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
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/37—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also hexavalent chromium compounds

Definitions

• This invention relates to the art of producing chemical conversion. coatings on the surface of aluminum or alloys .thereof in which aluminum is the principal or predominant ingredient and isprimarily concerned with aluminum coating. processes inwhich the, surface of. the metal is treated with acidic solutions containing fluorine bearcompounds and hexavalent chromium.
• the invention is particularly useful. in. thetreatment of large areas of aluminum surface in relatively short periods of time and particularly under conditions of heavy'bath loading, i.e.,
• the bathor-solution which is used in the process be kept in proper operating condition and the presentinvention has for its primary object the provision of certain improvements in the replenishing technique whereby the bathmay beHmaint-ained in suitable condition for the *rapid production of useful and uniform coatings both as to color andcorrosion resistance even in situations where the bath is being subjected to heavy loading.
• Alodine may be initially prepared and subsequently replenished. Examples of these which are well known to ,the trade are sold under the trademarks Alodine, I1idite and Bonderite.
• the Alodine solutions for this purpose are known as Alordine 600 and Alodiue 1200, the Iridite isv known as Iridite 14, and Iridite 14-2 and the Bonderite is known as Bonderite 721, and all of these consist primarily and essentially of hexavalent chromium, a fluorine bearing compound containing either a simple or a complex salt, and a cyanide selected from .the class consisting of ferro and .ferricyanides, together withmineral acids. as may be necessary to produce the desiredrange of pH in the coating bath.
• the present invention overcomes the previous difliculties and is based upon the discoverythat, if coating solutions of the type described are replenished from time to time by reston'ng'the hexavalent' chromium content to the desired level and, further, that if the fluoride and the fern'cyanide are-added to the bath in a definite relaing such solutions, it has been customary, if necessary, to
• compositions of coating solutions of the character described are wellknown in the art, and by way of -.example reference may be had to United States Patents 2,276,353; 2,507,956; 2,851,385; 2,796,370 and 2,796,- 371. It will be. noted from the last two patentsdisted that the presenceof ferro or. ferricyanide ions-:inthe .bath significantly improves the coating characteristics of a; bath which consists principally and .essentiallyjfof fluorship to the amount of hexavalent chromium that is added, it is possible to maintain the bath in such condition that it will produce uniformly.
• the source of fluoride is hydrofluoricacid, best results are secured by adding betweenv 0.5 and 0.75 part of. fluoride. for .eachpartof chromate.
• hydrofluoric acid isthessource of the fluorideynot more than" .8.-part F shouldabe added for each partof hexavalent" chromium.
• fluoridei is .introduced by additions of alkali or ammonium salts
• bestresults are secured by adding 0.75 part to parts of salt-for each partof. chromate added.
• lesszthan 0.5 'part fluoride is added :for each part of hexavalent l chromium (calculated as CrO it will benoted that' the uniformity of the coatings'produced by thebathrv'vill be rapidly impaired or no visible coatings whatsoever will be produced.
• the amount offluoride ,added-t' is more than 1 to 2 parts for every part of hexavalentichromium added, thecolor of the coating gradually-becomes lighter and lighter until no visible coating is produced.
• thebath should also contain ferricyanide and in replenishing this ingredient there should be added t least 005 part offerricyanide and preferably .from 0.2 to 1 part of ferricyanide for each part'of chromate V which is added to, the bath. .When less than 0.05 part; of ferricyanide. is added; the color-imparted to 'the coating :will, overv aygiven-period of time, become lighter and lightersand theucoating weight ;;will graduallydiminish.
• the acidity of the invention be controlled. This may be done in whole or in part by replenishment of the hexavalent chromium content as chromic acid and of the fluorine content as hydrofluoric acid or a simple salt thereof.
• the preferred method of controlling the bath pH is through the use of chromic acid (CrO and hydrofluoric acid, but in situations where this is not effective an amount of mineral acid, preferably nitric acid, may be added as needed to maintain the desired degree of acidity.
• Solutions replenished in accordance with the foregoing instructions are capable of coating large areas of a1uminum surfaces with a remarkable degree of uniformity both as to color and corrosion resistance and this in relatively short periods of time during a continuous operation.
