The effect of tempering temperature on pitting

The effect of tempering temperature on pitting corrosion resistance of 420 stainless steels Moch. Syaiful Anwar, Siska Prifiharni, and Efendi Mabruri Citation: AIP Conference Proceedings 1725, 020005 (2016); doi: 10.1063/1.4945459 View online: http://dx.doi.org/10.1063/1.4945459 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1725?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The effect of inhibitor sodium nitrate on pitting corrosion of dissimilar material weldment joint of stainless steel AISI 304 and mild steel SS 400 AIP Conf. Proc. 1717, 040009 (2016); 10.1063/1.4943452 Corrosion resistance of kolsterised austenitic 304 stainless steel AIP Conf. Proc. 1653, 020003 (2015); 10.1063/1.4914194 Molecular dynamics simulation to studying the effect of molybdenum in stainless steel on the corrosion resistance by lead-bismuth AIP Conf. Proc. 1448, 185 (2012); 10.1063/1.4725454 Zirconia-alumina nanolaminate for perforated pitting corrosion protection of stainless steel J. Vac. Sci. Technol. A 22, 272 (2004); 10.1116/1.1642650 Enhanced pitting corrosion resistance of 304L stainless steel by plasma ion implantation J. Vac. Sci. Technol. B 12, 940 (1994); 10.1116/1.587332

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The Effect of Tempering Temperature on Pitting Corrosion Resistance of 420 Stainless Steels

Moch. Syaiful Anwar1,a), Siska Prifiharni1,b), Efendi Mabruri1,c) 1

Research Center for Metallurgy and Materials – Indonesian Institute of Sciences (LIPI) Kawasan PUSPIPTEK Building 470 – South of Tangerang – Banten – 15314, Indonesia a)

Corresponding author: [email protected] b)[email protected] c)[email protected]

Abstract. The AISI Type 420 stainless steels are commonly used to steam generators, mixer blades, etc. These stainless steels are most prone to pitting in dissolved Cl- containing environments. In this paper, the effect of tempering temperature on pitting corrosion resistance of AISI Type 420 stainless steels was studied. The AISI Type 420 stainless steels specimens were heat treated at the temperature of 1050°C for 1 hour to reach austenite stabilization and then quench in the oil. After that, the specimens were tempered at the temperature of 150, 250, 350 and 450°C for 30 minutes and then air cooled to the room temperature. The electrochemical potentiodynamic polarization test was conducted at 3.5% sodium chloride solution to evaluate corrosion rate and pitting corrosion behaviour. The Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) were used to evaluate the pitting corrosion product. The result have shown that highest pitting potential was found in the sample tempered at 250°C and corrosion pits were found to initiate preferentially around chromium carbides.

INTRODUCTION Pitting corrosion is one of the most destructive and costly problem in the chemical process and desalination plants, water storage tank and pipeline, pump and valves, petroleum refineries etc [1,2]. Depending on the environment and material, the resultant pits can be wide and shallow or narrow and deep which can easily pierce the wall thickness of metal [3]. A typical example of pitting corrosion is the pit that occurs on a steam turbine blade working with neutral chloride. The pitting corrosion of a material can be prevented through appropriate material selection, cathodic protection and change of environment. Among them, appropriate material selection has been considered the best way to prevent pitting [4]. The selection of martensitic stainless steel for steam turbine blade application is based on hardened and strengthened ability by heat treatment, high temperature and corrosion resistance ability [5]. However, It is still having susceptibility to pitting corrosion in a neutral chloride environment. For example Li et al. mentioned that when NaCl concentration is increase, the pitting potential of the domestic super martensitic stainless steel is decrease [6]. In the recent year, such studies have been further extended to improve pitting potential of martensitic stainless steel. The study of Marcuci et al indicate that martensitic stainless steel with austenizing temperature more than 900 °C, the dissolution of carbide can influence protective film stability and increasing the pitting corrosion resistance [7]. The study by Ashkan Reza Gholi indicate that the stainless steel by applying different tempering temperature result in different corrosion resistance [8]. The studies by Isfahanya et al. [9] and Candelaria et al. [10] indicated that the martensitic stainless steel by applying hardening and heat treatment process will improve hardness and corrosion resistant. The 3rd International Conference on Advanced Materials Science and Technology (ICAMST 2015) AIP Conf. Proc. 1725, 020005-1–020005-8; doi: 10.1063/1.4945459 Published by AIP Publishing. 978-0-7354-1372-6/$30.00

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The present work present a detailed study on the effect of tempering temperature on the pitting corrosion resistant of the martensitic stainless steel type AISI 420. The aim of this work was to insvestigate of the effects of tempering at different temperatures on corrosion properties exposed in the sodium chloride solution.

