Phosphate-free cleaner for metallic surfaces with reduced pickling erosion

Abstract

Described herein is a water-based, alkaline cleaner concentrate for producing a cleaner for metallic surfaces, the concentrate including a) at least one (meth)acrylic acid homopolymer having a weight-average molar mass in the range from 3 000 to 19 000 g/mol and b) at least one (meth)acrylic acid copolymer having a weight-average molar mass in the range from 50 000 to 100 000 g/mol. Also described herein are a corresponding cleaner for metallic surfaces with reduced pickling erosion, a process for anticorrosive treatment of metallic surfaces that includes a corresponding cleaning step, a metallic surface obtained by the process, and a method of use thereof in the sector of the metalworking industries.

Claims

1. A water-based, alkaline cleaner concentrate for producing a cleaner for metallic surfaces, which comprises a) at least one (meth)acrylic acid homopolymer, wherein the at least one (meth)acrylic acid homopolymer is polyacrylic acid having a weight-average molar mass in the range from 5000 to 15000 g/mol and b) at least one (meth)acrylic acid copolymer, wherein the at least one (meth)acrylic acid copolymer is poly(acrylic acid-alt-maleic acid) having a weight-average molar mass in the range from 55000 to 90000 g/mol, wherein the at least one (meth)acrylic acid homopolymer of component a) and the at least one (meth)acrylic acid copolymer of component b) are present in a weight ratio in the range from 1.0:1 to 2.5:1; and wherein the at least one (meth)acrylic acid homopolymer of component a) is present in a concentration of at least 1.0 wt % but of at most 2.5 wt %, whereas the at least one (meth)acrylic acid copolymer of component b) is present in a concentration of at least 0.5 wt % but of at most 1.5 wt %.

2. The cleaner concentrate according to claim 1, which is phosphate-free.

3. The cleaner concentrate according to claim 1, which further comprises at least one water-soluble boron compound c).

4. The cleaner concentrate according to claim 3, wherein the at least one water-soluble boron compound c) is present in a concentration of at least 10.5 wt %, calculated as boric acid.

5. A water-based, alkaline cleaner for metallic surfaces, which comprises the water-based, alkaline cleaner concentrate of claim 1 and optionally at least one nonionic surfactant; wherein, if the cleaner is a fresh cleaner, component a) is present in a concentration of at most 0.65 g/l, calculated as polyacrylic acid and component b) is present in a concentration of at most 0.35 g/l, calculated as poly(acrylic acid-alt-maleic acid).

6. A process for anticorrosive treatment of a metallic surface, which comprises contacting the surface in succession with the following compositions: i) at least one water-based, alkaline cleaner according to claim 5, ii) a first water-based rinsing composition, iii) optionally a second water-based rinsing composition, iv) a water-based acidic conversion composition, v) optionally a third water-based rinsing composition, and vi) a water-based composition comprising a (meth)acrylate-based and/or epoxy-based cathodic or anodic electrocoat material and/or a water-based or solvent-based wet or powder coating material.

7. The process according to claim 6, wherein the acidic conversion composition in step iv) comprises a nickel-free zinc phosphating composition which, as well as zinc ions and manganese ions, further comprises phosphate ions, and to which no nickel ions have been added.

8. The process according to claim 6, wherein the acidic conversion composition in step iv) comprises a composition for applying an organosilane-based thin-film system which, as well as at least one organosilane, its hydrolysis and condensation products included, optionally further comprises at least one titanium compound, zirconium compound and/or hafnium compound.

9. The cleaner concentrate according to claim 1, wherein the at least one (meth)acrylic acid homopolymer of component a) comprises at least one (meth)acrylic acid homopolymer having a weight-average molar mass in the range from 6 000 to 12 000 g/mol, calculated as polyacrylic acid.

10. The cleaner concentrate according to claim 1, wherein the at least one (meth)acrylic acid copolymer of component b) comprises at least one (meth)acrylic acid copolymer having a weight-average molar mass in the range from 60 000 to 80 000 g/mol, calculated as poly(acrylic acid-alt-maleic acid).

11. The cleaner concentrate according to claim 1, which further comprises at least one water-soluble boron compound c) selected from the group consisting of boric acid and alkali metal borates.

