Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/07—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing phosphates
  • C23C22/08—Orthophosphates
  • C23C22/12—Orthophosphates containing zinc cations
  • C23C22/16—Orthophosphates containing zinc cations containing also peroxy-compounds
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/36—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates
  • C23C22/362—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates containing also zinc cations
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/36—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates
  • C23C22/364—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates containing also manganese cations
  • C23C22/365—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates containing also manganese cations containing also zinc and nickel cations

Definitions

• This invention relates to zinc phosphate-based conversion treatment baths which can be applied to a variety of metal substrates, for example, steel, steel sheet, galvanized steel sheet, and the like. More particularly, this invention re- s lates to a zinc phosphate-based conversion bath and to a surface treatment method that are able to form a fine, dense, and uniform conversion coating on metal surfaces and that are also able to induce fine-crystal formation in the con ⁇ version coating.
• Background Art 0 The execution of a zinc phosphate-based conversion treatment on various metals prior to the coating or plastic working thereof is known at present for the purpose of improving the paint adherence and post-paint corrosion resistance and improving the lubrication during plastic working.
• the conversion treatment baths used for zinc phosphate-based conver- sion treatment are essentially acidic aqueous solutions that contain zinc ions, phosphate ions, and oxidizing agent(s). Nitrite salts, chlorate salts, hydrogen peroxide, organic nitro compounds, hydroxylamine, and the like, are ordinarily considered for this oxidizing agent. These oxidizing agents are typically called conversion “accelerators" because they function to accelerate the conversion re- actions.
• Nitrate salts may be present in conversion baths, but — because, in the concentrations usually present in zinc phosphate-based conversion baths, nitrate ions do not exercise an oxidizing activity sufficient to convert ferrous ions sub ⁇ stantially completely to ferric ions — nitrate ions must be distinguished from the conversion accelerators.
• One important role of conversion accelerators during the zinc phosphate based conversion treatment of ferriferous metals is to oxidize the divalent iron ions eluting into the conversion bath to trivalent iron ions. For example, the conversion reactions are inhibited when divalent iron ions accumulate in the conversion bath during the continuous conversion treatment of ferriferous metals, and the role of the conversion accelerator in inhibiting this accumulation of divalent iron ions is thus crucial.
• chloride salts When chlorate salts are used as conversion accelerators, chloride ions are produced as a decomposition product during conversion treatment and ac- cumulate in the conversion treatment bath. The corrosion resistance of the metal substrate is substantially impaired when even a trace of chloride ions from the conversion treatment bath remains on the surface of the metal workpiece.
• chlorate salts are ordinarily used in combination with another conversion accelerator, such as nitrite salts, and when used alone provide only a significantly reduced conversion reaction rate.
• Stability in the conversion treatment bath is also a problem for the use of hydrogen peroxide as a conversion accelerator: Hydrogen peroxide is readily decomposed by oxygen dissolved in the conversion bath. In addition, hydrogen peroxide has a narrow optimal concentration range for conversion treatment, which makes it difficult to manage the conversion treatment bath. When the dissolved concentration is too high, a powdery, poorly adherent conversion coating is deposited on the metal surface.
• Nitroguanine for example, has a low solubility in water and as a result cannot be formulated as a concentrate for addition to the conversion bath. It is also difficult to control the divalent iron ions concentration in the conversion bath using nitroguanine because this compound has a weak capacity to oxidize the divalent iron ions.
• sodium m-nitrobenzenesulfonate provides a poor conversion performance when used by itself, and for this reason this compound must ordinarily be used in combination with another, more powerful conversion accelerator.
• Hydroxylamine compounds are another type of nitrogenous organic compounds used as conversion accelerators. These compounds, however, in order to achieve the best results, must be added to give concentrations of at least 1,000 parts per million by weight (hereinafter usually abbreviated as "ppm") in the conversion bath, giving rise to the possibility of a large and economically undesirable consumption of the conversion accelerator.
• ppm parts per million by weight
• results have been reported from an investigation into the use of chromic acid and permanganate salts as conversion accelerators for zinc phosphate-based conversion treatment baths (Norio Sato, et al., Boshoku Gijutsu [English title: Corrosion Engineering], Volume 15, No. 5 (1966)). These authors report that the formation of conversion coatings was not observed at concentrations of 5 miliimoles per liter (hereinafter usually abbreviated as "mmol/L”) or 10 mmol/L
• the present invention seeks to solve the problems described above for conversion accelerators. More specifically, the present invention introduces a zinc phosphate-based conversion bath for metals and a metal surface treatment method which are able to deposit a fine, dense, and uniform zinc phosphate-type conversion coating on the surface of the metal substrate and which are able to induce fine-crystal formation in the conversion coating.
