Corrosion behavior of rare earth cerium based conversion coating on aluminum alloy

JOURNAL OF RARE EARTHS, Vol. 28, Spec. Issue, Dec. 2010, p. 371

Corrosion behavior of rare earth cerium based conversion coating on aluminum alloy ZHAO Dan (䍉Ѝ), SUN Jie (ᄭᵄ), ZHANG Lili (ᓴББ), TAN Yong (䈁࢛), LI Ji (ᴢᄷ) (School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110168, China) Received 18 June 2010; revised 17 October 2010

Abstract: The present paper focused on the use of the salt of rare earth cerium as corrosion inhibitor of aluminum by using cathodic electrolytic passivation method. The corrosion resistance and the microphology of the cerium passivation film were studied by the methods of electrochemical method, scanning electron microscopy (SEM), and energy dispersive spectroscopic analysis. From the results, it was shown that good corrosion resistance of cerium-based passive coating was obtained when the compositions were as follows: CeCl3·7H2O, 0.05 mol/L; H2O2, 30 ml/L; current density, 1.1 mA/cm2; temperature, 40 ºC; time, 9 min. SEM and EDS revealed that the cerium conversion coatings formed on the surface of aluminum alloy were related to cerium hydroxide/hydrated oxide depositions. Keywords: aluminum alloy; cerium-based passive film; electrochemistry; corrosion resistance; rare earths

Aluminum and its alloy are used widely in engineering applications such as in the field of aeronautics industry, construction business for their low density, favorable mechanical properties and relatively good corrosion resistance. The resistance of aluminum alloy against corrosion in aqueous or atmospheric media can be attributed to a rapid formed surface oxide film, which consists of Al2O3, Al(OH)3 and AlO(OH) phases[1,2]. However, the natural oxide ¿lm on aluminum does not offer sufficient protection against aggressive anions. In presence of chlorides or other halides, localized corrosion such as pitting corrosion is promoted when passivity breaks down at local points of the surface leaving the inner substrate exposed to media containing these aggressive ions[3,4]. To avoid corrosion, different methods have been used for years. Currently, chromates are still used widely in anti-corrosive pre-treatment of aluminum alloy[5–7]. The chemical conversion coatings on aluminum alloy by chromate processes can improve paint adhesion and impart corrosion protection. One of the main actions of chromates, which explain their efficiency, is their self-repairing effect. Their strong oxidizing power stops corrosion by forming a new and efficient passive layer on the aluminum alloy surface[8]. Since the beginning of the 1990s, the high toxicity associated with chromates has imposed restrictions on their use in industry and food applications. The Directives of the European Union did not allow chrome hexavalent compounds in car industry since July 1, 2003. As a consequence, intense research efforts have been undertaken to find new environmentally friendly compounds as corrosion inhibitors of aluminum alloys. In recent years, a great deal of research efforts have been done to develop new environmentally friendly corrosion in-

hibitor as an alternative and no toxic compound to replace the classic systems based on chromates. Molybdates, tungstates, permanganates, and vanadates, including chromium-like elements, were the chemical elements to replace the hexavalent chromium. Many alternative coatings have been developed based on zirconium, titanium salts, cobalt salts, and organic conductive polymers[9–13]. Among the numerous studies concerning the replacement of hexavalent chromium element, some compounds have presented interesting corrosion protection activity. However, despite the promising performances, these compounds do not offer the same level of protection as chromates, particularly in surface treatments. To find an alternative treatment to hexavalent chromium conversion coatings, several treatments which present a good anti-corrosion behavior, a high benefit/cost relation, and mainly, low environmental impacts are still to be developed. Recently the research has been focused on the use of rare earths metals as a green alternative for the chromates species, not only because of their effectiveness but also due to the null toxicity for most of them[14–22]. Rare earths inhibitors can be used as pigments in conversion coatings for different alloy systems. These inhibitors work well in high pH condition and therefore they can be used in alkaline environment. A large amount of research work is underway to replace chromate coating by cerium and lanthanum conversion coatings. Among all the lanthanide compounds, cerium salts have successfully been used as corrosion inhibitors on different metals and alloys. The present paper focused on the use of the salt of rare earth cerium as corrosion inhibitor of aluminum. The corrosion resistance of the rare earth metal conversion passivation was studied in this paper.

