United States Patent
`
`[19]
`
`‘[1 1] Patent Number:
`
`5,266,412
`
`Bartak et al.
`
`[45] Date of Patent:
`
`Nov. 30, 1993
`
`U5005266412A
`
`[54] COATED MAGNESIUM ALLOYS
`
`[75]
`
`Inventors: Duane E. Bartak, Grand Forks, N.
`Dak.; Brian E. Lemieux, East Grand
`Forks, Minn; Earl R. Woolsey,
`Grand Forks, N. Dak.
`
`[73] Assignee:
`
`Technology Applications Group, Inc.,
`Grand Forks, N. Dak.
`
`[21] Appl. No.: 943,325
`
`[22] Filed:
`
`Sep. 10, 1992
`
`Related U.S. Application Data
`
`[62]
`
`Division of Ser. No. 729,612, Jul. 15, 1991.
`
`Int. Cl.5 ................................................ 8328 9/00
`[51]
`[52] U.S. Cl. .................................... 428/472; 428/469;
`428/696; 428/699; 428/701; 428/697; 428/702;
`205/321
`[58] Field of Search ................ 428/321; 428/469, 472,
`428/701, 699, 696, 697, 702
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`1.574289
`2/1926 Keeler ................................. 205/321
`3.834.999
`9/1974 Hradcovsky et a1. .......... 205/321
`
`3,956.080
`5/1976 Hradcovsky et a1. .......... 205/321
`4,082,626 4/1978 Hradcovslcy ........................ 205/321
`4.184.926
`1/1980 Kojak .................................. 205/321
`4,620,904 11/1986 Kozak ............. 205/321
`
`4.659440 4/1987 Hradcovsky .................... 205/321
`5/1987 Habermann et a1. ........... 205/321
`4,668,347
`
`5/1988 Kobayashi et al. ............. 205/321
`4.744.872
`
`................. 205/321
`4.976.830 12/1990 Schmeling et a1.
`
`Primary Examiner—John Niebling
`Assistant Examiner—Kishor Mayekar
`Attorney, Agent, or Firm—Merchant, Gould, Smith,
`Edell, Welter & Schmidt
`
`[57]
`
`ABSTRACT
`
`A two-step process for the coating of magnesium and its
`alloys is disclosed. The first step comprises immersing
`the magnesium workpiece in a first electrochemical
`solution comprising about 3 to 10 wt-% of a hydroxide
`and about 5 to 30 wt-% of a fluoride having a pH of at
`least about 12. By controlling a current density to about
`10 to 200 mA/cmz, an increasing voltage differential is
`established between an anode comprising the pretreated
`article and a cathode also in contact with the electro-
`lytic solution. Next, the article is immersed in an aque-
`ous electrolytic solution having a pH of at least about 11
`and which solution is prepared from components com-
`prising a water soluble hydroxide, a fluoride source and
`a water soluble silicate in amounts to result in an addi—
`
`tion of about 2 to 15 g of a hydroxide per liter of solu-
`tion, about 2 to 14 g of a fluoride per liter of solution and
`about 5 to 40 g ofa silicate per liter of solution. Again,
`by comrolling the current density to about 5 to 100
`mA/cmz, an increasing voltage differential of at least
`about 150 volts is established between an anode com-
`
`prising the pretreated article and a cathode also in
`contact with the electrolytic solution. This process
`results in a superior coating which has increased abra-
`sion and corrosion resistance.
`
`4 Claims. 2 Drawing Sheets
`
`'2
`14
`
`”Illnlllllllllllnllla "I.
`'k““““\\\\\‘\\\\\\\\\\\\‘\\\\\\‘
`
`
`
`
`
`
`VIII/[llllll
`
`I0
`
`
`
`UNTREATED
`ARTICLE
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 1 of 2
`
`5,266,412
`
`
`
`FIG. I
`
`.4
`
`,2
`
`
`
`
`W
`
`
`”Innlnllllllllllllfl 7’1.
