evaluation of corrosion performance of two mn-containing stainless steel alloys

Ihsan-ul-Haq ToorDept. of Mechanical Engineering, King Fahd University of Petroleum & Minerals (KFUPM), Kingdom of Saudi Arabia

Evaluation of corrosion performanceof two Mn-containing stainless steel alloys

300 series austenitic stainless steels (SSs) are widely usedas structural materials in various corrosive environmentsbecause of their excellent corrosion resistance and goodmechanical properties. However, due to the rapid increasein Ni cost over the last few years, there have been attemptsto develop cheap SSs while still maintaining their relativelyhigh corrosion resistance. The possible replacements for Ni(a gamma stabilizer) in SSs are Mn, N and Cu etc. In thispaper, the effect of Mn addition on the corrosion propertiesof two SS alloys, i. e. 202M2 and 202M4, is discussed andresults are compared with those of 304 SS alloy. Results ofdifferent tests showed that overall corrosion resistanceproperties (measured in terms of pitting potential (Epit), me-tastable pitting events density and time to failure etc.) de-creased with an increase in Mn content of the alloys.

Keywords: Stainless steels; Nickel; Manganese; Pitting;SCC

1. Introduction

Austenitic SSs are replacing carbon steel in the automobileindustry and other structural applications because of theirhigher strength, excellent corrosion resistance, ease ofweldability and toughness, which makes automobiles light-er & corrosion resistant. Ni in SSs is a structure-determin-ing element, so the SSs are categorized as ferritic, austeniticand duplex based on their Ni content. Ni is also known tohave a significant effect on the stress corrosion cracking(SCC) resistance of SSs. However due to their high cost(because of 8*12 wt.% Ni), austenitic SSs cannot be usedin many applications where they would otherwise be anideal choice. Ferritic SSs, on the other hand, do not containNi and are thus much cheaper than the austenitic grades,however they have very poor weldability. So cheaper SSsare being developed in which Ni is being replaced withother austenite forming elements such as Mn, Cu and N asreported by Wataya et al. [1]. According to Kostina et al.[2], Mn is not only 7*8 times cheaper per ton than Ni, butit also increases the nitrogen solubility, and these two to-gether compensate for Ni deficiency in high Mn SSs. Nitro-gen in addition of functioning as an austenite stabilizer, alsoimproves the corrosion resistance properties. It also hasseveral other benefits, such as increased strengthening andretardation of sensitization.Mn in SSs decreases their corrosion resistance, mainly

due to the formation of manganese sulphide (MnS) inclu-sions as shown in Refs. [3–11]. But recently with advance-

ments in steel-making technology such argon oxygen dec-arburization (AOD) and vacuum oxygen decarburization(VOD), sulphur content in SSs has been decreased consid-erably, which will ultimately decrease the MnS inclusionsin SSs. Lunarska et al. [12] reported that austenitic SSs suchas Fe–Cr–Mn–Ni–N (AISI 200 Series) have corrosion resis-tance similar to those AISI 300 series SSs during atmo-spheric exposure and under oxidizing conditions. TheFe–Cr–Mn and Fe–Cr–Mn–N SSs are, however, inferior to300 series with respect to general corrosion resistance espe-cially under reducing conditions, as found by Wranglen[13]. According to Szklarska [14], this is probably due tothe fact that Cr and Ni have beneficial effects on the corro-sion resistance of SSs, while Mn is generally considered tohave a detrimental effect. The effect of Mn on the pitting re-sistance of Fe–Cr–Mn alloys is rather controversial withvarious effects being observed. Lunarska et al. [12] is ofthe opinion that this controversy is mainly due to the factthat these steels are extremely sensitive to the presence ofmicro-constituents such as carbon, nitrogen, sulphur andphosphorus. It is believed that Mn decreases the corrosionproperties of SSs considerably by forming MnS inclusions.These sulphide inclusions are known to dissolve and pro-duce an aggressive environment for pit initiation in chloridesolution as shown in Refs. [15–17].There have been few studies on the role of Mn in SSs,

and rarely has any of them investigated the effect of Mnon the corrosion resistance properties comprehensively. Inthis paper, we have used differential experimental techni-ques such as potentiodynamic polarization (PD), electro-chemical noise analysis (ENA), critical pitting temperature(CPT) measurement and stress corrosion cracking (SCC)together to investigate the effect of Mn on the overall corro-sion performance of two high Mn stainless steel alloys. Inorder to confirm the role of Mn further, the experimental re-sults of high Mn SS alloys were compared with that of tradi-tional SS alloy 304.

