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Formation Mechanism and Properties of Fluoride–Phosphate Conversion

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Formation Mechanism and Properties of Fluoride–Phosphate Conversion RSC Advances View Article Online PAPER View Journal | View Issue Formation mechanism and properties of fluoride– phosphate conversion coating on titanium alloy Cite this: RSC Adv.,2017,7,16078 Shuaixing Wang,*a Xiaohui Liu,a Liqiang Wang,b Qingjie Wen,b Nan Dua and Jianhang Huanga A uniform grey conversion coating with the thickness of 4–5 mm was prepared on titanium alloy by a fluoride–phosphate treatment. Scratch testing results indicated that the conversion coating greatly improved the adhesion strength of the paint coat to the titanium substrate. The growth mechanism of the conversion coating was analyzed comprehensively by E–t curve, SEM, EDS, XPS and XRD. In the conversion process, the titanium substrate was first dissolved (0–45 s), then Na3TiF6 nucleation occurred via a series of reactions (45–120 s), whereby the conversion coating grew steadily with the nucleation Received 23rd November 2016 and growth of Na TiF grains (120–600 s); after 600 s, the conversion coating reached dynamic Accepted 28th February 2017 3 6 equilibrium of growth/dissolution. In the drying process at 100 Æ 5 C, some Na3TiF6 grains reacted with DOI: 10.1039/c6ra27199e O2 to form Na3TiOF5. The components of the conversion coating were Na3TiF6,Na3TiOF5, a little TiO2, Creative Commons Attribution-NonCommercial 3.0 Unported Licence. rsc.li/rsc-advances TiF4 and phosphate derivatives. 1. Introduction phytic acid (PA) treatment17 and Zr/Ti-based treatment18–20 may also be used for the pretreatment of titanium alloy. In general, Titanium and its alloys are increasingly applied in the aerospace the uoride–phosphate conversion technique is the most effec- industry, such as in airframes, engine components, fasteners and tive pretreatment system for titanium alloy, and has even been others, owing to their excellent combination of low density, adopted as an official aerospace material specication (AMS high strength-to-weight ratio and good corrosion resistance.1,2 2486D).12 However, so far there has been no report about the This article is licensed under a However, drawbacks, such as low surface hardness, high friction chemical composition, microstructure and formation mecha- coefficient and poor adhesion with organic coatings, limit the nism of the uoride–phosphate conversion coating on titanium direct application of titanium alloys. A variety of surface treat- alloy. Open Access Article. Published on 13 March 2017. Downloaded 10/1/2021 8:07:53 PM. ment technologies, including anodizing, micro-arc oxidation, In many case, the precipitation of these ultra-thin conversion chemical conversion, surface alloying and physical vapour coatings is an electrochemically driven process and requires the – deposition, have been employed to improve the surface perfor- dissolution of alloy and a subsequent lm deposition.5,17,20 24 mance of titanium alloys.3 Amongst the mentioned surface The literature20,21 reports that the formation of a Ti/Zr-based treatment technologies, chemical conversion coating is the conversion coating on aluminium alloy can be deemed as primary surface pretreatment for titanium alloy in the aerospace a nucleation, followed by the growth of Na3AlF6 crystal and industry, and it provides good corrosion protection, promotes formation of a metal–organic complex. In the formation process the adhesion with paints and has a low processing cost.4,5 of a zinc phosphate (ZPO) layer on aluminium, ZPO crystals However, an excellent coating is difficult to convert on titanium primarily occur on and around the intermetallic particles.22 using the traditional chemical conversion technology based on Zhao9 indicated that ZPO conversion coating on pure titanium chromates and phosphates,6–8 due to the presence of the passive experiences a similar nucleation step, and that ultrasonic irra- oxide lm. Therefore, zinc phosphating,9 trivalent chromium- diation can improve the nucleation and growth of ZPO. Besides, phosphated conversion10,11 and uoride–phosphate conversion Li17 found that PA induced the effective deposition of reaction coating12 are gradually being developed in line with the existing products and the formation of the conversion coating in and emerging requirements. Besides, the eco-friendly pretreat- a mixed conversion system of PA and NH4F for Ti-6Al-4V alloy. ment techniques for aluminium alloy, including rare-earth Although the formation of a uoride–phosphate conversion conversion,13,14 molybdate conversion,15 silane treatment,16 coating on titanium alloy may undergo similar processes, the basic theories about the coating composition and the coating– forming reactions are not yet clear, until now. aNational Defense Key Discipline Laboratory of Light Alloy Processing Science and The aim of the present work was to gain an insight into the Technology, Nanchang