Enhancement of corrosion resistance of AISI 420 stainless steels by nitrogen and silicon plasma immersion ion implantation

Enhancement of corrosion resistance of AISI 420 stainless steels by nitrogen and silicon plasma immersion ion implantation

Surface & Coatings Technology 201 (2007) 4879 – 4883 www.elsevier.com/locate/surfcoat Enhancement of corrosion resistance of AISI 420 stainless steel...

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Surface & Coatings Technology 201 (2007) 4879 – 4883 www.elsevier.com/locate/surfcoat

Enhancement of corrosion resistance of AISI 420 stainless steels by nitrogen and silicon plasma immersion ion implantation Ricky K.Y. Fu a , D.L. Tang a,b , G.J. Wan a,c , Paul K. Chu a,⁎ a

Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b Southwestern Institute of Physics, Chengdu 610041, China c School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China Available online 14 August 2006

Abstract The effects of nitrogen and silicon plasma immersion ion implantation (PIII) on the surface corrosion resistance of martensitic stainless steel AISI 420 were investigated. Nitrogen was plasma-implanted at elevated temperature and diffused to form a thick and continuous nitrided surface layer, followed by silicon cathodic arc plasma implantation to produce an additional Si-rich oxynitride region near the stainless steel surface. X-ray photoelectron spectroscopy was used to delineate the elemental depth distribution and determine the layer composition and structure. Surface microhardness and corrosion measurements were performed using Vicker indentation and potentiodynamic polarization in a NaCl solution, respectively. The experimental results demonstrate the passivation behavior and dramatic reduction of the corrosion current after nitrogen and silicon PIII. Scanning electron microscopy results reveal differences in the pitting distribution and appearance suggesting that the improvement in the corrosion resistance arises from the change in the surface chemical composition due to the formation of a Si-rich region and oxynitride layer. The mechanism of the enhancement is discussed. © 2006 Elsevier B.V. All rights reserved. PACS: 68.55.Ac; 61.50.-f Keywords: Plasma immersion ion implantation; Corrosion resistance; Silicon; Elevated temperature nitriding

1. Introduction Ion implantation is an effective surface modification technique. Incident ions with high energy can result in the formation of a near-surface alloy of graded composition without an abrupt interface with the substrate. Such a graded surface layer can improve specific surface properties while the desirable properties of the substrate can be retained. This graded layer can be made quite thick (more than 1 μm) by using post-implantation annealing and can better withstand harsh conditions in the field. In contrast, conventional thin coatings are more prone to interfacial stress and film delamination. Thin films may also suffer from pitting and crevice corrosion if the film structure is not sufficiently robust and the adhesion between the film and substrate is not strong enough. In these cases, ion implantation

⁎ Corresponding author. Tel.: +852 27887724; fax: +852 27889549, +852 27887830. E-mail address: [email protected] (P.K. Chu). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.077

can be used to produce a stable layer without a discrete interface to more effectively combat corrosion. Conventional nitriding methods involve high temperature (close to the melting point of the bulk materials) to enhance nitrogen diffusion. However, some metallic alloys may degrade under high temperature. Their bulk properties may be affected and surface oxide may be formed to hinder diffusion. It has been reported that the efficiency of nitriding can be enhanced if the materials are treated using nitrogen plasma [1,2]. In this case, diffusion is promoted even at low temperature. Plasma nitriding has been applied to many metallic materials such as aluminum [3], iron [4], steels [5,6], and alloys [7], and improvements on both the mechanical and chemical properties have been achieved. For instance, Rossi et al. studied the nitrogen implantation effects on the adhesion and friction properties between polymers and mould steels [8]. Austenitic stainless steels after nitrogen treatment have been shown to form multiple nitride phases that resist surface corrosion [9–11]. Several studies on nitriding of martensitic steels also show the formation of nitrides and the enhancement of both the mechanical and chemical properties [12,13]. However, the

