WO2000004207A1

WO2000004207A1 - Degreasing and zinc phosphate conversion treatment of oily metal substrates in a single process operation

Info

Publication number: WO2000004207A1
Application number: PCT/US1999/014149
Authority: WO
Inventors: Hitoshi Ishii
Original assignee: Henkel Corporation
Priority date: 1998-07-16
Filing date: 1999-07-16
Publication date: 2000-01-27
Also published as: ID23055A

Abstract

A zinc phosphating composition that contains zinc ions, phosphate ions, and conversion accelerant makes it possible to degrease oil coated metal substrates and form a high quality phosphate coating on them in a single process operation when the composition contains from 0.1 to 10 g/l of mineral oil that is emulsified by polyoxy-ethylene alkyl ether surfactant with an HLB of 12 to 17.

Description

DEGREASING AND ZINC PHOSPHATE CONVERSION TREATMENT OF OILY METAL SUBSTRATES IN A SINGLE PROCESS OPERATION

FIELD AND BACKGROUND OF THE INVENTION

The invention generally relates to liquid compositions and processes for zinc phosphate conversion treatment (hereinafter abbreviated in some cases as zinc phosphating) of oil¹ -contaminated surfaces of metal substrates, for example, cold-rolled

5 steel sheet, hot-rolled steel sheet, castings, steel wire and cable, steel tube and pipe, etc. More particularly, the invention relates to the degreasing and zinc phosphate conversion treatment in a single process operation, while still providing a zinc phosphate conversion coating whose appearance and coating weight are the same as when de- greasing and zinc phosphate conversion treatment are carried out in separate processes 0 according to the prior art. The zinc phosphate conversion coatings produced with the treatment bath according to the present invention can function, for example, as an un- derpaint coating, as an undercoating for lubrication in plastic working processes, and as an antirust coating.

Zinc phosphating is currently in wide use for the purpose of equipping steel with s high levels of rust resistance, paintability, and lubricity. The phosphate conversion treatment sequence typically consists of (1 ) degreasing, (2) water rinse, (3) conversion (zinc phosphating), (4) water rinse, and (5) drying in the sequence given. In some cases the water rinses (2) and (4) may take the form of a multistage water rinse or a hot water rinse. 0 When a zinc phosphate conversion coating is to function as an underpaint coating, it desirably takes the form of a uniform, fine, and dense coating present at a coating weight of about 2 to 5 grams of coating per square meter of coated surface, this unit of coating weight being hereinafter usually abbreviated as "g/m²". Since a coating of uniform, fine, and dense coating crystals is quite difficult to achieve using only variations in 5 conditions during the zinc phosphating process itself, a titanium colloid surface conditioning treatment will often be carried out just before zinc phosphating. The zinc phosphating process itself will generally be run using an aqueous liquid treatment composition, often hereinafter called a "bath" for brevity, even though it may be brought into contact with the metal substrate to be coated by any suitable mechanical means 0 such as spraying instead of or as well as by immersion, that contains 10 to 20 grams per

¹E.g., antirust oil, press oil, cutting oil, or the like. liter (hereinafter usually abbreviated as "g L") of phosphate ions and 1.5 to 5.0 g/L of zinc ions. This treatment is typically implemented at 35 to 60 °C by spraying or dipping the steel stock.

When a zinc phosphate conversion coating is to function as an undercoating in support of lubrication, it desirably takes the form of a thick coating with a coating weight of about 5 to 20 g/m². In addition, a lubrication treatment, such as treatment with a soap or solid lubricant, will normally be carried out after the water rinse process (4) or drying process (5). Zinc phosphating is usually run in this field of application by dipping the steel stock into a 60 to 90 °C bath containing 10 to 50 g/L of phosphate ions and 5 to 20 g/L of zinc ions. The treatment conditions used for the formation of an antirust coating are about the same as for formation of a lubrication undercoating, but in this latter field of application an antirust treatment, such as painting with antirust oil, is carried out after conversion coating in place of the lubrication treatment.

Regardless of the field of application, the execution of zinc phosphating almost inevitably must accommodate a workpiece whose surface carries antirust oil or an oil originating with such upstream processes as cutting or press operations. In such cases the degreasing process (1 ) has generally been regarded in the prior art as essential for practical success.

A first problem associated with treatment using the above-described treatment processes is that the overall sequence necessitates large-scale treatment facilities and large space requirements. While the overall treatment is composed of 5 or 6 processes, the alkaline degreasing and water rinse processes are themselves frequently implemented as multistage processes in order to improve the cleaning efficiency. This necessitates additional equipment expenses and also impairs the productivity since relatively long periods of time are required to pass through all of the treatment processes.

