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
  • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
  • C23F1/00—Etching metallic material by chemical means
  • C23F1/10—Etching compositions
  • C23F1/14—Aqueous compositions
  • C23F1/32—Alkaline compositions
  • C23F1/36—Alkaline compositions for etching aluminium or alloys thereof

Definitions

• ABSTRACT An etching process is provided for aluminum metal which produces a uniform surface with low reflectance and hides surface imperfections while at the same time increasing the whiteness of the surface.
• the etching process provides successive or multiple stage short-time etching treatments in which the first treatment employs special etching solutions based on alkali metal hydroxide, chelating agent and special oxidizing agents.
• the second or successive etching treatment employs conventional alkali metal hydroxide etching solutions.
• the multiple stage etching process produces aluminum metal suitable for architectural purposes without the necessity of using mechanical buffing prior to etching.
• Aluminum finishers using the conventional available etching processes, find that they may have to etch their aluminum products for as long as 15 minutes to achieve the desired improvement in appearance. Such prolonged treatments add considerably to the cost of aluminum metal finishing, in that large amounts of metal may be removed, and large amounts of etching chemicals may be consumed by the prolonged etching.
• the conventional aluminum etching solutions generally employ aqueous alkali metal hydroxide solutions and a chelating agent.
• the conventional etching solution usually comprises a strong sodium hydroxide solution of about 15 to 250 grams per liter and a chelating agent to keep the dissolved aluminum from redepositing sodium aluminate on the metal surface.
• These etching solutions normally produce a silver to silver gray metallic appearance. In addition to the silver color such surfaces will have a high reflectivity and may be shiny. In addition the etched surface will be non-uniform in character and there will be no uniformly grained appearance. The resulting overall surface will not hide the die or extrusion marks and scratches are often visible.
• the conventional etching solutions remove large amounts of aluminum metal from the surface without providing a good hiding or obscuring of the surface imperfections.
• the surface produced by a chelated caustic solution treatment gives an etched surface slightly more pleasing than the raw metal while hiding only a small amount of surface imperfections.
• Highly reflecting surfaces may be obtained'by using a combination of high nitrate and low chlorite concentrations so as to produce a mirror surface in U.S. Pat. No. 2,673,143. Similar brightening effects were obtained in U.S. Pat. No. 2,671,717 by using high concentrations of nitrate.
• the aluminum processor first mechanically buffed the aluminum to remove scratches, pits, die lines, rolling marks or any surface irregularity. This mechanical buffing left the alu-- SUMMARY OF INVENTION.
• My improved etching process for unbuffed aluminum metal comprises successive short-time etching treatments employing as the etchant aqueous alkali metal hydroxide solutions.
• My invention is also characterized in that the first etching treatment takes place in alkali metal hydroxide solutions containing one or more particular oxidizing agents.
• the second etching treatment takes place in conventional alkali metal hydroxide etching solutions.
• etching treatments that are required to produce frosty white low reflecting aluminum surfaces.
• additional etching cycles may be employed if necessary.
• the total etching time of a two etch-cycle is generally less'than the time employed in a single etching of aluminum metal which'has been mechanically buffed before etching.
• the first etching solution comprises one or more oxidizing agents in solution with the alkali metal hydroxide.
• the second or successive etching treatment usually employs the conventional aqueous alkali metal hydroxide etching solution and need not contain the oxidizing agent of the first etching solution for reasons of economy. However, from the standpoint vof surface obtained, the second etching solution may also contain one or more of the selected oxidizing agents.
• the oxidizing agent which is'employed in the first etching solution is selected from the group consisting of persulfates, peroxides, perborates, periodates, chlorites, chlorates, perchlorates, iodates, hypochlorites, nitrites, peroxyphosphates, and bromates.
• the oxidizing agent is used at concentrations of about 2 to about grams per liter.
• the alkali metal hydroxide concentration will be at about 15 to about 50 grams per liter.
• Special chelating agents are required for use in the etching solution containing the oxidizing agent and consist of one or more compounds selected from the group consisting of sorbitol, gluconic acid and glucoheptonic acid and their alkali metal salts.
• the first etching process using the oxidizing agent is operated at a temperature of about 130 to about 200F.
