improvement of corrosion resistance by plasma surface ... · improvement of corrosion resistance by...

Trends in Corrosion Research

Vol. 3,2004

Improvement of corrosion resistance by plasma surface modification Liru Shen1p2and Paul K. Chu19* '~e~artment of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong. *southwestern Institute of Physics, Chengdu, Sichuan, 610041, China

Abstract Corrosion is one of the prevalent causes of materials

failure and waste. The corrosion mechanisms can typically be divided into chemical, electro-chemical, and energetic particle induced corrosion. Plasma surface modification is a novel concept to combat surface corrosion by producing new components, phases, and structures on the materials surface to improve surface properties such as corrosion resistance, mechanical strength, electrical and optical performances, etc. This technique has many unique and attractive advantages compared with simple surface coating and electroplating and can be applied more extensively to different types of materials. This paper reviews the recent research activities and results with regard to the enhancement of corrosion resistance of materials by plasma surface modification techniques including plasma immersion ion implantation (PIII), ion beam enhanced deposition (IBED), and plasma electrolysis processing. The results are compared with those by conventional beam-line ion implantation. The improved surface properties achieved for materials such as Cr4Mo4V used in aerospace bearings, stainless steels, aluminum alloys, and titanium alloys are discussed in this paper.

1. Introduction

Corrosion, wear, or the combined effect of these

*Corresponding author. e-mail: [email protected]

destructive failure modes costs industries hundreds of billions of dollars each year. One of the effective means to mitigate damages due to corrosion and wear is to treat of the surface of the materials and plasma surface modification has thus attracted much attention. Plasma is a quasi-neutral particle system in the form of a gas or fluid-like mixture composed of fiee electrons and ions, fiequently also containing neutral particles (atoms, molecules). Plasmas are fiequently subdivided into low and high temperature ones. Low temperature plasmas (LTP) can be further categorized into thermal LTP (T, = Ti % T d 2x10~ K) and non-thermal LTP (Ti = T d 300 K, Ti << T, d lo5 K). Low-temperature plasmas are widely used in surface processing of materials such as etching, ion implantation, physical vapor deposition, plasma nitriding, ion beam assisted deposition, and so on.

During the last two decades, there have been tremendous developments in the field of surface modification including plasma surface modification processes developed in our laboratory [I-31. In this technique, plasma immersion ion implantation (PID), plasma nitriding, sputter deposition are conducted in the same multi-functional ion implanter that is equipped with gas plasma sources, metal plasma sources, gas-solid mixing plasma sources, evaporation sources and sputtering sources to meet the needs of various surface treatment techniques catering to a variety of components made of different materials. Alternatively, the plasma electrolysis process has been developed based on electrolysis and anodic oxidation to

Lim Shen & Paul K. Chu

yield a ceramic or metal coating to improve the surface properties on aluminum alloys, titanium alloys, tantalum alloys and steels [4]. All these techniques are used to enhance the surface properties of materials such as wear resistance, corrosion resistance, mechanical strength, as well as electrical and optical characteristics. In this paper, we will review recent progress pertaining to the improvement of corrosion resistance by plasma surface processing.

2. Improvement of Corrosion Resistance by Ion Implantation

2.1 Plasma Immersion Ion Implantation

Plasma immersion ion implantation (PIII) is an advanced technique for surface modification of materials. A typical PI11 system consists of a vacuum chamber with a sample stage, plasma sources, pulsed high-voltage power supply, and vacuum system. Fig. 1 depicts the multi-functional PIII instrument in the City University of Hong Kong [2,3,5]. The instrument has a radio-frequency (RF) plasma source, a filament discharge plasma source, four metal arc plasma sources and a sputtering electrode, so that single or multiple implantation and deposition steps can be performed in succession without breaking vacuum in the same equipment. In PIII, the component is immersed in plasma and biased by a pulsed negative voltage. This negative bias creates an ion sheath around the surface, and thereby accelerates ions from the plasma towards the substrate surface. Plasma immersion ion implantation thus does not suffer from the line-of-sight restriction inherent to beam-line ion implantation and overcomes the retained dose problem [6-141. Plasma immersion ion implantation & deposition (PIII&D) is a hybrid versatile technique encompassing both ion implantation and deposition. The process is suitable for the synthesis of thin films and modification of thesurface properties of materials and industrial components. Unlike conventional beam-line ion implantation, PIII-D excels in the processing of large and irregular specimens. This burgeoning technology has found many applications spanning the enhancement of aerospace and industrial

Fig. 1: Schematic of the multi-purpose plasma immersion ion implantation equipment in the City University of Hong Kong: (1) rf antenna system; (2) sputtering electrode system; (3) shutter; (4) rf plasma chamber; (5) rf generator; (6)gas inlet; (7) MEVVA plasma sources; (8) trigger power supply; (9)arc power supply for MEVVA plasma sources; (10) filament; (11) filament power supply; (12) discharge bies power supply; (13)HV target stage; (14) workpiece; (15) turbo-molecular pumping system; (16) gate valve; (17) vacuum chamber.

components to the fabrication of new materials used in the microelectronics industry.

