Study of Zn-Ni and Zn-Co alloy coatings electrodeposited on steel strips I: Alloy electrodeposition and adhesion of coatings to natural rubber compounds

Surftice and Coatings Technology, 52(1992)17—30

17

Study of Zn—Ni and Zn—Co alloy coatings electrodeposited on steel strips I: Alloy electrodeposition and adhesion of coatings to natural rubber compounds J. Giridhar and W. J. van Ooij Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401 (USA) (Received January 23, 1991; accepted in final form July 29, 1991)

Abstract The electrodeposition ofZn—Ni and Zn—Co binary alloy coatings on low carbon steel plates from simple acid baths has been studied. Coatings were examined for their composition, phase structure, morphology and deformability. Their properties and plating conditions were optimized such that these coatings would serve to formulate an improved composite coating system for steel tire cords, replacing the conventional brass coating. In view of this, adhesion of several as-deposited and deformed coatings to a commercial natural rubber compound was evaluated. A dual-layer coating system consisting of a zinc-rich Zn—Co alloy underlayer and a nickelrich Ni—Zn alloy top layer was found to exhibit characteristics most suitable for application on steel tire cords.

1. Introduction Since the advent of steel-belted radial tires, brasscoated steel cord is the material most widely used for the reinforcement of automobile tires [I]. The conventional brass coating offers several significant advantages such as ease of electroplating, excellent drawability of the coated wire and high brass to natural rubber (NR) compound adhesion strength that often exceeds the tear strength of vulcanized NR compound. However, two major deficiencies of the brass coating arise from the electrochemical corrosion behavior of the brass—steel couple under salt corrosion and humidity aging [2, 3]. Salt is widely used on icy road surfaces during winter months. Cuts in the tire tread surface may be produced in service. Deep cuts in the tread down to the steel cords expose the latter to corrosion through the formation of oxygen concentration cells [3]. At low pH levels (existing at oxygen-depleted regions deeper in the cut), the brass coating is cathodic to steel; hence it accelerates the corrosion of the underlying steel, thus reducing the strength of reinforcing steel cords, Under conditions of high pH (existing at oxygen-rich regions near the surface of the exposed cord), the brass coating itself corrodes through dezincification followed by dissolution of brass. This in turn results in the deterioration of the adhesion between brass coating and vulcanized NR compound. Further, the fully deformed brass coating on the final cord filaments is porous and does not offer a barrier protection to the steel core,

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A copper-free alloy coating system based on Zn—Ni and Zn—Co binary alloys that can overcome the drawbacks of conventional brass coatings was conceived by van Ooij [4]. A few preliminary experiments were carried out to examine the possibility of using this new coating system in place of brass [4, 5]. Ideally, the coating would consist of a zinc-rich first alloy layer on steel to provide galvanic protection against corrosion, followed by a nickel-rich second alloy layer for improved initial and aged adhesion to NR compounds. Results of aged adhesion experiments reported here in Part I and of electrochemical corrosion studies to be discussed in Part II have demonstrated that sacrificial corrosion of zinc does not lead to any appreciable adhesion loss in the corroding area. An improved adhesion retention upon aging is expected based on a significantly lower dezincification rate claimed for nickel-containing coatings in comparison with that of brass coatings [2]. However, in order for the new coating system to qualify as an industrially viable candidate to replace the brass coating, it must satisfy the following criteria: (i) it must be amenable to direct alloy electroplating onto steel at high current densities from simple cyanide-free acid baths; (ii) it must have sufficient ductility and low friction so as to withstand heavy cold deformation during wet fine drawing and cording of the coated wire; (iii) it should exhibit high initial and aged adhesion to NR compounds; (iv) it must offer galvanic protection against the pitting corrosion of steel cords often observed in actual tire samples after on-the-road testing [5, 6].