A second problem is the large number of parameters requiring management. A wide range of parameters must be managed, for example, the alkalinity (total alkalinity, free alkalinity) of the degreasing bath in the case of alkaline degreasing, and the concentration (total alkalinity, titanium concentration) of the surface conditioning bath when a titanium colloid surface conditioning is used. This situation imposes a high burden from a process standpoint. Moreover, a relatively high cost burden derives from the fact that reagents are consumed in each process. The degreasing bath is also consumed by its entrainment by the workpiece and resulting carry over to the next process (water rinse) and by its periodic disposal/renewal. The surface conditioning bath is also consumed by carry over and disposal/renewal. In addition, the treatment solutions are themselves unstable and frequently must be continuously partially renewed (e.g., by autodrainage), again producing another source of consumption.

A third problem is the large amount of waste water discharge produced by the water rinses. A post-degreasing water rinse must be implemented due to the problems that would be caused by carry over of degreasing bath into the surface conditioning bath or zinc phosphate conversion bath. A post-conversion water rinse must also be provided because the presence of the treatment bath on the workpiece surface has negative implications for use as an antirust coating, underpaint coating, or lubrication undercoating. Large amounts of rinse water are discharged from these two systems (post-degreas- ing and post-zinc phosphating), which imposes a fairly large load on waste water treatment.

Each of these issues, i.e., saving on equipment costs, saving on space, simplification of process management, and reduction in the load on waste water treatment, has recently been under intense scrutiny in the surface treatment industry, and a resolution of these issues would accrue major benefits.

Methods that run degreasing at the same time as zinc phosphating have already been investigated in order to solve the three problems discussed above. One such method is disclosed in Japanese Laid Open (Kokai or Unexamined) Patent Application Number Sho 63-227786 (227,786/1988). Disclosed therein is a phosphating method in which a zinc- or zinc alloy-plated steel sheet molding is dipped in an acidic treatment bath that contains 0.3 to 1.0 g/L of Zn²⁺, 0.4 to 3.5 g/L of Ni ⁺, 0.1 to 3.5 g/L of Mn²⁺, 10 to 20 g/L of PO₄ ^3", 0.5 to 1.5 g/L of P, < 15 g/L of O₃-, 0.7 to 6 g/L of surfactant, and 2 to 6 points of accelerant (NO₂ ^')-

Degreasing and conversion treatment can be carried out in a single process operation using this method, and it can also provide improvements in the cationic electro- coating performance. This method, however, is not applicable to materials other than zinc-containing metals. Attempted application of this teaching to steels has provided only an inadequate coating formation and a corresponding inability to satisfy the various property requirements. Otherwise, Japanese Laid Open (Kokai or Unexamined) Patent Application Number Hei 8-302477 (302,477/1996) discloses a conversion bath that contains zinc ions, phosphate ions, conversion accelerant in the form of 50 to 1 ,500 ppm of organoperoxide, and optionally surfactant. This document also discloses a method that uses this conversion bath to coat metal surfaces with a zinc phosphate conversion coating that con- tains micro sized crystals. However, the simultaneous execution of degreasing and zinc phosphate conversion on heavily oil-contaminated steel using this method still does not result in a satisfactory film formation.

In sum, then, although there is strong desire in the art to shorten the current de- greasing-conversion-drying treatment sequence and thereby reduce equipment and reagent costs and simplify bath management, this desire remains unsatisfied at present as the result of various technical obstacles. In other words, although the concept of degreasing and zinc phosphating in a single process operation has been proposed in the past, a conversion bath that can provide a satisfactory degreasing and a satisfactory zinc phosphate conversion treatment in a single operation has yet to be obtained with respect to steel, especially when the steel has been heavily treated with oil. This invention is directed to solving the above-described problems with the prior art. One major object of the invention is to provide a bath for degreasing and zinc phosphating that enables the degreasing and conversion on oil-contaminated steels in one and the same process operation and that thereby achieves at least one, or most preferably all, of the following objectives: shortening of the treatment processes, space conservation, enhanced productivity, and a reduction in reagent costs. BRIEF SUMMARY OF THE INVENTION

It has been discovered, and forms part of the basis of this invention, that the addition of surfactant with a specific chemical structure, specifically polyoxyethylene alkyl ether, resulted in better degreasing and zinc phosphating than did the addition of other surfactants. However, even with the use of some polyoxyethylene alkyl ether surfactants, the degreasing performance and properties of the zinc phosphate coating (uniformity, coverage, etc.) were still unsatisfactory compared to the separate execution of degreasing and zinc phosphating. It was eventually found that in order to improve the aforementioned properties the hydrophile-lipophile balance, hereinafter usually abbrevi- ated as "HLB", must be in the range from 12 to 17. The HLB is determined, in a manner known in detail to those skilled in the art, by the number of carbons in the alkyl moiety and by the number of moles of polyoxyethylene addition in the polyoxyethylene alkyl ether.

