Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
  • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used
  • C25D11/08—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used containing inorganic acids

Definitions

• This invention relates to the production of hard, smooth, wear and corrosion resistant aluminum oxide films on aluminum and aluminum alloys by electrolytic oxidation of the aluminum and the aluminum alloys.
• obtaining a .005" oxide coating can be out say from about 70 minutes to about 45 minutes.
• an initial coating of .0005" can be deposited in from 4 to 5 minutes compared to the about 25 minutes or more required according to the procedure of my co-pending application.
• my process involves anodizing the aluminum or alui minum alloy part in an electrolyte maintained at about' 40 to about F. until a .0005" oxide coat is formed, and then transferring the part to another electrolyte bath maintained at a lower temperature preferably between about 0 F. and about 35 F. to complete oxidation to the desired thickness of coating.
• a lower temperature preferably between about 0 F. and about 35 F. to complete oxidation to the desired thickness of coating.
• the process may be continued in the initialelecw trolyte at the higher temperature of about 40.65 F. until a .002 thick coating is formed, and the additional coating should be formed in the electrolyte at reduced. temperature preferably not in excess of 35 F., e.g., about,
• aluminum anodizing generally causes an increase in surface roughness of the oxide coating over its initial degree of roughness prior to anodizing.
• the technique of the instant process has the additionaladvantage that it results in a smoother and harder coat-. ing than that obtained in prior art processes, including that of my co-pending application.
• the anodized coating may be almost as smooth as the metal surface prior to anodizing, or only slightly rougher, whereas according to prior techniques, the final anodized surface is ordinarily considerably rougher than the original surface.
• sulphuric acid chromic acid
• oxalic acid or mixtures thereof.
• the latter acids are considered equivalents for the anodic ox idation of aluminum and its alloys and are termed elec tro-anodizing acids herein.
• Other acids, in addition to sulphuric, oxalic and chromic acids, have been suggested by the prior art for the anodic oxidation of aluminum, and those skilled in the art will understand the nature and type of such acids contemplated herein.
• I may use up to say about H in the electro lyte, but I prefer to use dilute H 80 in an amount corresponding to a range of from about 1 part to 20 parts by volume of concentrated sulphuric acid dissolved in 100 parts by volume of water.
• I may employ an amount of H 80 in the electrolyte corresponding to from about to 10% by volume of 66. Baum sulphuric acid per 100 parts by volume of water.
• I also preferably employ in the bath from 1 to 6 parts, preferably about 3 to about 6 parts, by volume of an aqueous extract of peat per 100 parts by volume of water in the anodizing acid solution.
• the extract additive can be obtained by extracting peat obtained from various localities, with water.
• the extraction is particularly made more rapid and yield is increased by extraction at elevated temperature, preferably the atmospheric boiling point of the mixture or more elevated temperature, and most desirably by extraction at elevated temperature under pressure for a period sufiicient to produce an aqueous acid solution.
• the peat used in practice of my invention may be derived from various locations in the United States, for example, from Georgia, Florida, California or Michigan.
• the peat is first mixed with water in a proportion of say one part by weight of the ground, flaked or fibrous .peat with, for example, six parts of water. These proportions may vary, however.
• This aqueous mixture is then fed to an autoclave when high extraction temperatures are desired for practical reasons and wherein the mixture is cooked at autogenous pressure and at temperature above its normal boiling point (i.e., the boiling point of the mixture at atmospheric pressure).
• the peat is first mixed with water in a proportion of say one part by weight of the ground, flaked or fibrous .peat with, for example, six parts of water. These proportions may vary, however.
• This aqueous mixture is then fed to an autoclave when high extraction temperatures are desired for practical reasons and wherein the mixture is cooked at autogenous pressure and at temperature above its normal boiling point (i.e., the boiling point of the mixture at atmospheric pressure).
