Corrosion behavior of Ce-oxide/hydroxide coated AA7075-T6 prepared by dip immersion method

Turkish J Eng Env Sci (2014) 38: 363 – 372 Turkish Journal of Engineering & Environmental Sciences http://journals.tubitak.gov.tr/engineering/ c TÜBİTAK ⃝ doi:10.3906/muh-1403-12 Research Article Corrosion behavior of Ce-oxide/hydroxide coated AA7075-T6 prepared by dip immersion method Iman DANAEE∗, Maryam KANAANI, Mohammad Hosein MADDAHY, Soudabeh NIKMANESH Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran Received: 31.03.2014 • Accepted/Published Online: 28.08.2015 • Printed: 04.03.2016 Abstract:In this study, cerium-based conversion coating was deposited on aluminum 7075-T6 by dip immersion method. Cerium oxide/hydroxide is an environmentally friendly conversion coating. Its corrosion resistance in 3.5 wt.% NaCl solution was investigated by means of electrochemical impedance spectroscopy, potentiodynamic polarization, and surface techniques. The coated samples showed a significant decrease in corrosion rate and the coating resistance increased with increasing immersion time up to 1200 s. In addition, electrochemical impedance data showed that in the presence of cerium oxide/hydroxide conversion coatings, the charge transfer resistance of aluminum increased. Surface morphology and its chemical composition were analyzed by means of scanning electron microscopy and energy dispersive spectroscopy. Key words: Aluminum 7075, cerium oxide/hydroxide coatings, impedance, corrosion, scanning electron microscopy, potentiodynamic polarization 1. Introduction Aluminum is widely used as a structural material because of its favorable properties such as a high strength to weight ratio, corrosion resistance, and low cost [1,2] and in military and aerospace industries [1–3] owing to its low density [3,4]. Alloying of aluminum is necessary to promote it to a high strength level [1–3] and hence reduce its vulnerability to corrosion. The presence of the second phase particles in alloys leads to a potential difference between the aluminum matrix and alloy element, which results in the formation of a galvanic cell. This galvanic cell causes a decrease in aluminum’s corrosion resistance, particularly against halide ions [5–7]. In the past, chromate conversion coatings have usually been employed to protect aluminum alloys against corrosion [8–13]. The highly carcinogenic and toxic [11,12] properties of hexavalent chromium compounds forced researchers to search for more benign alternatives. Amongst the various alternatives, recently rare-earth coatings, and particularly cerium, have attracted significant attention [14–18] as cerium forms a stable oxide and hydroxide film. Furthermore, cerium-based oxide hydroxide film is nontoxic and inexpensive [16–18]. The commonly employed deposition mechanism involves both the oxidation of aluminum and the reduction of H 2 O 2 in the reaction [14–19]: ∗ Correspondence: Al → Al3+ + 3e (1) H2 O2 + 2e → 2OH (2) Ce3+ + OH − + 1/2H2 O2 → Ce(OH)2+ 2 (3) 363 DANAEE et al./Turkish J Eng Env Sci Ce(OH)2+ 2 + 2OH → Ce(OH)4 (4) Ce(OH)4 → CeO2 + 2H2 O (5) H 2 O 2 is added to the coating bath as an effective accelerator additive. The acceleration of the cerium conversion coating process may be attributed to the rapid increase in pH, in turn caused by H 2 O 2 reduction in cathodic sites. Local increase of pH leads to improving the deposition of cerium oxide and cerium hydroxide [6,19]. At lower pH, according to the Pourbaix diagram, Ce(III) is more stable, so Ce(IV) reduces to Ce(III). However, in the higher pH range, Ce(IV) is more stable, particularly when oxidizing agents like O 2 or H 2 O 2 are available [8]. Previous research has described in detail the effect of different deposition methods such as sol gel, brush, and spray coatings on the corrosion resistance of cerium-based conversion coatings on aluminum 7075-T6 [20–23]. The present work, on the other hand, concentrates on the experimental investigation of the electrochemical properties of 7075-T6 aluminum alloy covered with cerium obtained by dip immersion method by impedance spectroscopy and potentiodynamic polarization. Surface morphology and chemical composition of cerium-based conversion were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). 2. Experimental work 2.1. Substrate preparation For substrate preparation we followed exactly the same steps employed in an earlier work [20]. First, ceriumbased conversion coating was deposited on AA7075-T6 aluminum alloy with a chemical composition of Si: 0.4, Fe: 0.4, Cu: 1.4, Mn: 0.4, Mg: 2.2, Cr: <0.5, Zn: 5.8, Ti: <0.5, and others: < 0. 5 wt.%, with Al is the balance. The specimens were cut to 1 cm × 1 cm samples from larger panels. Prior to the surface pretreatment process, all samples were mounted in epoxy resin. Aluminum coupons were then mechanically abraded with abrasive papers (400–2000 mesh). The specimens were desmutted by rinsing with acetone and alkaline cleaning by soaking in a NaOH solution, followed by their acid activation in an H 2 SO 4 solution. Between each sequential step of the pretreatment process the samples were rinsed with deionized water. Pretreated samples were coated by immersion in a cerium solution at room temperature for periods ranging from 30 s to 1800 s. The coating bath contained 1 g of CeCl 3 , 3 drops of glycerin, and 2 mL of H 2 O 2 in 100 mL of deionized water. After the coating process, they were stored at room temperature in the ambient laboratory air for 24 h. 2.2. Methods The employed methods were also borrowed from an earlier work [20] as follows. Electrochemical impedance spectroscopy (EIS) and polarization curves were employed to evaluate the corrosion behavior of cerium-based conversion coatings. The electrolyte was 3.5 wt.% NaCl solution and the solution pH was adjusted by adding hydrochloric acid or sodium hydroxide. A three-electrode cell system (PGSTAT 302N) was used for the electrochemical tests. Ag/AgCl, platinum, and AA7075 electrodes were used as the reference, counter, and working electrodes, respectively. The potential was scanned at a rate of 1 mV s −1 . Polarization parameters were calculated by the Tafel extrapolation method [24,25]. A frequency sweep from 100 kHz to 10 MHz was used for EIS measurements. EIS data were curve-fitted to the proposed equivalent circuit by in-house least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [26,27]. SEM (VEGA, TESCAN-LMU) equipped with EDS was employed to characterize the surface morphology and average chemical composition. 364 DANAEE et al./Turkish J Eng Env Sci 3. Results and discussion 3.1. Surface analysis Figure 1a presents the cerium conversion coatings of aluminum obtained in 1200 s of immersion time in cerium solution. The cerium conversion coating exhibits a uniform surface with mud-crack morphology, and some external growth and an enriched zone (...truncated)