2.2 Improvement of Corrosion Resistance of High Cr-Content Steels

2.2.1 Modification of AISI304 steels

A number of different techniques are used in industrial production such as salt bath nitriding,

Improvement of corrosion resistance by plasma surface modification 43

nitriding from the gas phase, and carburizing. Nitrogen incorporation is broadly used to harden the surface of metallic components, particularly steel parts. Plasma immersion ion implantation (PIII) and plasma nitriding are relatively new and effective surface modification techniques. Their enhancement mechanisms are quite different, and each one possesses distinct advantages and drawbacks. PI11 is a non-thermal, non-equilibrium, physically-driven, and ballistic-alloying process, whereas plasma nitriding is a thermally-driven, equilibrium, and diffusion-based process. Both of these techniques have been adopted to improve the surface properties of AISI304 stainless steel [15-221.

The experiments described here were carried out in the multi-functional PI11 instrument in the City University of Hong Kong. Low-voltagelhigh-voltage PI11 at low and elevated temperatures, hybrid elevated-temperature, low-voltagehigh-voltage nitrogen plasma immersion ion implantation were researched to improve the surface erosion resistance of AIS1304 stainless steels [19,21]. The composition of the AISI304 steels is: 19wt%Cr, 9wt%Ni, O.OSwt%C, 2.0wt%Mn, l.Owt%Si, 0.045wt%P, and 0.03 wt%S (the balance being Fe).

The results acquired by X-ray diffraction indicate that a new phase, expanded austenite v ' (Fe, Cr, Ni)N, is formed in the surface layer by means of PIII at low voltage and medium temperature of 300-350°C [23,24], but no expanded austenite v N appears at low temperature [25]. This new phase is different from the main component of E -Fe3N and (Fe, Cr)2N formed by nitridation. The new patterns are typical of the high nitrogen f.c.c. phase with a series of broad peaks (labeled v N) left of each austenite peak. The peak shift and modified layer thickness depend on the processing parameters, e.g. temperature and incident ion dose [7] as shown in Fig. 2. The subsequent high-voltage implantation process increases nitrogen incorporation and lattice expansion, but they do not increase linearly with the implantation dose. Elevated-temperature implantation can facilitate thermal diffusion and increase nitrogen retention in the substrate, and the lattice expands linearly with

increasing treatment time (dose). It is found that the surface micro-hardness of all the treated samples is enhanced significantly, especially for the low-voltage, elevated-temperature process which poses an increase of 1.5 [26]. In addition, a thicker and smoother modified layer is formed on the sample surface, and it is in contrast with the results obtained by low-temperature PIII. Among the low-voltage samples, a higher implantation dose leads to better wear resistance. The fXction property of the high-voltage sample is superior to that of the low-voltage sample. It can perhaps be attributed to nitrogen solution-strengthening and a greater compressive stress induced by the higher bombardment energy. The medium-temperature PIII process gives rise to a wear resistance enhancement of a maximum of 11.25 times. The degree of improvement is quite substantial.

After the treatment, the equilibrium potential in 3 wt. % NaCl solution increases from -524 mV to -200 mV, the corrosion current at the equilibrium potential decreases from 1 pA / cm2 to 0.06 pA / cm2, and the dissolution current is very low, and as shown in Fig. 3, the outcome depends on the treatment temperature and implantation dose. The sample treated by hybrid low-voltage / high-voltage nitrogen PI11 at elevated-temperature possesses the highest corrosion potential and pitting potential as well as the smallest dissolution current. The morphology of samples after the corrosion test is exhibited in Fig. 4 which indicates that the untreated samples have been severely eroded in the corrosion test. The corrosion pits have dimensions of a several hundred pm's, but the pits are relatively small and less visible on the samples treated using the high-voltage, high-dose, elevated temperature process. The pit shape depends on the surface properties. The higher the dose, the more corrosion resistant is the surface and the smoother the pits.

