Method for corrosion-protective serial surface treatment of metallic components

Abstract

The present invention relates to a method for serial surface treatment of metallic components comprising aluminum surfaces, wherein an alkaline pretreatment is followed by a conversion treatment. According to the invention, the intention during the alkaline pretreatment is that a maximum value for the concentration of dissolved zinc is not exceeded, in order to ensure a sufficient quality of the corrosion-protective coating on the aluminum surface of the components following the surface treatment. In a preferred embodiment, the content of dissolved zinc is effectively held below the respective bath-typical maximum value of dissolved zinc by the addition of compounds constituting a source of sulfide ions. The functionality of the surface treatment can be additionally increased by likewise controlling the content of dissolved aluminum in the alkaline pretreatment such that, by adding compounds constituting a source for silicate anions, a threshold value for dissolved aluminum is not exceeded.

Claims

1. A method for serial wet chemical surface treatment of metallic components, in which method metallic components having surfaces of aluminum as well as components having surfaces of zinc are subjected to wet chemical pretreatment by bringing them into contact with an alkaline bath solution which is stored in a system tank, and a wet-on-wet conversion treatment of at least the surfaces of aluminum of the metallic components subsequently takes place, the pH of the alkaline bath solution in the wet chemical pretreatment being greater than 10, and the free alkalinity being at least 0.5 points but less than 50 points, wherein the following maximum value for the concentration of dissolved zinc in the alkaline bath solution of the system tank is not exceeded:
Zn.sub.max=0.0004(pH9)[FA]+0.6[Y] pH: pH value Zn.sub.max: maximum value of the concentration of dissolved zinc, in mmol/L [FA]: free alkalinity in mmol/L [Y]: concentration in mmol/L of complexing agents Y in the form of water-soluble condensed phosphates calculated as P.sub.2O.sub.6 and/or in the form of water-soluble organic compounds which contain at least one functional group selected from COOX.sub.1/n, OPO.sub.3X.sub.2/n, and/or PO.sub.3X.sub.2/n, where X represents either a hydrogen atom or an alkali and/or alkaline earth metal atom having the respective valence n; wherein the serial wet chemical surface treatment of the metallic components takes place at least for such a number of metallic components that a total surface area comprising solely zinc surfaces of the metallic components, in square meters, which is greater than the following term: $\frac{V_B{\mathrm{Zn}}_{\mathrm{ma}x}{M}_{\mathrm{Zn}}}{m_{\mathrm{Zn}}}$ V.sub.B: bath volume in m.sup.3 Zn.sub.max: maximum concentration of dissolved zinc in mmol/L M.sub.Zn: molar mass of zinc in g/mol M.sub.Zn: surface-normalized pickling removal, based on the zinc surfaces of the metallic components in g/m.sup.2, is subjected to wet chemical pretreatment with the alkaline bath solution of the system tank.

2. The method claim 1, wherein an exceedance of the maximum value Zn.sub.max in the wet chemical pretreatment is prevented by continuously or discontinuously removing dissolved zinc from the alkaline bath solution of the system tank, wherein this does not take place solely by discarding at least a portion of the alkaline bath solution of the system tank containing dissolved zinc, and adding another portion of an alkaline solution, which contains only the active components of the alkaline bath solution essential to adjusting alkalinity, to the system tank.

3. The method according to claim 2, wherein an exceedance of the maximum value Zn.sub.max in the wet chemical pretreatment is prevented by adding a water-soluble compound, which represents a source of sulfide ions, to at least a portion of the alkaline bath solution of the system tank, and optionally separating a solid portion, which forms in this portion of the alkaline bath solution, from the alkaline bath solution.

4. The method according to claim 3, wherein the portion of the alkaline bath solution of the system tank to which the water-soluble compound, which represents a source of sulfide ions, is added has a temperature of at least 40 C., but less than 65 C.

5. The method according to claim 4, wherein partial volumes are continuously withdrawn from the alkaline bath solution of the system tank, to which partial volumes of the alkaline bath solution containing the water-soluble compound, which represents a source of sulfide ions, is added, after which the solid portion, which is formed in these partial volumes of the alkaline bath solution is separated from the alkaline bath solution by filtration thereby forming a filtrate, and the filtrate is then recirculated into the alkaline bath solution of the system tank.

6. The method according to claim 5, wherein prior to being recirculated into the alkaline bath solution of the system tank, in order to reduce excess water-soluble compounds which represent a source of sulfide ions, a water-soluble oxidizing agent is added to the filtrate, the standard reduction potential of which oxidizing agent is greater than +0.6 V (SHE).

