The formation of phosphate coatings on nitrided stainless steel

The formation of phosphate coatings on nitrided stainless steel

Corrosion Science 43 (2001) 1711±1725 www.elsevier.com/locate/corsci The formation of phosphate coatings on nitrided stainless steel J. Flis a,*, J...

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Corrosion Science 43 (2001) 1711±1725

www.elsevier.com/locate/corsci

The formation of phosphate coatings on nitrided stainless steel J. Flis a,*, J. Ma nkowski a, T. Zakroczymski a, T. Bell b a

Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44/52, 01 224 Warsaw, Poland b School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, UK Received 29 March 2000; accepted 7 December 2000

Abstract X10Cr18Ni9Ti stainless steel was plasma nitrided at 600°C or 575°C for 9 h and then subjected to the phosphating in zinc and manganese/iron phosphate baths. Depth pro®le analysis by glow discharge optical emission spectrometry (GDOES) showed that the coatings obtained in the Mn/Fe phosphate bath were about 10 lm thick and were enriched in chromium. Surface analyses by GDOES and Auger electron spectroscopy indicated that the outer layers of the coatings were composed mainly of the components from the bath, whereas the constituents from the steel prevailed in deeper layers. Anodic behaviour of the phosphated and post-treated steel was examined in 0.1 M Na2 SO4 of pH 3.0 or 6.4 by measuring polarisation curves and linear polarisation resistance. It was found that the phosphating with a subsequent chromate passivation and oil impregnation signi®cantly improved the corrosion resistance of the nitrided steel, imparting the resistance up to an order of magnitude higher than that of the unnitrided stainless steel. The ability of the nitrided stainless steel to undergo the phosphating can be related to its enhanced anodic reactivity. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Stainless steel; Auger electron spectroscopy; Polarisation; Phosphate coatings

1. Introduction It is envisaged that the combination of nitriding and phosphating should improve the corrosion resistance and tribological properties of metal surfaces. Nitriding [1,2] *

Corresponding author. Tel.: +48-22-6320-768; fax: +48-3912-0238. E-mail address: j¯[email protected] (J. Flis).

0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 1 7 9 - 7

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is a common treatment used for increasing the fatigue strength, hardness and wear resistance of steels, whereas phosphating of metal surfaces [3±7] is widely applied to improve paint adhesion, corrosion resistance, and to provide electrical insulation and lubrication; phosphate coatings decrease friction and wear, and facilitate cold forming of metals. It can be expected that phosphate coatings will exhibit a similar e€ect on nitrided surfaces. Nitriding increases the corrosion resistance of low-alloy steels [8±11], however, it can deteriorate the resistance of chromium-bearing steels [8,10±19]. Strong deterioration occurs especially in di€usion zones due to the precipitation of chromium nitrides and to the resultant chromium depletion of the matrix [12]. The outer layers of nitrided steels are usually composed of nitride phases c0 (M4 N) and/or e (M2±3 N) and have a corrosion resistance much better than that of the di€usion zones [12,19,20]. The e€ects of nitrogen on the corrosion properties of stainless steels and the proposed mechanisms have been reviewed in Refs. [21±23]. A signi®cant improvement in the corrosion resistance of nitrided X10Cr18Ni9Ti steel (AISI 321 SS) has been achieved by various chemical passivation treatments, especially by passivation in a hot NaOH/KNO3 solution or a molten NaNO3 / KNO3 mixture, followed by oil impregnation [18]. The formation of thick oxide ®lms on the nitrided chromium-bearing steel was possible due to the high reactivity of the chromium-depleted matrix. It is suggested that due to this reactivity, nitrided stainless steel can be also subjected to other chemical treatments involving dissolution of the metal with the formation of conversion coatings. Phosphating would be of a particular interest, because in addition to the increase in corrosion resistance it may impart an improved lubrication of the hardened surface. The phosphating process involves dissolution of a base metal in an acidic solution of soluble primary phosphates, with the subsequent hydrolysis of these phosphates and the precipitation of insoluble tertiary phosphates. The metal should dissolve at a moderate rate to allow the necessary neutralisation and supersaturation of the nearsurface solution. Among the common alloying elements (Ni, Cr, Mo, Mn), chromium impairs the phosphatibility to the largest extent, and therefore its content in the alloy should not exceed about 4% [7]. Stainless steels are not normally phosphatable, however, they can be phosphated after surface activation by chemical, plasma or mechanical pre-treatments, which are the subject of patents. Patents cover the methods of phosphating to increase coatability of stainless steel [24±29], or to improve lubrication during cold plastic working [30]. Activation can be attained by blasting with cast-iron grit [31], by nitriding [32] as well as other methods; nitriding with subsequent phosphating has been applied to 13Cr±0.3C steel for cyclic sliding service [32]. In the present work, phosphating treatments were applied to unnitrided and nitrided X10Cr18Ni9Ti steel (AISI 321 SS) with the purpose of increasing the corrosion resistance of nitrided chromium-bearing steels. Anodic behaviour of the steel and elemental composition of the phosphate coatings have been examined.

