PICKLING METHOD FOR PROFILES, ROLLED STRIPS, AND SHEETS MADE OF ALUMINIUM ALLOYS

Abstract

In a method for cleaning a non-machined aluminum alloy product involving alkaline degreasing and treatment thereof after alkaline pickling with an acidic solution, the aluminum alloy product undergoes (a) initial acid cleaning before alkaline degreasing or (b) alkaline degreasing, followed by a subsequent acid rinse, alkaline treatment anew and an additional acid rinse thereafter. The aluminum alloy product may be a rolled aluminum alloy strip, a rolled aluminum alloy sheet and an aluminum alloy profile.

Claims

1. A method for cleaning an aluminum alloy product which has not been machined, comprising: cleaning the aluminum alloy product beforehand with an acid, subsequently degreasing with an alkaline solution, and thereafter rinsing with an acid.

2. The method according to claim 1, wherein the aluminum alloy product is treated with an alkaline solution before initial acid cleaning.

3. The method according to claim 1, wherein the alkaline solution used for degreasing the aluminum alloy product has a temperature of 50 C. to 85 C.

4. The method according to claim 1, wherein the aluminum alloy product is selected from rolled aluminum alloy strips, from rolled aluminum alloy sheets and from aluminum alloy profiles.

5. The method according to claim 1, wherein the aluminum alloy product used is produced from an aluminum alloy of type AA 5xxx, AA 1xxx, AA 3xxx, AA 6xxx and AA 8xxx according to the International Alloy Designations of The Aluminum Association.

6. The method according to claim 1, wherein the aluminum alloy product that is to be cleaned has undergone intermediate or final annealing.

7. The method according to claim 1, wherein the aluminum alloy product is acid cleaned beforehand for 0.5 to 15 seconds, in particular for 1 to 8 seconds.

8. The method according to claim 1, wherein alkaline degreasing of the aluminum alloy product lasts 1 to 25 seconds, in particular 1.5 to 15 seconds.

9. The method according to claim 1, wherein the aluminum alloy product is acid rinsed for 0.5 to 15 seconds, in particular for 1 to 8 seconds.

10. The method according to claim 1, wherein the surface of the aluminum alloy product is passivated after the acid rinse.

11. An aluminum alloy product made of an aluminum alloy of class AA 5xxx, AA 1xxx, AA 3xxx, AA 6xxx and AA 8xxx according to the International Alloy Designations of The Aluminum Association, obtainable by any one of the processes of claim 1, wherein the aluminum alloy product does not possess any gray to grayish-brown irregularities or imperfections on the surface.

12. The aluminum alloy product according to claim 11, wherein the aluminum alloy product has been annealed before it undergoes cleaning.

11. The aluminum alloy product according to claim 11, wherein the magnesium content is homogeneously distributed across the width of the aluminum alloy product.

14. The aluminum alloy product according to claim 11, wherein the aluminum alloy product is selected from rolled aluminum alloy strips, from rolled aluminum alloy sheets and from aluminum alloy profiles.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0048] FIG. 1 is a photograph of an aluminum alloy strip coil with the previously described annealing defect after alkaline degreasing and acid rinsing according to WO 2013/113598 A1.

[0049] FIG. 2 is a photograph of an aluminum alloy strip coil of the same batch after complete elimination of the annealing defect by implementing a three-stage treatment process involving initial acid cleaning followed by alkaline degreasing and subsequent acid rinsing.

[0050] FIG. 3 shows the results of a corrosion test in a metal sheet obtained according to the invention and a comparison metal sheet made from the same aluminum alloy strip.

[0051] FIG. 4 depicts the curve of the mass fraction of magnesium on the surface of an aluminum alloy strip (AA 5182) after annealing and after the three-stage pickling method according to the invention. The magnesium content was measured by glow-discharge optical emission spectroscopy. The upper curve shows the measurement results after annealing and the lower curve shows the measurement results after the three-stage method according to the invention.

[0052] FIG. 5 shows the composition of the first 0 to 500 nm of two aluminum alloy strips over half the width of the strip-shaped aluminum alloy product.

[0053] FIG. 6 shows the content of further alloying components of the surface composition from 0 to 500 nm of two aluminum alloy strips over half the width of the strip-shaped aluminum alloy product.

[0054] FIG. 7 shows the composition of the first 0 to 500 nm of two aluminum alloy strips over the full width of the strip-shaped aluminum alloy product.

[0055] FIG. 8 shows the efficient removal of surface accumulations in aluminum alloy strips according to the invention in relation to the alkaline treatment time.

