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
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/31—Coating with metals
  • C23C18/32—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron
  • C23C18/34—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron using reducing agents
  • C23C18/36—Coating with nickel, cobalt or mixtures thereof with phosphorus or boron using reducing agents using hypophosphites

Definitions

• the present invention relates to chemical nickel plating of magnesium and its alloys, and more particularly to processes for effecting such plating with aqueous chemical nickel plating baths of the nickel cation-hypophosphite anion type.
• the metal surface is highly susceptible to oxidation and filmsover prior to or upon contact with the aqueous chemical nickel plating bath; whereby there is a film at the interface between the magnesium metal surface and the coating of nickel-phosphorus alloy that is inherently produced by a plating bath of this type; which film at the interface prevents proper initiation of the plating and intimate bonding between the magnesium alloy metal surface and the coating, with the result that the coating is characterized by blisters and other defects evidencing lack of continuity and uniformity of adhesion thereof with respect to the metal surface of the workpiece.
• Another object of the invention is to provide a process of coating the magnesium alloy surface of a workpiece with a chemical nickel plating bath of the nickel cationhypophosphite anion type so that the coating thus produced is intimately bonded thereto and devoid of blisters or other defects characteristic of lack of adhesion.
• Another object of the invention is to provide a process of chemical nickel plating upon a previously prepared magnesium alloy surface of a workpiece that is productive of a continuous smooth coating of nickel-phosphorus alloy that is devoid of blisters and other defects characteristic of lack of adhesion.
• a further object of the invention is to provide a process of the character noted that employs a chemical nickel plating bath of the nickel cation-hypophosphite anion type of improved composition which has a pH in the alkaline range and that is entirely stable notwithstanding this circumstance.
• Another object of the invention is to provide a process of the character noted that utilizes an improved lowtemperature heat-treating step following the chemical nickel plating of the coating upon the metal surface of the workpiece, whereby the coating is intimately bonded to the underlying metal surface of the workpiece.
• a further object of the invention is to provide a chemical nickel plating bath of the nickel cation-hypophosphite anion type and of improved composition which has a pH in the alkaline range and that is entirely stable notwithstanding this circumstance.
• a still further object of the invention is to provide an improved process of producing, as an article of manufacture, a workpiece having a metal outer surface formed essentially of magnesium alloy and carrying a smooth and continuous coating intimately bonded thereto and essentially comprising a nickel-phosphorus alloy.
• a workpiece or article of manufacture having an outer surface formed essentially of magnesium or an alloy thereof; and ordinarily the workpiece is first ma chined, or otherwise finished, and thereafter subjected to the process so as to provide on the metal surface thereof a continuous uniform and smooth coating intimately bonded thereto and formed of a nickel-phosphorus ally that is inherently produced by chemical deposition from a plating bath of the nickel cation-hypophosphite anion type.
• the workpiece 'is machined, or otherwise finished it is first subjected to certain pretreatment steps, then it is subjected to chemical deposition from a first plating bath of the type noted having a pH in the alkaline range, then it is subjected to chemical deposition from a second plating bath of the type noted having a pH in the acid range, and ultimately it is subjected to a heat-treatment step in order to render the composite coating product in the two chemical nickel plating steps mentioned intimately bonded thereto and of finished character.
• the present process has been applied in coating the typical magnesium alloys: AZ-3l, ZK-60A, AZ-9l, AZ-6l, AZ-92 and AZ-80.
• These typical magnesium alloys have the following compositions by weight.
• AZ-31 Percent A1 3 Zn 1 Mg Balance ZK-60A:
• the alloy AZ-91 is also identified as AZ91A and comprises a casting alloy; the alloy AZ-61 corresponds to Dow alloy K-l and comprises an extrusion alloy; and the alloy AZ-8l corresponds to Dow alloy C l and comprises a forging alloy.
