corrosion behaviour of selected high manganese...

15. - 17. 5. 2013, Brno, Czech Republic, EU

CORROSION BEHAVIOUR OF SELECTED HIGH MANGANESE AUSTENITIC STEELS

Stanislav LASEK, Eva MAZANCOVÁ

VŠB-TU Ostrava, 17. Listopadu 15/2172, [email protected]

Abstract

High-manganese steels (20-30% Mn, suitable for cars) have higher toughness and ductility compared to

unalloyed structural steels at comparable strength properties. The corrosion resistance of these steels can

be reduced by a high content of manganese which is not electrochemically noble element. In this paper are

compared the results of corrosion tests of TWIP (X70Mn22) and TRIPLEX (X100MnAl28-12) steels in rolled

and annealed state with structural carbon steel C17 (CSN 411375). The cyclic potentiodynamic method for

pitting sensitivity in neutral NaCl solution and potentiodynamic polarization method for assessing the

resistance to uniform corrosion in acidic conditions (sulfuric acid water solution) were used. Exposition

standard salt spray test, gravimetric method as well as microscopic observations of structure and corrosion

were also applied. Based on the exposure tests and measurements the average corrosion rates (non-

uniform and pitting) were determined and the ranking of steel resistance was established. On the basis of the

polarization test in chloride solutions there were found much higher values of corrosion rate of X70Mn22

(TWIP) steel compared with X100MnAl28-12 (TRIPLEX) one, while a positive effect of aluminum for

corrosion resistance was confirmed. In the sulfuric acid solution slightly lower resistance TWIP steels

compared with TRIPLEX was measured, but both Mn steels have unacceptably high corrosion rate.

Keywords: manganese steel, corrosion resistance, polarization method, salt spray test

1. INTRODUCTION

The new high manganese TRIPLEX steels have been developed from Hadfield (X120MnCr13) and TWIP

(twinning–induced plasticity) steels. The structure of TRIPLEX is formed by stable austenitic phase with

higher stacking fault energy (110 mJ/m2) and very fine coherent k–carbides (FeMn)3AlC, dispersed in matrix

[1]. Small volume fraction of δ-ferrite (< 10 %) can be also formed in structure after rolling. High manganese

steels have higher strength values and simultaneously high plasticity, which is desired for vehicles. The

advantage of stated steels is also higher absorption energy and lower density as compared with commercial

carbon steel. Worse corrosion resistance of high manganese steels is connected with low standard potential

(-1.2 V SHE) and without the ability to form protective layer. It is often part of the sulphides and carbides,

favoring certain types of localized corrosion.The corrosion resistance of TWIP (Fe-25Mn, Fe-25Mn-3Al) steel

is low and the electroless coating Ni-P can be used for protection [2]. The potentiodynamic polarization tests

in 3.5 % NaCl and 0.1 M H2SO4 showed that Ni-P coating significantly improved the corrosion resistance in

both media. The decisive influence on corrosion resistance of Mn steels (X7MnSiAlNbTi26-3-3,

X5MnSiAlNbTi24-3-2) has their chemical composition, which determines high rate of dissolution in acidic

media, i.e. general corrosion and formation of pitting [3].

The aim of the work is comparison and evaluation of corrosion resistance of TRIPLEX steel X70MnAl28-9

and TWIP X70Mn22 by standard laboratory tests.

2. MATERIALS AND EXPERIMENTAL TECHNIQUE

For electrochemical corrosion tests were prepared samples of TWIP austenitic steel containing 0.7% C, 21-

23% Mn (wt. %, hot rolled in air, dimensions 34 x 32 x 3 mm) and samples of TRIPLEX steel (18x18x3 mm)

with composition 1.0% C, 28% Mn and 12% Al, with 8% -ferrite content and the tensile strength of 1100

MPa. Commercial carbon steel C 17 (CSN 411373, sheet 1,0 mm) was also used as reference material.

mailto:[email protected]


Before electrochemical corrosion tests, unequal and scaled surfaces of samples on one side were ground

gradually by wet SiC grit papers No. 60-1500 and these samples were degreased by alcohol-petrol and then

rinsed with demineralized water. For salt spray test of supplied steels, the corrosion products (scales) were

removed on all surfaces by wet grinding gradually (SiC papers No. 60 - 2000). Samples were cleaned in

alcohol and weighed by digital scales Sartorius CP224S-OCE.

Before polarization test corrosion potential values (Ecor) were measured and then polarization measurements

were started from approx. 100 mV below the corrosion potential values.

