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
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/12—Light metals
  • C23G1/125—Light metals aluminium

Definitions

• the present invention relates to a pickling kit suited to the degreasing and desmutting of aluminum articles such as aluminum can bodies and to an associated pickling method.
• an aluminum article such as a beverage can, just as formed from aluminum or alloyed aluminum has been covered with aluminum oxide and oil.
• the aluminum can ih particular, is usually fabricated by the drawing technique called “drawing and ironing (DI)" and the surface of the can produced by this technique has deposits of the fine aluminum shavings (smut) generated on drawing and the lubricating oil used. Therefore, in order that an intimate chemical conversion film and/or a coating film may be formed on such an aluminum substrate, its surface must be cleaned beforehand by removing the aluminum oxide film or smut and oil.
• the surface cleaning of an aluminum article is generally made using a chromate type, hydrofluoride type, chromium-free type or fluorine-free type pickling agent which etches off the metallic surface in a suitable degree.
• the etching reaction of aluminum which takes place in such an acidic cleaning agent comprises an anodic reaction by which aluminum is converted to aluminum ion (Al 3+ ) and a cathodic reaction by which H + in the agent is reduced to 1/2 H 2 .
• oxidized metal ion or oxoanion for example a ferric ion (Fe 3+ )
• Fe 3+ ferric ion
• the anodic reaction reducing Fe 3+ to Fe 2+ takes place simultaneously with the above-mentioned reduction of H + so that the etching reaction of aluminum is encouraged to proceed.
• an oxidizing agent is concomitantly used.
• the invention disclosed in Japanese Kokai Publication Hei-7-41973, for instance comprises using a pickling agent containing an inorganic acid in a sufficient amount to establish a pH value not over pH 2, an oxidized metal ion, a surfactant, and a polyhydric alcohol having at least two hydroxyl groups directly attached to adjacent carbon atoms of its main chain in an amount of 0.1 to 5 g/l, with the intent of preventing decomposition of the surfactant with the polyhydric alcohol.
• the pickling agent is supplementary added to the pickling bath using a first aqueous solution containing an inorganic acid and an oxidized metal ion and a second aqueous solution containing a surfactant, an oxidizing agent and a polyhydric alcohol with reference to the monitored redox potential of the pickling agent.
• the present invention has for its object to provide a pickling kit such that it can be timely supplemented with a surfactant for upholding its degreasing power and be also timely supplemented with an oxidizing agent for reoxidizing the reduced ion for upholding its etching power and an associated pickling method.
• the aluminum substrate pickling kit according to the present invention comprises a solution (A) containing sulfuric acid, a surfactant and an oxidized metal ion or anion and a solution (B) containing nitric acid and an oxidizing agent.
• the oxidized metal ion mentioned above is preferably a ferric ion.
• aluminum can bodies can be mentioned.
• the pickling method for aluminum substrate according to the invention comprises supplementing, in the course of pickling of an aluminum substrate, a pickling agent with a solution (A) and a solution (B)
• the timing of supplementation with said solution (A) is determined according to the electric conductivity of the pickling bath and that of said solution (B) is determined according to the redox potential of the pickling bath.
• pickling kit means the pickling agent which is used by separately adding the component solutions.
• the (A) solution contains sulfuric acid, a surfactant and an oxidized ion.
• the surfactant is used for the purpose of removing the oil adhering to the aluminum surface, for example the lubricating oil when the substrate to be cleaned is a DI-process aluminum can body.
• the surfactant any of nonionic, cationic, anionic and amphoteric surfactants can be employed. However, it is preferable to use a nonionic surfactant such as an alkyl alcohol-ethylene oxide adduct, an alkylphenol-ethylene oxide adduct, or an abietic acid derivative.
• the addition amount of the surfactant is preferably 1 to 50 mass parts, more preferably 5 to 30 mass parts, based on the total weight of the (A) solution. If the amount of the surfactant is less than 1 mass part, the degreasing power will be insufficient. If it exceeds 50 mass parts, the pickling bath will foam copiously to made cleaning difficult and increase the waste water treatment load.
• the oxidized ion is used for oxidizing the aluminum substrate to aluminum ion (Al 3+ ) and thereby etching off the metal surface to facilitate formation of a chemical conversion film and improve adhesion of the film to the metal substrate.
• oxidized metal ions such as ferric ion (Fe 3+ ), ceric ion (Ce 4+ ), cobaltic ion (Co 5+ ), stannic ion (Sn 4+ ), etc. and oxidized metal oxoanions such as metavanadate ion (VO 3- ).
• oxidized ions are preferably supplied in the form of water-soluble salts such as sulfates and nitrates .