`IL“\““‘\\‘\\‘\\\\‘\‘\\\\\\\\\\\\‘
`
`
`
`
`l0
`
` UNTREATED
`
`ARTICLE
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`U.S. Patent
`
`Nov. 30, 1993
`
`Sheet 2 of 2
`
`5,266,412
`
`FIG. 4
`
`
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`1
`
`COATED MAGNESIUM ALLOYS
`
`5,266,412
`
`This is a division of application Ser. No. 07/729,612,
`filed Jul. 15, 1991.
`
`FIELD OF THE INVENTION
`
`The invention relates to a process for forming an
`inorganic coating on a magnesium alloy In particular,
`the invention relates to a two-step method comprising a
`first electrochemical treatment in a bath comprising a
`hydroxide and a fluoride and a second electrochemical
`treatment in a bath comprising a hydroxide, a fluoride
`source and a silicate.
`
`BACKGROUND OF THE INVENTION
`
`The use of magnesium in structural applications is
`growing rapidly. Magnesium is generally alloyed with
`any of aluminum, manganese,
`thorium,
`lithium,
`tin,
`zirconium, zinc and rare earth metals or other alloys or
`combinations of these to increase its structural ability.
`Such magnesium alloys are often used where a high
`strength to weight ratio is required. The appropriate
`magnesium alloy can also offer the highest strength to
`weight ratio of the ultra light metals at elevated temper-
`atures. Further, alloys with rare earth or thorium can
`retain significant strength up to temperatures of 315° C.
`and higher. Structural magnesium alloys may be assem-
`bled in many of the conventional manners including
`riveting and bolting, arc and electric resistance welding,
`braising, soldering and adhesive bonding. The magnesi-
`um-containing articles have uses in the aircraft and
`aerospace industries, military equipment. electronics,
`automotive bodies and parts, hand tools and in materials
`handling. While magnesium and its alloys exhibit good
`stability in the presence of a number of chemical sub-
`stances, there is a need to further protect the metal,
`especially in acidic environments and in salt water con-
`ditions. Therefore, especially in marine applications, it
`is necessary to provide a coating to protect the metal
`from corrosion.
`
`There are many different types of coatings for mag—
`nesium which have been developed and used. The most
`common coatings are chemical treatments or conver-
`sion coatings which are used as a paint base and provide
`some corrosion protection. Both chemical and electro-
`chemical methods are used for the conversion of mag-
`nesium surfaces. Chromate films are the most com—
`monly used surface treatments for magnesium alloys.
`These films of hydrated, gel-like structures of polychro-
`mates provide a surface which is a good paint base but
`which provide limited corrosion protection.
`Anodization of magnesium alloys is an alternative
`electrochemical approach to provide a protective coat-
`ing. At least two low voltage anodic processes, Dow 17
`and HAE, have been commercially employed. How-
`ever, the corrosion protection provided by these treat-
`ments remains limited. The Dow 17 process utilizes
`potassium dichromate, a chromium (V1) compound,
`which is acutely toxic and strictly regulated. Although
`the key ingredient in the HAE anodic process is potas-
`sium permanganate, it is necessary to use a Chromate
`sealant with this coating in order to obtain acceptable
`corrosion resistance. Thus in either case, chromium
`(VI) is necessary in the overall process in order to
`achieve a desirable corrosion resistant coating. This use
`of chromium (V1) means that waste disposal from these
`processes is a significant problem.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`2
`More recently, metallic and ceramic-like coatings
`have been developed. These coatings may be formed by
`electroless and electrochemical processes. The electro-
`less deposition of nickel on magnesium and magnesium
`alloys using chemical reducing agents in coating formu-
`lation is well known in the art. However, this process
`results in the creation of large quantities of hazardous
`heavy metal contaminated waste water which must be
`treated before it can be discharged. Electrochemical
`coating processes-can be used to produce both metallic
`and nonmetallic coatings. The metallic coating pro-
`cesses again suffer from the creation of heavy metal
`contaminated waste water.
`
`Non-metallic coating processes have been developed,
`in part,
`to overcome problems involving the heavy
`metal contamination of waste water. Kozak, U.S. Pat.
`No. 4,184,926, discloses a two-step process for forming
`an anti-corrosive coating on magnesium and its alloys.