• I have found that satisfactory results according to the invention are obtained by cooking the peat at a temperature of from about the boiling point of the mixture
• I employ an aqueous extract of Georgia peat made according to the example described below.
• This extract is in the form of an aqueous solution of organic acids of a complex nature, and is characterized by the following properties.
• the aqueous extract has a pH of from about 4 to about 6 and may contain as little as 2% of dissolved or dispersed solids, depending on the amount of dilution of the material.
• the extract upon evaporation to dryness leaves a dark brown, glossy residue which is amorphous and has a total nitrogen content of about 4.5 to 5%, for example, 4.8% as determined by the Kieldahl method.
• the extract solids are essentially water soluble and form a clear dark brown solution or dispersion.
• the aqueous extract is preferably kept refrigerated or a small amount of sulfuric acid can be added or a fungicidal material such as Dowicide A, believed to be essentially sodium o-phenyl phenate and marketed by the Dow Chemical Company, may be added to prevent mold growth.
• the part to be coated is connected to the anode of an electrolytic cell and immersed in the electrolyte bath which is maintained during the first stage of the reaction at the above noted temperature of between about 40 and about 65 F., preferably 45 to 55 F.
• the second stage of the reaction is carried out at electrolyte temperatures ranging from about 0 F.- to about 35 F. ordinarily, and preferably 10 to 30 F.
• Direct current for anodizing is applied in both the first and second stages. If desired, however, an alternating current component m y added to the direct current. As the coating becomes thicker, its electrical resistance requires higher voltages for penetration. I have found that the addition of peat extract to the electrolyte is necessary in conjunction with my technique for obtaining the improved results noted herein.
• the presence of the peat extract aids in preventing burning of the part at high voltages and amperages, and also permits use of low electrolyte temperatures, particularly in the second stage electrolyte of the invention.
• the rapid increase of voltage at the end of the coating period produces the proper thickness quickly with little softening or solution of the aluminum oxide coating; hence, harder and thicker and also smoother coatings are obtained by my process than are obtainable by the prior art.
• I raise the voltage say to about 20 volts and thereafter increase the voltage until the initial coating is formed.
• I preferably raise the voltage gradually from the initial voltage of 20 volts to 26 volts in about 1 minute, measuring the voltage across the cell electrodes and depending somewhat on the cell resistance. Thereafter the voltage is raised in approximately 1 volt increments, each for a period of between about 15 and 45 seconds until the initial coat is formed.
• the voltage at which a .0005" thick oxide coat is formed is between about 31 and 34 volts and will be higher if the coat is greater in thickness or lower if the initial coat is thinner than .0005".
• a coat of .0005 can be formed in from about 4 to about 7 minutes from the commencement of the electrolytic oxidation.
• each voltage step as the voltage is raised the current is increased, and while during said voltage step the voltage is maintained substantially constant the current decreases during each voltage increment, as the voltage is raised above 26 volts in the aforesaid cell, up to the voltage at which the initial, e.g., .0005? coat is formed.
• the voltage increment is adjusted so that amperage at the start of each of the successive voltage steps is less than the amperage at the start of the previous voltage step, until at the voltage at which, for example, a .0005" coat is formed, amperage falls so that the current density is down to about 20-25 amps. per square foot in the above system.
• the reason for adjusting the maximum amperage at each voltage step so that amperage decreases in the latter stage of this operation, e.g., from 60 amps. per square foot at about 26 volts to say about 20 amps. per square foot at the voltage at which a .0005" coat is formed, e.g., 31 to 34 volts, is that the oxide coat which commences to form at about 26 volts continues to increase in thickness as voltage increases.
• amperage where permitted to increase materially or to remain constant at 50-60 amps. during this build up of oxide coating above, for example, 26 volts, the coating would tend to dissolve and the part would begin to burn.
• burning is prevented and the coating builds up in a satisfactory manner with improved hardness and smoothness.