Potenciodynamic cyclic polarization tests of resistance to pitting corrosion were carried out according to

standard [4] in aqueous solutions of 0.1 to 1.0 mol/l NaCl at room temperature with polarization rate at 1.0

mV/s, under free access to air. Polarization measurements were performed using a potentiostat PGP201

with a special corrosion cell with three-electrode connection: sample as the working electrode, reference

saturated calomel electrode (SCE) and platinum auxiliary external electrode. Aqueous solutions of sodium

chloride are used for testing or comparing the corrosion resistance of steels and surface layers or coatings

used in the automotive industry.

Further electrochemical corrosion tests were performed in solution of 0.1 mol / l H2SO4 [5] under the same

conditions as the test using NaCl solution.

Exposure tests were performed according to ISO standard [6] in the salt spray cabinet LIEBISH S400 M-TR,

where they were placed on plastic racks (the largest samples area under the angle of 20° to the vertical) and

then continuously exposed for 12 hours in neutral 5% NaCl salt spray at 35 °C. The corrosion weight gains

and losses per unit area were determined after the exposure time (the gravimetric method). After performed

tests the macroscopic images of exposed surfaces were made (using digital camera Canon A700) and

microscopic and metallographic observation.

3. DESCRIPTION AND DISCUSSION OF RESULTS

3.1 Polarization tests

On the basis of the cyclic potentiodynamic polarization method [2] the polarization curves were recorded

(Fig. 1) and following parameters of pitting and uniform corrosion were determined, see Table 1:

Ed - depassivation potential set at the conventional current density Jd = 100 A/cm2,

Er - repassivation potential at current density Jr = 10 A/cm2 ,

Ev - reversible potential established when current density Jv = 1.0 mA/cm2,

Ecor - corrosion potential,

Jcor - corrosion current density,

Rp - polarization resistance (inversely proportional to the uniform corrosion rate rc = B/Rp)

rc - mean corrosion rate (determined according to the Stern methods and Faraday's law).

Potential E [mV] SCE

Fig. 1 Cyclic potentiodynamic

polarization curves of tested

steels (0.1M NaCl, 25°C)

TRIPLEX

TWIP

C17

Curr

ent d

ensity

j

[A/c

m2]


In general, the higher the value of potentials (Ed, Er) the higher the resistance to pitting corrosion. It is also

necessary to consider the differences between these potentials and corrosion one (the higher values Ed -Ecor

and/or Er - Ecor, the better resistance). Further parameters (Ecor, Jcor, Rp, rc) are related to uniform corrosion

before pitting initiation. The uniform corrosion is lower, if parameters Jcor and rc have smaller values.

Table 1 Comparison of corrosion parameters of tested and compared steels

steel sample solution Ecor Ed Ev Er Rp Jcor rc note

wt. % No. NaCl mV mV mV mV Ωcm2 A/cm2 mm/r cycle

TWIP 0.7 % C 22 % Mn original surface

5.3 1.0 M -633 -604 -501 -708 560 29.1 0.340 1

5.3 0.1 M -740 -698 -542 -723 624 17,4 0.203 1

8.3 0.1 M -673 -653 -482 -701 519 18.5 0.216 1

8.3 0.1 M -718 -682 -540 -712 452 24.0 0.281 2

8.3 0.1 M -710 -662 -506 -684 267 21.8 0.260 1

TRIPLEX 1.0 % C 28 % Mn 12 % Al

1.0 0.1 M -420 -289 -126 -447 7350 1.37 0,015 1.

1.0 0.1 M -428 -228 -114 -462 2248 0.46 0.005 2

6A 3.5% -426 -302 -206 -602 7890 1.15 0.013 1

6A 3.5% -456 -365 -234 -609 5380 2.35 0.028 2

Steel C17 0.17% C

NS 0.1 M -473 -412 -262 -523 1400 17.8 0.209 1

0.1 M -513 -450 -273 -534 1160 31.4 0.368 2

Notes: The values of potentials are given relative to a saturated calomel electrode (SCE). For the sample

TRIPLEX 6A crevice corrosion was observed, which resulted in lower values of the measured potentials (Ed,

Er), compared with the sample TRIPLEX 1.0. Content of 1.0 M NaCl = 1.0 mol/l NaCl = 5.84% wt. NaCl.

For comparison, the cheapest type of Cr13 stainless steel under the same conditions has value of potential:

Ecor = -200 mV, Ed = +10 mV, Er = -270 mV (25 °C, 0.1 M NaCl).