Description

EXAMPLES

(1) i) Determination of Picking Erosion:

(2) Principle of Measurement:

(3) The pickling erosion indicates the weight loss of the bare metal during a cleaning step. It is tested by immersing a defined standard sheet of AA6014 aluminum with dimensions of 105190 mm (Gardobond test sheet, Chemetall GmbH, Germany) into the solution under testin this case, corresponding cleaner solution. The loss of mass is then determined gravimetrically using an analytical balance. The test in this case was confined to aluminum surfaces, as they are the most sensitive to a pickling attack.

(4) Preparation of Test Sheets:

(5) The test sheets were first preliminarily degreased with petroleum spirit in order to remove any kind of organic impurity. This allows the direct attack of the test solution on the base substrate itself to be assessed and compared.

(6) Measurement of Pickling Erosion:

(7) The mass of the respective test sheet having undergone preliminary degreasing was determined on an analytical balance. Directly afterwards the test sheet was immersed for 10 minutes at 55 C. in a 31 beaker containing the corresponding test solution. Stirring took place with a 40 mm magnetic stirrer at the bottom of the beaker with a speed of 500 rpm.

(8) After 10 minutes, the test sheet was withdrawn from the test solution, rinsed with fully demineralized (FD) water, and dried using compressed air. The loss of weight was then determined on the analytical balance.

(9) In each case a reference was tested in parallel, to allow comparison of the values obtained.

(10) ii) Determination of Minimum Cleaning Time:

(11) Principle of Measurement:

(12) The minimum cleaning time (MCT) indicates the minimum duration of cleaning step that is necessary in order to remove organic impurities from a standard sheet of 1.0312 steel with dimensions of 105190 mm (test sheet) under constant conditions. The quality of the cleaning must attain a certain minimum value, which is determined from the percentage water wetting of the metal surface. For this case, steel surfaces were looked at exclusively, as they are typically the surfaces that are most difficult to degrease.

(13) Preparation of Test Sheets:

(14) The test sheets used had a constant oil burden (1.7+/0.2 g/m.sup.2). The purpose of this was the compatibility of the results.

(15) Measurement of Minimum Cleaning Time:

(16) To determine the minimum cleaning time, the respective test sheet was immersed for 1 minute at 55 C. in a 31 beaker containing the corresponding cleaner solution. Stirring took place with a 40 mm magnetic stirrer at the bottom of the beaker with a speed of 500 rpm. The test sheet was subsequently rinsed with reciprocal movements (around 15 back-and-forth movements) in the immersive rinse, with the sheet always being withdrawn completely from the rinsing water, and was held vertically for assessment after 10 seconds (in order to rule out false wetting).

(17) The minimum cleaning time is reached when the water wetting of the surface amounts to at least 95%, i.e., when there is a coherent water film. If this condition is not met, the test sheet is immersed in the cleaner solution for a further minute, as described above, and then rinsed in the immersive rinse. This is repeated until the condition is met.

(18) The number of repetitions is added up and, as a control, an identical oil test sheet is left in the cleaner solution for the entire time. This is necessary since the intermediate rinsing steps improve the cleaning performance. If the sheet is wetted to an extent of at least 95% after the added-up time, this time is recorded as the MCT. If this is not the case, cleaning and rinsing are repeated in steps of 1 minute until the surface is at least 95% water-wettable without intermediate rinsing steps. This time is then the MCT.

(19) iii) Investigation of Different Cleaner Solutions:

(20) In order to test the effect of different polymers in terms of pickling erosion and MCT, a standard cleaner concentrate (VB1) having a pH of 12.9 was first prepared as reference, this concentrate containing FV water and also the following components:

(21) TABLE-US-00001 % by Component weight Potassium 12.5 hydroxide Boric acid 14.5 Potassium 10 carbonate Sodium gluconate 3

(22) By adding the polymers below to this standard, different cleaner concentrates were obtained (VB2 to VB6 and also B1):

(23) TABLE-US-00002 Weight-average molar weight Polymer Chemical designation [g/mol] Polymer 1 Polyacrylic acid 4.000 Polymer 2 Polyacrylic acid 8.000 Polymer 3 Polyacrylic acid 20.000 Polymer 4 Poly(acrylic acid-alt-maleic acid) 70.000

