GB2199047A

GB2199047A - Forming phosphate coatings

Info

Publication number: GB2199047A
Application number: GB08724339A
Authority: GB
Inventors: Atsunori Yoshida; Toshi Miyawaki; Takanobu Saito
Original assignee: Nihon Parkerizing Co Ltd
Current assignee: Nihon Parkerizing Co Ltd
Priority date: 1986-10-16
Filing date: 1987-10-16
Publication date: 1988-06-29
Anticipated expiration: 2007-10-16
Also published as: EP0264811B1; AU7978387A; GB8724339D0; EP0264811A1; DE3771026D1; JPH055899B2; NZ222135A; BR8705531A; GB2199047B; DE3734596A1; ES2022854B3; JPS63100185A

Description

1 1 n. rl -;U r 1 219 --- Processes for Forming Phosphate Coatings It is

well known in the automotive and other industries to form phosphate coatings on iron surfaces, such as surfaces of cold rolled steel sheet or galvanised steel sheet. Such coatings serve as a base -for paint and also give corrosion resistance.

Traditional high performance methods required treatment at a temperature of 50 to 550C, generally using a zinc phosphate bath containing nickel, nitrite and chlorate or fluoride. More recently it has been possible to obtain high performance coatings at lower temperatures, 40 to 450C, using a bath containing nickel, nitrite, fluoride and manganese. Such a process is suitable on various types of plated steel sheet and gives improved paint adhesion and corrosion resistance, despite the low temperature of operation.

The presence of zinc in the bath is essential in order to permit the formation of -the desired zinc phosphate base coating, and the concentration of zinc can have a significant influence on the performance of the resultant coating, including the performance of a paint film applied over the zinc phosphate coating. Generally the concentration of zinc is in the range 0.9 to 1. 5g/1.

It is known that the use of the lower concentrations is desirable as -it tends to increase the ratio P/P+H (P: Phosphophyllite, Zn 3 (PO 4)2 4H 2 0), paint film.

Although it would have been obviously desirable, from the point of view of energy considerations, to use the lowest possible temperatures, in pratice performance deteriorates if the temperature is too low. As a result 400C has proved to be the lower limit of what is practicable in most commercial processes currently in Zn 2 Fe (PO 4) 2 4H 2 01, H: hopite.

thereby improving the performance of the 2 use. In any particular process on any particular steel, if the temperature is too low the resultant phosphate coating may be incomplete and/or too light and/or of a crystal size that is too coarse and/or may be such as to result in yellow rust.

Some of the problems of existing processes can be overcome by increasing the amount of zinc but this reduces the ratio of P/P+H and leads to inferior paint film performance.

In the invention a phosphate coating is formed on an iron surface by treatment with a coating solution that contains phosphate, nitrate and fluoride and in which the amount of zinc is from 1.5 to 2. 5g/l, the weight ratio Zn/PO 4 is 0.08 to 0.21 and the free acidity value is from -0.1 to 0.1 when the amount of zinc is 1.5g/l and is from 0.1 to 0.5 when the amount of zinc is 2.5g/l and is at an interpolated intermediate value at intermediate amounts of zinc, and the solution includes accelerator to accelerate the dissolution of iron from the iron surface and to promote the deposition- of Phosphophyl lite. By the invention it is possible to obtain good coatings at low temperatures, for instance in the range 15 to 39C, preferably 20 to 35C, and so preferably the process is conducted at such a temperature.

Despite performing the process at such a temperature it is possible, by the invention, to obtain a phosphate coating that has a coating appearance, coating weight, coating crystal size and P/P+H ratio, and a paint film performance that has corrosion resistance and adhesion substantially equivalent to those obtained by normal present day processes conducted at a temperature of 40 to 4511C.

The iron surface that is treated in accordance w ith the invention is normally a steel sheet, and in particular is preferably a cold rolled steel sheet or is j A 3 a galvanised steel sheet that may have been galvanised on one side only or on both sides.

Before treatment with the coating solution, the iron surface should be clean and any of the conventional processes for cleaning iron surfaces prior to zinc phosphating may be used, including the conventional final water rinse. The cleaned surface is preferably subjected to surface conditioning and again this can be in conventional manner, for instance using an aqueous phosphate solution containing colloidal titanium.

The cleaned and conditioned metal surface is then treated with the zinc phosphate coating solution. Treatment is preferably by dip, preferably at temperatures of 15 to 391C, most preferably 20 to 350C.

is The coating solution is a zinc phosphate coating solution containing nitrate and fluoride. Particularly important parameters of the solution are its concentration of zinc, the inclusion of an appropriate amount of an accelerator that is additional to the nitrate that is in the solution, and the relationship between its.free acidity and the amount of zinc.