©

1992

Elsevier Sequoia. All rights reserved

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Zn Ni and Zn (a co hogs un ~tceIstrip; I

The objectives of the research described here are the development and characterization of dual-layer coatings based on Zn -Ni and/or Zn Co binary alloy layers and the selection of a specific combination of alloy layers that best satisfies the criteria listed above. The experiments and results reported here in Part I of the twopart publication involve (i) the deposition of Zn -Ni and Zn Co binary alloys on flat steel strips with a laboratory—scale electroplating cell, (ii) optimization of the alloy coatings, plating baths and electrodeposition conditions (in terms of compositions and phase structures of coatings, as related to their deformability and adhesion to NR compounds), and (iii) investigation of failure modes of special joints prepared by bonding an NR compound (through vulcanization) to the co ited steel strips (co ited ~sith optimized dual I iyer coatings) in comparison to Cu 35 wt % Zn brass strips Studies of electrodeposition of the dual-layer coating system on actual running stccl wires at the pilot plant level and of its deformation behavior during subsequent wire-drawing and cording operations are to be published in due course. Although the primary motive for the development of this dual—layer coating system is its intended application in steel tire cords, such a coating system is suited to use in other applications involving sheet steels. For example. use of Zn--Ni- or Zn—Co-alloy-coated automotive sheet steels is already widely practiced by many major automotive industries world wide [7]. The dual-layer coating design studied here offers sacrificial protection (zinc-rich layer), barrier protection (nickel-rich layer) and better overall formability. Because of the cathodic nature of the nickel-rich surface layer, it is reasonable to expect excellent phosphatability and paintability as well. Thus the coating system may offer significant advantages in automotive applications,

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absorption spectroscopy (AAS). The pH of each bath was adjusted and maintained with sulfuric acid and sodium hydroxide additions. The pre-treatment procedure applied to the steel strips prior to every deposition consisted of the following sequence of steps: (a) pre-rolling the steel strip to lO% reduction in thickness. (b) forming the strip into a cylindrical ring. (c) cleaning the steel surface with a detergent and rinsing, (d) alkaline anodic electrocleaning and rinsing with water, (e) acid dipping and rinsing with deionized water and (f) final rinsing with a pure volatile organic solvent. The cathode rotation speed was fixed at 207 rev mm so that it would be close to the typical running speed (50 m mm i) of a line 1.4 mm in diameter during plating in an industrial line.

Electrodepositions of single- and dual-layer coatings of Zn—Ni (more than 50 wt.% Zn), Ni Zn (more than 50 wt.% Ni). Zn—Co (more than 50 wt.% Zn) and Co Zn (more than 50 wt.% Co) binary alloys were made on cold-rolled low carbon steel strips. Each strip was first formed into a ring and mounted on a rotating sample holder (Fig. I). Each time, a precisely marked area (either 0.5 or 1.0 dm2) on the outer cylindrical surface of the cathodic strip was exposed for deposition. The rest of the area was masked off with plastic adhesive tape. Four 99.85% pure zinc sheets arranged concentric to the cathode ring served as anodes. Simple acid plating baths of various formulations were utilized for the deposition experiments (Table 1). The composition of each plating bath was analyzed routinely using atomic

2.2. Coating analysis and characterization The coating composition was analyzed routinely by scanning electron microscopy (SEM)—energy-dispersive X-ray analysis (EDXA) using a JEOL JX 840 instrument with Tracor Northern EDXA system TN-5500. The thickness of thick coatings (above 1.5 l.tm) was directly measured from an SEM image of a coating cross-section. Thickness estimates of thinner coatings were made using a proprietary thin film analysis program available with the EDXA system. In some cases, coating weights and compositions were determined by dissolving the coating in concentrated nitric acid and performing elemental analyses of the solution using AAS. These AAS results were utilized in cross-checking the results of SEM

2. Experimental procedure

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EDXA. Coating surface morphology and homogeneity were also examined with SEM EDXA. Phase structures and qualitative preferred orientations of the coating crystals were investigated using X-ray diffraction (XRD). XRD was performed on a Rigaku200 system using Cu K~radiation. Deformahility of the coatings was assessed qualitatively by cold rolling the coated steel strip to a total of 50% reduction in thickness in five l0% steps and examining the rolled coating surface under SEM for cracks in the coating.

