Elsevier

Applied Surface Science

Volume 458, 15 November 2018, Pages 350-359
Applied Surface Science

Full Length Article
Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy

https://doi.org/10.1016/j.apsusc.2018.07.044Get rights and content

Highlights

  • This conversion coating is environmentally friendly, containing no Cr6+ and Cr3+.

  • Aminotrimethylene phosphonic acid is used to improve adhesion strength.

  • Instead of acids, salts are used to improve the quality of conversion coating.

  • Effect of the conversion coating on adhesion strength is specially explored.

  • Corrosion resistance is enhanced by more compact conversion coating.

Abstract

In this paper, a chromate free conversion coating based on titanium/zirconium salts and aminotrimethylene phosphonic acid (ATMP) was prepared on 7A52 aluminum alloy. The morphology and composition of conversion coating were investigated by test measurements of SEM、EDS、XRD and FT-IR. The adhesion strength between aluminum matrix and subsequent epoxy primer was tested directly. The roughness and wettability of conversion coating were also detected to indirectly characterize the adhesion strength from another side. The electrochemical tests and neutral salt spray tests were also conducted to measure the corrosion resistance of the prepared conversion coating. Results reveal that a uniform and dense conversion coating consisted by metal oxides and other salts is obtained on the surface of 7A52 aluminum alloy. Significantly, the adhesion strength between matrix and epoxy primer increases from 7.97 MPa to 16.23 MPa, which dues to the increase of surface roughness and the improvement of wettability. In addition, the neutral salt spray test time of the conversion coating reaches as long as 120 h. Three orders of magnitudes decrease of current density proves the excellent corrosion resistance of conversion coating, which is also confirmed by electrochemical impedance spectroscopy test. This dues to the barrier property of conversion coating, preventing the corrosive solution to have a direct contact with aluminum matrix. Anyhow, both the adhesion strength and corrosion resistance are greatly reinforced by the existence of conversion coating.

Graphical abstract

The conversion coating based on titanium/zirconium salts and aminotrimethylene phosphonic acid was prepared to improve the adhesion strength and the corrosion resistance of 7A52 aluminum alloy.

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Introduction

Due to the combination of excellent mechanical properties and superior strength to weight ratio [1], aluminum alloy is considered as the ideal material for many aspects. However, the uneven distribution of intermetallic particles in aluminum alloy could induce different electrochemical characteristics and accelerate the process of localized corrosion [2], [3], [4]. Commonly, the thin layer of aluminum oxide formed naturally on its surface can increase the resistance to corrosion. However, the adhesion strength between aluminum alloy matrix and subsequent anticorrosive coating will reduce at the same time.

To solve this problem, appropriate surface treatment is urgently required and chemical conversion treatment is the most suitable method to be adopted. For a long time, chromate conversion coating has been widely used owing to its self-healing ability and outstanding feature of effective corrosion protection [5] as well as the ability to enhance the adhesion strength between subsequent organic anticorrosive coating and aluminum matrix [6]. However, it has been strictly forbidden to be taken into practical application for the toxicity and carcinogenicity of chromium ion [7]. Therefore, numerous chromate free conversion coatings have been investigated and it was found that inorganic conversion coatings such as molybdate-based conversion coating [8], cerium-based conversion coatings [9], [10], titanium-based conversion coating [11], or zirconium conversion coating [12] and also the phosphoric acid modified boric/sulfuric acids conversion coating [13] with the incomparable advantage of environmentally friendly could be used as suitable alternatives. With the further development of research, a great deal of literatures have pointed out that composite conversion coatings based on titanium and zirconium are optimal to achieve the same performance as the chromate conversion coatings [14], [15], [16], [17], [18], [19], [20], [21]. However, these previous studies mainly focused on the improvement of corrosion resistance and the formation mechanism of the conversion coating. It is worth noting that organic coatings are often painted to further improve the corrosion resistance after the chemical conversion treatment. Thus, the adhesion strength is a key property to be investigated, which also has a close relationship to the long lasting corrosion resistance of aluminum alloy. In this area, relevant researches has been reported. Zhu et al. [15], [22] had made special studies on the improvement of adhesion strength enhanced by inorganic conversion coating. It was found that the prepared Ti/Zr/V conversion coating had enhanced all factors relating to adhesion strength, and obviously, the adhesion strength was significantly enhanced. In Mirabedini’s work [23], the prepared organic polyacrylic acid/hexafluorozirconic acid conversion coating gave the marginally second best performance behind the chromate/phosphate conversion coating during the pull-off tests. In addition, the effect of different pretreatments such as polishing, acid and alkali etching on the adhesion strength was also investigated [24], [25], [26] and even the laser ablation pretreatment was specially adopted to study the adhesion strength of adhesive-bonded aluminum joints in detail [27].

