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
  • C23F1/44—Compositions for etching metallic material from a metallic material substrate of different composition

Definitions

• An improved method for removing an aluminide coating from an article includes a sequential treatment of an aqueous mixed acid solution of nitric and phosphoric acids, an alkaline permanganate aqueous solution and then the mixed acid solution repeated.
• the method combines a mechanical pretreatment with the chemical stripping operation.
• An optional final mechanical removal of residue can be included.
• the mechanical pretreatment includes the use of angular metallic particles, such as steel, which are soluble in a subsequently applied solution, such as the mixed acids.
• This invention relates to the removal of coatings from metallic surfaces and, more particularly, to an improved method for removing from a metallic surface an aluminide coating, in one particular form, one which has been subjected to high temperature operating conditions.
• the invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the US. Department of the Air Force.
• the most common, reliable and least expensive method of providing the necessary protection is through the use of an aluminide diffusion coating. While the coating provides adequate protection, it is normally life-limited to the range of up to about 5000 hours. Therefore, the necessity arises to renew the protection provided by the coating several times during the life of the part. The removal of the coating is made more difficult since the coating has been designed and developed to resist attack. A significant problem, therefore, is one of removing the coating without affecting the structural material of the article.
• a principal object of the present invention is to provide an improved method for removing all types of aluminide coatings while maintaining the integrity of and avoiding intergranular attack on the coated article.
• Another object is to provide, for use with articles having cooling passages, such an improved method which avoids cooling hole blockage and dimensional changes.
• the method of the present invention comprises subjecting the coating to an aqueous mixed acid solution consisting essentially of greater than 25 percent to less than 75 percent by volume nitric acid of about 70 weight percent HNOa and greater than 25 percent to less than 75 percent by volume phosphoric acid of about 85 weight percent H PO
• an alkaline permanganate solution consisting essentially of, by weight, about 8 ll percent sodium hydroxide, about 8 11 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities.
• the alkaline permanganate solution is then rinsed from the coating and the coating is again subjected to the mixed acid solution.
• the method of the present invention in a form in which it is used for removing an aluminide coating from an article which has been exposed to air and products of fuel combustion at elevated temperatures, provides a mechanical pretreatment involving abrading the coating to a surface finish of no greater than about rms (root mean square) with angular particles, preferably metallic, which are soluble in a subsequently applied liquid.
• a mechanical pretreatment involving abrading the coating to a surface finish of no greater than about rms (root mean square) with angular particles, preferably metallic, which are soluble in a subsequently applied liquid.
• the surface on which the coating existed is mechanically treated to remove any chemical residue.
• the method of the present invention provides an improved balance between speed and safety for stripping aluminide coatings from a variety of high temperature operating superalloys including those based on nickel as well as those based on cobalt.
• the process avoids termination of aluminide coating removal due to passivation yet avoids detrimental attack on the base material which was coated.
• the method of the present invention is useful in removing aluminide coatings which have been exposed to elevated temperature operating conditions in air and in the presence of products of combustion. Such coatings are particularly difiicult to remove because of an oxide layer which is created under such operating conditions.
• the first is an aqueous mixed acid solution of mixed nitric and phosphoric acid.
• the amounts of acids included are equivalent to those which consist essentially of greater than 25 to less than 75, and preferably 45 55, volume percent nitric acid of about 70 weight percent HNO and greater than 25 to less than 75, and preferably 45 55, volume percent phosphoric acid of about 85 weight percent H PO
• the second solution is an alkaline permanganate solution consisting essen tially of, by weight, about 8 ll percent sodium hydroxide, about 8 11 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities.
• tion in a jet engine for example, at least about 100 hours, to remove all of the coating. This was due to the presence of oxides and products of fuel combustion on or in the coating surface.
• a nickel base superalloy in the form of a turbine blade and including an aluminide coating applied through a pack diffusion method was evaluated for coating removal after the turbine blade had been used in a jet engine.
