MXPA00009759A - Method and composition for a metal coloring process. - Google Patents

Abstract

The present invention discloses a method to form a coating of chemical conversion over substrates of ferrous metal, using chemical solutions for the coating and the articles coated with the same. When modifying and combining the characteristics of two existent coating techniques, but to date not related, a hybrid conversion coating is formed. Specifically, an intermediate coating enhanced with molecular iron/oxygen, such as a dicarboxylate or phosphate, to a ferrous substrate by means of a first oxidation. The intermediate coating preconditions the substrate to form a surface rich in molecular iron and oxygen in an easy and accessible way for an ulterior reaction. This oxidation procedure is followed by a coloration procedure employing a hot oxidation solution (approximately 48.4 to 103.4°C (120 to 220°) which contains an alkali metal hydroxide, an alkali metal nitrate, an alkali metal nitrite or their mixtures, reacting with the iron and oxygen enhanced intermediate coating to form m agnetite, (Fe3O4). The result is the formation of a brown or black finishing under more favorable, benign and safe conditions than the ones previously observed with the conventional processes of caustic blackening, as consequence of the chemical reaction between the intermediate coating and the solution of the second oxidation. When sealed with an appropriate preventive upper layer of the oxidation, the final result is an attractive ultra-thin protective finishing applied through a simple immersion technique. The finishing is a coating of final protection over a fabricated metal article and likewise provides a grade of lubricity to help assembling the sliding surfaces or provides an anti-scratch protection. The finishing also provides an adherent base for paint finishings.

Description

COMPOSITION AND METHOD FOR A PROCESS FOR COLORING METALS Background of the Invention Field of the Invention This invention relates to the formation of a hybrid chemical conversion coating on iron or ferrous metal substrates, which consist of an enriched iron / oxygen intermediate coating. and an upper layer of magnetite. This invention also relates to coated ferrous metal substrate in accordance with the process disclosed herein. This invention further includes a seven step process for preparing the ferrous metal substrate with a coating containing magnetite. Description of Related Art The established technique for coloring ferrous metals has revolved mainly around the methods for producing black coatings. Since in the 50s, the commercial method most commonly used to blacken ferrous metals has been the oxidation process of caustic black. This method will be examined, along with the ferrous oxalate conversion coating on the ferrous metal substrate and the iron phosphatization process. Black Oxidation Caustiqa: This process uses sodium hydroxide, sodium nitrate and sodium nitrite as oxidation agents, which operate at approximately a pH of 14, at a temperature of approximately 139 to 150 ° C (285 to 305 ° F) . A black coating is formed during the exposure of approximately 10 .3 minutes. This process forms a deposit of magnetite (Fe304), approximately 1 meter thick, by reacting with a metallic iron substrate in situ. Although the process produces high quality black fines, when properly operated, it has the disadvantage of requiring high temperatures and highly concentrated solutions (700-1000 grams per liter) to carry out the reaction. During the course of operation, this reaction consumes oxidation salts and the boiling solution expels significant amounts of water. These materials must be added with black color to the solution to maintain the proper operating conditions. However, it is quite dangerous to add sodium hydroxide to water, which is a highly exothermic reaction, because the operation of the solution is already boiling. Similarly, adding water to compensate for a solution that is already at temperatures of 139 to 150 ° C (285 to 305 ° F) causes the water to boil instantaneously, in case it is not added slowly and carefully, consequently, the The operation of the process has several safety hazards for the personnel, due to the dangers involved in the normal operation and maintenance of the system. These dangerous conditions can be difficult to justify in the manufacturing environments of modern industry. In addition, normal operating conditions typically result in the formation of dense sludge in the processing tank, making it difficult to remove spent solutions (due to extremely high concentrations) and variable amounts in certain metals, including steel alloys. of tools, sintered iron articles and other porous substrates. Unless they are highly experienced operator employees, this process can result in very poor quality finishes. It is common to see undesirable brown / red finishes in certain alloys or salts that are subjected to leaching on porous substrates. As a result, the process is relegated to using professional finishers who possess the specialized knowledge and experience to deal with hazardous materials. Conversion of Ferrous Oxalate conversion.

This coating was originally developed to be used as lubricant forming metals or an anti-chaff coating for corresponding parts or counterparts. The finish is generally applied at temperatures close to the ambient, is approximately 1 miera of thickness and an opaque gray color. When a superior preventive layer is sealed against oxidation, Oxalate offers some degree of protection against corrosion. The oxalate process was most commonly used in the 1950s, is rarely used now, since these have given their place in the market to several phosphate processes, which offer more beneficial properties in terms of lubrication and / or adhesion of the paint. Iron Phosphate Conversion Coating: These coatings are widely used in the metal finishing industry as pre-treatments to increase paint adhesion and corrosion resistance of ferrous metal substrates. With a coating thickness of about 1 miera, the amorphous deposit is formed at temperatures of about 20.9 to 53.9 ° C (70 to 130 ° F) by a soft acid solution, which may also contain cleaning agents. The process of iron phosphate has proven to be a very versatile and effective option in paint lines and other lines of metal finishing processes. There are several patents issued during the years that are related to blackening processes. The purpose of this invention, however, is to refer to prior patents that are directly related to oxalate and phosphate conversion coatings on ferrous metal substrates and related to caustic black oxidation of ferrous metal substrates. Patent of Object Date the US. 2,774,686 18/12/56 Oxalate Coatings on Substrates of Chromium Alloys. 2,791,525 07/05/57 Accelerated Oxalate Coatings with Chlorate on Ferrous Metals to Form Lubricants and Adhesion to Paint. 2,805,696 10/09/57 Accelerated Oxalate Coatings with Molybdenum. 2, 835, 616 20/05/58 Method of Processing Ferrous Metals to Form Coatings and Oxalate. , 850,417 02/09/58 Oxalates Accelerated with Nitrobenzene Sulfonate on Ferrous Metals. , 960,420 11/15/60 Composition and Process of Black Oxidation of Ferrous Metals Using Accelerators based on Mercaptos and Naphthalenes based on Moistening Agents. , 121,033 11/02/64 Oxalates on Stainless Steels, 481,762 02/12/69 Manganese Oxalates Sealed with Graphite and Petroleum to form Lubricating Characteristics. 3,632,452 9/17/58 Accelerated Oxalates with Stabilized Products on Stainless Steels. 3,649,371 03/14/72 Oxalates Modified with Fluoride. 