untreated 301SS have the highest corrosion rate values, starting at 6.21 mm/y (0.75% NaCl) and peaking at 1.75% NaCl with corrosion rate value of 13.45 mm/y before gradual decrease to 12.42 mm/y at 2.25% NaCl. The corrosion rate values of annealed 301SS are relatively lower than the values obtained for the untreated counterpart, though the corrosion rate values at 0.75% NaCl for both steel samples are comparatively the same, annealed 301SS proves to be more corrosion resistant than untreated 301SS with increase in NaCl concentration. The corrosion rate of annealed 301SS increased at lower rates than the annealed steel. Both untreated and annealed 301SS showed good thermodynamic stability under applied potential after 0.75% NaCl from observation of corrosion potential values, with annealed 301SS maintaining lower corrosion rates at higher corrosion potentials due to the strength of its passive protective film. However, with the potential of the metal being more electronegative, the rate of hydrogen evolution and oxygen reduction reactions may increase to the level at which the rate of diffusion of the reducible species dominates the rate of reduction. This is proven from the cathodic Tafel slopes of the untreated and annealed 301SS which are generally constant and higher than the anodic counterpart signifying dominant hydrogen evolution and oxygen reduction reactions. The values of the anodic Tafel slope of annealed 301SS, though relatively smaller than the cathodic Tafel slope, varied with NaCl concentration signifying increased oxidation of the steel. Quenched 301SS proves to be the most corrosion resistant with corrosion rate values significantly lower than the untreated and annealed 301SS samples. The values correspond to lower corrosion current density on the sample surface as a result of lower redox electrochemical process occurring therein. Changes in NaCl concentration had little influence on the active-passive behavior of the polarization plot as the corrosion potential shifts to anodic values due to the reaction of the metal surface with water to form the passive film. The resulting

Table 1 Percentage nominal (wt. %) composition of 301SS. Mo

(1)

D 3

Experimental methods

Element symbol

0.00327 × Cd × Eq

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(a)

(b)

(c)

Fig. 1. Potentiodynamic polarization plots of untreated 301SS in 2 M H2SO4/0.75%–2.25% NaCl solutions (a) 0.75%–1.75% NaCl, (b) 2% NaCl, and (c) 2.25% NaCl.

concentration before breakdown of the passive film at the transpassive region due to dissolution of the passive film and the resulting ion exchange mechanism [30]. The metastable regions of the polarization plots are significantly influence by the Cl− ions following anodic polarization due to the formation and disappearance of transient corrosion pits before repassivation of the metal alloy. The metastable region increased leading to delayed repassivation with respect to Cl−; however the overall passivation range (strength of the passive) remained unchanged due to the excellent pitting corrosion resistance of the quenched 301SS. Observation of the polarization plots of untreated 301SS [Fig. 1(a–c)] shows its passivation behavior and pitting corrosion resistance is significantly influenced by Cl− ion concentration. Slight decrease in passivation range was observed from 1% and 1.5% NaCl concentration [Fig. 1(a)], after which there was a visible decrease in passivation range at 1.75% NaCl following metastable pitting due to adsorption of Cl− ions. It must be noted that the corrosion resistance of stainless steels largely depends on the stability of the protective film on their surface. Decrease in the passivation range shows the steel is more prone to pitting as seen on untreated 301SS polarization plot (1.5%

metallurgical structure of the alloy caused significant resistance to anodic dissolution. The higher corrosion rate of the annealed 301SS compared to the quenched sample is due to the formation of larger grain sizes which are responsible for the precipitation of carbides at the grain boundaries regions. This significantly decreases the chromium content of the annealed 301SS, exposing it to localized corrosion [9,29]. Temperature alteration of 301SS changes the ratio and distribution of the austenite phase, and the metal alloy constituents which eventually influence the corrosion resistance of the sample, hence quenching tends to remove the alloy segregation, sensitization and sigma phase in the quenched 301SS. Passivation behavior and pitting corrosion analysis Quenched 301SS showed strong resistance to pitting corrosion before transpassivity from observation of the polarization plots [Fig. 3]. Variation in Cl− ion concentration had no significant effect on the pitting potential of the alloy. However, increase in the current density at the breakdown potential can be observed at higher Cl− ion 572

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Table 2 Potentiodynamic polarization results for untreated, annealed and quenched 301SS in 2 M H2SO4/0.75%–2.25% NaCl solution. NaCl concentration (%)