• organoperoxide conversion ac ⁇ celerators need not be used in combination with nitrate salts or another conver- sion accelerator and thereby make possible the elimination of nitrogenous com ⁇ pounds from the conversion bath; the use of organoperoxide conversion acceler ⁇ ators yields fine, dense, and uniform crystals in the coating even without the ap ⁇ plication of a surface-conditioning treatment; and the use of organoperoxide con- version accelerators results in the formation of high-quality conversion coatings on metal substrates without being subject to narrow limitations of temperature, zinc concentration, and the like
• the present invention was developed as a result of these discoveries.
• treatment baths according to the present invention can also satisfy environmental regulations concerning the amount of nitrogenous compound in the effluent.
• environmental regulations concerning the amount of nitrogenous compound in the effluent.
• the present invention relates to an acidic zinc phosphate-based conversion treatment aqueous liquid composition, usually here ⁇ inafter called a "bath" for brevity, for treating metal substrates, wherein said bath is characterized in that it contains zinc ions and phosphate ions as its main com- ponents and also contains organoperoxide(s) as a conversion accelerator.
• the organoperoxide concentration in said conversion treatment bath is preferably 50 to 1 ,500 ppm.
• the zinc phosphate-based treatment method according to the present in ⁇ vention for application to metal surfaces is characterized by contacting the metal surface with the above-described zinc phosphate-based conversion treatment bath according to the present invention after the pH of the conversion bath has been adjusted to 2.0 to 4.0.
• the described zinc phosphate-based surface treatment according to the invention method is preferably carried out by subjecting the preliminarily degreased surface of the metal to a water rinse and consecutively thereafter to the conversion treatment.
• the appropriate range for zinc ions concentration 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 range for this concen ⁇ tration is 0.5 to 15.0 g/L.
• the formation of a conversion coating with a coating weight of around 0.5 to 10.0 g/m 2 is preferred when the conversion treatment bath according to the present invention is to be used to provide an underpaint coating for metals.
• the corresponding preferred zinc ions concentration range in the conversion bath will therefore be 0.5 to 5.0 g/L.
• the zinc ions concentration falls below 0.5 g/L, the resulting zinc phosphate-type conversion coating will exhibit a reduced coverage ratio, which can cause an unsatisfactory post-paint coating adherence and an unsatisfactory post-paint corrosion resistance.
• Zinc ions concentrations in excess of 5.0 g/L cause a coarsening of the crystals in the coating, which can in particular cause a reduced post-paint coating adherence.
• the conversion treatment bath is to be used in support of the plastic working of metals, the formation of a thick conversion coating with a coating weight of around 5.0 to 15.0 g/m 2 is preferred in order to produce a conversion film capable of following the plastic deformation of the workpiece.
• the preferred zinc ions concentration range in the conversion bath will be 5.0 to 15.0 g/L. It becomes difficult to obtain the prescribed coating weights for this application at zinc ions concentrations below 5.0 g/L. The coating weight no longer increases at above 15.0 g/L, which makes such values economically undesirable.
• the zinc ions can be provided by dissolving zinc oxide or zinc hydroxide in the acid 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 phosphate ions concentration in the conversion bath according to the present invention is preferably 5.0 to 30.0 g/L. Obtaining a normal conversion coating can become problematic at below 5.0 g/L. No additional benefits are obtained at above 30.0 g/L, which makes such values uneconomical.
• the phosphate ions can be generated by the addition of phosphoric acid or its aqueous solutions to the conversion bath or by dissolving a salt of phosphoric acid, such as the sodium, potassium, magnesium, zinc, or the like salt, in the conversion bath.
• the zinc phosphate-based conversion treatment bath according to the present invention is an acidic aqueous solution whose pH preferably is from 2.0 to 4.0 and more preferably from about 2.5 to 3.5.
• ortho- phosphoric acid H 3 PO 4
• H 2 PO 4 dihydrogen phosphate ions
• HPO 4 '2 hydrogen phosphate ions
• PO 3 phosphate ions
• the concentrations specified herein as those of "phosphate ions” are intended to include the stoichiometric equivalent as phosphate ions of any of the chemical species from undissociated orthophosphoric acid to completely ionized phosphate ions.