Significantly higher resistance TRIPLEX steel (X100MnAl28-12 type) compared with TWIP steel (X70Mn22)

was demonstrated in NaCl solution. Based on the polarization tests, it was found the significant increase in

depassivation Ed and repassivation Er potential and the order of magnitude decrease in the corrosion rate,

which can be explained by a certain passivation effect of aluminum (12%) in the high-Mn-Al steel. The

polarization curves recorded for TWIP steel have continuous increase in current density, whereas on steel

TRIPLEX slight current increase in quasi-passive area and then sharp increase after breakdown (damage) of

protective layer by pitting corrosion.

For technical practice and required lifetime is usually permissible uniform corrosion rate lower than 0.1 mm/a

(annual, yea)r, which is met with steel TRIPLEX (corrosion rate from 0.01 to 0.03 mm/a). In contrast, the

TWIP steel was predicted much higher corrosion rate 0.2 - 0.4 mm/a.

By stereo and metallographic microscope on TWIP steel after corrosion test were observed the uneven

spots with microscopic pits (10-40 m), on TRIPLEX steel elongated pits (300 x150 m) and several

smaller clusters of corrosion pits (30-100 m).

The results for the acidic environment in Table 2 show a significantly higher corrosion rate (1-2 order) in

comparison with neutral solution (Table 1). In the measured range of potential (Ecor ± 50 mV) polarization

curves had almost linear trends.

The differences in corrosion resistance between steel TWIP and TRIPLEX are relatively small in aggressive

acidic solution of 0.1 M H2SO4, see Table 2. While slightly lower mean corrosion rate was found to TRIPLEX

steels alloyed with aluminum, as compared to TWIP. High Mn-steels have in average many times higher

corrosion rates than carbon steel.


Table 2 Comparison of corrosion parameters TWIP steel (X70Mn22) and TRIPLEX (X100MnAl 28-12) in a

solution of 0.1 M H2SO4.

steel sample Ecor Rp Jcor rc note

wt. % No. mV Ωcm2 A/cm2 mm/a cycle

TWIP 0,7 % C 22 % Mn

8.3 -560 23,4 443 5,19 1

8.3 -554 23,9 507 5,93 1

5.3 -557 23,7 475 5,56 1

TRIPLEX 1,0 % C 28 % Mn 12 % Al

4.0 -620 34,6 341 3,99 1

4.0 -617 29,8 363 4,25 2

1.0 -606 64,3 599 5,01 1

1.0 -610 24,4 383 4,48 2

Carbon steel C17

NS1 -518 488 31,6 0,370 1

NS1 -527 308 54,5 0,638 2

NS2 -500 108 77,6 0,908 1

NS2 -514 95 124 1,41 2

Lower values of corrosion potential for steel TRIPLEX (X100MnAl 28-12) can be explained by a higher

content of Mn and Al alloys because both elements have a low equilibrium (standard) potential compared to

Fe. The standard potential Eo for Al, Mn and Fe have the following values: -1.66 V, -1.18 V and -0.44 V SHE.

Note: Testing in acidic solutions can be usefull in terms of reduction in steel pickling [3] and also in assessing

the impact of polluted industrial atmosphere due to SO2 or action of acid rain.

Under acidic conditions, the intensity of etching both steels significantly higher (than in NaCl) and in some

places the grain boundaries and differently etched crystallographic planes were observed using a

metallographic microscope

3.2 Salt spray test

In this paragraph are described and compared the results of exposure tests in salt spray cabinet according to

standard, Table 3.

Table 3 The results of accelerated exposure test in salt spray

steel dimensions surface mass mass gain mass loss notes

sample LxBxH [mm] S [cm2] mo [g] ∆m/S [g.m-2] ∆m/S [g.m-2] surface

TWIP 5.3 8.3

33,6x32,2x2,4 24,80 20,5866 15,9 16,6 ground up to

33,6x33,8x2,4 26,00 21,5723 16,2 17,9 No. 2000

16,1 17,3 mean value

TRIPLEX 1.0 4,0

18,8x18,4x2,4 8,70 4,6063 7,59 4,48 ground up to

18,0x17,3x2,2 7,78 4,2551 7,97 3,21 No. 2000


Carbon

steel C17

50,8x29,0x3,0 34,3 33,2801 14,0 11,4 ground up to

43,4x28,9x2,5 28,0 22,9444 16,6 11,4 No. 2000

102x50,8x1,5 107,9 53,8024 13,8 16,6 ground no.120

98,7x49,7x1,2 101,7 40,4491 19,6 22,7 original surface


Measured average corrosion mass gains were approximately 2x higher for TWIP steel than TRIPLEX one,

and corrosion losses were more than 4 times higher for TWIP steel, Table 3. For carbon steel have been

also found somewhat lower corrosion mass changes in comparison with TWIP steel (with the same surface

quality). On the roughly brushed or slightly corroded surface, on carbon steel were found out a higher

corrosion weight changes. Similar corrosion behavior can be expected for the TWIP or TRIPLEX steel with

respect to different surface quality. After chemical remove of rust in aqueous hydrochloric acid solution 1:1


with inhibitor (Hexamethylentetraminum, C6H12N4) corrosion losses were measured on unalloyed carbon

steel of 12.8 gm-2 but for TWIP steel 41.7 gm-2, which also confirms the lower corrosion resistance

of Mn-TWIP steel.

