Method for the surface treatment of parts made of an aluminum or magnesium alloy

Abstract

A method for surface treatment of a part made of aluminum, magnesium, or one of the alloys thereof, to protect the part from corrosion. The method comprises consecutively immersing the part in a first aqueous bath containing a corrosion-inhibiting metal salt and an oxidizing compound, and a second aqueous bath containing an oxidizing compound and a corrosion-inhibiting rare-earth salt. The method can be carried out for the chemical conversion of aluminum or the alloys thereof, and of magnesium or the alloys thereof, on parts that have not been previously treated, or after anodizing the part to seal the anodic layer.

Claims

1. A method for surface treatment of a part made of aluminum, magnesium or one of their respective alloys, comprising the steps of successively: immersing the part in a first aqueous bath containing a trivalent chromium salt, excluding a hexavalent chromium salt, and an oxidizing compound; and immersing the part in a second aqueous bath containing an oxidizing compound chosen from a group consisting of substances based on permanganate and hydrogen peroxide, and a rare-earth-salt corrosion inhibitor.

2. The method as claimed in claim 1, wherein a trivalent chromium salt in the first aqueous bath is selected from fluorides.

3. The method as claimed in claim 1, wherein a trivalent chromium salt in the first aqueous bath is selected from sulfates.

4. The method as claimed in claim 1, wherein a temperature of the first aqueous bath is between 10 and 80 C.

5. The method as claimed in claim 1, wherein a temperature of the first aqueous bath is between 20 and 50 C.

6. The method as claimed in claim 1, wherein a pH of the first aqueous bath is between 1 and 7.

7. The method as claimed in claim 1, wherein a pH of the first aqueous bath is between 2 and 5.

8. The method as claimed in claim 1, further comprising the step of immersing the part in the first aqueous bath for a duration between 1 and 60 minutes.

9. The method as claimed in claim 1, further comprising the step of immersing the part in the first aqueous bath for a duration between 5 and 30 minutes.

10. The method as claimed in claim 1, further comprising the step of immersing the part in the first aqueous bath for a duration between 10 and 20 minutes.

11. The method as claim in of claim 1, wherein a concentration of the trivalent chromium salt in the first aqueous bath is between 0.5 and 50 g/L.

12. The method as claim in of claim 1, wherein a concentration of the trivalent chromium salt in the first aqueous bath is between 1 and 20 g/L.

13. The method as claim in claim 1, wherein the rare-earth-salt corrosion inhibitor present in the second aqueous bath is a lanthanum salt.

14. The method as claim in claim 1, wherein the rare-earth salt corrosion inhibitor present in the second aqueous bath is a cerium salt.

15. The method as claim in claim 1, wherein the rare-earth salt corrosion inhibitor present in the second aqueous bath is a cerium salt in the +3 oxidation state.

16. The method as claim in claim 1, wherein the rare-earth salt corrosion inhibitor present in the second aqueous bath is a cerium nitrate.

17. The method as claim in claim 1, wherein a concentration of the rare-earth salt corrosion inhibitor in the second aqueous bath is greater than 0 and less than 50 g/L.

18. The method as claim in claim 1, wherein a concentration of the rare-earth salt corrosion inhibitor in the second aqueous bath is between 1 and 10 g/L.

19. The method as claimed in claim 1, wherein a temperature of the second aqueous bath is between 10 and 80 C.

20. The method as claimed in claim 1, wherein a temperature of the second aqueous bath is between 15 and 40 C.

21. The method as claimed in claim 1, wherein a temperature of the second aqueous bath is between 20 and 30 C.

22. The method as claimed in claim 1, wherein a pH of the second aqueous bath is between 1 and 7.

23. The method as claimed in claim 1, wherein a pH of the second aqueous bath is between 2 and 5.

24. The method as claimed in claim 1, further comprising the step of immersing the part in the second aqueous bath for a duration between 1 and 60 minutes.

25. The method as claimed in claim 1, further comprising the step of immersing the part in the second aqueous bath for a duration between 2 and 20 minutes.

26. The method as claimed in claim 1, further comprising the step of immersing the part in the second aqueous bath for a duration between 5 and 10 minutes.

