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Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy
Accepted Manuscript Full Length Article Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy Qi Liu, Xiaoming Cao, An Du, Ruina Ma, Xiaoran Zhang, Tingting Shi, Yongzhe Fan, Xue Zhao PII: DOI: Reference:
S0169-4332(18)31935-4 https://doi.org/10.1016/j.apsusc.2018.07.044 APSUSC 39859
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
11 April 2018 6 June 2018 6 July 2018
Please cite this article as: Q. Liu, X. Cao, A. Du, R. Ma, X. Zhang, T. Shi, Y. Fan, X. Zhao, Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.044
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Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy Qi Liu, Xiaoming Cao, An Du, Ruina Ma, Xiaoran Zhang, Tingting Shi, Yongzhe Fan*, Xue Zhao* School of Materials Science and Engineering, Hebei University of Technology, Key Lab for New Type of Functional Materials in Hebei Province, Tianjin Key Lab Materials Laminating Fabrication and Interface, Tianjin 300132, People's Republic of China *Corresponding author: Yongzhe Fan: Tel: +86 22 60204527; E-mail: [email protected] Xue Zhao: Tel: +86 22 60204527; E-mail: [email protected]
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 hours. 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.
Key words: titanium-zirconium salts and organic phosphonic acid composite chromate free conversion coating; adhesion strength; corrosion resistance; aluminum alloy
1. 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 [2-4]. 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 has 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 [14-21]. However, these previous studies mainly focused on the improvement of corrosion resistance and the formation mechanism. 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 for 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 [24-26] 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 [28-30]. 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.
2. Experiment 2.1 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 carbonate (Na2CO3) and sodium fluoride (NaF) were purchased from Tianjin Bodi Chemical Co. Ltd. and Tianjin Fengchuan chemical reagents Technology Co., Ltd, respectively. NJ epoxy primer was purchased from ShiJiazhuang paint factory. The 7A52 aluminum alloy was processed into small specimens with the size of 70 mm × 50 mm × 3 mm. The composition of aluminum alloy is shown in Table 1. 2.2 Surface pretreatment Firstly, all aluminum alloy specimens were polished by abrasive papers of 600 meshes, 1200 meshes and 2000meshes. Secondly, specimens was degreased by alkaline etching and acid etching. The composition of alkaline solution was NaOH 1-2 g/L, Na2CO3 60-70 g/L, Na3PO4 10 g/L. The specimens were immersed into the alkaline solution for 3 minutes at 55°C and then washed by distilled water thoroughly. After that the specimens were immersed into pickling solution containing HNO3 7-9 ml/L and HF 3-10 ml/L for 3 minutes at room temperature and then washed by
distilled water. 2.3 Chemical conversion treatment Specimens after etching pretreatment were immersed into the chemical conversion solution with the composition shown in Table 2. The pH was adjusted to 3.5 by dilute concentrated nitric acid. The optimal immersion time was 7 minutes and the temperature was 40°C. After chemical conversion treatment, specimens were immediately dried at 100°C in the oven for 1 hour. 2.4 Epoxy primer treatment In order to further study the adhesion strength between conversion coating and organic epoxy primer, specimens were then painted with epoxy primer through air spraying and then dried naturally after the chemical conversion treatment. The epoxy primer, solvent and curing agent were formulated at a weight ratio of 5:3:1. This epoxy primer was also adopted for the measurement of contact angle. 2.5 Test and characterization method 2.5.1 Basic characterizations of the conversion coating The microscope morphology of conversion coating was studied by Nova Nano SEM450 and its element composition was studied by the equipped EDS with model of X-MaxN. The phase composition of conversion coating was studied by X ray diffractometer with model of X 'Per Pro, which target is copper target. The scanning speed was 0.001 °/s and the step length was 0.02 °. The scanning range was from 10 ° to 80 °. The FT-IR test of conversion coating was conducted on VECTOR 22 and the
type of accessory is transmission FT-IR. The surface roughness was measured by using roughness tester with model of TIME 3200. Three points were measured at intervals of 2 cm on each specimen and multiple specimens were tested to reduce the contingency. Contact angles were measured on JJC-1 wetting angle measuring instrument using the epoxy primer mention above as probe liquid at room temperature. A small drop of epoxy primer was dripped on each specimen. The shape of probe liquid was recorded after 30 s. Contact angles could be calculated automatically by the system software. 2.5.2 Characterization of the adhesion strength The pull-off test was conducted on adhesion tester with model of Posi Test AT-M. When conducting the test, three testing spindles were glued to each specimen coated with epoxy primer at the interval of 2 centimeters by the AB glue. When the glue was completely dry, the epoxy primer coating was cut on the edge of the spindle with a 20 mm sleeve cutter. Then the specimen was connected to the test instrument and the testing process was carried out automatically and the adhesion strength was displayed on the screen directly. The test was repeated for three times for specimen with different treatment to ensure the repeatability. 2.5.3 Characterization of the corrosion resistance The corrosion resistance was investigated by NSST and electrochemical test. The NSST was conducted on the cyclic corrosion tester with model of Q-FOG/CCT600. All specimens were exposed in salt fog containing 5 wt. % sodium chloride at temperature of 35±2°C and the pH value was about 6.5-7.2. Potentiodynamic
polarization curves and electrochemical impedance spectroscopy (EIS) were performed in 3.5 wt. % sodium chloride solution on the CHI-660E electrochemical working station at room temperature. The electrochemical cell with three electrodes were employed for all electrochemical tests, where the specimen (exposed area is 1 cm2) acted as working electrode, platinum as auxiliary electrode and saturated calomel electrode as reference electrode. Open circuit potential was considered to be stable when its fluctuation was less than 10 mV for a period of 600 s. Test of potentiodynamic polarization curves were carried out in the potential range of 300 mV on either side of open circuit potential and at a scan rate of 1 mV/s. Impedance tests were carried out at the corrosion potential. The range of scanning frequency was from 100 kHz to 10 mHz. What’s more, in order to investigate the effect of immersion time on the corrosion resistance of prepared conversion coating, the chemical conversion treated sample was firstly immersed into 3.5 % NaCl solution for 12hours, 24 hours and 48 hours. After immersing enough time, the sample was pick out and the test of EIS was conducted. The results were fitted and analyzed by ZSimDemo software. 