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Standard lap-shear samples in the unaged and the humidity-aged conditions were subjected to the usual lap-shear test for adhesion strength. The special samples, however, were immersed in liquid nitrogen for a few minutes and then quickly broken by pulling apart the free ends of the bonded plates. Fracture surfaces of both the separated pieces were inspected visually and under SFM—EDXA to analyze their compositions and to identify the failure modes. Further, the humidity-aged samples were inspected for red-rust formation on both the coated and the bare-steel sides.

2.3. Adhesion testing and the anal v.si.s of joint’failurc modes

Four different dual-layer coatings resulting from the combinations of four single-layer alloy types, i.e. (a) Ni Zn on Zn Ni, (b) Ni--Zn on Zn Co. (c) Co Zn on Zn Ni and (d) Co--Zn on Zn--Co. were deposited and tested first for ductility and interlayer adhesion integrity upon cold rolling and then for adhesion to NR compounds. Preliminary tests on the adhesion of as-deposited and cold-rolled coated strips to a few proprietary NR compounds (Table 2) were conducted with standard lapshear test samples (Fig. 2(a)). These tests were performed using an Instron mechanical testing system at a constant cross-head velocity of 10mm min The level of adhesion was estimated by both the maximum force at joint failure and the percentage rubber coverage of the joint area after failure. After finding that an NR compound (NRC-3) containing a cobalt organic salt exhibited the highest level of adhesion, further tests were conducted using only this specific compound. Attempts were made to investigate the rubber—coating interface of samples by separating the rubber from the coating surface. Samples that show poor adhesion had failed interfacially and hence the interface was readily available for analysis. However, samples exhibiting good adhesion and a cohesive failure in the rubber defied all attempts at exposure of the interface. Efforts to fracture the rubber at liquid-nitrogen temperature or swelling it in toluene did not succeed in separating the rubber from the coating. For reasons described in the following section, dual-layer coatings (a) Ni- 20 wt.%Zn on Zn--S wt.%Ni and (h) Ni—20 wt.%Zn on Zn—I wt.%Co showed the greatest promise for use in steel tire cords, Several samples of these two dual-layer coatings in asdeposited and cold-rolled conditions were bonded to NR compound NRC-3 to obtain standard lap-shear and special specimens (Fig. 2). The latter have a configuration similar to standard lap-shear geometry except that one of the plates was rotated through I 80~(Fig. 2(b)). These test specimens with the special configuration were prepared in an attempt to induce interfacial failure during breakage. Some of the bonded specimens of both eonfIgurations were subjected to humidity aging at 65 ~C and 9O°/o relative humidity for 4 days and for 8 days. ~.

3. Results and discussion 3.1. Optimization of electrodeposition and coating properties 3.1.1. Plating bath composition and deposition condition.s

Several plating bath formulations were tested and compared based on their ability to deposit coatings with properties required of a steel cord coating. Some of these bath compositions tested are given in Table I. The effects of deposition conditions such as plating temperature. bath pH, plating current density and cathode rotation speed on the composition and structure of the deposited coatings were also investigated in detail. The important results of these plating experiments are summarized below. In general, the results obtained from these experiments on the electrodeposition of Zn Ni and Zn -Co binary alloys are in good agreement with the results of similar investigations that have been published by other researchers in this area [8 II]. Table I also gives the bath compositions and plating conditions that were used for deposition of the optimized dual-layer coatings. It was found that the presence of chloride ions in the bath improved the brightness of all the alloy coatings. However, the coatings (especially Ni--Zn) exhibited brittleness and poor adherence to steel upon cold rolling (Fig. 3). Hence only pure sulfate baths were used in later depositions. Addition of sodium lauryl sulfate (SLS) to the bath improved coating adhesion to steel but the brittleness of the coatings persisted. Hence SLS was also discarded from the bath formulation. Pure sulfate baths with boric acid buffer gave duller coatings with relatively improved ductility. However, coatings with maximum deformability and adhesion to steel (as indicated by the absence of exposed steel after rolling) were obtained from baths free of boric acid, chloride ions and SLS. These baths contained only sodium sulfate as the supporting electrolyte to improve the conductivity of the bath for operation at high current densities. Considering the general effects of plating conditions for any specific bath, it was found that an increase in plating temperature significantly reduced the amount of

