PROCESS FOR TREATING THE SURFACE OF A PART MADE OF ALUMINIUM OR ALUMINIUM ALLOY OR OF MAGNESIUM OR MAGNESIUM ALLOY
20210262107 · 2021-08-26
Inventors
Cpc classification
C23C22/44
CHEMISTRY; METALLURGY
International classification
Abstract
The invention relates to a method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy, comprising a step of treatment by oxidation of said part and a step of applying an aqueous composition to the surface of said part.
Claims
1. A method for the surface treatment of a part made from aluminum or aluminum alloy or from magnesium or magnesium alloy, the method comprising: oxidation treating said part; and, applying, to the surface of said part, an aqueous composition free of chromium, the aqueous composition containing: fluorozirconate ions, molybdate ions, and at least one component selected from lithium ions and permanganate ions.
2. The method according to claim 1, wherein said oxidation treatment of said part is an anodization treatment.
3. The method according to claim 1 wherein said oxidation treatment of said part is a micro-arc oxidation.
4. The method according to claim 1 wherein said application is carried out by immersing said part in a bath of said aqueous composition.
5. The method according to claim 1 wherein said aqueous composition applied during the application has a temperature of between 10 and 60° C.
6. The method according to claim 1 wherein said application is carried out for a time greater than or equal to 5 minutes, preferably between 5 and 20 minutes.
7. The method according to claim 1 further comprising preliminarily pretreating said part by at least one of chemical degreasing and pickling.
8. The method according to claim 1 wherein said aqueous composition contains lithium ions and permanganate ions.
9. The method according to claim 1 wherein said aqueous composition contains cerium ions.
10. The method according to claim 1 wherein said aqueous composition contains nitrate ions.
11. The method according to claim 10, wherein said aqueous composition contains lithium nitrate.
12. The method according to claim 1 wherein said aqueous composition has a pH of between 3 and 7.
13. The method according to claim 1 wherein said aqueous composition contains at least one of the following concentrations: between 5 and 30 g/l of fluorozirconate salt, between 2.5 and 10 g/l of molybdate salt, between 2 and 8 g/l of lithium salt, between 4 and 19 g/l of permanganate salt, and/or between 0.1 and 0.5 g/l of cerium salt.
14. A part made from aluminum or aluminum alloy or from magnesium or magnesium alloy obtained by a surface treatment comprising: oxidation treating said part; and, applying, to the surface of said part, an aqueous composition free of chromium, the aqueous composition containing: fluorozirconate ions, molybdate ions, and at least one component selected from lithium ions and permanganate ions.
Description
[0104] The features and advantages of the invention will emerge more clearly in the light of the embodiments below, which are provided purely by way of illustration and in no way limit the invention, with the support of
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[0108] and
EXAMPLE 1
[0109] AA2024 aluminum alloy test pieces (with the following composition: 1.2 to 1.8% Mg, 0.3 to 0.9% Mn, max. 0.5% Fe, 3.8 to 4.9% Cu, max. 0.25% Zn, max. 0.1% Cr, max. 0.15% Ti, Al for the remaining %), rolled or machined, with dimensions 25×100×3 mm (for microstructural characterizations) and 150×80×3 mm (for salt spray tests) were treated using the method according to the present invention as per the following operating conditions.
[0110] The test pieces were first subjected to surface preparation steps. For this purpose, they were successively soaked in the baths below: [0111] aqueous bath containing Turco® 4215 NCLT 50 (50 g/l) and Turco® 4215 additive pH 9 (10 g/l), at 60° C., for 20 min (alkaline degreasing) [0112] demineralized water at ambient temperature for 5 min (rinsing) [0113] aqueous bath containing Turco® Smut Go NC (19% v/v), at 20° C., for 5 min (pickling) [0114] demineralized water at ambient temperature for 5 min (rinsing).
[0115] The test pieces were then subjected to a sulfuric anodic oxidation treatment, in a conventional manner in itself, according to the following parameters: [0116] 200 g/l aqueous sulfuric acid electrolytic bath [0117] duration 21.33 min [0118] bath temperature 19° C. [0119] voltage increase at a speed of 3 V/m in, up to a plateau value of 16 V, and hold at this plateau value for 16 min.