3. Results and Discussion 3.1 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 seen in Fig. 1(b). At larger magnification in Fig. 1(c), it is much clear that the conversion coating is mainly composed of nodule-like particles with irregular shapes. The average size of these nodules is under 3 μm and each one is tightly coupled with its surrounding nodules. The cause of this structure can be attributed to the electrochemical and/or catalytic effects of the alloy elements [32]. Besides, there are also some tiny cracks which are attributed to the inter stress associated with dehydration during drying process [33]. In addition, Fig. 1(c) also delivers the information that the unflat conversion coating has a fluctuating arrangement, leading to the fluctuation of conversion coating thickness. This phenomenon should be explained as that the cathodic intermetallic particles are the preferred deposition locations for membrane salt containing Ti or Zr and the deposition of conversion coating spreads around these locations until the whole surface is covered [18]. The bulge and depression of nodules are expected and will be beneficial for the improvement of surface roughness. Fig. 1(d) shows EDS result of the conversion coating. The existence of C and P elements can be easily found. For the reason that other reagents used in conversion process do not contain these two elements but only in ATMP, so we can put forward that the ATMP is surely involved into the conversion coating forming reaction. The presences of Al, K, Na, F, Ti and Zr in the conversion coating are mainly derived from the complex salts produced by the reaction between aluminum matrix and chemical conversion solution. Moreover, Fig. 2 shows the cross
section morphology of the prepared conversion coating. It could be clearly seen from Fig. 2(a) that the prepared conversion coating is relatively homogeneous and the average thickness is about 700 nm. The slightly bulge structure can also be seen, which is consistent with the SEM result. The line scanning of aluminum element in Fig. 2(b) also delivers the information of the conversion coating thickness. The distance between the decreasing ending point of platinum content and the increasing starting point of aluminum content is approximate 700 nm, which correspond to the result of direct measurement in the cross section picture. XRD result in Fig. 3 shows the exact phase composition of the prepared conversion coating. Although we adopt the method of small angle grazing, a strong diffraction peak of Al in XRD spectrum is also seen which is ascribed to that the X-ray passes through the conversion coating and directly irradiates on the aluminum matrix. The peaks of metallic oxide, such as Al2O3 and TiO2 as well as ZrO2 are also clearly observed in XRD spectrum. A diffraction peak of K2NaAlF6 is also observed and this is consistent with mass ratio of K, Na and F observed in EDS test. However, no phase related to ATMP is observed in the conversion coating. Transmission FT-IR results are shown in Fig. 4. The broad stretching vibration peaks of OH and the characteristic peaks of CH2 can be seen in both spectra of ATMP and conversion coating at 3398 cm-1 and 3008 cm-1, respectively [34]. The peaks appearing in both spectra at 1672 cm-1 are the characteristic peaks of O-H in P-OH and the peaks at 1055 cm-1 are the characteristic peaks of P-O in ionic species PO32[35]. As the key feature of ATMP, the C-N peak is observed in all spectra at 1155 cm-1
[36]. It is worth noting that the peak considered as characteristic peak of P-OH in the structure of O=P (OH)2 appears in the spectrum of pure ATMP at 930 cm-1 [37], while it not appears in the spectrum of conversion coating and the peak at 961 cm-1 is considered to represent the characteristic peak of P=O in alkylphosphonic groups [38]. This can forcefully confirm the involvement of ATMP in the membrane formation. The peaks observed at 876 cm-1 and 833 cm-1 are for titanium oxide and zirconium oxide [21]. According to the above results, mechanism for the formation of conversion coating can be explained as follow: Reactions firstly occurred surrounding intermetallic particles when aluminum alloy was immersed into the chemical conversion solution [18]. The reactions of anode and cathode were confirmed to take place according to Eq. 1 and Eq. 2, leading to the formation of Al3+ and OH- [17]. And then, the TiF62- and ZrF62- in chemical conversion solution reacted with Al3+ as shown in Eq. 3 and Eq. 4 [17, 31], causing the formation of AlF63-. Afterwards, the generated AlF63- reacted with Na+ and K+ and eventually formed NaF2 AlF6 (Eq. 5) [17]. At the same time, due to the ongoing cathodic reaction, concentration of OH - locally increased near the cathode and this local alkalinity would favor reactions based on Eq. 6 and Eq. 7 [17]. Consequently, these hydrous metal oxide dehydrated in the next drying process and eventually formed TiO2 and ZrO2. Al3+ would also react with enriched OH - on surface to form metal oxide (Eq. 8). Finally, with the prolongation of reaction time, the conversion coating was generated gradually until completely covered the whole surface of
aluminum matrix. Besides, it should be also announced that the added corrosive free F in sodium fluoride played an important role not only in accelerating the process of anode and cathode reaction, but also in expediting the dissolution of aluminum matrix and the formation of conversion coating [31, 39]. As for the added ATMP, the phosphoric acid groups in its structure react with aluminum matrix or metal oxide in the conversion coating to form hydrophobic amino trimethonyl phosphonate, which is demonstrated by the disappearance of hydroxyl groups in phosphate and the presence of phosphate ions in phosphate in the FT-IR result of conversion coating. The generated complex phosphate deposit into the defect of the prepared conversion coating and then comes to the enhancement of corrosion resistance.
3.2 Investigation on adhesion strength 3.2.1 Surface roughness Fig. 5 shows the surface roughness of specimens with different treatments. With the further treatment for aluminum alloy, the roughness shows an increasing trend.
The roughness of untreated specimen is the minimum, only 452.2 nm, and the roughness of etching treated specimen is 673.3 nm, which is 1.5 times of the untreated one. Comparing with the etching treated specimen, the application of chemical conversion treatment brings out an increased roughness of 12 %, which reaches as high as 751.4 nm. As is known to all, the aluminum matrix is covered by a layer of oxide and many other impurities, which will be completely removed during the etching process. Meanwhile, some of the intermetallic particles also peel off from the matrix, leaving behind scallops observed in SEM. Also the different corrosion rates at different places cause the different dissolution amount of aluminum and further leads to the formation of corrosion pits. All of these contribute to the increase of surface roughness after etching treatment. Moreover, the observed bulge and depression of nodules on conversion coating is the crucial factor contributing to the further improvement of surface roughness when comparing with the etching treated one. 3.2.2 Contact angle and surface free energy Contact angles of epoxy primer on different specimens are shown in Fig. 6. The surface free energy (
) and adhesion work (
) are calculated according to Young’s
equation (Eq. 9) and Neumann equation (Eq. 10) [40]. In theoretical calculation, the value of
is 24.13 mN/m (This surface tension of used epoxy primer is measured
by the experiment) and the value of β equals to (1.247 ± 0.100) × 10 -4 (mN/m) -2. The values of θ and
as well as
are shown in Table 3.
