WO1991003582A1

WO1991003582A1 - Method of conditioning surfaces before phosphating

Info

Publication number: WO1991003582A1
Application number: PCT/US1990/004998
Authority: WO
Inventors: Yoshiji Shirogane; Shinji Takagi; Norio Kozuka
Original assignee: Henkel Corporation
Priority date: 1989-09-04
Filing date: 1990-09-04
Publication date: 1991-03-21
Also published as: AU6422890A; JPH0394075A

Abstract

Stabilization of the phosphating quality achieved after a surface conditioning treatment with an otherwise conventional composition containing dispersed colloidal titanium, and other benefits, is achieved by causing the aqueous composition to flow at a rate of at least 80 meters per minute for at least 10 minutes after make-up, in order to achieve optimum titanium particle size and homogeneity of the composition.

Description

METHOD OF CONDITIONING SURFACES BEFORE PHOSPHATING

Field of the Invention

This invention relates to the use of surface conditioning treatments on metal surfaces prior to applying phosphate conversion coatings on the metal surfaces. The surface conditioning treatments within the field of this invention comprise contacting the metal surface to be conditioned with an agueous dispersion of colloidal titanium. This aqueous dispersion preferably also contains phosphoric acid and/or anions derived from ionization of phosphoric acid and usually also contains magnesium ions. (In the remainder of this specification, the term "solution" may be used to include such dispersions as are described above.) The purpose of the conditioning treatment is to accelerate the formation of the conversion coating and to maintain the areal density of the phosphate conversion coating (hereinafter often denoted simply as "coating weight") and the average size of the crystals in the phosphate conversion coating film (hereinafter often denoted as "film crystals" or "film crystal size") within particular specified ranges. Statement of Related Art

Conditioning dispersions of the type described above are known generally in the art. Commercial immersion type colloidal titanium based surface conditioning treatment baths have heretofore been as large 1,000 to 100,000 liters ("L"). Treatment solution compositions have been described, for example, in (1) Japanese Patent Publication Number 58-55229 (55,229/83), (2) Japanese Patent Publication Number 62-9190 (9,190/87), (3) Japanese Patent Application Laid Open (Kokai) Number 61-99278 (99,278/86), and (4) Japanese Patent Application Laid Open Number 63-18084 (18,084/88) from the viewpoint of the stability of such surface conditioning solutions. In each case, these patent publications relate to a surface conditioning process or to the treatment solution or dispersion used in it. However, when using the inventions described in these patent publications, the performance falls off when the composition of the treatment solution fluctuates. It is therefore still true for them that continuous or frequent removal of part of the treatment solution volume, management of treatment solution renewal, and routine analysis of the treatment solution remain crucial. Accordingly, the surface conditioning performance may decline when there are variations in the available pressure and/or the impurity content of the water supply used for replenishing the volume of treatment dispersion withdrawn. This can cause the coating weight and film crystals of the phosphate conversion film to fall outside prescribed ranges.

Moreover, with regard to treatment solution analysis, the surface-conditioning effect cannot itself be directly correlated to an adequate degree with any known chemical or physical analytical technique or combination of techniques applied to the conditioning treatment solution alone, without actually performing and evaluating a subsequent phosphate conversion coating. The best methods known are probably those based on control of the ratio of effective titanium/total titanium and of the Mg/P₂O₇ ratio as described in reference (2) above, but even these are often inadequate. Otherwise, the aforementioned reference (4) as well as (5) Japanese Patent Application Laid Open Number 61-25748 (25,748/86) disclose that the stability of the colloidal titanium in the treatment solution can be increased through the addition of organic compounds; however, these teachings are not directed to regulation or control within prescribed ranges of the coating weight or film crystals obtained by phosphate conversion coating after conditioning.

In addition, (6) Japanese Patent Application Laid Open Number 63-76883 (76,883/88) discloses a management method based on measurement of the particle size of the colloidal titanium. This approach has been found to be generally impractical under ordinary commercial conditions because it is too difficult to distinguish the colloidal titanium particle size from contaminant particles found in the treatment solution under those conditions encountered industrially.

Thus, for each of these examples of related art, the problem of the long-term, stable maintenance of the performance of the surface-conditioning solution within specified ranges still remains to be solved satisfactorily. Description of the Invention

In the following description, except in the claims and in the working and comparative examples or where explicitly stated to the contrary, all numbers specifying amounts of materials or conditions of reaction or use are to be understood as modified by the term "about" in defining the broadest scope of the invention. Practice within the exact numerical limits given is generally preferred, however.