[0120] At the end of these steps, an anode layer was obtained on the surface of the test pieces.
[0121] The test pieces were then subjected to a sealing treatment in accordance with the present invention. For this, they were immersed in the following aqueous composition:
TABLE-US-00001 K.sub.2ZrF.sub.6 25 g/l Na.sub.2MoO.sub.4, 2H.sub.2O 5 g/l LiNO.sub.3 4 g/l KMnO.sub.4 9.5 g/l Ce(NO.sub.3).sub.3, 6H.sub.2O 0.1 g/l
[0122] The pH of this composition is equal to 6.
[0123] Certain test pieces were treated with the same composition, but devoid of Ce(NO.sub.3).sub.3 (“without Ce”).
[0124] The treatment temperatures and times are shown in Table 1 below.
[0125] At the end of these steps, a layer with the thickness indicated in Table 1 below was obtained on the surface of the test pieces.
[0126] After sealing, the test pieces were directly, i.e. without rinsing, exposed to salt spray for 750 or 1176 h, at a temperature between 15 and 25° C., according to the conditions in accordance with the NF EN ISO 9227 standard.
[0127] Each set of conditions was performed in triplicate.
[0128] By way of comparative example, test pieces were subjected to sealing by means of the method based on trivalent chromium sold under the name SurTec® 650.
[0129] The obtained results, expressed as the number of pits observed on the surface of the test piece after exposure to salt spray, are shown in Table 1 below.
TABLE-US-00002 TABLE 1 Results of a salt spray exposure test for anodized and sealed aluminum alloy parts Number Number Layer of pits of pits Temperature Duration thickness after after Sample (° C.) (min) (□m) 750 h 1176 h AA2024 50 8 6/6.2/6.1 6/7/7 10/6/10 rolled AA2024 ambient 8 6.3/6.2/5.6 1/1/1 1/2/2 rolled AA2024 ambient 8 5.3/5.5/5.4 1/1/1 1/1/1 rolled AA2024 50 40 / 3/5/3 / machined AA2024 ambient 8 5.2/5.3/5.2 1/1/1 1/1/1 machined AA2024 ambient 8 7.2/7.4/7.4 1/1/1 1/1/1 machined AA2024 50 8 5.6/3.8/3.5 0/0/1 0/1/3 machined - “without Ce” AA2024 ambient 8 4.2/3.0/2.5 1/1/0 / rolled SurTec ® 650 AA2024 ambient 8 4.6/4.9/5.9 3/3/3 / machined - SurTec ® 650
[0130] As can be seen, very satisfactory corrosion protection of the test pieces is obtained for all of the conditions tested. Particularly good results are advantageously obtained for the treatment with cerium at ambient temperature: after only 8 min, the corrosion resistance of the test pieces is particularly good, both for the rolled test pieces and for those which have been machined in bulk. It is in particular similar to, and for machined test pieces even better than, that obtained by the SurTec® 650 commercial method based on chromium proposed by the prior art.
EXAMPLE 2
[0131] Rolled AA2024 aluminum alloy test pieces with dimensions 25×10×1 mm were subjected to steps of surface preparation and then anodization as described in Example 1 above.
[0132] These test pieces were then sealed using compositions applied by a method according to the invention and described in Table 2 (C1 to C4).
[0133] By way of comparative examples, similar test pieces were sealed using compositions not in accordance with the invention, also described in Table 2 (Comp1 and Comp2).
[0134] Certain test pieces did not undergo any sealing treatment after anodization (Comp).
[0135] The sealing conditions were 15 min at ambient temperature (i.e. approximately 21° C.).
[0136] At the end of the sealing treatment, the test pieces were rinsed with water and dried at 60° C. for 10 min.
[0137] The thickness of the layer which was formed on their surface is shown in Table 2 below.