Contact angles, in order of untreated, etching treated and chemical conversion treated specimens show a reducing trend, while the surface free energy and adhesion work show an opposite trend. The values of contact angle and surface free energy for untreated specimen are 80.5° and 8.7 mN/m, indicating a low wettability. Comparatively, the decrease of contact angle (46.2°) and increase of surface free energy (17.5 mN/m) for chemical conversion treated specimen suggest an excellent lipophilicity for epoxy primer. As for the etching treated specimen, both the value of contact angle and surface free energy are between those of untreated and chemical conversion treated specimen. These results can intuitively show that the prepared conversion coating has favorable wettability and lipophilicity to epoxy primer. The reason for this phenomenon can be explained as follow: Firstly, the proper surface treatment removes not only the impurities but also the thin layer of oxide and this can efficiently enhance the wettability [22, 24]. Secondly, the superior roughness of conversion coating makes epoxy primer easier spread on the surface of aluminum alloy [22, 24, 25]. 3.2.3 Pull-off test and fracture mode The adhesion strength can be more intuitively characterized by the pull-off test and result is shown in Fig. 7. Two types of fracture modes are also observed in the test. The lowest adhesion strength of 7.97 MPa is observed on untreated specimen and the epoxy primer completely dissects from the specimen at the interface of matrix and epoxy primer in the form of adhesive failure. While, the adhesion strength of chemical conversion treated specimen increases about 103 %, reaching the maximum
of 16.23 MPa. This outstanding adhesion strength makes it strong enough to cope with the applied pulling force during the test and the fracture occurs at the inner layer of epoxy primer, which is regarded as the cohesion failure. As for the etching treated specimen, the adhesion strength value of 10.49 MPa is between those of untreated and chemical conversion treated specimens and part of the epoxy primer thoroughly peels off at the interface of matrix and epoxy primer and the remaining part still tightly couples with the substrate. That is to say the fracture mode of etching treated specimen is the mixture of adhesive failure and cohesion failure. The results of pull-off test have a close connection with previous research results. The increase of surface free energy caused by the removal of oxides and impurities makes the matrix much easier to be wetted, so the best adhesion strength is observed on chemical conversion treated specimen [22]. In addition, the increase of surface roughness means larger contact area for epoxy primer to adhere and this is beneficial for the formation of favorable mechanical interlocking between conversion coating and epoxy primer [25]. What’s more, the interaction between epoxy primer and conversion coating is more than a simple physic-sorption, the ATMP existed in conversion coating brings out more functional groups such as hydroxyl groups and these groups will react with the various polar functional groups in epoxy primer to form hydrogen bonds or even other forms of chemical bonds. The formation of chemical bonds also has an irreplaceable effect on the highest adhesion strength for chemical conversion treated specimen [15]. 3.3 Investigation on resistance results for as-prepared samples
3.3.1 Nneutral salt spray test Visual surface state of untreated and chemical conversion treated specimens after different time during the NSST is shown in Fig. 8. As for the untreated specimen, even though there exists a layer of oxides on its surface, corrosion traces can be clearly seen after 10 hours. The corrosion traces also spread at a very fast speed and it takes 24 hours before the whole surface is covered by corrosion products. Only a few and fewer matrix is not corroded after 72 hours’ test. While, only a slight of corrosion trace can be observed on the surface of chemical conversion treated specimen after 24 hours and it takes 72 hours before the corrosion became serious. Besides, the corrosion status of chemical conversion treated specimen after 120 hours’ NSST is very similar with that of the untreated specimen after 10 hours’ NSST. These show that the spread of corrosion for chemical conversion treated specimen is much slower than that of untreated one. The NSST results preliminarily prove that the prepared conversion coating can surely delay the corrosion of aluminum alloy to a certain extent. 3.3.2 Potentiodynamic polarization curve Fig. 9 shows the potentiodynamic polarization curves of untreated and chemical conversion treated specimens. Values of corrosion potential ( current density (
) and corrosion
) are listed in Table 4. As a key factor for measuring the
corrosion susceptibility,
changes form -1.103 V to -0.975 V after the chemical
conversion treatment and this shift towards the more positive direction shows that a favorable corrosion resistance is obtained after the conversion process. The
of
untreated and chemical conversion treated specimen are 2.694×10-5 A/cm2 and 9.323×10-8 A/cm2, respectively. The reduction of two orders of magnitude is observed in comparison and this means that the chemical conversion treated specimen has lower corrosion dynamic rate because of the
represents the intensity of cathodic
oxygen reduction and anodic dissolution of metal ions. What’s more, it’s worth noticing that three orders of magnitude of corrosion rate is observed, which also indicates the excellent corrosion resistance performance of the prepared conversion coating. Moreover, protection efficiency (η) of the prepared conversion coating is calculated according to Eq. 11 [41], where
and
are the corrosion current
densities in absence and presence of the conversion coating. The value is also shown in Table 4 and it can reach as high as 99.04 %.
3.3.3 Electrochemical impedance spectroscopy To further evaluate the effect of conversion coating on the corrosion resistance of aluminum alloy, EIS tests were conducted and results are shown in Fig. 10. In the Nyquist plot, the total diameter is the measurement of the corrosion resistance [41]. All specimens exhibit one single semicircle in Fig. 10, indicating the charge transfer takes place at the interface of specimen and electrolyte [42]. The diameter of chemical conversion treated specimen is 103 order of magnitude, which is only 102 for untreated specimen. The huge distinction is obvious and the increase of the arc diameter implies the decrease of the corrosion rate, which confirms the favorable corrosion resistance of conversion coating. Besides, the appraisal of impedance
modulus in low-frequency region corresponds to the total corrosion protection performance conferred by the prepared conversion coating [43]. As shown in Fig. 10(b), the impedance modulus of chemical conversion treated specimen is higher than that of the untreated one and this more resistance behavior in low-frequency reflects the remarkable corrosion resistance of the conversion coating. In Fig. 10(c), a higher phase angle at middle high frequency for the chemical conversion treated specimen corresponds to a capacitive behavior, that is to say the conversion coating has good dielectric property to avoid the ionic flow of electrolyte. Additionally, as the frequency decrease to 0.01 Hz, the phase angle tends to drop and lead to a second relaxation process, relating to the penetration of 3.5 % NaCl solution to the underlying aluminum matrix. Two time constant can be seen in low frequency region and medium-high frequency region, respectively. The EIS data is also fitted by electrical equivalent circuit (ECC), which is presented in Fig. 10(d). When choosing the suitable ECC, the layer of oxides on the surface of aluminum matrix was also taken into consideration. So the ECC of R(Q(R(QR))) was finally adopted [44]. The ECC is consisted of the R s modeling the solution resistance, in sequence with a constant phase element CPE1 in parallel with another resistance Rc modeling the coating resistance, and then another constant phase element CPE2 and the third charge transfer resistance of Rct associating with the corrosion process [9]. The CPE is chose to represent non-ideal capacitive behavior, considering the existence of surface defects [45]. The impendence of the constant phase angle element CPE can be represented by QCPE = [jω] -n/Y0, where ω is the