Foundation item: Project supported by the National High Technology Research Development Program of China (863 Project) (2009AA03z529) Corresponding author: SUN Jie (E-mail: [email protected]; Tel.: +86-24-24680345) DOI: 10.1016/S1002-0721(10)60338-9

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1 Experimental The measured specimens of 50 mm×100 mm were LY12 aluminum alloy. Before being used, the alloy surfaces were wet-polished by silicon carbide paper (No. 600, No.800, No. 1000 and 1500) respectively. The samples were then degreased with acetone, absolute alcohol and rinsed in distilled water. The conversion coating was formed by dipping the polished LY12 substrate in a CeCl3·7H2O and H2O2 solution in the condition of current density. The basic compositions of electrolytic passivation solution are as follows: CeCl3·7H2O: from 0.003 to 0.07 mol/L H2O2: 30 ml/L Current density: 1.1 mA/cm2 Temperature: 40 ºC Time: 9 min The corrosion resistance of the conversion coating was investigated using electrochemical test. The photograph of the conversion film surface was characterized by scanning electron microscopy (SEM). Electrochemical test with a conventional three-electrode corrosion cell were carried out in quiescent 3.5 wt.% NaCl aqueous solution at room temperature to evaluate the corrosion resistance of the different concentrations of CeCl3ǜ7H2O using the CHI660A electrochemistry workstation. The working electrode was 1 cm2 of the material under study. The linear polarization curves were measured in 3.5 wt.% NaCl solution with different cerium ion concentrations with a sweep rate of 1 mV/s. The reference electrode was standard saturated calomel electrode (SCE), and the platinum electrode was used as the counter electrode to ensure the homogeneous distribution of the current. The polarization curves were carried out at a sweep rate of 1 mV/s after 2 s stabilization of the open circuit potential. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) (SHIMADZU SSX-550 SEM/EDS) were used to characterize the surface morphology and elemental composition of the coating.

2 Results and discussion

Fig. 1 Linear polarization curves for aluminum alloy in 3.5% NaCl solution with different amounts of CeCl3·7H2O

Fig.2 Linear polarization impedance for aluminum alloy in 3.5% NaCl solution with different amounts of CeCl3·7H2O

out by mean of the corrosion rate using the expression:. IP=

i0 -iinh i0

×100%

(1)

where iinh is the corrosion current density with inhibitor in the solution, and i0 is the corrosion current for the blank solution. From the measurement results, the corrosion current density of blank solution was 2.028×10–5 A/cm2, and the corrosion current density of 0.05 mol/L cerium solution was 4.893× 10–7 A/cm2. The inhibition power is about 97.8%. The result suggests that cerium acts as corrosion inhibitor for aluminum alloy and the optimum concentration is about 0.05 mol/L.

2.1 Electrochemical measurements

2.2 Morphology and compositions of conversion coatings

The electrochemical polarization curves of aluminum alloy in 3.5 wt.% NaCl solution with different concentrations of CeCl3·7H2O are presented in Fig. 1. The linear polarization impedance are shown in Fig. 2. The curves and data reveal a dependence on the cerium concentration. As the amount of inhibitor in the solution increases, the linear polarization impedance shifts to higher value until the concentration of cerium reach to 0.05 mol/L. When the concentration of cerium continually increases, the linear polarization impedance changes from higher to lower value. To compare with the performance of corrosion resistance, the quantification of the inhibition power (IP) was carried

The SEM analysis that was performed after cathodic electrolytic passivation in the condition of 0.05 mol/L cerium solution, revealed the presence of precipitation deposited on the surface of aluminum alloy (Fig. 3). The observation at high magnifications showed a spherical structure, which is mainly composed by cerium, as was confirmed by the EDS spectrum plotted in Table 1. As can be seen from the micrograph, the white localized spots were observed on the surface of conversion film, with cerium conversion coatings that were related to cerium hydroxide/hydrated oxide depositions. Meanwhile element percent analysis indicated the existence of cerium in the coatings.