[0056] FIG. 9 shows the decrease in adhesive strength after weathering compared to an unweathered reference group in strips that have undergone pickling according to standard pickling procedures and according to the pickling method described in the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0057] The aluminum alloy strip shown in FIG. 1 has a high magnesium content and a composition that corresponds to that of AA 5182. The defect described earlier is visible at the edges as an irregular wavy grayish-brown streak. This strip initially underwent alkaline pickling (0.5% NaOH, 1.5% of a degreasing composition of non-ionic and anionic surfactants and of complexing agents; contact time 45 seconds) and thereafter acid pickling (2% HNO.sub.3+300 ppm F.sup.; contact time 11 seconds). The strip shown in FIG. 2 is produced from the same batch as the strip in FIG. 1. The strip in FIG. 2 was additionally acid cleaned (2% HNO.sub.3+300 ppm F.sup.; contact time 11 seconds) according to the method described in the invention before being sent for pickling whereby the process time was reduced by half (23 seconds). No defect is visible on this strip.

[0058] Aluminum alloy strips, which were treated on the one hand according to the three-stage pickling method described in the invention (samples CV2, CV3, CV5) and on the other according to the known two-stage pickling method (samples CV1, CV4), were compared in a laboratory test. The same aluminum alloy was used in both cases and the treatment of the strips before pickling was also identical. The aluminum alloy in question is of type AA 5182. The sheets were degreased with an organic solvent before the comparative test was performed. The sheets were immersed manually in the baths. The measured values are shown in Table 1 below.

TABLE-US-00001 TABLE 1 Weight Pickling stage Pickling Weight Weight per unit H.sup.+- OH.sup. H.sup.+ area before after Weight area after before in in in Sample in m.sup.2 pickling pickling loss pickling seconds seconds seconds CV-1 0.0470 134.06 g 133.82 g 0.24 g 5.0 g/m.sup.2 0 60 30 CV-2 0.0504 134.20 g 133.88 g 0.34 g 6.4 g/m.sup.2 12 60 12 CV-3 0.0472 133.31 g 133.08 g 0.23 g 4.9 g/m.sup.2 6 30 6 CV-4 0.0441 133.15 g 132.52 g 0.63 g 14.3 g/m.sup.2 0 180 60 CV-5 0.0470 133.58 g 133.39 g 0.19 g 4.0 g/m.sup.2 3 15 3

[0059] In the first row, H+ before indicates the acidic cleaning stage conducted beforehand, OH stands for alkaline pickling and H+ for the subsequent acidic rinse. The acidic pickling solution contained 5% by weight HNO.sub.3 at room temperature. The alkaline pickling solution contained 2% by weight NaOH and 2% by weight of a degreasing agent composition at a temperature of 70 C.

[0060] After pickling, sheet CV-1 presents the surface appearance as shown in the photograph in FIG. 1 with an inhomogeneous surface color. Sample CV-2 shows a good result with a matte and homogeneous surface. Sample CV-3 shows the best result with a matte and homogeneous surface appearance. Sample CV-4 is better than sample CV-1, but still has an inhomogeneous appearance. The surface of Sample CV-5 presents a slightly inhomogeneous appearance with slight discoloration.

[0061] A corrosion test was performed for comparison purposes, whereby sheets of two aluminum alloy strips were subjected to what is known as the accelerated filiform corrosion test. Both sheets were taken from the same aluminum alloy strip, type AA 5182. The samples marked Standard underwent pickling with the known two-stage process; the remaining samples are samples obtained according to the invention. After pickling, the sample sheets were coated with a clear lacquer. The sheets were not passivated. In accordance with the accelerated filiform corrosion test, grooves with a width of one millimeter were made in the sample sheets. The sample sheets were treated with HCl and then kept at 40 C. and a relative humidity of 80% for five days. The results are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Number of filaments Filiform no. Pickling Grooves per 50 mm Filaments/mm F = Freq. L Standard 1 65.2 1.3 0.7 Standard 2 70.0 1.4 0.7 Standard 3 52.1 1.0 0.5 Standard 4 54.3 1.1 0.5 Standard 5 55.9 1.1 0.6 Standard 6 39.3 0.8 0.4 Invention 1 50.9 1.0 0.5 Invention 2 57.5 1.1 0.6 Invention 3 55.5 1.1 0.6 Invention 4 49.0 1.0 0.5 Invention 5 44.6 0.9 0.4 Invention 6 50.6 1.0 0.5 Standard Mean 56.1 1.1 0.6 Invention Mean 51.4 1.0 0.5

[0062] Table 2 shows that with 56.1 filaments per 50 mm the number of corrosion filaments is greater after the two-stage standard pickling process than after the pickling process according to the invention with 51.4 filaments per 50 mm. The sheet metal obtained according to the invention thus shows a better resistance to filiform corrosion. On the whole, the filiform factor is better for the metal sheets according to the invention. The effects of the test are shown in FIG. 3.