`The first step is an acidic chemical pickling or treatment
`of the magnesium work piece using hydrofluoric acid at
`about room temperature to form a fluoro-magnesium
`layer on the metal surface. The second step involves the
`electrochemical coating of the work piece in a solution
`comprising an alkali metal silicate and an alkali metal
`hydroxide. A voltage potential from about 150-300
`volts is applied across the electrodes, and a current
`density of about 50—200 mA/cm2 is maintained in the
`bath. The first step of this process is a straight forward
`acid pickling step, while the second step proceeds in an
`electrochemical bath which contains no fluoride source.
`
`Tests of this process indicate that there is a need for
`increased corrosion resistance and coating integrity.
`Kozak, U.S. Pat. No. 4,620,904, discloses a one-step
`method of coating articles of magnesium using an elec~
`trolytic bath comprising an alkali metal silicate, an alkali
`metal hydroxide and a fluoride. The bath is maintained
`at a temperature of about 5°—70° C. and a pH of about
`12~l4. The electrochemical coating is carried out under
`a voltage potential from about 150-400 volts. Tests of
`this process also indicate that there remains a need for
`increased corrosion resistance.
`Based on the teachings of the prior art, a process for
`the coating of magnesium-containing articles is needed
`which results in a uniform coating with increased corro—
`sion resistance. Further, a more economical coating
`process is needed which has reduced apparatus de-
`mands and which does not result in the production of
`heavy metal contaminated waste water.
`
`SUMMARY OF THE INVENTION
`
`The present invention is directed to a process for
`coating a magnesium-containing article. The article is
`first immersed in an aqueous electrolytic solution com-
`prising about 3 to 10 g/L of a hydroxide and about 5 to
`30 g/L of a fluoride having a pH of at least about 11. By
`controlling a current density to about
`10 to 200
`mA/cmz, an increasing voltage differential
`is estab-
`lished between an anode comprising the pretreated
`article and a cathode also in contact with the electro-
`lytic solution. This pretreatment step cleans the article
`and creates a base layer comprising magnesium oxide,
`magnesium fluoride, magnesium oxofluoride, or a mix-
`ture thereof at the surface of the article. Next, the article
`is immersed in an aqueous electrolytic solution having a
`pH of at least about 11 and which solution is prepared
`from components comprising a water soluble hydrox-
`ide, a water soluble fluoride source and a water soluble
`silicate in amounts to result in an addition of about 2 to
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`5,266,412
`
`4
`
`TABLE I
`More
`Most
`
`Component
`Preferred
`Preferred
`Preferred
`Hydroxide (M)
`3 to 10
`5 to 8
`5 to 6
`
`Fluoride (g/L) 12 to 15 5 to 30 10 to 20
`
`
`
`3
`15 g of a hydroxide per liter of solution, about 2 to 14 g
`of a fluoride per liter of solution and about 5 to 40 g of
`a silicate per liter of solution. Again by controlling the
`current density to about 5 to 100 mA/cmz, an increasing
`voltage differential of at least about 150 volts is estab-
`lished between an anode comprising the pretreated
`article and a cathode also in contact with the electro-
`
`lytic solution to produce a spark discharge. Through
`this process,
`a silicon oxide-containing coating is
`formed on the base layer.
`In one preferred embodiment, a full wave rectified
`alternating current power source is used.
`The term “magnesium-containing article", as used in
`the specification and the claims, includes magnesium
`metal and alloys comprising a major proportion of mag-
`nesrum.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates a cross-section of the coated mag-
`nesium-containing article of the present invention.
`FIG. 2 is a block diagram of the present invention.
`FIG. 3 is a diagram of the electrochemical process of
`the present invention.
`FIG. 4 is a scanning electron photomicrograph of a
`cross-section through the magnesium-containing sub-
`strate and a coating according to the invention.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`FIG. 1 illustrates a cross-section of the surface of a
`
`magnesium-containing article having been coated using
`the process of the present invention. The magnesium-
`containing article 10 is shown with a first inorganic
`layer 12 comprising magnesium oxide, magnesium fluo-
`ride, magnesium oxofluoride, or a mixture thereof and a
`second inorganic layer 14 comprising silicon oxide. The
`layers 12 and 14 combine to form a corrosion resistant
`coating on the surface of the magnesium-containing
`article.