ZHAO Dan et al., Corrosion behavior of rare earth cerium based conversion coating on aluminum alloy

Fig. 3 SEM micrograph with 0.05 mol/L cerium conversion coating and EDS spectra of the sample Table 1 Compositions of 0.05 mol/L cerium conversion coating Element

Content/wt.%

OK

31.12

Al K

15.75

Ce L

53.13

Total

100.00

It is worth noting that the crack can be observed on the surface of the conversion coatings. The maximum width of some crack is around 1 ȝm. Most of the crack width is small. According to the study of scholars, the formation of cerium passivation film is carried out along the top of the membrane pore walls and not completely covered. The cathodic electrodeposition process causes the state of cerium passive film surface morphology[23,24]. 2.3 Discussion According to the analysis of corrosion, micrograph and electrochemistry, the cerium conversion coatings remarkably decreased with the corrosion current. The cathodic reactions (oxygen reduction) generated on an alkaline environment and lead to precipitation of cerium oxides and thus to the formation of a protective surface film. Hosseini et al. proposed the cerium conversion mechanism on the galvanized steel[15]. Based on his theory and the electrolysis passivation system, the mechanism of cerium conversion film can be described as follows. The color of the electrolysis solution changes from colorless to yellow when adding H2O2 to CeCl3 solution because of the oxidation of trivalent cerium by H2O2. The following reaction happens: 2Ce3++H2O2+2H2Oĺ2Ce(OH)22++2H+ (2) In the film-forming solution, the content of H2O2 is much more than the amount of reaction by the oxidation of trivalent cerium amount required. From the change of color in the solution, the existing state of cerium can be as the tetravalent and trivalent form. The mechanism for forming the cathode electrolytic, not yet a mature theory is widely accepted by academic circles. Hinton et al. used bipolar theory as the theoretical basis to describe the electron passing through the surface when in the

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process of film-forming. When applied cathodic current, the polarization phenomena lead to the formation of an internal electric field, and the electrochemical reduction reaction occurs at the side of electronegativity[25]. According to the bipolar theory, electrochemistry theory and the experimental results, the reduction reactions occur at the side of electronegativity at the beginning of cathode electrolytic: H2O2+2eĺ2OH– (3) O2+2H2O+4eĺ4OH– (4) As the reaction progressed, the pH value near the surface gradually increased. There is an OH– ion concentration gradient in the liquid layer of the cathode surface. When the pH value increased to be higher than 8 on the aluminum alloy surface, the chemical dissolution will occur for the existence of the surface oxide: Al2O3+2OH–ĺ2AlO2–+H2O (5) According to equation (2), film-forming solution of Ce (OH)22+ will participate in the electrochemical reduction reaction. In accordance with the bipolar theory and ceriumH2O potential-pH system, when the pH value of more than 12, the cerium passivation film deposition most likely represents by: Ce(OH)22++2OH–ĺCe(OH)4ĺCeO2+H2O (6) Thus, the existing state of cerium in the conversion coatings was related to cerium hydroxide/hydrated oxide depositions.

3 Conclusions The cerium conversion coatings were formed by using cathodic electrolysis passivation method. When the concentration of cerium reached 0.05 mol/L, the maximum linear polarization impedance could be obtained. The micrograph of the conversion coatings showed a spherical structure, which was mainly composed of cerium. The cerium conversion coatings were related to cerium hydroxide/hydrated oxide.

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