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Typical XRD patterns obtained from some of the coatings are presented in Fig. 4 together with the corresponding secondary-electron images of coating surfaces. The patterns from Ni Zn (more than 70 wt.°/~Ni) and Co Zn (more than 80 wt.%Co) showed that these coatings consisted of the single phase (Figs. 4(a) and 4(e) respectively). The composition range of the pure 2 phase was found to be between 10 and 30 wt.%Ni for the Ni Zn co stings (Figs 4(b) and 4(c)) Ni Zn coatings with compositions outside these two ranges were dual-phase structures, either (high nickel) or ~‘+tm (low data. nickel). The ~ phase was ~+;‘ not discernible from the XRD The ~ phase Ni Zn coating was either microcrystalline tending towards an amorphous structure or crystalline with a noticeable (Ill) preferred orientation with respect to the coating surface. The microcrystalline coating exhibited higher deformability. The subgrain size indicated by X-ray peak width (full width at half-maximum) varied depending on the coating thickness. its composttion, plating conditions and pre-treatment of the steel surface prior to deposition. It was observed that an -~

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zinc in binary alloy coatings and increased the size of grains. Bath pH variations within the narrow acidic region of I.0—4.0 did not affect the coating composition significantly. However, coating deformability was adversely affected if bath pH exceeded 2.5. At pH values less than 1.0, cathode current efficiency decreased rapidly owing to excessive hydrogen evolution. Deposition cur-

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increase in Ni--Zn coating thickness resulted in a corresponding increase in subgrain size. Ni --Zn coatings thicker than about 0.6 tim with 20 wt.%Zn or more tended to be more crystalline (larger subgrain sizes) and were more brittle than thinner coatings. Thus the optimum thickness for the Ni—Zn alloy layer was determined to be around 0.5 p.m. Pre-rolling the steel strip changed the texture of the cold-rolled steel substrate from (110) to (211). Deposition of Ni—Zn coating on this steel texture seemed to improve its deformability and its adhesion to steel. The preferred orientation of pure phase Zn Ni was found to be strongly dependent on the plating bath. plating conditions and method of preparation of the steel surface for plating. The preferred orientations ~ I),(4 lUand (33O~and (442) and (600) tions gave a coating ductile enough so as not to crack and expose the steel upon cold rolling. Hence, the composition of the Zn—Ni layer was changed from 12 wt.%Ni (pure ~‘) to 5 wt.%Ni (mj+~)of Fig. 4(d). The latter composition, while being less resistant to salt corrosion than pure 2 is, performed better in resisting corrosion than a pure zinc coating of equivalent thickness when immersed in a I M NaCl solution [12]. Since this Zn 5 wt.% Ni coating contained a sufficient amount of the ductile ~iphase, it was also adequately deformable. In the case of Zn—Co coating, it was observed that about

J. Giridhar, 14< J. van Ooij

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and other relevant studies were deposited so as to conform to the above specifications for alloy layer thickness. 3.1.3. Adhesion of coatings to natural rubber compounds Initial adhesion studies of Ni—Zn single-layer and Ni— Zn-on-Zn—Ni dual-layer coatings to several NR cornpounds (Table 2) showed that compound NRC-3 which contained a cobalt adhesion promoter gave the best results. It appears that cobalt in the NR compound is necessary to obtain an acceptable adhesion level to Ni—Zn coatings. Table 3 gives the adhesion test results

