Corrosion protection of TiN-coated low carbon steel with electroless Ni-P as an interlayer

SurJace and Coatings Technology, 53 (1992) 93—98

93

Corrosion protection of TiN-coated low carbon steel with electroless Ni—P as an interlayer J. L. He and M. H. Hon Department of Materials Engineering, National Cheng Kung university, Tainan (Taiwan) (Received February 4, 1992; accepted February 28, 1992)

Abstract The corrosion protection of TiN-coated low carbon steel using electroless Ni— P as an interlayer was investigated by anodic polarization and a.c. impedance tests in a 3.5% NaCl solution. Both cathodic arc plasma and hollow cathode discharge methods were used to coat TiN in order to reveal the flexibility ofthis hybrid-coating process. Experimental results showed that the corrosion potential (Eco,r) and polarization resistance (Rn) increased but the corrosion current (I~o,,)decreased as the Ni—P interlayer was introduced. The impedance of hybrid-coated specimens increased as a function of Ni—P layer thickness. The Ni—P layer with a lamellar structure acted as a serial connected resistor and changed TiN from a parallel connected resistor into a serial one. The corrosion resistance ofTiN-coated low carbon steel was improved by introducing the Ni—P interlayer.

1. Introduction Titanium nitride (TiN) is now widely used in decorative applications such as coatings on ornaments, watch parts and glass frames. Work to date shows that passive metals are generally available as substrates which would otherwise not be corrosion resistant. This could be due to the penetration of corrosive species along numerous pores and pinholes in the coated TiN layer [I]. Optimization of process parameters to achieve a better corrosion resistance has been done by several investigators [2—4].However, these pores and pinholes in the TiN coating could not be fully eliminated simply by optimizing the process parameters. Kado et al. [5] estimated that a sufficient film thickness of TiN coating on SS41 by plasma-enhanced chemical vapour deposition for corrosion resistance in hydrochloric acid solution using the anodic polarization test was up to 14 ~tm.This might not be commercially practical for decorative coating, not to mention that salt environments are often encountered when decorative coated metals contact the human body. An alternative way to achieve a better corrosion resistance is to introduce a metallic titanium interlayer between substrate and TiN coating as demonstrated by Massiani et al. [1] and Kurokawa eta!. [6]. Park eta!. [7] have used ion-plated nickel as an interlayer to improve the corrosion protection of SUS3O4 stainless steel. Recently, Chen and Dub [8] reported that electroless Ni—P as an interlayer on mild steel followed by TiN coating resulted in an increase in the surface hardness

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as well as in the adhesion strength. These findings support the possibility of improving the corrosion protection of TiN-coated low carbon steel using Ni—P as an interlayer. In this study electroless Ni—P was used as an interlayer between TiN coating and low carbon steel substrate to evaluate the corrosion performance by changing the film thickness of the Ni—P layer. Titanium nitride coatings were obtained by both cathodic arc plasma (CAP) and hollow cathode discharge (HCD) ion-plating methods in order to examine the coatings produced by these techniques and also the interlayer Ni—P coatings. Anodic polarization and a.c. impedance tests were used to evaluate the corrosion protection.

2. Experimental details

JIS G3l41 SPCC low carbon steel sheet substrate 0.8 mm thick was pressed to form a specimen 16 mm in diameter. Before electroless Ni—P plating, the specimens were degreased and ultrasonically cleaned in a hydrochloric acid solution. Electroless Ni—P plating was carried out with Nickora solution produced by the Schering company. The plating temperature was maintamed at 90°C with a pH value of 4.9. Plating was carried out for times of 10, 20, 40 and 60 mm to obtain film thicknesses of 4, 8, 15 and 25 ~tmrespectively. CAPTiN coatings were carried out in a Hauzer HC- 1000 system while HCD-TiN coatings were carried out in an ULVAC IPB-l5S system. The film thickness was about

~ 1992

—

Elsevier Sequoia. All rights reserved

94

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(orroa:on proleulion of 1 N—coated steel

1.5 ttm for CAP-TiN coatings and I ~tm for HCD-TiN

the charge transfer resistance (R1~), which has been defined in ref. 9 and elsewhere. X-ray diffractometry and scanning electron microscopy were used to characterize the microstructure of the coated films.