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2. Experimental 2.1. Material and nitriding Samples were delivered by S.C. Plasmaterm S.A., Romania [18]. They were of austenitic X10Cr18Ni9Ti steel (AISI 321 SS; further designated Cr18Ni9Ti) (C: 0.085±0.11, Cr: 17.2, Ni: 9.55, Si: 0.44, Mo: 0.15, and Ti: <1.00 wt.%) in form of discs 25 mm diameter and 4 mm thick, plasma nitrided in an ammonia atmosphere of pressure 3 torr at 600°C, or pressure 2 torr at 575°C, for 9 h. These two temperatures are above and below, respectively, the eutectoid temperature 590°C for the iron±nitrogen system with the a ‡ c0 eutectic at 2.35 wt.% N [33]. The samples had nitrided layers about 130 lm thick, with a thin (about 2 lm thick) compound-like zone at the outer surface, the rest being a di€usion zone with nitride precipitates. Electrochemical studies and phosphating treatments were performed on the asnitrided surfaces after cleaning with a cloth and then acetone in an ultrasonic washer.

2.2. Phosphating treatments Zinc, manganese±iron, and manganese phosphatings were performed in the following solutions: (a) Zinc phosphating was carried out in the bath containing 24.5 g/l ZnO, 45.0 g/l H3 PO4 (density 1.53 g/ml) and 23.0 g/l HNO3 (density 1.50 g/ml) at 96°C for 15 min; this phosphating is denoted in ®gures of the present paper as ``Zn phos''. (b) Manganese±iron phosphating was carried out in Mazhef salt, which is a mixture of manganese and ferrous primary orthophosphates. The reagent was manufactured by Polish Chemical Reagents Co. (POCH) and contained 46±52 wt.% P2 O5 , 14 wt.% Mn, 0.3±3 wt.% Fe. Treatments were performed in the baths with 15±100 g/l Mazhef and up to 20 g/l MnCO3 at temperature of 96°C for 30±90 min. This phosphating is denoted in the ®gures as ``MnFePO4 ''. (c) Manganese phosphating was made in a solution of manganese carbonate with H3 PO4 corresponding to 32.0 g/l Mn(H2 PO4 )2 , at 96°C for 30 min; this phosphating is denoted in the last ®gure as ``Mn phos''. Post-treatments included were (a) Passivation in 8% K2 Cr2 O7 at 80°C for 20 min. (b) Oil impregnation in an engine oil LotosÒ (Lotos Synthetic SH/CD/EC 5W/40, Gdansk Re®nery, Poland) at 90°C for 30 min. Measurements were made one day after the application of the oil.

