Bonding Properties between Hot-Dip Coated Steel Strip and Polyethylene in Corrugated Pipe

Abstract

We studied the effects of different types of hot-dip coatings and passivation treatments on cross-sectional morphology, adhesion to polyethylene (PE), and bonding energy. The results revealed that the bonding force of a hot-dip Zn–Al coating on a steel strip to PE is 196.3 N. The adhesion of Zn–Al and Zn–Al–Mg to PE before passivation was increased by 84.6% and −31.1% higher than that of Zn and PE. After passivation, the adhesion of Zn–Al and Zn–Al–Mg to PE was increased by 63.8% and −46.6% higher than that of Zn and PE, respectively. Passivation treatment can effectively improve the bonding between hot-dip coatings and PE. The bonding strengths of the hot-dipped Zn, Zn–Al, and Zn–Al–Mg to PE increased by 73.3%, 26.6%, and 33.3%, respectively, following passivation. The application of a silane coupling agent to the passivator formed chemical bonds on the surface of the hot-dip coatings, which improved the adhesion between the coatings and resin. Overall, utilising a hot-dip Zn–Al coating instead of a pure zinc coating (containing 0.01% Al) can improve the adhesion between the coating and PE in steel-strip-reinforced PE spiral-corrugated pipe (MRP), thereby improving the application performance of the MRP.

1. Introduction

A steel materials strip-reinforced polyethylene (PE) spiral-corrugated pipe (MRP) is composed of high-density PE as a base (inner and outer layers) and a wave-like steel strip coated with modified PE as a reinforcing support structure.¹⁾ The steel strip has an arched shape with high strength and is wound with the PE to form MRP. MRP has good corrosion resistance and high strength. It is widely used in urban underground pipeline networks and other large-scale projects.²⁾ The interface bonding between the steel strip and PE determines the service life of the MRP.³⁾ There are several factors that affect the adhesion between the steel strip and PE, including the amount of adhesive resin, thickness of the steel strip, and passivation treatment. To improve interfacial bonding, several studies have been conducted on the modification of metal and organic components. Regarding metal components, a silane coupling agent is typically applied as a passivator for hot-dip galvanising coatings. Silane with amine groups can promote interface bonding between a metal substrate and organic coating via chemical bonding.^4–8) Organic components are typically modified utilising PE, where maleic anhydride is often used to graft polyolefins because it is an effective compatibilizer in terms of improving the thermal and mechanical properties of composites.^9–12) Yuan¹³⁾ discovered that silane films can significantly enhance the adhesion between epoxy coatings and aluminium alloys. They also provide effective corrosion resistance. Rodošek¹⁴⁾ studied the protective properties of 1,2-di (trimethoxy silyl) ethane films on a 2024 aluminium alloy. Underhill¹⁵⁾ investigated the reaction of 3-glycidyl propyl trimethoxy silane on the surface of aluminium with silanol and epoxy groups, as well as the existence of a wide range of Si–O–Si bonds and potential Si–O–Al bonds. Alumina exhibits excellent surface passivation properties at Si/Al₂O₃ interfaces based on chemical passivation and a reduction of interface defect density.^16–23)

Typically, MRP is lined with hot-dip galvanised strip steel. To avoid oxidation of the strip from transport to coating, a clean interface is typically created by applying surface passivation. Silane coupling agents are widely used for metal pre-treatment based on their non-toxicity, low cost, and excellent adhesion.^24,25) There are many types of silane coupling agents with different pre-treatment processes and applicability. The silane coupling agent KH-560, which is often applied between metals and organic polymers, was optimised in this study. Although passivation treatment is widely used, if a metal strip is not passivated, but PE still maintains a strong bond, significant cost reduction can be achieved. In this study, the effects of passivation on the adhesion of substrates followed plastic coating were analysed.