• ferric ion as an example, ferric sulfate and ferric nitrate can be used with advantage.
• the addition amount of such oxidized ion in (A) solution is preferably 0.5 to 30 mass parts, more preferably 2 to 15 mass parts. When the addition amount is below 0.5 mass part, it is difficult to control the ion concentration in the pickling bath. Supplying the oxidized ion in excess of 30 mass parts tends to cause a precipitation problem due to excess supply.
• the addition amount of sulfuric acid in (A) solution is preferably 30 to 500 mass parts, more preferably 100 to 300 mass parts.
• the proportion of sulfuric acid is smaller than 30 mass parts, the necessary pH level of the pickling bath can hardly be established and upheld.
• the limit of 500 mass parts is exceeded, the increased carryover of the acid to the next step will interfere with the chemical conversion reaction and cause an economic loss as well.
• (B) solution contains nitric acid and an oxidizing agent.
• the oxidizing agent is used for reoxidizing the reduced ion formed on reduction of said oxidized metal ion by the oxidative etching of aluminum.
• the ferric ion only is supplementally added, the ferrous ion formed in the pickling agent accumulates in the pickling bath to encourage formation of a slurry and the precipitate derived from the ferrous ion adversely affects workability.
• the cleaned aluminum product carries Fe ions over to the next step, a precipitate may form in the chemical conversion bath used in the next step so that the chemical conversion treatment is adversely affected.
• these disadvantages can be obviated by using an oxidizing agent.
• the oxidizing agent which can be used in the present invention includes but is not limited to hydrogen peroxide (H 2 O 2 ), persulfates (e.g. NaS 2 O 8 2- ), ozone (O 3 ), cerium compounds (e.g. ceric ammonium sulfate (NH 4 ) 4 Ce(SO 4 ) 2 ) and nitrites (e.g. NaNO 2 and KNO 2 ).
• the amount of the oxidizing agent in (B) solution is preferably 0.5 to 30 mass parts, more preferably 2 to 15 mass parts. When the amount is below 0.5 mass part, it is difficult to uphold the oxidized ion at a necessary concentration in the pickling bath. On the other hand, supplying the oxidizing agent in excess of 30 mass parts causes decomposition of the surfactant so that the necessary surfactant concentration of the bath may not be upheld.
• the amount of nitric acid in (B) solution is preferably 0.5 to 30 mass parts, more preferably 2 to 20 mass parts.
• the amount is below 0.5 mass part, the aluminum substrate may not be uniformly etched.
• it exceeds 30 mass parts the waste water treatment load is increased and the economics of the process is sacrificed.
• the pickling solution according to the present invention may contain various additives in addition to said ingredients.
• a polyhydric alcohol may be added to either (A) solution or (B) solution for preventing the decomposition of the surfactant by the oxidizing agent.
• This polyhydric alcohol has at least two hydroxyl groups directly attached to adjacent carbon atoms of its main chain, thus including dihydric alcohols such as 1,2-ethanediol (ethylene glycol), 1,2-propanediol (propylene glycol), 1,2-pentanediol, 1,2-butanediol, etc., trihydric alcohols such as 1,2,3-propanetriol (glycerol), 1,2,4-butanetriol, etc., and tetrahydric alcohols such as 1,2,3,4-butanetetraol etc., among others.
• dihydric alcohols such as 1,2-ethanediol (ethylene glycol), 1,2-propanediol (propylene glycol), 1,
• the amount of said polyhydric alcohol in (A) solution or (B) solution is preferably 1 to 50 mass parts, more preferably 5 to 30 mass parts.
• a bromide ion may be added in a small proportion.
• said (A) solution and nitric acid are appropriately blended to prepare a cleaning solution in the first place.
• This cleaning solution is prepared to a pH value of 0.6 to 2, a surfactant concentration of 100 to 5000 ppm, preferably 500 to 3000 ppm, and an oxidized ion concentration of 50 to 3000 ppm, preferably 200 to 1500 ppm.
• the method of pickling an aluminum substrate according to the present invention can be carried into practice by whichever of the spraying technique and the dipping technique.
• the treatment temperature is set to preferably 35 to 85 °C, more preferably 50 to 75 °C.
• the treatment temperature exceeds 85 °C, overetching causes accelerated aging of the bath.
• the pickling time is preferably 30 to 300 seconds. If the duration of treatment exceeds 300 seconds, overetching will cause accelerated aging of the bath. If it is less than 30 seconds, insufficient etching will result in poor desmutting. More preferred pickling time is 45 to 120 seconds.
• the amount of supplemental addition of (A) solution is preferably selected according to the measured electrical conductivity of the pickling bath. Since the electrical conductivity depends on the electrolyte concentration of the bath, the conductivity value drops as the sulfuric acid is carried off with the aluminum substrate and the aluminum ion is eluted out of the aluminum substrate. Preferred electrical conductivity value is 10 to 100 mS/cm, preferably 30 to 70 mS/cm.
• the surfactant concentration can be upheld within the above-mentioned conductivity range.