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(joint separation force in newtons) for some of the first set of coatings that have been tested. This initial set of adhesion tests were performed on several ductile and brittle Ni—Zn single-layer coatings of various nickel contents as well as a few dual-layer coatings with the brittle ~ phase Zn—Ni alloy forming the bottom layer. It must be noted here that the first step in steel strip pre-treatrnent prior to coating deposition (i.e. cold rolling to 10% reduction in strip thickness) was omitted while preparing this first set of coating samples. Several of these samples exhibited good adhesion to NRC-3 in

the as-deposited condition, The adhesion of as-deposited coatings to NRC-3 was comparable with that of bulk brass to NRC-3. This confirms the feasibility of using Ni—Zn coatings in place of brass coatings. Cold-rolled coatings of the first set of samples, however, showed an adhesion level much lower than that of brass and comparable only with that of zinc-plated steel. In the case of brittle coatings, coating rupture upon cold rolling which exposed the underlying steel might have caused local debonding, thus reducing the effective bonded area and consequently the overall joint

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.1. Giri’d/ta,’, (1< .1. cot Ooij

Zn- Ni ititil Zn Co coatt’ng.s on steel s-trip; I

separation force. Further, the cold-rolled samples, being only half as thick as the as-deposited samples, were always noticeably bent after adhesion testing. This mdicates that the lap-shear joint specimens constructed with cold-rolled coated steel strips may have been subjected to bending moments as well (instead of just pure shear) during adhesion testing. A set of dual-layer coating samples consisting of the sufficiently ductile Zn—S wt.% Ni (i~-rich ij+;) or Zn I wt.%, Co (pure t~phase) alloy as the bottom layer was deposited from the optimized plating baths. Cornposition of the Ni Zn top layer was fixed at about 25 wt.°/ Zn. Severe interlayer adhesion problems were encountered between ti-rich Zn Ni underlayer and pure a Ni Zn top layer. unlike the previous combination of a-on--; dual-layer coatings. This might be due to a lattice mismatch between the h.c.p. t~ phase and the f.c.c. a phase at the interlayer interface. The a phase was seen to deposit in a loosely held powdery form on the ~+ underlayer. This interlayer adhesion problem was solved satisfactorily for the Ni ---Zn-on-Zn--Ni dual-layer system by rinsing the first layer’s surface thoroughly with distilled water and acetone and letting i dry completely before depositing the Ni Zn layer on it. The Ni—Zn-onZn—Co dual layer did not present this problem and its interlayer adhesion always seemed better than that of Ni--Zn on Zn Ni. 3.2. Tests on coatings deposited from optimized plating

baths 3.2.1. Adhesion tests Lap-shear adhesion test results of a fresh set of duallayer coatings deposited from optimized plating baths under fixed plating conditions (baths 4, 7, 8 and 9 in Table I) are presented in Table 4. Qualitative comparison of the higher joint failure force values in Table 3 (obtained from the unoptimized initial set of coating samples) with those in Table 4 (obtained from the optimized fresh set) shows that the latter values are somewhat lower than the former, This could be attributed to the 10% reduction in overall thickness of the coated strips in Table 4 with respect to those in Table 3. It is also seen that, in the case of samples exhibiting good adhesion (i.e. lOO% rubber coverage), the joint failure force for a coating in the as-deposited condition is always higher than that for the same coating in the cold-rolled condition. This could also be the effect of reduced sample thickness, the cold-rolled samples being only half as thick as the as-deposited samples. It is worth noting that the standard bulk (not electrodeposited!) brass sample whose adhesion performance is taken as the base reference for comparisons was significantly thicker than all the coated samples in Tables 3 and 4. The dual-layer Ni--Zn-on-Zn--Ni samples that show poorer adhesion in the as-deposited condition than in the rolled condition

