MXPA97002738A - Composition and coating process deconversion of z phosphate - Google Patents

Abstract

The present invention relates to an aqueous acidic phosphate conversion coating treatment composition for metal surfaces, the composition comprising water, zinc ions, phosphate ions and an acceleration component consisting essentially of one or more organoperoxides. A process for forming a phosphate conversion coating on a metal surface, wherein the metal surface is contacted with a liquid aqueous composition

Description

"COMPOSITION AND COATING PROCESS OF ZINC PHOSPHATE CONVERSION" TECHNICAL FIELD This invention relates to zinc phosphate-based conversion treatment baths that can be applied to a variety of metal substrates, for example, steel, steel sheet, galvanized steel sheet and the like. More particularly, this invention relates to a conversion bath based on zinc phosphate and with a surface treatment method which are capable of forming a thin, dense and uniform conversion coating on metal surfaces and which are also capable of inducing the formation of fine crystal in the conversion coating.

ANTECEDENTS OF THE TECHNIQUE The execution of the conversion treatment based on zinc phosphate on various metals before coating or working of plastic thereof is currently known in order to improve the adhesion of the paint and post-paint corrosion resistance and to improve the lubrication during the plastic working. The conversion treatment baths used for the zinc phosphate-based conversion treatment are essentially aqueous acidic solutions containing zinc ions, phosphate ions and an oxidizing agent (s). Nitrite salts, chlorate salts, hydrogen peroxide, organic nitro compounds, hydroxylamine and the like are generally considered to be this oxidizing agent. These oxidizing agents are typically called conversion "accelerators" because they function to accelerate the conversion reactions. Nitrate salts may be present in the conversion baths, but - because in the concentrations usually present in zinc phosphate-based conversion baths, the nitrate ions do not exercise sufficient oxidation activity to convert the ions ferrous almost completely in ferric ions - the nitrate ions should be distinguished from the conversion accelerators. An important role of the conversion accelerators during the zinc phosphate conversion treatment of the ferrous metals is to oxidize the divalent iron ions that are eluted in the conversion bath in trivalent iron ions. For example, conversion reactions are inhibited when divalent iron ions accumulate in the conversion bath during the continuous conversion treatment of the ferrous metals, and the role of the conversion accelerator to inhibit this accumulation of divalent iron ions is therefore crucial. However, each of these known conversion accelerators is associated with problems that must be addressed. For example, in the case of nitrite salts, which are the most widely used conversion accelerators at present, these compounds are unstable in the acidic region. As a result, these compounds undergo spontaneous decomposition and are consumed even when the conversion treatment (storage period) is not being carried out. The maintenance of a constant or marked concentration of these compounds, therefore, requires the continuous replenishment to replace the amount lost with respect to this consumption. It is also known that as a result of their oxidative activity and spontaneous decomposition, these nitrite salts are partially converted into NOx gas that diffuses into and contaminates the atmosphere. When chlorate salts are used as the conversion accelerators, the chloride ions are produced as a decomposition product during the conversion treatment and accumulate in the conversion treatment bath. The corrosion resistance of the metal substrate deteriorates considerably even when a vestige of conversion chloride bath chloride ions remains on the surface of the metal workpiece. In addition, the chlorate salts are usually used in combination with another conversion accelerator, such as nitrite salts, and when used alone provide only a significantly reduced conversion reaction rate. The stability in the conversion treatment bath is also a problem for the use of hydrogen peroxide