7. The method according to claim 6, wherein the alkaline bath solution of the wet chemical pretreatment contains aluminum dissolved in water, wherein a maximum value of 20 mmol/L, for the concentration of dissolved aluminum in the alkaline bath solution of the system tank is not exceeded due to the fact that a water-soluble compound which represents a source of silicate anions is added to at least a portion of the alkaline bath solution of the system tank, and a solid portion comprising aluminum silicate, which forms in this portion of the alkaline bath solution is optionally separated from the alkaline bath solution by filtration.

8. The method according to claim 7, wherein partial volumes are continuously withdrawn from the alkaline bath solution of the system tank, to which partial volumes the water-soluble compound which represents a source of silicate anions is added, after which the solid portion comprising aluminum silicate, which is formed in these partial volumes of the alkaline bath solution is separated from the alkaline bath solution by filtration, thereby forming a filtrate, and the filtrate is then recirculated into the alkaline bath solution of the system tank.

9. The method according to claim 8, wherein the conversion treatment following the wet chemical pretreatment using an acidic aqueous composition and the bringing into contact with same take place for a period of time in which the surfaces of aluminum of the metallic components undergo a pickling removal of less than 0.1 g/m.sup.2.

10. The method according to claim 9, wherein the conversion treatment following the wet chemical pretreatment takes place using an acidic aqueous composition containing water-soluble compounds of Zr, Ti, and/or Si, and optionally compounds which represent a source of fluoride ions.

11. The method according to claim 10, wherein metallic components having a composite design are pretreated, the metallic components having surfaces wherein at least 2% of the metallic components' surfaces are aluminum surfaces, and at least 5% of the metallic components' surfaces are zinc surfaces.

12. A method for serial wet chemical surface treatment of metallic components, comprising: a) contacting metallic components having surfaces of aluminum and surfaces of zinc with a wet chemical pretreatment comprising an alkaline bath solution which is stored in a system tank, said alkaline bath solution having a pH greater than 10 and free alkalinity of at least 0.5 points but less than 50 points, and b) after step a) performing a wet-on-wet conversion treatment of at least the surfaces of aluminum of the metallic components, wherein content of dissolved zinc in the alkaline bath solution of the system tank is held below a maximum value of dissolved zinc Zn.sub.max according to Formula (I):
Zn.sub.max=0.0004(pH9)[FA]+0.6[Y](I) where: pH: pH value; Zn.sub.max: maximum value of the concentration of dissolved zinc, in mmol/L; [FA]: free alkalinity in mmol/L; [Y]: concentration in mmol/L of complexing agents Y in the form of water-soluble condensed phosphates calculated as P.sub.2O.sub.6, and/or in the form of water-soluble organic compounds which contain at least one functional group selected from COOX.sub.1/n, OPO.sub.3X.sub.2/n, and/or PO.sub.3X.sub.2/n, where X represents a hydrogen atom, an alkali metal and/or an alkaline earth metal atom having a respective valence n; and further comprising a step of preventing exceedance of the maximum value Zn.sub.max in the wet chemical pretreatment by adding a water-soluble source of sulfide ions, to at least one portion of the alkaline bath solution of the system tank, and optionally separating a solid portion comprising zinc formed in said at least one portion of the alkaline bath solution, from the alkaline bath solution.

13. The method according to claim 12, wherein the alkaline bath solution of the wet chemical pretreatment contains aluminum dissolved in water, wherein concentration of dissolved aluminum in the alkaline bath solution of the system tank is held below a maximum value of 20 mmol/L, by adding a water-soluble source of silicate anions to at least a portion of the alkaline bath solution of the system tank, and optionally separating a solid portion comprising aluminum formed in said at least one portion of the alkaline bath solution, from the alkaline bath solution.

Description

EXEMPLARY EMBODIMENTS

(1) The influence of alkaline pretreatments on the effectiveness of the conversion treatment is described below with reference to individual exemplary embodiments. In particular, the positive influence on the filiform corrosion of aluminum which is surface-treated in methods according to the invention, and which has additionally been coated with a cathodic dip coating, is explained.

(2) The compositions of different alkaline pretreatments (systems A-E) are given in table 1. In addition to the prototype alkaline systems A-D, which vary with regard to their free alkalinity and pH, a cleaner for alkaline degreasing of metals (system E) and an alkaline steeling of the type described in DE 102010001686 were listed. Table 1 also lists the maximum value Zn.sub.max for dissolved zinc which is specific for the particular system for the alkaline pretreatment.

