Sherardizing and characteristic of zinc protective coating on high-strength steel bridge cable wires

JingHua Jianga, AiBin Mab, XinDu Fanc, MingZi Gongd and LiuYan Zhange College of Materials Science and Engineering, Hohai University, Nanjing 210098, China

a jinghua-jiang@hhu.edu.cn, b aibin-ma@hhu.edu.cn, c vanbryan@126.com, d gmz3705@tom.com, e zhangliuyanjust@163.com

Keywords: Zinc Protective Coating, Sherardizing, High-Strength Steel Wire, Bridge Cable.

Abstract. Zinc protective coatings on high carbon SWRH82B-1 steel were sherardized to markedly improve corrosion resistance of the high-strength steel bridge cable wires (SBCW). Sherardizing parameters have been optimized by optical microscopy (OM) /scanning electron microscopy (SEM), X-ray diffraction (XRD) and potentiodynamic polarization tests. The sherardizing coatings are composed of the loose outer layer (§-FeZn13 phase) and the dense inner layer (δ- FeZn7 phase) with higher hardness. Addition of Y2O3 activator slightly increases the corrosion resistance of sherardized steel wire in comparison with CeO2. A thicker coating corresponds to a higher sherardizing temperature or a longer heating duration, but an extra thick coating is unfavorable for thru-microcrack existed in the inner layer. Good quality of sherardized wires ( higher corrosion resistance and longer duration than conditional hot-dip-galvanized one) can be produced with the zinc-rich powder containing 7.5wt.% CeO2 activator and 25wt. % SiO2 filler under 400℃for 6h.

Introduction

In the past decade, the diversified steel cables are made from high-strength cold-drawn wires with new steel brands to meet the market requirements for developing modern cable-stayed bridges. Progressively steel cable products are widely used for its very high strength and acceptable level of stabilized toughness associated with light weight [1]. As an important load-bearing structure of cable-stayed bridges, steel cables should be protected to ensure the longevity of performance in increasingly hostile environments, with an attempt to reduce the alarmingly high cost and difficulty of cable replacement [2-4]. The hot-dip galvanized coating has become the standard practice for structural steel cables due to the undisputable protective effectiveness, ease of application and low cost [4-5], but the fume emissions during hot-dip galvanizing (mainly concluding ZnO, ZnCl2 and NH4Cl [6-7]) is so intense as to induce the expectation of alternative coating techniques more friendly to the environment.

Zn-rich coatings are well suited and widely used for corrosion control, resulted from a number of characteristic of Zn metal [8-11]. Sherardizing may be a promising technique to replace conditional hot-dip galvanizing for the corrosion protection of high strength SBCW (steel bridge cable wires), because of no fumes and lower energy consumption. In this thermochemical process, the treated steel parts are heated in contact with a mixture of powders containing Zn and thereby allowing the zinc to be absorbed onto the steel substrate via diffusion to form a sherardized coating with good bonding interface[12-14]. However, there is almost no literature data about the sherardizing process applied to the high carbon steels in high-strength structural steel cables.

With the view to scaling up of the sherardizing method to more industrial applications, the present work was designed to systematically investigate the microstructure characteristic and corrosion resistance of zinc diffusion coatings sherardized on a typical high carbon steel of SWRH82B-1, the world’s high-strength bridge steel for the increased strength to mass ratio over mild steel. The effect of different processing factors were studied (such as sherardizing temperature and time, powder composition), while the anticorrosive performance of the sherardized coatings were evaluated by a potentiodynamic polarization method in aqueous NaCl solution, since it is mainly used for corrosion protection.

Advanced Materials Research Vols. 97-101 (2010) pp 1368-1372Online available since 2010/Mar/02 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.97-101.1368

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Experimental procedure

SWRH82B-1 steel wires used for sherardizing were machined from the cold-drawn steel wire (supplied by China Fasten Group Co.) with a strength level of 1670 MPa. This steel contains 0.82wt%C, 0.25wt%Si, 0.73wt%Mn, 0.21wt%Cr, 0.03wt%V, 0.001wt%S, 0.011wt%P, Bal. Fe. The precleaning of steel wires is an essential prerequisite for the high-quality sherardizing coating, which depends largely on the surface contamination. Here, the samples (7mm in diameter and 20mm in length) were grinded by SiC paper, degreased by hot alkaline soak, rinsed in de-ionized water, pickled in hydrochloric acid, then placed in porcelain crucibles filled with zinc-rich powder mixtures. Sherardizing tests were carried out in a tubular argon-purged electric furnace. The process parameters for various samples are summarized in Table 1, including sherardizing temperature, heating duration and mix proportion of zinc-rich powders (7.5 wt.% rare-earth activator, 25-30 wt.% SiO2 filler, Bal. zinc dust ).

Cross-section morphologies, chemical composition and phases of the sherardized samples were analyzed by Olympus XJD-05 optical microscope (OM), JSM-5610LV scanning electron microscope (SEM) with energy dispersive spectroscope (EDS) and XD-3A X-ray diffraction phase analysis (XRD), respectively. The hardness distribution of the sherardizing coatings was measured with a Vickers microhardness tester (HXD-1000TC) at a load of 25g and a loading time of 15s. Corrosion resistance of the sherardized samples was estimated from the potentiodynamic polarization curves measured in 3.5wt.% NaCl solution using an advanced electrochemical system of PARSTAT2273.