• the workpiece is subjected to the following steps in the order named:
• This pickling bath employed in the pretreatment essentially comprises an aqueous solution of chromic acid, nitric acid and hydrofluoric acid and has the general composition:
• This bath may be conveniently formulated by dissolving in water about 280 g./l. of CrO and by adding thereto 10 ml./l. of 42 B. (technical) HNO and by adding thereto 1 ml./l. of an aqueous solution of 70% HF.
• This treatment bath employed in the pretreatment essentially comprises an aqueous solution of sodium hydroxide and has the general composition: NaOH-about 0.1 m.p.l.
• This bath may be conveniently formulated as a 10% sodium hydroxide aqueous solution.
• magnesium as well as its alloys, is an extremely reactive metal, which is substantially attacked by water at temperatures as low as 70 C., and that the rate of attack is substantially increased at elevated temperatures. Therefore,
• the pickling bath No. l dissolves all the magnesium oxides from the magnesium surface of the workpiece. Moreover, there is a chemical polishing action, due to the presence of chromic acid (an oxidizing agent), which, in conjunction with the nitric and hydrofiuoric acids, attacks the convex irregularities of the surface at a higher rate.
• chromic acid an oxidizing agent
• the pickling bath No. 2 first attacks the clean metal surface obtained by the pickling bath N0. 1, as shown by hydrogen evolution. Hydrogen evolution then subsides, and an insoluble film of magnesium fluoride is formed upon the clean magnesium surface of the workpiece.
• the treatment bath No. 3 effects an alternation of the film upon the magnesium surface that was deposited thereupon by the pickling bath No. 2; and more particularly, the treatment hath No. 3 first neutralizes any acid that may be present on the surface of the workpiece, and then alters the magnesium fluoride fihn upon the magnesium surface of the workpiece and forms a final protective film of basic magnesium fluoridemagnesium hydroxide thereon. It has been found that this film upon the magnesium surface of the workpiece initiates plating very rapidly in the first chemical nickel plating bath previously mentioned and is also highly useful in obtaining adhesion of the coating and lack of blistering thereof upon the workpiece.
• first chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH in the alkaline range and immersed therein throughout a first time interval sufficiently long to produce an initial nickel coating having a thickness of at least about 0.1 mil upon the exterior surface thereof; which first plating bath is normally maintained at a relatively high temperature in the general range 92 C. to 98 C.
• the initial plating or coating that is inherently produced by this first plating bath essentially comprises about 92% to 97% nickel and about 3% .to 8% phosphorus by weight; which plating proceeds at a rate of at least 1.0 mil/hour.
• This first plating bath essentially comprises an aqueous solution of nickel cations, hypophosphite anions, a first chelating and exalting agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof, a second chelating agent selected from the class consisting .of short chain aliphatic hydroxycarboxylic acids and salts thereof, ammonium cations, fluoride anions, sufiicient hydroxide anions (OH”) to produce a pH in the approximate range 7.0 to 9.0, a buffering system comprising alkali metal cations and boron oxide anions, and AsO anions.
• This first plating bath contains Ni++ and H Po, the absolute concentration of H2PO2 being in the range 0.15 to 1.20 moles per liter, and the ratio between Ni and H PO being in the rang 0.25 to 1.60.
• This first plating bath has the general composition:
• the fundamental purpose of the pretreatment is not only (AsO Stabilizing content.
• composition of this first plating bath is as follows:
• Suitable examples of this first plating bath are as follows:
• the Ni++ may be derived from nickel hypophosphite, nickel acetate, nickel sulfate, nickel carbonate, etc.
• the H POr may be derived from nickel hypophosphite, hypophosphorous acid, alkali metal hypophosphite, etc.
• the NH may be derived from ammonium hydroxide, ammonium fluoride, ammonium sulfate, ammonium bifluoride, etc.
• the F- may be derived from hydrofluoric acid, ammonium fluoride, ammonium bifluor-ide, alkali metal fluoride, etc.
• the short chain aliphatic aminocarboxylic acid radical may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes aminoacetic acid, alpha-aminopropionic acid, beta-aminopropionic acid, alpha-aminobutyric acid, aminosuccinic acid, etc.