In addition to pure zinc, the hot-dip coating of steel also utilises Galfan, ZAM, and other hot-dip coating alloys.^26–28) Compared to a hot-dip pure zinc coating, various hot-dip zinc-based alloy coatings have unique properties in terms of flexibility, adhesion, and microstructure that may affect their bonding properties.^29,30) The Galfan alloy is widely used in the construction industry based on its superior corrosion resistance, aesthetic appeal, and low thickness based the addition of Al and rare earth elements.^31,32) ZAM is a zinc-based alloy with added Al and Mg. It is widely used in home appliances and the automotive industry based on its excellent corrosion resistance.^26,33–35) In actual production, the adhesion between galvanised steel strip and PE is less than ideal. It typically only meets national standards with a peel strength of approximately 100 N cm⁻¹.³⁶⁾ Therefore, improving the performance of MRP is an urgent problem. Relatively few studies on the bonding properties between hot-dip coatings and modified PE have been published in the literature.³⁷⁾

In this study, the adhesion of Zn–Al (such as Galfan and ZAM) hot-dip coatings to PE was analysed. The effects of passivation, roughness, and chemical bonding on adhesion between the coatings and PE were considered.

2. Materials and Methods

2.1 Materials

Low carbon steel strip samples (15 cm × 5 cm × 0.5 mm) and maleic anhydride grafted PE (PE-g-MAH) were provided by Langfang Huachuangtianyuan Co., Ltd. The chemical compositions of the hot-dipped Zn, Zn–Al, and Zn–Al–Mg alloy ingots are listed in Table 1. There are three types of auxiliary plating agents, namely hot-dip Zn, Zn–Al, and Zn–Al–Mg auxiliary plating agents, as shown in Table 2. Passivator γ-glycidyl ether oxy propyl trimethoxy silane (KH560, hereafter referred to as a silane coupling agent) was provided by Shanghai Macklin Co., Ltd.

Table 1 Alloy composition (mass%).

Table 2 Compositions of plating agents.

2.2 Sample preparation

Samples were pre-treated via acid pickling at room temperature. The pickling process involved placing the steel samples in a 15% hydrochloric acid solution for 5 min, followed by washing with distilled water prior to plating. All samples were then dried to ensure safety. Finally, the samples were dipped into molten alloy baths at 460°C, 420°C, and 520°C for 20 s to produce hot-dip Zn, Zn–Al, and Zn–Al–Mg, respectively.

The passivation solution was prepared by mixing 5% silane, 5% deionised water, and 90% ethanol. This solution was then stirred for 1 h and stored for 2 d to allow for hydrolysis. The prepared samples were immersed in passivation solutions at room temperature for 2 h and then dried at 100°C for 30 min.³⁸⁾

A flat vulcanising machine (HB-350 × 350 × 202) was utilised to fold equal amounts of bonding resin into the upper and middle parts of two identical plates with their tails folded to 90° to facilitate tensile testing. The thickness of the resin was 1 mm and the thickness of the two steel plates was 0.5 mm. The conditions for the coating treatment were 240°C, 10 MPa of pressure, and a duration of 5 min. For simplicity, the details of the procedure are presented in Fig. 1. The coated specimens were uniformly cut into 15 cm × 3 cm × 2 mm strips.

Fig. 1

Plastic coating process.

2.3 Measurements and observations

The T-peel strength test was carried out by electronic universal testing machine (CMT6104). The tensile test speed was 50 mm/min at room temperature and three samples were tested simultaneously. The average of the measurements was taken as the result. According to the peel strength formula:³⁹⁾ G = 2F/w, F is the adhesion, in N; w is the sample width, in cm. The width of the all samples is 3 cm, so the change trend of the adhesion is consistent with the peel strength. So, this paper only compares the magnitude of the force. This process was followed by polishing of the cross sections of the samples to a mirror finish prior to etching. The microstructure of each sample was observed utilising scanning electron microscopy (SEM, QUANTA 450 FEG) and the surface roughness values of different alloy coating samples in the form of arithmetic averages (Ra) were assessed utilising a surface roughness tester (NDT150). Chemical bonds were analysed via Fourier transform infrared spectroscopy (FTIR, TENSOR 27) with a resolution of 0.5 cm⁻¹ in a scan range of 400–4000 cm⁻¹. The compositions of the passivated surfaces were measured utilising X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with Al Kα radiation. Charge correction was performed at 284.8 eV based on C1s peaks.