angular frequency (rad s-1), Y0 is the parameter and n is the dispersion index which indicates the homogeneity of the coating [46]. When the value of n is 1, the CPE is a pure capacitor with a capacitance of Y0. It is believed that the smaller the n value is and the more defects will be on the surface as well as the pitting corrosion is more likely to occur. The fitting results are also shown in Fig. 10 and the relating parameters fitting from ECC is shown in Table 5. There is a good agreement between the experimental and fitted data for both Nyquist and Bode plots. Usually, the bode plots can deliver the information about the impedance features of different specimens: the resistance behavior of electrolyte in high frequency region, resistance behavior in low frequency region and the mixed impedance behavior of faradaic resistance and capacitor in middle frequency region. The Rs values of 10.05 and 14.77 indicate the well conductivity of 3.5 wt % Sodium chloride solution. The larger value of n indicates the surface of specimen becomes more perfect after the chemical conversion treatment. Values of R c and Rt are greatly improved after the chemical conversion treatment. Especially for R c, it has increased for two orders of magnitude, suggesting a compact surface with few defects and the notable improvement of protective properties. Rt also becomes three times larger than that of untreated specimen, representing a favorable corrosion resistance of the whole material. Besides, the charge transmission between aluminum matrix and corrosive medium can also be effectively controlled during the electrochemical corrosion process. In a word, both the higher Rc and Rt for chemical conversion treated specimen make it isn’t easy for the corrosive ions in electrolyte to get across
the conversion coating. Fig. 11 shows the EIS results of chemical conversion coating versus the immersion time and the relating parameters are shown in Fig. 12. Both maximum phase angle and value of Z in low frequency region tend to decrease with the prolongation of immersion time and this shows that the degradation of corrosion resistance in a certain degree. As shown in Fig. 12, the evident decrease in an order of magnitude of Rc is the result of the degradation of the prepared conversion coating. The value of Rct almost linearly decreases from 2.869 × 10 4 Ω·cm-2 to 7.801 × 103 Ω·cm-2 with the increasing of immersion time. The variation has a close relationship with the porous and inhomogeneous nature of the prepared conversion coating. The decrease of Rct means the change of conversion coating thickness and homogeneity as well as density, taking place within microstructure [47]. Thus more corrosive solution can reach to the underlying aluminum matrix and the anticorrosive property consequently become worse. In addition, the Y1 parameter has a slight rising tendency and the more obvious change of Y2 parameter corresponds to the decrease in the dielectric property of conversion coating. That is to say the protection performance will get worse with the prolongation of immersion time. Based on the above results, we can give a reasonable explanation for the mechanism of the aluminum alloy corrosion resistance reinforced by the conversion coating. Firstly, the dense conversion coating can act as a super physical shielding layer and avoid direct contact between the aluminum matrix and corrosive ions. Few channels are provided to allow the corrosion ions to pass through and the penetration
of the corrosion ions is also inhibited. Even though the heavy corrosive chloride is also difficult to cause a negative effect on the aluminum alloy in a short time from the results of NSST. What’s more, during the electrochemical corrosion process of aluminum alloy itself, the charge transfer between the aluminum alloy and the corrosive solution is significantly affected by existence of the conversion coating with poor conductivity. So the corrosion resistance is finally enhanced. 4. 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 refered. The main conclusions of this paper are as follow: ▪The prepared conversion coating are mainly consisted by TiO2, ZrO2, Al2O3, K2NaAlF6, and organic phosphonate ATMP, and this uniform and dense conversion coating attributes to the excellent corrosion resistance. ▪The adhesion strength is significantly enhanced by the conversion coating, reaching as high as 16.23 MPa. This improvement is caused by the increase of surface roughness (750 nm) and surface free energy (17.5 mN/m) as well as the decrease of contact angle (46.2 °), which are beneficial for the well spreading of epoxy primer and the close connection between conversion coating and epoxy coating. ▪ NSST and electrochemical results demonstrate the outstanding corrosion resistance of conversion coating. The NSST time can reach 120 hours. Three orders magnitude
decrease of current density and the more positive corrosion potential are obtained. The enhancement of corrosion resistance has a close relationship to the favorable barrier property of conversion coating, which avoids the direct contact between electrolyte and aluminum matrix and the transformation of electric charge during the electrochemical corrosion process. In general, the existence of conversion coating can not only improve the adhesion strength between aluminum alloy and epoxy primer, but also effectively enhance the corrosion resistance of aluminum alloy. We hope that our research will play an important role in the pretreatment for aluminum alloy, especially in the aspect for the enhancement of adhesion strength between aluminum matrix and organic coating.
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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Figure caption Fig. 1 Results of SEM and EDS. (a) is the SEM image of etching treated specimen, (b) is the SEM image of conversion coating, (c) is the SEM image of conversion coating at larger magnification, (d) is the EDS result of the conversion coating. Fig. 2 Cross-section images of the prepared conversion coating. (a) is the intuitionistic cross section images, (b) is the result of element line scanning. Fig. 3 XRD pattern of the conversion coating. Fig. 4 FT-IR spectra of the aminotrimethylene phosphonic acid and conversion coating. Fig. 5 Surface roughness values of different specimens. Fig. 6 Contact angles of different specimens. (a) is the untreated specimen, (b) is the etching treated specimen, (c) is the chemical conversion treated specimen. Fig. 7 Adhesion strength and fracture mode of different specimens. Fig. 8 Result of neutral salt spray test. (a)-(e) are the untreated specimen, (f)-(j) are the chemical conversion treated specimen. Fig. 9 Potentiodynamic polarization curves of different specimens. Fig. 10 EIS and fitting results of different specimens. (a) and (b) are Nyquist plots, (c) and (d) are Bode plots. Fig. 11 EIS and fitting results of conversion coating versus the immersion time Fig. 12 Variation of relating parameters with time for chemical conversion treated specimen. (a) is Rc and Rct, (b) is Y1 and Y2.
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.
Table
Table 1 Composition of 7A52 aluminum alloy (wt. %) Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
Content
0.13
0.17
0.12
0.25
2.3
0.2
4.31
0.11
92.41
Table 2 Composition of the chemical conversion solution K2ZrF6
K2TiF6
NaF
ATMP
5 g/L
3.5 g/L
2.5 g/L
2.0 g/L
Table 3 Contact angle, adhesion work and surface free energy of different specimens. Specimens Contact angle (mJ) (mN/m) Untreated 80.5° 28.1 8.7 Etching treated 73.8° 32.1 10.4 Chemical conversion treated 46.2° 65.0 17.5
Table 4 Parameters obtained from potentiodynamic polarization curves. Specimen
icorr (A·cm-2)
Ecorr (V)
Corrosion Rate (mil/year)
η
Untreated
2.694×10-5
-1.103
1.156×101
-
Chemical conversion treated
9.323×10-8
-0.975
3.999×10-2
99.96%
Table 5 Parameters fitted from electrical equivalent circuit. Specimen Untreated Chemical conversion treated
Rs (Ω·cm-2) 10.05 14.77
CPE1 Y1 (S cm s ) 2.244×10-5 -2 n
1.543×10-5
n1 0.7844 0.8703
Rt (Ω·cm-2) 1.036×10
3
3.232×103
CPE2 Y2 (S cm-2 sn) 1.535×10-2
n2 1
2.946×10-3
1
Rc (Ω·cm-2) 7.783×102 3.129×104
Highlights
1. This conversion coating is environmentally friendly, containing no Cr6+ and Cr3+. 2. Aminotrimethylene phosphonic acid is used to improve adhesion strength. 3. Instead of acids, salts are used to improve the quality of conversion coating. 4. Effect of the conversion coating on adhesion strength is specially explored. 5. Corrosion resistance is enhanced by more compact conversion coating.