27. The method as claimed in claim 1, further comprising the step of performing anodizing treatment on the part prior to successively immersing the part in the first aqueous bath and second aqueous bath.

28. The method as claimed in claim 1, wherein the oxidizing compound in said second bath is hydrogen peroxide H.sub.2O.sub.2.

29. The method as claimed in claim 1, wherein the immersion is said second bath has a duration of more than 1 min and up to 60 min.

30. The method as claimed in claim 1, further comprising a rinsing step between said first and said second immersion steps.

31. The method as claimed in claim 1, wherein the part is immersed in the second aqueous bath without drying the part after being immersed in the first bath.

32. The method as claimed in claim 1, wherein the rare-earth-salt corrosion inhibitor is chosen from a group consisting of the salts of lanthanum, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) The features and advantages of the method according to the invention will become clearer from the embodiment examples given below, supplied purely for purposes of illustration and not limiting the invention in any way.

Example 1Treatment of Chemical Conversion of Parts Made of Aluminum Alloy

(2) 1.1. Methods of Treatment

(3) Parts made of rolled aluminum alloy 2024 T3, with dimensions of 120802 mm, are treated as follows.

(4) Steps of surface preparation of each part are first carried out successively: alkaline degreasing, by dipping the part in a mixture of TURCO 4215 NCLT at 50 g/L and TURCO 4215 additive at 10 g/L, at a temperature of 60 C., for 20 min; water rinsings; acid pickling, by dipping the part in a solution of SMUT-GO NC at 19% v/v, at a temperature of 20 C., for 5 min; water rinsings.

(5) The parts are then submitted to successive immersions in the following first aqueous bath, and respectively in one of the following second aqueous baths.

(6) The first bath, based on trivalent chromium, called Bath 1, corresponds to the composition: CrK(SO.sub.4).sub.2,6H.sub.2O at 2 g/L+K.sub.2ZrF.sub.6 at 5 g/L, in water.

(7) Its pH is fixed at 3.5, and its temperature is adjusted to 40 C.

(8) The duration of immersion in this first bath is equal to 10 min.

(9) The second aqueous bath, called Bath 2, corresponds to one of the compositions shown in Table 1 below. Three of these baths, comprising an oxidizing compound and a rare-earth salt, respectively of cerium (baths D1 and D2) or of lanthanum (bath D3) are according to the present invention, and two of them, Comp.1 and Comp.2, constitute comparative examples.

(10) TABLE-US-00001 TABLE 1 Compositions of the second aqueous baths (Bath 2) Oxidizing Bath compound Rare-earth salt pH D1 H.sub.2O.sub.2, 35% v/v, Ce(NO.sub.3).sub.3, 6H.sub.2O 3 50 mL/L 5 g/L D2 H.sub.2O.sub.2, 35% v/v, Ce(NO.sub.3).sub.3, 6H.sub.2O 3.5 50 mL/L 5 g/L D3 H.sub.2O.sub.2, 35% v/v, La(NO.sub.3).sub.3, 6H.sub.2O 3.5 50 mL/L 5 g/L Comp. 1 H.sub.2O.sub.2, 35% v/v, 3.5 50 mL/L Comp. 2 Ce(NO.sub.3).sub.3, 6H.sub.2O 3.5 5 g/L

(11) The temperature of each of these baths is room temperature, i.e. a temperature between about 18 and 25 C. The duration of immersion in each of these second baths is equal to 5 min.

(12) Some parts are also treated, after surface preparation, by immersion only in Bath 1 described above.

(13) As other comparative examples, identical parts, having undergone an identical surface preparation, are treated by the following commercial methods of chemical conversion proposed in the prior art: Alodine 1200 (Henkel) (using hexavalent chromium), SurTec 650 (SurTec) (using trivalent chromium), and Lanthane VS 613.3 (Coventya) (using trivalent chromium).

(14) The operating conditions for these comparative examples are shown in Table 2 below.

(15) TABLE-US-00002 TABLE 2 Operating conditions of methods of chemical conversion of the prior art Duration of Concentration Temperature immersion in Method in the bath ( C.) pH the bath (min) Alodine 1200 15 g/L 20 1.8 1 SurTec 650 20% v/v 40 3.9 4 Lanthane VS Part A 100 ml/L 38 3.5 5 613.3 Part B 75 ml/L

(16) 1.2. Corrosion Resistance Tests

(17) All of the parts thus treated are submitted to a salt spray test according to standard ISO 9227. Preliminary approximate average results, obtained on a small number of parts, are shown in Table 3 below.