3. Results and Discussion

3.1 Adhesion of different coatings to PE

Figure 2 presents the trends of the adhesion between the hot-dip Zn, Zn–Al, and Zn–Al–Mg layers to PE before and after passivation. This figure reveals that the adhesion between the plated parts and bonding resin increases following passivation. From 106.6 N to 184.7 N, the force increase for the Zn coating is 73.3%. From 196.3 N to 248.5 N, the force increase for the Zn–Al coating is 26.6%. From 73.5 N to 98 N, the force increase for the Zn–Al–Mg coating is 33.3%. However, based on the use of a silane coupling agent in the passivator, chemical bonding was initiated on the surfaces of the hot-dip coatings and chemical reactions occurred, which improved the adhesion between PE and silane.¹⁵⁾ The passivation treatment also increased the surface roughness of the hot-dip coatings, thereby increasing the contact area between the PE and the substrate, as shown in Fig. 3.

Fig. 2

Changes in binding force before and after passivation for hot-dip Zn, Zn–Al, and Zn–Al–Mg coatings.

Fig. 3

Roughness of hot-dip Zn, Zn–Al, and Zn–Al–Mg coatings before and after passivation.

Compared to hot-dip galvanising, the adhesion of the Zn–Al coating before passivation increased by 84.6%, while that of the Zn–Al–Mg coating decreased by 31.1%. Following passivation, the bonding force of the Zn–Al coating increased by 63.8%, while that of the Zn–Al–Mg coating decreased by 46.6%, as shown in Fig. 4. The Zn–Al alloy clearly improves the bond strength, which can be attributed to Al catalysis, which promotes the combination of the two elements.¹⁵⁾

Fig. 4

Growth rates of binding force before and after passivation compared to hot-dip galvanising.

In the Zn–Al–Mg alloy, on the one hand, the addition of Mg causes the formation of MgZn₂ and other intermetallic compounds^40,41) in the solidification process of the alloy coating, which is not easy to be corroded on the coating surface. MgO⁴²⁾ can be formed on the surface of the substrate, and the surface quality of the coating is lower than that of Zn–Al, so these factors affect the roughness. The surface roughness of the Zn–Al–Mg coating is too large to allow the adhesive resin to make contact with the dent in the bonding process, reducing the contact area and thus reducing the adhesion, as shown in Fig. 3. On the other hand, the existence of Ref. 43) MgZn₂ phase of primary hard-brittle leads to the decrease of the anti-cracking ability of the alloy coating. The Al tendency exists in the inner layer and the Al content in the outer surface is less.⁴⁴⁾ Even when Li⁴³⁾ detected ZAM surface by XRD, Al phase could not be detected, and Al did not play an active role. All these factors will have a negative effect on the adhesion between Zn–Al–Mg coating and resin.

3.2 Cross section of the failure areas

Figure 5 presents SEM cross-sectional images of the combinations of different passivated coatings with PE. Figure 5(a) reveals that most of the resin and matrix is bonded closely with few uneven areas. From the outside to the inside, the hot-dip galvanising coating consists of a pure zinc phase (η phase), FeZn₁₃ phase (ζ phase), FeZn₇ phase (δ phase), and Fe₅Zn₂₁ phase (Γ phase).⁴⁵⁾ The ratio of zinc to iron decreases gradually. The η phase and rod-like ζ phase account for most of the coating. In Fig. 5(b), the interface is flat and tightly bonded, and the coating and resin are interlocked. The coating is divided into two parts. The Fe–Al compound interlayer near the steel substrate is very thin with almost a straight line. The lamellar eutectic structure composed of Zn and Al is the main part of the coating. The brittle Zn–Fe is restrained and the ductility is improved by adding Al.⁴⁴⁾ In Fig. 5(c), there are many uneven areas with few tight joints. The structure of ZAM coating is complex,⁴⁶⁾ Mg tends to exist in the outer layer and form Zn–Mg eutectic structure surrounding Zn grains, while Al tends to exist in the inner layer and distribute in the interstices of dendrite grains, and part of it forms Al–Fe alloy structure.