EXPERIMENTAL PROCEDURE Material All specimens were machined from a section AISI Type 420 stainless steel rod, having dimension of 30 x 20 x 5 mm and the nominal chemical composition given in Table 1. The specimens was heat treated in the following manner: austenitized at1050°C for 1 hour to reach austenite stabilization, quench in the oil, then tempered at 150, 250, 350 and 450 °C for 30 minutes. TABLE 1. Chemical Composition AISI Type 420 Stainless Steel

C

Cr

Mn

Si

Ni

P

S

Fe

0.43

12.4

0.37

0.26

0.14

0.02

0.02

Bal. 2

The specimens for the anodic polarization curve were rectangular with surface areas from 0.9 to 1.5 cm . Before use, the specimens were connected with copper wire as an electrical contact and mounted with resin to cover unexposed area. Then, the specimens were grinded with silicon carbide paper from grid #80 to #1200, cleaned with etanol and dried with dryer.

Potentiodynamic Polarization Test The polarization test used three-electrode consists the specimens as working electrode, graphite as auxiliary electrode and Saturated Calomel Electrode (SCE) as reference electrode. The polarization cell was filled with 500 ml of the 3.5% Sodium Chloride solution. The working electrode, graphite auxiliarly electrode and SCE were then immersed into solution. The working electrode was allowed corroded freely for 2 h before beginning the polarization run. At the end of this period, the conditioning has conducted for 100 s before the open circuit potential (OCP) was recorded, and the anodic polarization test was started from value of OCP. The polarization was carried out in the potential −600 mV more active than the stable OCP and terminating at a potential +1200 mV more positive than OCP at the scan rate of 1 mV/s. From the E-log I plot, corrosion potential (Ecorr) and corrosion rate (CR) of AISI Type 420 stainless steel was determined by Tafel fitting the data in the region of ±600 mV from OCP.

Cyclic Polarization Test The breakdown potential of AISI Type 420 stainless steel was determined by conducting the cyclic polarization test. The polarization was carried out in the potential range between −600 mV and +1200 mV from OCP at the scan rate of 1 mV/s. After reaching the potential value of +1200 mV, reverse polarization was done to find the breakdown and repassivation potential of AISI Type 420 stainless steel in 3.5% Sodium Chloride solution. All polarization curves were run at room temperature.


SEM and EDS Measurement The appearance of pitting corrosion after cyclic polarization test was analyzed by SEM. It was carried out on corroded sample after cleaned with ethanol and dried. The chemical composition in the pit analyzed by EDS.

RESULTS AND DISCUSSION Potentiodynamic Polarization Studies Potentiodynamic polarization plot of AISI Type 420 stainless steel at various tempering temperature in the 3.5% sodium chloride solution is shown in Fig. 1.

1,00E+00 8,00E-01 6,00E-01

Potential (V vs SCE)

4,00E-01 2,00E-01 0,00E+00 -2,00E-01 -4,00E-01 -6,00E-01 -8,00E-01 -1,00E+00 -1,20E+00 -1,40E+00 1,00E-08 1,00E-07 1,00E-06 1,00E-05 1,00E-04 1,00E-03 1,00E-02 1,00E-01 1,00E+00

Current (A/cm2) Control

Temper 150

Temper 250

Temper 350

Temper 450

FIGURE 1. E – Log I plot of AISI Type 420 stainless steel at various tempering temperature

The overlays of polarization curve shows there are a passive region, a transpassive region and secondary passive. The value of current and potential of passive region, transpassive region, secondary passive region from potentiodynamic plot are given in the Table 2. The Fig. 1 and Table 2 shows passive region of sample tempered 150°C is longer than all sample and the shorter passive region was found on the sample tempered 350°C. The passive-transpassive-secondary passive of AISI 420 stainless steel indicates that steels having uniform corrosion resistance in the 3.5% sodium chloride solution.


TABLE 2. Result from potentiodynamic polarization studies Tempered 150°C

Control Current (A/cm2) Passive Transpassive Secondary Passive

Start End Start End Start End

4.83E-04 6.25E-04 6.25E-04 1.29E-01 1.29E-01 3.53E-01

Potential (mV vs. SCE) -613.2 -260.5 -260.5 114.7 114.7 676.4

Current (A/cm2) 1.84E-04 3.24E-04 3.24E-04 2.53E-01 2.53E-01 5.04E-01

Tempered 250°C

Potential (mV vs. SCE) -609 -240.6 -240.6 212.6 212.6 758.3



Tempered 350°C Current (A/cm2) 1.27E-04 1.19E-04 1.19E-04 2.77E-02 2.77E-02 1.78E-01