One embodiment of the present invention, as a means for solving the problems of the aforementioned related art, comprises management of the surface conditioner treatment solution so as to maintain the subsequently formed phosphate film crystals and coating weight within a desired range, by circulating the solution through the equipment used for conditioning, including the part of the equipment where actual contact with the metal surfaces to be condi tioned takes place, at a sufficient flow rate to develop' shear forces within the solution. The flow rate is preferably at least 80 meters per minute ("m/minute") and more preferably at least 110 m/minute) .

In a common commercial embodiment, using a colloidal titanium based surface conditioner treatment solution, a 1,000 to 100,000 L treatment bath, and an overflow bath, the surface conditioning treatment method is characterized by treatment of the workpiece by bringing it into contact with said surface conditioner treatment solution while the latter is being stirred by suction through and discharge from a pump with an output capacity of 800 to 3,500 L/minute. Moreover, the required flow rates may be accomplished in part by spraying and otherwise by circulation through a pump. Since the dispersibility of the colloidal titanium is thereby increased and its aggregation or coarsening is inhibited, the surface-conditioning performance can be stably maintained for long periods of time. The aforesaid pump preferably has a lift of at least 10 m and more preferably of at least 15 m.

Most preferably, the conditioning treatment solution used in a process according to the invention is kept in motion at the flow rates indicated above throughout its use. However, as shown below in connection with the working examples, much of the benefit of the invention can be obtained even by use of high speed flow rates only during make-up of the solutions and for a period of at least 10 minutes after make-up. Preferably, the high speed flow rates characteristic of the invention are also used during and for at least long enough after every subsequent replenishment of the active ingredients of the treatment conditioning bath.

Furthermore, a highly desirable benefit available from, and thus a preferable element within the scope of, the present invention is that the coating weight and film crystal size of a zinc phosphate film can be freely and stably controlled through the regulation and control of the stirring flow rate according to the aforementioned methods. Brief Description of the Drawings

Figure 1 is a schematic side view of one example of an actual treatment apparatus according to and for carrying out a method according to the present invention; Figure 2 is a schematic side view of another example of such an apparatus; and Figure 3 is a schematic side view of yet another example of such an apparatus.

Examples

The practice of the invention can be further appreciated from the following working and comparative examples. Apparatus and process embodiments of the invention may be understood with reference to the drawings of practical equipment for carrying out the invention. The equipment illustrated in Figure 1 is one example for an immersion or dipping method for surface conditioning processes. In the figure, 1 is a treatment bath, 2. is an overflow bath, 3. is a pump, 4. refers to a plural number of valves, 5 is a pipe arrangement which forms a circulation path on one side, 6. is a riser for spraying, 7 represents spray nozzles, and 8. is a pipe arrangement which forms a circulation path on the other side.

A prescribed quantity of colloidal titanium based surface conditioner treatment solution resides in treatment bath 1 , and a workpiece is treated by immersion or dipping in this treatment bath. The surface conditioner treatment solution is extracted, put through the pump 3, and circulated via the aforesaid pipe arrangements 5. and 8. The pump 3. used here should have a capacity of 800 to 3,500 L/minute and preferably 1,500 to 3,000 L/minute, and its lift should preferably be at least 10 m and more preferably is at least 15 m. The configuration is such that the treatment solution is sprayed via spray nozzles 7 from the aforesaid riser 6 while it is also circulated and ejected into overflow bath 2 via pipe arrangement 8. By this means the treatment solution is thoroughly sheared and stirred, and additional surface conditioner may also be added to the liquid when needed with stirring. When the output of the pump falls below 800 L/minute, the colloidal particles may adhere to one another and grow in size, and the surface conditioning performance would then deteriorate. When the pump output exceeds 3,500 L/minute, the shearing effect may be too great, so that the colloidal particles become smaller than optimum, and the surface conditioning performance would again be inferior.

One example of a suitable pump is a minimum caliber Ebara^TM FS pump (standard centrifugal pump), model number 100X 80 FS4K511, with a flow rate of 2,000 L/minute, 11 kilowatts of power draw (50 cycle current), and a lift of 22.5 m.