The analysis of SIMS indicates the enrichment of the oxygen due to residual oxygen recoiled into the substrate by energetic ion bombardment during plasma immersion ion implantation, particularly during high-voltage implantation. The improvement of corrosion resistance can be attributed to not only the effects of oxygen and nitrogen on the dense chromium

44 Liru Shen & Paul K. Chu

Fig. 2: Glancing XRD patterns acquired from four AISI304 samples: (1) 4 kV, 6 x 1 0 ' ~ ~ + - c m - ~ , 350°C; (2) 25 kV, 3 . 5 ~ 1 0 ' ~ ~ - c m - ~ , < 100°C following process (1); (3) 25 kV, 7 . 0 ~ 1 0 ' ~ ~ - c m - ~ , < 100°C following process (1); (4) 25 kV, 10.4x10'~ ~ - c m - ~ , <lOO°C following process 1.

Current ( pAJcm2 )

Fig. 3: Potentiodynamic polarization curves acquired from the AISI304 samples with a scan rate of 30 mV/min in 3 wt.% NaCl solution: (1) 4 kV, 6 x 1 0 ' ~ N+-cm-', 350°C; (2) 25 kV, 3 . 5 ~ 1 0 ' ~ ~ - c m - ~ , < 100°C following process (1); (3) 25 kV, 7 . 0 ~ 1 0 ' ~ N-cm-', < 100°C following process (1); (4) 25 kV, 1 0 . 4 ~ 1 0 ' ~ N-cm-', <lOO°C following process 1.


Fig 4: SEM micrographs showing the surface morphology after electrochemical corrosion test: (a) untreated sample, (b) 25 kV, 10.4~10" ~ - c m - ~ , <lOO°C following processing (1) described in Fig. 2.

oxide protective film but also probable structural thicker expanded austenite layer, increases the change such as amorphous layer formation. thickness of the chromium oxide containing layer, Consequently, the hybrid treatment process velds a enriches the amount of nitrogen and some of oxygen in

Liru Shen & Paul K. Chu

the top surface, and probably forms the amorphous layer. These increase the surface hardness and improve significantly the corrosion and wear resistance of AISI 304 stainless steels.

2.2.2 Modification of 9Crl8 steels

9Cr18 steels are often used in the aerospace industry as bearing materials as they possess good corrosion resistant properties. In aeronautic applications, bearings must be able to tolerate very harsh working conditions and unforgiving environment. They must be able to sustain a high continuous rotational velocity while carrying a heavy load in a surrounding medium containing oil, water, and corrosive species comprising S, C1, Na, K, Ca and other deleterious elements. The main failure mechanisms are surface wear and corrosion [27].

The typical chemical composition in wt% of 9Cr18 is: C-0.96, Cr-17.8, Si-0.8, Mn-0.72, P-0.035, S-0.03 and Fe-79.655. Because of the difference in chemical compositions, different treatment processes should be employed to improve the surface properties of different kinds of materials. The processing temperature ought to be controlled to be less than 200°C during ion implantation due to the lower tempered temperature of tearing materials. Plasma nitriding is thus not a suitable technique. Because of the high Cr content of 9Cr18 steels, nitrogen ion implantation has been proposed to be a good technique to enhance the surface properties [28]. Nitrogen PI11 with two plasma excitation conditions, hot electrons emiffed from

filaments and RF induction, has been performed [29]. In these experiments, the plasma is generated by two methods: low pressure gas discharge by hot electrons emitted from heated filament and radio-frequency glow discharge. The implantation parameters are: RF frequency = 13.56 MHz, RF power = 500kW, implantation voltage = 32 kV, pulse width = 30 us, pulse repetition rate = 300 Hz, nitrogen pressure =

4x10-' Pa, implant dose = 2x10'~ at/cm2 and 4x10'' at/cm2 for each process. The microhardness and pin-disk test results are displayed in Table 1. The wear property is improved considerably after nitrogen PI11 and as expected, a higher implantation dose gives rise to a higher microhardness, a larger cut - through cycle and a lower fiction coefficient.

