Enhancement in corrosion resistance of austenitic stainless steels by surface alloying with nitrogen and carbon

Enhancement in corrosion resistance of austenitic stainless steels by surface alloying with nitrogen and carbon

Materials Letters 59 (2005) 3410 – 3413 www.elsevier.com/locate/matlet Enhancement in corrosion resistance of austenitic stainless steels by surface ...

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Materials Letters 59 (2005) 3410 – 3413 www.elsevier.com/locate/matlet

Enhancement in corrosion resistance of austenitic stainless steels by surface alloying with nitrogen and carbon Y. Sun * School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore Received 14 February 2005; accepted 3 June 2005 Available online 1 July 2005

Abstract In this work, attempts have been made to alloy the surfaces of austenitic stainless steels simultaneously with nitrogen and carbon in the glow discharge of a plasma at temperatures below 450 -C. As a result of such a low temperature hybrid treatment, a dual-layer structure is produced, which comprises a nitrogen-enriched layer on top of a carbon enriched layer. Both nitrogen and carbon are supersaturated in the austenitic face-centred cubic structure in the respective sublayer. Electrochemical corrosion tests have been conducted potentiostatically in a 3 wt.% NaCl aqueous solution to measure the anodic polarisation curves of the alloyed surfaces. The results show that this hybrid treatment can significantly improve the corrosion resistance of austenitic stainless steels by several orders of magnitude over a wide range of potentials. D 2005 Elsevier B.V. All rights reserved. Keywords: Stainless steel; Surface alloying; Nitrogen; Carbon; Corrosion

1. Introduction Austenitic stainless steels are a class of technologically important materials widely used in various sectors of industry. The problem associated with the poor tribological properties of these materials has recently been tackled by many investigators. These efforts have led to the development of two promising and industrial viable surface alloying techniques which do not compromise the good corrosion resistance of austenitic stainless steels. These include low temperature nitriding [1 –6] and low temperature plasma carburizing [7 –11]. The former is carried out at temperatures lower than 450 -C for up to several tens of hours, resulting in the incorporation of a large amount of nitrogen in the nitrided layer up to 20 Am thick to form an expanded austenite structure which is free from nitride precipitates. Such a low temperature nitrided layer possesses not only

* Fax: +65 6790 9081. E-mail address: [email protected] 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.06.005

a high hardness up to 1500 HV but also good corrosion resistance which is even better than untreated austenitic stainless steels [1,4,5]. Similarly, during plasma carburizing, which is carried out at temperatures between 400 and 500 -C, carbon, rather than nitrogen, is introduced into the surfaces of austenitic stainless steels, forming a precipitation-free, carbon supersaturated austenite layer up to 40 Am thick [8 –10]. The carburized layer possesses a lower hardness (yet still as high as 1100 HV at the surface) but larger thickness than the low temperature nitrided layer [10]. A hybrid surface alloying process has recently been developed by the present author to integrate the low temperature plasma nitriding and carburizing processes by introducing nitriding and carburising species to the plasma media to effect the simultaneous incorporation of nitrogen and carbon into the surfaces of austenitic stainless steels, with the purpose to form a hybrid structure characteristics of both the nitrided layer and the carburized layer [12]. As a part of this new development, electrochemical corrosion tests have been conducted in 3.0% NaCl aqueous electrolyte on austenitic stainless steels surface

Y. Sun / Materials Letters 59 (2005) 3410 – 3413

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alloyed by this hybrid process. This paper reports the results obtained and demonstrates that this hybrid process can produce a dual-layer structure which possesses superior corrosion resistance.

N C

2. Experimental procedure

Table 1 Chemical compositions of the austenitic stainless steels investigated (wt.%) AISI

Cr

Ni

Mo

Ti

Mn

C

316 304 321

19.23 18.45 18.78

11.26 10.54 11.04

2.67 0.00 0.00

0.00 0.00 0.45

1.86 2.00 1.91

0.07 0.07 0.10

20µm Fig. 1. Optical micrograph of the cross sectional morphology of the alloyed zone produced by the hybrid treatment on AISI 316 steel at 415 -C for 15 h, showing the dual layer structure comprising a nitrogen-enrich layer on top of a carbon-enriched layer.

All the tests were carried out at room temperature (24 -C), open to the air.