2. Experimental procedure

The alloys were prepared in a vacuum induction furnaceand materials were subsequently hot rolled to 6 mm thick-ness as shown in Table 1. The hot rolled plates were coldrolled (35%) to 4 mm thickness, solution annealed at(1050 8C for 30 min) and subsequently water quenched.Scanning eletectron microscopy (SEM) together with en-ergy dispersive X-ray spectroscopy (EDS) was carried outto examine the surface morphology of alloys and for ana-lyzing the inclusions present in these alloys. For electroche-

Ihsan-ul-Haq Toor: Evaluation of corrosion performance of two Mn-containing stainless steel alloys

386 Int. J. Mater. Res. (formerly Z. Metallkd.) 105 (2014) 4

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mical experiments, specimens were ground to 2000 gritemery paper and then ultrasonically cleaned with distilledwater. Prior to electrochemical tests, the specimens werecathodically cleaned for 5 min at –0.8 VSCE to remove theair-formed oxide film. A three electrode cell (a specimenas a working electrode, a Pt counter electrode and a satu-rated calomel reference electrode) was used for the experi-ments. Potentiodynamic (PD) and other tests were carriedout at a scan rate of 0.5 mV s–1 in 0.5 M NaCl nitrogen dea-erated solution at room temperature, 25 8C. Specimens withan exposed surface area of 0.22 cm2were used for corrosionexperiments.The metastable pitting tendency was determined by

using electrochemical noise analysis (ENA) at room tem-perature in deaerated 0.5 M NaCl solution. The exposedsurface area was 0.22 cm2 and applied potential was–0.2 VSCE. Data were recorded for 7000 s at a sampling rateof 10 Hz. To minimize the external noise, experiments wereperformed in a Faraday cage.The critical pitting temperature (CPT) was measured in

deaerated 4 M NaCl solution at room temperature, 25 8Cusing zero resistance ammeter (ZRA) mode. Specimenswith an exposed surface area of 1 cm2 were used as a work-ing electrode after being cold mounted in epoxy resin. Thespecimen surface was mechanically polished using SiC pa-per up to 2000 grit, subsequently rinsed with distilled waterand dried. During CPT measurement the solution tempera-ture was continuously increased at a rate of 1 K min–1 andcurrent density was recorded throughout the experiment.The SCC tests were carried out in a Cortest slow strain

rate testing (SSRT) machine at a strain rate of 2.05 ·10–6 s–1. The Tensile specimens for SSRT were preparedwith a 20 mm gauge length of area (1.5 mm · 2.5 mm).The specimens were surface finished to 1200 grit emerypaper, ultrasonically cleaned in acetone and sealed with si-licon sealant for 8–10 h (complete drying). To get thestable corrosion potential, the specimens were left in thecell for 10–15 h at the required temperature before anyload or strain rate was employed. SCC tests were performedunder open circuit conditions in boiling 25 wt.% MgCl2 so-lutions (at 120 8C). The fractured surfaces after the experi-ments were examined using SEM to find out the fracturemode, (ductile, brittle, quasi-cleavage).

3. Results

3.1. Polarization response

Figure 1 shows the polarization response of the three alloysin deaerated 0.5 M NaCl solution at room temperature,25 8C. The addition of Mn in these alloys made them a littlebit more active, by lowering their corrosion potential (Ecorr)towards the active region. This is mainly because Mn is an

active element as compared to Fe or Cr. Ecorr of the alloyswas found to be in the order of –0.32 VSCE (202M4),–0.29 VSCE (202M2) and –0.28 VSCE for 304 SS. FromFig. 1, the values of pitting potential (Epit) were noted forthree alloys and it was clear that Epit of alloy 202M4 (withhigher Mn content) was lower than that of alloy 202M2.On the other hand, 304 SS alloy exhibited higher resistanceto localized corrosion as compared to both the high Mnalloys in terms of pitting potential. Epit followed the trendas follows, i. e. 0.34 VSCE (304) > 0.2 VSCE (202M2) >0.12 VSCE (202M4). The results demonstrate that the resis-tance to pitting corrosion of the alloys decreased with in-creasing Mn content at the expense of Ni content, althoughthere is a small difference in Cr content among the alloys.This harmful effect of Mn on pitting corrosion resistanceis attributed to the presence of inclusions in high Mn alloys,as shown in Fig. 2, and these inclusions were found to beoxides of Mn, Cr, and Fe.

3.2. Electrochemical noise analysis

The stage of pitting includes passive film breakdown, meta-stable pitting initiation and repassivation and stable pitgrowth. The metastable pitting stage is thought to be the mostimportant because only pits that survive this stage can be-come stable growing pits. The metastable pitting behaviorof the three SSs was measured by means of electrochemicalnoise analysis. Each current spike which appears duringENA means an initiation and repassivation of a single meta-stable pitting event. The number of current spikes per unitarea was defined as metastable pitting event density. Fig-


Int. J. Mater. Res. (formerly Z. Metallkd.) 105 (2014) 4 387

Table 1. Chemical composition (wt.%) of the two high Mn alloys along with type 304 SS. (Balance Fe).