S0169-4332(18)31935-4 https://doi.org/10.1016/j.apsusc.2018.07.044 APSUSC 39859
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
11 April 2018 6 June 2018 6 July 2018
Please cite this article as: Q. Liu, X. Cao, A. Du, R. Ma, X. Zhang, T. Shi, Y. Fan, X. Zhao, Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.044
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Investigation on adhesion strength and corrosion resistance of Ti-Zr aminotrimethylene phosphonic acid composite conversion coating on 7A52 aluminum alloy Qi Liu, Xiaoming Cao, An Du, Ruina Ma, Xiaoran Zhang, Tingting Shi, Yongzhe Fan*, Xue Zhao* School of Materials Science and Engineering, Hebei University of Technology, Key Lab for New Type of Functional Materials in Hebei Province, Tianjin Key Lab Materials Laminating Fabrication and Interface, Tianjin 300132, People's Republic of China *Corresponding author: Yongzhe Fan: Tel: +86 22 60204527; E-mail: [email protected] Xue Zhao: Tel: +86 22 60204527; E-mail: [email protected]
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 hours. 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.
Key words: titanium-zirconium salts and organic phosphonic acid composite chromate free conversion coating; adhesion strength; corrosion resistance; aluminum alloy
1. 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 [2-4]. 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 has 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 [14-21]. However, these previous studies mainly focused on the improvement of corrosion resistance and the formation mechanism. 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 for 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 [24-26] 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 [28-30]. 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.
2. Experiment 2.1 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 carbonate (Na2CO3) and sodium fluoride (NaF) were purchased from Tianjin Bodi Chemical Co. Ltd. and Tianjin Fengchuan chemical reagents Technology Co., Ltd, respectively. NJ epoxy primer was purchased from ShiJiazhuang paint factory. The 7A52 aluminum alloy was processed into small specimens with the size of 70 mm × 50 mm × 3 mm. The composition of aluminum alloy is shown in Table 1. 2.2 Surface pretreatment Firstly, all aluminum alloy specimens were polished by abrasive papers of 600 meshes, 1200 meshes and 2000meshes. Secondly, specimens was degreased by alkaline etching and acid etching. The composition of alkaline solution was NaOH 1-2 g/L, Na2CO3 60-70 g/L, Na3PO4 10 g/L. The specimens were immersed into the alkaline solution for 3 minutes at 55°C and then washed by distilled water thoroughly. After that the specimens were immersed into pickling solution containing HNO3 7-9 ml/L and HF 3-10 ml/L for 3 minutes at room temperature and then washed by
distilled water. 2.3 Chemical conversion treatment Specimens after etching pretreatment were immersed into the chemical conversion solution with the composition shown in Table 2. The pH was adjusted to 3.5 by dilute concentrated nitric acid. The optimal immersion time was 7 minutes and the temperature was 40°C. After chemical conversion treatment, specimens were immediately dried at 100°C in the oven for 1 hour. 2.4 Epoxy primer treatment In order to further study the adhesion strength between conversion coating and organic epoxy primer, specimens were then painted with epoxy primer through air spraying and then dried naturally after the chemical conversion treatment. The epoxy primer, solvent and curing agent were formulated at a weight ratio of 5:3:1. This epoxy primer was also adopted for the measurement of contact angle. 2.5 Test and characterization method 2.5.1 Basic characterizations of the conversion coating The microscope morphology of conversion coating was studied by Nova Nano SEM450 and its element composition was studied by the equipped EDS with model of X-MaxN. The phase composition of conversion coating was studied by X ray diffractometer with model of X 'Per Pro, which target is copper target. The scanning speed was 0.001 °/s and the step length was 0.02 °. The scanning range was from 10 ° to 80 °. The FT-IR test of conversion coating was conducted on VECTOR 22 and the
type of accessory is transmission FT-IR. The surface roughness was measured by using roughness tester with model of TIME 3200. Three points were measured at intervals of 2 cm on each specimen and multiple specimens were tested to reduce the contingency. Contact angles were measured on JJC-1 wetting angle measuring instrument using the epoxy primer mention above as probe liquid at room temperature. A small drop of epoxy primer was dripped on each specimen. The shape of probe liquid was recorded after 30 s. Contact angles could be calculated automatically by the system software. 2.5.2 Characterization of the adhesion strength The pull-off test was conducted on adhesion tester with model of Posi Test AT-M. When conducting the test, three testing spindles were glued to each specimen coated with epoxy primer at the interval of 2 centimeters by the AB glue. When the glue was completely dry, the epoxy primer coating was cut on the edge of the spindle with a 20 mm sleeve cutter. Then the specimen was connected to the test instrument and the testing process was carried out automatically and the adhesion strength was displayed on the screen directly. The test was repeated for three times for specimen with different treatment to ensure the repeatability. 2.5.3 Characterization of the corrosion resistance The corrosion resistance was investigated by NSST and electrochemical test. The NSST was conducted on the cyclic corrosion tester with model of Q-FOG/CCT600. All specimens were exposed in salt fog containing 5 wt. % sodium chloride at temperature of 35±2°C and the pH value was about 6.5-7.2. Potentiodynamic
polarization curves and electrochemical impedance spectroscopy (EIS) were performed in 3.5 wt. % sodium chloride solution on the CHI-660E electrochemical working station at room temperature. The electrochemical cell with three electrodes were employed for all electrochemical tests, where the specimen (exposed area is 1 cm2) acted as working electrode, platinum as auxiliary electrode and saturated calomel electrode as reference electrode. Open circuit potential was considered to be stable when its fluctuation was less than 10 mV for a period of 600 s. Test of potentiodynamic polarization curves were carried out in the potential range of 300 mV on either side of open circuit potential and at a scan rate of 1 mV/s. Impedance tests were carried out at the corrosion potential. The range of scanning frequency was from 100 kHz to 10 mHz. What’s more, in order to investigate the effect of immersion time on the corrosion resistance of prepared conversion coating, the chemical conversion treated sample was firstly immersed into 3.5 % NaCl solution for 12hours, 24 hours and 48 hours. After immersing enough time, the sample was pick out and the test of EIS was conducted. The results were fitted and analyzed by ZSimDemo software. 