`
`FIG. 2 illustrates the steps used to produce these
`coated articles. An untreated article 20 is first treated in
`a first electrochemical bath 22 which cleans and forms
`a layer comprising magnesium oxide, magnesium fluo-
`ride, magnesium oxofluoride, or a mixture thereof on
`the article. Next, the article is treated in a second elec-
`trochemical bath 24 resulting in the production of a
`coated article 26.
`
`The article is subjected to a first electrochemical
`coating process shown in FIG. 3. In the first electro-
`chemical step, the first electrochemical bath 22 com-
`prises an aqueous electrolytic solution comprising about
`3 to 10 g/L of a soluble hydroxide compound and about
`5 to 30 g/L of a soluble fluoride. Preferred hydroxides
`include alkali metal hydroxides and ammonium hydrox-
`ide. More preferably, the hydroxide is an alkali metal
`hydroxide, and most preferably, the hydroxide is potas-
`sium hydroxide.
`The soluble fluoride may be a fluoride such as an
`alkali metal fluoride, ammonium fluoride, ammonium
`bifluoride, and hydrogen fluoride. Preferably, the fluo-
`ride comprises an alkali metal fluoride, hydrogen fluo-
`ride or mixtures thereof. More preferably, the fluoride
`comprises potassium fluoride.
`Compositional ranges for the aqueous electrolytic
`solution are shown below in Table I.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`In both the first and second electrochemical opera-
`tions, the article 30 is immersed in an electrochemical
`bath 42 as an anode. The vessel 32 which contains the
`electrochemical bath 42 may be used as the cathode, or
`a separate cathode may be immersed in the bath 42. The
`anode may be connected through a switch 34 to a recti-
`fier 36 while the vessel 32 may be directly connected to
`the rectifier 36. The rectifier 36, rectifies the voltage
`from a voltage source 38, to provide a direct current
`source to the electrochemical bath. The rectifier 36 and
`switch 34 may be placed in communication with a mi-
`croprocessor control 40 for purposes of controlling the
`electrochemical composition. The rectifier provides a
`pulsed DC signal, which, in a preferred embodiment, is
`initially under voltage control with a linear increase in
`voltage until the desired current density is achieved.
`The conditions of the electrochemical deposition
`process are preferably as illustrated below in Table II.
`
`TABLE II
`More
`Most
`
`Component
`Preferred
`Preferred
`Preferred
`pH
`ill
`121013
`12.5 to 13
`Temperature (°C.)
`5 to 30
`10 to 25
`15 to 20
`Time (minutes)
`up to 8
`2 to 6
`2 to 3
`Current Density
`10 to 200
`20 to 100
`40 to 60
`(mA/cmz)
`
`The magnesium-containing article is maintained in
`the first electrochemical bath for a time sufficient to
`clean impurities at the surface of the article and to form
`a base layer on the magnesium—containin g articles. This
`results in the production of a magnesium-containing
`article which is coated with a first or base layer, com-
`prising magnesium oxide, magnesium fluoride, magne-
`sium oxofluoride, or a mixture thereof. Too brief a resi-
`dence time in the electrochemical bath results in an
`insufficient formation of the first layer and/or insuffi—
`cient cleaning of the magnesium-containing article. This
`will ultimately result in reduced corrosion resistance of
`the coated article. Longer residence times tend to be
`uneconomical as the process time is increased and the
`first layer will be thicker than necessary and may even
`become non-uniform. This base layer is generally uni-
`form in composition and thickness across the surface of
`the article and provides an excellent base upon which a
`second, inorganic layer may be deposited. Preferably,
`the thickness of the first layer is about 0.05 to 0.2 mi-
`crons.
`
`Although we do not wish to be confined to a particu-
`lar mechanism for the coating process, it appears that
`the first electrochemical step is beneficial
`in that
`it
`cleans or oxidizes the surface of the substrate and also
`
`provides a base layer which firmly bonds to the sub-
`strate. The base layer is compatible with the composi-
`tion which will form the second layer and provides a
`good substrate for the adhesion of the second layer. It
`appears that the base layer comprises magnesium oxide.