(24) Furthermore, a phosphate-containing standard cleaner concentrate (VB7) having a pH of greater than 11.5 was prepared as a reference, containing FV water and also the following components:

(25) TABLE-US-00003 Component % by weight Potassium 31.5 hydroxide Boric acid 17.0 Phosphoric acid 4.0

(26) All of the cleaner concentrates were subsequently diluted by a factor of 1:50 (corresponding to 20 g of concentrate for 1.01 of cleaner) with FV water and admixed with 2 g/l of an ethylene/propylene oxide fatty alcohol, in other words a nonionic surfactant.

(27) Additionally the pH of all of the cleaner concentrates was adjusted to 10.5 by addition of boric acid or aqueous potassium hydroxide solution.

(28) The cleaner solutions obtained were then tested for pickling erosion and MCT as described earlier on abovecf. i) and ii). The results obtained accordingly are collated in Tab. 1 (mean values for at least three sheets in each case, i.e., n3).

(29) TABLE-US-00004 TABLE 1 Concentration of the polymer in the Pickling (Comparative) concentrate erosion MCT Example Polymer [% by wt.] [g/m.sup.2] [min] VB1 none 0 0.14 10 VB2 Polymer 1 7.0 0.68 5 VB3 Polymer 2 7.0 0.83 4 VB4 Polymer 3 7.0 1.45 3 VB5 Polymer 4 7.0 1.15 3 B1 Polymer 2 + 2.8 0.78 2 Polymer 4 (1.8:1.0 *) VB6 Polymer 2 + 5.5 1.14 3 Polymer 4 3.5:2.0 *) VB7 none 0 0.35 3 *) Weight ratio of the two polymers

(30) The experimental results show that by adding a polymerpolyacrylic acid or poly(acrylic acid-alt-maleic acidit is possible to achieve a large improvement in the cleaning performance (see MCT; VB2 to VB6 and also 1B1 vs. V1B1). At the same time the aggressiveness of the medium on aluminum is increased (see pickling erosion). It is apparent that in the case of polyacrylic acid, the effect of adding the polymer becomes greater in terms of pickling attack and MCT as the chain length, and hence the molar mass, increases (VB2 to VB34).

(31) It was also found that no polymer on its own was able to achieve the performance of a standard phosphate-containing cleaner in terms of pickling erosion and MCT. Only the cleaner solution of the invention (1B1), comprising a specific mixture of two polymers, is capable of achieving or even exceeding the cleaning performance of a phosphate-containing cleaner (VB7)MCT below 4 minuteswith a generally low service concentration and a balanced pickling attackpickling erosion up to 0.8 g/m.sup.2. If the concentration of the two polymers is doubled, however, the cleaning performance continues to be satisfactory, but the pickling attack is too severe (VB6).

(32) iv) Determination of Multimetal Capacity:

(33) The multimetal capacity of the various solutions was likewise determined using the two above-described measurement principles of pickling erosion (see Tab. 2: n3) and MCT (see Tab. 3: n3), but using in each case a specific VDA 230-213 testing apparatus. The tests in question were conducted on the following four substrates encountered in the automobile industry: Cold-rolled steel (CRS), hot-dipped-galvanized steel (HDG), prephosphated electrogalvanized steel (ZEP), and automotive-grade aluminum (AA6014).

(34) TABLE-US-00005 TABLE 2 (Compar- Concentration Pickling erosion ative) of the polymer [g/m.sup.2] Example Polymer [% by wt.] CRS HDG ZEP AA6014 VB8 Polymer 2 3.5 0.0. 0.02 0.09 1.61 B1 Polymer 2 + 2.8 0.0 0.02 0.18 0.78 Polymer 4 1.8:1.0 *) VB7 None 0 0.0 0.01 0.03 0.35 *) Weight ratio of the two polymers

(35) TABLE-US-00006 TABLE 3 (Compar- Concentration MCT ative) of the polymer [min] Example Polymer [% by wt.] CRS HDG ZEP AA6014 VB8 Polymer 2 3.5 6 3 3 7 B1 Polymer 2 + 2.8 5 2 2 5 Polymer 4 1.8:1.0 *) VB7 None 0 7 10 5 10 *) Weight ratio of the two polymers

(36) As apparent from Tab. 2, the pickling attack in the case of the cleaner solution of the invention (B1) is in fact somewhat higher by comparison with a phosphate-containing cleaner (VB7). In contrast to a cleaner solution based only on one polymer, however, with a lower polymer concentration (VB8), values obtained on aluminum (AA6014) are also values acceptable for an ongoing operation, thus demonstrating the multimetal capacity of the cleaning solution of the invention.