3,806,375 04/23/75 Oxalates Accelerated with Hexamine / S02 3,829,237 04/22/75 Oxalates Accelerated with Manganese, Fluoride and Sulfur. 3,899,367 12/08/75 Composition and Process of Black Oxidation of Ferrous Metals Using Molybdic Acids on Tool Steels. 4,017.35 04/12/77 Composition and Method with Stabilized pH for Fosfatar Iron from Ferrous Metal Surfaces. 5,104,463 04/14/92 Caustic Oxidation Composition and Process of Stainless Steels Using Chromate Accelerators. Of all, only one of these patents related to oxalates belongs to the formation of a ferrous oxalate conversion coating on ferrous metal substrates using various accelerators. These oxalates are intended to be used as functional coatings to aid in the assemblies or to provide the formation of lubricating characteristics, etc. These coatings serve as boundary layers that can be deformed or can be squeezed into the metal surface, thus protecting the base metal during contact with other surfaces. The patents related to caustic black oxidation are focused in relation to the compositions and processes that oxidize the metal-to-magnetite iron substrate, Fe30, as described in U.S. Patent No. 2,960,420. Actually, when the stoichiometry of Fe30 is examined, one can observe that the iron is not in a state of oxidation only ferrous (II) or ferric (III). Perhaps a more accurate description of the material is that of a mixed salt, the ferrous-ferric oxide is FeO Fe203, which shows both irons, ferrous and ferric. Conventional caustic oxidation processes depend on the ability of the operating solution to oxidize metallic iron to the ferrous (II) or ferric (III) oxidation state and to form the Fe O Fe203 mixed oxide. The process described in the U.S. Patent. No. 4,017,335 is representative of the state of the art, focusing on the primary phosphatization mechanism that is well known to those skilled in the art. In addition, the same patent illustrates the incorporation of a cleaning agent and a pH stabilizer into the oxidation solution to effectively clean highly soiled ferrous articles and phosphatise them with iron in a single step.

BRIEF DESCRIPTION OF THE INVENTION This invention provides an alternative method and composition for forming protective and aesthetically pleasing magnetite coatings, as well as functionally useful on ferrous metal substrates. The mechanism comprises a first oxidation to provide an intermediate coating on the metallic iron substrate, such as ferrous oxalate (or other dicarboxylate) or an iron phosphate coating, whose primary purpose is to act as a precursor for magnetite. By providing an abundant surface both molecular and molecular oxygen, the intermediate coating facilitates the formation of magnetite (in a second oxidation), thus requiring a blackening solution with much less potential oxidation than is necessary with conventional oxidation solutions in terms of concentration, operating temperatures and contact times. It is important to note that the oxidation solution used in the second oxidation of this invention is not capable of blackening the metal substrate without the intermediate coating (of the first oxidation) in place. The overall potential oxidation of the second oxidation solution of this invention is much lower than that of the conventional solutions in which no reaction is carried out unless the intermediate coating (of the first oxidation) has been applied in the first place.

After the second oxidation, the coating may be an upper layer coated with a lubricant, an oxidation preventive compound or an upper layer based on a polymer appropriate for the final use of the article. A process according to this invention for forming a hybrid conversion coating on the ferrous metal substrate, which comprises applying to the substrate an intermediate coating rich in molecular iron and oxygen, and subsequently contacting the coated intermediate substrate with an aqueous solution of oxidation to form a surface that contains magnetite. The substrate is coated with an intermediate coating enriched with iron and water-insoluble molecular oxygen by a first oxidation comprising contacting the substrate with an aqueous solution of a dicarboxylic acid, or a reagent selected from phosphoric acid, pyrophosphoric acid and its salts, or their mixtures, at a concentration, pH, temperature and appropriate time to achieve an intermediate coating enriched with iron and water-insoluble molecular oxygen. The coated intermediate substrate is then subjected to a second oxidation by contacting it with an aqueous solution of an oxidizing agent at a concentration, pH, temperature and time to form the desired amount of magnetite. The coated substrate can then be sealed with a top layer. An article of colored ferrous metal coated in accordance with this invention has a surface formed by two treatments, wherein the first treatment is an oxidized intermediate coating enriched with iron / oxygen applied to a ferrous substrate, and the second treatment is an additional oxidation of the first magnetite coating. An oxidation solution for oxidizing at least a portion of an iron / oxygen enriched intermediate coating on a ferrous substrate in magnetite according to this invention comprises an aqueous solution of oxidation agents selected from alkali metal compounds of hydroxide, nitrate, and nitrite and its mixtures, and optionally further includes an additional component selected from an accelerator, a metal chelate former, a surface tension reducer, and mixtures thereof. The invention also provides a seven-step process for forming a hybrid conversion coating on a ferrous metal substrate, comprising the steps of: (1) subjecting the ferrous metal substrate to a selected treatment of cleaning, degreasing and descaling and its mixtures; (2) rinsing the substrate of step (1) with water; (3) subjecting the substrate of step (2) to a first oxidation to form an intermediate enriched iron / molecular oxygen; (4) rinsing the substrate of step (3) with water; (5) subjecting the substrate of step (4) to a second oxidation to form a surface that is predominantly magnetite, Fe304; (6) rinsing the substrate of step (5) with water; and (7) sealing the substrate with an appropriate top layer. DETAILED DESCRIPTION OF THE INVENTION A ferrous metal substrate is defined herein as a metallic substrate whose composition is mainly iron. This can include steel, stainless steel, cast iron, gray and ductile iron and sintered iron of all alloys. The enriched iron / oxygen intermediate coating applied to the substrate in the first oxidation can be formed using any of the water-soluble dicarboxylic acids, especially aliphatic dicarboxylic acids, generally up to about five carbon atoms, such as oxalic, malonic acids, succinic, tartaric, and other acids and their mixtures. There are advantages and disadvantages related to dicarboxylic acids. For example, oxalic acid is generally available at a lower cost and is the most reactive. However, oxalic acid tends to form intermediate coatings of a relatively coarse grain, with large crystals, and the intermediate coat generally benefits by the addition of a more refined grain in the first oxidation, such as the alkali tartrate metal compounds, tripolyphosphate, molybdate, citrate, polyphosphate and thiocyanate, including sodium potassium tartrate, sodium citrate, sodium molybdate, sodium polyphosphate, and sodium thiocyanate. An intermediate coating with a denser crystalline structure is most preferred, because it tends to produce a resultant finish in black (after the second oxidation) which is cleaner, less polished and also thinner, which is desirable for the most machine / tool applications. A mixture of two or more dicarboxylic