1. Field of the Invention
This invention relates to the formation of a hybrid chemical conversion coating on ferrous metal substrates, consisting of an iron/oxygen rich intermediate coating and a top layer of magnetite. This invention also relates to ferrous metal substrates coated according to the presently disclosed process. This invention further includes the oxidation solution used in oxidizing the iron/oxygen rich intermediate coating to the final magnetite containing top layer. This invention also includes a seven-step procedure for preparing a ferrous metal substrate with a magnetite containing coating.
2. Description of the Related Art
The established art of coloring ferrous metals has revolved principally around methods for producing black coatings. Since the 1950's, the most commonly used commercial method for blackening ferrous metals has been the caustic black oxidizing process. This method will be examined, along with the ferrous oxalate conversion coating on ferrous metal substrate and the iron phosphatizing process.
Caustic black oxidizing:
This process uses sodium hydroxide, sodium nitrate and sodium nitrite as oxidizing agents, operating at about pH 14, at temperatures of about 285-305.degree. F. A black coating is formed during exposures of about 10-30 minutes. This process forms a magnetite (Fe.sub.3 O.sub.4) deposit, approximately 1 micron thick, by reacting with the metallic iron substrate in situ. Although the process produces high quality black finishes when operated properly, 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 oxidizing salts and the solution boils off significant quantities of water. These materials must be added back to the solution to maintain proper operating conditions. However, adding sodium hydroxide to water, being a highly exothermic reaction, is quite hazardous because the operating solution is already boiling. Likewise, adding make-up water to a solution which is already at 285-305.degree. F. causes the water to instantly boil if not added very slowly and carefully. Consequently, the operation of the process poses severe safety hazards for personnel, due to the dangers involved in normal system operation and maintenance. These hazardous conditions may be difficult to justify in the manufacturing environments of modem industry. In addition, normal operating conditions typically entail heavy sludge formation in the process tank, difficulty in disposal of the spent solutions (due to extremely high concentrations), and variable quality on certain metals, including tool steel alloys, sintered iron articles or other porous substrates. Unless highly skilled operators are employed, this process may result in poor quality finishes. It is common to see undesirable red/brown finishes on certain alloys or salt leaching on porous substrates. As a result, the process is largely relegated to use by professional metal finishers who possess specialized knowledge and experience in dealing with hazardous materials.
Ferrous oxalate conversion coating:
This coating was originally developed for use as a metal forming lubricant and anti-galling coating for mating parts. The finish is generally applied at about ambient temperatures, is about 1 micron thick and opaque gray in color. When sealed with a rust preventative topcoat, the oxalate offers some degree of corrosion protection. Used more commonly in the 1950's, the oxalate process is rarely used today, having given way to the several phosphate processes on the market, which offer more beneficial properties in terms of lubrication and/or paint adhesion.
Iron phosphate conversion coating:
These coatings are widely used in the metal finishing industry as pretreatments to enhance paint adhesion and corrosion resistance on ferrous metal substrates. With a coating thickness of about 1 micron, the amorphous deposit is formed at temperatures of about 70-130.degree. F. by a mildly acid solution which may also contain cleaning agents. The iron phosphate process has proven to be a very versatile and effective option in paint lines and other metal finishing process lines.
There have been several patents issued over the years which relate to blackening processes. For purposes of this invention, however, reference is made to prior patents which are directly related to oxalate and phosphate conversion coatings on ferrous metal substrates and to the caustic black oxidizing of ferrous metal substrates:
U.S. Pat. No. Date Subject 2,774,696 12/18/56 Oxalate Coatings on Chromium Alloy Substrates 2,791,525 5/7/57 Chlorate Accelerated Oxalate Coatings on Ferrous Metals for Forming Lubricity and Paint Adhesion 2,805,696 9/10/57 Molybdenum Accelerated Oxalate Coatings 2,835,616 5/20/58 Method of Processing Ferrous Metals to Form Oxalate Coatings 2,850,417 9/2/58 m-Nitrobenzene Sulfonate Accelerated Oxalates on Ferrous Metals 2,960,420 11/15/60 Composition and Process For Black Oxidizing of Ferrous Metals Using Mercapto-Based Accelerators and naphthalene based Wetting Agents 3,121,033 2/11/64 Oxalates on Stainless Steels 3,481,762 12/2/69 Manganous Oxalates Sealed with Graphite and Oil for Forming Lubricity 3,632,452 9/17/58 Stannous Accelerated Oxalates on Stainless Steels 3,649,371 3/14/72 Fluoride Modified Oxalates 3,806,375 4/23/75 Hexamine/SO.sub.2 Accelerated Oxalates 3,879,237 4/22/75 Manganese, Fluoride, Sulfide Accelerated Oxalates 3,899,367 8/12/75 Composition and Process For Black Oxidizing Of Ferrous Metals Using Molybdic Acids On Tool Steels 4,017,335 4/12/77 pH Stabilized Composition and Method For Iron Phosphatizing Of Ferrous Metal Surfaces 5,104,463 4/14/92 Composition and Process For Caustic Oxidizing Of Stainless Steels Using Chromate Accelerators
All but one of these oxalate patents pertain to the formation of a ferrous oxalate conversion coating on ferrous metal substrates using various accelerators. These oxalates are intended for use as functional coatings to aid in assembly or provide forming lubricity, etc. These coatings serve as deformable or crushable boundary layers at the metal surface, thereby protecting the base metal during contact with another surface.
The caustic black oxidizing patents focus on compositions and processes which oxidize the metallic iron substrate to a magnetite, Fe.sub.3 O.sub.4, as described in U.S. Pat. No. 2,960,420. Actually, when examining the stoichiometry of the Fe.sub.3 O.sub.4, one can see that the iron is not in either a purely ferrous (II) or ferric (III) oxidation state. Perhaps a more precise description of the material is that of a mixed salt, ferrosoferric oxide, or FeO.Fe.sub.2 O.sub.3, which exhibits both ferrous and ferric iron. The conventional caustic oxidizing processes all depend on the ability of the operating solution to oxidize metallic iron to both ferrous (II) and ferric (III) oxidation states to form the mixed oxide FeO.Fe.sub.2 O.sub.3.
The process described in U.S. Pat. No. 4,017,335 is representative of the state of the art, focusing on the primary phosphatizing mechanism which is well known to those skilled in the art. In addition, this same patent illustrates incorporation of a cleaning agent and pH stabilizer into the oxidizing solution to effectively clean lightly soiled ferrous articles and iron phosphatize them in a single step.