(37) Tab. 3 shows that for the cleaner solution of the invention (B1), the minimum cleaning time (MCT), i.e., the cleaning performance, is better particularly in comparison to a phosphate-containing cleaner (VB7), but also to a cleaner solution based only on one polymer, with a lower polymer concentration (VB8), on each of the substrates used in a multimetal operation.

(38) As a result of the use of the specific VDA 230-213 testing apparatus, the outcome obtained for pickling erosion and for MCT are higher by comparison with the results ascertained manually, in Tab. 1, this being attributable to the lower circulation/bath movement within the apparatus.

(39) In the cleaner solution B1 of the invention, moreover, the borate concentration was varied, in order to ascertain the optimum in terms of pickling attacks within a multimetal operation. The cleaner solutions thus prepared (B1-1 to B1-3) were then tested as described earlier on above (cf. i) in relation to their pickling erosion on the following three substrates encountered in the automobile industry: Automotive-grade aluminum (AA6014), hot-dipped-galvanized steel (HDG) and electrogalvanized steel (MBZE). The metal sheets used for this purpose each underwent preliminary degreasing with an aqueous surfactant solution.

(40) The results obtained accordingly are collated in Tab. 4 (mean values of in each case at least 3 sheets, i.e. n3).

(41) TABLE-US-00007 TABLE 4 Concen- tration of boric acid Pickling Pickling Pickling in the erosion erosion erosion (Comparative) concentrate [g/m.sup.2] [g/m.sup.2] [g/m.sup.2] Example Polymer [% by wt.] AA6014 HDG MBZE B1 Polymer 2 + 14.5 0.32 0.03 0.04 Polymer 4 (1.8:1.0 *) B1-1 Polymer 2 + 5.0 2.9 0.05 0.08 Polymer 4 (1.8:1.0 *) B1-2 Polymer 2 + 10.0 2.7 0.04 0.07 Polymer 4 (1.8:1.0 *) B1-3 Polymer 2 + 16.5 0.06 0.03 0.02 Polymer 4 (1.8:1.0 *) *) Weight ratio of the two polymers

(42) As can be seen from looking at the experimental results for the cleaner solution 1B1 of the invention and also its variants 1B1-1 to 1B1-3, the pickling attack of aluminum (AA6014) is in the desired low range at a concentration of 14.5 and also 16.5 wt % boric acid in the concentratei.e., a concentration of 0.29 and 0.33 wt %, respectively, for a dilution of 1:50 in the cleaner solution. In this way, then, it is possible for all the substrates testedincluding aluminumto undergo optimal treatment in combination.

(43) v) Compatibility with Conversion Treatments:

(44) The compatibility of the cleaner solution of the invention (B1) with known conversion treatments was examined on the basis of an organosilane-based thin-film coating and also a trication phosphating.

(45) To investigate the effect of the cleaner solution of the invention (B1), the corresponding polymers were added in quantities larger than operationally usualof the kind which may enter a conversion bath as a result of cleaner medium being entrained by componentsto the two conversion baths (1B2 and 1B3). Thereafter the following substrates encountered in the automobile industrycold-rolled steel (CRS), hot-dipped-galvanized steel (HDG) and aluminum (AA6014)were pretreated in a standard treatment procedureorganosilane and zirconium compound or zinc manganese nickel phosphate (phosphating time: 180 s) following prior activation with zinc phosphate (activation time: 60 s).

(46) The effect of the polymers was assessed by determination of coat weight (CW) by X-ray fluorescence analysis (XRF) and also scanning electron microscopy (SEM) images of the surface structure of the resulting conversion coat. The coat weights determinedcalculated as zirconium metal (Zr)for the organosilane-based thin-film coating are collated in Tab. 5 (n3).