• the preferred operating range is about 150 to about 170F. In general, the higher the operating temperature, the faster is the rate of etching.
• the second or successive etching using the conventional etching solution is operated within the range of about 120 to about 190F.
• the improved etching process may comprise successive'or multiple stage etching treatments for aluminum such as contacting unbuffed aluminum with (l) etching solution containing oxidizing agent, (2) a conventional etching solution, (3) an etching solution containing oxidizing agent and finally, (4) a conventional etching solution.
• the etching cycle may consist of (l) etching in a solution containing oxidizing agent, (2) etching in conventional etching solution, and (3) etching in conventional etching solution.
• the work piece is rinsed with water, desmutted and rinsed again with water. Thereafter, it may go to an anodizing process or to paint preparation processes.
• Both the etching solution containing the oxidizer of the first etching stage and the conventional alkali metal hydroxide etching solution of the second or successive stage may contain other additives such as aluminum surface modifying agents.
• These may be surfactants such as fluoroalkyl surfactants as shown in U.S. Pat. No. 3,557,000 which provide long-lasting bath life.
• fluoroalkyl surfactant means a fluorine containing surfactant as disclosed and claimed in U.S. Pat. No. 3,557,000.
• Special surfactants to improve rinsing and to reduce fuming and foaming, fluorides and phosphates which produce special. surface effects on the aluminum as shown in U.S. Pat. No. 2,653,861 may be used.
• My new etching treatment is generally employed on the aluminum alloys used for architectural purposes. These alloys are generally 6061, 6062, 6063, 6463, and KB45 for extrusion purposes, and 3003, 5005, 5052, 6061 for sheet etching. The process is equally satisfactory on other aluminum alloys not generally used for architectural purposes such as high strength corrosion resistant alloys used in automobile and aircraft industry. Examples are 2024 and 7075.
• the aluminum metal should be carefully prepared for etching by first removing any cutting, rolling or stamping oils and greases. This is usually done by cleaning 6
• concentration of the alkali in the first or oxidizing etching solution is within the range of about 15 to about 50 grams per liter expressed as sodium hydroxide. Higher concentrations of alkali causes rapid depletion of the oxidizing agent. A concentration of 30 to 45 grams/liter is preferred for optimum efficiency.
• the caustic concentration is within the range of about 15 to about 200 grams per liter expressed as sodium hydroxide.
• the etching processes of my invention are usually performed by immersion of the work pieces in the etching solution contained in a steel tank.
• the tanks are equipped with coils for heating and/or cooling the etching bath. Agitation of the bath is desirable to maintain a uniform concentration of the etching solution, particularly at the interface with the work piece. Good ventilation is required to dispel the caustic fumes and byproduct hydrogen gas.
• My process for etching aluminum using successive or multiple stage etching treatments is applicable to immersion or spray operation. Immersion is most generally used.
• the first etching treatment containing the oxidizer is continued until a frosty, white appearing surface is obtained and there is a leveling of surface irregularities.
• the second etching treatment is continued until the aluminum surface has a uniform appearance and which has a low-reflectance. At this time nearly all surface irregularities will have been removed and the aluminum surface is frosty and white.
• the time for each etching treatment will be about l to 4 minutes in duration. However, etching treatments lasting from 1 up to 6 minutes have been used. When two successive etching treatments of 2 minutes each are employed in my process this 4 minutes of etching time compares with an etching time of 10 to 15 minutes using the convention etching processes and solutions on aluminum metal which has previously been mechanically buffed.
• the sources of oxidizing agents used in the first or oxidizing etching solution for etching unbuffed aluminum are as follows:
• persulfates ammonium 'persulfate, sodium persulfate and potassium persulfate; for peroxides hydrogen peroxide and sodium peroxide;
• chlorates sodium chlorate, potassium chlorate and ammonium chlorate
• hypochlorites sodium hypochlorite, potassium hypochlorite and calcium hypochlorite;
• nitrites sodium nitrite, potassium nitrite and ammonium nitrite
• the oxidizing agent in the alkali metal hydroxide solution is used at a concentration of about 2 to 120 grams/liter. About 2 grams/liter is the minimum con centration of oxidizing agent which will provide acceptable surface appearance. This is observed by visual inspection and/or by measuring both total and diffuse reflectivity with a Photo-Volt Reflectometer. A preferred concentration range for the oxidizing agent is about 30 to 60 grams per liter. At this concentration level the reflectance of the etched aluminum is generally nonimage forming. At concentrations higher than 120 grams/liter no increased benefits are obtained.