• the free acid content, measured as described in the examples below, of the compositions according to the invention preferably is at least, with increasing preference in the order given, 0.1, 0.3, 0.5, or 0.6 point and independently preferably is not more than, with increasing preference in the order given, 1.5, 1.3, 1.2, 1.1, 1.0, or 0.9 point(s).
• organoperoxides used by the present invention can be classified into, for example, organoperoxides, such as ethyl hydroperoxide, isopropyl hy- droperoxide, tert-butyl hydroperoxide, tert-hexyl hydroperoxide, diethyl peroxide, tert-butyl peroxymaleate, and the like, that contain a peroxy moiety without an adjacent carbonyl group; and percarboxylic acid types such as per- acetic acid, monoperphthalic acid, persuccinic acid, and the like.
• organoperoxides such as ethyl hydroperoxide, isopropyl hy- droperoxide, tert-butyl hydroperoxide, tert-hexyl hydroperoxide, diethyl peroxide, tert-butyl peroxymaleate, and the like, that contain a peroxy moiety without an adjacent carbonyl group
• Organoperoxide molecules are preferably used at concentrations of 50 to 1,500 ppm in a conversion bath according to the invention. Acceleration of conversion film formation can become unsatisfactory when the organoperoxide concentration in the conversion bath is below 50 ppm. Accordingly, the organ ⁇ operoxide molecules present in the conversion bath according to the present invention preferably contain C, to C 7 alkyl moieties, because a low water sol ⁇ ubility is exhibited by organoperoxides containing aromatic or higher molecular weight alkyl moieties, and this can result in a failure to obtain a satisfactory oxidizing activity. On the other hand, no additional effect is obtained at concentrations in excess of 1,500 ppm, and such values are therefore uneconomical.
• the instant conversion bath can produce a fine, dense, and uniform zinc phosphate-type conversion coating even in the absence of an immediately preceding surface-conditioning treatment for the specific purpose of inducing fine-crystal formation in the coating.
• the conversion bath according to the present invention does not require the addition of nitric acid, nitrous acid, an organic nitro compound, or the like, and thus can be formulated entirely free of nitrogenous compounds.
• Nitrogenous compounds may be added to the conversion treatment bath according to the present invention on an optional basis, but the nitrogen concentration is preferably held to no greater than 100 ppm and more preferably to 20 ppm or less.
• Suitable non-zinc metal ions are exemplified by nickel ions, manganese ions, cobalt ions, iron ions, magnesium ions, calcium ions, and so forth. Each of these ions can be provided by dissolution in the treatment bath of the oxide, hydroxide, carbonate, sulfate, phosphate, or the like, of the corresponding metal.
• Fluoride ions or complex fluoride ions e.g., fluosilicate ions, fluozirconate ions, and the like, can be used as etchant. 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, fluozirconic acid, fluotitanic acid, and the corresponding metal salts (e.g., sodium, potassium, magnesium).
• fluorine compounds hydrofluoric acid, fluosilicic acid, fluozirconic acid, fluotitanic acid, and the corresponding metal salts (e.g., sodium, potassium, magnesium).
• the following process steps preferably should be consecutively executed in the sequence given in order to form a conversion coating on metal surfaces using a zinc phosphate based conversion bath according to the present invention: alkaline degreasing, a water rinse, treatment with the zinc phosphate-based conversion bath, and a water rinse.
• the degreasing and water rinse processes may themselves each be implemented as multistage processes.
• a deionized water rinse is preferably used for the final water rinse when the conversion coating will be used as an underpaint coating.
• test materials were (1) 0.8 mm-thick cold-rolled steel sheets (SPCC- SD, abbreviated below as "SPC”) and (2) galvanized steel sheets (abbreviated below as “plated”) prepared by the zinc electroplating (20 g/m 2 ) of the aforemen- tioned cold-rolled steel sheets. These were in each case cut to 70 x 150 mm and subjected to treatment in the working and comparative examples described be ⁇ low.