acids tends to favor the formation of a denser microcrystalline structure on the metal surface, perhaps counteracting the need for a more refined grain. However, the costs of many of the other commercial grade dicarboxylic acids are significantly higher than those of oxalic acid, the solubilities are lower and also the reaction rates are significantly lower. In fact, these other longer chain aliphatic dicarboxylic acids may actually require the use of accelerators instead of grain refiners or in addition thereof in order to be viable in a practical sense. Accelerators suitable for use in the first oxidation include organic and inorganic nitroso compounds, alkali metal compounds of citrata, molybdate, polyphosphate, thiocyanate, chlorate and sulfur, such as sodium chlorate, sodium molybdate and organic nitroso compounds. Alternatively, the enriched iron / oxygen intermediate coating may consist of other coatings, such as iron phosphate. The iron phosphate coating does not appear to be as effective as the dicarboxylate coatings, because the deposit of iron phosphate tends to be amorphous instead of crystalline. Although the adhesion of the iron phosphate to the substrate is generally satisfactory, the wear factors appear in this way to have a more blackened polish in the prepared article. The advantages of phosphate coating, however, include a lower commercial cost for chemicals and the ability to operate at higher pH levels (less acidic). These advantages improve the safety aspects of the worker of the process line. Suitable reagents for the deposition of the water-insoluble phosphate-based coating include phosphoric acid, as well as an alkali metal acid phosphate, alkali metal pyrophosphates, primary alkanolamine phosphates and mixtures thereof. Typically, iron phosphate solutions are capable of operating at a pH of about 3. to 5.0 (dicarboxylates operate at a pH of about 1.0 to 2.0) at temperatures of approximately 20.9 to 53.9 ° C (70 to 130 ° F) and contact times of 1 to 3 minutes. An intermediate coating with a more densely formed crystalline structure tends to concentrate or increase the availability of iron and oxygen, and thus tends to favor the formation of the magnetite in the second oxidation. A more densely formed crystal structure tends to facilitate the blackening of certain ferrous alloys with lower reactivity, such as heat-treated steels or steels with higher alloys. Typically, these types of steels tend to be less reactive because the concentration of metallic iron on the surface is lower than that found with molten irons or softer steels. Accordingly, it is considered most preferable to design the composition of the iron / oxygen rich intermediate coating solution to maximize the density of the crystalline structure of the intermediate coating, thereby overcoming any lower initial reactivity of the iron substrate.

The operating temperature of the intermediate coating solution also has an effect on the speed of the reaction, at higher temperatures the reaction rate tends to decrease. Experimental evidence indicates that although many iron alloys can be processed successfully at ambient temperatures, certain less reactive alloys benefit from the application of the intermediate coating at temperatures of approximately 37.4 to 64.9 ° C (100 to 150 ° F) to overcome any lower initial reactivity of the metal surface. It has been found that the grain refiner suitable for the first oxidation is an alkali metal tartrate, typically at a concentration of about 0.1 to 1.0 grams per liter, the accelerator is selected from organic and inorganic nitrous compounds, alkali metal salts of citrate , molybdate, polyphosphate, thiocyanate, chlorate and sulfur at concentrations of 0.5 to 5.0 grams per liter. In summary, the composition of the intermediate coating solution (the first oxidation) can take many forms, depending on the cost, solubility and activity level of the chemicals used, the pH of the solution and the roughness of the crystalline structure, as well as the initial reactivity of the metallic iron alloy, the value or intended use of the article and other factors considered relevant to each application. After coating the article with the iron / oxygen rich intermediate coating, the article is blackened by contacting it with a second oxidation solution at high temperatures to form the magnetite. Experimental evidence indicates that most of the intermediate coating remains intact on the surface of the article after the second oxidation, with only a small part of the coating reacting to form magnetite. Although the mechanism of the exact reaction of the second oxidation is not clearly understood, it is considered that the parts of the intermediate coating react with the second oxidation solution to form magnetite sandwiched within the crystalline structure of the coating. Some magnetite can be chemically linked to the molecules of the intermediate coating. It is believed that the first oxidation converts metallic iron to Fe (II), when the coating is ferrous dicarboxylate, or to a mixture of Fe (II) and Fe (III) when the coating is an iron phosphate. Accordingly, in this specification the dicarboxylate coating is designated as "ferrous" because the iron is in the ferrous or Fe (II) oxidation state, while the phosphate coating is more broadly designated as "iron". , because the iron is in the ferrous, Fe (II) and ferric oxidation states, Fe (III). It is reasonable to consider that the primary iron oxide formed is Fe304, although it is possible that other iron oxides are formed, such as FeO and Fe203, and other compounds, such as FeS, SnS and SnO (due to the possible presence of sulfur and tin in the reagent solutions), which can be gray / black. Iron oxides tend to be non-stoichiometric and can quickly convert to each other. The tendency of each of the iron oxides that is not stoichiometric is due in some measure to the intimate relationship between their structures. The structure of each oxide can be visualized as a cubic arrangement of closed compact mass of the oxide irons with a certain number of Fe (II) and / or Fe (III) ions distributed between the octahedral and tetrahedral orifices. Each of the iron oxides can alter its composition in the direction of one or two of the others without there being any major structural change, only a redistribution of ions between the tetrahedron and octahedron interstices. Because of their rapid convertibility among themselves, their trend is not stoichiometric, and in general, the complexity of the Fe-O system. For a further discussion of iron oxides, see, for example, Cotton and Wilkinson, Advanced Inorganic Chemistry, Interscience Publishers, 1966, Second Edition, pages 847-862. Subsequently the second oxidation converts at least a part of the intermediate coating to magnetite. The mechanism of the exact reaction for the second oxidation has not been determined. However, the non-stoichiometric nature and rapid convertibility of these iron compounds, as recognized in the art and as discussed in Cotton and Wilkinson, it is reasonable to believe that the resulting black coating is composed of a mixture of iron and oxygen that it only vaguely resembles precise or discrete compounds. The composition of the second oxidation solution may vary, depending on the type, thickness and grain structure of the intermediate coating prepared. Generally, it is considered preferable to add at least one, two or even three oxidants and one accelerator to the second oxidation solution. The primary oxidants may be alkali metal compounds of hydroxide, nitrate and nitrite and their mixtures. Specific examples of suitable primary oxidants