GB2169620A - Phosphate coatings - Google Patents

Abstract

A heavy phosphate coating suitable as a base for a lubricant, prior to cold deformation of a ferrous surface, is formed by application of an aqueous acidic phosphating solution containing phosphate, organic nitro compound or nitrate (introduced as an inorganic nitrate), zinc, manganese or iron and, to increase the coating weight, hydroxylamine, is an amount of at least 0.01% by weight.

Description

SPECIFICATION Cold deformation process employing improved lubrication coating It is well known to form metal phosphate coatings by application of aqueous acidic phosphating solutions to ferrous surfaces. The surface of the metal reacts with the solution to form an integral layer of substantially insoluble crystalline phosphate on the surface of the metal.

These coatings can serve to improve corrosion resistance of the surface, as a base for the application of a paint, or as a preparation for cold forming (otherwise known as cold deformation) of the article.

The metal phosphate coating reduces the friction created during drawing or other cold forming operation, for instance between the metal surface and the die. In particular the conversion coating increases the ability of the metal to retain a uniform film of lubricant over the entire surface. This second lubricant reduces or prevents welding and scratching in drawing operations, and reduces metal to metal contact in cold forming operations. This reduction in friction allows shapes to be made by cold forming which would otherwise not be possible or practicable.

It is well known to form phosphate coatings using aqueous acidic solutions containing, for instance, zinc, phosphate and an accelerator. Often the accelerator is an organic nitro compound (for instance sodium nitro benzene sulphonate) or an inorganic nitrate. Many other oxidising accelerating agents are known.

Depending upon the ultimate use, in some processes it is desirable for the coating weight to be very low whilst in others it is desirable for the coating weight to be heavier.

In U.S. 2,743,204 it is described that the coating weight can be increased by adding a chelating agent to an aqueous acidic phosphating solution containing iron, zinc and/or manganese. It is stated that the solution may contain oxidising agents and examples that are given include potassium iodate, sodium metanitro benzene sulphonate, citric acid, hydroxylamine and nitrate although it is stated that nitrates are not the same most beneficial oxidising agents in zinc phosphate solutions. In an example the solution is formed of 20 g/l zinc hydrogen phosphate, 0.5 g/l hydroxylamine and up to 10 g/l acetic amino acid chelating agent.

In one commercial process for forming a phosphate coating on a ferrous surface, prior to application of a lubricant, it is known to use a solution of zinc dihydrogen phosphate, nitrate and chelating agent in order to give relatively heavy coating weights. Unfortunately the coating weight is difficult to control accurately and the coating is rather soft. If the amino acetic acid chelating agent is replaced by a hydroxy carboxylic chelating agent then a light coating is obtained.

In addition to being mentioned as an oxidising agent for use in the process described in U.S. 2,743,204, hydroxylamine has been proposed as a component of various other phosphating solutions. For instance U.S. Patent No.2,298,280 discloses the use of hydroxylamine in phosphate coating solutions as an accelerator to assist in depositing corrosion-resisting paint base phosphate coatings. However, the described compositions, and especially the relatively low levels of zinc and phosphate, render the exemplified solutions capable of providing lubricating coatings which are only marginally effective for undemanding cold forming processes (e.g., wire and tube pulling); such solutions would be wholly unacceptable for providing lubricating phosphate coating for metal surfaces about to undergo demanding cold deformation operations (e.g., the extrusion of large parts).

U.S. Patent No.2,702,768 describes the use of hydroxylamine in "noncoating phosphate" solutions, such as those which contain sodium, potassium and ammonium phosphates.

U.S. Patent No.2,928,762 discloses the use of hydroxylamine phosphate as a reducing agent in an orthophosphoric acid preliminary rinse solution in a phosphate coating process.

U.S. Patent No.4,003,761 discloses a process for applying a phosphate coating to a ferric surface. This process comprises spraying the surface with an aqueous acid solution having a pH of 4.3 to 6.5 and which contains an alkali metal or ammonium salt. The solution additionally contains an oxidizing or reducing agent accelerator, from 0.05 to 1.0 gram per liter of a C2-C4 alkylolamine, and a wetting agent.