• the blade was immersed in the 1:1 HNO /H PO mixed acid solution at 80C for 2 hours after which it was rinsed and immersed in the alkaline permanganate solution consisting essentially of, by weight, 10.9 percent sodium hydroxide, 10.9 percent anhydrous sodium carbonate, 5.5 percent potassium permanganate with the balance essentially water and incidental impurities at a temperature of 80C for 1 hour. After rinsing, it was again immersed in the above identified mixed acid solution for 2 hours. After rinsing it was mechanically abraded with a number 1250 grit silicon dioxide, suspended in water, to remove a chemical residue or smut which remained on the surface.
• test of example 2 was repeated on specimens of other nickel base superalloys such as those described in US. Pat. Nos. 3,155,501 issued Nov. 3, 1964, 3,642,469 issued Feb. 15, 1972 and 3,615,376 issued Oct. 26, 1971.
• the specimens were coated with different types of diffusion applied aluminides.
• the hydrogen pretreatment which is less desirable, both from the standpoint of high temperature requirements and lack of ability to reduce alumina, was eliminated even though hydrogen can be effective in chemically converting or reducing sulfidation reaction products from engine run components.
• the results showed that exposure to the mixed acid solution alone for up to about 2 hours at about 80C did not remove all of the coating.
• the pretreatment which is in accordance with one form of the method of the present invention, is to first abrade the coating to a surface finish of no greater than rms. Greater than 100 rms provides a surface which results in subsequent imperfect coating. In this example, the same grit suspended in water was used to abrade or blast the coating surface prior to treatment with the aqueous solutions.
• the present invention in one form, provides an improved method for removing an aluminide coating from an article surface including a pretreatment involving abrading the coating surface to a certain surface finish, treating the article with a particular combination of aqueous solutions and then, optionally, abrading the treated surface to remove any chemical smut or residue.
• a pretreatment involving abrading the coating surface to a certain surface finish treating the article with a particular combination of aqueous solutions and then, optionally, abrading the treated surface to remove any chemical smut or residue.
• the above comparative test procedures, with and without pretreatment including grit blast, in this example with a grit of number 220, applied to different areas of the same blade produced the same results: the coating was removed from those surfaces of the article which had been pretreated with a grit blast but was not completely removed from those areas on which no grit blast pretreatment had been applied. On such areas, 'there were remains in spots both of the difiusion zone as well as some of the originally
• various angular iron and angular steel grits were evaluated both from the standpoint of their ability to pretreat or abrade the coated surface to be stripped as well as their solubility in the subsequently applied aqueous solutions during practice of the present invention.
• iron and steel are completely soluble in the mixed acid solution and are not as hard as alumina abrasives which can have a detrimental effect on the article.
• One material tested was a chilled iron grit 325 mesh having a hardness in the range of about 50 56 R
• dry, angular steel abrasive in the range of 25 120 grit was used.
• an aqueous mixed acid solution consisting essentially of an aqueous solution of HNO in an amount greater than 25 to less than 75 volume percent nitric acid of about 70 weight percent HNO and an aqueous solution of mm, in an amount greater than 25 to less than 75 volume percent phosphoric acid of about 85 weight percent H PO
• an alkaline permanganate solution consisting essentially of, by weight, about 8 ll percent sodium hydroxide, about 8 l 1 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities;
• the aqueous solution of HNO is in an amount equal to 45 55 volume percent of the nitric acid.
• an aqueous mixed acid solution consisting essentially of an aqueous solution of HNO in an amount greater than 25 to less than 75 volume percent nitric acid of about weight percent HNO and an aqueous solution of l-l PO in an amount greater than 25 to less than volume percent phosphoric acid of about weight percent H PO
• an alkaline permanganate solution consisting essentially of, by weight, about 8 l 1 percent sodium hydroxide, about 8 l 1 percent anhydrous sodium carbonate, about 4 6 percent potassium permanganate, with the balance water and incidental impurities

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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)

Priority Applications (9)

Applications Claiming Priority (1)

Publications (1)

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Family Applications (1)

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Cited By (40)

Families Citing this family (8)

Citations (6)

• 1972
  • 1972-09-05 US US00286196A patent/US3833414A/en not_active Expired - Lifetime
• 1973
  • 1973-06-12 IL IL42488A patent/IL42488A/en unknown
  • 1973-07-03 CA CA175,457A patent/CA978837A/en not_active Expired
  • 1973-07-18 JP JP8040173A patent/JPS564628B2/ja not_active Expired
  • 1973-08-04 DE DE19732339608 patent/DE2339608A1/de not_active Withdrawn
  • 1973-08-24 IT IT28184/73A patent/IT993775B/it active
  • 1973-08-28 GB GB4042573A patent/GB1407106A/en not_active Expired
  • 1973-08-28 FR FR7331021A patent/FR2198004B1/fr not_active Expired
  • 1973-08-29 BE BE135040A patent/BE804141A/xx unknown

Patent Citations (6)

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