3. Results and Discussion 3.1 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 seen in Fig. 1(b). At larger magnification in Fig. 1(c), it is much clear that the conversion coating is mainly composed of nodule-like particles with irregular shapes. The average size of these nodules is under 3 μm and each one is tightly coupled with its surrounding nodules. The cause of this structure can be attributed to the electrochemical and/or catalytic effects of the alloy elements [32]. Besides, there are also some tiny cracks which are attributed to the inter stress associated with dehydration during drying process [33]. In addition, Fig. 1(c) also delivers the information that the unflat conversion coating has a fluctuating arrangement, leading to the fluctuation of conversion coating thickness. This phenomenon should be explained as that the cathodic intermetallic particles are the preferred deposition locations for membrane salt containing Ti or Zr and the deposition of conversion coating spreads around these locations until the whole surface is covered [18]. The bulge and depression of nodules are expected and will be beneficial for the improvement of surface roughness. Fig. 1(d) shows EDS result of the conversion coating. The existence of C and P elements can be easily found. For the reason that other reagents used in conversion process do not contain these two elements but only in ATMP, so we can put forward that the ATMP is surely involved into the conversion coating forming reaction. The presences of Al, K, Na, F, Ti and Zr in the conversion coating are mainly derived from the complex salts produced by the reaction between aluminum matrix and chemical conversion solution. Moreover, Fig. 2 shows the cross
section morphology of the prepared conversion coating. It could be clearly seen from Fig. 2(a) that the prepared conversion coating is relatively homogeneous and the average thickness is about 700 nm. The slightly bulge structure can also be seen, which is consistent with the SEM result. The line scanning of aluminum element in Fig. 2(b) also delivers the information of the conversion coating thickness. The distance between the decreasing ending point of platinum content and the increasing starting point of aluminum content is approximate 700 nm, which correspond to the result of direct measurement in the cross section picture. XRD result in Fig. 3 shows the exact phase composition of the prepared conversion coating. Although we adopt the method of small angle grazing, a strong diffraction peak of Al in XRD spectrum is also seen which is ascribed to that the X-ray passes through the conversion coating and directly irradiates on the aluminum matrix. The peaks of metallic oxide, such as Al2O3 and TiO2 as well as ZrO2 are also clearly observed in XRD spectrum. A diffraction peak of K2NaAlF6 is also observed and this is consistent with mass ratio of K, Na and F observed in EDS test. However, no phase related to ATMP is observed in the conversion coating. Transmission FT-IR results are shown in Fig. 4. The broad stretching vibration peaks of OH and the characteristic peaks of CH2 can be seen in both spectra of ATMP and conversion coating at 3398 cm-1 and 3008 cm-1, respectively [34]. The peaks appearing in both spectra at 1672 cm-1 are the characteristic peaks of O-H in P-OH and the peaks at 1055 cm-1 are the characteristic peaks of P-O in ionic species PO32[35]. As the key feature of ATMP, the C-N peak is observed in all spectra at 1155 cm-1
[36]. It is worth noting that the peak considered as characteristic peak of P-OH in the structure of O=P (OH)2 appears in the spectrum of pure ATMP at 930 cm-1 [37], while it not appears in the spectrum of conversion coating and the peak at 961 cm-1 is considered to represent the characteristic peak of P=O in alkylphosphonic groups [38]. This can forcefully confirm the involvement of ATMP in the membrane formation. The peaks observed at 876 cm-1 and 833 cm-1 are for titanium oxide and zirconium oxide [21]. According to the above results, mechanism for the formation of conversion coating can be explained as follow: Reactions firstly occurred surrounding intermetallic particles when aluminum alloy was immersed into the chemical conversion solution [18]. The reactions of anode and cathode were confirmed to take place according to Eq. 1 and Eq. 2, leading to the formation of Al3+ and OH- [17]. And then, the TiF62- and ZrF62- in chemical conversion solution reacted with Al3+ as shown in Eq. 3 and Eq. 4 [17, 31], causing the formation of AlF63-. Afterwards, the generated AlF63- reacted with Na+ and K+ and eventually formed NaF2 AlF6 (Eq. 5) [17]. At the same time, due to the ongoing cathodic reaction, concentration of OH - locally increased near the cathode and this local alkalinity would favor reactions based on Eq. 6 and Eq. 7 [17]. Consequently, these hydrous metal oxide dehydrated in the next drying process and eventually formed TiO2 and ZrO2. Al3+ would also react with enriched OH - on surface to form metal oxide (Eq. 8). Finally, with the prolongation of reaction time, the conversion coating was generated gradually until completely covered the whole surface of
aluminum matrix. Besides, it should be also announced that the added corrosive free F in sodium fluoride played an important role not only in accelerating the process of anode and cathode reaction, but also in expediting the dissolution of aluminum matrix and the formation of conversion coating [31, 39]. As for the added ATMP, the phosphoric acid groups in its structure react with aluminum matrix or metal oxide in the conversion coating to form hydrophobic amino trimethonyl phosphonate, which is demonstrated by the disappearance of hydroxyl groups in phosphate and the presence of phosphate ions in phosphate in the FT-IR result of conversion coating. The generated complex phosphate deposit into the defect of the prepared conversion coating and then comes to the enhancement of corrosion resistance.
3.2 Investigation on adhesion strength 3.2.1 Surface roughness Fig. 5 shows the surface roughness of specimens with different treatments. With the further treatment for aluminum alloy, the roughness shows an increasing trend.
The roughness of untreated specimen is the minimum, only 452.2 nm, and the roughness of etching treated specimen is 673.3 nm, which is 1.5 times of the untreated one. Comparing with the etching treated specimen, the application of chemical conversion treatment brings out an increased roughness of 12 %, which reaches as high as 751.4 nm. As is known to all, the aluminum matrix is covered by a layer of oxide and many other impurities, which will be completely removed during the etching process. Meanwhile, some of the intermetallic particles also peel off from the matrix, leaving behind scallops observed in SEM. Also the different corrosion rates at different places cause the different dissolution amount of aluminum and further leads to the formation of corrosion pits. All of these contribute to the increase of surface roughness after etching treatment. Moreover, the observed bulge and depression of nodules on conversion coating is the crucial factor contributing to the further improvement of surface roughness when comparing with the etching treated one. 3.2.2 Contact angle and surface free energy Contact angles of epoxy primer on different specimens are shown in Fig. 6. The surface free energy (
) and adhesion work (
) are calculated according to Young’s
equation (Eq. 9) and Neumann equation (Eq. 10) [40]. In theoretical calculation, the value of
is 24.13 mN/m (This surface tension of used epoxy primer is measured
by the experiment) and the value of β equals to (1.247 ± 0.100) × 10 -4 (mN/m) -2. The values of θ and
as well as
are shown in Table 3.