The amount of zinc is generally above the 1.5g/1 maximum that is conventional in many existing processes and as a result of choosing the quoted range it is possible- to increase the speed of depositing the appropriate phosphate coating.on the sheet steel or other surface. If the amount of Zn is less than 1.5g/1 a phosphate coating of the desired weight cannot be formed and if the amount is greater than 2.5g/1 the coating becomes too heavy and it is hard to obtain a high P/P+H ratio. Paint film adhesion and corrosion are consequently worse.

- The free acidity must be chosen having regard to the zinc concentration, and in particular the optimum free acidity increases as the zinc concentration increases.

4 As a result the pH of the bath can be increased and this broadens the range of conditions at which satisfactory precipitation of zinc phosphate will occur. In particular, the free acidity (measured as described below) should be f rom -0. 1 to +0. 1 when the amount of zinc is 1.5g/1 and should be from 0.1 to 0.5 when the amount of zinc is 2.5g/1 and at intermediate amounts of zinc the free acidity should be at interpolated intermediate values. This is demonstrated in the accompanying drawings in which the free acidity, in points, is plotted against a zinc concentration in the solution. The cross hatched area is the area within which the free acidity and zinc concentration should lie. If the free acidity is lower than the minimum shown in the drawing the pH of the bath could reach the point at which precipitation occurs and the bath composition becomes unbalanced. If the free acidity is above the maximum the rate of deposition of the coating is too slow and it becomes difficult to obtain dense coating crystals.

If the zinc and free acidity values are chosen in the manner indicated the resultant coating may have an unsatisfactory low ratio of P/P+H. For instance the ratio is preferably at least 0.7 and most preferably at least 0.8. A supplemental accelerator is therefore included in order to accelerate the dissolution of iron from the steel surface and to facilitate the deposition of Phosphophyllite on to the surface in the coating. In this way the reactivity of the process is such that the resultant phosphate coating gives high quality film performance. In particular, it is preferred to include nitrite accelerator in an amount of 3.5 to 6.5 points (measured as described below). This contrasts with the lower amounts of nitrite generally used in the conventional processes at 40 to 451C. If less than m 3.5pt accelerator- (NO 2 is used the rate of deposition of the coating can be too low, the dissolution of Fe may be suppressed and it can be hard to obtain a high P/P+H ratio. if more than 6.5 accelerator (NO 2 is used a blue colour, i.e., iron phosphate type coating tends to f orm.

The amount of phosphate should be below 10 and 20g/1. If it is below 10g11 the coating tends to become thin and non-uniform. Amounts above 20 911 tend to be uneconomic as they do not give further improvement.

The ratio Zn/PO should be between 0.08 and 0.21. 4 If the value is below 0.8 the tendency to form a phosphate coating is seriously worsened whilst if the value if above 0.21 there is no advantage, and the crystal size may become too coarse. Preferably the crystal size is mainly in the range 2 to 5pm.

Nitrate is present as oxidising agent, generally in an amount of 5 to 15g/1. If the amount is below 5g/1 1 there tends to be inadequate oxidising power and amounts higher than 15g/1 tend to provide no advantage and therefore to be uneconomic.

The presence of fluoride contributes to the uniform etching of the steel surface as well as to the densification of the phosphate crystals. The f luoride may be introduced as simple fluoride or as complex fluoride, for instance 'SiF 6 2- ' BF 4-' ZrF6 2- p TiF 6 2-, or HF 2 Mixtures may be used. The amount of fluoride is preferably from 0.5 to 1.5g/1 measured as F. If the amount is below 0. 5g/1 etching tends to be non-uniform and densification of the phosphate coating crystals may be rather poor. If the amount is above 1.5g/1 the phosphate coating tends to become too thin and the performance properties may be difficult to achieve.

In addition to containing zinc, phosphate, nitrate, fluoride and accelerator, all as described above, it is particularly preferred that the solution should also contain nickel and a small amount of ferrous iron, and often also manganese.

The inclusion of nickel contributes to the densification of the phosphate coating crystals and results in improved corrosion resistance and paint adhesion. The amount of nickel is preferably from 0.5 to 1.5g/1. If the amount is below 0.5 there is inadequate improvement in the density of the phosphate coating crystals and in the corrosion resistance and paint adhesion. Amounts above 1.5g/1 are uneconomic as they do not give any improvement.

Ferrous iron serves to elevate the pH value of the phosphate solution at which it starts to precipitate, and thus facilitates the formation of the phosphate coating.