The results of anode polarization test performed on five 9Cr18 samples in 0.1 M NaCl buffer solution and 0.1 M &So.$ solution are shown in Tables 2 and 3. It can be observed that the corrosion potentials of the implanted samples are higher than that of the untreated sample and the corrosion current densities of the implanted samples are lower than those of the untreated sample both in NaCl and solutions. The corrosion resistance of the samples treated by PI11 with RF excitation is slightly better than that with filament gas discharge, but the difference is not very large. The data demonstrate unambiguously the improvement of the surface chemical stability and wear resistance of 9Cr18 steels after PLU and different plasma excitation methods do not affect the electrochemical corrosion curves significantly. The XPS Cr 2p and N 1s results as well as AES depth profile measured from the

Table 1: Microhardness and cut-through cycle numbers of the five samples [29].

dose (a t -~m-~)

0 2x l0l7 (rf) 4x10'' (rf) 2x 10" (fila.) 4x10'' (fila.)

Microhardness

(HV) 430 577 598 490 550

Fiction coefficient 0.8-0.9 0.2-0.3 0.15-0.2 0.2-0.3 0.1-0.2

Cut-through cycle numbers 450 3260 9600 4200 10800


Table 2: Results of electrochemical corrosion test in NaCl solution [29].

Table 3: Results of electrochemical corrosion test in HzS041 solution [29].

0 2x l0l7 (rf) 4x10'~ (rf) 2x10'~ (fila.) 4x10" (fila.)

nitrogen-implanted 9Cr18 samples demonstrate that fine dispersion of CrN and Fe16N2 hard phases are formed in the deeper region, and dense Cr203, Cr(OH)3 films are formed near the surface due to residual oxygen in the processing vacuum chamber. This treatment process is generally effective to enhance the corrosion and wear resistances of high Cr-content steels such as 2Cr13 [30] that is commonly used to fabricate the pistons in oil pumps because of its good heat resistance and other beneficial properties.

resistance (kQ) 5.348 85.09 129.30 111.90 128.30

N+ dose (at-cm-')

0 4x10" (rf) 4x10" (fila.)

2.3 Improvement of corrosion resistance of low Cr-content steels and Cr-free steels

2.3.1 Modification of Cr4Mo4 steels

- Corrosion current density (@/crn2) 4.060 0.2552 0.1680 0.1940 0.1692

Corrosion resistance

@a

0.4499 60.58 60.08

The composition of Cr4Mo4V steel is: C-0.74, Cr-3.93, Mo-4.28, V-0.99, Mn-0.15, Si-0.15, P-0.025, S-0.03 and Fe-89.705. Due to the lower Cr content in the Cr4Mo4V steel, [Cr+N'], [Cr+Mo+] and

Corrosion potential (V) -0.3703 -0.2953 -0.2104 -0.2497 -0.2925

[c~+Mo++@] ion beam enhanced deposition (IBED) is conducted to improve the corrosion and wear resistance [31]. The deposition of chromium by sputtering is followed by the implantation of nitrogen at 50 kV, and the implantation dose is 3x 1017 in the [ ~ r + @ ] and [c r+~o++N' ] processes. The implanted Cr dose is -2x10I7 In the [c~+Mo+] process, Cr deposition is first conducted and then the Mo ions are produced from four MEVVA plasma sources and implanted into the samples at 50 kV. The Mo dose is -1 .5x10I7 cm-2. The purpose of Mo implantation is to alleviate corrosion stemming from C1 as the formation of molybdic acid radicals and molybdenates ties up free chlorine atoms and stop their penetration.

Corrosion current density

(Wcm2> 21.13 0.1792 0.1807

The anode polarization curves measured from the Cr4Mo4V samples in 0.5 M H2S04 and 0.1 M NaCl reveal that the surface corrosion and point-corrosion resistance are much improved compared with the untreated samples (Fig. 4), but similar results are obtained for all the treated samples. The results of the salt fog test indicate that the corrosion resistance of the three treated samples is drastically better than that of the untreated sample and the anti-conosion effectiveness of the [Cr+N] sample is slightly better among the treated samples. The coefficients of fiction of the treated Cr4Mo4V samples are found to decrease fiom -0.7 to -0.2. The needle-dish wear test also indicates that the wear resistance of all the treated Cr4Mo4V samples is improved by about 200 %.