as a conversion accelerator. Hydrogen peroxide is easily broken down by dissolved oxygen in the conversion bath. In addition, hydrogen peroxide has a critical optimum concentration scale for the conversion treatment, which makes it difficult to handle the conversion treatment bath. When the dissolved concentration is too high, a powdery, hardly sticky conversion coating is deposited on the metal surface. With respect to the use of the nitrogen-containing organic compounds as conversion accelerators, the following problems are associated with the use of organic nitro compounds, such as nitroguanine and sodium m-nitrobenzenesulfonate: nitroguanine, for example, has a low solubility in water and as a result can not be formulated as a concentrate to be added to the conversion bath. It is also difficult to control the concentration of divalent iron ions in the conversion bath using nitroguanine because this compound has a weak ability to oxidize the divalent iron ions. On the other hand, sodium m-nitrobenzene sulfonate provides poor conversion performance when used per se, and for these reasons this compound should generally be used in combination with another more powerful conversion accelerator. In addition, its concentration management requires the use of large-scale measurement equipment, such as an ion chromatograph. Another problem with the use of organic nitro compounds is that the accumulation of these compounds and their decomposition products in the conversion bath cause an increase in the chemical oxygen demand ("COD") of the effluent from the conversion treatment, which adversely affects the environment. Hydroxylamine compounds are another type of organic nitrogen compounds used as conversion accelerators, these compounds, however, in order to achieve the best results, they must be added to provide concentration of at least 1000 parts per million by weight (below usually abbreviated as "ppm") in the conversion bath, giving rise to the possibility of a large and economically undesirable consumption of the conversion accelerator. Likewise, the results of an investigation into the use of chromic acid and permanganate salts of conversion accelerators, for conversion treatment bath based on zinc phosphate (Norio Sato, et al., Boshoku Gi tutsu [English title: Corrosion Engineering], Volume 15, Number 5 (1966)). These authors report that the formation of the conversion coatings was not observed at concentrations of 5 millimoles per liter (usually abbreviated below as "mmol / L") or 10 mmol / L. Many of the conversion accelerators known as described above are nitrogenous compounds and as such resist removal by chemical wastewater treatment techniques, so that in practice they are usually removed through microbiological treatments. However, even with the use of microbiological treatments, the elimination of high concentrations of these nitrogenous compounds is highly problematic, while complete elimination can not be achieved even at low concentrations. Nitrogen compounds have recently been thought to be a factor in the eutrophication of bodies of water, and the discharge of nitrogen compounds has therefore become the object of an increasingly strict regulatory atmosphere. In view of these environmental considerations, the development of a conversion bath based on zinc phosphate free of the nitrogen compound would be highly desirable. Another inconvenience of each of the conversion accelerators described above is that, in order to obtain thin, uniform, thin and dense desired conversion coatings as a coating below the paint, the metal surface in each case must be conditioned. by treatment with a colloidal titanium system immediately before the execution of the conversion treatment. In addition, from the fact that the handling of the treatment bath is quite complicated in the case of surface conditioners, a surface conditioning step also requires the installation of corresponding treatment facilities and the expansion of the space dedicated to the treatment. As a result, there has recently been intense demand for the development of a conversion accelerator that is capable of forming high quality conversion coatings on metal surfaces even without the implementation of a surface conditioning step.