include sodium hydroxide, sodium nitrate and sodium nitrite at varying concentrations. However, in each of the cases, the entire concentration of the oxidants according to this invention is significantly lower than what is seen in conventional oxidation processes, as described in US Pat. mentioned previously. For example, U.S. Pat. No. 3,899,367 suggests the following concentrations in the oxidation solutions, sodium hydroxide 200-100 grams per liter sodium nitrate 12-60 grams per liter sodium nitrite 30-150 grams per liter along with lower concentrations of such additives as accelerators and wetting agents. The actual practice in the metal finishing industry indicates that only the upper end of the concentration range shown in the previous example of US Pat. No. 3,899,367 is effective to produce a satisfactory black coating of magnetite. Solutions with lower concentrations tend to boil at lower temperatures, leading to the formation of unwanted red and brown coatings less than satisfactory results. According to the present invention, the optimum concentrations used for the second oxidation solution to produce satisfactory final black magnetite coatings may be as follows: sodium hydroxide 25-200 grams per liter sodium nitrate 9-70 grams per liter nitrite Sodium 1-10 grams per liter Additional components that can be added to the second oxidation solution include accelerators, metal chelating agents, and surface tension reducers. Accelerators suitable for the second oxidation include the organic and inorganic nitroso compounds, the alkali metal compounds of citrate, molybdate, polyphosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfur and thiosulfate and stannous chloride and stannic chloride. Suitable accelerators are selected according to such considerations as cost and solubility. Suitable metal chelate formers include the alkali metal compounds of thiosulfate, sulfide, ethylenediamine tetraacetate, thiocyanate, gluconate, citrate and tartrate. Suitable chelate formers are selected according to such considerations as cost, solubility and reactivity. Suitable surface tension reducers include alkylnaphthalene sulfonate and related compounds that are stable in environments with high pH. A suitable accelerator for the second oxidation is selected from the alkali metal salts of molybdate, vanadate, tungstate, thiocyanate, dichromate, stannate, thiosulfate, stannous chloride and stannic chloride, preferably at concentrations of 0.05 to 0.5 grams per liter. A metal chelator for the second oxidation is selected from alkali metal salts of thiosulfate, sulfide, ethylenediamine tetraacetate, thiocyanate, gluconate, citrate or tartrate, preferably in concentrations of about 1.0 to 10.0 grams per liter. A suitable surface tension reducer for the second oxidation is selected from alkylnaphthalene sulfonate, at typical concentrations of about 0.025 to 0.2 grams per liter. The reaction parameters suitable for the second oxidation are as follows: the pH varies: approximately from 12.0 to 14.0, typically around 13 to 14; the operating temperature varies: around 48.4 to 103.4 ° C (120 to 220 ° F), typically around 70.4 to 92.4 ° C (160 to 200 ° F); The contact time varies: around 0.5 to 10 minutes, typically around 2 to 5 minutes. Temperatures as low as 20.9 to 26.4 ° C (70 to 80 ° F) have been used successfully in reaction times of 30 minutes or more. The iron / oxygen rich intermediate coating (from the first oxidation) is responsible for reducing the minimum oxidation potential necessary for satisfactory coatings. Since the metal substrate has already been oxidized by the intermediate coating solution (the first oxidation), it is easier for an effective oxidation solution to provide the oxidation finish at the level of black magnetite (the second oxidation). The solution of the second oxidation can not react with the metallic iron; the second oxidation solution reacts only with the previously existing, easily accessible iron and oxygen contained in the intermediate coating. Because the intermediate coating (of the first oxidation) facilitates the reaction of the second oxidation, a solution of the second oxidation much less effective than that which has been typically used in conventional blackening processes is required. In a similar manner, the operating temperature and contact time for the second oxidation are significantly reduced from similar parameters for conventional oxidation solutions. Again the patent of the U.S.A. No. 3,899,367 suggests an operating temperature of 105.15 to 161.15 ° C (225 to 325 ° F) and contact times of 10 to 25 minutes. In actual practice, it has been found that the optimum operating temperature for the process of U.S. Pat. No. 3,899,367 is approximately 139.15 to 144.65 ° C (285 to 295 ° F) with a contact time of 10 to 25 minutes. According to the present invention, the optimum temperature gradient for the second oxidation is approximately 86.9 to 103.4 ° C (190 to 220 ° F) for black coatings and approximately 70.4 to 86.9 ° C (160 to 190 ° F) for brown coatings. Contact times are approximately 2 to 10 minutes.

These parameters are significantly lower than those of the conventional oxidation solutions employed in U.S. Pat. No. 3,899,367. Among the important advantages of the process of this invention are the surprisingly low temperatures at which this second oxidation can operate successfully. Reactions at temperatures as low as 20.9 to 26.4 ° C (70 to 80 ° F) produce products with a surface finish with a highly acceptable color, generally by increasing the contact time, for example approximately 30 minutes or more. The ability to operate successfully at such low temperatures offers substantial advantages by providing a process that can be safe and carried out effectively by an end user. As the procedures of "low temperature with a longer time" produce attractive finishes for less demanded final products, including such decorative and artistic products as ornamental wrought iron work, locksmith finishes, sculptural works, handmade crafts and handicrafts and similar improvements. These finishes of the "low temperature and longer time" procedures can show the colors in the range from black to dark black to brown. In addition, the product's ornament with color may include the removal of a certain color finish to reveal the underlying metal's brightness, achieving a patina and antique effect. Although this is naturally known in kinetic reactions that lower the operating temperature that can demand increases in reaction times, the ability to operate at such surprisingly low temperatures has not been reported anywhere in the industry, with the knowledge of the inventors of the present. Together with the primary oxidation agents, the second oxidation solution may preferably contain an accelerator. In the present invention, the accelerators for the solution of the second oxidation may be alkali metal compounds of molybdate, vanadate, tungstate, thiocyanate, dichromate, stannate or thiosulfate or stannous chloride, stannic chloride, sodium stannate, sodium thiosulfate, molybdate of sodium and thiourea of ethylene and their mixtures. Other accelerators that have been mentioned in previous related literature, including sodium dichromate, sodium tungstate, sodium vanadate, sodium thiocyanate and benzothiacil disulfide. In addition, the surface tension reducing agents tend to increase the washing capacity and reduce the delay of the solution. Effective surface tension reducing agents include alkylnaphthalene sodium sulfonate, such as that manufactured by Witcho Corporation under the Petro AA brand, and similar surface tension reducing agents.