(18) TABLE-US-00003 TABLE 3 Salt spray durability of parts made of rolled aluminum alloy 2024 T3 treated by a method according to an embodiment of the invention and by methods of chemical conversion of the prior art Salt spray durability (appearance of the 1st corrosion pit) Method of treatment (h) Alodine 1200 168 SurTec 650 48 Lanthane VS 613.3 72 Immersion in Bath 1 only 96 Immersion in Bath 1 and then bath 408 D1 according to an embodiment of the invention

(19) More precise average results for appearance of the first corrosion pit and generalized corrosion, obtained on a larger number of parts (30 parts treated similarly), are shown in Table 4 below.

(20) TABLE-US-00004 TABLE 4 Salt spray durability, in terms of appearance of the first corrosion pit and generalized corrosion, of parts made of rolled aluminum alloy 2024 T3 treated by a method according to an embodiment of the invention and by methods of chemical conversion of the prior art Salt spray durability (h) Appearance of the Generalized Method of treatment 1st corrosion pit corrosion Alodine 1200 168 240 SurTec 650 24 48 Lanthane VS 613.3 48 72 Immersion in Bath 1 only 48 96 Immersion in Bath 1 and 120 192 then in bath Comp. 1 Immersion in Bath 1 and 96 144 then in bath Comp. 2 Immersion in Bath 1 and 192 288 then in bath D1 (cerium) Immersion in Bath 1 and 192 288 then in bath D2 (cerium) Immersion in Bath 1 and 216 312 then in bath D3 (lanthanum)

(21) The above results clearly show that the methods according to the invention, using trivalent chromium, are able to endow the treated part with corrosion resistance greater than that obtained by the conventional methods of chemical conversion, including that using hexavalent chromium (Alodine 1200). This resistance is also far greater than that conferred by a treatment only envisaging immersion of the part in the first bath, and not in the second, as well as that conferred by a treatment in which the second bath is without rare-earth salt (Comp.1), or without oxidizing compound (Comp.2).

(22) 1.3. Test of Adherence of Paint Systems

(23) A test of adherence of conventional paint systems on the conversion layer formed on the part, on the one hand by an aforementioned method according to the invention, comprising immersing the part in Bath 1 and then in Bath 2 designated D1 (cerium salt), and on the other hand by the method of the prior art Alodine 1200, is carried out as follows.

(24) Two paint systems are tested: a water-dilutable epoxy-based system (P60+F70) and a solvent-treated polyurethane-based system (PAC33+PU66). The tests are carried out according to standard ISO 2409, for dry adherence, after drying of the paint system, and for wet adherence: after drying of the paint system, the samples are immersed in demineralized water for 14 days, and then dried before undergoing the adherence test according to the standard.

(25) The results are shown in Table 5 below.

(26) TABLE-US-00005 TABLE 5 Results of tests of adherence of two paint systems on parts treated by a method according to one embodiment of the invention or by a method of chemical conversion of the prior art Method according to the invention Alodine 1200 (Bath 1 then Bath 2 D1) Dry ad- Wet ad- Dry ad- Wet ad- Paint system herence herence herence herence Solvent- PAC33 Grade 0 Grade 0 treated base PAC33 + Grade 0 Grade 0 Grade 0 Grade 0 PU66 Water- P60 Grade 0 Grade 0 dilutable base P60 + Grade 0 Grade 0 Grade 0 Grade 0 F70

(27) These results show that the parts treated by the method according to an embodiment of the invention display adherence of the paint systems, whether of the water-dilutable or solvent-treated type, comparable to that obtained for the parts treated by the method of the prior art Alodine 1200.

(28) 1.4. Test of Electrical Conductivity of the Layer Formed on the Surface of the Part by the Method of Treatment

(29) The parts treated by the method according to the invention, comprising immersing the part in Bath 1 and then in Bath 2 designated D1 (cerium salt), are submitted to a test of electrical conductivity according to standard MIL-DTL-81760B, which consists of measuring the resistivity of the layer/substrate/layer system.