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2.3. Electrochemical measurements Measurements were carried out at ambient temperature in non-deaerated solutions: (a) 0.1 M Na2 SO4 acidi®ed with H2 SO4 to pH 3.0; (b) 0.1 M Na2 SO4 without additions, pH 6.4. The solutions were prepared from double distilled water and analytical grade reagent chemicals. The Na2 SO4 solution of pH 3.0 was used for the measurement of polarisation curves, whereas the less aggressive solution of pH 6.4 was used for the measurement of linear polarisation resistance. Electrochemical measurements were performed in a conical glass cell in which the samples were positioned horizontally at the bottom. Potentials were measured relative to a Hg|Hg2 SO4 |0.1 M Na2 SO4 reference electrode (designated as MSE or Hg/ Hg2 SO4 ) and reported as such …EMSE ˆ ‡0:661 VH ˆ ‡0:420 VSCE †. A platinum strip above the sample served as a counterelectrode. Polarisation curves were recorded after 2-min holding at a potential of 1.0 VMSE for standardising the surface conditions. The curves were measured at a potential scan rate of 1 mV/s, starting from a potential of 1.0 VMSE . Open-circuit potential was measured immediately after immersion, and the polarisation resistance, Rp , was measured after 30 min since the immersion and was continued through 120 min. The polarisation resistance, Rp , makes it possible to determine the corrosion rate (ic ) from the Stern±Geary equation [34] ic ˆ BRp 1 with B ˆ ba bc =2:3…ba ‡ bc †. By using the Tafel slopes (ba and bc ) equal to 0.12 V/ decade, the approximation gives B ˆ 0:026 V [35]. For Cr18Ni9 steel, corrosion current of 1 lA/cm2 corresponds to corrosion rate of 0.0106 mm/year. Polarisation resistance, Rp , was determined as a slope, DE=Di, in potential scans at a rate of 0.1 mV/s from 10 mV (cathodic) to ‡10 mV (anodic) vs. the opencircuit potential. The measurements were made with an electrochemical measurement system CMS100/CMS105, manufactured by Gamry Instruments, Inc.

2.4. Surface analysis Elemental concentration/depth pro®les of phosphate coatings were obtained by glow discharge optical emission spectrometry (GDOES), using GDS-750 QDP of Leco Ltd., and by Auger electron spectroscopy (AES). The AES spectrometer was operated at a beam voltage of 3.0 kV. Depth pro®ling was performed by Ar‡ sputtering at 3.0 kV; the ion gun current was 10 lA for a sputtered area of 1.1 cm2 . Spectra were recorded in the pulse counting mode and atomic concentration, CX , was assessed using the relation:

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IX =SX CX ˆ P …Ii =Si † where IX is the peak height of element X, SX is a relative elemental sensitivity factor. Treated surfaces were analysed after washing in double distilled water.

3. Results 3.1. Surface analysis of phosphated steels Figs. 1 and 2 present the AES depth pro®les for surface ®lms on zinc phosphated unnitrided and nitrided steel, respectively. Concentration pro®les of the elements suggest that the composition of the phosphate coatings was as follows. Unnitrided steel (Fig. 1): The outer layer contained three elements from the phosphating bath (O, Zn and P) and only one element (Fe) from the steel. For the

Fig. 1. AES depth pro®les for surface ®lm on unnitrided Cr18Ni9Ti steel subjected to zinc phosphating.

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Fig. 2. AES depth pro®les for surface ®lm on Cr18Ni9Ti steel nitrided at 600°C for 9 h and subjected to zinc phosphating.

depth corresponding to the sputtering time of about 5 min, atomic concentration ratio of Zn:Fe:P:O was about 5:1:3:8, being close to the ratio 2:1:2:8 for Zn2 Fe(PO4 )2 . Hydrated Zn2 Fe(PO4 )2 (phosphophillite Zn2 Fe(PO4 )2  4H2 O) is formed on iron in zinc phosphating bath [7], and possibly this compound was also formed on the stainless steel. In deeper layers of the phosphate coating there was also present chromium. Nitrided steel (Fig. 2): Judging from the sputtering time till the disappearance of phosphorus and zinc, phosphate coating was about 20 times thicker than that on the unnitrided steel. The outer layer did not contain any element from the steel. At the depth corresponding to the sputtering time of 300 min, the atomic concentration ratio of Zn:P:O was about 4:1:14 which is fairly close to 3:2:12 for hopeite Zn3 (PO4 )2  4H2 O. In similarity with phosphophillite, hopeite is formed on iron in zinc phosphating bath [7]; the obtained AES pro®les suggest that it was also formed on the nitrided stainless steel. Constituents of the steel (Fe and Cr) appeared in deeper layers of the phosphate coating.