Contact angles, in order of untreated, etching treated and chemical conversion treated specimens show a reducing trend, while the surface free energy and adhesion work show an opposite trend. The values of contact angle and surface free energy for untreated specimen are 80.5° and 8.7 mN/m, indicating a low wettability. Comparatively, the decrease of contact angle (46.2°) and increase of surface free energy (17.5 mN/m) for chemical conversion treated specimen suggest an excellent lipophilicity for epoxy primer. As for the etching treated specimen, both the value of contact angle and surface free energy are between those of untreated and chemical conversion treated specimen. These results can intuitively show that the prepared conversion coating has favorable wettability and lipophilicity to epoxy primer. The reason for this phenomenon can be explained as follow: Firstly, the proper surface treatment removes not only the impurities but also the thin layer of oxide and this can efficiently enhance the wettability [22, 24]. Secondly, the superior roughness of conversion coating makes epoxy primer easier spread on the surface of aluminum alloy [22, 24, 25]. 3.2.3 Pull-off test and fracture mode The adhesion strength can be more intuitively characterized by the pull-off test and result is shown in Fig. 7. Two types of fracture modes are also observed in the test. The lowest adhesion strength of 7.97 MPa is observed on untreated specimen and the epoxy primer completely dissects from the specimen at the interface of matrix and epoxy primer in the form of adhesive failure. While, the adhesion strength of chemical conversion treated specimen increases about 103 %, reaching the maximum
of 16.23 MPa. This outstanding adhesion strength makes it strong enough to cope with the applied pulling force during the test and the fracture occurs at the inner layer of epoxy primer, which is regarded as the cohesion failure. As for the etching treated specimen, the adhesion strength value of 10.49 MPa is between those of untreated and chemical conversion treated specimens and part of the epoxy primer thoroughly peels off at the interface of matrix and epoxy primer and the remaining part still tightly couples with the substrate. That is to say the fracture mode of etching treated specimen is the mixture of adhesive failure and cohesion failure. The results of pull-off test have a close connection with previous research results. The increase of surface free energy caused by the removal of oxides and impurities makes the matrix much easier to be wetted, so the best adhesion strength is observed on chemical conversion treated specimen [22]. In addition, the increase of surface roughness means larger contact area for epoxy primer to adhere and this is beneficial for the formation of favorable mechanical interlocking between conversion coating and epoxy primer [25]. What’s more, the interaction between epoxy primer and conversion coating is more than a simple physic-sorption, the ATMP existed in conversion coating brings out more functional groups such as hydroxyl groups and these groups will react with the various polar functional groups in epoxy primer to form hydrogen bonds or even other forms of chemical bonds. The formation of chemical bonds also has an irreplaceable effect on the highest adhesion strength for chemical conversion treated specimen [15]. 3.3 Investigation on resistance results for as-prepared samples
3.3.1 Nneutral salt spray test Visual surface state of untreated and chemical conversion treated specimens after different time during the NSST is shown in Fig. 8. As for the untreated specimen, even though there exists a layer of oxides on its surface, corrosion traces can be clearly seen after 10 hours. The corrosion traces also spread at a very fast speed and it takes 24 hours before the whole surface is covered by corrosion products. Only a few and fewer matrix is not corroded after 72 hours’ test. While, only a slight of corrosion trace can be observed on the surface of chemical conversion treated specimen after 24 hours and it takes 72 hours before the corrosion became serious. Besides, the corrosion status of chemical conversion treated specimen after 120 hours’ NSST is very similar with that of the untreated specimen after 10 hours’ NSST. These show that the spread of corrosion for chemical conversion treated specimen is much slower than that of untreated one. The NSST results preliminarily prove that the prepared conversion coating can surely delay the corrosion of aluminum alloy to a certain extent. 3.3.2 Potentiodynamic polarization curve Fig. 9 shows the potentiodynamic polarization curves of untreated and chemical conversion treated specimens. Values of corrosion potential ( current density (
) and corrosion
) are listed in Table 4. As a key factor for measuring the
corrosion susceptibility,
changes form -1.103 V to -0.975 V after the chemical
conversion treatment and this shift towards the more positive direction shows that a favorable corrosion resistance is obtained after the conversion process. The
of
untreated and chemical conversion treated specimen are 2.694×10-5 A/cm2 and 9.323×10-8 A/cm2, respectively. The reduction of two orders of magnitude is observed in comparison and this means that the chemical conversion treated specimen has lower corrosion dynamic rate because of the
represents the intensity of cathodic
oxygen reduction and anodic dissolution of metal ions. What’s more, it’s worth noticing that three orders of magnitude of corrosion rate is observed, which also indicates the excellent corrosion resistance performance of the prepared conversion coating. Moreover, protection efficiency (η) of the prepared conversion coating is calculated according to Eq. 11 [41], where
and
are the corrosion current
densities in absence and presence of the conversion coating. The value is also shown in Table 4 and it can reach as high as 99.04 %.
3.3.3 Electrochemical impedance spectroscopy To further evaluate the effect of conversion coating on the corrosion resistance of aluminum alloy, EIS tests were conducted and results are shown in Fig. 10. In the Nyquist plot, the total diameter is the measurement of the corrosion resistance [41]. All specimens exhibit one single semicircle in Fig. 10, indicating the charge transfer takes place at the interface of specimen and electrolyte [42]. The diameter of chemical conversion treated specimen is 103 order of magnitude, which is only 102 for untreated specimen. The huge distinction is obvious and the increase of the arc diameter implies the decrease of the corrosion rate, which confirms the favorable corrosion resistance of conversion coating. Besides, the appraisal of impedance
modulus in low-frequency region corresponds to the total corrosion protection performance conferred by the prepared conversion coating [43]. As shown in Fig. 10(b), the impedance modulus of chemical conversion treated specimen is higher than that of the untreated one and this more resistance behavior in low-frequency reflects the remarkable corrosion resistance of the conversion coating. In Fig. 10(c), a higher phase angle at middle high frequency for the chemical conversion treated specimen corresponds to a capacitive behavior, that is to say the conversion coating has good dielectric property to avoid the ionic flow of electrolyte. Additionally, as the frequency decrease to 0.01 Hz, the phase angle tends to drop and lead to a second relaxation process, relating to the penetration of 3.5 % NaCl solution to the underlying aluminum matrix. Two time constant can be seen in low frequency region and medium-high frequency region, respectively. The EIS data is also fitted by electrical equivalent circuit (ECC), which is presented in Fig. 10(d). When choosing the suitable ECC, the layer of oxides on the surface of aluminum matrix was also taken into consideration. So the ECC of R(Q(R(QR))) was finally adopted [44]. The ECC is consisted of the R s modeling the solution resistance, in sequence with a constant phase element CPE1 in parallel with another resistance Rc modeling the coating resistance, and then another constant phase element CPE2 and the third charge transfer resistance of Rct associating with the corrosion process [9]. The CPE is chose to represent non-ideal capacitive behavior, considering the existence of surface defects [45]. The impendence of the constant phase angle element CPE can be represented by QCPE = [jω] -n/Y0, where ω is the