• coating baths, replenished as described have been in substantially continuous operation under conditions of extremely heavy bath loading in the coating of literally millions of square feet of aluminum and the coatings produced have been of remarkably uniform quality throughout both as to color and corrosion resistance.
• the foregoing bath as initially prepared, had a pH of 1.5 and was used to coat 38 aluminum alloy pieces utilizing a 1-minute immersion time at room temperature.
• the bath produced strongly corrosion resistant coatings having a beautiful golden brown color.
• a series of aluminum coils was processed in rapid succession through this bath also at room temperature until a total of 40 ft? of aluminum surface had been coated.
• the coating solution was analyzed and replenishment was effected after every 10 square feet of aluminum had been coated and this replenishment was carried out in accordance with the manufacturer's directions and the practices of the prior art and at the same time the pH was adjusted with suitable additions of nitric acid to maintain the same at substantially its original value.
• the coatings produced by the bath had greatly diminished in color intensity and were obviously inferior to the coatings produced by the bath as originally prepared.
• this bath was replenished in accordance with the teachings of the invention herein disclosed. Specifically, for every 10 ft. of surface coated the bath was restored to its original hexavalent chromium content and at the same time, for each part of chromate added, there was added 0.5 part of fluoride in the form of hydrofluoric acid and 0.6 part of ferricyanide. When necessary the acidity of the bath was adjusted to its original value with suitable additions of nitric acid. When the bath was handled in this way it produced uniformly colored coatings even after 100 ft? of aluminum surface per liter of bath volume had been processed. In fact, the coating produced on the hundredth square foot was practically identical with the coating first produced by the bath.
• This bath as initially prepared had a pH of 1.5 and was used to coat 38 aluminum alloy panels using a 1-minute immersion cycle at room temperature. Panels treated initially were uniformly golden brown in color and had coating weights of mg./ft.
• a series of aluminum coils was subsequently processed in rapid succession through this bath at room temperature until a total of ft. of aluminum surface area had been coated.
• the coating solution was analyzed and replenished every 20 sq. ft. with CrO a mixture of NaF and KP in the molar ratio of 2 parts K to 1 part Na and K Fe(CN)
• the ratios of the replenishing materials were:
• the pH was adjusted to the desired 1.5 to 1.7 range using small increments of concentrated nitric acid.
• This bath as initially prepared had a pH of 1.5 and was used to coat 35 aluminum panels using a 1-minute immersion cycle at room temperature. Panels treated initially were uniformly golden brown in color and had coating weights of approximately 80 mg./ft.
• a series of aluminum coils was subsequently processed in rapid succession through this bath at room temperature until a total of 100 ft. of aluminum surface area had been coated. Analysis was made of the coating solution and replenished every 20 sq. ft. with CrO F (as HF) and K Fe(CN) in the ratio of 1:0.5:0.12 by weight. No nitric acid was added during this run since the pH was found to have remained constant. The final pH was 1.5. Aluminum panels coated for one minute in this bath replenished in the above manner were found to have remained quite constant in color uniformity.
• the method of maintaining the solution in coating condition throughout its operating life which comprises periodically restoring the hexavalent chromium content of the solution to the desired level and replenishing fluorine by adding fluoride from the class consisting of hydrofluoric acid and alkali and ammonium salts thereof, the quantity of fluoride from said class which is added being from 0.4 to 2 parts for each part of chromate (calculated as CrO 2.
• the method of claim 1 wherein the fluoride is added as hydrofluoric acid in an amount of from 0.5 to 0.8 part for every part of chromate added.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)

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Cited By (3)

Citations (3)

Family Cites Families (1)

• 0
  • NL NL258036D patent/NL258036A/xx unknown
• 1959
  • 1959-11-19 US US854005A patent/US2988465A/en not_active Expired - Lifetime
• 1960
  • 1960-05-13 ES ES0258049A patent/ES258049A1/es not_active Expired
  • 1960-10-17 GB GB35537/60A patent/GB893771A/en not_active Expired
  • 1960-10-26 CH CH1203660A patent/CH429368A/de unknown
  • 1960-10-27 BE BE596469A patent/BE596469A/fr unknown
  • 1960-10-31 FI FI1786/60A patent/FI40763B/fi active
  • 1960-11-18 DE DEA36058A patent/DE1150256B/de active Pending

Patent Citations (3)

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