U.S. Patent No.4,149,909 discloses accelerated phosphatizing compositions containing hydroxylamine sulphate (as a source of hydroxylamine in the use solution) to provide a hydroxylamine plus chlorate/ bromate accelerator combination.

U.S. Patent No.4,220,486 discloses conversion coating solutions having a pH of 5.5 to 6.5 which optionally employ 0.2 to 5.0 grams per liter of pyrazole, hydroxylamine or hydrazine compounds. These compounds are added to stabilise the use solutions.

Thus, it is known that hydroxylamine, (including hydroxylamine salts or complexes), can be employed to assist in depositing coatings designed to serve as a paint base, or as a corrosion-inhibiting base.

We have now found that it is surprisingly possible to obtain a heavy and controllable coating weight without the use of a chelating agent, and the associated disadvantages of that, if hydroxylamine is used in combination with an organic nitro compound or, preferably, a nitrate as oxidising accelerator. Thus it has now been surprisingly discovered that moderate to heavy phosphate coatings applied from a phosphate coating solution which is substantially free of chelating agent but which contains hydroxylamine are particularly well suited to act as lubricating (or lubricant-retaining) coatings on ferrous-base metal surfaces which are about to undergo cold deformation.It has also been surprisingly discovered that phosphate coatings particularly suited for prelubrication of articles about to undergo cold deformation can be deposited even in the presence of ferrous, or ferrous and ferric, ions, when hydroxylamine is employed.

A phosphate coating is applied to the surface of a ferrous-base metal article by contacting the surface with an aqueous acidic phosphate coating solution which contains an amount of hydroxylamine which is effective in increasing the rate at which the phosphate coating deposits from the solution. The resulting coating article is then subjected to cold deformation.

The metal article is preferably contacted with a second lubricating compound after the phosphate coating is applied. In addition to phosphate, the coating solutions employed preferably contain one or more of the following ions: zinc, manganese, nitrate, nickel, ferrous, ferric, copper, fluoride, or mixtures thereof.

A solution according to the invention for forming a phosphate coating on a ferrous surface is an aqueous acidic phosphating solution that is substantially free of chelating agent and that contains at least 0.75% metal selected from zinc, manganese, or iron, at least 1% phosphate, at least 0.5% accelerator that is an organic nitro compound or nitrate (introduced as an inorganic nitrate) and at least 0.01% hydroxylamine. All percentages are by weight unless otherwise specified.

A process according to the invention comprises forming a phosphate coating on a ferrous article by applying such a solution, applying a lubricant over the coating, and then cold forming the article.

The hydroxylamine can be added to the phosphate coating solution from any conventional source.

Preferably, the hydroxylamine source is a shelf-stable hydroxylamine salt or complex; many of these are items of commerce and frequently exist in a hydrated form. More preferably, the hydroxylamine source is a coating solution concentrate formulated with hydroxylamine sulphate ("HAS"), a stable salt of hydroxylamine. HAS is also referred to as hydroxyl-ammonium sulphate. Hydroxylamine sulphate may be represented by the formulae (NH2OH)2H2SO4 or (NH3OH)2.SO4.

Any effective amount of hydroxylamine from any source may be employed in these phosphate coating solutions and is at least 0.01%. By the term "effective amount", as used herein, is meant sufficient hydroxylamine (regardless of the source) to accelerate the coating process. That is, when two substantially identical phosphate coating solutions (differing only in that one contains an amount of hydroxylamine and the other does not) are compared, the solution with hydroxylamine either (1) increases the coating weight deposited over a given period of time or (2) decreases the time it takes the solution to deposit a given coating weight.

The amount that gives best properties is generally in the range 0.01 to 10%, and is usually above 0.05%. Amounts of up to 3%, for instance 0.05 or 0.1% to 3% are often preferred, with best results often being in the range 0.05 to 1%.