Tempered 450°C



Potential (mV vs. SCE) -575.2 454.5 454.5 104.7 104.7 774.1

Inspite of passive region occurred, the corrosion potential and corrosion rate can be computed from potentiodynamic polarization plot and the result are shown in Fig. 2. In general, potential corrosion (Ecorr) shift to more positive value with increasing tempering temperature. 1.20E+00

0 -100 -200

8.00E-01

-300

6.00E-01

-400 -500

4.00E-01

-600 2.00E-01

Ecorr (mV vs. SCE)

Corrosion Rate (mmpy)

1.00E+00

-700

0.00E+00

-800 0

50

100

150

200

250

300

350

400

450

500

Tempered Temperature (°C) Corrosion Rate

Ecorr

FIGURE 2. Corrosion potential and corrosion rate of AISI Type 420 stainless steel at various tempering temperature

The corrosion rate decrease with increasing tempering temperature. The corrosion rate of sample with tempering temperature of 250°C is 0.015 lower than that of autenitized sample (control sample) in the 3.5% sodium chloride. It means the tempering treatment of AISI Type 420 stainless steel can effect corrosion rate in the presence of 3.5% chloride. This result is similiar to the previous reported paper [8].

Cyclic Polarization Studies The overlays of cyclic polarization curve for AISI Type 420 stainless steel with and without tempering in 3.5% chloride containing solution is shown in Fig. 3.


TABLE 3. Result from cyclic polarization studies Epit Eprot (mV) (mV) Austenized 1050 (Control) -137.4 -263.9 Austenized 1050,temper 150

-140

-362.6

Austenized 1050,temper 250

-107.1

-286.7


-152.6

-392.9


-137.4

-324.6

Table 3 lists the values of pitting potential (Epit) and protection potential (Eprot), which is defined as the potential where forward and reverse scans cross. 2,00E-01

Potential (V vs SCE)

0,00E+00 -2,00E-01 -4,00E-01 -6,00E-01 -8,00E-01 -1,00E+00 -1,20E+00 1,00E-07

1,00E-06

1,00E-05

1,00E-04

1,00E-03

1,00E-02

1,00E-01

1,00E+00

Current (A/cm2)

Control

Temper 150

Temper 350

Temper 450

Temper 250

FIGURE 3. Cyclic polarization curve for AISI Type 420 stainless steel at various tempering temperature

Pitting Potential (mV vs. SCE)

-40 -60 -80 -100 -120 -140 -160 0

100

200

300

400

500

Tempered Temperature (ºC) FIGURE 4. Pitting potential for AISI Type 420 stainless steel at various tempering temperature


All the pitting scans of these steels contain a substantial hysteresis loop between the forward and the reverse scans, indicating that these steels with and witout tempering would be susceptible to pitting corrosion in 3.5% chloride environment. The higher pitting potential were considered to have better pitting corrosion resistance [11]. Fig. 4 show pitting potential for AISI Type 420 stainless steel at various tempering temperature. In this figure, the highest pitting potential of -107.1 mV was found in the sample with tempering at 250°C. It means the sample with tempering at 250°C having greater resistance to pitting potential.

(a)

(b) FIGURE 5. SEM image of pitting corrosion (a), and EDS analysis on the square no. 002 of pitting corrosion (b) in the sample with tempering at 350°C


When potentiodynamic sample were examined under the electron microscope in Fig. 5a, it was found that the pitting corrosion had occured in the surface of sample. Some of the pits were seen to contain irregular shaped precipitates. An EDS analysis of these precipitate showed them to be rich in carbon of 47.5 % mass and chromium of 22.04 % mass (Fig. 5b and Table 4), indicating that they were probably chromium carbide of the type Cr23C6, as had been found in a previous investigation [12]. TABLE 4. Result from EDS analysis

Element

Mass %

C

47.50

O

7.89

Cl

1.17

Cr

22.04

Mn

1.41

Fe

19.98

The oxygen of 7.89 % mass and chloride of 1.17 % mass detected on the analyses of the precipitates probably originated from the pitting-test solution, but could also have been associated with the precipitate. This could be a possible reason why the sample with tempering at 350°C had the lowest pitting potentials.

CONCLUSIONS The effect tempering treatment on the AISI Type 420 stainless steel reduces uniform corrosion rate of the alloys but can not prevent the susceptibility to pitting corrosion. The increment of tempering temperature of 150°C to 450°C has no effect linearly with pitting corrosion resistant. The highest pitting potential was found in the sample with tempering at

250°C. The lowest pitting potential was found in the sample with tempering at 350°C. Corrosion pits are found to initiate preferentially around chromium carbides.

ACKNOWLEDGMENTS The authors thank to Research Center for Metallurgy and Materials, Indonesian Institute of Sciences (LIPI) for funding this research in fiscal year 2015.

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