Table 1. Process parameters and coating characteristics of the sherardized samples Sample

No. Sherardizing process Coating characteristic

Zinc dust [wt.%]

RE [wt.%]

SiO2 [wt.%]

T

[℃] Time [h]

Thickness [µm]

Ecorr

[V] icorr

[mA/mm2] S1 67.5 7.5 CeO2 25 380 4 41 -0.7475 2.01 S2 67.5 7.5 CeO2 25 380 6 50 -0.6355 1.13 S3 67.5 7.5 CeO2 25 380 8 57 -0.6405 1.18 S4 67.5 7.5 CeO2 25 400 4 45 -0.6865 1.55 S5 67.5 7.5 CeO2 25 400 6 53 -0.6115 0.98 S6 67.5 7.5 CeO2 25 400 8 61 -0.6385 1.09 S7 67.5 7.5 Y2O3 25 400 8 63 -0.6285 1.01 S8 62.5 7.5 CeO2 30 400 8 45 -0.7135 1.86

Results and Discussion

Table 1 lists the average coating thickness of various sherardized samples, and some typical optical micrographs of sherardizing coating cross-sections are presented in Fig. 1. Obviously, the coating is composed of a loose outer layer and a dense inner layer with an indented interface (see Fig.1(b)), while the inner layer is more corrosion resistant. Comparing Fig. 1(a) with (d), it can be concluded that the coating quality formed at 400℃ is better than that at 380℃ because of homogeneous thickness and less thru-micropore in the inner layer. Fig.1(b), (c) and (d) show that prolonging the heating duration results in the thicker inner layer and the smoother interface between the inner layer and the outer layer, but some thru-microcracks are presented in the dense inner layer. Fig.1(e) shows that the coated sample with Y2O3 activator is produced a thicker and more homogeneous coating than that with CeO2 activator (see Fig.1(d)). The coating quality of sherardized sample with 30 wt.% SiO2 filler is very poor (see Fig.1(f)) for an inhomogeneous and thin inner layer, which results in the large-area direct contact of steel substrate with the loose outer layer. Apparently, the better appearance of sherardizing coating correlates with Y2O3 activator, small fraction of SiO2

filler, appropriate sherardizing temperature (400℃) and longer heating duration.

Advanced Materials Research Vols. 97-101 1369

Fig.1 Optical micrographs of sherardized coating formed with CeO2 activator at (a) 380℃/8h, (b) 400℃/4h, (c) 400℃/6h, (d) 400℃/8h;with (e) Y2O3 activator and (f) 30%SiO2 filler at 400℃/8h.

Fig.2 presents SEM micrographs of sherardized coatings formed with CeO2 activator at 380℃/6h (a), 400℃/8h (b), and with Y2O3 activator at 400℃/8h(c). It further proves that the inner layer and outer layer are tight combined together with an uneven interface, while some micro-cracks merely exist in the dense inner layer. The combination between the inner layer and steel substrate is very well with the formation of a white thin interlayer (see Fig.2(a)), which means the good metallurgical bonding of the sherardizing coating with the steel substrate and the interface produce being more corrosion-resistant. More thru-microcracks are presented with longer heating duration due to a thicker inner layer. The sherardized sample with Y2O3 activator is not ideal for plenty of thru-microcracks.

Fig.2 SEM micrographs of sherardized coatings formed with CeO2 activator at (a) 380℃/ 6h(S3) and (b) 400℃/8h(S6), and with Y2O3 activator at (c) 400℃/8h(S7).

EDS analyses for various sherardized samples showed that the atomic rate of Zn/Fe in the loose outer layer is 10.37 with 5.83% Fe for S3, 10.34 with 6.57% Fe for S7 and 10.27 with 7.75% Fe for S8, respectively. While the Zn/Fe rate in the dense inner layer is 8.21 with 9.18 % Fe for S3, 8.07 with 9.50% Fe for S7 and 8.24 with 8.40 % Fe for S8. Referring to the Fe-Zn diagram [15], it implies that the main phase of the outer layer is §-FeZn13 phase with about 6.7%Fe, and that of the inner layer is δ- FeZn7 phase with 8.3-13% Fe. The δ- FeZn7 phase is more brittle than §-FeZn13 phase, which is the reason that microcracks merely exist in the dense inner layer without growth across the interface. EDS line scan of the sherardized sample revealed that the Zn concentration on the interface between sherardizing coating and steel substrate is relative lower than δ- FeZn7 phase. The interface produce (several micrometre thick), as shown in Fig.2(a), is likely to correspond to Γ- Fe11Zn40 phase since the Fe concentration amounts to about 21-25%.