are those that exhibited poor interlayer adhesion. It is possible that the interlayer adhesion was slightly improved by the cold-rolling operation. The degrees of rubber coverage of the tested adhesion samples, as indicated in Table 4, agree well with these observations. Several samples exhibited I00% rubber coverage after the adhesion test, indicating cohesive failure in rubber. This shows that at least these coatings exhibit excellent adhesion to rubber and that the rubber--coating interfacial adhesion strength is higher than the cohesive strength of the vulcanized NR compound. Co--Zn coatings containing 70 and 90 wt.%Co were electrodeposited and tested for deformability. These coatings seemed brittle and exhibited severe and moderate cracking (respectively), exposing the steel substrate upon cold rolling. Dual-layer coatings of Co--tO wt,%Zn on Zn I wt.%Co and Co--- 10 wt.%Zn on Zn--S wt.%Ni were also prepared. These did not show any interlayer adhesion problems but the top layer cracked severely upon cold rolling. The joint failure force and rubber coverage of a Co --Zn single layer and a Co- Zn-on-Zn Co dual layer were quite comparable with that of Ni Zn on Zn Co and with that of brass but they were significantly poorer in the case of Co—Zn on Zn—Ni. This could be due to the more brittle nature of the latter. It could be seen from Tables 3 and 4 that percentage rubber coverage on the failed surfaces is a function of both the deformability and the cobalt and/or nickel content of the coating. The effect of humidity aging on the joint failure force and percentage rubber coverage of Ni Zn-on-Zn Ni and Ni Zn-on-Zn Co dual-layer coatings and bulk brass strip samples to NRC-3 is given in Table 5. As seen before, the improved rubber coverage of rolled samples of Ni Zn on Zn—Ni could be due to the increased interlayer adhesion resulting from the rolling operation. In the case of the Ni Zn-on-Zn Co coating, the rubber coverage was not at all affected by humidity aging up to 8 days. Hence, the observed drop in joint separation force does not reflect the degradation of rubber--coating adhesion but instead reflects a decrease in the cohesive strength of the vulcanized NR compound. -

-

3.2.2. Analysis of failure modes The results obtained from visual and SEM EDXA of failure surfaces of special samples that were broken in liquid nitrogen are summarized below in terms of each coating system. The uncoated steel back side of all the aged samples showed extensive red rust formation but the coated side did not show any red rust. This indicates that the Ni-- Zn top layer and the Zn—Ni or Zn- -Co underlayer cover the coated side sufficiently well and thus offer effective barrier and sacrificial protection to the coated side. --

J. Giridhar, W J. van Ooij

/

Zn—Ni and Zn—Co coatings on steel strip: 1

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J. Giridhar, W J. van Ooij

/

Zn—Ni and Zn—Co coatings on steel strip; I

3.2.2.1. Ni—Zn-on-Zn--Ni dual-layer system. Two types of sample were considered. (i) As-deposited coating. The failure in these samples in both unaged and aged conditions was in the Zn—Ni coating layer very close to the coating—steel interface as shown in Fig. 5(a). The failure was almost planar and parallel to the steel surface. The rubber side carried most of the coating and is almost completely covered by the back side of the coating. The metal side did not show any rubber and was seen to be only partly covered with a thin layer of Zn—Ni coating. This type of failure may occur owing to the inherently brittle nature of the dualphase Zn—Ni coating which contains some brittle y phase. It is also possible that a weak boundary layer may exist in the Zn—Ni coating close to the steel—coating interface as a result of non-steady state deposition conditions during the first few seconds of multiphase alloy deposition. The EDXA results confirmed this failure mode. The apparent lack of difference in observed

