Full Length ArticleInvestigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy
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.
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:
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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).
References (47)
- et al.
Influences of tool pin profile and tool shoulder diameter on the formation of friction stir processing zone in AA6061 aluminium alloy
Mater. Des.
(2008) - et al.
On the growth of conversion chromate coatings on 2024-Al alloy
Appl. Surf. Sci.
(2007) - et al.
Protective coatings on magnesium and its alloys—a critical review
J. Alloy. Compd.
(2002) - et al.
Protection of aluminium foil AA8021 by molybdate-based conversion coatings
Appl. Surf. Sci.
(2014) - et al.
Cerium-based conversion coatings to improve the corrosion resistance of aluminum alloy 6061–T6
Corros. Sci.
(2014) - et al.
Characterization of cerium-based conversion coatings for corrosion protection of aluminum alloys
Surf. Coat. Technol.
(2002) - et al.
Titanium conversion coatings on the aluminum foil AA 8021 used for lithium–ion battery package
Appl. Surf. Sci.
(2017) - et al.
The effect of surface pre-conditioning treatments on the local composition of Zr-based conversion coatings formed on aluminium alloys
Appl. Surf. Sci.
(2016) - et al.
The bonding strength and corrosion resistance of aluminum alloy by anodizing treatment in a phosphoric acid modified boric acid/sulfuric acid bath
Surf. Coat. Technol.
(2008) - et al.
Comparative study on Ti/Zr/V and chromate conversion treated aluminum alloys: anti-corrosion performance and epoxy coating adhesion properties
Appl. Surf. Sci.
(2017)
Preparation and properties of chrome-free colored Ti/Zr based conversion coating on aluminum alloy
Appl. Surf. Sci.
Formation and characterisation of Ti–Zr based conversion layers on AA6060 aluminium
Surf. Coat. Technol.
SKPFM and SEM study of the deposition mechanism of Zr/Ti based pre-treatment on AA6016 aluminum alloy
Surf. Coat. Technol.
Formation of a zirconium-titanium based conversion layer on AA 6060 aluminium
Surf. Coat. Technol.
Corrosion behavior of a zirconium-titanium based phosphonic acid conversion coating on AA6061 aluminum alloy
Acta. Metall. Sin. (Engl. Lett.)
Structural investigation of the zirconium-titanium based amino trimethylene phosphonate hybrid coating on aluminum alloy
Acta. Metall. Sin. (Engl. Lett.)
The adhesion performance of epoxy coating on AA6063 treated in Ti/Zr/V based solution
Appl. Surf. Sci.
Adhesive strength of powder coated aluminium substrates
Int. J. Adhes. Adhes.
Effect of surface pre-treatment on surface characteristics and adhesive bond strength of aluminium alloy
Int. J. Adhes. Adhes.
Effect of laser ablation surface treatment on performance of adhesive-bonded aluminum alloys
Surf. Coat. Technol.
Green chemistry in situ phosphatizing coatings
Prog. Org. Coat.
Molecular design of in situ phosphatizing coatings (ISPCs) for aerospace primers
Prog. Org. Coat.
An in situ phosphatizing coating on 2024 T3 aluminum coupons
Prog. Org. Coat.
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