Hangkong University, Nanchang 330063, P. R. China. E-mail: deposition mechanism of a uoride–phosphate conversion [email protected] coating on titanium alloy. For this purpose, the open circuit bAVIC Chengdu Aircra Industrial (Group) Co., Ltd., Chengdu 610091, P. R. China 16078 | RSC Adv.,2017,7,16078–16086 This journal is © The Royal Society of Chemistry 2017 View Article Online Paper RSC Advances potential (OCP) of titanium alloy in a conversion bath was typical result or the average of the three measurements is re- completely monitored. The transformation of the morphology ported in this paper. and the chemical composition of the coating during the conversion process were analyzed by SEM and EDS. In addition, 2.4 Characterization X-ray photoelectron spectroscopy (XPS) and X-ray diffraction The surface morphology and cross-sectional morphology of the (XRD) were used to accurately determine the chemical compo- conversion coating were analyzed by eld emission gun SEM sition of the conversion coating. Moreover, the corrosion (FE-SEM, Nova Nano-SEM 450). Cross-sections of the specimens resistance and the paint adhesion of the conversion coating were ground by successive grades of SiC paper, followed by were evaluated by electrochemical impedance spectroscopy nishing with a 1 mm diamond. The chemical compositions (EIS) and scratch testing, respectively. A possible formation were examined by energy dispersive X-ray spectroscopy (EDS, mechanism for a uoride–phosphate conversion coating on INCA 250). Phase compositions were investigated by X-ray TC1 alloy is proposed. diffraction analysis (XRD) using a Bruker D8 ADVANCE instru- ment equipped with Cu Ka. The intensity data were collected in À 2. Experimental a2q range from 10 to 70 with a scan rate of 0.02 s 1. X-ray photoelectron spectroscopy (XPS) measurements were con- 2.1 Materials ducted in an Axis Ultra DLD equipped with a standard Al Ka X- TC1 panels of 2.0 mm thickness were supplied by Baoti Corp., ray source (1486.6 eV) and a hemispherical analyzer. China. The main chemical composition of the TC1 alloy was – – 1.0 2.5 wt% Al, 0.7 2.0 wt% Mn, 0.30 wt% Si, and bal. Al. The 2.5 Paint adhesion specimens with a dimension of 25 mm  20 mm were cut from TC1 samples without and with chemical treatments were the panels, polished with abrasive papers, ultrasonically painted with S06-1010H polyurethane primer (Tianjin Light- cleaned in ethanol and nally dried in warm air. house Coating Co., Ltd., China). The thickness of the paint coat Creative Commons Attribution-NonCommercial 3.0 Unported Licence. was about 30–35 mm. The adhesion of the polyurethane coating 2.2 Preparation of conversion coating to the substrates was determined by a single scratch tester (WS- Prior to chemical conversion treatment, the samples were 2005, Lanzhou Institute of Chemical Physics, CAS, China). The sequentially treated by the following steps: degreasing in an measurements were done before (dry adhesion) and a er (wet À1 À1 adhesion) 7 days of the paint-coated samples exposure to aqueous solution containing 30 g L Na2CO3,20gL Na3PO4 À1 3.5 wt% NaCl solution. During the scratch testing, acoustic and 5 g L Na2SiO3 at 60 C for 2 min, rinsing in deionised water, pickling in 20 vol% HNO solution at room temperature emission (AE) signals were measured with a diamond indenter 3 (120 cone with a 200 mm radius tip) under a loading rate of 40 N for 1 min and nally rinsing with deionised water. À This article is licensed under a 1 – The cleaned samples were immediately immersed in a uo- min , loading range of 0 80 N and scratch length of 6 mm. ride–phosphate conversion solution at 30 Æ 3 C for different During the scratch testing, the load at which the coating was À1 removed from the substrate was designated as the critical load times. The conversion bath consisted of 40 g L Na3PO4,15g À1 À1 Open Access Article. Published on 13 March 2017. Downloaded 10/1/2021 8:07:53 PM. L NaF and 25 mL L HAc. The pH of the conversion baths (LC) and the moment was recorded by an acoustic emission (AE) 25 was adjusted to 4.0–5.0 by adding HF or NaOH solution. Aer signal. The pro les of the scratched coating were measured the surface treatment, the samples were rinsed thoroughly with using a KH-7700 3D video microscope. deionised water, and then dried in warm air or at 100 Æ 5 C for 30 min. 3. Results and discussion 3.1 Coating growth curve 2.3 Electrochemical tests Fig. 1 shows the variation curves of the OCP and the coating Electrochemical tests were conducted by using an Autolab weight in the uoride–phosphate conversion process. It was PGSTAT 302N. The tests were performed by using a three- found that the OCP decreased sharply before 45 s, and then electrode electrochemical cell. A saturated calomel electrode greatly increased at 45–600 s and then became stabile aer (SCE) and a platinum sheet were used as the reference and 600 s. The coating weight had a similar variation law with the counter electrodes, respectively.
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