The inventors also found that a uniform zinc phosphate coating can be formed — without impairing the degreasing performance — when mineral oil is present in a particular concentration in a treatment bath containing polyoxyethylene alkyl ether with the particular HLB value referenced above. This stands in contrast to the general trend that better degreasing performances are obtained when the concentration of oil is low. For example, when an alkaline degreaser is freshly prepared and contains no oil, it degreases more effectively than it does after a substantial concentration of oil is incorporated into it during the treatment of oil-contaminated steel. DETAILED DESCRIPTION OF THE INVENTION AND ITS PREFERRED EMBODIMENTS

A bath according to this invention for the degreasing and zinc phosphate conversion treatment of oil-contaminated steel in a single process operation (abbreviated in some cases below as the degreasing/zinc phosphating bath) comprises, preferably consists essentially of , or more preferably consists of water, zinc ions (Zn²⁺), phosphate ions (PO₄ ³ ), conversion accelerant and from 0.1 to 10 g/L of mineral oil emulsified by polyoxyethylene alkyl ether with an HLB of 12 to 17.

In order to obtain an even better degreasing performance and an even more uni- form zinc phosphate conversion coating, the aforesaid degreasing/zinc phosphating bath preferably contains from 0.5 to 5.0 g/L of the aforesaid polyoxyethylene alkyl ether and from 20 to 300 weight parts mineral oil per 100 weight parts of the polyoxyethylene alkyl ether.

When used to form an underpaint coating, the degreasing/zinc phosphating bath according to the present invention preferably contains from 1.5 to 5.0 g/L of zinc ions, 10 to 20 g/L of phosphate ions, and 0.5 to 4.0 points of free acidity. When used to form a lubrication undercoating or an antirust coating the said bath preferably contains from 5 to 20 g/L of zinc ions, 10 to 50 g/L of phosphate ions, and 4.0 to 15 points of free acidity.

When nitrite ions are employed as the conversion accelerant, the treatment bath according to the present invention preferably is prepared as a two-part formulation, using a concentrate or partial working solution that contains all of the needed ingredients except the nitrite ions and adding the latter immediately before the bath begins to be used, considering the instability of the nitrite ions in the presence of acid.

The degreasing/zinc phosphating bath according to the present invention comprises 0.1 to 10 g/L of mineral oil, which is emulsified by polyoxyethylene alkyl ether with an HLB of 12 to 17, and also essentially contains zinc ions, phosphate ions, and conversion accelerant. The specified mineral oil content refers to the mineral oil alone exclusive of the polyoxyethylene alkyl ether.

The alkyl group in the subject polyoxyethylene alkyl ether is not critical, but C₈ to C₁₄ straight-chain and branched alkyl is preferred.

With regard to the HLB of the subject polyoxyethylene alkyl ether, values below 12 result in poor degreasing, while values in excess of 17 cause uneven deposition of the zinc phosphate coating.

The mineral oil can be exemplified by machine oils, kerosene, light oils, cutting oils, turbine oils, antirust oils, press oils, and spindle oils. These can be specifically exemplified by the mineral oil products specified in various Japanese Industrial Standards ("JIS"), i.e., the machine oils in JIS K-2238, the kerosene in JIS K-2203, the light oils in JIS K-2204, the cutting oils in JIS K-2241 , the turbine oils in JIS K-2213, and the antirust oils in JIS K-2246. The presence of antirust additives and/or extreme- pressure additives in the antirust oils and press oils has no effect on the application of the present invention.

A mineral oil concentration below 0.1 g/L causes uneven deposition of the zinc phosphate coating, while a mineral oil concentration in excess of 10 g/L causes poor degreasing. The most preferred concentration range is 0.2 to 7.5 g/L.

In order to maintain an excellent degreasing performance and induce a more uniform deposition of the zinc phosphate coating, the polyoxyethylene alkyl ether concentration is preferably from 0.5 to 5.0 g/L and the mineral oil concentration is preferably adjusted into the range of 20 to 300 weight parts per 100 weight parts of polyoxyethylene alkyl ether.

The conversion accelerant functions to raise the etching capacity of the acid and accelerate the conversion reactions. The conversion accelerant can be, for example, nitrite ions, hydroxylammonium ions, chlorate ions, nitrobenzene sulfonate ions, hydrogen peroxide, and so forth, but the nitrite ions and hydroxylammonium ions are the most preferred. The overall preferred concentration range for the accelerant is from 0.05 to 2.0 g/L, while the nitrite ions are preferably used at from 0.050 to 0.20 g/L and the hy- droxylammonium ions are preferably used at from 0.5 to 2.0 g/L. The nitrite ions are preferably furnished by sodium nitrite, while the hydroxylammonium ions are preferably furnished by hydroxylammonium sulfate or hydroxylammonium phosphate.