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Phosphate coatings on the nitrided steel needed a long ion sputtering (Fig. 2), and therefore this method is not convenient for the depth pro®ling of thick phosphate coatings. For thick coatings obtained in Mn/Fe phosphate baths more appropriate was GDOES. Concentration depth pro®les determined by the GDOES analysis for nitrided steel subjected to Mn/Fe phosphating are shown in Fig. 3. Concentration pro®les of P, O and Mn indicated that the thickness of the phosphate coating was about 10 lm, being thus comparable with the thickness of the coatings on plain steel. Concentration pro®le of iron has been given in the upper part of the ®gure which presents elements from the steel, however, this pro®le also includes iron from the Mn/Fe phosphating bath. The concentration pro®les suggests that the outer layer (down to the thickness of about 1 lm) was composed mainly of the phosphates of Fe, Mn, and Cr, whereas in deeper layers there prevailed the components from the steel (Fe, Cr, Ni). Concentrations of Cr and N changed with depth in a similar way, indicating that these constituents occurred mainly as chromium nitrides; carbon was evidently also bound with Cr because its maximum was close to the maximum of Cr.

Fig. 3. GDOES depth pro®les for surface ®lm on Cr18Ni9Ti steel nitrided at 600°C for 9 h and subjected to manganese/iron phosphating. Concentration pro®le of iron in the upper part of the ®gure includes this element both from steel and bath.

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It is noteworthy that deeper layers of the phosphate coating were enriched in chromium (maximum appeared at the depth of about 5 lm). 3.2. Anodic polarisation curves Polarisation curves in the pH 3.0 solution for unnitrided and nitrided steel are shown in Fig. 4(a), and the curves after phosphating of nitrided steel in the zinc phosphate bath are shown in Fig. 4(b). The currents for the nitrided steel were approximately an order of magnitude higher than those for the unnitrided steel. Zinc phosphating without a post-treatment resulted in a decrease of anodic currents only in the active region (potentials up to about 0.5 VMSE ). Passivation in the chromate solution caused a slight decrease of the currents, whereas subsequent oil impregnation resulted in a very strong decrease of the currents to values below those for the unnitrided steel. The applied treatments diminished the currents in the active and passive regions, but they only slightly a€ected the transpassive region (around the potential of 0.5 VMSE ). Fig. 5 shows the e€ect of Mn/Fe phosphating followed by chromate passivation on the anodic currents of steel nitrided at 575°C (at this temperature the formation of the a ‡ c0 eutectic should be avoided). Anodic curve for the steel nitrided at 575°C (Fig. 5(a)) is in the active and passive regions similar to that for 600°C (Fig. 4(a)), but the transpassive peak for 575°C is narrower than that for 600°C. The Mn/Fe phosphating with chromate passivation (Fig. 5(b)) a€ected anodic currents similarly as did the zinc phosphating (Fig. 4(b)). The phosphating baths

Fig. 4. Anodic polarisation curves for Cr18Ni9Ti steel in 0.1 M Na2 SO4 , pH 3.0: (a) unnitrided steel and after nitriding at 600°C; (b) nitrided steel after zinc phosphating and post-treatment of chromate passivation with K2 Cr2 O7 and with subsequent oil impregnation.