Corrosion potential (V)

-0.074 +0.035 +0.075

2.3.2 Modification of 45' steels

Similarly, 45' carbon steels that are commonly used

Liru Shen & Paul K. Chu

18- 16- 14- 12- lo - 8 - 6 - 4 - 2 - 0 - -2- -4- -6

Fig. 5: Anode polarization curves of Cr4Mo4V samples in: (a) 0.5 M NaCl buffer solution and @) 0.5 M H2SO4 (1- untreated, 2- PIII).

in piston columns, piston sleeves and ball valves of oil pumps, precision gears, and so on in the industry can be treated by nitrogen ion beam enhanced deposition of Cr or Ti to enhance the corrosion and wear resistance [32, 331. After the treatment, the microhardness of the samples increases by 2 times, the mass loss due to wear decreases by a factor of 4, and the coefficient of friction is reduced by a factor of 2.3. The anode polarization curves of the treated samples show higher potentials and lower anodic current density. In the salt fog test, the untreated sample severely rusts after 30 min

whereas the rusted area on the treated samples is only 4% after 8 hours. These results corroborate that the corrosion resistance of 45' steels can be significantly improved by nitrogen ion beam enhanced deposition of Cr or Ti.

2.4 Improvement of the corrosion resistance of titanium alloys

Titanium and titanium alloys have three important advantages: low weight, good mechanical properties and chemical stability [34] and so they are increasingly used in the automotive, aerospace and medical fields [35]. In medical applications, Ti6A14V materials are often used as temporal and permanent implants. However, acidity in the mouth can vary greatly from near neutral to strongly acidic, some times reaching pH values below 2 at which the titanium undergoes enhanced corrosion reducing the lifetime of the implants and releasing titanium into the body. Therefore, deposition and Ar ion beam enhanced deposition of niobium on the titanium surface are used to improve the corrosion resistance [36].

The deposition of niobium is conducted by sputtering, followed by B E D with Ar. The high voltage pulses of 40 kV with a duration of 10 k and a repetition rate of 100 Hz are applied to achieve a total dose of 10" icm-2 during Ar bombardment. The niobium layer thickness is 10 - 20 nm. The temperature of samples is less than 100°C during B E D and the niobium layers are observed to be smooth and the grain sizes of niobium are on the order of 1-2 Pm.

After the pure deposition process, the corrosion potential of the sample surface increases and the corrosion current decreases by two orders of magnitude, as shown in Fig. 6. The native titanium oxide at the interface between the substrate and the layer apparently does not influence the corrosion properties negatively. The addition of an Ar PI11 step increases the adhesion properties and results in a very porous and dendritic protrusions structure with a grain size of some 3-5 P m, which is a very effective trapping center for residual oxygen from the plasma and the vacuum system. However, no beneficial effects on the corrosion


Nb dep. + Ar Pll l - Ti6AI4V

corrosion current (~1cn- t~)

Fig. 6: Polarization curves measured in 5 N NaCl solution against NHE acquired from untreated and treated titanium and Ti6A14V samples [361.

properties can be observed in contrast to pure niobium deposition [36]. These results indicate that niobium deposition onto titanium provides corrosion protection of dental implants, especially when combined with Ar PI11 for interface mixing and enhanced adhesion properties.

3. Improvement of corrosion resistance of material by electrolytic plasma processing

3.1 Electrolytic plasma processing

Plasma electrolytic processing (PEP) originates from electrolytic processes, which includes an anodic process such as plasma electrolytic oxidation (PEO) [37-391 and a cathodic process such as plasma electrolytic saturation (PES) [40]. The PEO process, also called microarc oxidation (MAO), micro-plasma oxidation, anode spark electrolysis, plasma electrolytic anode treatment, or anode oxidation under spark discharge, essentially combines electrochemical oxidation with a high voltage spark treatment. The coatings synthesized by the PEO process on valve metals (such as Al, Mg, Ti, etc.) possess superior

physical and chemical properties such as the excellent wear, friction, corrosion, electrical properties and thermal properties. Consequently, it has been applied to textile machine components, aerospace components, engineering equipment components and biomedical devices. The PES process can be applied to cleaning of steel surface and coating deposition [40]. The materials can be either immersed in the electrolyte or dripped with the electrolyte as shown in Fig. 7. Despite some differences in the materials that can be treated, the processing parameters and the results obtained, these techniques have some common characteristics. Electrolysis is conducted by application of different electrical potentials between the sample and a counter-electrode in a liquid environment and the process is based on the production of an electrical discharge on the sample surface.