During make up or replenishment of the bath, surface conditioner is added to overflow bath 2, and a stainless steel wire mesh 9. may be advantageously installed in the vicinity of the point of addition.

Figure 2 depicts another actual apparatus which as before is equipped with a high-capacity pump 3.. In a typical prior art installation, this pump would have had a flow rate sufficient to spray the treatment solution and would be able to pump the total capacity of a typical tank in about 90 to 120 minutes. However, in the present invention it can pump the total tank capacity in 20 to 60 minutes. Part of the flow is sprayed while the majority is returned to treatment bath 1 via return pipes 10, with the result that streams from both the spray nozzles and the return pipes stir the treatment bath. In this embodiment, the return pipes 10 may be placed at the side or bottom of the treatment tank, but must be placed with due consideration to avoiding any decline in the stirring performance within the tank even when the after-spray pressure is reduced.

In addition, in the embodiment depicted in Figure 2, the flow in treatment bath 1 may be regulated by the use of bypass 5a in pipe arrangement 5, in order to form an alternative circulation path, other than the path through bath 1, through pump 3.

Figure 3 depicts an embodiment of the invention in which the apparatus of Figure 1 has been supplemented withan auxiliary air-feed pipe arrangement 13. placed in overflow bath 2.. This agitates the solution in order to support rapid dissolution and dispersion of the surface conditioner.

The equipment shown in Figures l and 3 illustrates the importance of the means for dissolving the surface conditioner in water when the surface conditioner solution is being made up or replenished. It is known that the properties of the colloidal titanium in the surface conditioner are substantially influenced by the type of dispersing solvent. For example, as illustrated in the aforesaid related art reference (2), the particle size of the colloidal titanium varies with variations in the Mg/P₂O₇ ratio in the treatment solution, with the result that the coating weight and film crystal size vary. Moreover, the pH of the treatment solution is also extremely influential, and it has been confirmed that no surface-conditioning effect is observed at below pH 7 while the effect declines rapidly with time at above pH 9.5. Due to this, when the surface conditioner is dissolved or dispersed in water, the properties of the dispersion will vary unless each component in the surface conditioner is homogeneously dissolved or dispersed. Research results have shown that, in order to avoid this, all the components must reach a homogeneously dissolved or dispersed state within no more than 30 minutes after addition of the surface conditioner to water. This desirable condition is achieved by the high flow rates used in a surface-conditioning method according to the this invention. Furthermore, because shearing forces are applied to the treatment solution by high-capacity pump 3,, the colloidal titanium will be dispersed even more homogeneously than would otherwise be expected.

In a surface-conditioning method using a modification, not shown in the drawings, of the apparatus illustrated in Figure 2, a smaller part of the liquid delivered by the pump is discharged into treatment bath via the spray nozzles 7 , due to the installation of one or more valves in the riser 6. and/or the pipes leading directly to the spray nozzles 7 of the spray device. The spray pressure is thus adjusted to lower values and the output from the return pipes 10 is enlarged, and stirring within the bath is thereby strengthened. The reason for dropping the spray pressure is that this has the effect of suppressing reductions in the pH of the surface-conditioning solution. This feature differs from prior art devices, in which the pump outflow rate and spray flow are equal. When the flow rate is increased in such a case, the spray flow is thus increased, and the surface conditioning solution, absorbing carbon dioxide from the atmosphere, suffers from a decline in pH and the surface conditioning effect deteriorates.

The apparatus in Figure 3 provides for a remarkable increase in stability and in the speed with which a reliable conditioning effect is achieved at the time of surface conditioning bath make-up, by improving the rate at which all components of the surface-conditioning agent are homogeneously dissolved and/or dispersed. Since in most cases the pump is continuously operated during operations in an actual industrial plant, the surface-conditioning solution is stirred and, moreover, aggregation of the colloidal titanium with the passage of time is suppressed by the pump's shearing force.

Several specific working examples and comparison examples are explained below, and results are reported in Table 1.

Sample steel sheet

Cold-rolled steel sheet, cut to a size of 70 × 150 × 0.75 millimeters ("mm"), was used in all cases.

Treatment method steps

(1) Degreasing: spray, 40° C for 120 seconds with an aqueous solution containing 20 g/L of Finecleaner™ L 4480 from Nihon Parkerizing Company, Limited (Tokyo).