It is thought that the presence of hydroxylamine in the coating solutions contributes to the quality of lubricating or lubricant-retaining phosphate coatings by increasing the level of metal (especially zinc) which is present in the resulting coating. This increases the lubricating properties of the phosphate crystals themselves. More importantly, however, the increased level of zinc (or other metal) in the coating increases the ability of the coating to be reactive with a second lubricating agent, particularly those which contain a fatty acid or fatty acid soap. For example, when a phosphate coating containing zinc is contacted with a second lubricant containing sodium stearate, the available zinc reacts with stearate moeities. The resulting zinc hydroxy stearate is an excellent lubricant, much better than sodium stearate.

Thus, when more available zinc is brought down in the coating (per gram of coating weight), more zinc stearate will react when the coating is contacted with a sodium or potassium stearate (soap) containing lubricant. This increase in zinc hydroxy stearate significantly increases the ability of the surface to retain the second lubricant - the additional zinc hydroxy stearate also significantly increases overall lubricity.

The preferred phosphate coating solutions for use in the invention contain zinc, manganese, or mixtures thereof. Of these, zinc (and especially the so-called high-zinc phosphating treatment solutions) are most preferred.

The amount of the metal, and especially the amount of zinc, is preferably at least 1%, most preferably at least 2%. Typically the amount is up to 7.5% with best results generally being obtained in the range 1 to 5.5% zinc. For some processes amounts of 1 to 3% zinc are satisfactory.

The solutions typically contain at least 1.5%, and often at least 2%, phosphate. The amount is generally not more than 8%, usually not more than 7% and best results are generally obtained when the amount is from 2 to 6% although for some purposes 2 to 4% is satisfactory. These percentages are all by weight of H3PO4.

The oxidising accelerator is preferably an inorganic nitrate in an amount sufficient to give at least 0.5% by weight nitrate (NO3) in the solution. However other compounds that will be sufficiently stable and active in the presence of hydroxylamine may be used, especially the organic nitro compound accelerators that are known, for instance sodium nitro benzene sulphonate. It is, however, strongly preferred to use nitrate, generally as the sole accelerator with the hydroxylamine.

The amount of nitrate is generally at least 1%. It can be up to 10% but amounts of up to 7.5% are usually sufficient, and best results are generally obtained with amounts of 3 to 7%.

When both nitrate and phosphate are present in the phosphate coating solutions employed in the processes of the present invention, the quotient of the concentration of nitrate [NO3j over the concentration of the phosphate [P04], or [NO3j/[PO4] is about 0.3 to about 6.0, and more preferably about 0.5 to about 5.0 and most preferably it is about 0.9 to about 4.5.

One of the surprising features of the phosphate coating solutions employed in the practice of the present invention is the ability of these solutions to deposit coatings possessing a high concentration of zinc, even in the presence of ferrous (and ferric) ions. Thus, ferrous can be employed, either by deliberate addition, or by generation from the ferrous-base metal article being coated. If ferrous ions are present, it is preferred that they be present at a level of 0.05% to about 0.6% by weight. It will be appreciated that if no ferric iron is present, total iron can be used to determine this concentration; in the alternative, ferric and ferrous ion levels in solution must be determined.

It is often preferred for the solutions to contain nickel. The nickel is preferably present at a level of about 0.005% to about 0.1%, and even more preferably present at a level of about 0.01% to about 0.05%.

The solutions preferably have pH in the range 1.8 to 2.5.

The metal articles that may be coated in the invention are ferrous-based, and are those which can be deformed at temperatures below their recrystallization temperatures. Preferred articles are steel articles with a carbon content less than about 1.0%, and preferably about 0.05 to about 0.6%. However the improved lubricant coatings provided in the cold deformation processes of the present invention allow the deformation of steels with higher alloy content, and greater hardness, than would otherwise be practicable.

The articles that are to be coated are usually in the form of a blank, a slug or a preform.

Prior to contact with the phosphating solution the metal article may be subjected to one or more conventional pretreatment steps, such as cleaning, pickling or rinsing.