It is important to note that in the second oxidation of this invention the total concentration of the primary oxidants and the relative concentrations of each oxidant in the second oxidation solution are critical factors for success. It has been mentioned that the second oxidation solution of this invention can not react with metallic iron, because the oxidation potential of the solution is much lower. Similarly, when treating a ferrous substrate, as defined above, with a conventional oxidation solution and only reducing the concentration, the temperature and contract time will not result in satisfactory finishes. In general, the finishes obtained by treating a ferrous substrate with a conventional oxidation solution at a reduced concentration, temperature and contact time is a vaguely adherent coating with an undesirable brown color. For example, the oxidation solution described in U.S. Pat. No. 2,960,420, when operated at concentrations, contact times and reduced temperatures (around 86.9 to 92.4 ° C (190 to 200 ° F)) reacts poorly with the intermediate coating, producing finishes that are brown and adherent in a very vague way. In a similar manner, the oxidation solutions described in U.S. Pat. No. 3,899,367 also produce brownish, loosely adhering, undesirably thin coatings under similar operating conditions. The main benefits derived from the process according to the present invention are not related to the quality of the same black finish, but to the advantages of the processing. These improved advantages include lower operating temperatures, shorter process times, and lower solution concentrations, which lead to lower operating costs and an increase in worker safety. The resulting black finish is very comparable to that of the conventional blackening process, in terms of the corrosion resistance, wear resistance, appearance, thickness and applications in which the finished article is used. The process of the present invention causes the deposition of the conversion intermediate coating, which is rich in iron and oxygen and represents a first oxidation of the metallic iron substrate. This first oxidation (which forms the intermediate conversion coating) is followed by a second oxidation, which forms a magnetite compound by reacting with the intermediate coating. The precise chemical composition of the resulting black finish has not been identified. The chemical literature, as discussed above, suggests that there are three iron oxides, which are similarly present in the intermediate conversion coating: FeO, Fe203 and Fe304, which is a mixed salt of FeO and Fe203. In addition, these oxides are similar to other salts that are formed on the surface in smaller amounts, including FeS, SnS, SnO, due to the presence of tin-based and sulfur-based additives in the solution. The first oxidation and the intermediate conversion coating formed by this invention, which may be a dicarboxylate, a phosphate and its mixtures or certain other iron / oxygen rich materials, depending on the oxidation solution used, are not novel in themselves. The first oxidation and the intermediate conversion coating are in fact based on known chemicals. The novelty of the present invention is the use of these coatings (and the process of forming them) in the context of a blackening process. The novelty of the process, and the key to its success, lies in the solution of the second oxidation and its reaction with the intermediate coating. The concept of an initial oxidation of metallic iron to form an intermediate coating of dicarboxylate, phosphate or other iron / oxygen enriched intermediate coating, followed by further oxidation of the intermediate coating is a novel concept in the industry and depends on the composition and the operating parameters of the second oxidation solution To date our research does not indicate that all the dicarboxylate, phosphate and other intermediate coating enriched with iron / oxygen of the first oxidation becomes iron magnetite, Fe304 in The second oxidation, rather, our experimental work suggests that the second oxidation solution reacts with the iron and molecular oxygen of the intermediate coating.Although the entire intermediate coating is rich in iron and molecular oxygen, it is reasonable to assume that the area in the which these materials are more accessible is in their upper surfaces of the crystal structure of the intermediate coating. Truly, our tests have indicated that the black finish formed by the entire process (the first and second oxidations) of this invention can be removed from the steel article with hydrochloric acid, leaving behind a finish that looks gray. This finish that looks gray is the coating intermediate. The article can immediately be re-blackened by immersion in the second oxidation solution. We have experimentally determined that the second oxidation solution has no effect on metallic iron. The removal and re-blacking experiment reasonably suggests that only the upper surface of the intermediate coating turned black. In case all the intermediate coating becomes black iron magnetite, the removal operation with hydrochloric acid will remove all the coating, reducing the metallic iron and it would be impossible to re-black the article without first re-coating it with the intermediate coating. The invention will now be further illustrated by the description of certain specific examples of the practice which is intended to be illustrative only and not limiting in any way. Example 1 First Oxidation: A 1018 steel article is cleaned by conventional means. The clean article is then immersed for 1 minute at room temperature in an aqueous solution containing: Oxalic acid 14 g / 1 Phosphoric acid 1.2 g / 1 Sodium sulfonate m-Nitrobenzene 6 g / 1 Sodium-potassium tartrate 0.4 g / 1 The previous immersion produces an opaque gray intermediate coating on the surface of the steel. Second Oxidation: After mopping, the coated intermediate article is immersed for 4 to 5 minutes at a temperature of 92.4 ° C (200 ° F) in an aqueous solution containing: Sodium Hydroxide 100 g / 1 Sodium Nitrate 35 g / 1 Sodium Nitrite 5 g / 1 Sodium Thiosulfate 5 g / 1 Sodium Molybdate 5 g / 1 Stale Chloride 0.2 g / 1 Petro AA 0.1 g / 1 During this second immersion, the article gradually turns black due to the formation of magnetite on the surface. The article is then rinsed in water and sealed with a top layer of an oil that prevents the oxidation that displaces the water. The resulting coating is of an opaque black color, tightly adherent with a corrosion resistance equal to that provided by the oil sealer of the upper