The zinc phosphating bath used by the present invention may contain the various additive components heretofore used in the art, such as an auxiliary etchant for the pur- pose of rupturing the oxide film on the metal surface and assisting the etching reaction and auxiliary metal ions for the purpose of improving the adherence of the coating and improving its chemical stability with respect to acid and base.

The said auxiliary etchant can be, for example, fluoride ions or fluorosilicate ions, which may be added to the bath as the sodium or ammonium salt or as the free acid (hydrofluoric acid, fluorosilicic acid, etc.). The preferred concentration for the auxiliary etchant is 0.1 to 2.0 g/L.

In regards to the auxiliary metal ions, one can cite the use of the nickel ions (Ni²⁺), copper ions (Cu²⁺), and cobalt ions (Co2⁺), which function mainly to improve the adherence of the coating, and/or the use of the manganese ions (Mn²⁺), magnesium ions (Mg²⁺), and calcium ions (Ca²⁺), which function mainly to improve the chemical stability of the coating (resistance to acid and base). These can be furnished to the bath as their nitrates, phosphates, and the like. The concentration of the auxiliary metal ions is preferably 0.005 to 0.050 g/L in the case of the copper ions and 0.1 to 3.0 g/L for the rest.

The presence of these components in the treatment bath according to the present invention is entirely unproblematic because this bath permits each component to fully manifest its particular functional activity.

The generally preferred concentrations in the treatment bath according to the present invention are 10 to 50 g/L for the phosphate ions, 1.5 to 20 g/L for the zinc ions, and 0.5 to 15 points for the free acidity.

However, there are also even more preferred ranges for these concentrations depending on the particular application for the produced coating.

Thus, when the treatment bath according to the present invention will be used to form an underpaint coating, the deposition is desired of a thin, homogenous, fine, and dense zinc phosphate coating with a coating weight of about 2 to 5 g/m², and in such a case the phosphate ions concentration is preferably 10 to 20 g/L, the zinc ions concen- tration is preferably 1.5 to 5.0 g/L, and the free acidity is preferably adjusted to about 0.5 to 4.0 points.

When, on the other hand, the treatment bath according to the present invention will be used to form a lubrication undercoating, the deposition of a thick film with a coating weight of about 5 to 20 g/m² is desired, and in this case the phosphate concentration is preferably 10 to 50 g/L and the zinc ions concentration is preferably 5 to 20 g/L. When the instant treatment bath will be used as an antirust coating, the phosphate ions and zinc ions concentrations should be about the same as for formation of a lubrication undercoating. In both cases the free acidity is preferably adjusted to about 4.0 to 15 points and particularly preferably is adjusted to about 5.0 to 13 points. The free acidity is a substitute value employed in place of the pH to indicate the acidity of the treatment bath. The free acidity is used because the accurate and reproducible measurement of pH with a pH meter (glass electrode) is quite difficult at low pH values on the order of 2 to 3. The free acidity is measured by taking a 10 milliliters (hereinafter usually abbreviated as "mL") sample of the treatment bath and titrating the sample to neutrality with 0.1 N aqueous sodium hydroxide, using bromphenol blue as the indicator. The free acidity is the number of mL of titrant required to change the color from yellow to blue. For example, a free acidity of 1 point indicates that 1 mL of titrant was consumed in the titration.

The free acidity of the treatment bath according to the present invention can as a general matter be adjusted into the desired range, if any such adjustment is needed, by using nitric acid or sodium hydroxide, although the particular method of adjustment is not critical so long as the desired free acidity value is obtained.

The method for preparing the treatment bath according to the present invention is also not critical. As an example of bath preparation, an aqueous solution can first be prepared that contains the phosphate ions, zinc ions, conversion accelerant, auxiliary etchant (optional), auxiliary metal ions (optional), and so forth; the polyoxyethylene alkyl ether (HLB 12 to 17) and mineral oil can be added to this aqueous solution; and emulsification can then be carried out on the entire mixture to give the treatment bath. The conversion accelerant can also be added after emulsification, and this approach is in fact preferred when nitrite ions are used as the conversion accelerant. The post-emul- sification addition of nitrite ions is preferred because the nitrite ions spontaneously decompose in acidic aqueous solutions such as zinc phosphate treatment baths and their decomposition can be accelerated by the agitation carried out during emulsification.

Because of this instability associated with use of the nitrite ions as the conversion accelerant, the treatment bath according to the present invention may be formulated as a two-part bath for degreasing and zinc phosphate conversion treatment of oil-contaminated steel in a single operation. In such a two part formulation, one of the parts is an aqueous solution that contains nitrite ions, the other part is an aqueous solution that contains zinc ions, phosphate ions, and 0.1 to 10 g/L of mineral oil emulsified with polyoxyethylene alkyl ether with an HLB of 12 to 17, and intermixing of the two parts affords a single degreasing/zinc phosphating bath as described hereinabove. Such a two-part degreasing/zinc phosphating bath will typically be mixed by the user at the point of use to give the single, fully constituted bath.