EXHIBITION OF THE INVENTION PROBLEMS THAT WILL BE RESOLVED THROUGH THE INVENTION The present invention seeks to solve the problems described above for the conversion accelerators. More specifically, the present invention introduces a zinc phosphate-based conversion bath for metals and a metal surface treatment method that are capable of depositing a thin, dense, zinc phosphate-type conversion coating. uniform on the surface of the metal substrate, and which are capable of inducing fine crystal formation in the conversion coating.

DESCRIPTION OF THE INVENTION, INCLUDING PREFERRED MODALITIES As a consequence of the research focused on the organoperoxides within the domain of organic oxidizing agents, the inventors discovered that fine, dense and uniform zinc phosphate conversion coatings can be formed through the use of soluble organoperoxides in the conversion bath as conversion accelerators. The following findings were also made: organoperoxide conversion accelerators do not need to be used in combination with nitrate salts and another conversion accelerator and thus make it possible to remove the nitrogenous compounds from the conversion bath; the use of organoperoxide conversion accelerators yields fine, dense and uniform crystals in the coating even without the application of a surface conditioning treatment; and the use of organoperoxide conversion accelerators results in the formation of higher quality conversion coatings on metal substrates without being subject to critical temperature, zinc concentration and the like limitations. The present invention is developed as a result of these discoveries. Since nitrogen compounds are not included among the essential components of conversion baths in accordance with the present invention, the treatment baths in accordance with the present invention can also satisfy environmental regulations related to the amount of the nitrogen compound in the effluent. It should be noted in this regard that there is very little risk of damage to the environment when the concentration of nitrogen in a conversion treatment bath is less than or equal to 20 parts per million. In specific terms, then, the present invention relates to an aqueous liquid composition treatment composition based on acidic zinc phosphate, usually referred to below in a "bath" for reasons of abbreviation, to treat metal substrates, in where the bath is characterized because it contains zinc ions and phosphate ions as its main components and also contains organoperoxide (s) as a conversion accelerator. The concentration of the organoperoxide in the conversion treatment bath is preferably 50 to 1500 parts per million. The zinc phosphate-based treatment method according to the present invention for applications to metal surfaces is characterized by contacting the metal surface with the zinc phosphate-based conversion treatment bath described above, in accordance with the present invention, after the pH of the conversion bath has been adjusted to a value of 2.0 to 4.0. Finally, the zinc phosphate-based surface treatment described according to the invention is preferably carried out by subjecting the preliminarily defatted surface of the metal to a rinse with water and consecutively after the conversion treatment. The appropriate scale of concentration of zinc ions in a bath according to the invention will vary as a function of the service intended for the conversion coating produced, but as a general matter, the preferred scale for this concentration is 0.5 to 15.0 grams per liter . For example, the formation of a conversion coating with a coating weight of about 0.5 to 10.0 grams per square meter is preferred when the conversion treatment bath, in accordance with the present invention, is to be used to provide a coating for metals below the paint. The scale of concentration of the corresponding preferred zinc ions in the conversion bath will therefore be 0.5 to 5.0 grams per liter. When the concentration of the zinc ions decreases to less than 0.5 gram per liter, the resulting zinc phosphate-type conversion coating will exhibit a reduced coverage ratio, which can result in unsatisfactory post-coating coating adhesion and strength to the unsatisfactory post-paint corrosion. Concentrations of zinc ions in excess of 5.0 grams per liter, cause thickening of the crystals in the coating, in particular can cause a reduced post-coating coating adhesion. As another example, when the conversion treatment bath is to be used to support the metalworking of metals, the formation of a coarse conversion coating with a coating weight of about 5.0 to 15.0 grams per square meter is preferred, In order to produce a conversion film capable of following the plastic deformation of the work piece. In this case, the preferred zinc ion concentration scale in the conversion bath will be 5.0 to 15.0 grams per liter. It has been difficult to obtain the indicated coating weights for this application of zinc ion concentrations of less than 5.0 grams per liter. The weight of the coating no longer increases above 15.0 grams per liter, which makes these values economically undesirable. Zinc ions can be provided by dissolving zinc oxide or zinc hydroxide in the acidic component of the conversion bath or by dissolving a water soluble zinc salt, such as the phosphate salt, sulfate salt or the like, in the conversion bath . The concentration of phosphate ions in the conversion bath according to the present invention is preferably 5.0 to 30.0 grams per liter. Obtaining a normal conversion coating can be problematic to less than 5.0 grams per liter. No additional benefits are obtained at more than 30.0 grams per liter, which makes these values ineconomic. The phosphate ions can be generated by the addition of phosphoric acid or its aqueous solutions to the conversion bath, or by dissolving a phosphoric acid salt, such as sodium, potassium, magnesium, zinc or a similar salt, in the conversion bath. . The zinc phosphate-based conversion treatment bath according to the present invention is an aqueous acidic solution whose pH is preferably 2.0 to 4.0 and more preferably about 2.5 to 3.5. In this pH region, orthophosphoric acid (H3PO4) exists in equilibrium mainly with the dihydrogen phosphate ions (H2PO4) but also with smaller amounts of hydrogen phosphate ions (HP? 4 ~ 2) and phosphate ions ( PO43-); however, the concentrations specified herein are those of the "phosphate ions" which are intended to include the stoichiometric equivalent as phosphate ions of any of the chemical species of the orthophosphoric acid not dissociated to the