This invention relates to articles having integral chemically-formed surface coatings that provide an improved combination of adherence and corrosion resistant properties to such products and to a process for making same. More particularly, the articles of this invention have on their surfaces an integral, chemically-formed coating that is strongly adherent and resistant to chipping or flaking at elevated temperatures and provides to the product a unique combination of corrosion properties including commercially satisfactory resistance to oxidation during use in gases at elevated temperatures such as encountered in the engine compartments of vehicle engines, resistance to corrosion from humidity, from organic solvents such as ethylene glycol, oils and gasoline, from acidic or alkaline solutions such as salt spray to the extent that is required of a base for paint or other protective organic or water-based protective coating on parts used within the engine compartments of vehicles.
Chemical coatings on aluminum for various purposes including oxides, chromate-phosphates, chromates, and phosphates have long been known and have been commercially employed since the 1930's when the original Bauer-Vogel process of German patent 423,758 for chemically forming oxide coatings on aluminum was improved in 1937 by reducing the time required from hours to minutes but still produced only gray coatings at near boiling temperatures, see Aluminum, 1937, 19, 608-11 (hereby expressly incorporated by reference). Colorless oxide coatings suitable for a wider range of aluminum alloys were later developed but were less desirable as a base for paint than the Bauer-Vogel products and could not be successfully dyed, see Aluminum, 1938, 20, 536-8 (hereby expressly incorporated by reference). Chromate-phosphates were developed in the 1940's as paint base coatings and disclosed in U.S. Pat. No. 2,438,877 and later modified as disclosed in British Patent 1,114,645 and French Patent 1,477,179 (all of which are hereby expressly incorporated by reference). Chromate processes developed during the 1960's and 1970's have been asserted to provide improved paint bases relative to the chromate-phosphate coatings and are disclosed in a number of United States patents, including U.S. Pat. Nos., 3,009,482, 3,391,031, 3,404,043, 3,410,707, 3,447,972, 3,446,717, 3,982,951, 4,036,667, 4,146,410 and British Patent 1,409,413, all of which are hereby expressly incorporated by reference. A number of additional patents discuss various types of chemical coatings, protective layers or processes, and include U.S. Pat. Nos. Re. 28,015, Re. 29,827, 1,811,298, 1,840,562, 1,946,151, 1,995,225, 2,035,380, 2,059,801, 2,060,192, 2,106,227, 2,106,904, 2,134,830, 2,440,969, 2,680,081, 2,694,020, 2,825,697, 3,175,931, 3,214,287, 3,400,021, 3,950,575, 3,967,984, 3,982,951, 4,070,193, 4,141,758, 4,200,475, 4,341,878, 4,569,699, and 4,657,599, all of which are hereby expressly incorporated by reference.
Even though extensive development of chemical coatings for aluminum and its alloys has resulted from worldwide research efforts each heretofore known process and product present some problem or lack a particular set of properties needed for use in specific applications. Thus, there is a continuing need for other efficient, low cost processes for providing corrosion resistant coatings on aluminum and its alloys to satisfy specific commercial needs. For example, there are needs for uses other than bases for paints or other organic finishes, other needs for coating aluminum alloy substrates which contain alloy constituents known to hinder coating formation on alloys such as magnesium, silicon, copper, chromium and manganese. There remains a need for coating aluminum alloy sand castings which contain silicon, copper and magnesium and may contain other heavy metals such as nickel, chromium, titanium or silver to provide coatings that resist thermal and gaseous engine fume degradation and development of localized white corrosion products during long periods of use such as in commercial truck and automobile engine compartments. There also remains a need for improved coatings for zinc-based, cadmium-based, and magnesium-based materials.
The present invention provides articles that are coated with a new integral coating that results in good corrosion resistance and resistance to dislodgment during use in environments, such as vehicle environments. This invention also provides an economic, continuous process for producing the new coated articles of this invention, as will be described hereinafter.