The amount of ferrous iron is preferably in the range 2 to 20mg/1. Amounts below 2mg/1 give inadequate increase in the pH at which precipitation formation occurs and 1 1 results in retarded formation of the phosphate coating. If the amount is above 20mg/1 there is a tendency towards the formation of iron phosphate sludge and to the destruction of the balance of the treatment solution.

The inclusion of manganese results in' the improvement of the secondary adhesion of the paint film after water soaking and so is particularly preferred when such properties are required. If the amount of Mn 2+ is above lg/1 the formation of the phosphate coating may become harder to achieve and, in particular, its rate of formation may be reduced. If the amount is below 0.2g/1 there may be no benefit, and so preferably the amount of manganese is from 0.2 to lg/1.

The preferred process according to the invention is for the formation of a phosphate conversion coating on cold rolled steel sheets or galvanised steelsheets (galvanised on one or both sides) which has been cleaned and surface conditioned, and in t his process the sheet is dip treated with an acidic zinc phosphate solution containing 10 to 20g11 phosphate, 5 to 15g/1 nitrate. 0.5 to 1 5g/1 fluorine compound measured as F, 0.5 to 1.5g/1 divalent nickel, 2 to 20mg/1 trivalent iron, and 1.5 to 2.5g/1 divalent zinc, and the weight ratio zinc/phosphate is 0.08 to 0.21, the free acid is dependent on the concentration of zinc (generally in accordance with the drawing), the solution contains accelerator (generally nitrite) at a concentration of 3.5 to 6.5 points, and the process is conducted. at a temperture of 15 to 39C, preferably 20 to 30 or 35'C.

The dip treatment is generally conducted for 1.5 to 3 minutes on cold rolled steel sheet and for from 0.5 to 3 minutes on galvanised steel sheet. When a combination of cold rolled and galvanised steel sheets are being treated it is generally best to select a duration of dip treatment that is suitable for treating the cold rolled steel sheet component. Examples In the following examples two types of steel sheets were tested namely JIS-G-3141, SPCC (hereinafter referred to as SPC) and two-side, hot dip, galvanealed steel sheet (hereinafter referred to as PLATED). Their surfaces were cleaned using Fine Cleaner L4435 (product of Nihon Parkerizing Co.Ltd.), which is an alkaline cleaning solution, pH about 12 and total alkalinity 19 1 pt, as measured by titrating 10m1 sample solution with 0.1N H 2 so 4 solution as titrating solution and bromophenol blue as indicator.

The surfaces were then conditioned using lg/1 conditioner (Preparene 4028A, product of Nihon Parkerizing Co.Ltd.).

A phosphate coating solution was formed containing 16g/1 PO 4 3- 1 10g/1 NO 3' 1.0g/1 Ni 2+, 10mg/1 Fe 3+ and 1.0g/1 F (introduced as SiF 6) and containing Zn 2+ and Mn 2+ in amounts shown in Table 1 and having a zinc/phosphate ratio, a free acid pointage and a nitrite accelerator pointage as shown in Table 1.

Free acidity pointage is measured on a 10m1 sample, using Bromophenol Blue indicator. When colour development is yellow to yellow green, the figures correspond to the titration by 0.1N NaOH (end point:blue) and when it is blue violet the titration is made by 0.1N H 2 so 4 solution (end point:blue) where the figures are indicated negative.

Nitrite accelerator pointage is measured by saccharometric method (Sample solution is taken into a saccharometer with 50m1 volume for the determination). After adding 2-5g of sulfamic acid, turn the apparatus upside down so that sulfamic acid reaches to the top, then set it up as beofre. The volume in ml of gas f V generated at the inspection tube is the so-called pointage.

The overall process comprised cleaning the surfaces by dip for 3 mintues at 4WC 211 followed by water rinsing by spraying for 20 seconds at room temperature with city water followed by surface conditioning by dipping for 30 seconds at room temperature followed by phosphating by dipping for 2 minutes (120 seconds) at the temperature shown in Table 1. The coated surface was then water rinsed by spraying for 20 seconds with city water at room temperature and was then water rinsed by spraying for 20 seconds with deionised water having electrical conductivity 0.2liS/cm, followed by drying at 110C for 3 minutes.

The coated surface was then painted with three coats as follows. (1) Electrodeposit coating b.

C.

ELECRON 9400 (cationic electropaint: productof Kansai Paint Co.Ltd.) was used.

Bath temperature:. 2811C Voltage: 250 V Time: 180 sec. Coating thickness: 20g Water rinsing: city water, spray for 20 sec.

Deionised water rinsking:

DI water of 0.2gS/cm electric conductivity Room temperature, Spray for 5 sec.

d. Baking: 1750C, 30 min.