(30) As comparative examples, parts treated by the commercial method of chemical conversion proposed in the prior art Alodine 1200, as described in Table 2 above (Alodine 1200 thick layer), as well as parts treated by the same method of chemical conversion Alodine 1200, but comprising immersion in the treatment bath for 30 seconds only (Alodine 1200 thin layer), are also submitted to the same test.

(31) According to the prior art, the thick layer of Alodine 1200 is recommended when good properties of corrosion resistance are required, at the expense of the properties of electrical conduction. Conversely, the thin layer of Alodine 1200 is recommended when good properties of electrical conduction are required, but with a halving of the anticorrosion performance of the treatment.

(32) The results obtained are shown in Table 6 below.

(33) TABLE-US-00006 TABLE 6 Results of tests of electrical conductivity on parts treated by a method according to an embodiment of the invention or by methods of chemical conversion of the prior art Resistivity of the layer (m) Alodine 1200 thin layer 59 Alodine 1200 thick layer 84 Method according to the invention 69 (Bath 1 then Bath 2 D1)

(34) These results show that the layer formed on the part by the method according to the invention has good properties of electrical conduction, close to those obtained by the method Alodine 1200 thin layer of the prior art.

(35) The method according to the invention thus makes it possible to form a layer on the part that advantageously combines performance of corrosion protection better than that obtained by the method of the prior art Alodine 1200 thick layer, with good electrical conductivity.

Example 2Treatment of Chemical Conversion of Parts Made of Aluminum Alloy

(36) Several operating parameters of the method according to the invention are varied relative to the above Example 1.

(37) 2.1. Variants of Oxidizing Compounds in Bath 2

(38) Parts made of aluminum similar to those used for Example 1 are submitted to the preliminary steps of surface preparation described in Example 1.

(39) These parts are then submitted to a first immersion in the following Bath 1: CrK(SO.sub.4).sub.2,6H.sub.2O at 2 g/L+K.sub.2ZrF.sub.6 at 5 g/L, in water, pH=3.5, temperature=40 C.; the duration of immersion in this first bath is 10 min.

(40) They are then submitted to immersion in a Bath 2 according to the invention, more particularly either in bath D1 described above, or in an aqueous bath D4 of composition: Ce(NO.sub.3).sub.3,6H.sub.2O at 5 g/L; KMnO.sub.4 at 10 ml/l in water; pH=3.

(41) For each of these methods, the temperature is room temperature, and the duration of immersion in Bath 2 is 5 min.

(42) The parts thus treated are submitted to a salt spray test according to standard ISO 9227. The results obtained are shown in Table 7 below.

(43) TABLE-US-00007 TABLE 7 Salt spray durability, in terms of appearance of the first corrosion pit and generalized corrosion, of parts made of rolled aluminum alloy 2024 T3 treated by two variants of methods according to the invention Salt spray durability (h) Appearance of the Generalized 1st corrosion pit corrosion Bath 1 then bath D1 240 336 Bath 1 then bath D4 216 312

(44) It can be seen from these results that the method according to the invention, using potassium permanganate as oxidizing compound in the 2nd bath, displays, just as when this oxidizing compound is hydrogen peroxide, very high performance in terms of corrosion protection of the treated parts.

(45) 2.2. Variants of Trivalent Chromium Salts in Bath 1

(46) Parts made of aluminum similar to those used for Example 1 are submitted to the preliminary steps of surface preparation described in Example 1.

(47) These parts are then submitted to a first immersion, for 10 min, in the Baths 1 indicated in Table 8 below, whose pH is fixed at 3.5 and the temperature is adjusted to 40 C.

(48) TABLE-US-00008 TABLE 8 Composition of the first aqueous baths (Bath 1) Bath 2 Metal salt Oxidizing compound P1 CrF.sub.3, 4H.sub.2O at 6 g/l K.sub.2ZrF.sub.6 at 1 g/l P2 CrK(SO.sub.4).sub.2, 6H.sub.2O at 2 g/l K.sub.2ZrF.sub.6 at 5 g/l P3 Cr.sub.2(SO.sub.4).sub.3 at 2 g/l K.sub.2ZrF.sub.6 at 1 g/l

(49) Each part is then immersed in Bath 2 according to the invention D1 described above, at room temperature, for 5 min.