Fig. 5

Cross-sectional SEM micrographs of different coating and PE: (a) Zn (b) Zn–Al (c) Zn–Al–Mg.

Figure 6 presents cross-sectional SEM images of the failure sections for different coatings and PE. The steel matrices and bonding resins are highlighted. Figure 6 reveals that failures between the coating and PE are cohesive failures. The damage on the PE side is consistent with that observed on the polyurethane-bonded PE and aluminium plate studied by Huang.⁴⁷⁾ Surface passivation treatment has a positive effect in terms of adhesion. It has been proven that samples treated with passivation have more adhesive resins with a filamentous appearance at failure sites.

Fig. 6

Cross-sectional SEM images of failure locations between different coatings and PE: (a1) Zn, (a2) passivated Zn, (b1) Zn–Al, (b2) passivated Zn–Al, (c1) Zn–Al–Mg, (c2) passivated Zn–Al–Mg.

In Fig. 6(a1) and (a2) the filiform adhesive resin is significantly less prominent than that in Fig. 6(b1) and (b2), indicating that the bonding force of the hot-dip Zn–Al alloy to the adhesive resin is greater than that of the hot-dip Zn. The addition of Al plays a positive role in promoting the combination of elements. The filament resin in Fig. 6(c1) and (c2) is the least prominent, indicating that the bonding effects of the hot-dip Zn–Al–Mg alloy are poor. The addition of Mg has a negative effect. This further demonstrates that passivation and a hot-dip Zn–Al alloy can promote adhesion between a matrix and resin.

3.3 Infrared analysis following passivation treatment

To determine the cause of the changes in adhesion, the reactions between PE-g-MAH and the coating were investigated utilising FTIR spectroscopy. The black line in Fig. 7 represents the spectrum of the resin and the other lines are the surface infrared spectra of the coating after tensile testing. The peaks at approximately 3000 cm⁻¹, 1500 cm⁻¹, and 750 cm⁻¹ in Fig. 7 are the characteristic peaks of the adhesive resin.⁴⁸⁾ Because these test results were obtained following adhesive failure, the FTIR spectra contain the characteristic peaks of the resin. The absorption band at 1790 cm⁻¹ is attributable to the carbonyl group of maleic anhydrides.⁴⁹⁾

Fig. 7

Infrared spectra of passivation coating surface following sample failure.

However, in addition to the absorption peak at 1790 cm⁻¹, a new peak is visible following passivation. The new peak at 1720 cm⁻¹ corresponds to the carbonyl stretching vibrations of carboxylic acid or ester, which are caused by reactions between the passivation film and PE-g-MAH.⁹⁾ The reaction mechanism is illustrated in Fig. 8(a). The tensile vibrations corresponding to the –C–O–C– group⁴⁹⁾ in MAH at 914 cm⁻¹ disappear during this reaction. This indicates that a ring-opening reaction occurred in the MAH, followed by further reactions with the passivation film. Therefore, following passivation treatment, adhesion between the coating and PE increases.

Fig. 8

(a) Reaction of PE-g-MAH with silane and (b) possible reaction between the coupling agent and the surface layer of the matrix.

3.4 XPS analysis of failure surface

The metal side of the failure site was subjected to XPS analysis. Figure 9 presents the full spectrum resulting from the tests and Fig. 10 presents the results of the peak separation of C, O, and Al. The locations of the peaks for each element are listed in Table 3.^49–54) The full spectrum in Fig. 9 reveals an increase in the relative strengths of the C1s, O1s, and Si2p peaks of the hot-dip Zn–Al alloy. This indicates that more resin remains on the metal surface compared to the hot-dip Zn alloy following tensile testing. These results reveal that the bonding force between the matrix and resin is enhanced by the hot-dip Zn–Al alloy.