(2) Intermediate coating Melamine-alkyd base resin paint: AMILAC N-2 Sealer (pr duct of Kansai Paint Co.Ltd.) was coated by air spray to 30g dry film thickness. After setting for - 20 min., it was baked at 1400C for 30 min.

(3) Top coating Malamine-alkyd base resin paint: AMILAC WHITE M3 (product of Kansai Paint Co.Ltd.) was coated by air it spray to 404 dry film thickness. After setting for 10 - 20 min., it was baked at 140C for 30 min.

The appearance was observed visually. In all instances it was a dense and uniform phosphate coating.

Coating weight was measured by observing the difference in weight before and after stripping the coating with a stripping solution. The stripping solution was 50g/l anhydrous chromic acid for SPC and, for plated steel sheet, was 20g/l ammonium bichromate and 480g/l concentrated ammonia. The coating crystal size was determined by scanning electron microscope (JSM-T-100, product of Japan Electron Optics Laboaratory Co.Ltd.) in 4m units.

The ration P/P+H was calculated from P: diffraction intensity (cps) of phosphophyllite face (100) H: diffration intensity (cps) of hopeit face (020).

Diffraction intensities were determined by X-ray diffraction method (GEIGERFLEX 2028, product of RIGAKU DENKI CO.Ltd.) for the faces (100) of Phosphophyllite and (020) of Hopeit.

The performance of the paint film was recorded in various ways.

(1) Salt spray test Electropainted panels, after cross-hatching of the paint film, was subjected to 5% salt spray test (JISZ-2371) for 1,000 hr. The result was indicated with the width (mm) of blistering observed along the hatched. line.

7 (2) Composite cycle test Test pieces treated with phosphate conversion coating, cathodic electrodeposit coating, intermediate and top coatings then left stand at room temperature for 24 hr were immersed in deionized water for 120 hr, wherein attention was made so as to keep the water temperature at 40 + '16C and to-avoid the test pieces from coming into contact each other. Test pieces were then taken out and air-dried for 1 hr at room temperature. Thereafter, they were fixed at an angle 450 to horizontal level. with painted face upward. Next, the test pieces were subjected to paint film chipping damage caused by spontaneous falling down of 100 pieces of 1/4 inch nut from the height measuring 4.5 m from the centre of the line that crosses the centre of test piece and at rectangular direction to the horizontal face, where a guide tube with V' diameter served to pass the nuts through it. (total weight: 198 - J. 0.5 g).

These panels were subjected to 5% salt spray test (JIS-Z-2371) for 72 hr. Thereafter they were taken out and exposed to out-door atmosphere for 96 hr. The above procedure was repeated for 4 times, then the test pieces 'were further subjected to the same salt spray test for 72 hr as abovementioned. Next, the test pieces were taken out, scraped away from corrosion products developed on painted surface as well as from blisterd portions of paint film with a metallic scraper and then visually inspected for the degree of paintfilm peeling off.

0 A --- Peeled-off paint film to very slight extent B --- of to slight extent c to some extent 35 D to very great extent 1 All the examples of the invention were category A. (3) Secondary paint adhesion with water soaking Painted panels prepared by electrodeposit coating, intermediate coating and top coating were immersed into deionized water for 20 days at 4CC, then cross-scribed with cutter knife at 2 mm interval so as to reach the substrate metal surface and to form 100 squares. The panels were then subjected to cellophane tape peelingoff test. The result was indicated by the number of squares remaining without being peeled off. All the examples of the invention gave a value of 100 after 20 days and all the examples of the invention gave a value of 100 after 30 days except for example 2 plated (92) example 6 plates (90) and example 7 plated (95). The materials and conditions used are shown in the following table.

4 11 k W (Ai K) N tn 0 f- f-i ul Q ul Table 1 ul Example

SPC SPC Plated 71ai' d SPC Spc SPC Plated SPC Plat" --e Z nW 2.0 2.5 2.0 _2. 0 2.0 2.0 2.o n4M 04 0.09 0.13 If) 0.13 0.13 0.13 0.13 0. 13 M n1M 9 1k 0 0 0 0 0 - 0 0.5 Free acid Pointage 0 0. IS 0.30 0.15 0.15 0.15 0.15.0.15 Accelerator Pointage. 5.0 5.0 5. 0 3.5 6.5 5.0 5.o 5.0 reatment temp. Or- 28 28 28 28 28 20 34 2 coating 2 ight,S/' 2.1 2.2 -3.4 2.4 2.3 2.1 2.2 3.5 2.4 3.0 -2.2 3.3 crystal size. um 2-4 2-5 2-5 3-4 3-5 2-4 3-5 3-5 2-5 2-5 2-4 2-4 P/P+H ratio '6.93 0.87 0.82 0. W2 0.90 0.85 0.86 0.85 Salt. spray tent 2.0 2.1 1.0 -2.2 2.1 2.0 2.1 1.2 2.0 1.2 L, 1. . 1 2.2 1.1 If, 1 P.' Cl,Jk In comparative examples that differed by, for instance, having zinc or zinc/phosphate ratios or free acid pointage values outside the defined ranges coatings are obtained that are inferior in various respects and that, in some instances, give extremely poor paint film performance results.