Alloy C Si Mn P S Cr Ni Mo Cu N

304 0.119 0.58 0.89 0.28 0.003 17.18 6.76 0.14 0.26 0.0479202M2 0.059 0.50 7.43 0.031 0.019 15.86 4.79 0.07 1.16 0.0619202M4 0.053 0.49 9.89 0.029 0.001 14.47 1.02 0.06 1.13 0.163

Fig. 1. Potentiodynamic polarization behavior of the three alloys indeaerated 0.5 M NaCl solution at room temperature, 25 8C (scan rate =0.5 mV s–1).

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ure 3 shows the metastable pitting events density of the threealloys measured potentiostatically at –0.2 VSCE in 0.5 MNaCl solution at room temperature and at a sampling rate of10 Hz. Each current spike means an initiation and repassiva-tion of a single metastable pitting event. A high value of me-tastable events density means higher susceptibility to meta-stable pitting, which means that alloy 202M4 is the mostsusceptible to pitting as it showed the highest pitting eventsdensity among all SS alloys. Frankel et al. [18] showed thatthe metastable pits may grow into stable pits, only if the pitdepth is by itself a sufficient barrier to maintain an environ-ment that is sufficiently aggressive to prevent repassivationinside the pit. It is clear from Fig. 3 that an increase in Mncontent of the alloys increased their metastable pitting eventdensity as well, i. e., 202M4 > 202M2 > 304 SS in the orderof susceptibility to metastable pitting.Critical pitting temperature (CPT), is the temperature be-

low which pitting does not take place and this technique is



Fig. 2. SEM micrographs showing the non-metallic inclusions (Mn, Cr) oxides in 304, 202M2 and 202M4 alloys.

Fig. 3. Metastable pitting events density of the alloys (304, 202M2and 202M4), in deaerated 0.5 M NaCl at room temperature and at–200 mVSCE showing highest density for 202M4 alloy.

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widely used investigate the pitting resistance of SSs. CPTresults of two high Mn alloys together with SS 304 are pre-sented in Fig. 4a, which shows a similar trend to that ob-served in PD and ENA tests, i. e. high Mn containing alloyswere more susceptible to corrosion. CPT values followedthe same trend, i. e. 304 > 202M2 > 202M4. Figure 4bshows the pitting resistance (measured in terms of Epit andCPT values) versus pitting resistance equivalent (PREMn).In the literature there are different formulas to consider therole of Mn in PREMn calculation, however in this paper,we have used the formula, PREMn = Cr% + 3.3Mo% +30N%–1.0Mn% to calculate PREMn [19–21]. It is clearthat both Epit and CPT values decreased with an increasein Mn content of the alloys, i. e. pitting corrosion resistancewas decreased as Mn content was increased.

3.3. Slow strain rate test

SCC experiments were carried out by SSRT in boiling25 wt.% MgCl2 at 120 8C. The specimens for SSRT werecold rolled and solution annealed before the test and testswere performed under open circuit conditions at a strainrate of 2.05 · 10–6. Figure 5a shows the load-elongationcurves of the high Mn alloys along with 304 SS conducted

at 120 8C. It is quite clear that 304 SS did not show any sus-ceptibility to SCC, as it exhibited enough elongation ofaround 60%. However, the other two alloys (202M2 and202M4) were susceptible to SCC as the elongation was lessthan 20%. SSRT results also suggest that with an increasein Mn content of the alloys, SCC susceptibility (measuredin terms of time to failure (TTF), % elongation and reduc-tion of area (ROA)), was increased considerably as shownin Table 2. These results were further confirmed by micro-scopic investigations of failed specimens. SEM fracto-graphs (Fig. 5b) exhibited a dimple fracture for 304SS,which is a typical of ductile behavior, while a qausi-clea-vage fracture, typical of SCC fracture, was observed forboth 202M2 and 202M4 alloys. From these results it is clearthat SCC susceptibility was increased with increase in Mncontent, i. e. (202M4 > 202M2 > 304) of the alloys and thisis in accordance with previous results, which showed thatan increase in Mn content of the alloys is detrimental forthe corrosion resistance properties of SSs.

