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
  • C25D11/06—Anodisation of aluminium or alloys based thereon characterised by the electrolytes used

Definitions

• the present invention relates to a novel method of the production of protective coatings upon 1 aluminum and aluminum alloys and more par- I ticularly, to the production of novel oxide coatings upon aluminum or aluminum alloys during an anodizing treatment.
• many processes have been developed for the production of corrosion resistant coatings upon aluminum and aluminum alloys which depend in most cases upon the production of an oxide coating which is thicker than that ordinarily obtained by the oxidation of aluminum or aluminum alloys in air.
• oxide coatings are produced by the treatment of aluminum or aluminum alloys with special chemicals such as, for example, the treatment of aluminum or aluminum alloys with a boiling solution of soda and silicates; and electrolytic processes in which the aluminum or aluminum alloys are subjected to an anodizing treatment in aqueous solutions of sulfuric, chromic or oxalic acid.
• oxide coating During the heating of aluminum or aluminum alloys in air or in salt baths commonly used in the heat treatment of aluminum or aluminum alloys, a very thin oxide coating is obtained.
• the oxide coating roduced in the salt baths is usually spoken of as a nitride coating and is thought to render it difficult subsequently to apply anodic coatings and is consequently normally removed before the aluminum is subjected to anodic treatment.
• These oxidic coatings do not contain hydroxide in view of the high temperatures at which they are formed, but they are not suitable for practical purposes as they are not thick enough.
• N02 is produced in accordance with the following equation:
• the process in accordance with the invention also has the'advantage that it may be combined with a heat treatment of the aluminum to be coated and furthermore that such process prevents a deleterious attack of the aluminum surfaces during the heat treatmentin fused salt baths.
• Ammonium nitrates and nitrites are also suited but are not aspractical as they) tend to vaporize.
• the relatively non-expensive sodium and potassium nitrates which. respec-. tively have melting points of 310 C. and 337 C; are will suitedas electrolytes in accordance with the invention. Potassium diohromate has also been found suitable,
• the special properties of the coatings produced in accordance with the invention are believed to a certain degree to be caused by the relatively high temperatures of the salt baths during the electrolys s-- Transparent glass-clear coatings may be obtained upon pure aluminum and when special precautions are observed, which will subsequently be described in detail, such transparent coatings ma cliche obtained 1 alumimlm a ys N pores are visible in the coatings obtained in accordance. with th n niiq i. en Pen inspection by highpcwered microscopes even up to the limits of resolution of light microsccpes
• the coatings may in some instances contain very fine crack swhose incidence depends upon the manner in, which the coated articles are cooledwhen withdrawn from the fused electrolytes.
• the alum num be tr a ed sh ws; a: cal l ent potential of 2 to 2.51011 5.
• ainiasthaamde This potential rapidly falls off andis reduced to under 0.5 volt in about a minute, and at the same tim he gasbubbles.
• h chv a stillp odu s at the anode after th u ren is c o e rease. If; for-exampl ure alumi uml s elec ro y ically rea edin ccordan e; w th. the inventioo with; a
• alytic materials to; the nitrate-containing elecro yti ath nml r fin. a cordan e. with the invention, which materials act to increase the rapidity of the oxidation of the aluminum.
• copper and iron compounds may be easily introduced by sending a direct or alternrting current through the bath employing iron or copper electrodes whereby iron or copper compounds are introduced into the baths which, after-a period of time, evidently are converted to their oxides. It is also known that the reduction of nitrates may be catalytically accelerated by the presence of copper or lead oxides.
• This coloration is caused as may be seen from microscopic study, by the alloying elements which separate out at the edges of the grains and color under the influence ofthe electric current. If higher initial current densities are used the aluminum oxide coating is evidently produced so rapidly that the alloying elements present at the edges of the grains are covered and separated from the direct influen'ce of the electrolysing current. This aforementioned coloration does not substantially affect the corrosion resistant properties of the coating obtained, and consequently the following precautions need only be observed in carrying out the process in accordance with the inventionupon aluminum alloys, if alight metal tone is desired upon the alloys treated.
• One method of insuring the product-ion of light transparent coatings upon aluminum alloys is to degenerate the local element effect by periodically cutting oi the electrolysing current so that a high average current density is obtained as indicated above.
• alight colored coating is produced upon an aluminum alloy of the constitution 24S when treated as follows: a sheet of such alloy which has been pretreated by normal etching in'causticsoda, followed by dipping in nitric acid and drying, is dipped into a fused salt bath composed of; sodium nitrate and a small amount of sodium nitrite. If the sheet has.
• anodizing current flow can be instituted immediately, however, should bubblesoccur on the sheet due to the presence of traces of fat thereon, it is advisable to destroy the bubbles by withdrawing the sheet from the bath and allowing thesait coating to solidify before instituting the anodizing treatment.
• An anodizing. current of eleven volts is used-and is preferably initially permitted to flow only for about one to ten seconds and is then interrupted until the voltag occurring at the aluminum, when measured by a voltmeter, has. degenerated tO- one-half its original value when the anodizing current was cut off,- or one can continue the interruption of; the externalianodizins; cur ent flow un il no noteworthy bubble for:-