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effects of plasma nitriding and high-energy ion bombardment on martensitic steels at elevated temperature have not been widely studied. The surface properties can conceivably be enhanced further by introducing new elements such as silicon into the surface. Silicon is known to help prevent materials from general corrosion due to the formation of insoluble silicon oxide [14,15]. In this work, nitrogen plasma immersion ion implantation (PIII) was performed at 40 kV and elevated temperature followed by silicon plasma implantation to modify the surface properties and corrosion resistance of AISI martensitic 420 (AISI 420) stainless steel. As PIII is a non-line-of-sight process, industrial moulds that are made of martensitic steel and possess a complex shape can be implanted without complex beam rastering or sample manipulation [8]. 2. Experimental details The substrate was AISI 420 martensitic stainless steel. The materials were cut into 1 × 1 cm2 squares, polished, and cleaned before PIII. The samples were treated using different nitrogen PIII and silicon cathodic arc conditions (Table 1) to assess the efficacy of the treatment process. Prior to nitrogen PIII, the samples were subjected to argon plasma cleaning using 1000 W radio frequency (RF) power and a negative sample DC bias of − 500 V. Pure nitrogen gas was subsequently bled into the vacuum chamber to maintain a working pressure of 6 × 10− 4 Torr to conduct N-PIII [16]. The samples were pulse-biased to − 40 kV with a pulse width of 30 μs and repetition rate of 200 Hz. The specimens were placed on a 150 mm sample stage with temperature control and implantation was conducted at various temperatures. One set of the nitrogen-implanted samples subsequently underwent silicon ion implantation using a cathodic arc plasma source [17,18]. A silicon rod (cathode) with a diameter of 12.7 mm was pulse triggered to 2 to 3 kV and the current between the cathode and anode was about 70 A. An electromagnetic field was applied to the plasma transportation duct (duct bias of 20 V and magnetic field strength of 100 G). The sample was subjected to 25 kV pulsed high voltage which was in phase with the voltage pulses applied to the cathode so that the process was implantationdominant. The repetition rate of the trigger pulse was 25 Hz and the arc duration was about 250 μs. The silicon implantation time was 1 h and the treatment conditions and sample description are listed in Table 1. XPS analysis was performed in an ultra-high vacuum using a PHI-5600. The excitation source was monochromatic Al Kα radiation operated at 14 kV and 350 W and the detection angle

Fig. 1. Depth profiles of silicon and nitrogen implanted sample at substrate temperature of 300 °C. The implantation depth is calibrated using an approximate sputtering rate of 2.5 nm/min calculated from previous experiments.

was 45°. Because the surface of the stainless steel samples was not very smooth, crater depths were not measured. Instead, the sputtering rate was estimated to be 2.5 nm/min based on that of SiO2 measured under similar sputtering conditions. The surface microhardness was measured using Vicker indentation with the load of 5 N where 100 HV = 1 GPa. Each data point is the average of twenty measurements conducted on different parts of the specimen for statistical accountability. The potentiodynamic polarization corrosion tests were conducted in 0.1 mol NaCl solution for a scanning range from 20% below the corrosion potential Ecorr to + 1.5 V at a scanning rate of 1 mV/s. The surface topography was assessed by secondary electron microscopy (SEM). 3. Results and discussion Plasma implantation introduces nitrogen into the sub-surface of the steel sample and diffusion of nitrogen to a greater depth is

Table 1 Sample labels and treatment conditions Sample number

Nitrogen PIII

Silicon cathodic arc

Temperature (°C)

Implantation

#1 #2 #3 #4 #5 #6

Untreated Room temperature 300 400 500 300

No No No No No Yes

Fig. 2. N 1s core level XPS spectra acquired from samples #2 and #6 after argon sputtering for 10 min.