3.2 Improvement of corrosion resistance of aluminum alloys by microarc plasma oxidation

3.2.1 Al2O3 coatings

Microarc plasma oxidation (MAO) can form quality


Sample Plasma envelope

Counter electtode with holes

Samles (Cathode)

Counter electtode and bath

(a) @)

Fig. 7: Schematic of plasma electrolysis processing of materials.

ceramic coatings on Al, Mg, Ti Nb, Ta and their alloys to enhance the surface physical and chemical properties including microhardness, wear resistance and corrosion resistance. In the MA0 process, the materials are usually immersed in the electrolyte as shown in Fig.7 (a). Based on the results described in reference #41, an A1203 coating with a thickness of 250 pm on BS A6082 aluminum alloy substrate (nominal composition of the alloy is: 1.3% Si, 0.5% Fe, 0.1% Cu, 0.4-1% Mn, 0.6-1..2% Mg, 0.25% Cr, 0.2% Zn, 0.1% Ti and A1 balance) possesses excellent corrosion resistance in the solution as well as good microhardness and abrasive wear resistance. The electrolyte is prepared from a solution of sodium silicate (2-10 g/l) in distilled water with addition of KOH (1-2 g/l) to adjust the pH and conductivity. The electrolyte temperature is between 70-80°C during the process. A bipolar AC voltage pulsed at a frequency of 50 Hz is applied. The voltage is between 400 and 600 V during the positive half cycle and -200 V in the negative half cycle. The predefined current density is 100 m ~ l c m ~ . The A1203 coating consists of phases comprising a- Alz03 and y-A1203. An amorphous and nanocrystalline inner layer and a nanocrystalline intermediate layer are formed in the coating.

The corrosion test results acquired after immersion in

a 0.5 M NaCl solution for 1 h, 1 day and 2 days indicate that the corrosion potential and resistance, respectively, increase from -692 mV and 3.25~10' 01cm~ for the aluminum alloy substrate to -185 mV and 2.21~10' ~ l c m ~ for the alumina coating. With increasing immersion time, the corrosion current of the alumina coating does not increase significantly, but the corrosion current of the uncoated samples increases drastically after several days of immersion in the corrosion solution, as shown in Table 4. Therefore, the Alz03 coating can enhance the corrosion resistance of A1 alloys in the solution, and it is even considerably better stainless steel. In addition, the surEace microhardness can be as high as 18 GPa and the maximum cross-sectional hardness is typically as high as 23 GPa [42]. The structure with nano-sized grains can improve the strength and ductility of the coating. Consequently, the synthetic corrosion resistance of the A1 components is enhanced and the components can tolerate harsh working conditions.

3.2.2 Incorporation of MA0 with other plasma processing for aluminum alloys

A1203 coatings consisting of a higher content of a-A1203 have a relatively high fiction coefficient (0.6-0.9) against WC-Co and AISI 52100 in dry sliding


Table 4: The results of corrosion test A1203 coating in in 0.5 M NaCl solution.

rests but serious pitting corrosion can occur due to micro-pores in the coatings [42,43]. Therefore, deposition of duplex A1203/DLC coatings [41] and duplex MA0 with magnetron sputtering (MS) A1203/TiN coatings [44] can decrease the friction coefficient and enhance the corrosion resistance. The latter has been canied out in the MA0 treatment device and the PSII-IM device [45].

Samples are made of LY2 aluminum alloys and they consist of aluminum as a base and approximately 2.6-3.2% Cu, 2.0-2.4% Mg, 0.45-0.7% Mn and <0.8% impurities. They are subjected to micro-arc discharge oxidation in an alkaline aqueous electrolyte of primarily sodium tungstate and ti-sodium phosphate (Na2W04 12H20: Na3P04 12H20 =1:12). During deposition of the TiN film on the MA0 treated samples, a titanium sputtering target is used and the working pressure is 8x los2 Pa. The ratio of argon to nitrogen is 7:l and the sputtering current is 2 A. The samples are biased with a DC potential of -200 V, the deposition time is 45 minutes, and the substrate temperature is less than 200°C. The thickness of the TiN film is approximately 2 pm.

The friction coefficient of the MAOMS treated sample is almost constant at a value of approximately 0.2 throughout the test. The cross-sectional profile of the wear track after 180 minutes shows that the wear track is the biggest with a depth of 12 pm and width of 1.2 mm on the LY12 Al-alloy substrate. It is the smallest with the depth of 1.6 pm and width of 0.3 mm

on the MAOMS treated sample.

The polarization curves acquired from the potentiodynamic polarization method are plotted in Fig. 8. The free corrosion potential of the MA0 coated sample is higher than that of the Al-alloy substrate but lower than that of the duplex MAOMS coated sampIe. The free corrosion current for the Al-alloy substrate linearly increases with the applied voltage due to the dynamic solvation process, but the free corrosion current density for the coated samples is reduced significantly at the same applied voltage in comparison with that of the Al-alloy substrate. This implies a lower corrosion rate of the coated samples and furthermore, a passive process is observed. The passive process raises the free corrosion potential of the coated samples. It can also be observed that pitting corrosion occurs on the coated samples since there are many micro-pores in the coatings. The salt spray test indicates that corrosion occurs after 48 hours, 312 hours and 504 hours for the Al-alloy substrate, MA0 and duplex MAOMS coated samples, respectively. The results are consistent with the electrochemical measurements.