angular frequency (rad s-1), Y0 is the parameter and n is the dispersion index which indicates the homogeneity of the coating [46]. When the value of n is 1, the CPE is a pure capacitor with a capacitance of Y0. It is believed that the smaller the n value is and the more defects will be on the surface as well as the pitting corrosion is more likely to occur. The fitting results are also shown in Fig. 10 and the relating parameters fitting from ECC is shown in Table 5. There is a good agreement between the experimental and fitted data for both Nyquist and Bode plots. Usually, the bode plots can deliver the information about the impedance features of different specimens: the resistance behavior of electrolyte in high frequency region, resistance behavior in low frequency region and the mixed impedance behavior of faradaic resistance and capacitor in middle frequency region. The Rs values of 10.05 and 14.77 indicate the well conductivity of 3.5 wt % Sodium chloride solution. The larger value of n indicates the surface of specimen becomes more perfect after the chemical conversion treatment. Values of R c and Rt are greatly improved after the chemical conversion treatment. Especially for R c, it has increased for two orders of magnitude, suggesting a compact surface with few defects and the notable improvement of protective properties. Rt also becomes three times larger than that of untreated specimen, representing a favorable corrosion resistance of the whole material. Besides, the charge transmission between aluminum matrix and corrosive medium can also be effectively controlled during the electrochemical corrosion process. In a word, both the higher Rc and Rt for chemical conversion treated specimen make it isn’t easy for the corrosive ions in electrolyte to get across
the conversion coating. Fig. 11 shows the EIS results of chemical conversion coating versus the immersion time and the relating parameters are shown in Fig. 12. Both maximum phase angle and value of Z in low frequency region tend to decrease with the prolongation of immersion time and this shows that the degradation of corrosion resistance in a certain degree. As shown in Fig. 12, the evident decrease in an order of magnitude of Rc is the result of the degradation of the prepared conversion coating. The value of Rct almost linearly decreases from 2.869 × 10 4 Ω·cm-2 to 7.801 × 103 Ω·cm-2 with the increasing of immersion time. The variation has a close relationship with the porous and inhomogeneous nature of the prepared conversion coating. The decrease of Rct means the change of conversion coating thickness and homogeneity as well as density, taking place within microstructure [47]. Thus more corrosive solution can reach to the underlying aluminum matrix and the anticorrosive property consequently become worse. In addition, the Y1 parameter has a slight rising tendency and the more obvious change of Y2 parameter corresponds to the decrease in the dielectric property of conversion coating. That is to say the protection performance will get worse with the prolongation of immersion time. Based on the above results, we can give a reasonable explanation for the mechanism of the aluminum alloy corrosion resistance reinforced by the conversion coating. Firstly, the dense conversion coating can act as a super physical shielding layer and avoid direct contact between the aluminum matrix and corrosive ions. Few channels are provided to allow the corrosion ions to pass through and the penetration
of the corrosion ions is also inhibited. Even though the heavy corrosive chloride is also difficult to cause a negative effect on the aluminum alloy in a short time from the results of NSST. What’s more, during the electrochemical corrosion process of aluminum alloy itself, the charge transfer between the aluminum alloy and the corrosive solution is significantly affected by existence of the conversion coating with poor conductivity. So the corrosion resistance is finally enhanced. 4. 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 refered. The main conclusions of this paper are as follow: ▪The prepared conversion coating are mainly consisted by TiO2, ZrO2, Al2O3, K2NaAlF6, and organic phosphonate ATMP, and this uniform and dense conversion coating attributes to the excellent corrosion resistance. ▪The adhesion strength is significantly enhanced by the conversion coating, reaching as high as 16.23 MPa. This improvement is caused by the increase of surface roughness (750 nm) and surface free energy (17.5 mN/m) as well as the decrease of contact angle (46.2 °), which are beneficial for the well spreading of epoxy primer and the close connection between conversion coating and epoxy coating. ▪ NSST and electrochemical results demonstrate the outstanding corrosion resistance of conversion coating. The NSST time can reach 120 hours. Three orders magnitude
decrease of current density and the more positive corrosion potential are obtained. The enhancement of corrosion resistance has a close relationship to the favorable barrier property of conversion coating, which avoids the direct contact between electrolyte and aluminum matrix and the transformation of electric charge during the electrochemical corrosion process. In general, the existence of conversion coating can not only improve the adhesion strength between aluminum alloy and epoxy primer, but also effectively enhance the corrosion resistance of aluminum alloy. We hope that our research will play an important role in the pretreatment for aluminum alloy, especially in the aspect for the enhancement of adhesion strength between aluminum matrix and organic coating.
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).
Reference
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Figure caption Fig. 1 Results of SEM and EDS. (a) is the SEM image of etching treated specimen, (b) is the SEM image of conversion coating, (c) is the SEM image of conversion coating at larger magnification, (d) is the EDS result of the conversion coating. Fig. 2 Cross-section images of the prepared conversion coating. (a) is the intuitionistic cross section images, (b) is the result of element line scanning. Fig. 3 XRD pattern of the conversion coating. Fig. 4 FT-IR spectra of the aminotrimethylene phosphonic acid and conversion coating. Fig. 5 Surface roughness values of different specimens. Fig. 6 Contact angles of different specimens. (a) is the untreated specimen, (b) is the etching treated specimen, (c) is the chemical conversion treated specimen. Fig. 7 Adhesion strength and fracture mode of different specimens. Fig. 8 Result of neutral salt spray test. (a)-(e) are the untreated specimen, (f)-(j) are the chemical conversion treated specimen. Fig. 9 Potentiodynamic polarization curves of different specimens. Fig. 10 EIS and fitting results of different specimens. (a) and (b) are Nyquist plots, (c) and (d) are Bode plots. Fig. 11 EIS and fitting results of conversion coating versus the immersion time Fig. 12 Variation of relating parameters with time for chemical conversion treated specimen. (a) is Rc and Rct, (b) is Y1 and Y2.
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.
Table
Table 1 Composition of 7A52 aluminum alloy (wt. %) Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Al
Content
0.13
0.17
0.12
0.25
2.3
0.2
4.31
0.11
92.41
Table 2 Composition of the chemical conversion solution K2ZrF6
K2TiF6
NaF
ATMP
5 g/L
3.5 g/L
2.5 g/L
2.0 g/L
Table 3 Contact angle, adhesion work and surface free energy of different specimens. Specimens Contact angle (mJ) (mN/m) Untreated 80.5° 28.1 8.7 Etching treated 73.8° 32.1 10.4 Chemical conversion treated 46.2° 65.0 17.5
Table 4 Parameters obtained from potentiodynamic polarization curves. Specimen
icorr (A·cm-2)
Ecorr (V)
Corrosion Rate (mil/year)
η
Untreated
2.694×10-5
-1.103
1.156×101
-
Chemical conversion treated
9.323×10-8
-0.975
3.999×10-2
99.96%
Table 5 Parameters fitted from electrical equivalent circuit. Specimen Untreated Chemical conversion treated
Rs (Ω·cm-2) 10.05 14.77
CPE1 Y1 (S cm s ) 2.244×10-5 -2 n
1.543×10-5
n1 0.7844 0.8703
Rt (Ω·cm-2) 1.036×10
3
3.232×103
CPE2 Y2 (S cm-2 sn) 1.535×10-2
n2 1
2.946×10-3
1
Rc (Ω·cm-2) 7.783×102 3.129×104
Highlights
1. This conversion coating is environmentally friendly, containing no Cr6+ and Cr3+. 2. Aminotrimethylene phosphonic acid is used to improve adhesion strength. 3. Instead of acids, salts are used to improve the quality of conversion coating. 4. Effect of the conversion coating on adhesion strength is specially explored. 5. Corrosion resistance is enhanced by more compact conversion coating.