Significant differences were observed between the upper and lower surface with respect to salt fog flowing.

The TRIPLEX steel on the upper surface has formed macroscopic corrosion pits of different shapes (Fig. 2),

the other surrounding areas (90%) remained clear with a slight color appearance. On the lower surface were

not observed corrosion pits and metallic surface remained clean. TWIP steel was considerably and non-

uniformly corroded (over 90% of the above surface covered with corrosion products), after removing the

rusted areas there were observed numerous microscopic corrosion pits, usually shallow and oval in shape,

Fig. 3. On the reference steel C17 the similar corrosion type as on TWIP were observed, but to a lesser

extent. After their removal (by brushing), irregular cavities and microscopic holes were observed, with

different morphology compared to TWIP steel.

Fig. 2 Samples after removing of rust (TWIP, TRIPLEX, C17) Fig. 3 Small pits on TWIP steel

In metallographic polished section the non-uniform and pitting types of corrosion has been observed, Fig 4,

5. On TRIPLEX steel, the special undermining tape of corrosion has been found out, probably along series of

aluminum oxides. Regular (polyhedron) grains in structure has been observed after etching. To increase

corrosion resistance TWIP steels, some coatings are used in surface protection [2].

Fig. 4 Cross section of TRIPLEX after salt spray test Fig. 5 Metallographic section of TWIP steel after

Corrosion pit and attack along impurities exposure in salt fog. Non-uniform corrosion type

TWIP 32 x 34 mm

TRIPLEX

C17


4. CONCLUSIONS

The corrosion properties of high manganese steels TWIP (X70Mn22) and TRIPLEX (X100MnAl 28-12) were

tested and compared with carbon steel C17 as reference material. Based on the polarization tests in water

solutions with sodium chloride there were observed one order of magnitude higher corrosion rate of steel

TWIP (X70Mn22) compared with steel TRIPLEX (X100MnAl 28-12). Positive effect of aluminum for

increasing in corrosion resistance was also demonstrated by higher values of measured pitting potentials.

After salt spray test on TWIP steel the non-uniform corrosion was observed in the form of spots, small pits

and areas with different color shade, while on TRIPLEX steel were formed deeper and larger pits.

Gravimetric method confirmed higher corrosion of TWIP than TRIPLEX steel. Unacceptably high uniform

corrosion on both high manganese steels was found out in the sulfuric acid solution.

ACKNOWLEDGEMENTS

This paper was created in the project No. CZ.1.05/2.1.00/01.0040 "Regional Materials Science and

Technology Centre" within the frame of the operation programme "Research and Development for

Innovations" financed by the Structural Funds and from the state budget of the Czech Republic.

REFERENCES

[1] MAZANCOVÁ, E., New types of materials for automotive industry. Physical engineering properties of high strength

steels alloyed by manganese. VSB - Technical University of Ostrava, 2007, pp. 24-35. ISBN 978-80-248-1647-0.

[2] HAMADA, A. S, et.al., Corrosion behavior of high Mn-TWIP steels with electroless Ni-P coating. Acta University of

Oulu, C281, 2007, 6 p.

[3] GRAJCAR, A, Corrosion behavior of plastically deformed high-Mn austenitic steels. Journal of Achievements in Materials and Manufacturing Engineering, 43/1 (2010) 228-235.

[4] ASTM G-61 Standard test method for conducting cyclic potentiodynamic polarization measurements for localized

corrosion susceptibility of iron-, nickel-, or cobalt-base alloys, Annual Book of ASTM Standard, Vol.03.03, 2001, p.

223-227.

[5] ASTM G-59 Standard test method for conducting potentiodynamic polarization resistance measurements. Annual

Book of ASTM Standards, Vol.03.02, 2001, p. 237-239

[6] ČSN EN ISO 9227, Corrosion tests in artificial atmospheres – Salt spray tests. UNMZ, 2012, 23 p.

corrosion behaviour of selected high manganese...

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