(47) TABLE-US-00008 TABLE 5 Concentration CW of Zr (Comparative) of the polymer [mg/m.sup.2] Example Polymer [g/l] CRS HDG AA6014 VB9 None 0 45 56 48 B2 Polymer 2 + 0.48 48 52 51 Polymer 4 1.8:1.0 *) B3 Polymer 2 + 0.96 52 50 54 Polymer 4 1.8:1.0 *) *) Weight ratio of the two polymers

(48) The deviations attained within the different variants (VB9, B2 and B33) are situated within the possible error tolerance of the CW determination. The SEM images of the surface structure of the conversion coat showed no peculiarities in any case. The polymers used in the invention therefore have no adverse effects on the optimal development of coatings of an organosilane-based thin-film system, and are therefore compatible with said system.

(49) The coat weights determinedcalculated in each case as Zn.sub.3(PO.sub.4).sub.2.4H.sub.2Oare collated for the zinc phosphate activation and for the subsequent trication phosphating in Tab. 6 and, respectively, Tab. 7 (in each case n3).

(50) TABLE-US-00009 TABLE 6 Concentration CW of the zinc of the phosphate coat (Comparative) polymer [g/m.sup.2] Example Polymer [g/l] CRS HDG AA6014 VB10 None 0 2.9 2.8 3.2 B4 Polymer 2 0.01 2.9 2.7 3.0 B5 Polymer 4 0.006 2.8 2.7 2.8

(51) TABLE-US-00010 TABLE 7 Concentration CW of the of the zinc phosphate (Comparative) polymer coat [g/m.sup.2] Example Polymer [g/l] CRS HDG AA6014 VB10 None 0 2.0 2.1 4.0 B4 Polymer 2 0.01 2.7 2.8 4.2 B5 Polymer 4 0.006 2.3 2.2 4.6

(52) The coat weights obtained show that the two polymers exhibit no effects at all on the zinc phosphate activation (cf. Tab. 6) and only minor influences on the trication phosphating (cf. Tab. 7) (B34 and B5 vs. VB10), and these effects can be compensated in the ongoing operation by adaptation to the phosphating parameters. The SEM images of the surface structure of the trication conversion coat show no peculiarities.

(53) It has therefore been possible to demonstrate the compatibility of the cleaner solution of the invention not only with organosilane-based thin-film coatings but also with trication phosphating systems.

(54) vi) Corrosion Behavior in Organosilane-Based Thin-Film Coating:

(55) In order to investigate the effect of the cleaner solution of the invention 1B1 on the corrosion behavior, the material HDG in sheet form was treated within a standardized process.

(56) Process: 1.) Spray cleaning, 60 seconds 2.) Immersion cleaning, 180 seconds 3.) Immersive rinse, 30 seconds 4.) Immersive conversion, 180 seconds 5.) Immersive rinse, 30 seconds 6.) Drying with compressed air

(57) Cleaning steps 1.) and 2.) were carried out using the phosphate-free cleaner solution B1 of the invention at a dilution of 1:50 from the concentrate and of 2 g/l of an ethylene/propylene oxide fatty alcohol. For comparison, two standard phosphate-containing cleaners (VB11 and VB12) were also tested.

(58) For the conversion in step 4.) an organosilane based thin-film system (Chemetall, Germany) was used. After step 6.), the treated sheets were tested for paint adhesion and corrosion by means of a cyclical corrosion test (VDA 621-415) customary within the automobile sector.

(59) The results for corrosive undermining and also for paint adhesion after stone chipping are collated in in Tab. 8 (in each case n 2 3).

(60) It is apparent that the phosphate-free cleaner solution B1 of the invention in combination with an organosilane-based conversion system significantly improves both the corrosion behavior and the paint adhesion properties of the surface by comparison with a standard phosphate-containing cleaner (VB11 and VB12).

(61) TABLE-US-00011 TABLE 8 Corrosive Characteristic (Comparative) Concentration undermining stone chip Example of cleaner [g/l] pH [mm] value VB11 15 10.5 3.0 3.0 VB12 15 10.0 2.5 2.5 B1 20 10.0 2.0 2.0 B1 20 9.5 1.0 1.0