coatings. Anodic polarization tests were carried out in a threeelectrode cell using an EG&G PAR 273 system with platinum as the counterelectrode surrounding the working and reference electrodes. The potential of the working electrode was measured with respect to a saturated calomel electrode (SCE) through a KCI salt bridge. A 3.5°A NaCI solution was used as the test bath. It was not deoxygenated before testing in order to simulate a general corrosion environment. Polarization curves were plotted from 500 to 1500 mV at a sweep rate of I mV s A.c. impedance tests were carried out in a system similar to that used for the anodic polarization tests, except an additional 1255 frequency analyser was employed. Measurements were conducted at open-circuit potential with a voltage amplitude of ±5mV. Both Bode and Nyquist plots have been employed to evaluate

3. Results and discussion Figure 1(a) shows the polarization curves of the low carbon steel specimen and Ni --P-plated substrates. Significant changes in Ecorr, ‘corr and R~are observed. Ecorr increased from —724 to —-300 mV, ‘corr decreased and R5 increased with increasing Ni --P layer thickness. It is obvious that the Ni P layer protected low carbon steel from corrosion to a certain degree. Figure 1(b) shows the polarization curves of the single-CAP-TiN- and hybrid-CAP-TiN-coated substrates. The single-CA PTiN-coated specimen showed an increase in Ecotr, a

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Fig. I. Anodic polarization curves of )a) uncoated and Ni P-plated. )h) CAP-TiN-coated and hybrid-CAP-TiN-coated and Ic) hyhrid-HCDTiN-coated low carbon steel specimens.

J. L. He, M. H. Hon / Corrosion protection of TiN-coated steel

95

TABLE I. Results of anodic polarization tests for uncoated and coated low carbon steel specimens. The data were obtained at a sweep rate of lmVs’ Specimen

Ecor,

‘corr

(mY)

(

1sA cm 14.0

2) )

Peel-off voltage (mV)

(k~cm 0.8

Low carbon steel

—724

CAP-TiN coated

—432

5.9

0.2

10 20 40 60

mm mm mm mm

Ni—P Ni—P Ni—P Ni—P

plated plated plated plated

—331 —360 —349 —311

7.6 2.7 4.5 0.1

12.5 13.5 20.0 26.2

10 20 40 60

mm mm mm mm

Ni—P Ni—P Ni—P Ni—P

plated +CAP-TiN plated+CAP-TiN plated +CAP-TiN plated+CAP-TiN

coated coated coated coated

—425 —411 —373 —388

1.7 0.9 0.3 0.5

20.2 18.1 30.5 20.4

500 600 680 800

10 mm Ni—P plated+HCD-TiN coated 20 mm Ni—P plated+HCD-TiN coated 40 mm Ni—P plated+HCD-TiN coated 60 mm Ni—P plated+HCD-TiN coated

—370 —403 —354 —407

1.8 0.7 1.0 0.5

18.3 33.8 42.0 45.2

490 700 850 900

comparable increase in ‘cor. and a decrease in R~ as compared to the uncoated specimen. This indicates that the single TiN coating cannot effectively protect low carbon steel from corrosion, which has been mentioned elsewhere [1,10]. The hybrid-coated specimens showed an increase in both Ecorr and R~accompanied by a large decrease in ~ as expected. Note that ‘cor. decreased by at least an order of magnitude as compared to the low carbon steel substrate and ‘cort decreased as the Ni—P layer thickness was increased. This implies that the corrosion rate of hybrid-coated low carbon steel was at least one-tenth that of the uncoated specimen in practice. The hybrid-HCD-TiN-coated specimens had a similar performance as shown in Fig. 1(c), in which Ecorr fell into the same range as for hybrid-CAP-TiN-coated specimens and ‘corr decreased and R~increased as the Ni—P interlayer thickness was increased, During the polarization test, peel-off of the TiN layer was observed. The peel-off voltage, which could be used as an indication of the degree of corrosion protection, together with other important polarization results are shown in Table 1. It is seen that the thicker the Ni—P layer, the higher the peel-off voltage value is. All the Ni—P-plated specimens without the TiN layer showed a few tiny pits on the surface after the anodic polarization test (Fig. 2(a)), whereas the hybrid-CAPTiN-coated specimens have very large pits (Fig. 2(b)) and the hybrid-HCD-TiN-coated specimens have no TiN left at all (Fig. 2(c)). X-ray diffraction (XRD) revealed that peaks corresponding to HCD-TiN disappeared after the polarization while peaksof from CAP-TiN were still present (Fig. test 3). The results the polarization test indicate that the Ni—P interlayer provided corrosion