fully ionised phosphate ions. The free acid content, which is measured as described in the examples that will be presented below of the compositions according to the invention, is preferably at least, preferably increased to the order of 0.1, 0.3, 0.5 or 0.6. point and independently and preferably no more than preferably increased in the order provided of 1.5, 1.3, 1.2, 1.1 and 1.0 or 0.9 point (s). The organoperoxides used by the present invention can be classified, for example, in organoperoxides such as ethyl hydroperoxide, isopropyl hydroperoxide, tertiary butyl hydroperoxide, tertiary hexyl hydroperoxide, diethyl peroxide, tertiary butyl peroxyalate, and the like, which contain a residue of peroxy without an adjacent carbonyl group; and the types of percarboxylic acid, such as peracetic acid, monoperphthalic acid, persuccinic acid and the like. The organoperoxide molecules are preferably used at concentrations of 50 to 1500 parts per million in a conversion bath according to the invention. The acceleration of the formation of the conversion film can become unsatisfactory when the concentration of the organoperoxide in the conversion bath is less than 50 parts per million. Accordingly, the organoperoxide molecules present in the conversion bath according to the present invention preferably contain alkyl residues of 1 to 7 carbon atoms because of the low water solubility exhibited by the organoperoxides containing alkyl residues. aromatics or higher molecular weight, and this can result in the failure to obtain a satisfactory oxidant activity. On the other hand, no additional effect is obtained at concentrations in excess of 1500 parts per million and these values, therefore, are ineconomic. Because the conversion treatment bath in accordance with the present invention also functions to induce the formation of fine crystal in the part where the zinc phosphate type crystals are deposited, the present conversion bath can produce a coating of fine, dense and uniform zinc phosphate type conversion even in the absence of an immediately preceding surface conditioning treatment, with the specific object of inducing the formation of fine crystal in the coating. In addition, the conversion bath according to the present invention does not require the addition of nitric acid, nitrous acid in an organic nitro compound or the like, and therefore, can be formulated entirely free of nitrogen compounds. In this form, therefore, it offers the advantage of not requiring the inclusion of a treatment step for nitrogen compounds in the effluent treatment process. The nitrogen compounds can be added to the conversion treatment bath according to the present invention on an optional basis, but the nitrogen concentration is preferably maintained at no more than 100 parts per million and more preferably at 20 parts per million or more. less. Metal ions other than zinc ions can be added to the zinc phosphate-based conversion bath according to the present invention. These metal ions can act as etching acids in order to induce uniform etching of the surface of the metal workpiece, or they can act as paint improvers when the conversion coating is being used as a coating below the painting. Metal ions that are not suitable zinc are exemplified by nickel ions, manganese ions, cobalt ions, iron ions, magnesium ions, calcium ions and so on. Each of these ions can be provided by dissolution in the treatment bath of the corresponding metal, oxide, hydroxide, carbonate, sulfate, phosphate or the like. Fluoride ions or complex fluoride ions, e.g., fluosilicate ions, fluozirconate ions and the like can be used as an acid to burn. These ions can be provided, for example, by dissolving in the conversion treatment bath one or more of the following fluorine compounds: hydrofluoric acid, fluosilicic acid, fluorozirconic acid, fluotitanic acid and the corresponding metal salts (e.g. , sodium (potassium, magnesium) The following steps of the preference process must be carried out consecutively in the sequence provided in order to form a conversion coating on metal surfaces, using a conversion bath based on zinc phosphate in accordance with the present invention: alkaline degreasing, enamelling with water, treatment with the conversion bath based on zinc phosphate and a rinse with water.The processes of defatting and rinsing with water can each finally be implemented as multi-stage processes. A rinse with deionized water is preferably used for the final water rinse when the conversion coating will be used or a coating below the paint. Further, when the conversion coating is produced on a metal surface to be used as a coating below the paint, it is preferred that the conversion treatment be immediately preceded by a surface conditioning process using a titanium-containing surface conditioner. colloidal, in order to induce a thin crystal formation in the coating. The metals subjected to the conversion treatment described above can be painted after rinsing with final water as described above or after a drying step following the final water rinse. When the plastic work is the service intended for the conversion film formed on the metal substrate using the conversion bath according to the present invention, after the degreasing and rinsing with water described above, the metal workpiece preferably it is subjected to a pickling step for the purpose of surface deoxidation. Again with reference to the production of the conversion film for the plastic working service, the lubricity of the coating can be further improved by a soap treatment (lubrication treatment) after the formation of the conversion film. Since the surface treatment using the zinc phosphate-based dip bath in accordance with the present invention, it is generally carried out by dipping, spraying or a combination thereof. When the conversion film is intended as a coating under the paint, the desired coating can be formed and preferably formed by a conversion treatment at a treatment temperature from about room temperature to about 60 ° C, and a time of Treatment from about 0.5 minute to about 5 minutes. When the conversion film is intended for the plastic working service, the desired coating can be formed and preferably formed by conversion treatment at a treatment temperature of about 50 ° C to about 90 ° C and a treatment time of about 1 minute to about 15 minutes. The invention will be explained in more detail below with reference to the following working examples and comparison of effective treatment. The scope of the present invention is in no way limited by these examples.