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Fig. 5. Anodic polarisation curves for Cr18Ni9Ti steel in 0.1 M Na2 SO4 , pH 3.0: (a) unnitrided steel and after nitriding at 575°C; (b) nitrided steel after manganese/iron phosphating in the bath with 35 and 40 g/l Mazhef (MnFePO4 ) followed by chromate passivation and with subsequent oil impregnation.

with the Mazhef concentrations of 40±50 g/l gave smaller anodic currents than those with lower or higher concentrations. Similarly as for the zinc phosphating, subsequent oil impregnation resulted in a very strong decrease in anodic currents. After the Mn/Fe phosphating with chromate passivation and oil impregnation, anodic currents in the passive region were over an order of magnitude lower than those for unnitrided steel. In the near-neutral solution of pH 6.4 (Fig. 6), anodic polarisation curves did not exhibit peaks. Nitriding temperature noticeably a€ected only the transpassive region: after nitriding at 575°C there occurred one transpassive peak at the potential of 0.70 VMSE , whereas after nitriding at 600°C there appeared another peak at the potential of 0.95 VMSE (Fig. 6(a)). Probably, the second peak was associated with the presence of the a ‡ c0 eutectic. Phosphating resulted in a shift of corrosion potentials in the noble direction (Fig. 6(b)). Similarly as with the solution of pH 3.0, the anodic currents for phosphated

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Fig. 6. Anodic polarisation curves for Cr18Ni9Ti steel in 0.1 M Na2 SO4 , pH 6.4: (a) unnitrided steel and after nitriding at 575°C and 600°C; (b) steel nitrided at 575°C and phosphated in Zn bath and in 35 g/l Mazhef (MnFePO4 ) with subsequent chromate passivation.

surfaces without a further post-treatment were higher for Mn/Fe bath than those for Zn bath. 3.3. Polarisation resistance and open-circuit potential Polarisation resistance Rp and open-circuit potentials in near-neutral 0.1 M Na2 SO4 solution of pH 6.4 for unnitrided and nitrided steel and after phosphating of nitrided steel in Zn bath and Mn/Fe bath are presented in Figs. 7 and 8, respectively, as a function of immersion time. In this solution, the unnitrided steel was in the passive state (potentials 0.6 to 0.4 VMSE ); Rp was high (above 105 X cm2 ) and it increased with immersion time. The potential of steel nitrided at 600°C attained the active state in which Rp was by two orders of magnitude lower than that for the unnitrided steel (Fig. 7); steel nitrided at 575°C showed nobler corrosion potential and higher Rp (Fig. 8). These data indicate a strong decrease in the corrosion resistance of stainless steel after nitriding. After phosphating, Rp was in the range of about 104 X cm2 . Subsequent oil impregnation led to a strong rise in the resistance; in the case of Zn phosphating it attained a value similar to the unnitrided steel (105 X cm2 (Fig. 7)), whereas for Mn/ Fe phosphating it exceeded this value by almost an order of magnitude (Fig. 8). Fig. 9 presents a plot of polarisation resistance as a function of the open-circuit potential for an immersion time of 1 h. The potential dependence of Rp has a

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Fig. 7. Electrode potential (E) and polarisation resistance (Rp ) in 0.1 M Na2 SO4 , pH 6.4, for unnitrided and nitrided Cr18Ni9Ti steel and for Zn phosphating treatments with subsequent chromate passivation of steel nitrided at 600°C.

maximum around the potential of 0.35 VMSE . The drop in the resistance at nobler potentials coincides with the increasing current of the anodic polarisation curves (Fig. 6(b)).

4. Discussion This work shows that the phosphating can be applied e€ectively to nitrided stainless steel. In order to be phosphated, a base metal should undergo a moderate dissolution in an acidic bath of primary phosphates [7]. Untreated stainless steels do not undergo a proper phosphating due to their high corrosion resistance, allowing only a negligible dissolution in the phosphating baths. Good phosphatability of nitrided stainless steel can be ascribed to its diminished corrosion resistance. Owing to its enhanced reactivity, the nitrided surface undergoes a moderate dissolution which is a prerequisite for the phosphating process. The results of this work indicate that phosphate coatings subjected to oil impregnation can impart very good corrosion resistance to the nitrided steel. It is