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failure mode before and after aging indicates that humidity aging does not affect the adhesion of rubber to the coating. (ii) Cold-rolled coating. The failure in these samples in both unaged and aged conditions started in the Zn---- Ni coating layer (as in as-deposited samples) but quickly transferred to the rubber. Most of the failure occurred partly in the rubber cohesively and partly at the coating-rubber interface, at regular intervals of about 0.1 mm. Only a small region of the rubber-side surface consisted of some pulled-out coating. This situation is depicted in Fig. 5(b). This type of failure can occur because of an increase in coating-to-steel adhesion by the cold rolling of the coated steel strip. It was also noted that this mode of failure was very similar to that observed in unaged bulk brass plate samples. No significant sulfide film formation at the rubber—coating interface could be detected by SEM—EDXA even after humidity aging for 8 days.

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Fig. 5. Schematic representations ofjoint failure modes observed in (a) unaged and humidity-aged special samples (see Fig. 2(b)) of as-deposited Ni—22 wt.%Zn-on-Zn—5 wt.%Ni dual-layer coatings,(b) unaged and humidity-aged special samples of cold-rolled Ni—22 wt.%Zn-on-Zn- 5 wt.%Ni dual-layer coatings (and in unaged special samples of cold-rolled Ni—21 wt.%Zn-on-Zn—l wt.%Co dual-layer coatings), (c) unaged and humidityaged special samples of as-deposited Ni—21 wt.%Zn-on-Zn—l wt.%Co dual-layer coatings, (d) humidity-aged special samples of cold-rolled Ni—21 wt.%Zn-on-Zn—l wt.%Co dual-layer coatings, (e) unaged special samples ofCu—35 wt.%Zn bulk brass strips and (f) humidity-aged special samples of Cu—35 wt.%Zn bulk brass strips.

J. Giridhar, W J.

van

Ooij

Zn—Ni and Zn—Co coating,s on steel

3.2.2.2. Ni—Zn-on-Zn--Co dual-layer system. Two types of sample were considered, (i) As-deposited coating. The failure in these samples in both unaged and aged conditions were fully cohesive in rubber and the failed pieces both contained about equal thicknesses of rubber, indicating that the failure was fully controlled by the cohesive strength of rubber alone. This failure mode is illustrated in Fig. 5(c). Unlike the Ni—Zn-on-Zn—Ni system, the absence of any detectable failure in the Zn—Co layer is probably because the Zn—Co layer with only I wt.%Co is far more ductile than the Zn—Ni layer. This further indicates that the coating—steel adhesion is sufficiently strong. Neither the rubber—coating interface nor the coating—steel interface seems to be affected by humidity aging. (ii) Cold-rolled coating. The failure in these samples in unaged condition was similar to unaged brass and coldrolled Ni—Zn-on-Zn—Ni coatings, as depicted in Fig. 5(b). The aged samples, however, failed in a manner similar to the as-deposited samples, i.e. the failure was fully cohesive in rubber, but failure was closer to one of the rubber—coating interfaces as indicated in Fig. 5(d), rather than closer to the center plane bisecting the rubber bulk. This could be just a mechanical effect due to the thinner and hence more flexible steel plates. The aging treatment seemed to promote cohesive failure in rubber and suppress interfacial failure between rubber and coating. Again, SEM—EDXA could not detect any significant sulfide film formation at the exposed coating-rubber interfacial regions in unaged samples.

Fig. 6. Secondary-electron image of a corner of the rubber side surface of a fractured humidity aged (8 days) special joint sample (see Fig. 2(b)) made with cold-rolled Cu—35 wt.%Zn bulk brass strips bonded to NRC-3; the Joint s center is to the top left corner of the image; the presence of a thick sulfide film (brighter grey region) near the joint edges is seen clearly,