Examples The test materials were cold-rolled steel sheets (1) of 0.8 mm thickness (SPCC-SD, which will be abbreviated below as "SPC") and (2) galvanized steel sheets (abbreviated below as "galvanized"). ) prepared by zinc electrodeposition (20 grams per square meter) of the aforementioned cold-rolled steel sheets. These in each case were cut to 70 x 150 millimeters and subjected to the treatment in the working and comparison examples which will be described below. The following steps of the treatment process which are a typical example of the treatment for the purpose of producing a coating under the paint, were used in the working and comparison examples: (1) defatting (alkaline degreaser, factory name: FINECLEANER ™ of L4460 from Nihon Parkerizing Company, Limited, 20 grams per liter of constituent A, 12 grams per liter of constituent B); immersion at 43 ° C, 120 seconds; (2) rinse with water (tap water); spraying at room temperature, 30 seconds; (3) surface conditioning (colloidal titanium surface conditioner, factory name PREPALENE® ZN from Nihon Parkerizing Company, Limit, aqueous solution of 1 gram per liter); spraying at room temperature, 30 seconds; (4) zinc phosphate-based conversion treatment (as described and continued for the individual work and compilation examples); immersion at 43 ° C, 120 seconds; (5) rinse with water (tap water); spraying at room temperature, 30 seconds; (6) rinse with deionized water (deionized water, conductivity = 0.2 microS / cm); rinse at room temperature, 20 seconds; (7) drain and dry; hot air at 110 ° C, 180 seconds, except that the surface conditioning step was not carried out in Examples 5 and 7 or in Comparative Example 3, and in these cases, the step (4) of treatment of Zinc phosphate-based conversion was therefore carried out directly after the defatting steps (1) and following that of the rinsing with water (2). The free acidity in the zinc phosphate-based conversion baths in Examples 1 to 8 and Comparative Examples 1 to 4 was adjusted to specific values, using sodium hydroxide. Free acidity was measured by evaluating 10 milliliters (abbreviated below usually as "mL") from the specific treatment bath to neutrality with 0.1 N aqueous sodium hydroxide, using bromophenol blue as the indicator. The number of mL of the 0.1 N aqueous sodium hydroxide required for the color change from yellow to blue was determined and disclosed as "points" of free acidity. The concentration of the fluoride ions in the conversion bath was measured using a fluoride sensitive electrode. The weight of the coating was measured as follows: the pso (Wl) in grams of the treated sheet after the conversion treatment was measured first, and the treated sheet was then subjected to a film purification treatment using the purification solution and debugging conditions that are disclosed below. The weight of the cleaned sheet was measured to give W2 in grams and the weight of the coating was calculated from the following equation: Coating weight (in g / m2) = (W1-W2) / O .021.

