ARTICLES WITH ZINCIFEROUS SURFACES WITH IMPROVED UNPAINTED CORROSION RESISTANCE, AND PROCESSES FOR MAKING AND USING THE SAME

Abstract: The resistance to atmospheric corrosion of unpainted articles with zinciferous outer surfaces is substantially improved by providing the surface with a phosphate conversion coating layer consisting essentially of zinc and magnesium phosphates and containing from 1-7 percent by weight of Mg.

Description: ARTICLES WITH ZINCIFEROUS SURFACES WITH IMPROVED UNPAINTED CORROSION RESISTANCE, AND PROCESSES FOR MAKING AND USING THE SAME Field of the Invention This invention relates to articles of manufacture that have at least an outer surface region or layer that is zinciferous, i.e., which consists predominantly of zinc, and that have better corrosion resistance when left un- painted or otherwise protected with organic chemical over¬ coatings than otherwise similar articles of the prior art. More particularly, this invention relates to structural elements which use zinc base coated materials such as zinc-plated steel, zinc alloy plated steel, galvannealed steel, hot-dipped galvanized steel, and the like. Such articles may be employed unpainted as automobile parts, household electrical appliance exterior surfaces, construction bolts, and the like. Moreover, the present invention relates to zinc base coated materials for which a primary rust preventive treatment is desired. The invention also relates to processes for making articles according to the invention, by contacting articles having zinciferous surfaces with zinc and magnesium containing phosphating compositions, and to processes for using the articles according to the invention. Statement of Related Art The application of phosphate conversion treatments and various chro ate treatments (in particular, reactive chro - ate treatments, chromate coating treatments, electrolytic chromate treatments, etc.) to zinc based surfaces has been widely investigated as a method for improving the corrosion resistance of zinc base coated materials through chemical " treatment. It is known that when zinc-based coating is subjected to an ordinary phosphate conversion treatment, the result¬ ing phosphate conversion film is almost entirely hopeite. To improve this film's poor alkali resistance and paint ad- herence, metal ions such as Ni, Mn, etc. may be added to the surface treatment agent. This metal component becomes incorporated into the hopeite film, and the alkali resist¬ ance, paint adherence, and corrosion resistance are there¬ by improved (refer to Nikkei New Material, 1989, 2-20, page 59) . Japanese Patent Application Laid Open [Kokai] Number 60-50175 [50,175/85] is an example of a patent relating to the addition of Ni and Mn. U. S. Patent 4,713,121 of Dec. 15, 1987 to Zurilla et al. teaches the use of zinc phosphating solutions contain- ing magnesium or other divalent cations along with zinc to improve the resistance to alkaline dissolution of the coat¬ ings (column 5 lines 57 - 61) . However, like most other prior art, the teachings of this reference were aimed at improving corrosion of articles after painting (column 1 lines 10 - 14) . Also, although the reference teaches in general terms that it is applicable to zinciferous surfaces along with those of iron and aluminum (column 12 lines 9 - 11) , the specific examples including magnesium in the phosphate conversion coating formed were prepared on steel substrates only (column 30 lines 22 - 24) . Conversion treatments in which the treatment bath con¬ tains manganese ions and nickel ions (in specified propor- tions) as heavy metal ions in addition to the zinc ions, such as are currently in commercial use, have excellent properties in terms of post-painting corrosion resistance and paint adherence. However, such films do not have a satisfactory corrosion resistance when left unpainted. Moreover, the various chromate treatments already known in the art also suffer from a variety of problems. For example, when the substrate has a complex shape, chrom¬ ate coating treatments are encumbered by the problem of " drainage of the treatment liquid after application, while electrolytic chromate treatments suffer from the problem of uneven coating on such a substrate. It is thus diffi¬ cult to obtain a good quality film by either treatment. Reactive chromate treatments solve this problem, but they cause hexavalent chromium pollution unless expensive count- ermeasures are taken. Description of the Invention Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quant- ities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the exact numeri¬ cal limits stated is generally preferred. It has now been found that excellent film uniformity, processability, and unpainted corrosion resistance can be given to zinciferous surfaces, preferably to surfaces con¬ taining at least 85 percent by weight (hereinafter "w/o") zinc, by providing the surfaces with an outermost layer of a phosphate conversion coating containing at least 1 but not more than 7 w/o of magnesium and also containing zinc. The zinc base