(50) The parts thus treated are submitted to a salt spray test according to standard ISO 9227. The results obtained are shown in Table 9 below.

(51) TABLE-US-00009 TABLE 9 Salt spray durability, in terms of appearance of the first corrosion pit and generalized corrosion, of parts made of rolled aluminum alloy 2024 T3 treated by three variants of methods according to the invention Salt spray durability (h) Appearance of the Generalized 1st corrosion pit corrosion Bath P1 then bath D1 216 312 Bath P2 then bath D1 240 360 Bath P3 then bath D1 240 336

(52) It can be seen from these results that the method according to the invention gives high performance in terms of corrosion protection of the treated parts whatever the trivalent chromium salt used in the 1st bath.

Example 3Treatment of Chemical Conversion of Parts Made of Magnesium Alloy

(53) A part made of extruded magnesium alloy Elektron 21, with dimensions of 120806 mm, is treated as follows.

(54) Steps of surface preparation of the part are first carried out successively: alkaline degreasing, by dipping the part in a mixture of Na.sub.3PO.sub.4 at 20 g/L and of Na.sub.2CO.sub.3 at 40 g/L, at a temperature of 60 C., for 10 min; water rinsings; acid pickling, by dipping the part in a solution of nitric acid at 50 g/L, at a temperature of 30 C., for 40 seconds; water rinsings.

(55) The part is then immersed successively in the following first and second aqueous baths.

(56) The first bath, based on trivalent chromium, called Bath 1, corresponds to the composition:

(57) CrK(SO.sub.4).sub.2,6H.sub.2O at 2 g/L+K.sub.2ZrF.sub.6 at 5 g/L, in water.

(58) Its pH is fixed at 3.5, and its temperature is adjusted to 40 C.

(59) The duration of immersion in this first bath is 10 min.

(60) The second bath, based on cerium, called Bath 2, corresponds to the composition: Ce(NO.sub.3).sub.3,6H.sub.2O at 5 g/L; H.sub.2O.sub.2, solution at 35% v/v, 50 mL/L, in water.

(61) Its pH is fixed at 3, and its temperature is room temperature, i.e. a temperature between about 18 and 25 C.

(62) The duration of immersion in this second bath is 5 min.

(63) As a comparative example, identical parts, having undergone identical surface preparation, are treated by a method of chemical conversion proposed in the prior art: Mordanage [Mordanting] (using hexavalent chromium), carried out in the following conditions: composition: K.sub.2Cr.sub.2O.sub.7 at 40 g/L+KCr(SO.sub.4).sub.2,12H.sub.2O at 2.2 g/L+KOH at 2 g/L temperature: 75 C. duration of immersion: 5 min.

(64) All of the parts thus treated are submitted to a salt spray test according to standard ISO 9227. Preliminary approximate average results, obtained on a small number of parts, are shown in Table 10 below.

(65) TABLE-US-00010 TABLE 10 Salt spray durability of parts in extruded magnesium alloy Elektron 21 treated by a method according to an embodiment of the invention and by a method of chemical conversion of the prior art Salt spray durability (appearance of the 1st corrosion pit) (h) Mordanage 24 Immersion in Bath 1 and then Bath 48 2 according to an embodiment of the invention

(66) More precise average results relating to appearance of the first corrosion pit and generalized corrosion, obtained on a larger number of parts (30 parts treated similarly), are shown in Table 11 below.

(67) TABLE-US-00011 TABLE 11 Salt spray durability, in terms of appearance of the first corrosion pit and generalized corrosion, of parts in extruded magnesium alloy Elektron 21 treated by a method according to an embodiment of the invention and by a method of chemical conversion of the prior art Salt spray durability (h) Appearance of the Generalized 1st corrosion pit corrosion Mordanage 4 24 Immersion in Bath 1 and then Bath 2 7 48 according to an embodiment of the invention

(68) The above results show that the method according to an embodiment of the invention, using trivalent chromium, makes it possible, for magnesium alloy just as for aluminum alloy in Example 1 above, to endow the treated part with corrosion resistance far greater than that obtained by the conventional method of chemical conversion.