Fig. 9

XPS full spectra of the metal sides of the failure sites: (a) Zn, (b) Zn–Al, and (c) Zn–Al–Mg.

Fig. 10

XPS spectra of elements on the metal sides following tensile testing: (a1)–(c1) Zn–Al and (a2)–(c2) Zn–Al–Mg.

Table 3 Binding energies of XPS elements.

In Fig. 10 and Table 3, one can see that C1s peak is fitted to the four peaks of C–Si (283.9 eV), C–C (284.7 eV), C–O/C–O–Si (285.6 eV), and C–O/C–O–Si (289.0 eV).^50–54) In the silane coupling agent, C1s only exhibits three peaks of C–Si, C–C, and C–O/C–O–Si, while the C–O bond in PE-g-MAH is detected on the side of the steel matrix, indicating that there is residual adhesive resin on the metal surface following tensile testing. In other words, the Zn–Al alloy coating and bonding resin form a strong bond. The O1s peak is fitted to C–O–C (534.0 eV), Si–O–Si (532.7 eV), O–C=O (530.2 eV), and C–O/C–O–Si (532.3 eV), but the O–X (531.94 eV) peak is also visible for the Zn–Al alloy. The new peak at 531.94 eV does not have a standard spectrum for comparison. Therefore, it can be concluded that the interactions between the silane coupling agent or bonding resin and Al or Al₂O₃ on the surface of the matrix may lead to the appearance of this chemical state. For the Al2p peak, two peaks of Al (72.7 eV) and Al–O (74.5 eV) were fitted.^49,50) However, the Al–X (75.26 eV) peak is also visible for the Zn–Al alloy, whereas the new peak at 75.26 eV does not have a standard spectrum. If the Al on the surface of the matrix could form Si–O–Al bonds, the appearance of this peak could be explained. The greater the bond energy, the more stable the bonding and the more stable the overall performance. According to the Pauling electronegativity principle, C (2.5) > Si (1.95) > Al (1.5),⁵⁵⁾ so Al is most likely to combine with Si–O to form Si–O–Al. Due to the accumulation of Mg in the outermost layer of Zn–Al–Mg alloy and the tendency of Al to the inner layer of the coating, the Al content on the surface of Zn–Al–Mg alloy is very low, so the possible Al–O–Si bond can’t be detected. Therefore, there is no unknown peak separated from Al and O. Based on the FTIR spectra, it can be concluded that the silane coupling agent and Al or Al₂O₃ on the surface of the substrate may influence adhesion.

4. Conclusion

The bonding forces between hot-dip alloys and adhesive resin range from large to small in the order of Zn, Zn–Al, and Zn–Al–Mg. The Al in the hot-dip Zn–Al alloy may form Si–O–Al chemical bonds, which increase the bonding force with PE. Compared to the bonding force between hot-dip Zn an PE, the bonding forces between hot-dip Zn–Al and PE, and Zn–Al–Mg and PE change by 84.6% and −31.1% before passivation, and 63.8% and −46.6% after passivation, respectively.

Following passivation by a coupling agent, the adhesion of the metal matrix to the resin can be improved. Through passivation, roughness increases and the coupling agent can react with the resin to form chemical bonds. Following passivation, the adhesion of Zn, Zn–Al, and Zn–Al–Mg to PE increase by 73.3%, 26.6%, and 33.3%, respectively.

The combination of hot-dip Zn–Al alloys and grafted PE is expected to improve the corrosion resistance and strength of steel-strip-reinforced PE helical bellows in the future.

Acknowledgments

The authors gratefully acknowledge the financial supports by the National Natural Science Foundation of China under Grant numbers 51601056, the Natural Science Foundation of Hebei Province of China under project number E2017202012, as well as Innovation and the Entrepreneurship Training Program for College Students of Hebei University of Technology under Grant numbers X201910080113.

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