I

29

3.2.2.3. Brass plate samples. In the unaged condition, the failure was partly cohesive in the rubber and partly at the brass—rubber interface as in Fig. 5(e). A very thin layer of sulfide film was detected by SEM—EDXA at the exposed brass—rubber interface, indicating that the sample was in an undercured state. The aged samples clearly showed the formation of a thick copper sulfide film near the edges where the moisture had penetrated; the failure path in these edge regions was between the sulfide film and brass alloy. In the rest of the central joint area where there was no significant moisture-assisted sulfide film growth, failure was fully cohesive in rubber (Figs 5(f) and 6). The sample that was aged for 8 days showed a thicker sulfide film covering a larger area near the edges of the joint than did the film aged for 4 days. These results strongly suggest that both Ni--Zn-onZn—Ni and Ni—Zn-on-Zn—Co dual-layer coating systems are not affected by the humidity aging treatment. Further, the mechanism of adhesion of NR compounds to these coatings seems to be different from that of NR compound—brass adhesion in that the former probably involves the formation of a much thinner sulfidized interfacial layer. The failure modes clearly indicate the more brittle nature of Ni--Zn-on-Zn—Ni system than Ni—Zn on Zn—Co; hence the latter is the most promising candidate to replace the brass coating on steel tire cords.

Conclusions

4.

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strip;

(I) The Ni—Zn (about 80 wt.%Ni) on Zn—Co (about I wt,% Co) dual-layer alloy coating exhibits properties of the Zn—Ni and Zn—Co binary alloy systems best suited to steel tire cord applications. (2) These alloy layers can be electrodeposited from simple stable acid plating baths under conditions that are industrially viable. (3) The alloy layers in the coating consist of tj (h.c.p.) phase (Zn—Co underlayer) and ~ (f.c.c.) phase (Ni--Zn top layer) as determined by XRD analysis. (4) Interlayer adhesion between the two alloy layers and the adhesion of Zn—Co underlayer to steel are sufficiently high; hence, localized loss of coating due to flake-offeffects during cold-rolling operations is minimal and insignificant. (5) The coating is sufficiently ductile and withstands severe cold deformation. (6) Excellent adhesion of coating to NR compound is obtained when the compound contains small amounts of a cobalt adhesion promoter. (7) Loss of interfacial adhesion under high humidity aging conditions that is characteristic of brass is absent with the new Ni—Zn-on-Zn—Co dual-layer coating. .

,

.

.

.

.

.

.

.

.

.

30

.1. Giridhar, 147 J.

van Ooij

Zn—Ni and Zn

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coatings on steel .strtp; /

Acknowledgments

3 Y. Ishikawa and S. Kassakami. Rubber (‘bent. Jecltttol.59 (1986) 1. 4 W ivan Ooij, Pirelli Coordinamento Pneumatici SpA..

The authors gratefully acknowledge Mr. Marco Nahmias of Pirelli PCP, for carrying out the adhesion tests and providing vulcanized test specimens. The research work described here was supported by a grant from Pirelli PCP, Milan, Italy.

Milan. Italy. Lur. Patent EP (1 283 738, September 28. 5 W. J van Ooij, i Giridhar and 3 H Ahn. kautsch GuntnII, Kun.st.st., 44(1991) 348. 6 1. H. Ahn. MS. Thesis, Department of (‘hemistry, Colorado School

References I R. S. Bhakuni and S. K. Mowdood, Tires, The Goodyear Tire and Rubber Co., Akron, OH, 1987. 2 W. J. van Ooij, Rubber Chern. Technol., 57(1984) 421.

of Mines. Golden, CO 80401. 7 T. F. Sharples. Prod. ttntssh., 54 (1990) 38. 8 D. F. flaIl, P/cit Sun. Finis/t., 70(1983) 59. 9 S. Swathtrajan. J - Electroanal. (‘Item. Intericciwl Elect rochem. - 221 (1987) 211. 10 V. M. I. C’. Verbernc, ‘Trcots. Inst. ~Iel. littisit -, 64 (986) 30. II W. Siegert, Metalloherfldi-lte, 4/(1987) 259. 12 1. Giridhar and W..I.~’an Ooij. Stir). Coat. 7cc-Itnol.. ((992). submitted for publication.