(2) Water wash: tap water spray at room temperature for

20 seconds.

(3) Surface conditioning: immersion at room temperature for 20 seconds in an aqueous dispersion containing 1.4 g/L of Prepalene™ ZTH from Nihon Parkerizing Company, Ltd.

(4) Phosphating: immersion at 42° C for 120 seconds in an aqueous solution of type Palbond™ L3080 from Nihon Parkerizing Company, Ltd., with total acidity = 23 points, free acidity = 0.8 points, and accelerator = 3.0 points.

(5) Water wash: tap water spray for 20 seconds at room temperature.

(6) De-ionized water wash: spray for 20 seconds with de-ionized water (conductivity = 2 microSiemens/cm) . (7) Drain and dry: in air at 110° C for 180 seconds.

Conditions for the surface conditioning solution

Control of Mg/P₂O₇ ratio: adjusted by the addition of reagent grade MgSO₄ or Na₄P₂O₇;

stirring conditions: homomixer Model SL, 5 L tank, 3 L liquid capacity

Evaluation of the surface-conditioning performance

(1) Film appearance

The appearance of the phosphate film was evaluated visually after drainage and drying.

+ + good uniformity

+ film on the thick side or film is uneven

X poor formation in some parts

X X poor formation over the entire surface (2) Coating weight

The film was stripped by immersion for 15 minutes at 50° C in 5% aqueous chromic acid. The difference in weight before and after film stripping is reported in units of grams per square meter ("g/m²").

(3) Film crystal size

This was evaluated as the average crystal size from a scanning electron micrograph (x 1,000).

Upon examining the results of the Examples and Comparison Examples in Table 1, one observes that the external appearance in Example 1 is clearly better than in Comparison Example 1, while the former also has preferred values for the coating weight and film crystal size. These are effects deriving from the high flow rates maintained according to the invention.

A comparison of Example 2 with Comparison Example 2 demonstrates the continuation, even after the passage of 5 days, of the surface-conditioning effect obtained through the effect of initial high speed stirring.

Examples 1, 3, and 5 and Examples 2, 4, and 6 demonstrate that the coating weight and film crystal size vary, even at a constant Mg/P₂O₇ molar ratio, according to the speed (revolutions per minute or "rpm") of the homomixer, and also that the coating weight and film crystal size both decline with increasing rpm.

It is thus demonstrated that stirring at the time of surface conditioner dissolution is critical and that the effect of the surface conditioning solution varies with the stirring rate and force.

When the Mg/P₂O₇ molar ratio was set at 1.2, one generally observed a substantial deterioration in the surface conditioner solution with the passage of time in Comparison Examples 3 and 4, while a good performance was observed due to stirring in Examples 7, 8, and 9. A comparison of Exampies 8 and 9 shows that stirring after the passage of time had an additional beneficial effect. Examples 10 through 14 demonstrate that the coating weight and crystal size can be adjusted by varying the stirring force.

The preceding results show that even at the level of practical industrial devices the surface-conditioning effect can be adjusted or regulated by controlled stirring.

Table 2 reports the results from an apparatus used in industrial practice.

The pump flow rate for a 60,000 L-capacity tank was varied from 500 L/min to 3,000 L/min, and the results confirmed that the Mg/P₂O₇ ratio had little effect at increasing flow rates. In other words, even with a variation in Mg/P₂O₇ in the practical process due to variations in water quality, it would be possible stably to maintain the coating weight and film crystal size within prescribed ranges. Benefits of the Invention

The present invention achieves such excellent effects, as stabilization of the surface-conditioning performance over long periods of time and prevention of aggregation or coarsening of the colloidal titanium. Moreover, since the constituents of the colloidal titanium-based surface conditioner treatment solution are uniformly and stably dispersed and aggregation of the colloidal titanium is prevented, surface conditioning of the workpiece is uniform. Accordingly, the present invention also achieves such excellent effects as an acceleration of the conversion reaction in the following conversion treatment and stable coating weights and film crystal size in the conversion coating.

Claims

What is claimed is: CLAIMS

1. A process for conditioning a metal surface to promote the subsequent formation of a phosphate conversion coating thereon, said process comprising contacting the metal surface with an aqueous composition containing colloidal titanium dispersed therein, wherein the improvement comprises utilizing an aqueous composition that has been subjected to a flow rate of at least about 80 meters per minute at the time of make-up of the aqueous composition for a sufficient time after make-up to disperse all solid constituents of the aqueous composition homogeneously therein.