The major targets for application of the treatment bath according to the present invention are electrochemically active metals, more particlularly carbon steel, other non- stainless steels, and alloys that are at least 50 % of either zinc or aluminum, in all instances having surfaces at least partially coated with oil. The invention is most particularly applicable to non-stainless steels, and the oil on the surface can be, for example, a deliberately used antirust oil or press oil or any kind of oily or greasy soil of unknown origin, and the oil-coated steel can be, for example, cold-rolled steel sheet, hot-rolled steel sheet, castings, steel wire or cable, or steel tube or pipe. The steel is preferably plain steel, and the invention is not as advantageously applicable to alloy steels that contain large amounts of alloying component, such as stainless steels. The adhering oil may be mineral oil, vegetable oil, animal oil, etc., but the oils found coated on steels are most typically mineral oils. The adhering oil is preferably a mineral oil since it can then function as a source of the oil in the treatment bath. The amount of oil adhering to the steel is not critical. However, this invention is most advantageous for, and therefore is preferably used on, substrates on which the amount of surface oil is at least, with increasing preference in the order given, 0.05, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, or 1.00 grams of oil per square meter of surface. (If the soil is spotty rather than uniform, these values should be applied only to part of the surface that is coated with oil.) Moreover, while the treatment bath according to the present invention acts with particularly good effect on steels, it can also fully manifest its activities on other metals, such as zinc-plated materials, zinc stock, and aluminum stock.

Spray and dipping techniques are preferably used as the techniques for treating oil-contaminated steel using the treatment bath according to the present invention. The treatment temperature, treatment time, and coating weight will vary as a function of the particular application for the coating.

When the treatment bath according to the present invention is used to form an underpaint coating, the deposition is desired of a thin, uniform, fine, and dense zinc phosphate coating at a coating weight of about 2 to 5 g/m², more preferably from 3.1 to 4.5 g/m², or still more preferably from 3.1 to 3.7 g/m². In this case the temperature of the treatment bath during treatment is preferably 35 to 60 °C. Degreasing and/or deposition of the zinc phosphate coating may be inadequate at a temperature below 35 °C, while temperatures in excess of 60 °C are uneconomical. The treatment time in this case is preferably from 30 seconds to 10 minutes, or more preferably, if the treatment method is by dipping, from 90 to 180 seconds. Times below 30 seconds can result in inadequate degreasing and/or conversion. Degreasing and the conversion reactions are completely finished by 10 minutes, and contact between the treatment bath and workpiece for longer times provides no additional development of the reactions and is thus economically fruitless and impairs the productivity. When the treatment bath according to the present invention is used to form a lubrication undercoating, the deposition of a thick zinc phosphate coating with a coating weight of about 5 to 20 g/m² is desired. In this case the temperature of the treatment bath during treatment is preferably from 60 to 90 °C. The desired coating weight cannot usually be obtained within an economically reasonable time at temperatures below 60 °C, while temperatures in excess of 90 °C are virtually impossible to maintain in a water based bath in a container open to the atmosphere and are also uneconomical. The treatment time is preferably from 30 seconds to 10 minutes in this case, and if the treatment is by dipping, the treatment time is more preferably at least 200, or still more preferably at least 300, seconds. Times below 30 seconds can result in an inadequate degreasing and/or conversion. Degreasing and the conversion reactions are completely finished by 10 minutes, and contact between the treatment bath and workpiece for longer times provides no additional development of the reactions and is thus economically fruitless and impairs the productivity.

The treatment temperature, treatment time, and coating weight for formation of an antirust undercoating with the treatment bath according to the present invention are about the same as for formation of a lubrication undercoating.

After treatment with the treatment bath according to the present invention, the steel stock is submitted to a water or hot water rinse in order to wash off treatment bath adhering on the workpiece post-treatment. This is followed by drying as required in the particular case, and then by painting when painting is the intended application, or by treatment with a reactive soap or application of a solid lubricant when lubrication is the intended application. When the intended application is rust resistance, the workpiece will generally be dried and then used directly or after coating with an antirust oil.

The advantageous effects of the present invention will be explained more specifically through the following working and comparative examples of practical treatment. However, the invention is in no way limited by the following working examples.

The following test materials were used in the examples: cold-rolled steel sheet

(SPCC-SD) with a thickness of 0.8 mm, coated with 1 g/m² of a detergent antirust oil

(NOX-RUST530-40); carbon steel cylinders (S45C) with an outer diameter of 30 mm and a height of 34 mm, coated with 2 g/m² of a press oil (NOX-RUST320). Both of these oils were products of Parker Kosan Kabushiki Kaisha and had a mineral oil base.