• the part being treated is preferably removed to 2,905,:eoo
• the temperature of the bath may be lowered to 35 F. or below, and anodic oxidation continued in the same bath at the lower temperature. From this point on, the coating is increased in thickness to the desired depth according to the procedure in my copending application as described more fully below.
• the electrolytic oxidation may be continued in the first stage electrolyte at about 40 to about 65 F. until the initial coating is formed before carrying out the second stage operation at the lower temperature usually below 35 F.
• voltage is increased in one volt increments, each for a duration of about 15 to 45 seconds until the desired initial thickness, e.g., up to .0015" to .002 is reached.
• amperage decreases moderately from the amperage at the .0005" coat thickness, and at a thickness say of .0015", the amperage will decrease until during the last voltage increment amperage may range from about to 20 amperes per square foot. At the start of each voltage increment the amperage will rise but will then start to fall, the overall trend being to gradually decrease the amperage to the above noted values. With alloys such as 248 and 758, a thickness of .0015" is attained at a voltage of between about 40 and 50 volts.
• the second stage operationinan electrolyte at-lower' temperature is carried out in an. increasing stepwise series of substantially constant voltage steps, said voltage steps. varying from about 2 to about 5 volts between. successive steps, each successive increment of constant voltage being maintained for an interval ranging from about 1 to 3 minutes.
• Such increments of voltage may be about 2 to 3 volts at first, but such increments may be of greater value as the oxide formation grows.
• the amperage during each voltage step in the second stage operation at lower bath temperature should be permitted to drop materially while maintaining the voltage substantially constant between voltage steps to, for example, about 30% to 50% of the current value initially attained at each such voltage step after the incremental increase in voltage.
• the amperage should rise and start to fall after but a small interval of time within about the first 30 seconds after the voltage increment has been applied. If this phenomenon does not occur, the usual consequence is that the amperage will steadily incerase, usually rapidly, and burning results. This is indicative that the voltage increment was too great, unless some'mechanical or electrical failure is the cause of this rise.
• the voltage increment should in each case be less than that which permits such excessive current flow.
• the voltage increment should be less than that which gives the burning phenomenon previously described.
• the voltage is increased to reestablish in rough approximation the value of the current observed at the commencement of the previous voltage step.
• the amperage value is dropped to approximately 30% to 50% of the initial value during each voltage'step', that a voltage increment of about 2 or 3 volts is generally sufiicient during the early portions of the second stage operation at lower temperature below about 40 F., to re-establish the aforementioned current value which will give good oxide coating without burning.
• Subsequent voltage changes are adjusted to re-establish at the initiation of each voltage step increase the amperage found. safe, i.e., in order to obtain deposition without destruction of the oxide coat.
• voltage increments of say 3 to 5 volts are usually required to approximately re-establish such current value.
• the voltage may be finally increased in the second stage operation to and above 100 volts, and as high as about 130 volts, to wit, to the voltage at which the coating no longer increases. in thickness.
• current density is generally maintained at less than 20 amps/sq. it, often dropping below 10 amps/sq. ft. at the end of each of the voltage increments.
• Oxide coatings according to the invention can be prolduced on'various alloys, a few of which are illustrated elow;'
• a voltage of about 20 volts is applied, and the ,voltage is gradually increased toabout 26 volts over a perind of aboutl minute. During this period amperage rises from a current density of about 20 amps. per square foot to about 55 amps. per square foot. At the end of this period, voltage is increased one volt for about each 30 to 45 seconds. When the voltage reaches about 34 volts, the coating thickness of .0005" is obtained. While proceeding from 26 volts to 34 volts, amperage at the start of each successive voltage step is lower than at the start of the previous voltage step, and also amperage decreases during each voltage step. During the final voltage step at about 34 volts, amperage ranges from a high of about 20 arnps. per square foot to a low of about amps. per square foot at the end of this voltage step. The total timerequired to produce this .0005" coat is about 5 to 7 minutes.