4. Discussion

As discussed in the results section, an increase in Mn con-tent of the alloys is not good for corrosion resistance prop-erties. This effect can be attributed mainly to non-metallicinclusions (NMIs) such as oxides and sulphides of Mn inthese SSs. It has been reported previously by a few re-searchers [15–17] that these inclusions act as potential sitesfor the initiation of metastable or stable pitting corrosion ofSSs. Park et al. [22] investigated, whether NMIs act as pitinitiation sites or not, so they examined the surfacemorphologies of metastable pits and it was demonstratedthat metastable pits occurred at the edge of an NMI. In ourstudy, manganese sulphide (MnS), which is consideredone of the most deleterious inclusion in SSs [3–11] wasnot found and most of the inclusions were (Fe, Cr, Mn)-oxides, as confirmed by the EDS data shown in Fig. 2. Asthe Mn content of the alloys was increased, both the Epitand metastable pitting events density was increased, whichmeans even Mn-oxide type of NMIs can be the pitting in-itiation sites, as has been reported by Park et al. [22]. Thismeans protectiveness of the passive film was seriously de-graded by the presence of Mn in these alloys.Electrochemical noise analysis (ENA) is a powerful tech-

nique to study the initiation, propagation and repassivationof microscopic pits [23–25]. Usually corrosion pit growthtakes place in two stages, characterized by metastablegrowth initially, which is followed by stable growth. Outof these two, metastable pitting is considered to be the mostimportant stage because any pit that survives this stage willbecome a stable pit later on. This metastable pitting beha-vior of the alloys was measured in terms of pitting eventsdensity, Fig. 3. It was clear that as the Mn content of the al-loys was increased, metastable pitting events density wasalso increased, which ultimately means that susceptibilityto pitting was increased mainly due the presence of NMIsobserved in Mn-containing SSs (Fig. 2) [26, 27]. A similartrend was obtained in CPT tests, which determined the low-est potential independent of temperature, below which pit-ting does not occur. The principal advantage of the test isthe rapidity with which the CPT can be measured in a singletest. So, in short, it can be said the Mn content is an impor-tant factor in determining the resistance to pitting corrosion



(a)

(b)

Fig. 4. (a) Critical pitting temperature of three alloys in 4 M NaCl so-lution, (b) pitting resistance equivalent number of the three alloys ver-sus CPT.

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of stainless steel alloys and with an increase in Mn contentof the alloys, corrosion resistance degraded. However, anoptimized combination of Mn and N together, can be usefulin many applications where moderate corrosion resistanceis required or in oxidizing environments.

5. Conclusions

The corrosion behavior of the two high-Mn SSs (202M2,202M4) was studied and results were compared with thoseof 304 SS. The results showed that Mn content was an im-



(a)

(b)

Fig. 5. (a) Load–elongation curves for thealloys at 120 8C using the slow strain ratetechnique in deaerated 25 wt.% MgCl2 so-lution, and (b) SEM fractographs of the al-loys after SSRT testing in 25 wt.% MgCl2solution at 120 8C.

Table 2. Results of SSRT tests conducted in boiling MgCl2 solution at 120 8C.

SSRT in boiling MgCl2 solution at 120 8C

# UTS (MPa) Elongation (%) TTF (h) ROA (%)

304 98 65 81 31.5202M2 25 17 19 17.0202M4 30 9 11 5.0

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portant factor in determining the resistance to localized cor-rosion and SCC of high-Mn alloys.The resistance to pitting corrosion of the alloys was de-

creased with an increase in Mn content of the alloys, whichwas confirmed by the decrease in Epit and CPT values. In-creasing Mn content increased the metastable pitting eventsdensity of high-Mn alloys. The decrease in the resistance topitting corrosion was associated with the presence of non-metallic inclusions such as (Mn, Cr) oxides, which act aspitting initiation sites.Stress corrosion cracking experiments performed in

25 wt.% MgCl2 solution at 120 8C showed that the resis-tance to SCC (measured in terms of % elongation andTTF) of the alloys decreased significantly with an increasein the Mn content of the alloys.

The author gratefully acknowledges the support provided by KingFahd University of Petroleum &Minerals (KFUPM) Saudi Arabia, un-der the research grant # IN111048 in the successful completion of thisresearch work.

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(Received August 27, 2013; accepted November 25, 2013;online since March 19, 2014)

Bibliography

DOI 10.3139/146.111035Int. J. Mater. Res. (formerly Z. Metallkd.)105 (2014) 4; page 386–391# Carl Hanser Verlag GmbH & Co. KGISSN 1862-5282

Correspondence address

Ihsan ul haq ToorAssistant ProfessorDept. of Mechanical EngineeringKing Fahd University of Petroleum & Minerals (KFUPM)P.O. Box. 1308Dhahran 31261Kingdom of Saudi ArabiaTel: +966-13-860-7493Fax: +966-13-860-2949E-mail: [email protected]



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evaluation of corrosion performance of two mn-containing stainless steel alloys

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