After salt spray tests in 3.5% NaCl solution @H = 7), the pitting holes are observed to be regular, small and deep on the treated samples, but the pitting holes are irregular, big and shallow on the Al-alloy substrate. It discloses that the corrosion processing is preferentially passed onto the Al-alloy substrate where pits initiation occurs on the coating surface on the treated samples

52 Lim Shen &Paul K. Chu

Fig. 8: Polarization curves of samples in 3.5% NaCl solution: (1) substrate; (2) MA0 coating with a thickness of 2 pm; (3) MA0 coating with a thickness of 2 pm +TIN coating; (4) M A 0 coating with a thickness of 10 pm coating: (5) MA0 coating with a thickness of 10 pm+TiN coating.

and corrosion develops and propagates along the improvement of the corrosion resistance. boundary of the aluminum grains in the Al-alloy substrate sample. 3.3 Microarc plasma oxidation of titanium alloys.

The TiN film produced by magnetron sputtering exhibits good adhesion with the M A 0 alumina thin layer. The Vicker's microhardness increases from HV 78 to HV 150. The friction coefficient has a low value of 0.2, and this low value is retained for a long time in the pin-on-disc wear test. The potentiodynamic polarization measurement indicates that the MAOMS coated samples have the highest free polarization potential (-350 mV) and lowest corrosion current as well as a passive regime. In the salt spray test, the longest time (504 hours) is achieved for the MAOMS coated samples. SEM micrographs disclose that pitting corrosion is the prominent form of corrosion on the treated samples. Moreover, the alumina grains and TIN film produced on the coated samples as well as the micro-pores in the MAOMS coatings being partially or l l l y covered by the TiN films can account for the

The corrosion properties of BaTi03 films prepared by microarc oxidation at an anodic voltage of 90 V for 55OC have been investigated [46]. Barium titanate is an important functional material in the electronics industry on account of its superior dielectric, ferroelectrics piezoelectric, pyroelectric, and electro-optical properties. The microstructure of the film fabricated by M A 0 resembles a crater and the large-grain cubic BaTi03 film is about 10 pm thick. The open-circuit potential measured from the BaTiO3 film up to 1 h in a deaerated 0.1 M NaOH solution is very stable within the range of 0.01-0.02 V, but that of the control Ti sample shows a large variation from -0.35 to 0.02 V. Fig. 9 shows the polarization curves of the BaTi03 films and Ti substrate in a 0.1 M NaOH solution acquired by potentiodynamic polarization tests at a scan rate of 10 mV1min. The pure titanium


Current Density [~lcm']

Fig. 9: Current density versus potential curves of BaTi03/Ti, TiOZ/Ti, and Ti in 0.1 M NaOH solution by potentiodynarnic polarization measurements using a scan rate of 10 mV/min [46].

sample shows a passivation effect at a nearly 0-0.4 V, and then the current increases when the potential exceeds 0.4 V. The anodic current density is approximately 3 ~ 1 0 - ~ A/cm2 at 0.4 V and it increases gradually from the corrosion potential. The free corrosion potential increases from 4 . 1 4 4 V for pure titanium to 0.02 V for BaTiO3, whereas the corrosion current decreases from 15.43 W c m 2 for pure titanium to 0.38 for BaTiO3. These results show considerable improvement on the corrosion resistance of BaTiO, films compared to pure titanium in highly alkaline NaOH solutions.

The formation of an oxidized film on Ti6A14V alloy by AC microarc oxidation in different electrolytes is shown in Table 5. The oxidized films have a favorable combination of properties including thickness of 50-60 pm, hardness of 575 kg/mm2, as well as high adhesion and a low wear rate (3.4~10-' mm3/Nm). However, they possess a relatively high fiction coefficient of p = 0.6-0.7 against steel [47]. The free corrosion potentials of the MA0 films after 1 hour exposure in different solutions all increases compared with the substrate as shown in Table 6. This oxide

film shows good corrosion resistance in different corrosion solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate (Fig. 10). The SiOz/Ti02-based films with thickness of 70-90 pm and high bulk porosity produced from silicate and silicate-aluminate electrolytes demonstrate better corrosion behavior in H2S04 solution due to the greater chemical stability of the film phase components.