Corrosion rate (mm/y)

Corrosion current density (A/cm2)

Corrosion potential (V)

Polarization resistance, Rp (Ω)

Cathodic Tafel slope, Bc (V/dec)

Anodic Tafel slope, Ba (V/dec)

Untreated 301SS 0.75 1 1.25 1.5 1.75 2 2.25

6.21 7.94 13.96 14.16 13.45 10.72 12.42

5.77E−04 7.30E−04 1.28E−03 1.30E−03 1.24E−03 9.86E−04 1.14E−03

−0.315 −0.327 −0.327 −0.327 −0.326 −0.346 −0.345

52.99 41.88 23.83 23.48 24.72 33.50 26.61

−7.109 −7.144 −8.169 −8.247 −8.179 −9.819 −8.496

5.364 7.120 5.520 5.412 5.928 8.840 6.879

Annealed 301SS 0.75 1 1.25 1.5 1.75 2 2.25

5.72 6.02 6.38 6.97 7.35 9.07 10.36

5.31E−04 5.53E−04 5.86E−04 6.41E−04 6.76E−04 8.34E−04 9.53E−04

−0.309 −0.373 −0.373 −0.373 −0.431 −0.372 −0.372

53.13 50.11 46.62 41.08 35.29 30.84 27.63

−8.383 −8.392 −8.94 −9.081 −9.289 −8.669 −8.988

3.349 3.791 4.652 6.046 6.416 8.328 7.859

Quenched 301SS 0.75 1 1.25 1.5 1.75 2 2.25

1.39 2.06 2.91 3.31 4.82 5.34 5.94

1.29E−04 1.90E−04 2.68E−04 3.04E−04 4.43E−04 4.91E−04 5.46E−04

−0.389 −0.374 −0.378 −0.367 −0.374 −0.373 −0.331

80.91 148.9 38.72 92.87 63.74 46.13 51.71

−8.054 −8.581 −8.766 −9.470 −9.055 −8.938 −12.23

2.659 4.371 4.693 9.441 8.560 8.030 4.032

(a)

(b)

Fig. 2. Potentiodynamic polarization plots of annealed 301SS in 2 M H2SO4/0.75%–2.25% NaCl solutions (a) 0.75%–1.25% NaCl, (b) 1.5%–2.25% NaCl.

NaCl). Further increase in Cl− ion concentration to 2% [Fig. 1(b)] significantly reduced the passivation range, beyond which [2.25% NaCl, Fig. 1(c)] passivation is completely absent. The annealed 301SS [Fig. 2(a) and (b)] showed more resilient passive film and pitting corrosion resistance than the untreated 301SS but lower than the quenched 301SS. The heat treatment on the annealed 301SS caused corrosion pits to possibly initiate at carbides, grain boundaries and other flaws on the alloy surface; as a result increase in Cl− concentration caused a consequential decrease in the passivation range as the anodic dissolution of the metal alloy dominates the electrochemical process [31]. The annealed 301SS retained its passivation at all NaCl concentration studied though at decreasing potentials before pitting, due to delayed passivation of the alloy following metastable pitting activity. Fig. 3

Optical microscopy analysis Optical microscopic images (mag. 40×) of untreated, annealed and quenched 301SS before corrosion, and after corrosion from 0.75% and 2.25% NaCl/2M H2SO4 are shown from Fig. 4(a) to Fig. 6(c). The 301SS samples have generally the same surface morphology [Fig. 4(a–c)] before corrosion. Fig. 5(a–c) show the morphological response of the 301SS samples to chloride attack within the electrolyte. The untreated 301SS [Fig. 5(a)] appears to have generally deteriorated (worn out) without any visible markings in 0.75% NaCl/2M H2SO4 solution, while on the annealed steel [Fig. 5(b)] grain boundaries appeared with visible signs of microscopic corrosion pits. The grain boundary phenomena observed is a function of the crystallographic nature and atomic rearrangement of the steel’s metallurgy at the grain boundary region [32]. As earlier mention in the introductory section larger grain sizes 573

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Fig. 3. Potentiodynamic polarization plots of quenched 301SS in 2 M H2SO4/0.75%–2.25% NaCl solutions.

(a)

(b)

(c)

Fig. 4. Optical microscopic image (mag. 40×) of 301SS before corrosion (a) untreated 301SS, (b) annealed 301SS and (c) quenched 301SS.