3. Results and discussion The alloyed zones produced on the three investigated steels by the hybrid treatment exhibit similar layer structure and morphology. Fig. 1 shows the cross sectional morphology of a typical alloyed zone on AISI 316 steel produced by the hybrid treatment. The corresponding nitrogen and carbon concentration profiles measured by GDS across the alloyed zone are given in Fig. 2. It can be seen that the alloyed zone comprises a dual-layer structure: the outer layer is rich in nitrogen and the inner layer is rich in carbon. The hybrid process thus successfully integrates the low temperature nitriding and carburising processes and results in the formation of an alloyed zone characteristic of these two individual processes. Both the N-enriched layer and the C-enriched layer are resistant to the etchant (50% HCl + 25% HNO3 + 25%H2O) used to reveal the microstructure of the substrate material, such that the two sublayers appear ‘‘bright’’ under optical microscopy. X-ray diffraction could not detect any nitride and carbide formation in the alloyed zone, instead two expanded austenite phases, gN and gC, were detected (Fig. 3). The former (gN) is obviously from the Nenriched layer and is similar to that produced by low temperature 35 AISI 316 415oC/15h

25

4 3

20 N

15

2

10

C %wt

30

N %wt

Three grades of austenitic stainless steel, AISI 304, 316 and 321, were used as the substrate materials. The chemical compositions of these steels are listed in Table 1. Disc specimens of 25 mm in diameter and 5 mm in thickness were cut from the as-received hot-rolled steel bars. The specimens were manually ground using SiC emery papers down to 1000 grade to achieve a fine surface finish. The hybrid plasma surface alloying process is described in detail elsewhere [12]. Briefly, the process is similar to that of individual plasma nitriding and plasma carburizing. The major difference between the hybrid process and the individual processes lies in that in the hybrid process, both nitrogen and carbon species are introduced into the plasma chamber. The processing temperature is sufficiently low to avoid nitride and carbide precipitation in the alloyed zone, which will otherwise deteriorate the corrosion resistance of the alloyed zone. Although this process has similarity with the conventional nitrocarburising process for ferrous alloys, it is carried out at much lower temperatures. In this work, the hybrid process was carried out in a DC plasma nitriding unit at temperatures between 380 and 430 -C for 15 to 40 h. The gas mixture used is 95% N2 + 5% CH4, where N2 acts as the nitriding species and CH4 acts as the carburizing species. The treated specimens were characterized by X-ray diffraction for phase identification, metallography for layer morphology and thickness examination, glow discharge optical spectrometry (GDS) for composition profiling, and microhardness tests for hardness measurements. Electrochemical corrosion tests were performed using an EG and G Parc three electrode electrochemical flat cell connected to an ACM GILLAC potentiostat equipped with a computer data login, requisition and analysis system. The specimen was clamped to the cell, sealing against a PTFE knife-edge gasket which expose 1.0 cm2 of the specimen surface to the electrolyte. The electrolyte used for the tests was 3.0 wt.% NaCl in deionised water. DC polarization was performed potentiodynamically and anodically. The anodic polarization curves were recorded with a sweep speed of 2 mVs 1.

C 1

5

0

0 0

10

20

30

40

Distance From Surface (µm) Fig. 2. Nitrogen and carbon concentration profiles measured by GDS across the alloyed zone shown in Fig. 1.

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Y. Sun / Materials Letters 59 (2005) 3410 – 3413

γN(111)

1000

AISI 316

316SS

o

750

γN(200)

Hybrid

500

E, mV/SCE

γC(111)

Intensity (arbitary unit)

415 C/15h

untreated

250 0

Nitrided

-250 40

45

50

55

60

Two-Theta (Degree) Fig. 3. X-ray diffraction pattern generated from the surface of the alloyed zone shown in Fig. 1.

nitriding, where nitrogen atoms dissolve in the face centred cubic (fcc) austenite lattice, causing lattice expansion and distortion [2,4]. Similarly, the gC phase arises from the underlying Cenriched layer, characteristic of low temperature carburising of austenitic stainless steels [8,9], where carbon atoms dissolve in the fcc lattice, also causing lattice expansion. The lattice parameter measured for the gN and gC phase is about 0.395 and 0.371 nm, respectively, as compared to 0.359 nm for the unalloyed substrate austenite phase. This registers a lattice expansion by about 10% and 3% in the N-enriched layer and C-enriched layer, respectively. It is also noted from Fig. 2 that although the lower part of the Nenriched layer is nearly free from carbon, quite a large amount of carbon is incorporated in the outer part of the N-enriched layer. This carbon incorporation obviously leads to further lattice expansion of the gN phase and contributes to the extremely high hardness (2000 HV0.025) measured for the hybrid treated surface, as compared to the individually nitrided (1500 HV0.025) and carburised (1100 HV0.025) surfaces. The morphology and thickness of the alloyed zone and the sublayers depend on processing conditions, such as gas composition, processing temperature and time. A general observation is that if the processing temperature is sufficiently low (< 430 -C) and processing time is not too long (< 40 h), a high quality dual layer structure can be produced without the formation of the harmful chromium nitrides and carbides. The thickness of the alloyed zones produced for corrosion tests in this work ranges between 20 and 30 Am. Electrochemical tests were first conducted to compare the corrosion behaviour of untreated, individually nitrided, individually carburised and hybrid treated AISI 316 stainless steel. The measured anodic polarization curves are shown in Fig. 4, from which several interesting observations can be made. First, as expected, the untreated AISI 316 stainless steel substrate suffers from an abrupt increase in current density at potentials above around 50 mV, due to the occurrence of pitting corrosion as confirmed by microscopic examination after the test. Second, both individual nitriding and carburising reduce the current density of the steel in the anodic region, indicating improved corrosion resistance, in agreement with previous observations [1,4,5,8]. No pitting corrosion was observed in the individually treated surfaces after testing up to a potential as high as 1000 mV, instead, generally corrosion was evident. This indicates that individual low temper-