The treatment with the phosphating solution is preferably conducted whilst the present invention the solution is maintained at a temperature of about 55"C to about 96"C, and more preferably at a temperature of about 72"C to about 88"C. The solution is preferably maintained at a pH of about 1.8 to about 2.5 while in this temperature range.

The coating solution can be applied by conventional method. It is preferably applied by flooding or immersion, and most preferably immersion. The time of exposure or contact times for immersion can be from about 0.5 minutes to about 30 minutes, and is preferably about 5 minutes to about 15.

After the surface of the ferrous-base metal article has been contacted with the phosphate coating solution, it is preferably subjected to a dilute, alkaline neutralising rinse. It can be dried.

Following rinsing, the coated article is contacted with a second, conventional cold forming lubricant.

This can be done immediately after coating (or rinsing), at press side immediately before formation, or during part or all of the cold deformation process (conjointly).

The second lubricant can be a soap, oil, drawing compound, or an emulsion of an oil and fatty acid, fatty acid salt, or soap. The second lubricant preferably contains c C8-C,s fatty acid or fatty acid salt or soap, generally at a level of about 3 percent to about 15 percent by weight. Most preferably, the second lubricant contains a soap selected from sodium stearate, potassium stearate, or mixtures thereof. These soaps are preferred because of their ability to react with the increased zinc levels found in the phosphate coatings employed in the present invention. The resulting zinc hydroxy stearate provides a highly preferred lubricant for cold deformation processes.

Drying after processing or between operations may be effected by conventional techniques such as forced air or flash drying.

The lubricated article may be subjected to conventional cold formation. For instance the article (e.g., blank, slug or preform) that is to be deformed deformation enters the deformation process at a temperature appreciably below the recrystallisation temperature, and preferably within 100 C of room temperature. Any subsequent rise in temperature is primarily due to the friction and/or heat from work hardening caused during deformation. Typical methods are cold extrusion, cold heading, and wire and tube pulling deformation operations.

The weight of the phosphate coating will be chosen having regard to the severity of the deformation process and the size of the article. The desired weight may be achieved by appropriate formulation of the phosphate coating solution and by maintaining contact for an appropriate duration. Preferably the coating weights are in the range of about 250 to about 6000 milligrams of coating per square foot of metal surface. coating weights of about 350 to about 4500 milligrams per square foot are most preferred, with coating weights of about 500 to about 3500 milligrams per square foot being even more preferred.

The following are examples.

Example 1 The following example demonstrates the preparation and use of a phosphate coating solution for use in the practice of the present invention.

A fresh phosphating solution containing 4.05 percent zinc, 5.00 percent phosphate (PO4), 5.55 percent nitrate (NO3), 0.01 percent nickel and a total acid of 60 points (5.0 ml sample titrated with 0.1N NaOH to phenolphthalein endpoint, total acid points being equal to milliliter of 0.1 N NaOH used to endpoint) was heated to 830C. Once at 830C, 5.0 gm/I (0.5 percent) hydroxylamine sulphate (H.A.S.) was added. After allowing 5 minutes for equilibrium to be reached, 0.007 percent sodium nitrate (NaNO3) was added. After another 5 minutes, 10cm x 15cm x 14 gauge, cold rolled steel, pickled panels, two (2) at a time, were coated for five (5) minute immersions every ten (10) minutes (approximate loading rate of 3.8 ft2 surfacer.

gal). Zinc phosphate coating weight analysis revealed that not only was there a two-fold increase in coating weight, but also that the coating weights do not decrease as rapidly as seen using the same bath and conditions but without H.A.S.Crystal morphology/composition remained the same when using H.A.S., with P-ratios [ratio of phosphophyllite: (phosphopyllite + hopeite) as measured by x-ray crystalogrophy)] being about 0.105 versus 0.165 for the process without H.A.S.This indicates an acceptable level of zinc in the phosphate coating. [Reactivity with reactive soap lubricants, however, was lower when H.A.S. was incorporated into the phosphating solution when the second lubricant was applied approximately 30 days after coating].