(3) To explain the positive influence of such methods for surface treatment comprising alkaline pretreatment and subsequent conversion treatment, for which zinc and aluminum ions are held below the respective specific maximum value according to the present invention in the pretreatment, defined quantities of dissolved zinc and aluminum were added to systems A-E. For alkaline pretreatments which contained more dissolved zinc or aluminum than stipulated by the respective maximum value, comparative tests were conducted, after a quantity of precipitation reagent had been added to these pretreatment solutions which was sufficient to bring the zinc concentration below the maximum value. Exclusively aluminum sheets were surface-treated. The aging of the alkaline bath solution of a system tank in a serial wet chemical treatment of metallic components was simulated, as already stated, by the defined addition of water-soluble zinc salts and aluminum salts. The determination of the actual value, stated in table 2, of the concentration of dissolved zinc and aluminum took place immediately before bringing the aluminum sheets into contact with the particular alkaline system. The analytical determination method is discussed further below.

(4) The alkaline pretreatment was always followed by an inorganic conversion treatment free of chromium, using the wet-on-wet method, with a rinsing step in between. A cathodic electro-dip coating was subsequently applied, and the filiform corrosion of aluminum sheets surface-treated in this way was assessed. The associated results are summarized in table 2.

(5) The process for the wet chemical surface treatment of aluminum sheets (Aluminum AA 6014) consisted of the following detailed individual steps: 1. alkaline pretreatment with compositions corresponding to examples A1-3; B1-4; C1-4; D1-5; E1-2; and F1-6 (see table 2 and the respective base formulation from table 1) by immersing the sheets for 3 minutes at 60 C.; 2. rinsing with deionized water (<1 Scm.sup.1) by immersing the sheets for 30 seconds at 25 C.; 3. inorganic conversion treatment with an acidic aqueous composition containing 0.15 g/L H.sub.2ZrF.sub.6 40 ppm free fluoride (measured with an ion-selective combination electrode) pH 4.5 by immersing for 2 minutes at 30 C. (pickling removal from aluminum sheet<0.05 g/m.sup.2, determined by differential gravimetric analysis); 4. rinsing with deionized water (<1 Scm.sup.1) by immersing the sheets for 30 seconds at 25 C.; 5. depositing the cathodic dip coating (Cathoguard 500, BASF) in a dry film thickness of 202 m. The dip coating was burned in for 25 minutes at 180 C.

(6) The actual concentration of dissolved zinc and aluminum in the alkaline pretreatment according to table 2 was determined by means of optical emission spectroscopy with inductively coupled argon plasma (ICP-OES).

(7) After the sampling from the alkaline system solution, the portion of dissolved zinc may be further reduced by post-precipitation of poorly soluble hydroxides or phosphates. Therefore, for determining the actual concentration, and therefore the concentration according to the invention, of dissolved zinc and aluminum, immediately after the sample is withdrawn (within 5 minutes), it must be initially filtered over a filter having an exclusion limit of 0.1 m and subsequently acidified. Samples prepared in this way may be analytically measured at an arbitrary later point in time, since the portion of dissolved zinc or aluminum in the acidic sample volume is invariable.

(8) Accordingly, a sample of 2 mL of the alkaline system solution was withdrawn using a syringe, and filtrated, after placing a syringe filter, over a cellulose acetate membrane, having a porosity of 0.1 m, integrated into the syringe filter. Enough drops of 50% by weight nitric acid were then added to the filtrate of the filter syringe, with thorough shaking, until the pH was less than 2.

(9) After calibrating the measuring instrument (Optima 7300 DV, PerkinElmer) with standard solutions of standard titrimetric substances containing 1 ppm, 5 ppm, and 10 ppm of dissolved zinc and 4 ppm, 20 ppm, and 40 ppm of dissolved aluminum, respectively, the portion of zinc and aluminum in the acidified filtrate was determined by means of ICP-OES in the sample volume worked up in this way, which portion in turn corresponds to the actual concentration of these elements in the alkaline system solution at the time of sampling.

(10) The pickling removal from the aluminum sheets in the alkaline pretreatment with the system solutions listed in table 2 was determined by differential gravimetric analysis. For this purpose, the aluminum sheets were initially freed of organic residues such as fats and oils, using acetone, and were weighed after being blown dry. The sheets cleaned in this way were then pretreated with the alkaline system solution corresponding to the above-described process sequence, and after the subsequent rinsing operation with deionized water likewise blown dry and re-weighed. The surface-normalized mass difference then corresponds to the pickling removal.

(11) The filiform corrosion was evaluated after the aluminum sheets were stored for 42 days in accordance with DIN EN 3655. In each case, the average thread length and the longest thread of the occurring filiform corrosion was determined.

(12) The results clearly show that a distinct worsening in the filiform corrosion is observed as soon as the maximum value Zn.sub.max for dissolved zinc is exceeded (A3; B3; C3; D4-5; E2; F3; and F5). The worsening is accompanied by a reduction in the pickling rate with respect to the aluminum substrate to be coated. As soon as a quantity of a compound which releases sulfide ions (thioacetamide, sodium sulfide) is added and the actual dissolved zinc content is thereby reduced below the maximum value Zn.sub.max, the surface treatment according to the invention produces very good results in the filiform test (B4; C4; F6).