In this paper, potassium fluotitanate and potassium fluozirconate were used as the main membrane salts instead of the fluotitanic acid or/and hexafluorozirconic acid used in previous researches. The morphology and composition of the prepared conversion coating were investigated by means of scanning electron microscope (SEM)、energy dispersive X-ray spectrum (EDS)、X-ray diffraction (XRD)、transmission Fourier transform infrared spectroscopy (FT-IR). A kind of organic phosphonic acid was added to further reinforce the positive effect of conversion coating on the adhesion strength. The organic phosphonic acid can closely chelate with most metals by self-assembly and some groups contained in organic phosphonic acid will form strong chemical bonds with the polar groups in the subsequent organic coating and this is particularly beneficial for the enhancement of adhesion strength [28], [29], [30]. It should be announced that, the adhesion strength is not only significantly affected by the surface chemical state, but also is greatly changed by the surface topography. For example, it was pointed out that suitable surface roughness and wettability were two key parameters for the achievement of superior adhesion strength in practice [15], [22], [25]. In this paper, except for the properties of surface roughness and wettability of the conversion coating were tested, the surface free energy and adhesion work were also calculated in order to theoretically illustrate the improvement of adhesion strength enhanced by the conversion coating in detail. Moreover, we give a reasonable explanation for this enhancement. Besides, the corrosion resistance of conversion coating was investigated by neutral salt spray test (NSST) and more objective electrochemical test. The specific explanation for the improvement of corrosion resistance is also given.

Section snippets

Materials

All chemical reagents used in the present work are all analytical reagent level. 68% concentrated nitric acid (HNO3), 40% hydrofluoric acid (HF), trisodium phosphate anhydrous (Na3PO4) and sodium hydroxide (NaOH) were purchased from Tianjin Da Mao chemical reagents factory. Potassium fluotitanate (K2TiF6) and potassium flurozate (K2ZrF6) were purchased from Aladdin Reagent Co., Ltd. Amino trimethylene phosphonic acid (ATMP) was purchased from Qingdao usolf Chemical Technology Co., Ltd. Sodium

Basic characterizations and formation mechanism of the conversion coating

Fig. 1 shows the microscope morphology of specimens after etching pretreatment and chemical conversion treatment. As shown in Fig. 1(a), some tiny white spots, representing the existence of intermetallic particles are observed and meanwhile, some scallops are also observed, which are closely to the exfoliation of intermetallic particles caused by the local dissolution of aluminum matrix surrounding them [31]. After the chemical conversion treatment, a uniform and dense conversion coating can be

Conclusion

In this paper, a conversion coating based on Ti/Zr and ATMP was prepared on the surface of 7A52 aluminum alloy through a simple chemical conversion treatment. Basic characterizations and formation mechanism of the conversion coating are discussed. Adhesion strength was specially investigated and corrosion resistance was also referred. The main conclusions of this paper are as follow:

  • The prepared conversion coating with the thickness of 700 nm are mainly consisted by TiO2, ZrO2, Al2O3, K2NaAlF6

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant numbers 51501055, 51601056); and the Natural Science Foundation of Hebei Province of China (grant number E2017202012).

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