Articles treated with the solutions described above can then be subjected to conventional weakly alkaline rinsing. A reasonable time after rinsing (within about 5 hours) a second sodium stearate-containing lubricant is applied. After application of the second lubricant, the article is subjected to flash drying, and the excess soap is allowed to remain. The article is then subjected to a conventional cold deformation process such as extrusion with excellent results. Such articles are more efficiently subjected to conventional cold deformation processes than articles coated by conventional phosphate-coating solutions.

Example 2 A phosphate coating solution concentrate is prepared for use in the process of the present invention as follows: Concentrate A INGREDIENT PARTS BY WEIGHT Water 360.5 Zinc Oxide (80.3% Zn) 159.0 Nitric Acid (42" Be' 67% HNO3) 255.5 Phosphoric Acid (75% H3PO4) 216.7 Hydroxylamine Sulfate 6.0 Nickel Nitrate (13.9% Ni; 29.37.NO3) 2.3 1000.0 This concentrate is then diluted to prepare a use solution; this dilution is preferably done by using a ratio of 175 pounds of concentrate for every 100 gallons (U.S.) of final use solution, or 210 grams per liter of use solution.

During use, the use solution prepared from Concentrate A can be replenished or revitalised with the following concentrate.

Concentrate B Water 302.2 Zinc Oxide (80.3% Zn) 132.3 Nitric Acid (42" Be' 67% HNO3) 143.1 Phosphoric Acid (75% HaPO4) 400.8 Nickel Nitrate (13.9% Ni; 29.3% NO3) 1.6 Hydroxylamine Sulfate 20.0 1000.0 This replenisher can be employed when the ratio of total acid:free acid in the use solution rises above the desired level; Concentrate A can be used to revitalise the use solution if the ratio falls below the desired value.

Claims (18)

1. A solution for forming a phosphate coating on a ferrous surface and which is an aqueous acidic phosphating solution that is substantially free of chelating agent and that contains at least 0.75% metal selected from zinc, manganese and iron, at least 1% phosphate, at least 0.5% accelerator that is an organic nitro compound or nitrate (introduced as an inorganic nitrate) and at least 0.01% hydroxylamine, all percentages being by weight.

2. A solution according to claim 1 in which the said metal is zinc, the accelerator is nitrate and the solution contains 0.05 to 10% hydroxylamine.

3. A solution according to claim 2 in which the amount of zinc is from 1 to 7.5%.

4. A solution according to claim 2 or claim 3 in which the amount of nitrates from 1 to 7.5%.

5. A solution according to any of claims 2 to 4 in which the amount of phosphate is from 1.5 to 8%.

6. A solution according to claim 2 containing 1 to 5.5% zinc, 2 to 6% phosphate and 3 to 7% nitrate.

7. A solution according to any of claims 2 to 6 containing 0.1 to 3% hydroxylamine.

8. A solution according to any of claims 2 to 7 containing from 0.05 to 0.6% ferrous iron.

9. A solution according to any of claims 2 to 8 containing at least 0.005% nickel.

10. A solution according to claim 9 containing from 0.005 to 0.1% nickel.

11. A solution according to any preceding claim having pH 1.8 to 2.5.

12. A solution according to any preceding claim in which the quotient NO3/PO4 is 0.3 to 6.

13. A solution according to claim 12 in which the quotient is 0.9 to 4.5.

14. A process comprising forming a phosphate coating on a ferrous article by applying a solution according to any preceding claim to the article, applying a lubricant over the coating and then cold forming the article.

15. A process according to claim 14 in which the lubricant contains a fatty acid or fatty acid ssalt or soap.

16. A process according to claim 14 in which the lubricant contains sodium or potassium stearate.

17. A process according to any of claims 14 to 16 in which the phosphate coating weighs 250 to 6000 mg/ft2.

18. A process according to claim 17 in which the phosphate coating weighs 500 to 3500 mg/ft2.