`magnesium fluoride, magnesium oxofluoride, or a mix-
`ture thereof which strongly adheres to the metal sub-
`strate. It appears that the compatibility of these com-
`pounds with those of the second layer permits the depo-
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`5
`sition of a layer comprising silicon oxide, in a uniform
`manner, without appreciable etching of the metal sub-
`strate. In addition, both the first and second layers may
`comprise oxides of other metals within the alloy and
`oxides of the cations present in the electrolytic solution.
`The base layer provides a minimum amount of pro-
`tection to the metal substrate, but it does not provide
`the abrasion resistance a complete, two-layer coating
`provides. However, if the silicon oxide-containing layer
`is applied directly to the metallic substrate without first
`depositing the base layer, a non-uniform, poorly adher-
`ent coating, which has relatively poor corrosion-resist-
`ant properties, will result.
`Between the first and second electrochemical baths,
`22 and 24 respectively, the pretreated article is prefera-
`bly thoroughly washed with water to remove any con-
`taminants.
`
`The article is then subjected to a second electrochem-
`ical coating process as also depicted in FIG. 3 and gen-
`erally discussed above. The details of the second elec-
`trochemical coating step follows. The second electro-
`chemical bath 24 comprises an aqueous electrolytic
`solution comprising about 2 to 15 g/L of a soluble hy-
`droxide compound, about 2 to 14 g/L of a soluble fluo-
`ride containing compound selected from the group
`consisting of fluorides and fluorosilicates and about 5 to
`40 g/L of a silicate. Preferred hydroxides include alkali
`metal hydroxides and ammonium hydroxide. More
`preferably, the hydroxide is an alkali metal hydroxide,
`and most preferably, the hydroxide is potassium hy-
`droxide.
`
`5,266,412
`
`6
`
`TABLE III-continued
`More
`Most
`
`Component
`Preferred
`Preferred
`Preferred
`Fluoride
`2 to 14
`6 to 12
`7 to 9
`Source (g/L)
`
`10 to 255 to 40Silicate (g/L) l5 to 20
`
`
`
`
`
`The conditions of the electrochemical deposition
`process are preferably as illustrated below in Table IV.
`
`10
`
`
`TABLE IV
`More
`Most
`
`Component
`Preferred
`Preferred
`Preferred
`pH
`:11
`11.5 to 13
`12 to 13
`Temperature ('C.)
`5 to 35
`10 to 30
`15 to 25
`Time (minutes)
`'
`5 to 90
`10 to 40
`15 to 30
`Current Density
`5 to 100
`5 to 60
`5 to 30
`(mA/cmz)
`
`These reaction conditions allow the formation of an
`
`15
`
`20
`
`25
`
`3O
`
`65
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`inorganic coating of up to about 40 microns in about 90
`minutes or less. Maintaining the voltage differential for
`longer periods of time will allow for the deposition of
`thicker coatings. However, for most practical purposes,
`coatings of about 10 to 30 microns in thickness are pre-
`ferred and can be obtained through a coating time of
`about 10 to 30 minutes.
`the coating is
`In the first electrochemical bath,
`formed through a spark discharge process. The current
`density applied through the electrochemical solutions
`establishes an increasing voltage differential, especially
`at the surface of the magnesium-containing anode. A
`spark discharge is established across the surface of the
`anode during the formation of the coating. Under re—
`duced light conditions, the spark discharge is visible to
`the eye. Of course, as the coating increases in thickness,
`its resistance increases, and to maintain a given current
`density,
`the voltage must
`increase. Similar sparking
`procedures are disclosed in Hradcovsky et al., U.S. Pat.
`Nos. 3,834,999 and 3,956,080, both of which are hereby
`incorporated by reference.
`The second coating produced according to the
`above-described process is ceramic-like and has excel-
`lent corrosion and abrasion resistance and hardness
`characteristics. While not wishing to be held to this
`mechanism, it appears that these properties are the re-
`sult of the morphology and adhesion of the base and the
`second coating to the metal substrate and the base coat-
`ing, respectively. It also appears that the preferred sec-
`ond coating comprises a mixture of fused silicon oxide
`and fluoride along with an alkali metal oxide, most
`preferably, this second coating is predominantly silicon
`oxide. “Silicon oxide” here includes any of the various
`5 forms of silicon oxides of silicon.