• degreasing alkaline degreaser, brand name: FINECLEANERTM L4460 from Nihon Parkerizing Company, Limited, 20 g/L of constituent A, 12 g/L of constituent B); 43 "C, 120 seconds, immersion; (2) water rinse (tap water); ambient temperature, 30 seconds, spray;
• the free acidity in the zinc phosphate-based conversion baths in Ex ⁇ amples 1 to 8 and Comparative Examples 1 to 4 was adjusted to specific o values using sodium hydroxide.
• the free acidity was measured by titrating 10 milliliters (hereinafter usually abbreviated as "mL") of the particular treatment bath to neutrality with 0.1 N aqueous sodium hydroxide, using bromophenol blue as the indicator.
• mL milliliters
• bromophenol blue bromophenol blue
• the number of mL of the 0.1 N aqueous sodium hydroxide required for the color change from yellow to blue was determined s and is reported as "points" of free acidity.
• the fluoride ions concentration in the conversion bath was measured using a fluoride sensitive electrode.
• the coating weight was measured as follows: The weight (W1) in grams of the treated sheet after conversion treatment was first measured, and the treated sheet was then subjected to a film stripping treatment using the 0 stripping solution and stripping conditions reported below. The weight of the stripped sheet was measured to give W2 in grams, and the coating weight was calculated from the following equation:
• Coating weight (in g/m 2 ) (W1 - W2)/0.021.
• wt% ammonium dichromate + 49 wt% of 0 28 wt% aqueous solution of ammonia + 49 wt% pure water stripping conditions: ambient temperature, 15 minutes, immersion.
• SEM scanning electron microscope
• Example 1 Composition of the conversion bath phosphate ions 15 g/L (from addition of 75% phosphoric acid) zinc ions 1.3 g/L (from addition of zinc oxide) nickel ions 1.0 g/L (from addition of nickel carbonate) manganese ions 0.5 g/L (from addition of manganese carbonate) fluoride ions 100 ppm (from addition of 55 % hydrofluoric acid) 450 ppm of tert-butyl hydroperoxide (organoperoxide component) was added to the conversion bath with the above composition, and the free acidity of the conversion bath was then adjusted to 0.9 point.
• phosphate ions 15 g/L (from addition of 75% phosphoric acid) zinc ions 1.3 g/L (from addition of zinc oxide) nickel ions 1.0 g/L (from addition of nickel carbonate) manganese ions 0.5 g/L (from addition of manganese carbonate) fluoride ions 100 ppm (from addition of 55 % hydrofluor
• the resulting conversion coating weight was 1.2 g/m 2 .
• the coating crystals were plates with an average grain size of 6 micrometers.
• the conversion coating was grayish black and was uniform, fine, and dense.
• a galvanized steel test sheet was subjected first to the same surface conditioning treatment as in Example 1 and then to conversion treatment using the same conversion treatment bath as in Example 1.
• the resulting conversion coating weight was 2.8 g/m 2 .
• the crystals were plates with an average grain size of 4 micrometers.
• the conversion coating was grayish white and was uniform, fine, and dense.
• Example 3 A cold-rolled steel test sheet was subjected first to the same sur ⁇ face-conditioning treatment as in Example 1 and then to conversion treatment using the same conversion treatment bath as in Example 1 , except that the or ⁇ ganoperoxide addition consisted of 80 ppm tert-butyl hydroperoxide and the free acidity was adjusted to 0.6 point.
• the resulting conversion coating weight was 0.9 g/m 2 .
• the coating crystals were plates with an average grain size of 8 micrometers.
• the conversion coating was grayish black and was uniform, fine, and dense.
• Example 4 5 A cold-rolled steel test sheet was subjected first to the same surface conditioning treatment as in Example 1 and then to conversion treatment using the same conversion treatment bath as in Example 1 , except that 1 ,200 ppm of tert-butyl hydroperoxide was added as the organoperoxide and sufficient 65.5% nitric acid was added to give a nitrogen component content of 500 ppm. o The free acidity of the conversion bath was adjusted to 0.9 point. The resulting conversion coating weight was 1.1 g/m 2 . The coating crystals were plates with an average grain size of 7 micrometers. The conversion coating was grayish black and was uniform, fine, and dense.
• Example 5 A cold-rolled steel test sheet was subjected to conversion treatment as in Example 1 , except that there was no surface-conditioning treatment and only 400 ppm of tert-hexyl hydroperoxide was added as the organoperoxide. The free acidity was adjusted to 0.9 point. The resulting conversion coating weight was 1.0 g/m 2 . The coating crystals were plates with an average grain 0 size of 6 micrometers. The conversion coating was grayish black and was uniform, fine, and dense.