surfaced articles of the present inven¬ tion evidence a high corrosion resistance at high atmos¬ pheric relative humidities and in environments which con- tain corrosive media, such as chloride and the like. Un¬ painted articles according to the present invention, having the properties described above, generally have satisfactory corrosion resistance in most atmospheric environments and may be used for, inter alia, automobile parts, components for household electrical appliances, and architectural bolts which are susceptible to rusting. The magnesium component which characterizes the exter¬ nal phosphate coating film on articles according to the present invention is normally present in a phosphate salt whose main component is hopeite. This magnesium can be detected and quantified by atomic absorption spectroεcopic analysis and/or x-ray fluorescence analysis after stripping the coating with hydrochloric acid. The magnesium in the coatings is thought to be present in the form of compounds such as magnesium phosphate. In order to improve the un¬ painted corrosion resistance, the magnesium content must be at least 1.0 w/o (equivalent as magnesium metal) of the total weight of the phosphate film. Preferably the magnes¬ ium content of the film is at least 2.0 w/o. It is tech¬ nically adverse to achieve magnesium contents in excess of 7.0 w/o of the conversion coating. More commonly used additive elements, known for improving the paint adherence, such as manganese and nickel, offer almost no improvement in the unpainted corrosion resistance as compared with their omission, while their addition in large quantities has a moderately adverse effect. Accordingly, these ele- ments preferably are not included in the phosphate conver¬ sion films on zinciferous surfaced articles for unpainted service. The thickness or areal density of the conversion film is not usually a critical feature of this invention and may be selected according to the experience of the prior art of phosphate conversion coatings generally, by considering the application for the articles to be treated. An areal density within the range from 2 to 10 grams per square meter (hereinafter "g/m 2") for the conversion coating layer will be satisfactory for most applications. The above described magnesium containing phosphate conversion film is preferably produced from a magnesium and zinc ion containing aqueous liquid phosphate conversion bath or solution having a composition within the following ranges: (A) from 0.2 to 10 grams per liter (hereinafter "g/L") of zinc ions: (B) from 0.5 to 5.0 g/L of magnesium ions; (C) from 5 to 50 g/L of phosphate ions; (D) at least one component selected from the group consisting of: (1) a total of from 0.05 to 5 g/L of one or more species selected from fluoride ions and complex fluoride anions, and (2) 0.2 to 10 g/L of chlorate ions; and (E) from 3 to 20 g/L of nitrate ions. In the above quantification of the preferred concentration ranges, phosphoric acid and any anions produced by the par¬ tial ionization of phosphoric acid are considered as their stoichiometric equivalent as phosphate ions in determining the concentration of phosphate ions. Zinc ions and phosphate ions are sources of film com¬ ponents for formation of the phosphate based film. In particular, the corrosion resistance and finish of the film may be affected by adjusting the zinc ion concentration. Within the range of treatment bath temperatures of 25 to 55 degrees Centigrade, it is difficult to obtain a good quality film with an excellent corrosion resistance at zinc ion concentrations in excess of 10 g/L. On the other hand, at below 0.2 g/L, the areal density of the film usually drops below 1.0 g/m 2, and it is then di.fficult to obtain a satisfactory corrosion resistance. The preferred range for phosphate ion is 5 to 50 g/L. A good-quality film cannot usually be produced below the lower value, while exceeding the upper value is economically disadvantageous because it does not achieve any further improvement in performance. A film formation accelerator can also be added in ord¬ er to increase the conversion reaction rate during the ini¬ tial stage of this treatment and to rapidly form a uniform film. While nitrite ion is the preferred film formation accelerator, nitrobenzenesulfonate ion and hydrogen perox¬ ide may also be used, either individually or in combina¬ tion. Any fluorine containing component is preferably added in the form of complex silicofluoride ions (SiF 6 ~2 ) , hydro¬ fluoric acid (HF) , or borofluoride ions (BF 4 " ) , using these either individually or in combination. The total fluoride should preferably fall within the range of 0.05 to 5 g/L for the stoichiometric equivalent as fluorine atoms. When chlorate is used, the chlorate ion concentration preferably falls within the range of 0.2 to 10 g/L. When neither of these lower limits is satisfied, neither a uniform etching of the surface to be coated nor a fine textured, dense film are usually obtained to a satisfactory degree. When the upper limits are exceeded, the film is then too