• nitrites When nitrites are employed in the first etching solution it is desirable but not necessary to have nitrate present at about 5 percent by weight of the nitrite content.
• the nitrate is supplied as one of the compounds selected from the group consisting of sodium nitrate, potassium nitrate and ammonium nitrate.
• a preferred concentration of alkali metal nitrites is 5 to 25 grams per liter of water.
• the oxidizing agent may be supplied to the bath in the form of a powder or as an aqueous solution.
• the chelating agent is conveniently formulated and supplied in combination with the caustic alkali and no separate analysis for this agent is necessary.
• the second or conventional etching solution should be analyzed for caustic etchant at least every four hours and replenishment of etchant should be made as indicated by the analyses.
• the chelating agent is conveniently supplied in combination with the caustic.
• the chelating agents which are efficient and compatible in the first etching solution containing the oxidizer are sorbitol. gluconic acid and glucoheptonic acid and their alkali metal salts. These chelating agents are used at concentrations well known in the art and are generally within the range of 0.5 to grams/liter. As a general rule, the minimum amount of chelating agent may be calculated as 2 percent by weight of the alkali metal hydroxide content.
• the chelating agent may be any agent useful for suppressing alumina precipitation during etching.
• agents include sorbitol, gluconic acid, glucoheptonic acid, mannitol, ascorbic acid, sorbose, tannic acid, ethylenediamine tetraacetic acid, sodium chrom glucosate, diglycolic acid, picolinic acid, aspartic acid, dithiooxamide, d-glyconolactone, and l-rhamnose.
• These chelating agents are used at concentrations well known in the art and are generally within the range of 0.5 to 15 grams/liter. As a general rule, the minimum amount of chelating agent may be calculatedas 2 percent of the alkali metal hydroxide content.
• Diffuse reflectivity was measured by setting the diffusion head of the PhotoVolt Reflectometer at 25 on the scale when reading the reflection from cleaned, deoxidized but unetched aluminum metal. At this reading the surface of the aluminum appears slightly frosty, slightly white with poor hide. of surface imperfections. Any reading about 25 represented an increase in diffuse reflectivity. This increase would correspond to an increase in the number of corrosion pits in the aluminum metal caused by the etching process.
• a diffuse reflectivity of 30 represents a minimum level of diffuse reflectivity which corresponds to an acceptable level v for use of the aluminum for architectural purposes. At adiffuse reflectivity of 30 there is good hide of surface imperfections and the aluminum appears uniform, frosty and moderately white in color.
• the expression low reflectance means that the metal surface has a diffuse reflectivity of 30 or higher as measured in the manner set forth above. Any diffuse reflectivity reading higher than 30 also represents a satisfactory etch. At a diffuse reflectivity of about 45 virtually all surface imperfections have been hidden and the aluminum surface appears frosty and uniformly white.
• the aluminum metal was prepared for etching by cleaning in an inhibited-mild alkaline cleaner, rinsed'with with water, deoxidized in an inhibited acid deoxidizer, again rinsed with water and then etched.
• the same aluminum alloy and the same chelating agent were used to'assure comparative results.
• the metal was rinsed with water, desmutted in an inhibited acid desmutter, rinsed again with water and dried in air. The etched surface was then immediately examined visually and with the reflectometer.
• EXPERIMENT 1 This experiment represents a conventional etching treatment with a conventional etching solution.
• Aluminum extrusions were first etched in an aqueous solution containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol for 4 minutes at 160F. The extrusions were rinsed with water, desmutted and then contacted with a second etching solution by immersion containing 29.4 grams/liter sodium hydroxide and 0.6 grams/liter of sorbitol for 4 minutes at 160F.
• EXPERIMENT 3 This experiment represents a conventional etching treatment in which the etching solution was modified by inclusion of sodium chromate and a surfactant which provides more efficient alkali consumption and extends the bath life.