• the short chain aliphatic hydroxycarboxylic acid radical may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes hydroxyacetic acid, monohydroxysuccinic acid, dihydroxysuccinic acid, gluconic acid, citric acid, hydroxymalonic acid, trihydroxyglutaric acid, alpha-hydroxypropionic acid, beta-hydroxypropionic acid, l-beta-hydroxybutyric acid, etc.
• the hydroxyl ions may be derived from ammonium hydroxide, or a suitable alkali metal hydroxide, sodium hydroxide being preferred; and any H+ adjustment of the pH is preferably derived from H
• the AsO,,-- anions may be conveniently derived by dissolving As 0 into the bath, the AsO anion being stable in the alkaline range of the bath.
• the boron oxide anions may be derived from H BO H B O Na BO or Na B O or any desired combination thereof.
• the aminocarboxylic acid radical constitutes a first chelating agent, as well as an exaltant increasing the rate of deposition; and. the hydroxycarboxylic acid radical constitutes a second chelating agent; which two chelating agents named are present in amounts sufficient to form mixed chelates of all of the nickel ions in the bath.
• the NH constitutes a complexing agent, thereby to insure that all of the nickel ions are tied-up or complexed either by way of the mixed chelates mentioned or by way of the ammoniacal complexes mentioned.
• the FT is employed fundamentally for the purpose of preventing blistering of the first coating as it is applied to the magnesium metal surface of the workpiece.
• This first plating bath includes the fundamental buffering system comprising alkali metal cations (Na+) and boron oxide anions.
• the plating bath inherently contains a substantial concentration of Na+ cations by virtue of the utilization of the sodium salts of the various acids involved in composing the same and by the utilization of NaOH in establishing the pH thereof in the alkaline range.
• the concentration thereof may be readily adjusted utilizing a suitable sodium salt such, for example, as sodium sulfate.
• this first plating bath includes two auxiliary buffering systems.
• the first auxiliary buffering system comprises alkali metal cations (Na+) and the aminocarboxylic acid radical; and the second auxiliary buffering system comprises alkali metal cations (Na+) and the hydroxycarboxylic acid radical.
• this first plating bath is highly buffered so as accurately to maintain the desired pH thereof within the range 8.0 to 9.0 as previously noted.
• this first plating bath comprises a boron oxide-sodium oxide-water system; whereby the source of the boron oxides present therein is not only immaterial, but the ultimate composition of the ionic species of the boron oxides in the plating bath is always the same, irrespective of the source of boxon oxide employed, under the same controlling equilibria conditions of concentration, temperature and pH.
• a 0.6 M aqueous solution of H BO essentially comprises:
• a 0.1 M aqueous solution of H BO essentially comprises:
• a dilute aqueous solution prepared by a Na B4O addition essentially comprises:
• the plating bath contains boron oxide anions that are equivalent to an addition of Na B O in the range 0.05 to 0.20 m.p.l.
• This content of boron oxide is also equivalent to an addition of H 80 in the range of 0.20 to 0.80 m.p.l. While any one of these sources of boron oxide may be employed, as previously explained, the utilization of ordinary borax is recommended, because of factors pertaining to simplicity and economy.
• the AsOpanions are employed for the purpose of achieving stability thereof so as to prevent spontaneous decomposition of the first plating bath by the rapid and random formation therethrough of black precipitate.
• the stabilizing content of ASO4 anions is ordinarily within the range to 150 parts per 1,000,000 parts of the bath by weight.
• nickel cations and hypophosphite anions are depleted; whereby either continuously or periodically the bath is regenerated in use by the addition of these ingredients noted in order to maintain the ranges thereof set forth.
• the workpiece After the workpiece has been given an initial coating having a thickness of at least about 0.1 mil in the first chemical nickel plating bath described above, it is transferred to a second chemical nickel plating bath of the nickel cation-hypophosphite anion type having a pH in the acid range and immersed therein throughout a second time interval sufficiently long to produce a final composite nickel coating having the desired thickness upon the exterior surface thereof; which second plating bath is normally maintained at a relatively high temperature in the general range 93 C. to 98 C.
• the plating or coating that is inherently produced by this second plating bath essentially comprises about 88% to 94% nickel and about 6% to 12% phosphorus by weight; which plating proceeds at a rate of about 0.9 mil/hour.