-500 0.0000001 0.00001

0.001

0.1

10

1000

Current Density (mA/cm2) Fig. 4. Anodic polarization curves measured electrochemically for the untreated, low temperature nitrided (380 -C/40 h), carburized (470 -C/20 h) and hybrid treated (380 -C/40 h) AISI 316 stainless steel.

ature nitriding and carburising can improve the pitting corrosion resistance of austenitic stainless steels, but the treated surfaces suffer from relatively high general corrosion rates, as can be seen from the relatively high current density. After the hybrid treatment, the anodic polarization curve is shifted towards lower current density by several orders of magnitude as compared to those for the untreated and individually nitrided and carburised steel. This registers an improvement in corrosion resistance by several orders of magnitude and signifies the excellent corrosion resistance of the hybrid treated surface, which is in the passive state even at a potential as high as 1000 mV in the tested electrolyte. Microscopic examination after testing did not reveal any sign of pit formation and noticeable change in surface appearance of the specimen. Further electrochemical tests were conducted to compare the corrosion behaviour of the specimens hybrid treated at various temperatures below 450 -C. The temperatures were so selected that only gN and gC phases were formed without chromium nitride and carbide formation as determined by XRD. The results summarised in Fig. 5 show that all hybrid treated specimens show much improved corrosion resistance as compared to the untreated and individually treated AISI 316 steel. The specimens hybrid treated at 380 and 415 -C show similar polarization behaviour, charac1000 316SS 800 600

E, mV/SCE

35

Carburised

380oC/40h 415oC/15h 430oC/15h

400

untreated

200 0 -200 -400 0.0000001

0.00001

0.001

0.1

10

Current Density (mA/cm2) Fig. 5. Anodic polarization curves measured electrochemically for the untreated, hybrid treated AISI 316 stainless steel under various conditions.

Y. Sun / Materials Letters 59 (2005) 3410 – 3413

1200 1000

380oC/40h

3413

and better corrosion resistance as compared to those achieved by individual nitriding and carburising.

321SS 304SS

E, mV/SCE

800 600

4. Conclusions 316SS

400 200 0 -200 -400 0.000001

0.0001

0.01

1

100

Current Density (mA/cm2) Fig. 6. Anodic polarization curves measured electrochemically for the hybrid treated AISI 304, 316 and 321 steels under the same conditions.

terized by the absence of pitting corrosion and low corrosion current density. The slightly higher corrosion rate measured for the specimen hybrid treated at 430 -C is probably due to the formation of a small amount of chromium nitride in the nitrogen-enriched layer, which could not be detectable by XRD. Fig. 6 shows the polarization curves measured for the three grades of austenitic stainless steels listed in Table 1 after the hybrid treatment under the same condition. Although these untreated stainless steels exhibit different corrosion resistance in the tested solution (not shown) due to the difference in chemical composition, after the hybrid treatment, the treated surfaces of the three steels exhibit quite similar corrosion behaviour. It is thus evident that the hybrid treatment can be applied to all three investigated steels to achieve much enhanced corrosion resistance. Although the corrosion behaviour of low temperature nitrided and carburised stainless steels has been investigated by several investigators [1,4,5,8], the reason for the enhancement in corrosion resistance due to nitrogen and carbon supersaturation in austenite has not been fully understood. A possible mechanism is that supersaturation of nitrogen and carbon in austenite helps to improve the passivation ability of austenite, and this beneficial effect seems to increase with increasing degree of supersaturation [13]. Thus, the much enhanced corrosion resistance observed for the hybrid treated surfaces may be attributed to the extremely large supersaturation of the upper part of the nitrogen-enriched layer with both nitrogen and carbon. This would contribute to the observed higher hardness

A hybrid plasma surface alloying process has been developed for austenitic stainless steels, which facilitates the simultaneous incorporation of nitrogen and carbon into the steel surface at temperatures below 450 -C. This hybrid process results in the formation of an alloyed zone with a dual-layer structure: a nitrogen-enriched layer on top of a carbon-enriched layer, both being free from nitride and carbide precipitation. Such an alloyed zone possesses not only a very high hardness, but also much enhanced corrosion resistance which is much superior to that measured for the untreated stainless steels and the individually low temperature nitrided and carburized layers.

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