[0063] The pickling results of a four-stage pickling method according to the invention are shown in Table 3 below. The treatment of an aluminum alloy sheet of an AA 5182 aluminum alloy, as described in detail in the publication International Alloy Designations issued by The Aluminum Association, involved dipping in a bath with a first alkaline pickling solution, rinsing with water, dipping in a bath with a first acid rinse, rinsing with water, dipping in a bath with a second alkaline pickling solution, rinsing with water, dipping in a bath with a second acid rinse and rinsing with water. Both alkaline baths have the same composition and both acid rinses have the same composition. The alkaline pickling solution contains 2% by weight NaOH and 2% by weight of a degreasing agent composition at a temperature of 70 C. The acid rinse contains 5% by weight HNO.sub.3 at room temperature.

[0064] The results in Table 3 below show that the four-stage method according to the invention facilitates a surprisingly shorter treatment time compared to the known two-stage process (V1, V2) and still leads to the good result of V1, in which the treatment times are comparatively very long. The long treatment time indicated for V1 is, however, uneconomical.

TABLE-US-00003 TABLE 3 1st pickling stage 2nd pickling stage Pickling Decapitation Pickling Decapitation [sec] [sec] [sec] [sec] Evaluation/comments Sheet 60 60 0 0 Very good pickling result with uniform, matte surface V1-1 Sheet 16 6 0 0 Poor pickling result, surface only slightly stripped V2-18 Sheet 60 60 60 60 Same as for Sheet V1-1 V3-2 Sheet 5 20 5 20 Poor result after the first pickling stage, hardly any stripping on the surface. V4-9 Slightly better result after the second pickling stage (light pickling). The surface is slightly more matte; the pickling time, however, is clearly too short. Sheet 2 15 20 15 Poor result after first pickling stage. Very good result after second pickling stage. V5-10 Matte and uniform surface comparable with V1 Sheet 1 15 20 15 Poor result after first pickling stage. Very good result after second pickling stage. V6-11 Matte and uniform surface comparable with V1 Sheet 1 15 15 15 Poor result after first pickling stage. Very good result after second pickling stage. V7-12 Matte and uniform surface comparable with V1 Sheet 1 3 15 3 Poor result after first pickling stage. Very good result after second pickling stage. V8-16 Matte and uniform surface comparable with V1 Sheet 1 3 15 3 Same as for Sheet V7-16; an intermediate rinse, however, was omitted. The result V9-19 is still very good.

[0065] Further pickling tests were performed in a spray booth. The quantity of material removed was determined for aluminum alloy strips consisting of four different alloys after said strips were treated using the pickling method according to the invention and the standard pickling process. Furthermore, both procedures involved alkaline pickling for 10 seconds and 20 seconds. The results in Table 4 show that, by implementing the pickling method according to the invention, less time is required for removing an equal quantity of material with impurities. The removal quantity (by pickling) was determined by differential gravimetric analysis before and after chemical treatment. Subsequently, the removal rate was determined for the treated surface.

TABLE-US-00004 TABLE 4 Initial acid Alkaline Subsequent Alkaline cleaning pickling acid rinse Removal pickling 3.0% H.sub.2SO.sub.4 0.6% NaOH 3.0% H.sub.2SO.sub.4 quantity removal rate 400 ppm F.sup. 0.6% surfactant 400 ppm F.sup. per unit area per unit area Process Alloy Room temperature 60 C. Room temperature [g/m.sup.2] [g/m.sup.2s] Standard AA 6451 10 s 10 s 0.30 0.030 (short) Standard AA 6451 20 s 10 s 0.69 0.035 (medium) Invention AA 6451 10 s 10 s 10 s 0.76 0.076 (short) Invention AA 6451 10 s 20 s 10 s 1.10 0.055 (medium) Standard AA 6016 10 s 10 s 0.36 0.036 (short) Standard AA 6016 20 s 10 s 0.67 0.033 (medium) Invention AA 6016 10 s 10 s 10 s 0.49 0.049 (short) Invention AA 6016 10 s 20 s 10 s 0.82 0.041 (medium) Standard AA 6060 10 s 10 s 0.23 0.023 (short) Standard AA 6060 20 s 10 s 0.45 0.022 (medium) Invention AA 6060 10 s 10 s 10 s 0.39 0.039 (short) Invention AA 6060 10 s 20 s 10 s 0.80 0.040 (medium) Standard AA 5182 10 s 10 s 0.50 0.050 (short) Standard AA 5182 20 s 10 s 0.72 0.036 (medium) Invention AA 5182 10 s 10 s 10 s 0.76 0.076 (short) Invention AA 5182 10 s 20 s 10 s 1.18 0.059 (medium)