(a)

(b)

(c)

Fig. 5. Optical microscopic image (mag. 40×) of 301SS after corrosion from 0.75% NaCl/ 2 M H2SO4 (a) untreated 301SS, (b) annealed 301SS and (c) quenched 301SS.

(a)

(b)

(c)

Fig. 6. Optical microscopic image (mag. 40×) of 301SS after corrosion from 2.25% NaCl/ 2 M H2SO4 (a) untreated 301SS, (b) annealed 301SS and (c) quenched 301SS.

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(a)

(b)

Fig. 7. Variation of OCP values versus exposure time for untreated, annealed and quenched 301SS (a) 2 M H2SO4/0.75% NaCl and (b) 2 M H2SO4/2.25% NaCl.

polarization hence the anodic reactions associated with metal dissolution ceases. The OCP curves obtained agree with the results from potentiodynamic polarization; however they all tend to be thermodynamically unstable in the electrolyte solution with 0.75% NaCl. In 2 M H2SO4/2.25% NaCl untreated and annealed 301SS showed relative thermodynamic stability compared to the quenched which whose corrosion potential increased progressively throughout the exposure hours.

are formed during annealing and chromium carbides precipitates along on the grain boundaries regions leading to chromium depletion. This is responsible for the grain boundaries in the annealed 301SS. The quenched 301SS [Fig. 5(c)] appears to have slightly deteriorated with faintly visible grain boundaries. This indicates that quenching increases the susceptibility of 301SS to intergranular corrosion despite its relatively higher pitting and general corrosion resistance. The morphology of the steel samples from 2.25% NaCl/2M H2SO4 contrast the earlier observed images. The untreated 301SS [Fig. 6(a)] showed an etched morphology with the different phases of the steel microstructure visible. The macro pits in Fig. 6(b) seem to have enlarged from Fig. 5(b) in addition to intergranular corrosion at the grain boundaries and greater degree of surface deterioration. Fig. 6(c) shows the absence of corrosion pits though the morphology appears to be worn out and grain boundaries are much more visible. Quenching heat treatment results in the formation and enlargement of retained austenite phase on 301SS being an austenitic steel. Increasing the proportion of the austenitic phase improves corrosion resistance of 301SS [24]. The corrosion resistance observed so far has been proven to be general and pitting corrosion resistance for the quenched 301SS. The presence of grain boundaries on the morphology of the annealed and quenched 301SS indicates susceptibility to intergranular corrosion compared to the untreated 301SS which displayed good resistance to that corrosion type. There is strong possibility that the chromium content close to the grain boundaries remained above the passivity limit (12 wt%) hence absence of intergranular cracks on the steel despite its higher corrosion rate from electrochemical test [33].

Conclusion Quenched 301SS displayed the lowest corrosion rate in the acid chloride solution from potentiodynamic polarization. Its passivation range remained unchanged at all chloride concentrations studied, compared to the annealed whose passive film weakened at higher chloride concentrations while the untreated steel lost its passivation at relatively low chloride concentration. The quenched steel shifts the OCP in the cathodic direction by large margin due to cathodic polarization. Anodic shift due to metal dissolution was observed for the untreated 301SS. Morphological observation showed the presence of different metallurgical phases on the untreated steel, while macro/ micro pits and intergranular cracks coupled with general surface deterioration were visible on the annealed 301SS. The morphological deterioration of the quenched 301SS was limited to faint intergranular cracks due to its comparatively higher corrosion resistance. Acknowledgement The author is grateful to Covenant University, Ota, Ogun State, Nigeria for the provision of research facilities for this project.

Open circuit potential measurement Appendix A. Supplementary material Fig. 7(a) and (b) shows the open circuit potential (OCP) versus exposure time curves obtained with samples of untreated, annealed and quenched 301SS in 2 M H2SO4/0.75% and 2.25% NaCl solution. Generally similar active-passive behavior was observed the steel specimens in both solutions. The untreated 301SS remained at more positive potentials compared to the annealed and quenched 301SS due to anodic polarization responsible for corrosion and dissolution of the steel. Both the annealed and quenched 301SS experienced significant cathodic shift compared to the untreated 301SS. The cathodic shift coupled with lower corrosion rates than the untreated 301SS signifies cathodic protection resulting from the reduction of corrosion tendency by cathodic

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