• the amount of supplemental addition of (B) solution is preferably selected according to the measured redox potential of the pickling bath. Since the redox potential depends on the ratio of oxidized ion to reduced ion in the bath, it drops as the oxidized ion is consumed by reduction. Taking ferric oxide as an example, the preferred redox potential is 0.4 to 0.7 mV (vs. Ag/AgCl), more preferably 0.5 to 0.6 mV. When this redox potential is less than 0.4 mV, the oxidized ion is deficient so that the amount of etching of the aluminum surface tends'to be decreased. On the other hand, when it exceeds 0.7 mV, the oxidizing agent is excessively supplied to accelerate decomposition of the surfactant and, moreover, an economic loss results.
• the oxidizing agent is added by supplemental addition of (B) solution, the reduced ion is oxidized to uphold the etching power.
• the amount of the (B) solution to be supplementary added can be mechanically determined according to the change in redox potential.
• the oxidizing agent and surfactant are not supplied in one and the same solution unlike in the prior art, it does not happen that a change in amount of the oxidizing agent affects the surfactant concentration to cause poor degreasing and associated black discoloration of the aluminum can bottom in the sterilization step.
• the aluminum surface cleaned with the pickling kit of the present invention can be rinsed with water and subjected to a chemical conversion treatment using, for example, a chromating agent such as chromic acid-chromate, phosphate-chromate and the like system or a chromium-free treating agent such as zirconium phosphate, titanium phosphate and the like, in the routine manner.
• a chromating agent such as chromic acid-chromate, phosphate-chromate and the like system
• a chromium-free treating agent such as zirconium phosphate, titanium phosphate and the like
• a solution (A) contains sulfuric acid, a surfactant and an oxidized metal ion or anion while a solution (B) contains nitric acid and an oxidizing agent. Therefore, it does not course the decrease in degreasing power and the associated black discoloration due to the effect of the variation of the amount the oxidizing agent on the surfactant concentration.
• the surfactant concentration can be stabilized, while, when the timing of supplementation with said solution (B) is determined according to the redox potential of the pickling bath, the oxidizing agent concentration can be stabilized.
• Lid-free can bodies carrying lubricating oil and smut'as obtained by DI processing of the 3004 aluminum alloy plate were used.
• a pickling agent was prepared according to the recipe; sulfuric acid: 20000 ppm, a surfactant: 2000 ppm, Fe 3+ : 500 ppm and nitric acid: 1000 ppm, and used as a treating bath.
• a solution (A) containing 200 mass parts of sulfuric acid, 20 mass parts of a surfactant, 5 mass parts of Fe 3+ and 300 mass parts of water and a solution (B) containing 10 mass parts of nitric acid, 10 mass parts of hydrogen peroxide and 150 mass parts of water were prepared for use as supplemental agents.
• the electric conductivity and redox potential of the pickling bath were measured, and the solution (A) was added when the electric conductivity value had reached 35 mS/cm or less while the solution (B) was added when the redox potential had dropped to 520 mV or less.
• the can bodies were treated continuously at the rate of 48000 cans/hr, 72000 cans/hr, or 96000 cans/hr for 24 consecutive hours.
• the temperature of the pickling bath was 75 °C and the pickling time was 50 seconds (spray treatment) .
• the can bodies were rinsed with tap water for 15 seconds and, then, subjected to chemical conversion treatment with a conversion reagent ("Alsurf 440", Nippon Paint) at 40 °C for 12 seconds.
• the treated can bodies were rinsed with tap water for 15 seconds, spray-washed with deionized water for 5 seconds, and dried at 190 °C for 3 minutes .
• the pickling bath composition after 24 hours of treatment was analyzed by Cesibor titrimetry for the surfactant, redox titrimetry for Fe 3+ , and TOC (total organic carbon) -metry ("TOC 5000", Shimadzu Corporation) for oil.
• TOC 5000 total organic carbon
• a solution (A) containing 200 mass parts of sulfuric acid, 10 mass parts of nitric acid, 5 mass parts of Fe 3+ and 300 mass parts of water and a solution (B) containing 20 mass parts of surfactant, 10 mass parts of hydrogen peroxide and 150 mass parts of water were respectively prepared. Otherwise, the procedure of Example 1 was faithfully followed to treat can bodies, and the surfactant concentration, oil (grease) concentration and boiling water resistance were measured and evaluated. The results are shown in Table 1.
• Treatment load (cans/hr) Concentrations of agent components after treatment of can bodies Evaluation of cans (boiling water resistance) Surfactant Oil Fe 3+

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Citations (5)

• 2000
  • 2000-02-15 JP JP2000036022A patent/JP2001226790A/ja active Pending
• 2001
  • 2001-02-15 EP EP01103735A patent/EP1126048A3/fr not_active Withdrawn

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