Five aqueous solutions were used as the sources for all of the active ingredients other than accelerant, oil, and surfactant in the treatment baths used in the working and comparative examples. The compositions of these five aqueous solutions were as fol- lows, except that the free acidity was adjusted when needed as specified at the end of all these descriptions and the balance not shown was water in all instances. - Aqueous Solution 1 - phosphate ions 12.0 g/L (added as 75% phosphoric acid) zinc ions 1.7 g/L (added as zinc oxide) nniiccKkeeil i ioonnss : l 1..υ0 α g//LL (added as nickel carbonate) fluosilicate ions 1.2 g/L (added as sodium fluosilicate) free acidity 1.2 points

- Aqueous Solution 2 - phosphate ions 15.0 g/L (added as 75% phosphoric acid) zziinncc i ioonnss 2 __..5s g α//LL (added as zinc oxide) cobalt ions 0.5 g/L (added as cobalt carbonate) free acidity 1.5 points

- Aqueous Solution 3 - phosphate ions 18.0 g/L (added as 75% phosphoric acid) zinc ions 4.5 g/L (added as zinc oxide) fluoride ions 0.3 g/L (added as 55% hydrofluoric acid)) free acidity 2.0 points

- Aqueous Solution 4 - phosphate ions 13.0 g/L (added as 75% phosphoric acid) zinc ions 7.0 g/L (added as zinc oxide) free acidity 5.5 points

- Aqueous Solution 5 - phosphate ions 40.0 g/L (added as 75% phosphoric acid) zinc ions 15.0 g/L (added as zinc oxide) free acidity 12.0 points

The free acidity of Aqueous Solutions 1 to 5 was adjusted to the specified values using nitric acid or sodium hydroxide.

The following treatment process operation sequence was used in the working and comparative examples:

(1 ) treatment with the bath specified for the particular working or comparative example (the conditions are reported below in the working and comparative examples);

(2) water rinse (tap water), ambient temperature, 30 seconds, spray;

(3) rinse with deionized water (with an electrical conductivity of < 0.2 microSiemens per centimeter), ambient temperature, 20 seconds, spray;

(4) water drain and dry in a 110 °C air current, 180 seconds.

The coating weight was calculated from the surface area of the treated article and the weight of the article before and after the coating was stripped off by dipping the coated article for 15 minutes in a solution of 5 % chromic acid (i.e., CrO₃) heated to 75 °C.

The coverage of the basis metal by the conversion coating crystals (ratio of the area covered by the coating to the overall area of the substrate) was calculated by inspecting the deposited coating crystals using a scanning electron microscope at a magnification of 1 ,000 X). The appearance of the coating was visually evaluated at the same time.

Example 1

0.9 g/L of polyoxyethylene alkyl ether (HLB = 12.3) and 0.2 g/L of NOX- RUST530-40 (i.e., 22 weight parts of oil per 100 weight parts of surfactant) were added to Aqueous Solution 1 followed by emulsification (6,000 rpm, 5 minutes) using a homog- enizer. Sodium nitrite was then added at 100 mg/L as nitrite ions and the bath temperature was brought to 55 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 120 seconds. The resulting coating weight was 2.8 g/m², the coating coverage area was 90 %, and the coating had a uniform appearance.

Example 2

3.3 g/L of polyoxyethylene alkyl ether (HLB = 14.2) and 9.7 g/L of NOX-

RUST530-40 (294 weight parts of oil per 100 weight parts surfactant) were added to Aqueous Solution 2 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Hydroxylammonium phosphate was then added at 0.7 g/L as hydroxylammonium ions and the bath temperature was brought to 50 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 120 seconds. The resulting coating weight was 3.6 g/m², the coating coverage area was 100 %, and the coating had a uniform appearance.

Example 3

2.0 g/L of polyoxyethylene alkyl ether (HLB = 14.2) and 2.0 g/L of NOX-

RUST530-40 (i.e., 100 weight parts of oil per 100 weight parts of surfactant) were added to Aqueous Solution 3 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Hydroxylammonium sulfate was then added at 1.5 g/L as hydroxylammonium ions and the bath temperature was brought to 58 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 90 seconds. The resulting coating weight was 3.2 g/m², the coating coverage area was 100 %, and the coating had a uniform appearance. Example 4

4.8 g/L of polyoxyethylene alkyl ether (HLB = 16.6) and 7.2 g/L of NOX-

RUST530-40 (i.e., 150 weight parts of oil per 100 weight parts of surfactant) were added to Aqueous Solution 3 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Sodium nitrite was then added at 70 mg/L as nitrite ions and the bath temperature was brought to 45 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by spraying for 40 seconds. The resulting coating weight was 3.0 g/m², the coating coverage area was 90 %, and the coating had a uniform appearance.