Example 4Anodizing and Sealing Treatment of Parts Made of Aluminum Alloy

(69) Parts made of rolled aluminum alloy 2024T3 with dimensions of 120802 mm are treated by anodizing, then sealing, according to the methods given below.

(70) They first undergo steps of surface preparation, by alkaline degreasing and acid pickling, as indicated in Example 1 above.

(71) For the anodizing step, three different methods of anodizing, namely OAS dilute, OAST and OASB, are used, to obtain an anodic layer of thickness from 2 to 5 m on the surface of the parts.

(72) The operating parameters for OAS dilute, OAST and OASB are shown in Table 12 below.

(73) TABLE-US-00012 TABLE 12 Operating parameters employed for the different anodizing steps OAS dilute OAST OASB Bath composition H.sub.2SO.sub.4: 62 g/L H.sub.2SO.sub.4: 40 g/L H.sub.2SO.sub.4: 45 g/L C.sub.4H.sub.6O.sub.6: 80 g/L H.sub.3BO.sub.3: 8 g/L Bath temperature 22 37 27 ( C.) Voltage cycle 14 V - 24 min 14 V - 25 min 15 V - 23 min

(74) At the end of the anodizing step, the parts obtained are submitted to a sealing step, either of the hydrothermal type, or of the hydrothermal type with nickel salts, or by the method according to the invention carried out in the conditions indicated in Example 1 above, as regards immersion in Bath 1 and Bath 2.

(75) The operating conditions for hydrothermal sealing and for hydrothermal sealing with nickel salts are as follows: hydrothermal sealing: immersing the part in demineralized water at a temperature of 98 C. for 40 min; hydrothermal sealing with nickel salts: immersing the part in demineralized water with addition of nickel acetate (CH.sub.3COO).sub.2Ni at 10 g/L, at a temperature of 98 C. and a pH of 5.5, for 30 min.

(76) A sealed anodic layer with thickness between 2 and 5 m is obtained on each treated part.

(77) All of the parts thus treated are submitted to a salt spray test according to standard ISO 9227. Preliminary approximate average results, obtained on a small number of parts, are shown in Table 13 below.

(78) TABLE-US-00013 TABLE 13 Salt spray durability of parts made of rolled aluminum alloy 2024 T3 treated by anodizing and sealing, sealing being carried out by a method according to an embodiment of the invention or by methods of sealing of the prior art Salt spray durability (appearance of the 1st corrosion pit) (h) OAS dilute OAST OASB Hydrothermal sealing 72 96 48 Hydrothermal sealing with 312 336 240 nickel salts Sealing by a method 696 744 552 according to the invention

(79) More precise average results, in terms of appearance of the first corrosion pit (1st) and generalized corrosion (G.sup.on), obtained on a larger number of parts (30 parts), are shown in Table 14 below.

(80) TABLE-US-00014 TABLE 14 Salt spray durability, in terms of appearance of the first corrosion pit (1st) and generalized corrosion (G.sup.on), of parts made of rolled aluminum alloy 2024 T3 treated by anodizing and sealing, sealing being carried out by a method according to an embodiment of the invention or by methods of sealing of the prior art Salt spray durability (h) OAS dilute OAST OASB 1st G.sup.on 1st G.sup.on 1st G.sup.on Hydrothermal sealing 72 168 72 192 48 168 Hydrothermal sealing with 312 792 336 840 288 744 nickel salts Sealing by a method 432 1272 480 1344 384 1128 according to the invention

(81) The above results clearly demonstrate that the method according to an embodiment of the invention, using trivalent chromium, carried out after an anodizing step, of whatever type, makes it possible to endow the treated part with corrosion resistance far greater than that obtained by the conventional methods of sealing, regardless of what method of anodizing was carried out beforehand.

(82) The foregoing description clearly illustrates that owing to its various features and their advantages, the present invention achieves the objectives that were set. In particular, it provides a method for the surface treatment of parts made of aluminum or of aluminum alloy, or of magnesium or of magnesium alloy, which, without using hexavalent chromium, makes it possible to obtain performance in terms of protection of the part against corrosion that is superior to that obtained by the methods of the prior art.