3.4 Electrolytic plasma cleaning and deposition on steel surface

Electrolytic plasma processes can be used to modify the surface properties of steels in the industry. A method of cleaning and deposition of metal coatings on steel surfaces for corrosion protection has been studied [48]. Low carbon steels (AIAI 1010) are selected as the substrate materials in the experiments. The samples are used as the cathode in PEP by dipping in an electrolyte of NaHC03 for cleaning and ZnS04 or ZnS04 + A12(S04)3 for deposition of the Zn or Zn+Al coating, respectively. The stainless steel anode and Zn anode are, respectively, used as anode in cleaning


Table 5: MA0 treatment processes and film thickness

[471.

concentration

Table 6: Free corrosion potentials after 1-h exposition of MA0 films in different solution [47].

1

2

3

4

5

1 solution

(dl) Na2Si03: 150; KOH: 2-4 Na3P04:13-15

KA102:10-15; KzSO4: 3-4 KA102: 10-15; NazSi03: 5

KA102: 25-30; Na3P04: 4-5

Corrosion solution 0.5 N HzS04 3.5% NaCl Physiological

and deposition of Zn for the study of the difference between the effects of two kinds of anodes.

After electrolytic plasma cleaning, the outermost surface layer shows a fine microstructure with a grain size of 10-20 nn? that may be formed by quenching effects during freezing of locally melted surface material. The corrosion potential in the background steel in tap water is at about -590 mV, while for

Ti02 (rutile), a-SiOz

A12Ti05, Ti02 (anatase and rutile), Some

of A1203, Alp04

Ti02 (anatase and rutile)

Ti02 (rutile), A12TiO5, a-SiO2 --

Ti02 (rutile), A12Ti05, Some of

Al203, Alp04

EPP-cleaning steel, it is significantly higher (at about 50 mV) than that of the background steel during the entire testing period. It is consistent with the formation of a clean, pure iron surface layer with no presence of FeIC microgalvanic cells. The anodic polarization behavior of the PEP-cleaned steel in tap water exhibits a more stable corrosion potential compared to the background steel. The corrosion rate of the PEP cleaned steel is significantly lower (by a

90

5-7

2.5

60-70

50-60

Pure Ti 1 1 2 0.46 -0.19 -0.15

-0.19 -0.21 -0.30

0.62 0.05 0.03

3 0.28 -0.03 -0.04

4 0.62 0.12 0.08

5 0.28 0.01 0.06


factor of 2.5) than that of the background steel. In the 3.5% NaCl solution, a similar c~rrosion rate is obtained for both types of steel surfaces. However, the corrosion potential difference remains at the same levels as in tap water indicating that even in the NaCl solution, the PEP-cleaning steel surface is thermodynamically more stable than the background steel. In tap water, the Zn and Zn-Al coatings and hot dip galvanized coatings exhibit activated polarization and corrosion rates comparable to those of the PEP coatings and possess a little higher corrosion potential.

1$ In the NaCl solution, the behavior is along the same

lo4 1 ~ 1 r n 3 line with the PEP coatings exhibiting somewhat higher corrosion potential. The hot dip galvanized coating exhibits a little lower corrosion rate. Table 7 gives the summary of the corrosion properties of steels &om anodic polarization. The results show that PEP is an emerging cleaning and coating technology with high potential.

Table 7: Summary of corrosion properties of steel &om anodic polarization.

/ Material /l Corrosion I I Corrosion rate I

4 Conclusion

steel

Plasma surface modification is a very effective means to improve the corrosion and wear resistance of materials. By using single or multiple processes encompassing plasma immersion ion implantation and

I

1 w'O 1 Wg 104 j, (M~~I ) plasma electrolysis, fine, superhard, and new phases and special compounds such as nitrides or oxides of Cr,

Fig. Corrosion potentiodynamic curves of PEO Ti Mo and ceramic layers can be produced to enhance

films using a scan rate of40 rnV/min in: (a) 0.5 N the wear resistance properties. The presence of these &so4; (b) 3.5 % NaC1; 3.5 % NaCl; and (c) new and chemically stable structures and phases can be physiological solutions [47]. applied to industrial components and new

EPP-cleaning 1 -196 1 -385 0.14 1 9.5

L~IU Shen & Paul K. Chu

developments pertaining to plasma surface modification of materials are constantly emerging.

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