thin for most applications and a good quality finish is not usually obtained. The nitrate ions can be added in the form of the ni- trate salt of the metal ions which are to be added. The nitrate ion concentration preferably is from 3 to 20 g/L: a stable bath is not usually obtained when this lower limit is not met, while exceeding the upper limit usually causes a deterioration in the corrosion resistance obtained after coating. Magnesium ions, which act to improve the phosphate conversion film in the present invention, preferably are present in the treatment bath within the concentration range from 0.5 to 5.0 g/L. When the treatment bath con- tains less than 0.5 g/L magnesium ion, it is usually diffi¬ cult to obtain as much as 1.0 w/o of magnesium in the film, and a satisfactory corrosion resistance is then not ob¬ tained. Exceeding 5.0 g/L may cause the magnesium compon¬ ent in the film to exceed 7.0 w/o and inhibit the conver- sion process, and it is also economically unfavorable. The benefits of the invention are developed regardless of whether the treatment bath is applied by spraying, im- mersion, a combination, or other methods, and irrespective of the method of contacting used, contact times of at least one minute at a temperature of at least 25° C, more prefer¬ ably in the range from 25 - 55 ° C, most preferably in the range from 35 - 55° C, are preferred. The Mg content of the coating increases with increasing treatment tempera¬ ture. The practice of the invention may be further appreci¬ ated from the following, non-limiting, working examples and comparison examples. Examples Substrate Material Steel sheet, two-sided electrogalvanized, 20 g/m 2 total zinc coating density. Process Sequence (1) Degreasing: Spray at 42 degrees Centigrade for 120 seconds with a solution containing 16 g/L of agent A and 12 g/L of agent B of commercial cleaner FC-L4460™ (available from Nihon Parkerizing Company, Ltd. , Tokyo, Japan) . (2) Water Rinse: Tap water at room temperature for 20 second spray. (3) Surface Activating: Room temperature spray for 20 seconds with an aqueous solution containing 1 g/L of commercial activator concentrate PL-ZN (available from Nihon Parkerizing Company, Ltd.). (4) Phosphate Conversion: Immersion for 120 seconds in a bath with a composition and at a temperature as shown in Table 1. (The fluoride shown in the Table was add- ed as SiF 6 '2 .) (5) Water Rinse: Tap water at room temperature, 20 second spray. (6) Deionized Water Rinse: Twenty second spray with de- ionized water (conductivity = 0.2 microSiemens/cm) . Table 1: TREATMENT CONDITIONS, FIUM COMPOSITIONS, AND CORROSION RATINGS FOR THE EXAMPLES AND COMPARATIVE EXAMPLES note 1: 3.0 g/L CIQj- note 2: 1.0 g/L Ni'*.05 g/L Mn 2+ note 3: LOg/ C ^O-*** g/ Mn 2 * note4: 0.45 g/L Mg 2 *, 1.0 g/L Ni + (7) Drain and dry at 110 degrees Centigrade for 180 seconds. Measurement of free acidity A sample in the amount of 10 milliliters of the treat- ment bath was taken and titrated to neutrality with 1/10 N NaOH using Bromophenol Blue as the indicator. The number of milliliters of 1/10 N NaOH required for the yellow-to- blue transition is defined as the number of "points" of free acid or "FA". The FA of the phosphating solutions used for the examples and comparison examples was adjusted, depending on the zinc ion concentration in the treatment solution, in the conventional manner known in the art (the free acid must be higher for higher zinc concentrations, to prevent precipitation) . Evaluation of the phosphate film (1) Film weight: This was calculated from the weights measured before and after stripping with an aqueous solution made by dissolving 20 grams of ammonium bichromate and 480 grams of 29 w/o aqueous ammonia in sufficient dis¬ tilled water to give a total volume of 1 liter. (2) Amount of magnesium in the film: This was measured with an x-ray fluorescence analyzer (System 3070E model from Rigaku Denki Kabushiki Kaisha) . (3) Magnesium percentage in the film: Calculated from the results for (1) and (2) . Performance evaluation The edges of the coated steel sheet were sealed, and the time required for the development of red rust in salt spray testing was measured. Results are reported in Table 1 according to the scale shown in Table 2. Example 2 (film Mg content = 1.05%) had a score of "+" for the red rust development time, while Comparison Example 1 (film Mg content = 0.94%) had a score of "x" for the red rust development time. This indicates that the boundary Table 2 SCALE FOR SCORING TIME REQUIRED FOR DEVELOPMENT OF RED RUST IN THE SALT SPRAY TEST Grade Time for Development of Red Rust at least 145 hours 97 to 144 hours 48 to 96 hours less than 48 hours for obtaining an improvement in the unpainted corrosion re¬ sistance lies between these two Mg contents. Example 5 (film Mg content * = 6.93%) had a 'score of "++" for the red rust development time, while Comparison Example 2 (film Mg content = 7.10%) had a score of "x x" for the red rust development time. This indicates that the boundary for obtaining an improvement in the unpainted