TABLE-US-00003 TABLE 2 Sealing compositions used K.sub.2ZrF.sub.6 Na.sub.2MoO.sub.4, LiNO.sub.3 KMnO.sub.4 Ce(NO.sub.3).sub.3, Thickness Composition (g/l) 2H.sub.2O (g/l) (g/l) (g/l) 6H.sub.2O (g/l) pH (□m) C1 12 10 8 — — 6.04 6.9 C2 12 10 8 19 — 5.34 5.9 C3 12 10 8 — 0.2 5.91 6.8 C4 12 10 8 19 0.2 5.94 6.2 Comp1 12 — — 19 — 3.96 5.8 Comp2 — 10 — 19 — 8.44 5.2
[0138] The test pieces were subjected to the following tests.
[0139] Salt Spray Resistance Test
[0140] This test was carried out as described in Example 1 above, for 750 h, for the test pieces treated with composition C4 according to the invention. After 750 hours of exposure to the salt spray, very little pitting corrosion is observed on the surface of the test pieces, as can be observed in the photographs shown in
[0141] For these test pieces, the appearance of 3 corrosion pits per dm.sup.2 is observed on average after 750 h of exposure to salt spray.
[0142] Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.
[0143] Electrochemical Test
[0144] The technique used to characterize the behavior of the treated test pieces with respect to corrosion is that of polarization curves. The anodic and cathodic curves were obtained on different samples for each of the studied sealing compositions.
[0145] To this end, a thermostatically controlled cell with 3 standard electrodes was used. The medium was a solution of 0.1 M NaCl in water, pH 5.67. The measurements were carried out at 25° C. The counter electrode was made of platinum and the reference electrode was made of silver/silver chloride/3M potassium chloride (E(Ag/AgCl)=+0.210 V vs standard hydrogen electrode).
[0146] Potentiodynamic anodic and cathodic polarization curves were obtained by a Gamry potentiostat/galvanostat, with a potential sweep speed of 0.5 mV/s.
[0147] The recording of the potentiodynamic curves was carried out from the potential of the open circuit (E.sub.ocp), measured in the absence of external current both in the anodic and cathodic directions. Individual samples were used for each recorded potentiodynamic curve. The open circuit potential of the studied samples was established by direct measurement of the function “E.sub.ocp-τ” relative to the same reference electrode after immersion in 0.1 M NaCl solution for up to 15 min. Corrosion current values i.sub.corr were determined by Tafel extrapolation of the linear region of the anodic polarization curves to corrosion potential E.sub.corr.
[0148] For the unsealed comparative example Comp, a corrosion current i.sub.corr=6.10.sup.−8 A.Math.cm.sup.−2 was thus obtained. The comparison of the anodic and cathodic potentiodynamic curves obtained for the test pieces before and after their anodization showed that the formed anode layer is an effective barrier for the cathodic reaction as well as for the anodic reaction of the corrosion process.
[0149] Regarding the sealed test pieces, the results obtained are shown in Table 3 below.
TABLE-US-00004 TABLE 3 Electrochemical parameters of anodized and sealed aluminum alloy test pieces E.sub.l E.sub.corr i.sub.corr Composition (V vs SSC) (V vs SSC) (A .Math. cm.sup.−2) C1 −0.759 −0.852 4 .Math. 10.sup.−9 C2 −1.006 −0.986 1 .Math. 10.sup.−8 C3 −0.877 −0.883 5 .Math. 10.sup.−8 Comp2 −0.656 −0.639 ≥6 .Math. 10.sup.−8
[0150] SSC denotes the standard silver electrode, E.sub.corr denotes the corrosion potential, and i.sub.corr denotes the corrosion current.
[0151] It is noted that for all of the compositions applied by the method according to the invention and tested, the corrosion current is lower than that obtained for the unsealed comparative example, as well as for the comparative example not according to the invention Comp2. Particularly good results are obtained for the compositions applied by the method according to the invention C1 and C2.
[0152] Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.
[0153] Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX)
[0154] The morphology, structure and surface composition of the sealed test pieces were examined by scanning electron microscopy (SEM) using a JEOL JSM 6390 electron microscope (Japan) equipped with an ultra-high resolution scanning system (ASID-3D), under secondary electron image (SEI) conditions, and backscattered electron image (BEI) conditions. The electron microscope was equipped with an Oxford Instruments INCA x-sight energy-dispersive spectrometer, which enables X-ray analysis by EDX microprobe of the samples studied at a fixed point.