TREATMENT FOR LAMINATED STEEL SHEETS IN FRIÓ purification solution: 5 percent solution of aqueous chromic acid. purification conditions: immersion at 75 ° C, 15 minutes.

TREATMENT OF GALVANIZED STEEL SHEETS cleaning solution: 2 weight percent (hereinafter usually abbreviated as "wt%") of ammonium dichromate + 49 weight percent of a 28 percent by weight aqueous solution of ammonia + 49 percent by weight 100 percent by weight of pure water. purification conditions: immersion, at room temperature, 15 minutes. The appearance of the coatings was visually inspected and the morphology and size of the grains in the conversion coating was evaluated by inspection with a scanning electron microscope (SEM).

Example 1 COMPOSITION OF THE CONVERSION BATH Phosphate ions: 15 grams per liter (from the addition of 75 percent phosphoric acid) zinc ions: 1.3 grams per liter (from the addition of zinc oxide) Nickel ions: 1.0 gram per liter (from the addition of nickel carbonate) manganese ions: 0.5 gram per liter (from the addition of manganese carbonate) fluoride ions: 100 parts per million (from the addition of 55 percent hydrofluoric acid) 450 parts per million of the tertiary butyl hydroperoxide (organoperoxide component) was added to the conversion bath with the aforementioned composition, and the free acidity of the conversion bath was then adjusted to 0.9 point. A group of cold-rolled steel test sheets was first subjected to the colloidal titatio surface conditioning treatment and then to the conversion treatment (conversion temperature = 43 ° C, treatment time = 120 seconds) using the conversion bath previously described. The weight of the resulting conversion coating was 1.2 grams per square meter. The coating crystals were plates with an average grain size of 6 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Example 2 A galvanized steel test sheet was first subjected to the same surface conditioning treatment as in Example 1 and then the conversion treatment using the same conversion treatment bath as in Example 1. The weight of the conversion coating resulting was 2.8 grams per square meter. The crystals were plates with an average grain size of 4 microns. The conversion coating was grayish-white and was uniform, thin and dense.