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Fig. 8. Electrode potential (E) and polarisation resistance (Rp ) in 0.1 M Na2 SO4 , pH 6.4, for unnitrided and nitrided Cr18Ni9Ti steel and for Mn/Fe phosphating treatments with subsequent chromate passivation of steel nitrided at 575°C.

possible that the protectiveness of phosphate coatings on nitrided stainless steel is better than that on plain steel owing to the presence of chromium. Chromium was found in deeper layers of the coatings (Figs. 1±3); in Zn phosphate coatings the concentration of chromium increased uniformly with the depth (Figs. 1 and 2), whereas in the Mn/Fe phosphate coating it showed a maximum at which the chromium concentration exceeded that in the nitrided substrate (Fig. 3). In the latter coating, the concentration pro®les of chromium and nitrogen were roughly similar, suggesting that the accumulated chromium was mainly in the form of nitrides. It can be expected that the accumulation of chromium might have a positive e€ect on the protective properties of the phosphate coating. It is suggested that the accumulation of chromium in the phosphate coating can be due to the preferential dissolution in the phosphating bath of the more reactive phases (c(N) and a(N) [17]). Less soluble chromium nitrides will remain on the

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Fig. 9. Polarisation resistance as a function of open-circuit potential for a 1-h immersion in 0.1 M Na2 SO4 , pH 6.4, for unnitrided and nitrided steel and for various treatments of nitrided steel. Phosphatings were followed by chromate passivation.

surface, or will be taken o€ by the ¯ux of the dissolving metal and redeposit on the surface. The outer layers of phosphate coatings were composed mainly of the components from the bath, with none or small amounts of the constituents from the steel. In Zn phosphate coating on the unnitrided steel (Fig. 1) the outer layer did not contain chromium, and on the nitrided steel it did not contain either chromium or iron (Fig. 2); in Mn/Fe phosphate coating these constituents appeared in the outer layer at lower concentrations than in deeper layers (Fig. 3). It means that in the ®nal stage of phosphating, the precipitation occurred mainly from the bath, whereas the precipitation of the components being dissolved from the substrate was much smaller or negligible. The phosphate coatings acquired good protective properties especially after the oil impregnation; this post-treatment is often used to increase the corrosion resistance of phosphated plain steels [7]. The dependence of the linear polarisation resistance on open-circuit potential showed a maximum for the open-circuit potential of about 0.35 VMSE (Fig. 9). The decrease in the polarisation resistance at nobler potentials

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might be associated with porosity of the coatings. It occurred mainly for the coatings without oil impregnation, and in particular for Mn phosphate coatings which are known to be less protective than Mn/Fe phosphate coatings. Dissolving of the steel in the phosphating process causes a thinning of an outer compound-like layer, and as a result a more reactive di€usion zone will be exposed. Probably, the observed decrease in the polarisation resistance at nobler potentials (Fig. 9) is due to the exposure of this zone through the pores in the coatings.

5. Conclusions (1) Thick phosphate coatings were obtained on plasma nitrided Cr18Ni9Ti stainless steel from the zinc and manganese/iron phosphate baths. The coatings from the Mn/Fe phosphate bath were about 10 lm thick and were enriched in chromium, probably in the form of nitrides. The outer layers of the coatings were composed mainly of the components from the bath, whereas the constituents from the steel prevailed in deeper layers. (2) Oil impregnated phosphate coatings exhibited very good protective properties. In near-neutral and acidi®ed 0.1 M Na2 SO4 solution, corrosion resistance of the coatings formed in the Mn/Fe bath signi®cantly exceeded the resistance of the unnitrided stainless steel. (3) The ability of the nitrided Cr18Ni9Ti stainless steel to undergo the phosphating can be related to its enhanced anodic reactivity.

Acknowledgements This work was supported under a Copernicus project from The European Commission, contract no. CIPA CT 94-0151. The authors express their thanks to Dr. Z. Kolozsvary and Mr. S. Janosi for providing nitrided samples, and to Ms. B. Narowska for technical assistance.

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