`The superior coating of the invention is produced
`without a need for chromium (VI) in the process solu-
`tions. Therefore, there is no need to employ costly pro-
`cedures to remove this hazardous heavy metal contami-
`nant from process waste. As a result, the preferred coat-
`ings are essentially chromium (VI)-free.
`The adhesion of the coating of the invention appears
`to perform considerably better than any known com-
`mercial coating. This is the result of coherent interfaces
`between the metal substrate, base coating, and second
`coating. A scanning electron photomicrograph cross—
`section view of the coating on the metal substrate is
`shown in FIG. 4. The photomicrograph show that the
`
`35
`
`45
`
`50
`
`The fluoride containing compound may be a fluoride
`such as an alkali metal fluoride, hydrogen fluoride,
`ammonium bifluoride or ammonium fluoride, or a
`fluorosilicate such as an alkali metal fluorosilicate or
`mixtures thereof. Preferably, the fluoride source com-
`prises an alkali metal fluoride, an alkali metal fluorosili-
`cate, hydrogen fluoride or mixtures thereof. Most pref-
`erably, the fluoride source comprises an alkali metal
`fluoride. The most preferable fluoride source is potas-
`sium fluoride.
`The electrochemical bath also contains a silicate. By
`“silicate", both here in the specification and the claims,
`we mean silicates, including alkali metal silicates, alkali
`metal fluorosilicates, silicate equivalents or substitutes
`such as colloidal silicas, and mixtures thereof. More
`preferably, the silicate comprises an alkali metal silicate,
`and most preferably, the silicate is potassium silicate.
`From the preceding paragraphs it
`is apparent a
`fluorosilicate may provide both the fluoride and the
`silicate in the aqueous solution. Therefore, to provide a
`sufficient concentration of fluoride in the bath only
`about 2 to 14 g/L of a fluorosilicate may be used. On the
`other hand,
`to provide a sufficient concentration of
`silicate, about 5 to 40 g/L of the fluorosilicate may be 5
`used. Of course, the fluorosilicate may be used in con-
`junction with other fluoride and silicate sources to pro-
`vide the necessary solution concentrations. Further, it is
`understood that, in an aqueous solution at a pH of at
`least about 11, the fluorosilicate will hydrolyze to pro-
`vide fluoride ion and silicate in the aqueous solution.
`Compositional ranges for the aqueous electrolytic
`solution are shown below in Table III.
`
`TABLE III
`More
`Most
`
`Component
`Preferred
`Preferred
`Preferred
`Hydroxide (g/L)
`2 to 15
`4 to 9
`5 to 6
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`5,266,412
`
`8
`
`EXAMPLES
`
`7
`metal substrate 50 has an irregular surface at high mag—
`nification, and a coherent base layer 52 is formed at the
`surface of the substrate 50. The silicon oxide-containing
`layer 54 which is formed on the base layer 52 shows
`excellent integrity, and both coating layers 52 and 54
`therefore provide superior corrosion resistant and abra-
`sion resistant surface.
`
`Abrasion resistance was measured according to Fed-
`eral Test Method Standard No. 141C, Method 6192.1.
`Preferably coatings produced according to the inven-
`tion having thickness of 0.8 to 1.0 mil will withstand at
`least 1000 wear cycles before the appearance of bare
`metal substrate using a 1.0 kg load on CS-17 abrading
`wheels. More preferably, the coating will withstand at
`least 2000 wear cycles before the appearance of the
`metal substrate, and most preferably, the coating will
`withstand at least 3000 wear cycles using a 1.0 kg load
`on CS-17 abrading wheels.
`Corrosion resistance was measured according to
`ASTM standard methods. Salt fog test, ASTM B117,
`was employed as the method for corrosion resistance
`testing with ASTM D1654, procedures A and B used in
`the evaluation of test samples. Preferably, as measured
`according to procedure B, coating on magnesium alloy
`AZ91D produced according to the invention achieve a
`rating of at least 9 after 24 hours in salt fog. More pref-
`erably, the coatings achieve a rating of at least 9 after
`100 hours, and most preferably, at least 8 after 200 hours
`in salt fog.