2. A process according to claim 1, wherein is utilized an aqueous composition that has been subjected to a flow rate of at least about 80 meters per minute for at least 10 minutes after make-up.

3. A process according to claim 2 wherein said flow rate is at least about 110 meters per minute.

4. A process according to claim 1 wherein said flow rate is at least about 110 meters per minute.

5. A process according to claim 4, wherein a total amount of aqueous composition of between about 1000 liters and about 100,000 liters is used and said flow rate is achieved by passing the aqueous composition through a pump having an output capacity of between about 800 and about 3,500 liters per minute.

6. A process according to claim 3, wherein a total amount of aqueous composition of between about 1000 liters and about 100,000 liters is used and said flow rate is achieved by passing the aqueous composition through a pump having an output capacity of between about 800 and about 3,500 liters per minute.

7. A process according to claim 2, wherein a total amount of aqueous composition of between about 1000 liters and about 100,000 liters is used and said flow rate is achieved by passing the aqueous composition through a pump having an output capacity of between about 800 and about 3,500 liters, per minute.

8. A process according to claim 1, wherein a total amount of aqueous composition of between about 1000 liters and about 100,000 liters is used and said flow rate is achieved by passing the aqueous composition through a pump having an output capacity of between about 800 and about 3,500 liters per minute.

9. A process according to claim 8, wherein said pump has a lift of at least 10 meters.

10. A process according to clai.m 7, wherein said pump has a lift of at least 10 meters.

11. A process according to claim 6, wherein said pump has a lift of at least 10 meters.

12. A process according to claim 5, wherein said pump has a lift of at least 10 meters.

13. A process according to claim 2 , wherein a specific flow rate is selected for the process and the flow rate during at least 10 minutes after make-up is continuously monitored and adjusted if necessary to maintain said specific flow rate.

14. Apparatus for conditioning a metal surface to promote the subsequent formation of a phosphate conversion coating thereon by a process comprising contacting the metal surface with an aqueous composition containing colloidal titanium dispensed therein, wherein the improvement comprises means for subjecting the aqueous composition to a flow rate of at least about 80 meters per minute at the time of makeup of the aqueous composition and for a sufficient time after make-up to disperse all solid constituents of the aqueous composition homogeneously therein.

15. Apparatus according to claim 14, comprising:

(A) a treatment bath container;

(B) an overflow container so arranged as to receive any of the aqueous composition that exceeds a fixed height in the treatment bath container;

(C) motion impeller means within a housing, said motion impeller means being capable of imparting to all of said aqueous composition that passes through said housing a flow rate of at least 80 m/minute;

(D) means for conveying a portion of said aqueous composition from said overflow container into the housing surrounding said motion impeller; and

(E) means for conveying at least a portion of the aqueous composition that has flowed through the housing surrounding said impeller means to the treatment bath container.

16. Apparatus according to claim 15, additionally comprising means for partitioning the portion of the aqueous composition that has flowed through the housing surrounding said impeller means into a first stream that flows into the treatment bath container before passing again through the housing surrounding said motion impeller means and a second stream that makes at least one additional passage through the housing surrounding said impeller means before flowing into the treatment bath container.

17. Apparatus according to claim 16, wherein the combined working capacity of the treatment bath container and the overflow container is from about 1000 to about 100,000 liters and the motion impeller means is a pump that produces an output flow rate between 800 and 3,500 liters per minute.

18. Apparatus according to claim 15, wherein the combined working capacity of the treatment bath container and the overflow container is from about 1000 to about 100,000 liters and the motion impeller means is a pump that produces an output flow rate between 800 and 3,500 liters per minute.

19. Apparatus according to claim 15, wherein the means for conveying at least a portion of the aqueous compositionr that has flowed through the housing surrounding said motion impeller means to the treatment bath container includes means for dividing said portion into a first sub-portion and a second sub-portion, means for conveying the first sub-portion to the treatment bath container via spray nozzles that spray the aqueous composition into the atmosphere above the treatment bath container, and means for conveying the second sub-portion into the treatment bath container via a discharge conduit that discharges the second subportion below the level of the liquid already present in the treatment bath container.

20. Apparatus according to claim 15, additionally comprising gas bubble agitation means within the overflow container.