(13) The actual analytically determined content of dissolved zinc resulted in the following values for these alkaline system solutions:

(14) B4: <0.24 mg/L (<0.004 mmol/L)

(15) C4: 1.2 mg/L (0.018 mmol/L)

(16) F6: 484 mg/L (7.4 mmol/L).

(17) It was thus shown that, regardless of the specific type of alkaline system solution, i.e. regardless of whether a cleaner for degreasing or an alkaline steeling is involved, controlling the maximum value of dissolved zinc in the alkaline pretreatment during the subsequent conversion treatment ensures that good corrosion protection of the aluminum surfaces of the components is achieved.

(18) In addition, tests F2 and F3 demonstrate that excessively high contents of dissolved aluminum are likewise detrimental to the corrosion resistance of the surface-treated aluminum sheets. Here as well, the exceedance of the maximum value for dissolved aluminum in the alkaline pretreatment is accompanied by a drastic decrease in the pickling removal from the aluminum sheet (see F3). The metered addition of such a quantity of silicates to this alkaline pretreatment, which theoretically causes a reduction in the dissolved aluminum content to 500 mg/L, once again results in increased pickling removal from the aluminum sheet, and after surface treatment is completed, gives a very good result in the filiform test (cf. F3 and F4).

(19) Thus, it is shown in general that pickling removal of at least 0.5 g/m.sup.2 from the surfaces of the aluminum in the wet chemical pretreatment with the alkaline bath solution must take place in order to be able to achieve sufficiently good quality of the corrosion-protective surface treatment with a subsequent conversion treatment.

(20) TABLE-US-00001 TABLE 1 Compositions of various alkaline systems for pretreatment Phos- Carbon- Y FA pH Zn.sub.max Sys- phate ate * Addi- [mmol/ [mmol/ val- [mmol/ tem [g/L] [g/L] tives L] L] ue ** L] A 4.10 5.4 77 11.0 0.062 B 3.60 1.3 34 11.5 0.034 C 3.60 10.8 155 11.5 0.155 D 4.10 5.4 49 12.0 0.059 E 5.0.sup.# 43.9.sup. 40 10.5 26.4 F 15.0.sup.# Fe(NO.sub.3).sub.3 43.9.sup. 15 11.0 26.5 * as K.sub.2CO.sub.3 .sup.#as NaHCO.sub.3 .sup.2.2 g/L K.sub.4P.sub.2O.sub.7; 7.1 g/L HEDP; 0.6 g/L sodium gluconate ** The pH was set using potassium hydroxide

(21) TABLE-US-00002 TABLE 2 Filiform corrosion test in accordance with DIN EN 3665 on pretreated, conversion-coated aluminum sheets (Alu AA 6014) after layer build-up by cathodic dip coating (Cathoguard 500, BASF) Zn Al Sul- Sili- Pickling Thread Sys- [mmol/ [mmol/ fide .sup.1 cate .sup.2 rate .sup.3 length .sup.4 tem No. L] L] [g/L] [g/L] [g/m.sup.2] [mm] A 1 0.66 0.3/2.0 2 0.012 0.53 0.4/2.3 3 0.076 0.01 3.5/6.5 B 1 0.70 0.2/1.9 2 0.008 0.50 0.4/2.2 3 0.153 0.04 3.2/6.0 4 0.153 0.15 * 0.75 0.2/1.8 C 1 1.27 0.1/1.1 2 0.018 0.69 0.2/1.8 3 0.275 0.00 3.7/6.8 4 0.275 0.15 * 1.25 0.1/1.2 D 1 1.54 0.2/1.6 2 0.032 0.71 0.3/1.9 3 0.041 0.58 0.3/2.4 4 0.105 0.19 2.3/4.8 5 0.550 0.01 3.6/6.1 E 1 7.644 0.90 0.3/1.1 2 30.576 0.00 3.1/7.1 F 1 7.644 0.83 0.2/1.2 2 7.644 18.649 0.60 0.4/1.7 3 7.644 37.297 0.20 2.5/5.2 4 7.644 37.297 4.00 # 0.59 0.4/1.9 5 30.576 0.22 2.5/5.1 6 30.576 1.79** 0.81 0.3/1.5 .sup.1, 2 Theoretical concentration values based on the respective quantity of precipitation agent added in a metered manner. .sup.3 Pickling removal determined by differential gravimetric analysis immediately after pretreatment with the respective composition according to table 1. .sup.4 Given as the average thread length and the thread length of the longest thread. * As thioacetamide; **as sodium sulfide; # as sodium water glass 40/42 (29% SiO.sub.2).