[0066] FIG. 5 shows a comparison of the effect of the method according to the invention on the composition of the alloying components in the surface layer of two aluminum alloy strips of the same alloy. The standard pickling process (alkaline pickling followed by acid rinse) was implemented for treating one strip, whereas the pickling method according to the invention was implemented to treat the other strip. The aluminum alloy strip treated by the standard pickling process shows the usual grayish-brown streaks which are located at the points marked on the strip treated by the standard pickling process in FIG. 1 (1=light, 2=dark, 3=light, 4=dark, 5=light, 6=dark, 7=light).

[0067] The concentrations of the alloying components across half the width of the strip (from the edge to the center of the strip) were determined to a depth of 500 nm from the surface of both aluminum alloy strips. The concentrations of the alloying components in the measured range are shown in FIG. 5, whereby Graph A shows the concentrations in the strip for which the standard pickling process was implemented and Graph B shows the concentrations in the strip for which the pickling method according to the invention was implemented. Only one half of the strip was analyzed across its width, as it can be assumed that a mirror image of the observations will be repeated on the other half of the strip.

[0068] The strip that has undergone pickling according to the method described in the invention (also see FIG. 2) shows a uniform distribution of the aluminum alloy components over the entire measurement area with a significantly reduced concentration of oxygen in contrast to the strip that has undergone pickling according to the standard process. No grayish-brown streaks occur in this strip.

[0069] The fluctuations in the proportions of the alloying components in the strip with grayish-brown streaks that has undergone pickling according to the standard process (also see FIG. 1) are significantly higher in contrast to the concentration of the alloying components measured in the strip that has undergone pickling according to the method of the invention.

[0070] The concentration of alloying components in the strip was determined by means of glow-discharge optical emission spectroscopy (GDOES). During the analysis, the elements of the surface are sputtered in a plasma in nanometric steps. The optical emission of the individual elements is then used to determine the element composition for each layer. The surface composition of the uppermost 500 nm was calculated by integrating the element compositions of all layers between 0 and 500 nm. The GDA 750 spectrometer from Spectruma Analytik GmbH was used to determine the concentration of the alloying components.

[0071] The graphs in FIG. 6 show the different concentrations of the other alloying components silicon, manganese, magnesium and copper on the surface of the aluminum alloy strips after treatment with the standard pickling process and after treatment with the method according to the invention. These analysis values also show an even distribution of the alloying components over the strip after pickling according to the method described in the invention in contrast to the strip that underwent pickling in the standard pickling process.

[0072] FIG. 7 shows the analysis results of the concentration of two aluminum alloy strips of an alloy composition AA 5018. The standard process was implemented for pickling one strip, while the method according to the invention was used for the other strip. With this alloy, no grayish-brown streaks appear on the strips. The advantage of using the pickling method according to the invention, however, is the higher efficiency of the alkaline pickling step during the same contact time, which is shown, for example, by the reduced oxygen content and the relatively reduced concentration of the alloying element magnesium on the surface.

[0073] FIG. 8 shows the surface composition of two aluminum alloy strips made of an aluminum alloy of type AA 6451. The strips underwent pickling according to the standard process and according to the method of the invention in a spray booth where the pickling agents were sprayed onto the strips. The concentrations of alloying components on the surface of the strips were determined by means of glow-discharge optical emission spectroscopy (GDOES). The sheets that underwent pickling according to the method of the invention achieve a lower concentration of alloying elements and oxides on the surface in a shorter period. A state of equilibrium is then achieved.

[0074] FIG. 9 shows the results of the adhesive strength analysis after 500 hours in the neutral salt spray test. The test parameters and procedure are described in DIN EN ISO 9227. The tensile lap-shear strength of bonded assemblies was determined for sheets of type AA 5182, one that underwent pickling according to the standard process and one that underwent pickling according to the method of the invention. The bonded assembly and subsequent lap-shear test was performed in accordance with DIN EN 1465:2009-07 with an overlap length of L=100.25 mm, which deviates from the standard. FIG. 9 shows in particular the loss of adhesive strength after weathering, which is lower in the case of the sheet that underwent pickling according to the method of the invention than in the sheet that underwent pickling according to the standard process.

[0075] All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0076] The use of the terms a and an and the and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including, and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to,) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0077] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.