By the invention it is therefore possible to obtain phosphate coatings having characteristics and paint film performance as good as those obtainable by conventional processes even though the coatings in the invention can be obtained at much lower temperatures than the temperatures of 40 to 450C or higher that are conventionally required in prior processes.

1 Qi i 1

Claims

1. A process in which a phosphate conversion coating is formed on an iron surface by treatment with a coating solution that contains zinc, phosphate, nitrate and fluoride and in which the amount of zinc is from 1. 5 to 2.5g/1, the weight ratio Zn:PO 4 is 0.08 to 0.21 and the free acidity value is from -0.1 to +0.1 when the amount of zinc is 1.5g/1 and is from 0.1 to 0.5 when the amount of zinc is 2.5g/1 and is at an interpolated intermediate value at intermediate amounts of zinc, and in which the solution contains accelerator to accelerate the dissolution of iron from the iron surface and to promote the deposition of Phosphophyllite.

2. A process according to Claim 1 in which the solution contains phosphate in an amount of 10 to 20g11, nitrate in an amount of 5 to 15g/1 and fluoride in an amount of 0.5 to 1.5g/1, measured as F.

3. A process according to claim 1 or claim 2 in which the solution also contains nickel in an amount of 0.5 to 201.5g/1.

4. A process according to any preceding claim in which the solution also contains ferrous iron in an amount of 2 to 20mg/1.

5. A process according to any preceding claim in which the said accelerator is nitrite and is present in an amount of from 3.5 to 6.5 points.

6. A process according to any preceding claim conducted at a temperature of 15 to 39C.

7. A process according to any preceding claim in which 30 the treatment is effected by dipping the iron surface in the solution.

8. A process according to any preceding claim in which the iron surface is cold rolled steel sheet or galvanised steel sheet.

10.. 't- Ib

9. A process in which a phosphate conversion coating is formed on a cold rolled steel sheet or galvanised steel sheet that has been cleaned and surface conditioned, the process comprising dipping the sheet at a temperature of 15 to 391C in an acidic solution containing 10 to 20g/1 phosphate, 5 to 15g/1 nitrate, 0.5 to 1.5g/1 fluoride compound, measured as F, 0.5 to 1. 5g/1 nickel, 2 to 20mg/1 ferrous iron and 1.5 to 2.5g/1 zinc and in which the solution contains nitrite accelerator in a concentration of 3.5 to 6.5 points and in which the ratio Zn/PO 4 is between 0.08 and 0.21 and the free acidity has a value of from -0.1 to +0. 1 when the amount of zinc is 1.5g/1 and a value of from 0.1 to 0.5 when the amount of zinc is 2.5g/1 and has an interpolated intermediate value at intermediate amounts of zinc.

10. A process according to any preceding claim in which the solution contains manganese in an amount of 0.2 to lg/1.

11. A solution suitable for forming a phosphate coating 20 and which is an acidic solution containing 10 to 20g/1 phosphate, 5 to 15g/1 nitrate, 0.5 to 1.5g/1 fluoride compound, measured as F, 0.5 to 1.5g/1 nickel, 2 to 20mg/1 ferrous iron and 1.5 to 2.5g/1 zinc and in which the solution contains nitrite accelerator in a concentration of 3.5 to 6.5 points and in which the ratio Zn/PO 4 is between 0.08 and 0.21 and the free acidity has a value of from -0.1 to +0.1 when the amount of zinc is 1.5g/1 and a value of from 0.1 to 0.5 when the amount of zinc is 2.5g/1 and has an interpolated intermediate value at intermediate amounts of zinc.

12. A solution according to clasim 11 and which contains 0.2 to lg/1 manganese.

Published 1988 all The Patent O:Mce. State House. 66 71 High Hollborn, London WC1R 4TP Further copies may be obt-q1ned from The Patent Office. Sajes Branch, St Ma-y Cray, Orpington, Kent BR5 3RD Printed by Multiplex techruques Itd. St Mary Cray, Kent. Con. 1/87.