• the part is then removed from the electrolyte and made the anode of a second electrolytic cell having an electrolyte with the same composition as the first bath, but maintained at a temperature of about F. to about 25 F.
• the voltage is then raised in steps starting at about 35 volts, the successive voltage steps ranging from 2 to 5 volts between steps, each step being maintained substantially constant for a period varying from about 1 to about 3 minutes until a .005" coating is obtained.
• current density is maintained less than about amps. per square foot, the amperage dropping below this value during each 1 to 3 minutes constant voltage period.
• the current density at the start of each new increment of voltage ranges from about 10 to about 18 amperes per square foot, the amperage dropping to from about 3 to about 6 amperes per square foot at the end of each voltage increment.
• the coating produced according to Example '1 above is smoother than a .005 coat formed by the procedure of my co-pending application or by the prior Ex'ampIeZ ji Re sults similar to those noted in Example 1 are obtaiua'ble employing 618 or 758' alloy instead of 124s alloy.
• 61S andS alloy panels having an initial "smoothness corresponding to an 8 micro finish
• my process following the procedure of Example 1 up to formation of a coating .001" thick, produces a part having a smoothness not exceeding a 12 micro finish surface for such .001" oxide coating, whereas employing the technique of my co-pending application, wherein the part is anodized throughout at a single low temperature level, the surface exhibits a 24 micro finish at a coating thickness of .001, and the prior art techniques produce a still rougher surface on the order of about a 32 micro finish.
• Example 3 j The procedure of Example 1 is carried out up through formation of a .0005" oxide coating in the first cell at between about 50 and 55 F. But instead of thereafter transferring the panel to an electrolyte at lower temperature, the panel is kept in the initial electrolyte at about 50-55 'F. and voltage is continued to be increased 1 volt about every 30 to 45 seconds until a coating of .0015" is obtained at about 44 volts. During this period amperage decreases as the thickness of'coat increases, and when a .0015" coat is formed, the current, density is reduced to about 20 amperes per square .foot or less. ; The total time for formation of a .0015 coat in the initial bath at the higher temperature of 50-55 F. is on the order of about 15 minutes.
• the panel is then transferred to another electrolyte of the same composition but maintained at about 15 F. to 25 F.
• the voltage is then raised in steps as in'the second stage of Example 1, beginning at a voltage of between about 45 and 50 volts, until a .005" oxide coat is formed.
• the .005" coating thus formed has improved smoothness over a .005 coat produced by the prior art, but is not quite as smooth as the .005" coat formed in Example 1, wherein the part has an initial coat of only .0005" when it is transferred to the second ce
• other additives may be used which permit the technique and procedure of the invention as described above to be applied to the electrolyte for obtaining the results of the invention.
• a process for coating aluminum and aluminum alloy metal articles with a hard and tough coating of oxide of aluminum which comprises forming an initial oxide coating on the article by passing an electric current through an electrolytic cell containing an electrolyte maintained at a temperature of between about 40 and about 65 F., with said article forming the anode, said electrolyte comprising a water solution of an electro-anodizing acid, and the procedure for forming said initial oxide coating including increasing the voltage applied across said cell during the formation of said initial coating at a rate which does not cause the current density of said cell to increase, forming an additional oxide'coating by continuing to electrol-ytically-oxidize saidarticle in an electrolyte of the aforementioned composition maintained at a temperature less than about 40 F. but not less than F., the procedure for forming said additional coating including increasing the voltage applied across said cell during the formation of said additional coating at a rate which results in maintaining current densities having values not substantially in excess of that obtained at the end of the initial coating procedure.