layer. Example 2 First Oxidation: A heat treated 4140 steel cutting tool is cleaned and descaled by conventional means. The tool is then submerged for 90 seconds at a temperature of 48.4 ° C (120 ° F) in an aqueous solution containing: Oxalic acid 14 g / 1 Phosphoric acid 1.2 g / 1 Sodium sulphonate m-Nitrobenzene 6 g / 1 Previous immersion produces an opaque gray intermediate coating on the surface of the steel. Because the 4140 steel is less reactive than the steel 1018 used in Example 1, the above oxidation solution has been modified from the first oxidation solution of Example 1 to remove the grain refiner (Sodium-Potassium Tartrate) and raise the operating temperature to make the reaction more aggressive. Second Oxidation: After mopping, the tool is immersed for 8 minutes at a temperature of 92.4 ° C (200 ° F) in an aqueous solution containing: Sodium Hydroxide 100 g / 1 Sodium Nitrate 35 g / 1 Sodium Nitrite 5 g / 1 Sodium Thiosulfate 5 g / 1 Sodium Molybdate 5 g / 1 Static Chloride 0.2 g / 1 Petro AA 0.1 g / 1 During this second immersion, the tool gradually takes on an opaque black color. The tool is then rinsed in water and sealed with an oil that prevents the oxidation that displaces the water. Example 3 First Oxidation: A decorative mild steel article is cleaned by conventional means and immersed for 1 minute at room temperature in an aqueous solution containing: Oxalic acid 14 g / 1 Phosphoric acid 1.2 g / 1 Sodium sulphonate m- Nitrobenzene 6 g / 1 Sodium-Potassium Tartrate 0.4 g / 1 Previous immersion will produce an opaque gray intermediate coating on the surface of the article after rinsing. Second Oxidation: The intermediate article is then immersed for 6 minutes at a temperature of 81.4 ° C (180 ° F) in an aqueous solution containing: Sodium Hydroxide 100 g / 1 Sodium Nitrate 27 g / 1 Ethylene Thiourea 0.6 g / 1 Tin Chloride (IV) 2 g / 1 Sodium Dichromate 0.3 g / 1 Petro AA 0.1 g / 1 During this second previous immersion, the article gradually takes on an opaque brown color. The article is then rinsed in water and sealed with an upper layer based on a transparent acrylic polymer. The resulting coating can serve as an aesthetic finish for decorative locksmith, etc. Example 4 First Oxidation: A metallic sintered iron article is cleaned by conventional means and immersed for 3 minutes at a temperature of 48.4 ° C (120 ° F) in an aqueous solution containing: Phosphoric Acid 28 g / 1 Hydrofluorosilicic Acid 8 g / 1 Xylene sulfonic acid 3 g / 1 Dodecylbenzenesulfonic acid 2 g / 1 Monoethanolamine 17 g / 1 Sodium sulphonate m-Nitrobenzene 1 g / 1 Molybdenum trioxide 0.2 g / 1 After this immersion, the article has an intermediate deposit coating of opaque gray iron phosphate. Second Oxidation: After drying, the article is immersed for 5 minutes at a temperature of 92. 4 ° C (200 ° F) in an aqueous solution containing: Sodium Hydroxide 100 g / 1 Sodium Nitrate 35 g / 1 Sodium Nitrite 5 g / 1 Sodium Thiosulfate 5 g / 1 Sodium Tungustate 5 g / 1 Sodium Stanate 0.2 g / 1 Petro AA 0.1 g / 1 During the previous dive, the article gradually takes on a black color. Later it was rinsed in water and sealed with oil that prevents the oxidation that displaces the water. The resulting finish is somehow more fragile than that deposited in Examples 1 and 2, but may be considered preferable for certain applications due to the lower expected operating cost. In addition, the extremely porous substrate produced by this process may tend to make its fragile nature unimportant, depending on the uses of the article. Due to the potentially dangerous nature of the known prior concealment processes, many manufacturers have found it more convenient to ship the parts to an outside vendor for the application of the black finish. Naturally, this is inefficient and is added to the total cost of production. A particular feature of this invention is a seven-step process that can be provided in an arrangement of seven batches or containers, so that a metal fabricator can safely and conveniently carry out the metal blackening process inside his or her establint without a risk for employees placed in such previous blackening procedures. The inventive process can be carried out commercially as a seven step process as follows: Step 1: The article is cleaned, degreased and descaled (if necessary) to remove foreign materials such as manufacturing oils, coolant, lubricants strangers, oxidation, lamination flakes, heat treatment flakes, etc. The help with this is to generate a metallic surface that is free of oils and oxides, which exposes a uniform reactive metal surface. Any known method for providing such a surface in the metal finishing industry is convenient. Acceptable methods include conventional cleaning in an alkaline detergent cleaner, a solvent degreaser or electrolysis. Descaling can be achieved by acid or caustic descaling methods. Abrasive cleaning methods such as blowing beads, mincing with ammunition and polishing with steam can be used with good results. All these methods are well known in the metal finishing industry. Step 2: The article is rinsed in clean water to remove any of the cleaning residues from the surface. Step 3 (First Oxidation): The article is then subjected to a first oxidation to provide an intermediate coating on the metallic iron substrate. The oxidation reagent is an aqueous solution of either dicarboxylate or phosphate or mixtures thereof, optionally with a grain refiner to provide the water-insoluble dicarboxylate-based deposit or a water-insoluble phosphate-based deposit, or mixtures thereof. Suitable dicarboxylic acids include aliphatic dicarboxylic acids generally of up to about five carbon atoms, such as oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, tartaric or citric acid and mixtures thereof. When the intermediate coating is a ferrous oxalate, the appropriate reaction parameters are as follows: the pH varies: approximately 0.5 to 2.5, typically around 1.6; the operating temperature varies: approximately 9.9 to 64.9 ° C (50 to 150 ° F), typically around 23.65 ° C (75 ° F), the contact time varies: approximately 0.5 to 5.0 minutes, typically around 2 minutes Suitable reagents for the deposition of the water-insoluble phosphate-based coating include phosphoric acid, as well as, alkali metal acid phosphates, alkali metal pyrophosphates or primary alkanolamine phosphates. When the intermediate coating is an iron phosphate, the suitable reaction parameters are as follows: the pH varies: about 3.0 to 5.5, typically about 4.0 to 50; the operating temperature varies: approximately 15.4 to 26.4 ° C (60 to 80 ° F), typically around 48.4 to 53.9 ° C (120 to 130 ° F); The contact time varies: approximately 1 to 10 minutes, typically around 3 to 5 minutes. Suitable grain refiners include alkali metal compounds of tartrate, tripolyphosphate, molybdate, citrate, polyphosphate and thiocyanate, such as sodium-potassium tartrate. A grain refiner is sodium-potassium tartrate. A solution of the first suitable oxidation according to this invention is prepared as follows: Component Concentration Acceptable Range Oxalic Acid 14 g / 1 3-35 g / 1 Phosphoric Acid 1.2 g / 1 0.5 to 3.0 g / 1 Sodium Sulfonate m-Nitrobenzene 6 g / 1 1 to 15 g / 1 Sodium potassium tartrate 0.4 g / 1 0.1 to 2.0 g / 1 The contact time in this solution is usually around 1 to 3 minutes at a temperature approximately 9.9 to 64.91 ° C (50 to 150 ° F). The resulting deposit is an opaque gray dicarboxylate intermediate coating. Alternatively, a solution can be used to phosphatize the iron to deposit an intermediate coating that is also effective. A suitable composition and an acceptable range of concentrations for this option is shown below: Component Concentration Acceptable Range Phosphoric acid 28 g / 1 7-70 g / 1 Fluorosilicic acid 8 g / 1 2 to 20 g / 1 Xylene sulfonic acid 3 g / 1 1-7.5 g / 1 Dodecylbenzenesulfonic acid 2 g / 1 1-5.0 g / 1 Monoethanolamine 17 g / 1 4-43 g / 1 Sodium Sulfonate m-Nitrobenzene 1 g / 1 0.25 to 2.5 g / 1 Molybdenum Trioxide 0.2 g / 1 0.05 to 0.5 g / 1 The contact time in this solution is usually around 1 to 3 minutes at an approximate temperature of 26.4 to 64.9 ° C (80 to 150 ° F), resulting in a deposition of an opaque gray iron phosphate intermediate coating. Step 4: The article is rinsed in clean water to remove any of the residues from the solution from the surface. Step 5 (Second Oxidation): The article is then oxidized on a colored surface by a second oxidation with an aqueous solution of oxidizing agents for a sufficient time to achieve the desired surface color. The composition of this second oxidation solution may include the primary oxidants together with such additional components as accelerators, metal chelate formers and surface tension reducers. Suitable oxidants include the alkali metal compounds of hydroxide, nitrate and nitrite. The oxidation solution for the blackening reaction (the second oxidation) preferably contains three oxidants, sodium hydroxide, sodium nitrate and sodium nitrite. In case one of these oxidants is omitted, it has been found that the blackening reaction proceeds less efficiently. Accelerators suitable for the second oxidation include the organic and inorganic nitroso compounds, the alkali metal compounds of citrate, molybdate, phosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfur and thiosulfate and stannous chloride and stannic chloride. Suitable accelerators are selected according to such considerations as cost and solubility. Suitable metal chelate formers include alkali metal thiosulfate, sulfide, ethylenediamine tetraacetate compounds; thiocyanate, gluconate, citrate and tartrate. Suitable metal chelate formers are selected according to such considerations as cost, solubility and reactivity. Suitable surface tension reducers include alkylnaphthalene sulfonates and related compounds that are stable in environments with high pH. The reaction parameters suitable for the second oxidation are as follows: the pH varies: approximately from 12 to 14, typically around 13 to 14; the operating temperature varies: approximately from 48.4 to 103.4 ° C (120 to 220 ° F), typically around 70.4 to 92.4 ° C (160 to 200 ° F); The contact time varies: approximately 0.5 to 10 minutes, typically around 2 to 5 minutes. The typical composition and range of concentrations for the process solution for Step 5 are as shown below: Component Concentration Acceptable Range Sodium Hydroxide 100 g / 1 25-200 g / 1 Sodium Nitrate 35 g / 1 8.75 a 70 g / 1 Sodium Nitrite 5 g / 1 1 to 10 g / 1 Sodium Thiosulfate 5 g / 1 1 to 10 g / 1 Sodium Molybdate 5 g / 1 1 to 10 g / 1 Tin Chloride 0.2 g / 1 .05 to 0.4 g / 1 Petro AA 0.1 g / 1 .025 to 0.2 g / 1 The normal contact time for the second oxidation is approximately 2 to 1Q minutes at a temperature of approximately 70.4 to 103.4 ° C (160 to 220 ° F). The resulting coating can be black or brown, depending on the exposure time, temperature and composition of the oxidation solution. Step 6: The article is rinsed in clean water to remove any of the residues of the oxidation solution from the surface. Step 7: The article is then sealed with a top layer suitable for the final use of the product, such as a lubricant, an oxidation preventive compound or a polymer-based top coat. The cleaning and rinsing techniques, such as those described for Steps 1, 2, 4 and 6, can vary widely and are well known in the metal finishing industry. Many different techniques can be used, depending on the condition of the metal surface before the blackening, the volume of work to be done, the finishing requirements, etc. As a result, alternate cleaning and rinsing techniques, recognized within the metal finishing industry, can be used and determined by the process operator. The specific cleaning and rinsing techniques described above will be considered only illustratively. Below is a description of the parameters of the seven-step sequence as described above used to produce a black finish on a substrate of a steel panel 1018 with low carbon content, which exemplifies the operation of the process of this invention. at extraordinarily low temperatures of 26.4 ° C (80 ° F): Step 1: The panel is cleaned as described above. Step 2: The panel is rinsed as described above. Step 3 (First Oxidation): A coating of dicarboxylate is provided.

Step 4: The panel is rinsed as described above.

Step 5: (Second Oxidation): The panel is oxidized to produce a black finish. The reaction parameters suitable for the second oxidation are as follows: the pH varies: approximately from 12 to 14, typically around 13 to 14; The operating temperature varies: approximately 26.4 ° C (80 ° F); the contact time varies: approximately 30 minutes. The composition and concentrations for this process solution are shown below: Component Concentration Sodium Hydroxide 175 g / 1 Sodium Nitrate 60 g / i Sodium Nitrite 10 g / s Sodium Thiosulfate 10 g / i Sodium Molybdate 8 « g / i Tin Chloride (IV) 0., 5 g / l Petro AA 0.2 g / 1 Step 6: The panel is rinsed as described above. Step 7: The panel is then sealed with an appropriate topcoat for final use as described above, such as a lubricant, an oxidation preventive compound or a polymer based topcoat.