Example 3 A cold rolled steel sheet was first subjected to the same surface conditioning treatment as in Example 1, and then to the conversion treatment using the conversion treatment bath as in Example 1, with the exception of addition of the organoperoxide it consisted of 8 parts per million of tertiary butyl hydroperoxide and the free acidity was adjusted to 0.6 point. The weight of the resulting conversion coating was 0.9 gram per square meter. The coating crystals were plates with an average grain size of 8 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Example 4 A cold rolled steel test sheet was first subjected to the same surface conditioning treatment as in Example 1 and then to the conversion treatment using the same conversion treatment bath as in Example 1, with the exception that they were added. 1200 parts per million tertiary butyl hydroperoxide as the organoperoxide and a sufficient amount of the nitric acid at 65.5 percent was added to provide a nitrogen component content of 500 parts per million. The free acidity of the conversion bath was adjusted to 0.9 point. The weight of the resulting conversion coating was 1.1 grams per square meter. The crystals of the coating were plates with an average grain size of 7 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Example 5 A cold rolled steel test sheet was subjected to the conversion treatment as in Example 1, with the exception that there was no surface conditioning treatment and only 400 parts per million of the tertiary hexyl hydroperoxide was added as the organoperoxide. The free acidity was adjusted to 0.9 point. The weight of the resulting conversion coating was 1.0 gram per square meter. The coating crystals were plates with an average grain size of 6 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Example 6 A cold rolled steel test sheet was first subjected to the same surface conditioning treatment as in Example 1, and then to the conversion treatment using the same conversion treatment bath as in Example 1, with the exception that 100 parts per million of peracetic acid was added as the organoperoxide, and the free acidity was adjusted to 0.6 point, the The resulting conversion coating weight was 1.3 grams per square meter. The coating crystals were plates with an average grain size of 10 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Example 7 A cold rolled steel test sheet was subjected to the conversion treatment using the same conversion bath as Example 1 with the exception that the surface conditioning was not used, 500 parts per million of the butyl hydroperoxide was added. tertiary as organoperoxide and free acidity was adjusted to 0.6 point. The weight of the resulting conversion coating was 1.1 grams per square meter. The coating crystals were plates with an average grain size of 10 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Example 8 COMPOSITION OF THE CONVERSION BATH Phosphate ions: 15 grams per liter (from the addition of 75 percent phosphoric acid) zinc ions: 1.3 grams per liter (from the addition of zinc oxide) Nickel ions: 1.0 gram per liter (from the addition of nickel nitrate) manganese ions: 0.5 gram per liter (from the addition of manganese carbonate) fluoride ions: 100 parts per million (from the addition of 55 percent hydrofluoric acid) nitrate ions: 7.2 grams per liter (from the addition of sodium nitrate and nickel nitrate) (nitrogen concentration = 1.4 grams per liter) 450 parts per million of tertiary butyl hydroxide (organoperoxide component) was added to the conversion bath with the aforementioned composition and the free acidity of the conversion bath was then adjusted to 0.9 point. A group of cold-rolled steel test sheets was first subjected to the surface conditioning treatment with colloidal titanium and then to the conversion treatment (conversion temperature = 43 ° C, treatment time = 120 seconds) using the conversion bath previously described. The weight of the resulting conversion coating was 1.1 grams per square meter. The coating crystals were plates with an average grain size of 5 microns. The conversion coating was grayish-black and was uniform, thin and dense.

Comparison Example 1 A cold rolled steel test sheet was subjected to the same surface conditioning treatment as in Example 1 and then subjected to the same conversion treatment as in Example 1 with the exception that the organoperoxide addition consisted of 5 parts by weight. million tertiary butyl hydroperoxide. The weight of the conversion coating was 0.5 gram per square meter and a development of yellow corrosion was observed.

Comparison Example 2 A galvanized steel test sheet was subjected to conversion treatment as in Example 1 with the exception that the organoperoxide addition consisted of 5 parts per million tertiary butyl hydroperoxide. The conversion coating weight was 0.9 grams per square meter, the average grain size was 15 microns, and the coating was sparse.

Comparison Example 3 A cold rolled steel test sheet was subjected to the conversion treatment as in Example 8 with the exception that there was no surface conditioning treatment and 150 parts per million nitrite salt was added to the water bath. conversion instead of the organoperoxide. The weight of conversion coating was 0.1 gram per square meter which indicated that deposition of the conversion coating had hardly occurred. A yellow corrosion had developed across the entire surface.

Comparison Example 4 A cold rolled steel test sheet was subjected to the conversion treatment as in Example 1 with the exception that sodium chlorate was added to the conversion bath instead of the organoperoxide. Sodium chlorate was added to provide a concentration of chlorate ions of 1.5 grams per liter. The weight of the conversion coating was 0.9 gram per square meter. The coating crystals were columnar and the average grain size was 15 micrometers. The conversion coating was poorly deposited and yellow corrosion was observed.

