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Ti-Based Conversion Coating for the Corrosion Protection of Electrogalvanized Steel

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Ti-Based Conversion Coating for the Corrosion Protection of Electrogalvanized Steel a *8 *9 */01- . - / w"#$ - 9 TI-BASED CONVERSION COATING FOR THE CORROSION PROTECTION OF ELECTROGALVANIZED STEEL Aleksey ABRASHOV 1, Nelya RI ORIAN 2, Tigran VA RAMIAN 3, Vladimir MEN‘SHIKOV 4, Anastasiya KOSTIUK 5 1MUCT, - D.Mendeleev University of Chemical Technology of ,ussia, Moscow, ,ussian 1ederation 1abr-ale [email protected], [email protected] , [email protected] , [email protected], 5nasten [email protected] Abstract 2rocess 0as de.eloped for free-chromate passi.ation of 4inc coatings. An optimal composition of the solution 0as de.eloped. It 0as sho0n that titanium-based passi.ating coatings on the electrogal.ani4ed steel articles compare 0ell in corrosion resistance and protecting capacity 0ith iridescent chromate coatings. It 0as re.ealed that, unlike chromate coatings, the de.eloped titanium-based coatings 0ithstand the thermal shock 0ithout degrading of characteristics. Keyw rds: Corrosion protection, con.ersion coatings, Ti-based coatings, chromate-free passi.ation 1. INTRODUCTION To impro.e the corrosion resistance of gal.anic 4inc and cadmium protecti.e coatings, as 0ell as aluminum surfaces, chromate processes are applied. 2rotecti.e chromate films inhibit corrosion processes on the surfaces of these metals and, besides, ha.e the ability to self-heal in case of mechanical .iolations of the integrity of the film 91:. Unfortunately, chromate solutions are .ery to6ic because of the constituent ions of he6a.alent chromium. The problem of replacing the chromating processes became more acute after the adoption of the European Directi.e 20003533EC in 2000 limiting the presence of Cr 7VI8 compounds in con.ersion coatings 92:, and in 2002 the additions to the directi.e, 0hich completely prohibits the presence of Cr 7VI8 in con.ersion coatings applied to parts of cars from July, 2007 93:. The RoHS 94: and WEEE 95: directi.es also prohibit the presence of Cr 7VI8 in coatings of electrical and electronic e5uipment. In connection 0ith this, the use of he6a.alent chromium has recently been se.erely restricted. Another significant disad.antage of the chromating processes is the lo0 thermal stability of the coatings formed: 0hen heated to temperatures of 1C0 M C or more, their protecti.e ability sharply decreases, 0hich is unacceptable for parts 0orking, for e6ample, in the engine compartment or other "hot spots" of the car. In addition, as a result of thermoshock, chromate films lose the ability to self-heal 91:. A possible alternati.e to chromating is the chromite process - the formation of a protecti.e film in solutions based on tri.alent chromium 9C,7:. The disad.antages of chromite films include the lack of the effect of "self- healing", as a result of 0hich "0hite" corrosion appears much faster on 4inc coatings 0ith chromium passi.ation. In a number of cases, chromating processes are proposed to be replaced by passi.ation processes in cerium-containing 98,9:, silicon-containing 910,11:, or molybdate solutions 912:. According to some authors, the most promising replacement for chromate films are con.ersion 4irconium o6ide and titanium o6ide coatings 913,14:. In the scientific articlessome technologies for applying these con.ersion coatingsare mentioned, ho0e.er, the composition of solutions and process parameters are not disclosed by the authors. Domestic publications or patents on these processes in the scientific and technical literature, as 0ell as in the Internet resources are absent. 105C a *8 *9 */01- . - / w"#$ - 9 The present 0ork is de.oted to the de.elopment of the process of applying protecti.e con.ersion titanium- containing coatings to gal.ani4ed surfaces. 2. E2PERIMENTAL MATERIALS The samples 0ere of4inc-gal.ani4ed plates of cold rolled steel grade 08ps, 0idely used in the automoti.e industry. Einc coating of steel samples 0as carried out in a 0eakly acidic electrolyte of the composition: EnSO 4 7H 2O 120 g3lB Na 2SO 4 18H 2O 20 g3lB NaCl 30 g3lB CH 3COONa 25 g3lB TsKN-3 40 ml3lB TsKN-4 50 ml3lB iX2 A3dm 2B tX20MC. For the preparation of solutions in this study the chemical reagents of pro analysis, pure grades, and distilled 0ater 0ere used. An e6press method 7drop method8 using a solution containing 2b7CH 3COO8 2 50 g3l 0as used to accelerate the e.aluation of the protecti.e ability of the con.ersion titanium-containing coatings. According to this method, the protecti.e ability of the coating 0as estimated in seconds, like the time before the color change of the control section from the transparent to black under a drop of a solution on a 4inc base. The morphology of the surface 0as studied using an INTE RA 2rima atomic force microscope 