600 —

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protection of the specimens. The lamellar structure of Ni—P inhibited the penetration of corrosion species which cannot be blocked by the TiN layer itself. The impedance spectra of the low carbon steel and Ni—P-plated specimens (Fig. 4(a)) show that the estimated R~,value of uncoated low carbon steel was as low as 175 Q cm2 only. R~,reached a maximum value of 16 k~cm2 as the Ni—P layer thickness was increased to 25 jim, which corresponds to 60 mm of plating. Nyquist plots for the low carbon steel and Ni—P-plated specimens are shown in Fig. 4(b). The curve for low carbon steel is invisible because it formed a very small circle around the origin. There seems to be no characteristic of the Warburg diffusion phenomenon which usually occurs in the case of a porous coating or a porous passive layer on a metal surface. The lamellar structure of Ni—P again proved to be a solid interlayer. The impedance of the CAP-TiN-coated specimen (Fig. 5(a)) dropped at a frequency lower than 10-’ Hz, which was beyond the range of Elsener et al’s [10] experiment. This impedance drop might be due to the penetration of corrosion species through the pores of the TiN layer. Since the penetration of corrosion species took some time, the impedance drop was only observed at low frequencies. From this point of view it was not surprising that R 0 of the CAP-TiN-coated specimen without the Ni—P layer was lower than that of low carbon steel. R~, of the hybrid-CAP-TiN-coated specimens increased significantly as compared with either the Ni-P-plated or CAP-TiN-coated substrates (Fig. 25(a)). It as the reachedlayer a maximum about 160 cm which Ni—P thicknessvalue was of increased to k1~ 25 jim, corresponds to 60 mm of Ni—P plating. The maximum

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and hybrid-CAP-TiN-coated specimens. Despite the fact

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value is a factor of 2 greater than that for the singleCAP-TiN-coated specimen and a factor of 90 greater than that for the low carbon steel substrate. The impedance values remained unchanged at low frequency for the hybrid-coated specimens because of the dense interlayer and therefore high impedance was obtained. Figure 5(b) shows Nyquist plots for the CAP-TiN-coated

that the impedance spectra of hybrid-coated specimens were similar to the case mentioned by Tavi ef a!. [9]. who claimed that it was sometimes difficult to decide between an ascending line (Warburg impedance) and a circular arc (high polarization resistance), we believe that it was almost impossible for the Warburg impedance to manifest itself since an Ni---P interlayer of lamellar structure existed beneath the TiN coating. Similar results for the hybrid-HCD-TiN-coated specimens are tllustrated in Figs. 6(a) and 6(b). The estimated maximum R~1was as high as 200 kQ cm2. A comparison of R~,for all specimens is given in Table 2. The above discussion enabled us to construct an equivalent circuit of such a hybrid-coated low carbon steel in this electrochemical system. The thin TiN coating shown in Fig. 7(a) acted as a parallel resistor in which a short circuit might occur as proposed by Elsener e~a!. [10] and Tavi et al. [9]. However, the Ni P interlayer with a lamellar structure hindered the penetration of corrosion species. Thus it acted as a serial-connected resistor and changed the TiN layer from a parallelconnected resistor into a serial one as illustrated in Fig. 7(b). Hence high impedance was observed and corrosion protection was obtained.

J. L. He, M. H. Hon

A

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Corrosion protection of’ TiN-coated steel

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Fig. 4. (a) Bode plot and (b) Nyquist plot of uncoated and Ni—Pplated low carbon steel specimens.

Fig. 6. (a) Bode plot and (b) Nyquist plot of uncoated and hybridI-lCD-TiN-coated low carbon steel specimens.