• the period during which the external anodizing current is cut off is at least as long as the period during which such current is on. This. sequence of switching on and off of the anodizing current is continued until 0.05 to 0.1 kwh./m. have been consumed when direct current is used for the anodizing current.
• This interrupted anodizing is expediently effected by anodizing a plurality of aluminum articles in one electrolysing bath and sequentially supplying the anodizing current to each article, so that the load upon the source of current remains substantially constant.
• the on period for the current is about five seconds, and the oif period is about fifty-five seconds, twelve articles can be arranged to be anodized in the bath with one cathode and supplying the anodizing current to each article for five seconds every minute with a rotating distributor.
• the interrupted anodizing current may also be produced by the discharge of suitable condensers.
• the desired interruption of the anodizing current may also be achieved by withdrawing the article treated from the bath.
• a pulsating direct current that is, a current'that pulses between 0 and the desired voltage.
• the local element effect is constantly weakened by the constant pulsation between 0 volt and the desired voltage.
• a current may be employed which fluctuates between 2 volts and the desired voltage.
• This type of current may be provided by superimposing a direct current of (N2) volts upon an alternating current of N volts. For example, if a direct current of 8 volts is superimposed upon an alternating current of ten volts, the voltage at the anode side of the direct current will fluctuate between +18 and -2.
• alternating current for the process in accordance with the invention.
• Protective coatings are obtained which are of the same thickness as the coatings obtained with direct current, but the current used in obtaining them is somewhat greater than when direct current is used because of slight disintegration of the coating being produced during the negative phase.
• the use of alternating current or alternating current followed by direct current has been found advantageous.
• a bright rolled aluminum sheet having deep grooves which are very diihcult to clean is advantageously first treated with an alternating current and then subjected to a normal direct current treatment.
• the corrosion resistance obtained is substantially better than when only direct current has been used. This is probably caused by the cleaning action upon the surface obtained with the alternating current for during the negative phase alkali is found at such surface.
• the voltage employed for the process in accordance with the invention should preferably not be below ten volts for otherwise too little current will flow because of the resistance of the electrolytic bath.
• the voltage used should not exceed about fifty volts to avoid the production of rough and dull coatings which are obtained when the voltage is high enough to cause dielectric breakdown in the coating produced.
• the consumption of current for the production of a corrosion resistant coating in accordance with the invention is about 0.1 kwh./m. when direct current is used. This very low current consumption may be explained in that the current flow decreases very rapidly from its high initial value of about 5 to 25 amp./dm. (depending upon the voltage used and the distance between the electrodes).
• the current flowing through the layer formed was only about 1 ampL/dm. and after five minutes, below 0.2 amp./dm
• the temperature of the fused bath which is employed as electrolyte depends upon the use to which the aluminum or aluminum alloy is to be put. It can vary between about C. and the temperature at which the aluminum or aluminum alloy melts. It has been found preferable for some purposes to maintain the temperatures of the salt baths between 300 C. and 550 C., as corrosion resistance is increased at these higher temperatures.
• the process in accordance with the invention may, for example, be combined with a homogenizing heat treatment.
• An Al-Mg-Cu alloy corresponding to alloy 24st may be provided with a glossy corrosion resistant coating and simultaneously be subjected to a homogenizing heat treatment in the following manner.
• the alloy was immersed and anodized in a bath of fused sodium nitrate containing a small quantity of potassium nitrate and sodium nitrite (about 5%) which was maintained at a temperature of 500 C. to provide the homogenizing temperature.
• the anodizing current was supplied in surges of one to two seconds, with intermediate rest periods of about twenty seconds, and the anodizing treatment continued until the current con sumption was about 0.2 kwh./m.
• the current used was fifty cycle alternating current of about twenty volts. The current was switched on and off on the primary side of the transformer used in supplying the current.
• the process in accordance with the invention may also be combined with an artificial ageing of aluminum alloys.
• an Al-Mg-Si sheet may be aged at, for example, 75 C. in a bath composed of sixty parts potassium nitrate and forty parts sodium nitrate, and while in such bath anodized in accordance with the invention for six minutes without interruption with a direct current of eleven volts.
• Ammonium nitrate could also be used in the bath a; these temperatures, but it evaporates rap- 1 y.
• the aluminum or aluminum alloys which are to be subjected to the anodizing treatment in accordance with the present invention may be subjected to the same pretreatments which are customary in the previously employed anodizing processes. It has, however, been found that in most instances a short etching in a caustic soda solution of about 5% to 15% concentration which is not too hot is suflicient. If the articles to. be treated have a smooth polished surface, it is normally only necessary to defat such surfaces'before treatment. Etching of the aluminum surface is not observed during the treatment in accordance with invention as customarily occurs during the previously employed anodizing processes employing aqueous electroiytes. To the contrary, it has been found that the present process results in the disappearance of etched surfaces under the clear coating which is obtained. On lightly etched pure aluminum,