(e) S7 (d) S6

(c) S5

(f) S8

thru-microcrack

(b) S4 (a) S3

substrate

inner layer

outer layer

indented interface

（（（（a）））） （（（（b）））） （（（（c））））

substrate

inner layer

outer layer

white interlayer

1370 Manufacturing Science and Engineering I

Fig.3 shows the XRD pattern of S6 sample sherardized at 400℃/8h with CeO2 activator, which verifies the structure of the sherardizing coating. Three alloy phases were observed in the sherardizing coating, i.e. δ-FeZn7, ξ-FeZn13 and Г- Fe11Zn40. The relative intensity of peaks presents the fraction of δ-FeZn7 is higher than that of ξ-FeZn13 phase, while Г- Fe11Zn40 is rare and no zinc solution were observed. The result of XRD is consistent with EDS analysis above and hardness measurement. It was found that the microhardness ranges of the outer layer and the inner layer are 142-208HV and 266-530HV, respectively.

Fig.3 XRD pattern of the S6 sample sherardized Fig.4 polarization curves of the sherardized

at 400℃/8h with a CeO2 activator samples with various heating durations

Corrosion potentials (Ecorr) and current densities (I corr) of various sheradized samples were obtained from experimental potentiodynamic polarization curves and listed in Table 1. Fig.4 shows typical polarization curves of three sherardized samples with various heating durations. Clearly, the Ecorr value of sample S5 (sherardized for 6 h) is higher than that of S4 (for 4h) and S6( for 8h) sample. From Table 1, the I corr value of S5 is obviously lower than that of S4 and S6. It implies that the corrosion resistance of sample S5 is much better, while a thicker coating is not always favorable for corrosion protection. The phenomenon can be attributed to the presence of microcracks in the inner layer. SEM micrographs of sherardized coatings (in Fig.2) proved that more thru-microcracks are formed with increase the thickness of sherardizing coating. From table 2, Y2O3 activator is better than CeO2 activator and the sherardizing temperature of 400℃ is superior to 380℃. Good quality of sherardized wires are produced with the sherardizing agent containing 7.5wt.% Y2O3 catalyzer and 25wt.% SiO2 additives under 400℃ for 6h.

Obviously, the corrosion resistance of the sherardizing coating is closely related to the electrochemical behavior of Fe-Zn intermetallic phases. Lee et al.[17] found out that the polarization resistance value of Fe-Zn alloy ( in a NaCl + ZnSO4 solution at pH5) increases with increase in Fe content, while the corrosion potential of all three Fe-Zn alloy phases (§-FeZn13, δ- FeZn7 and Γ- Fe11Zn40 phases) are lower than that of Fe (-0.60 V vs. SSE) but higher than the value of Zn (-1.00 V vs. SSE). The results were supported by the finding of Rout et al. about the electrochemical and corrosion behavior of galvanized coatings [18]. Therefore, the sherardized coating can be expected a higher corrosion resistance than hot-dip-galvanized coating, attributed to the two-layered structure consisted of §-FeZn13 phase and dominant δ- FeZn7 phase. Actually, the corrosion results on the life of various zinc coatings to 5% rust (carried out in Sheffield British Iron and Steel Research Association: Source Sherardizing by Zinc Alloy Rust-Proofing Co., Ltc) showed four out of five results with sherardizing are above the other coatings produced by flame spraying and hot dip galvanizing [12]. Sherardizing coating is claimed to be the hardest of all zinc finishes, which is highly wear and abrasion resistant and thus be able to withstand a fair measure of rough handling. Although sherardizing has a longer duration than hot-dip galvanizing, the treatment can be united with stabilizing annealing of SBCW and thereby easily applied to the manufacturing process without increased cost. It means that a sherardizing coating can economically replace an assembly of hot-dipping galvanized coating, due to attractive properties (higher corrosion resistance) and economic or technical advantages (lower operating temperature, less pollutant).

Advanced Materials Research Vols. 97-101 1371

Conclusions

(1) Sherardizing zinc protective coatings on high carbon steel have a two-layered structure: loose outer layer of §-FeZn13 phase and dense inner layer of δ- FeZn7 phase. Some thru-microcracks exist in the inner layer with extra thickness. A small quantity of Γ- Fe11Zn40 phase is formed on the interface of sherardizing coating with steel substrate, contributed to the good metallurgical bonding and corrosion barrier.

(2) Component of solid additives, sherardizing temperature and time should be controlled for improving coating quality. For the high-carbon steel substrate of SWRH82B-1, the addition of Y2O3 activator is better than CeO2 and containing <30wt.% SiO2 filler in zinc power is appropriate. A thicker coating corresponds to a higher temperature or a longer heating duration.

(3) Good quality of sherardized wires are produced with the sherardizing agent containing 7.5wt.% Y2O3 activator and 25wt.% SiO2 filler under 400℃ for 6h. An extra thick coating is unfavorable to improve corrosion behavior of sherardized wires, due to the presence of some thru- microcracks in the inner layer.

(4) Sherardizing zinc protective coating can be used for improving corrosion resistance of high-strength steel wires for structural cables. The new procedure is more attractive than traditional hot-dip galvanizing, due to less pollutant, lower price, easily-operating and better quality.

Acknowledgments

This work was supported by Qing Lan project(Jiangsu, China) and Hohai University. The authors are grateful to Fasten Group Co. (Jiangyin, China) for providing the raw materials.

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