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Fig. 3. Si 2p XPS spectra obtained from sample #6.

accomplished at elevated temperature. The projected range of 40 keV nitrogen ions in steel is about 45 nm as determined by TRIM [19]. However, for sample #6 (Fig. 1), nitrogen diffusion takes place concurrently during implantation at a substrate temperature of 300 °C. It can be observed that the nitrogen distribution has extended to about 3 times of the projected range. Subsequent silicon implantation leads to more ion mixing and yields a high silicon concentration on the steel surface. Fig. 2 shows the XPS spectra of the N 1s core level acquired from samples #2 and #6 after argon sputtering for 10 min. The spectra can be deconvoluted into two components by applying Shirley background subtraction and Gaussian–Lorentzian peak fitting. The peaks at binding energies (BE) of 397.27 eV (sample #2) and 397.31 (sample #6) can be attributed to Cr–N and Si–N bonds. The components at higher binding energies of 398.54 eV (sample #2) and 398.17 eV (sample #6) can be assigned to different metal nitride states. Similarly, the Si 2p spectrum acquired from sample #6 (Fig. 3) can be resolved into 3 components. The first component at a low binding energy of 99.26 eV arises from Si–Si while the other components at 101.73 eV and 103.40 eV represent the Si–N and Si–O bonds, respectively. Thus, multiple silicon-based species are embedded in the steel matrix after silicon and nitrogen implantation.

Fig. 4. Microhardness of untreated and treated samples measured using Vicker indentation.

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The mechanical properties such as microhardness of the untreated and treated samples were measured and the results are displayed in Fig. 4. All the treated samples exhibit a marked increase in the microhardness from 573 HV to around 661727 HV. The observed increase is believed to be due to implantation-induced defects, dislocation effects, and newly formed nitrides hard phases as revealed by XPS. However, the surface hardness diminishes in general when the substrate temperature is increased as a result of reduced density of defects and change in the microstructures. It has been shown that metallic microstructures can be altered when steels are subjected to high temperature treatments and so the mechanical properties can be influenced [20,21]. The chemical properties of the samples were measured using the potentiodynamic polarization corrosion test. Fig. 5 plots the corrosion potential versus current obtained from the samples plasma-implanted at different substrate temperatures. All the treated samples exhibit improvement when compared to the untreated sample #1 as shown in Fig. 6. After nitrogen PIII, the nitride phases (shown in Fig. 2) are formed on the surface to provide higher corrosion resistance. On the other hand, implantation at high substrate temperature seems to have unfavorable effects in martensitic steels with regard to corrosion resistance. As shown in Fig. 5, samples implanted at substrate temperature over 300 °C exhibit lower corrosion resistant potentials. This degradation is similar to the trend of the mechanical properties as shown in Fig. 4. These unfavorable effects may be due to the degradation of the martensitic phases of steel worsening the mechanical properties and dispersing the newly formed nitride phases on the near surface under high substrate temperature implantation. As shown in Fig. 6, the sample implanted with nitrogen and silicon at 300 °C shows the biggest improvement in the corrosion resistance. The corrosion current is 5 orders of magnitude lower and the corrosion potential rises from − 500 mV to − 430 mV. One of the primary

Fig. 5. Potentiodynamic polarization corrosion curves of nitrogen implanted samples at different substrate temperatures.

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more homogenous state gives rise to smaller differences in the energies of the various microstructures. A fine distribution of coherent nitrides or oxynitrides coupled with the small size of some of the nitrides and the screening effect caused by the high dislocation density in martensite may be responsible for the palliated intergranular corrosion risks [22]. This modified surface layer accounts for the lower corrosion current in the wide anodic scanning potential range, and so pittings are not exacerbated in the plasma-implanted samples whereas the untreated surface undergoes intergranular corrosion. 4. Conclusion

Fig. 6. Potentiodynamic polarization corrosion curves of the two plasma immersion ion implanted (PIII) samples, (Si + N) PIII and N PIII as well as the untreated samples.