• This second chemical nickel plating bath is preferably of the composition of that disclosed in US. Patent No. 2,822,294, granted on February 4, 1958, to Gregoire Gutzeit, Paul Talmey and Warren G. Lee; which plating bath is essentially of the nickel cation-hypophosphite anion type also containing lactic anion and propionic anion and having a pH in the acid range 4.0 to 6.0.
• this plating bath essentially comprises nickel cations, hypophosphite anions within the range 0.15 to 1.20 moles/ liter, a ratio between nickel cations and hypophosphite anions Within the range 0.25 to 1.60, lactic anions within the range 0.25 to 0.60 mole/liter, and propionic anions within the range 0.025 to 0.060 mole/ liter.
• a typical example of this second chemical nickel plating bath has the following composition:
• the workpiece carrying the composite plating or coating is heated to a temperature of about 400 F. for a time interval of about 1 hour; which heat-treatment effects intimate bonding between the chemically deposited composite plating and the magnesium metal surface of the workpiece so that an adherent, smooth, continuous and uniform coating is provided thereon.
• the initial coating be deposited upon the magnesium surface of the workpiece in the first chemical nickel plating bath having a pH in the alkaline range, and that this step be continued throughout a suitable time interval in order to obtain a thickness of at least about 0.1 mil of the initial coating mentioned, thereby to prevent attack of the magnesium metal surface of the workpiece.
• the heat-treatment step is very important as it not only integrates the composite coating upon the magnesium meal surface of the workpiece, but it also materially enhances the adhesion so that such a workpiece having a coating thickness from 1 to 3 mils has an average salt spray life in the approximate range 120 to 250 hours, depending upon the thickness of the final coating.
• An aqueous chemical nickel plating bath comprising nickel ions, hypophosite ions in the range 0.15 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.12 mole per liter, a second chelating agent selected from the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.06 to 0.10 mole per liter, ammonium ions in the range 0.05 to 0.20 mole per liter, fluoride ions in the range 0.10 to 0.40 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7.0 to 9.0, and a buffering system comprising alkali metal cations and boron oxide anions and equivalent to an addition of alkali metal tetraborate in the
• An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range 0.15 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of arni-noacetic acid and salts thereof and in the range 0.08 to 0.12 mole per liter, a second chelating agent selected from the class consisting of citric acid and salts thereof and in the range 0.06 to 0.10 mole per liter, ammonium ions in the range 0.05 to 0.20 mole per liter, fluoride ions in the range 0.10 to 0.40 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7.0 to 9.0, and a buffering system comprising alkali metal cations and boron oxide anions and equivalent to an addition of alkali metal tetraborate in the range 0.05 to 0.20 mole per liter
• An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range 0.15 to 1.20 moles per liter, the ratio between nickel ions and hyposphophite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.12 mole per liter, a second chelating agent selected trom the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.06 to 0.10 mole per liter, ammonium bifluoride in the range 0.05 to 0.20 mole per liter, sufficient hydroxyl ions to produce a pH in the approximate range 7.0 to 9.0, and a. buffering system comprising alkali metal cations and boron oxide and equivalent to an addition of alkali metal tetraborate in the range 0.05 to 0.20 mole per liter.

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• Chemically Coating (AREA)
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• Chemical Treatment Of Metals (AREA)

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Citations (8)

• 1961
  • 1961-12-15 US US159745A patent/US3121644A/en not_active Expired - Lifetime
• 1962
  • 1962-11-08 GB GB42228/62A patent/GB989250A/en not_active Expired
  • 1962-11-24 ES ES282772A patent/ES282772A1/es not_active Expired
  • 1962-11-26 FI FI2122/62A patent/FI42656B/fi active
  • 1962-11-29 CH CH1403962A patent/CH419771A/fr unknown
  • 1962-11-30 DK DK519862AA patent/DK105308C/da active
  • 1962-12-03 LU LU42802D patent/LU42802A1/xx unknown
  • 1962-12-04 SE SE13057/62A patent/SE304892B/xx unknown
  • 1962-12-04 AT AT950462A patent/AT247683B/de active

Patent Citations (8)

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