Example 5 0.6 g/L of polyoxyethylene alkyl ether (HLB = 14.2) and 1.2 g/L of NOX-

RUST530-40 (i.e., 200 weight parts of oil per 100 weight parts surfactant) were added to Aqueous Solution 2 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Sodium nitrite was then added at 180 mg/L as nitrite ions and the bath temperature was brought to 38 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 180 seconds. The resulting coating weight was 3.4 g/m², the coating coverage area was 100 %, and the coating had a uniform appearance.

Example 6

1.2 g/L of polyoxyethylene alkyl ether (HLB = 12.3) and 0.6 g/L of NOX-RUST320

(i.e., 50 weight parts of oil per 100 weight parts of surfactant) were added to Aqueous Solution 4 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer.

Sodium nitrite was then added at 120 mg/L as nitrite ions and the bath temperature was brought to 80 °C.

The resulting treatment bath was used to treat the oil-contaminated carbon steel by dipping for 480 seconds. The resulting coating weight was 7.5 g/m², the coating cov- erage area was 100 %, and the coating had a uniform appearance.

Example 7

2.5 g/L of polyoxyethylene alkyl ether (HLB = 16.6) and 7.0 g/L of NOX-RUST320 (i.e., 280 weight parts of oil per 100 weight parts of surfactant) were added to Aqueous Solution 5 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Hy- droxylammonium sulfate was then added at 1.0 g/L as hydroxylammonium ions and the bath temperature was brought to 70 °C.

The resulting treatment bath was used to treat the oil-contaminated carbon steel by dipping for 300 seconds. The resulting coating weight was 12.5 g/m², the coating coverage area was 100 %, and the coating had a uniform appearance. Comparative Example 1

3.6 g/L of polyoxyethylene alkyl ether (HLB = 14.2) and 11.0 g/L of NOX- RUST530-40 (i.e., 306 weight parts of oil per 100 weight parts of surfactant) were added to Aqueous Solution 1 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Sodium nitrite was then added at 100 mg/L as nitrite ions and the bath tempera- ture was brought to 55 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 120 seconds. The resulting coating weight was 1.1 g/m², the coating coverage area was 10 %, and the coating had a nonuniform appearance.

Comparative Example 2 2.0 g/L of polyoxyethylene nonylphenyl ether (HLB = 13.6) and 2.0 g/L of NOX-

RUST530-40 (100 weight parts per 100 weight parts surfactant) were added to Aqueous Solution 1 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Sodium nitrite was then added at 80 mg/L as nitrite ions and the bath temperature was brought to 55 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 120 seconds. Although the sample could be wetted by the water rinse executed after treatment, absolutely no coating deposition had occurred.

Comparative Example 3

1.0 g/L of polyoxyethylene alkyl ether (HLB = 11.5) and 2.0 g/L of NOX-

RUST530-40 (200 weight parts per 100 weight parts surfactant) were added to Aqueous Solution 2 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer.

Sodium nitrite was then added at 30 mg/L as nitrite ions and the bath temperature was brought to 50 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by dipping for 90 seconds. Crawling by the water was observed during the post-treatment water rinse, and in addition absolutely no coating deposition had occurred.

Comparative Example 4 0.3 g/L of polyoxyethylene alkyl ether (HLB = 17.5) and 0.05 g/L of NOX- RUST530-40 (17 weight parts per 100 weight parts surfactant) were added to Aqueous Solution 3 followed by emulsification (6,000 rpm, 5 minutes) using a homogenizer. Hydroxylammonium sulfate was then added at 0.4 g/L as hydroxylammonium ions and the bath temperature was brought to 33 °C.

The resulting treatment bath was used to treat the oil-contaminated cold-rolled steel sheet by spraying for 40 seconds. The resulting coating weight was 0.6 g/m², the coating coverage area was 10 %, and the coating had a nonuniform appearance.

Comparative Example 5 Sodium nitrite was added at 120 mg/L as nitrite ions to Aqueous Solution 4 and the bath temperature was brought to 80 °C.

The resulting treatment bath was used to treat the oil-contaminated carbon steel sheet by dipping for 480 seconds. The resulting coating weight was 1.5 g/m², the coating coverage area was 10 %, and the coating had a nonuniform appearance.

As may be understood from the preceding results, the deposition of uniform zinc phosphate conversion coatings on oil-contaminated steel was obtained in Examples 1 to 7 using the treatment method according to the present invention. In contrast to this, in the comparative examples, either no film deposition occurred, or, when film deposition did occur, a satisfactory coating weight was not obtained and the coating did not have a uniform appearance. The upper limit for the mineral oil concentration and the upper limit on the mineral oil-to-surfactant ratio were exceeded in Comparative Example 1. Comparative Example 2 used a polyoxyethylene nonylphenyl ether as the surfactant. The HLB of the polyoxyethylene alkyl ether did not come up to the lower limit in Comparative Example 3. In the case of Comparative Example 4, the mineral oil-to-surfactant ratio did not reach the lower limit, the treatment bath temperature did not reach the lower limit, and the HLB of the polyoxyethylene alkyl ether exceeded the upper limit. The treatment bath contained neither mineral oil nor surfactant in Comparative Example 5. The following two tests were also run in order to evaluate the functionality of the coatings produced in the working and comparative examples.