• Example 6 A cold-roiled steel test sheet was subjected first to the same surface conditioning treatment as in Example 1 and then to conversion treatment using 5 the same conversion treatment bath as in Example 1, except that 100 ppm of peracetic acid was added as the organoperoxide, and the free acidity was ad ⁇ justed to 0.6 point.
• the resulting conversion coating weight was 1.3 g/m 2 .
• the coating crystals were plates with an average grain size of 10 micrometers.
• the conversion coating was grayish black and was uniform, fine, and dense.
• a cold-rolled steel test sheet was subjected to conversion treatment us ⁇ ing the same conversion bath as in Example 1 , except that the surface condi- tioning treatment was not used, 500 ppm of tert-butyl hydroperoxide was added as the organoperoxide, and the free acidity was adjusted to 0.6 point.
• the resulting conversion coating weight was 1.1 g/m 2 .
• the coating crystals were plates with an average grain size of 10 micrometers.
• the conversion coating was grayish black and was uniform, fine, and dense.
• tert-butyl hydroperoxide organoperoxide component
• conversion coating weight was 1.1 g/m 2 .
• the coating crystals were plates with an average grain size of 5 micrometers.
• the conversion coating was grayish black and was uniform, fine, and dense.
• Comparative Example 1 A cold-roiled steel test sheet was subjected to the same surface condi ⁇ tioning treatment as in Example 1 and was then submitted to the same conver ⁇ sion treatment as in Example 1 , except that the organoperoxide addition con ⁇ sisted of 5 ppm of tert-butyl hydroperoxide.
• the conversion coating weight was 0.5 g/m 2 , and the development of yellow rust was observed. Comparative Example 2
• a galvanized steel test sheet was subjected to conversion treatment as in Example 1 , except that the organoperoxide addition consisted of 5 ppm of tert-butyl hydroperoxide.
• the conversion coating weight was 0.9 g/m 2 , the average grain size was 15 micrometers, and the coating was sparse.
• Comparative Example 3 A cold-rolled steel test sheet was subjected to conversion treatment as in Example 8, except that there was no surface-conditioning treatment and 150 ppm of nitrite salt was added to the conversion bath in place of the organoper ⁇ oxide. The conversion coating weight was 0.1 g/m 2 , which indicated that almost no conversion coating deposition had occurred. Yellow rust had developed over the entire surface. Comparative Example 4
• a cold-rolled steel test sheet was subjected to conversion treatment as in Example 1 , except that sodium chlorate was added to the conversion bath in place of the organoperoxide.
• the sodium chlorate was added to give a chlorate ions concentration of 1.5 g/L.
• the conversion coating weight was 0.9 g/m 2 .
• the coating crystals were columnar and the average grain size was 15 micrometers.
• the conversion coating was sparsely deposited, and yellow rust was observed.
• Example 2 plated 15 1.3 0 yes a
• Example 8 SPC 15 1.3 1400 yes a
• Example 1 450 0.9 1.2 black plates 6 grayish
• Example 2 450 0.9 2.8 white plates 4 grayish
• Example 3 80 0.6 0.9 black plates 8 grayish
• Example 4 1200 0.9 1.1 black plates 7
• Example 5 400 0.9 1.0 black plates 6 grayish
• Example 6 100 0.6 1.3 black plates 10 grayish
• Example 7 500 0.6 1.1 black plates 10 grayish
• Example 8 450 0.9 1.1 black plates 5
• the zinc phosphate-based conversion bath according to the present in ⁇ vention for application to metal substrates contains appropriate concentrations of organoperoxide as conversion accelerator. In consequence thereof, this bath yields uniform, fine, and dense conversion coatings with coating weights appropriate for the intended applications. This bath at the same time also acts to induce fine crystal formation in the conversion coating. As a result, the bath has such good effects that a surface-conditioning treatment is no longer a necessity.
• the organoperoxides used by the present invention react under mild con ⁇ ditions and are more stable than the generally used inorganic accelerators, and as a consequence have very good economic attributes. Since the presence of nitrogenous compounds in the conversion treatment bath is also no longer a ne ⁇ cessity, environmental regulations relating to the levels of nitrogenous compound discharge can now be fully satisfied, and on this point the conversion bath ac ⁇ cording to the present invention represents a major practical development.

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• 1995
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