4. Conclusions

deposited as an interlayer showed that ‘corr decreased and R~increased by at least an order of magnitude. Maximum corrosion resistance was obtained when the thickness of the Ni—P layer was above 15 jim. Hybrid-CAP-TiN-coated specimens became pitted while HCD-TiN-coated specimens peeled off in the polarization test.

The corrosion protection of hybrid-TiN-coated low carbon steel with electroless Ni—P acting as an interlayer has been evaluated and the following conclusions can be made, (1) Anodic polarization testing of the Ni—P coatings

A

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S CAP—TIN * 10 ruin Ni—P+CAP—TIN + 20 ruin Ni-P+CAP-TiN

~CAP-TIN 10 ruin Nl—P+CA.P—T1N

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Fig. 5. (a) Bode plot and (b) Nyquist plot of uncoated, CAP-TiN-coated and hybrid-CAP-TiN-coated low carbon steel specimens.

98

.1. L. lie, ti. II - Hon

( ‘oI’rIISIIni plot e~-tIon of TiN—i ‘outed at eel

Rs/F

TABLE 2. Charge transfer resistance (R~)values for uncoated and coated low carbon steel specimens Re

2)

Specimen

Cs’ F

Rhl, )k12 cm

Low carbon steel

Rc/(1 — F)

0.175

CAP-TiN coated IS 6.0 6,))

40 miii Ni P plated 60 mm Ni--P plated ID mm Ni—P 21) miii Ni--P 40 mitt Ni--P 60 mm Ni—P

plated + CAP-TiN plated +CA P-TiN plated+CAP-TiN plated + CA P—TiN

coated coated coated coated

43.0 30.)) 50.0 16)).))

10 mm Ni P plated + HCD-TiN 20 mirs Ni-P plaled+HCD-TiN 4)) mm Ni—P plated+HCD-TiN 60 mm Ni-—P plated+ HCD-TiN

coated coated coated coated

22.0 19.0 79.3 200.0

(2) The impedance of hybrId-coated specimens increased as a function of the Ni P layer thickness. The Ni--- P coating with a lamellar structure acted as a serialconnected resistor and changed the TiN from a parallelconnected resistor into a serial one, resulting in high impedance (3) The results of anodic polarization and ac. impedance tests of hybrid-coated specimens revealed an improved corrosion resistance. Electroless Ni P can therefore significantly improve the corrosion resistance of TiN-coated low carbon steel.

Acknowled ment The authors wish to thank the China Steel Co. who provided the electrochemical test facility.

(a)

Rs

Rn

Rt

Cs

Cn

Ct

Re

h ) Fig. 7. Lquivalent circuits of )a) single-TiN—coated and )h) hybridcoated low carbon steel specimens.

References I

. \‘.

-

-

J. ( rousier, V. Fedrizzi, A. C avallcri and P. V. Bonora. lc’clino/ .33 (19871 309 317 2 A. Telama. T. Mantyla and P. Kettunen. .1. Foe. Sci, Technol..’). 4 (6) (1986) 291 1-2914. 3 H. Freiler and H. P. Lorenz. J. Foe. Sd. Technol.,’). 4 (6) (1986) Massiani.

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4 F. K. Wiiala, 1. M. Penttmen. A. S. Korhonen. J . Aromaa aisd F Ristolainen. SunS Coat Technol., 41(1990) 191—204 S T. Kado, R. Makabe. S. Mochizuki. S. Nakajima and M. Araki. Boshoku G)/ut.s’u. 36 (1987) 551—558. 6 K. Kurokawa. T. Odaira. Y. Odaira. Y. Nakavama, K. Takis’awa and H. ln’iai, Surf. Technol. I’ Jpn. I, 4/(51)1990) 64--- 69. 7 M. J. Park. A. Leyland and A. Matthews. Surf. (‘oat. Technol.. 43—44 (1990) 481—492. 8 Y. I. Chen and J. C. Duh. Stir/S Coat. Technol., 45 (1991) 163-- 168. 9 M. Tavi. 0. Forsen and J. Aromaa. Mat er. Sc). Forum. 44—45 (19891 15-28. 10 B. Elsener, A. Rota and H. Bohni, Mater. Sc’). Forum. 44 (19891 29-38.