The first test was a test of the performance as an underpaint coating and involved evaluation of the post-painting corrosion resistance. The cold-rolled steel sheets treated in Examples 1 to 5 and Comparative Examples 1 to 4 were coated with a cationic electrocoating (Elecron™ 9200 from Kansai Paint) so as to provide a paint film thickness of 20 μm. A cross was then scribed using a sharp cutter and the samples were subjected to salt-spray testing for 1 ,000 hours. The cross cut was subsequently peeled with tape and the width of peeling was measured. The results are shown in Table 1 below. In the case of the cold-rolled steel sheets treated in Examples 1 to 5, the results showed that the peeling width from the cross cut was in all cases less than 3 mm. In contrast to this, in the case of the cold-rolled steel sheets treated in Comparative

Examples 1 to 4, the peeling width exceeded 3 mm in all cases. One may conclude from these results that the zinc phosphate conversion coatings afforded by the treatment methods of Examples 1 to 5 gave an entirely satisfactory performance as underpaint coatings.

The second test was a test of the performance as a lubrication undercoating and evaluated the performance in plastic working. The carbon steel cylinders treated in Examples 6 and 7 and Comparative Example 5 were immersed for 3 minutes at 80 °C in a 7 % aqueous solution of a sodium soap lubrication treatment agent (Palube® 235 from Nihon Parkerizing Co., Ltd.) and were thereafter submitted to backward punch extrusion testing.

Backward punch extrusion testing is a test that evaluates the plastic workability in cold forging. This test was run using the following parameters: punch material = HAP40, punch diameter = 21.21 mm, punch tip R = 6 mm, stroke rate = 30 strokes per minute, punch hole depth = 44 mm, cross section reduction ratio = 50 %.

The results showed that the carbon steel treated in Examples 6 and 7 was entirely undamaged and was very workable. In contrast, the carbon steel treated in Comparative Example 5 was strongly scratched by the punch and hence was unworkable under the conditions used in this test. Thus, the zinc phosphate conversion coatings afforded by Examples 6 and 7 were confirmed to perform well as lubrication undercoatings.

Claims

1. An aqueous liquid composition for degreasing and zinc phosphate conversion treatment of oil-contaminated steels in a single process operation, said aqueous liquid composition being an aqueous solution that comprises zinc ions, phosphate ions, and conversion accelerant, wherein the improvement comprises the presence in said aqueous liquid composition of from 0.1 to 10 g/L of mineral oil that is emulsified by polyoxyethylene alkyl ether surfactant with an HLB of 12 to 17.

2. An aqueous liquid composition according to claim 1 , said aqueous liquid composition comprising from 0.5 to 5.0 g/L of said polyoxyethylene alkyl ether surfactant and from 20 to 300 weight parts of said mineral oil per 100 weight parts of the polyoxyethylene alkyl ether surfactant.

3. An aqueous liquid composition according to claim 2, said aqueous liquid composition comprising from 1.5 to 5.0 g/L of zinc ions, from 10 to 20 g/L of phosphate ions, and from 0.5 to 4.0 points of free acidity.

4. An aqueous liquid composition according to claim 2, said aqueous liquid composition comprising from 5 to 20 g/L of zinc ions, from 10 to 50 g/L of phosphate ions, and from 4.0 to 15 points of free acidity.

5. A process of forming a zinc phosphate conversion coating on a metal substrate, wherein the improvement comprises forming said conversion coating by contact with an aqueous liquid composition according to claim 1.

6. A process according to claim 5, wherein the metal substrate surface is selected from the group consisting of non-stainless steel and alloys that contain at least 50 % by weight of either zinc or aluminum and the substrate surface, before being contacted with said aqueous liquid composition is coated with at least 0.3 g/m² of oil.

7. A process according to claim 6, wherein: the substrate is carbon steel; the zinc phosphate conversion coating formed is subsequently painted; and said aqueous liquid composition comprises from 1.5 to 5.0 g/L of zinc ions, from 10 to 20 g/L of phosphate ions, and from 0.5 to 4.0 points of free acidity.

8. A process according to claim 6, wherein: the substrate is carbon steel; the zinc phosphate conversion coating formed is subsequently coated with a lubricating coating and then cold worked or is subsequently coated with an antirusting oil treatment; and - said aqueous liquid composition comprises from 5 to 20 g/L of zinc ions, from 10 to 50 g/L of phosphate ions, and from 4.0 to 15 points of free acidity.