The conditions and results of these examples are summarized in Table 1 presented below. The concentrations of the organoperoxide used in Examples 1 to 8 were 50 to 1500 parts per million. It was demonstrated in this way that this concentration scale produced a good quality conversion coating on a cold rolled steel sheet as well as the galvanized steel sheet. A uniform, dense and thin coating was obtained even when a surface conditioning treatment was not used. In contrast, Comparative Examples 1 and 2 used organoperoxide concentrations of less than 50 parts per million it was found that in these cases, the oxidation activity by the conversion accelerator was inadequate, resulting in the deposition of scattered coating crystals. The uniformity of coatings on the base metal, therefore, decreased. Comparison Examples 3 and 4 used non-organoperoxide conversion accelerators. In Comparative Example 3, a nitrite salt was used as the conversion accelerator and no surface conditioning treatment was carried out. It was found that in this case the deposition of the conversion coating was entirely absent. TABLE 1 Examples and Ions of Ions Conditioning Concentration- Examples of zinc phosphate nitrogen agent Comparison Substrate gr / lt gr / lt ppm Oxidizing surface * Example 1 SPC 15 0 if Example 2 plating 15 0 SI Example 3 SPC 15 0 if Example 4 SPC 15 500 if Example 5 SPC 15 0 no Example 6 SPC 15 0 if Example 7 SPC 15 0 no Example 8 SPC 15 1400 if Example 1 Comparison SPC 15 1.3 if Example 2 Comparison plating 15 1.3 if Example 3 Comparison SPC 15 1.3 1400 no Example 4 Comparison SPC 15 1.3 if Example 1 450 0.9 1.2 grayish-black plates 6 Example 2 450 0.9 2.8 grayish-white plates 4 Example 3 80 0.6 0.9 grayish-black plates 8 Example 4 1200 0.9 1.1 grayish-black plates 7 Example 5 400 0.9 1.0 grayish-black plates 6 Example 6 100 0.6 1.3 grayish-black plates 10 Example 7 500 0.6 1.1 grayish-black plates 10 Example 8 450 0.9 1.1 grayish-black plates 5 Example 1 corrosion appeared Comparison 5 0.9 0.5 columnar yellow color 13 Example 2 Coating Comparison 5 0.9 0.9 Columnar 15 Example 3 appeared corrosion Comparison 150 0.9 0.1 granulated yellow color 80 Example 4 Coating Comparison 1500 0.9 0.9 columnar 15 * Type of oxidizing agents added: to tertiary butyl hydroperoxide b tertiary hexyl hydroperoxide c peracetic acid nitrite ions and chlorate ions A chlorate salt itself was used as the conversion accelerator in Comparative Example 4. It was found that in this case the conversion reaction rate had decelerated considerably.

BENEFITS OF THE INVENTION The zinc phosphate-based conversion bath according to the present invention for application to metal substrates contains appropriate concentrations of the organoperoxide as the conversion accelerator. As a result of this, this bath yields uniform, fine and dense conversion coatings with coating weights suitable for the applications to which they are intended. This bath at the same time also acts to induce the formation of fine crystal in the conversion coating. As a result, the bath has good effects in such a way that the surface conditioning treatment is no longer a necessity. The organoperoxides used by the present invention react under mild conditions and are more stable than the inorganic accelerators generally used as a consequence they have very good economic attributes. Since the presence of nitrogenous compounds in the conversion treatment bath is also no longer necessary, environmental regulations related to the levels of the nitrogen compound discharge can now be fully met and on this point the conversion bath in accordance with the present invention represents a main practical development.

Claims (8)

R E I V I N D I C A C I O N E S;

1. An aqueous acidic phosphate conversion coating composition for metal surfaces, the composition comprises water, zinc ions, phosphate ions and an acceleration component consisting essentially of one or more organoperoxides.

2. A composition according to claim 1, comprising from 50 to 1500 parts per million of the organoperoxide acceleration component.

3. A composition according to claim 2, which has a pH value of 2.0 to 4.0.

4. A composition in accordance with re-classification 3, which has a pH value from about 2.5 to about 3.

5. 5. A composition according to claim 1, having a pH value of 2.0 to 4.0.

6. A composition according to claim 5, having a pH value from about 2.5 to about 3.5.

7. A process for forming a phosphate conversion coating on a metal surface, wherein the metal surface is contacted with an aqueous liquid composition according to any of claims 1 to 6.

8. A compliance process with claim 7, wherein the metal surface before the formation of a phosphate conversion coating thereon is defatted and then rinsed with water.