Claims (1)

1. CLAIMS 1. A process for forming an intermediate conversion coating on a ferrous metal substrate comprising the steps of: (a) applying to the substrate an intermediate coating rich in iron and molecular oxygen: (b) contacting the coated substrate of the step (a) with an aqueous solution of oxidizing agents to form a surface that is predominantly of magnetite, Fe304. Z. The process of claim 1, wherein in step (a) the substrate is coated with a coating of water-insoluble dicarboxylate by contacting the substrate with an aqueous solution of a dicarboxylic acid at a concentration, pH, temperature. and time to achieve the desired dicarboxylate coating. The process of claim 1, wherein in step (a) the substrate is coated with a water-insoluble iron phosphate coating by contacting the substrate with an aqueous solution of a reagent selected from phosphoric acid, pyrophosphoric acid and its salts and mixtures thereof, at a concentration, pH, temperature and time to achieve the desired phosphate coating. The process of claim 1, wherein in step (b) the substrate coated in step (a) is contacted with an aqueous solution of an oxidizing agent at a concentration, pH, temperature and time to form a coating with the desired amount of magnetite. The process of claim 2, wherein in step (a), the dicarboxylic acid is selected from oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, maleic acid, malic acid, tartaric acid , citric acid and its mixtures. The process of claim 5, wherein the dicarboxylic acid is oxalic acid in a concentration of about 3 to 35 grams per liter, a pH of about 0.5 to 2.5, at a temperature of about 9.9 to 64.9 ° C (50 at 150 ° F) and an approximate contact time of 0.5 to 5.0 minutes. The process of claim 4, wherein in step (b) the aqueous oxidation solution contains oxidizing agents selected from alkali metal hydroxides at approximate concentrations of 25 to 200 grams per liter, the alkali metal nitrate at approximate concentrations from 9 to 70 grams per liter, and alkali metal nitrite at concentrations of approximately 1 to 10 grams per liter, with an approximate pH of 13 to 14, an approximate temperature of 48.4 to 103.4 ° C (120 to 220 ° F) and an approximate contact time of 2 to 10 minutes. The process of claim 1, further comprising the step of sealing the substrate with a top layer after step (b). The process of claim 2, wherein in step (a) the substrate is coated in the presence of an additive selected from a grain refiner and an accelerator. The process of claim 9, wherein the grain refiner is an alkali metal tartrate at an approximate concentration of 0.1 to 1. grams per liter. The process of claim 9, wherein the accelerator is selected from organic and inorganic nitroso compounds, alkali metal salts of citrate, molybdate, polyphosphate, thiocyanate, chlorate and sulfur in concentrations of approximately 0.5 to 5 grams per liter. The process of claim 3, wherein in step (a) the substrate is coated in the presence of an accelerator. The process of claim 12, wherein the accelerator is selected from organic and inorganic nitroso compounds in concentrations of approximately 0.1 to 5.0 grams per liter. The process of claim 1, wherein 15. The process of claim 14, wherein the substrate coated from step (a) is contacted in step (b) with an aqueous solution of oxidizing agents in the presence of an additive selected from an accelerator, a metal chelate former and a surface tension reducer. 16. The process of claim 14, wherein the metal chelate former is selected from alkali metal salts of thiosulfate, sulfide, ethylenediamine tetraacetate, thiocyanate, gluconate, citrate or tartrate in concentrations of approximately 1 to 10 grams per liter. The process of claim 14, wherein the surface tension reducer is selected from alkylnaphthalene sulfonate in concentrations of approximately 0.025 to 0.2 grains per liter. 18. A ferrous metal article prepared according to any of claims 1 to 17. 19. A ferrous metal article coated with color having a surface formed by two treatments, wherein the first treatment comprises an oxidized, enriched intermediate coating. iron / oxygen formed on the ferrous metal article, and the second treatment comprises an additional oxidation of the first coating to convert the first coating into a magnetite coating on the ferrous metal article. 20. An oxidation solution for oxidizing at least one part of an iron / oxygen enriched intermediate coating on a ferrous magnetite substrate comprising an aqueous solution of oxidizing agents selected from alkali metal compounds of hydroxide, nitrate and nitrite and its mixtures 21. The oxidation solution of claim 20, and further including an additional component selected from an accelerator, a metal chelator and a surface tension reducer and mixtures thereof. 22. The oxidation solution of the claim 20, wherein the oxidizing agents are sodium hydroxide, sodium nitrate and sodium nitrite. 23. The oxidation solution of the claim 21, wherein the accelerator is selected from organic and inorganic nitroso compounds, alkali metal compounds of citrate, molybdate, polyphosphate, vanadate, chlorate, tungstate, thiocyanate, dichromate, stannate, sulfur and thiosulfate, stannous chloride and stannic chloride and mixtures thereof . The oxidation solution of claim 21, wherein the metal chelator is selected from alkali metal thiosulfate, sulfide, ethylenediamine tetraacetate compounds; thiocyanate, gluconate, citrate and tartrate and their mixtures. 25. The oxidation solution of claim 21, wherein the surface tension reducer is selected from alkylnaphthalene sulfonate and related compounds, which are stable in environments with high pH. 26. The solution of claim 25, with a pH ranging from 12 to 14. 27. A process for preparing a hybrid conversion coating on a ferrous metal substrate, comprising the steps of: (1) subjecting the substrate to ferrous metal to a selected treatment of cleaning, degreasing and descaling and their mixtures; (2) rinsing the substrate of step (1) with water; (3) subjecting the substrate of step (2) to a first oxidation to form an intermediate coating enriched with iron / molecular oxygen; (4) rinsing the substrate of step (3) with water; (5) subjecting the substrate of step (4) to a second oxidation to form a coating predominantly of magnetite, Fe30; (6) rinsing the substrate of step (5) with water; and (7) sealing the substrate with an appropriate top layer 28. The process of claim 4, wherein in step (b) the aqueous oxidation solution is at a temperature of about 20.9 to 48.4 ° C (70 to 120 °). F), and an approximate contact time of 10 to 30 minutes. SUMMARY The invention is a method for forming a chemical conversion coating on ferrous metal substrates, chemical solutions are used for the coating and articles coated therewith. By modifying and combining the characteristics of two existing coating techniques, but to date unrelated, a hybrid conversion coating is formed. Specifically, an enriched iron / molecular oxygen intermediate coating, such as a dicarboxylate or phosphate, is applied to a ferrous substrate by first oxidation. The intermediate coating preconditions the substrate to form a surface rich in iron and molecular oxygen in a more easily accessible form for a subsequent reaction. This oxidation process is followed by a coloring process using a hot oxidation solution (approximately 48.4 to 103.4 ° C (120 to 220 ° F)) containing an alkali metal hydroxide, alkali metal nitrate, alkali metal nitrite. or its mixtures, which reacts with the enriched intermediate coating of iron and oxygen to form magnetite, (Fe304). The result is the formation of a brown or black finish under conditions much more favorable, benign and safe than those previously observed with conventional caustic blackening processes, by virtue of the chemical reaction between the intermediate coating and the solution of the second oxidation. When sealed with an appropriate top layer that is preventive of oxidation, the end result is an ultra-thin, protective attractive finish applied through a simple immersion technique. The finish is a final protective coating on a fabricated metal article and also provides a degree of lubricity to assist assembly, forcing sliding surfaces or providing anti-chafing protection. The finish also provides an adherent base for paint finishes.