reasons for the better corrosion resistance is believed to be the formation of a silicon oxynitride layer on the sample surface which reduces the direct exposure of metallic iron to the corrosive medium. Fig. 7 shows the surface morphology of the untreated and (Si + N) plasma-implanted samples after the potentiodynamic polarization corrosion test at a maximum corrosion potential of 1.5 V. Fig. 7A of the untreated sample shows large area damage caused by obvious intergranular corrosion failure manifesting as crevicelike cracks alongside of the grains boundaries. A high corrosion rate thus results under the anodic potential. In contrast, the sample with the silicon oxynitride structure shows different surface morphology as illustrated in Fig. 7B. The results indicate that the surface has undergone typical pitting corrosion failure characterized by small and deep cavities, while only few small cracks can be observed adjacent to the large cavities. From this point of view, the corrosion failure of the (Si + N) implanted and untreated martensite stainless steel samples have different mechanism. Even though most of the steel samples suffer from pitting corrosion when they are exposed to corrosive media, the corrosion rate and development are different. It is generally accepted that in martensitic stainless steels, precipitation and a Cr depletion zone adjacent to the grain boundaries promote intergranular corrosion that can further evolve into stress corrosion cracking (SCC) under some circumstances. SCC is more serious than the more common pitting corrosion failure in many situations. In order to reduce the risk of SCC, efforts should be made to alleviate the galvanic effects caused by Cr depletion and precipitation. This can sometimes be accomplished by heat treatments although it may not be easy. Energetic ion bombardment via plasma ion implantation offers an alternative means to transform and yield a homogeneous distribution of chromium in martensite as well as a lower number of coarse second-phase particles. According to our XPS results, co-implantation of nitrogen and silicon does form secondary phases and change the microstructures of the martensite surface possibly into a more homogenous state in addition to the formation of new buried secondary phases. This

Nitrogen plasma implantation into martensitic steel results in the formation of nitride phases which have higher mechanical strength and are relatively stable in corrosive medium. However, ion implantation conducted at very high substrate temperature does not yield high microhardness and good corrosion resistance. The combination of nitrogen and silicon plasma immersion ion implantation gives rise to better corrosion resistance in martensitic steels as the surface silicon oxynitride layer reduces the degree of corrosion and materials loss.

Fig. 7. SEM micrographs of corroded samples: (A) untreated sample and (B) (Si + N) PIII (plasma immersion ion implanted) sample.

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Acknowledgement The work was financially supported by the City University of Hong Kong Direct Allocation Grant No. 9360110. References [1] X.B. Tian, Y.X. Leng, T.K. Kwok, L.P. Wang, B.Y. Tang, P.K. Chu, Surf. Coat. Technol. 135 (2001) 178. [2] X.B. Tian, S.C.H. Kwok, L.P. Wang, P.K. Chu, Mater. Sci. Eng. A 326 (2002) 348. [3] K.C. Walter, R.A. Dodd, J.R. Conrad, Nucl. Instrum. Methods, B 106 (1995) 522. [4] C.A. Straede, Nucl. Instrum. Methods, B 68 (1992) 380. [5] G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Surf. Coat. Technol. 104 (1998) 212. [6] W. Wang, J.H. Booske, C. Baum, C. Clothier, N. Zjaba, L. Zhang, Surf. Coat. Technol. 111 (1999) 97. [7] Chen K. Sridharan, J.R. Conrad, R.P. Fetherston, Surf. Coat. Technol. 50 (1991) 1. [8] S. Rossi, Y. Massiani, E. Bertassi, F. Torregrosa, L. Fedrizzi, Thin Solid Films 416 (2002) 160. [9] T. Czerwiec, N. Renevier, H. Michel, Surf. Coat. Technol. 131 (2000) 267. [10] H. Pelletier, P. Mille, A. Cornet, J.J. Grob, J.P. Stoquert, D. Muller, Nucl. Instrum. Methods B 148 (1999) 824.

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