[0155] The results obtained by EDX analysis are shown in Table 4 below, for the full spectrum, with an amplification of ×500. For each element, the content is indicated in % by weight. It should be noted that this technique does not make it possible to detect the presence of lithium and cerium in the surface layer of the part.
TABLE-US-00005 TABLE 4 EDX analysis of anodized and sealed aluminum alloy test pieces Composition C1 C2 C3 C4 Comp1 Comp2 Comp Al 8.5 15.0 12.0 18.4 3.3 74.8 94.2 Cu 0.4 — — 0.4 — 0.8 3.8 Mg — — — 0.2 — 1.1 1.5 Mn — nd — nd nd nd 0.5 Zr 17.1 18.8 18.2 16.8 45.7 — — F 47.0 46.5 48.6 41.5 35.4 — — Mo 2.3 nd nd nd — 5.4 — S — 1.1 1.4 1.3 — 9.4 — K 17.9 18.7 14.8 16.3 15.7 nd — Na 6.7 nd 5.1 5.2 — nd — nd indicates an element which is present but of nondetermined quantity
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[0157] As can be seen, the test piece treated with composition C1 (a/) is characterized by a dense surface layer with a symmetrically ordered spheroidal structure. The integral analysis from a “large” area established the presence of zirconium, fluorine and molybdenum.
[0158] The test piece treated with composition C2 (b/) is covered with a dense surface layer. It also contains uniform spheroidal agglomerates in particular containing zirconium, fluorine, manganese and molybdenum.
[0159] The test piece treated with composition C3 (c/) is also covered with a dense layer of spheroidal agglomerates of two orders of different size and of different chemical composition. The integral EDX analysis of a “large” surface notably established the presence of zirconium, fluorine and molybdenum.
[0160] For the test piece treated with composition C4 (d/), the surface morphology is also a dense surface layer containing spheroidal agglomerates, and in particular containing zirconium, fluorine, manganese and molybdenum.
[0161] Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.
EXAMPLE 3
[0162] Rolled test pieces made from AA2024 aluminum alloy, with dimensions 150×80×3 mm, were treated by the method according to the present invention described in Example 1 above, but with the following aqueous composition according to the invention:
TABLE-US-00006 K.sub.2ZrF.sub.6 12 g/l Na.sub.2MoO.sub.4, 2H.sub.2O 10 g/l LiNO.sub.3 8 g/l KMnO.sub.4 19 g/l Ce(NO.sub.3).sub.3, 6H.sub.2O 0.2 g/l
[0163] The pH of this composition is 5.82 (measured at 19.9° C.).
[0164] The conditions for treatment with this composition are as follows: 19° C., 15 min.
[0165] At the end of the treatment, after rinsing with water and drying at 60° C., the test pieces are marked, in an X pattern, in accordance with the ISO 17872 standard, using a Van Laar tip made from tungsten carbonate. The marks are deep, so as to fully penetrate the surface layer, until they reach the basic metal alloy making up the test piece.
[0166] A photograph of a test piece thus marked is shown in
[0167] By way of comparison, marked test pieces are also produced after anodization and sealing treatment by means of an aqueous composition containing only K.sub.2ZrF.sub.6 at a concentration of 12 g/l.
[0168] The test pieces thus marked are subjected to a salt spray exposure test for 816 h, at a temperature between 15 and 25° C., according to the conditions in accordance with the NF EN ISO 9227 standard.
[0169] The results obtained, for each condition tested, are shown in
[0170] As can be seen in this figure, at the end of the salt spray exposure test, for the test piece treated with the comparative composition, containing only potassium hexafluorozirconate, pitting corrosion is observed in the marks formed on the test piece (examples of which are indicated by a box in c/ and d/ in the figure). On the contrary, no pitting corrosion was observed for the test piece treated in accordance with the present invention, and defects caused by the marking were even repaired. This clearly demonstrates the effectiveness of the composition used by the method according to the invention for the protection of parts against corrosion, and a self-healing effect of this composition: the defects induced on the surface of the part are advantageously effectively repaired.
[0171] Similar results are obtained for test pieces made from the same alloy and with the same dimensions, obtained by machining.