• the corrosion resistance of the coatings produced in, accordance with the invention may also, be seen from the following comparative tests:
• Test samples were coated in accordance with the invention and compared with samples of the same alloy anodically coated in an aqueous 30% sulfuric acid solution and sealed with nickel acetate.
• the following table shows the. weight losses of the samples in grams per square meter.
• Column A gives the valuesv for the samples treated in accordance with the invention and column B gives the values for the samples treated in aqueous sulfuric acid and sealed with acetate,
• Tests were also carried out to show the difference obtained; when different types of current were employed in producing a coating in accordance with the present invention. These test samples were all; of pure aluminum which was bright rolled but still contained deep grooves caused by a defective hot roll, The solution usedfor the corrosion test was the normal ox-iaa Anodizing treatment 122 6, minutes with 20 volt alternating current.
• the process in accordance with the invention has been found of special advantage in the treatment of copper-containing aluminum alloys.
• Such alloys upon such treatment with a caustic soda etch show a dark coating which may be wiped off, but which is preferably dissolved with dilute HNOs, before anodizing with they previously employed anodizing processes employing aqueous solutions. It has been found unexpectedly that. this dark colored coating need not be removed when the coatingprocess in accordance with the invention is employed.
• fused nitrate and nitrite baths While the invention has been described with particular reference to fused nitrate and nitrite baths, it is obvious that other fused salt baths may be employed as long as such baths have the proper melting point and boiling point, are substantially anhydrous and upon electrolysis will liberate oxygen at the anode. Fused salt baths which attack aluminum to a substantial degree should be avoided. Potassium dichromate has been found well suited for providing corrosion resistant coatings upon aluminum base metals in accordance with the invention. For example, an Al-Cu-Mg alloy was anodized in a fused bath of potassium dichromate at a temperature of 400 C., employing a voltage of eleven volts.
• the total time of treatment was a little less than one hour, but the current was interrupted several times for substantial periods so that the total electrolysing time was less than thirty minutes.
• a sample of the coated alloy was submitted to a corrosion test in four parts distilled water, four parts concentrated nitric acid and one part concentrated hydrochloric acid. The weight loss of the sample amounted to 042 gram per hour per square meter. Also another sample of the same alloy was anodized for ten minutes at eleven volts in a fused dichromate bath at a temperature of 500 C. A sample of the coated alloy showed a loss of 1.35 grams per hour per square meter in the same mixture of acids.
• a process for the production of an oxidic coating upon an aluminum base metal which comprises anodizing such aluminum base metal with direct current in a substantially water-free fused salt bath essentially consisting of at least one alkali metal nitrate and at least one alkali metal nitrite as the electrolyte, the anodizing current being intermittently supplied and the anodizing voltage being up to 50 volts, the individual periods of supply being from 1 to 10 seconds duration and the periods during which current is not supplied being at least as long as the periods in which the current is supplied.
• a process for the production of an oxidic coating upon an aluminum base metal which comprises anodizing such aluminum base metal in an electrolytic cell with direct current in a substantially water-free fused salt bath essentially consisting of at least one alkali metal ni trate and at least one alkali metal nitrite as the electrolyte, the anodizing current being periodically interrupted and the anodizing voltage being up to 50 volts, the periods during which the anodizing current is not supplied being sufficiently long that the cell voltage continuing after the anodizing current is interrupted falls to at least one half the value of such voltage when the anodizing current is first interrupted.
• a process for the production of an oxidic coating upon an aluminum base metal which comprises immersing said aluminum base metal in a substantially water-free fused salt bath cssentially consisting of at least one alkali metal nitrate and at least one alkali metal nitrite and supplying an anodizing current to said immersed aluminum base metal which anodizing current periodically varies from a voltage between 2.0 volts and 0 volt to a positive anodizing voltage up to 50 volts, the fused salt bath being maintained at a temperature between 140 C. and the melting point of the aluminum base metal during the treatment with the anodizing current.
• said fused salt bath also contains a catalyst which accelerates decomposition of nitrates.
• a process for the production of oxidic coatings upon an aluminum alloy containing copper which comprises etching such alloy in an alkali metal hydroxide solution whereby a dark colored coating is formed on such alloy, immersing said aluminum base metal in a substantially water-free fused salt bath essentially consisting of at least one alkali metal nitrate and at least one alkali metal nitrite without first removing such dark colored coating and supplying an anodizing current to said aluminum base metal carrying said dark colored coating which anodizing current periodically varies from a voltage between -2.0 volts and 0 volt to a positive anodizing voltage up to 50 volts.

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Cited By (7)

Citations (5)

• 1948
  • 1948-12-30 US US68421A patent/US2666023A/en not_active Expired - Lifetime
• 1949
  • 1949-12-29 CH CH292423D patent/CH292423A/de unknown

Patent Citations (5)

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