7NT-MDT, Russia8. Scanning mode - semi-contact, cantile.er - HA_NC Etalon 7NT-MDT, Russia8. The thickness of the coatings 0as determined by an ellipsometric method using an ellipsometer artner e5uipped 0ith a solid-state laser LSM-S-111 0ith a green filter. The 0etting angle at the surfaces ofmetal plates0as measured on the oniometer LK-1. The photos of drops 0ere taken 0ith the camera "Le.enhuk C310 N ", and then the 0etting angle 0as calculated using the DropShape soft0are. X2S-spectra 0ere obtained by means of a special chamber CLAM100, mounted on the Auger microscope HB100 7Vacuum enerators, B8. Corrosion tests of cerium and silicon-containing coatings 0ere carried out in a salt fogchamber Ascott S120i2 in accordance 0ith the international standard ASTM B117. 3. E2PERIMENTAL 5ORK According to the recommendations from other studies, solutions for the formation of titanium-containing coatings should contain he6afluorotitanic acid as the main component, as 0ell as hea.y metal ions, that platecontactly or in form of compounds on the surface of the metal substrate, initiating the subse5uent formation of protecti.e coatings 915,1C:. Taking this into account, Ni 2Y nickel ions 0ere introduced into solutions based on H 2TiF C in the form of a nitrate salt: Ni7NO 382zCH 2O. 3.1. Determinati n f the parameters f the pr cess f titanium-c ntaining c atings dep siti n The e6perimentscarried outre.ealed the concentration range of the solution components in 0hich it is possible 2Y to obtain homogeneous uniform coatings: 0.5-2.5 g3l H 2TiF C and 20-1C0 mg3l Ni . The protecti.e abilityof the coatings formed in this range determined by the drop method is about 7 seconds. Studies ha.e sho0n that the reasonable pH .alues of the solutions are in the inter.al of 4.0-5.5 units. Up to pH 4.0, no coatings are formed, and at a pH greater than 5.5, the coatings become non-uniform and discontinuous. Thus, the pH range 4.0-5.5 is optimal, 0hich is consistent 0ith the coating formation mechanism described in the literature. It is kno0n that in this region of the pH he6afluorotitanic acid reacts 0ith the 4inc- gal.ani4ed surface formingtitanium and 4inco6ides thatdepose and adsorb initially on the surface of the contactly deposited nickel.O.er time this coating e6pands to form a continuous film. 1057 a *8 *9 */01- . - / w"#$ - 9 En YH 2TiF C Y 4H 2O N TiO 2zEnOzH 2O Y CHF Y H 2õ 718 2En Y 2H 2TiF C Y CH 2O N Ti 2O3z2EnOzH 2O Y12HF Y H 2õ 728 En Y 2HF Y H 2O N EnF 2zH 2O Y H 2õ 738 EnO Y 2HF N EnF 2 Y H 2O 748 In this study, the protecti.e ability of coatings 0as in.estigated depending on the duration of the process. It 0as re.ealed that the protecti.e capacity of the coating increases during the first minute of the process and then stabili4es at the .alues of protecti.e ability. The immersion of coated sample in the solution for more than 4 minutes is undesirable, since this leads to a deterioration in the appearance of the coatings and a decrease in the protecti.e ability. Taking into account the obtained results, the optimal process duration 0as assumed to be 1 min. It 0as found that heating of solution to 40MC does not lead to significant changes in the appearance and protecti.e ability of the coatings, but at higher solution temperatures, the protecti.e ability decreases. Therefore, the range of 18-25MC 0as chosen for the operating range and it 0as noted that heating of the solution to 40MC, for e6ample in the summertime, is allo0ed. It 0as found that the protecti.e ability of the coatings is affected by the drying temperature of the coatings, and the dependence is of an e6treme nature: coatings dried at a temperature of 80-120MC ha.e the greatest protecti.e capacity 720 sec8, and outside the inter.al, the protecti.e ability of the coatings decreases. The sur.ey spectra of coatings obtained by the X2S re.ealed the presence of titanium, 4inc, o6ygen and fluorine compounds on the surface. Indi.idual spectra of the elements made it possible to establish that 4inc is included in the coating in the form of EnO, and titanium in the form of TiO 2-Ti 2O3, 0hich agrees 0ith the abo.e-described mechanism of coating formation. In.estigating the morphology of coatings using atomic-force microscopy allo0ed us to determine the grain si4e of the coating: it does not e6ceed 50 nm. The effect of hydro6ycarbo6ylic acids such as citric, malic, tartaric, lactic and o6alic acids has been studied. It 0as found that the introduction of tartaric acid in the amount of at least 0.4 g3l increases the protecti.e ability of the coatings obtained from 17 to 3C s. 3.2. F rmati n f t p-c at at titanium-c ntaining c atings In order to increase the protecti.e ability of chromite passi.ation coatings on 4inc, practically the additional protection or "sealer" are used, 0hich are thin organic or inorganic films that are applied as a finishing coat.
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