cor¬ rosion resistance lies between these Mg contents. Comparison Examples 4 and 5 (with addition of metal other than magnesium) have an unpainted corrosion resist¬ ance equal to that of Comparison Example 2 (which derives no benefit from its magnesium addition because the amount of magnesium is excessive) , thus showing that these metal ions other than magnesium are not effective in improving the unpainted corrosion resistance. Benefits of the Invention As a surface treatment for zinc base surfaced artic¬ les, the formation of a magnesium containing phosphate con¬ version film serves to impart an excellent unpainted corro- sion resistance. Furthermore, because the method of the invention for improving the corrosion resistance of zinc surfaces is a conversion coating method, the film uniform¬ ity is excellent and the method can be employed effectively with substrates with complex shapes. The resulting conver- sion film has an excellent ability to retain oil compon- ents, and a further increase in corrosion resistance can then be obtained by oiling. What is claimed is:

Claims: CLAIMS 1. An article of manufacture comprising: (A) an outermost surface layer consisting essentially of zinc and magnesium phosphates and containing from about 1.0 to about 7.0 w/o of magnesium; and (B) a second layer inward from and adjacent to said outer¬ most surface layer, said second layer containing at least about 50 w/o of metallic zinc. 2. An article of manufacture according to claim 2, ad- ditionally comprising a third layer inward from and adja¬ cent to said second layer, said third layer consisting essentially of carbon steel. 3. An article of manufacture according to claim 2, where¬ in said outermost surface contains at least about 2.0 w/o of magnesium. 4. An article of manufacture according to claim 1, where¬ in said outermost surface contains at least about 2.0 w/o of magnesium. 5. An article of manufacture according to claim 4, where- in said second layer contains at least about 85 w/o of Zn. 6. An article of manufacture according to claim 3, where¬ in said second layer contains at least about 85 w/o of Zn. 7. An article of manufacture according to claim 2, where¬ in said second layer contains at least about 85 w/o of Zn. 8. An article of manufacture according to claim 1, where¬ in said second layer contains at least about 85 w/o of Zn. 9. An article of manufacture according to claim 8, where¬ in said outermost layer has an areal density of at least 1 g/m 2 . 10. An article of manufacture according to claim 7, where¬ in said outermost layer has an areal density of at least 1 g/m 2 . 11. An article of manufacture according to claim 6, where¬ in said outermost layer has an areal density of at least 1 g/m 2 . 12. An article of manufacture according to claim 5, where¬ in said outermost layer has an areal density of at least 1 g/m 2 . 13. An article of manufacture according to claim 4, where¬ in said outermost layer has an areal density of at least 1 * g/m 2 . 14. An article of manufacture according to claim 1, where¬ in said outermost layer has an areal density of at least 1 g/m 2 . 15. A process for using, under conditions including expos¬ ure of the article to corrosive atmospheres, an article of manufacture having a zinciferous outer surface free from any solid organic chemical protective overcoating, wherein the improvement comprises providing said article of manu¬ facture with an outermost surface coating consisting essen¬ tially of zinc and magnesium phosphates and containing from about 1.0 to about 7.0 w/o of magnesium. 16. A process for providing an article of manufacture hav¬ ing a zinciferous outer surface with a protective conver¬ sion coating layer consisting essentially of zinc and mag¬ nesium phosphates and containing from about 1.0 to about 7.0 w/o of magnesium. 17. A process according to claim 16, comprising contacting the outer surface of said article of manufacture with an aqueous liquid composition consisting essentially of water and: (A) from 0.2 to 10 g/L of zinc ions: (B) from 0.5 to 5.0 g/L of magnesium ions; (C) from 5 to 50 g/L of phosphate ions; (D) at least one component selected from the group consisting of: (1) a total of from 0.05 to 5 g/L of one or more species selected from fluoride ions and complex fluoride anions, and (2) 0.2 to 10 g/L of chlorate ions; and (E) from 3 to 20 g/L of nitrate ions. 18. A process according to claim 17, wherein the time of contact between said article of manufacture and said aque¬ ous liquid composition is at least one minute. 19. A process according to claim 18, wherein the tempera¬ ture of said aqueous liquid composition during said con- tacting is between about 35 and about 55 ° C. 20. A process according to claim 17, wherein the tempera¬ ture of said aqueous liquid composition during said con¬ tacting is between about 35 and about 55 ° C.

Inventors: ISHII HITOSHI, MIYAWAKI TOSHI, MORI KAZUHITO, ISHII, HITOSHI, MIYAWAKI, TOSHI, MORI, KAZUHITO
Applicants: HENKEL CORP, HENKEL CORPORATION
Application year: 1990
Main classification: C23C22
IPC codes: C23C22