• a process for coating aluminum and aluminum alloy metal articles with a hard and tough coating of oxide of aluminum which comprises forming an initial oxide coating on the article by passing an electric current through an electrolytic cell containing an electrolyte maintained at a temperature of between about 40 and about 65 F., with said article forming the anode, said electrolyte comprising a water solution of an electro-anodizing acid, and an oxide coating accelerator material, the procedure for forming said initial oxide coating including increasing the voltage applied across said cell during the formation of said initial coating at a rate which does not cause the current density of said cell to increase, forming an additional oxide coating by continuing to electrolytically oxidize said article in an electrolyte of the aforementioned composition maintained at a temperature less than about 40 F. but not less than 0 F., the procedure for forming said additional coating including increasing the voltage applied across said cell during the formation of said additional coating at a rate which results in maintaining current densities having values not substantially in excess of that obtained at the end of the initial coating procedure.
• said coating accelerator material is chosen from the group consisting of an aqueous extract of peat, 2-aminoethyl sul-i furic acid, taurine, N-methyl taurine, N-cyclohexyltaurine, and sulfamic acid.
• said coat, ing accelerator material is an aqueous extract of peat having been obtained by an extraction of peat with waterat elevated temperatures.
• a process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum which comprises in a first stage passingan electriccurrent through an electrolytic cell containingan electrolyte maintained at a temperature of about 40 to about 65 F. with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extractbeing obtained by; extracting a mixture of said peat with water at elevated temperature, raising the voltage and amperage gradually, thereafter raising the voltage in increments and decreas-. ing current density until an oxide coating between. about .0005f and .0015" thick is formed, continuing electrolytic:
• oxidation in a second stage in an electrolyte of the aforementioned composition maintained ,at a lower temperature than about 40 F. but not less than about 0 F., raisingthe voltage in increments above the last voltage applied in the first stage electrolyte, and maintaining said highervoltage after the addition of each increment of voltage and oxidizing said metal article at a decreasing current value while maintaining said higher voltage after .each' increment of voltage has been applied.
• a process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum which comprises passing an electric current through a first electrolytic cell containing an electrolyte maintained at a temperature between about 40 F. and about 65 F. with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extract being obtained by extracting a mixture of said peat with water at elevated temperature, raising the voltage and amperage gradually to attain a preselected maximum current density, raising the voltage in increments at de creasing overall current density until an oxide coating about .0005" thick is formed, transferring said part to a second eletcrolytic cell containing an electrolyte of the aforementioned composition and maintained at a temperature of between about 0 and 35 F. with said article forming the anode, passing an electric current through said second cell, raising the voltage in increments above the ing current value while maintaining said higher voltage after each increment of voltage has been applied until a desired coating thickness greater than
• a process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum which comprises passing an electric current through an electrolytic cell containing an electrolyte maintained at a temperature of about 40 to about 65 F., with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extract being obtained by extractmosaics ingaunixtureof.
• said peat with waterzat elevatedtempera ture 'raising gradually the voltage to about 26 'volts and raising the amperage to a maximum .current density at said 26 volts, thereafter raising the voltage in increments ofabout 1 volt each for a period of from .about 15 .to 45 seconds at decreasing .current density not greater than said maximum value, until an oxide coating of between about 110$":a11d .0015" thick is formed, continuing electrolytic oxidation in an electrolyte of the aforementioned composition maintained at a lower temperature than about 40 F.
• a process for coating aluminum and aluminum alloy articles with a smooth, hard and tough coating of oxide of aluminum which comprises passing an electric current through a first electrolytic cell containing an electrolyte maintained at a temperature between about 40 F. and about 65 F. with said article forming the anode, said electrolyte comprising a water solution of sulfuric acid and an aqueous extract of peat, said extract being obtained by extracting a mixture of said peat with water at elevated temperature, raising gradually the volt- 12 agefrom about v20 volts to about 26 volts and raising the amperage to a preselected maximum current density at said 26 volts, and thereafter raising the voltage in in crementsot about 1 volt each for a period of from about 15 045 seconds at an overall decreasing current density not greater than said maximum value, until an oxide coating between about .0005" and about .0015" thick is formed, transferring said article to second electrolytic cell containing an electrolyte of the aforementioned composition and maintained at a temperature of between about 0 and

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