• Aluminum extrusions were immersed in an aqueous solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant as disclosed in U.S. Pat. No. 3,557,000 and 0.0525 grams/liter of sodium chromate for 8 minutes at a temperature of 160F.
• the surface measured a total reflectivity of 25.5 and a diffuse reflectivity of 30.0.
• the surface appeared frosty, moderately white with fair hide of surface imperfections.
• the etched surface was considered borderline for architectural uses.
• EXPERIMENT 4 This experiment shows the results of two 4 minute etches with an in-between water rinsing, desmutting and another water rinse as compared to a single 8 mintemperature was 160F for each etching.
• the surface of the aluminum measured 20.0 total reflectivity and 32.5 diffuse reflectivity.
• bitol 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate for 4 minutes at a temperature of F.
• EXPERIMENT 6 In this test sodium gluconate was used as the chelating agent. Aluminum extrusions were first immersed in an etching solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sodium gluconate, 45 grams/liter of sodium nitrite and 5.0 grams/liter of sodium nitrate for 4 minutes at 160F.
• the aluminum metal was then etched in a second solution containing 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate for 4 minutes at 160F.
• EXPERIMENT 8 This experiment shows the minimum concentration level of oxidizing agent to obtain a diffuse reflection of 30 which is considered the minimum level of acceptability for architectural purposes.
• Each oxidizing agent was tested in a sodium hydroxide solution at 29.4 grams/liter and 0.6 grams/liter of sorbitol at a temperature of 160F for 4 minutes.
• the second etching solution contained 29.4 grams/liter of sodium hydroxide, 0.6 grams/liter of sorbitol, 0.0015 grams/liter of fluoroalkyl surfactant and 0.0525 grams/liter of sodium chromate. Immersion of the work piece was 4 minutes at 160F.
• EXPERIMENT 12 This experiment shows varying levels of concentration for the chelating compound in the first etching solution containing the oxidizer.
• the first etching solution contained 45.0 grams per liter of sodium chlorite and 29.4 grams per liter of sodium hydroxide.
• the first etching by immersion was for 4 minutes at 160F.
• After a water rinse, desmutting and second water rinse the work pieces were etched for 4 minutes at l60F in the second etching solution described in Experiment H. The results appear in Table V.
• etched surfaces having a uniform appearance
• the improvement comprising subjecting unbufied aluminum metal to successive short-time etching treatments in which the unbuffed aluminum metal is contacted with a first aqueous alkali metal hydroxide etching solution comprising the following composition expressed in grams per liter of water:
• Alkali metal hydroxide 15 to 50 at least one chclating agent selected from the group consisting of sorhitol, gluconic acid, glucoheptunic acid. and their alkali metal salts and at least one oxidizing compound selected from the group consisting of nitrites, pcrsulfatcs, chlorites, pcroxyphosphates, chlorates, perchlorates, peroxides, iodates, hypochlorites, perborates, pcriodatesand bromates 2 to l20,
• Chelating agent selected from the group consisting of sorbitol, gluconic acid, glucoheptonic acid, and their alkali metal salts 0.5 to l5 Alkali metal nitrite 5 to 25 Alkali metal nitrate l to 5 S.
• the process for etching aluminum which comprises subjecting the aluminum work piece to at least two successive etching treatments as provided by claim 1.
• the oxidizing agent in the first etching solution is selected from the group consisting of sodium, potassium and ammonium nitrite, and the concentration of the nitrite compound is 5 to grams/liter.
• nitrate compound is selected from the group consisting of sodium, potassium, and ammonium nitrate.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• ing And Chemical Polishing (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

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Cited By (15)

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• 1972
  • 1972-08-09 US US00278891A patent/US3802973A/en not_active Expired - Lifetime
• 1973
  • 1973-06-12 CA CA173,807A patent/CA976065A/en not_active Expired
  • 1973-07-23 GB GB3501773A patent/GB1414874A/en not_active Expired
  • 1973-08-07 DE DE19732339992 patent/DE2339992A1/de active Pending
  • 1973-08-08 FR FR7328961A patent/FR2195701B3/fr not_active Expired
  • 1973-08-08 NL NL7310964A patent/NL7310964A/xx not_active Application Discontinuation
  • 1973-08-09 JP JP48088906A patent/JPS4958036A/ja active Pending

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