`After the magnesium-containing articles have been
`coated according to the present process, they may be
`used as is, offering very good corrosion resistant prop-
`erties, or they may be further sealed using an optional
`finish coating such as a paint or sealant. The structure
`and morphology of the silicon oxide-containing coating
`readily permit the use of a wide number of additional
`finish coatings which offer further corrosion resistance
`or decorativeproperties to the magnesium-containing
`articles. Thus, the silicon oxide-containing coating pro-
`vides an excellent paint base having excellent corrosion
`resistance and offering excellent adhesion under both
`wet and dry conditions, for instance, the water immer-
`sion test, ASTM D3359,
`test method B. Any paint
`which adheres well to glass or metallic surfaces may be
`used as the optional finish coating. Representative, non-
`limiting inorganic compositions for use as an outer coat-
`ing include additional alkali metal silicates, phosphates,
`borates, molydates, and vanadates. Representative, non-
`limiting organic outer coatings include polymers such
`as polyfluoroethylene and polyurethanes. Additional
`finish coating materials will be known to those skilled in
`the art. Again, these optional finish coatings are not
`necessary to obtain very good corrosion resistance;
`however, their use may achieve a more decorative fin-
`ish or further improve the protective qualities of the
`coating.
`Excellent corrosion resistance occurs after further
`
`application of an optional finish coating. Preferably, as
`measured according to procedure B, coatings produced
`according to the invention, having an optional finish
`coating, achieve a rating of at least about 8 after 700
`hours in salt fog. More preferably, the coatings achieve
`a rating of at least about 9 after 700 hours, and most
`preferably, at least about 10 after 700 hours in salt fog.
`
`The following specific examples, which contain the
`best mode, can be used to further illustrate the inven-
`tion. These examples are merely illustrative of the in-
`vention and do not limit its scope.
`
`EXAMPLE I
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`Magnesium test panels (AZ91D alloy) were cleaned
`by immersing them in an aqueous solution of sodium
`pyrophosphate, sodium borate, and sodium fluoride at
`about 70" C. and a pH of about 11 for about 5 minutes.
`The panels were then placed in a 5% ammonium bifluo-
`ride solution at 25° C. for about 5 minutes. The panels
`were rinsed and placed in the first electrochemical bath,
`which contained potassium fluoride and potassium hy-
`droxide. The first electrochemical bath was prepared by
`dissolving 5 g/L of potassium hydroxide and 17 g/L of
`potassium fluoride and has a pH of about 12.7. The
`panels were then placed in the bath and connected to
`the positive lead of a rectifier. A stainless steel panel
`served as the cathode and was connected to the nega-
`tive lead of the rectifier capable of delivering a pulsed
`DC signal. The power was increased over a 30 second
`period with the current controlled to a value of 80
`mA/cmz. After 2 minutes, the magnesium oxide/fluo-
`ride layer was approximately one to two microns thick.
`The panels were then taken out of the first electrochem-
`ical bath, rinsed well with water, and placed into the
`second electrochemical bath and connected to the posi-
`tive lead of a rectifier. The second electrochemical bath
`
`was prepared by mixing together potassium silicate,
`potassium fluoride, and potassium hydroxide. The sec-
`ond electrochemical bath was made by first dissolving
`150 g of potassium hydroxide in 30 L of water. 700
`milliliters of a commercially available potassium silicate
`concentrate (20% w./w $02) was then added to the
`above solution. Finally 150 g of potassium fluoride was
`added to the above solution. The bath had a pH of about
`12.7 and a concentration of 5 g/L potassium hydroxide,
`about 18 g/L potassium silicate and about 5 g/L potas-
`sium fluoride. A stainless steel panel served as the cath-
`ode and was connected to the negative lead of a rectifier
`capable of delivering a pulsed DC signal. The voltage
`was increased over a 30 second period to approximately
`150 V, and then the current was adjusted to sustain a
`current density of 25 mA/cml’. After approximately 30
`minutes,
`the coating was approximately 25 microns
`thick.
`
`EXAMPLES II—VIII
`
`Examples II—VII were prepared according to the
`process of Example I with the quantities of components
`as shown in Tables V and VIII shown below.
`
`55
`
`TABLE V
`Electrochemical Bath #1 30 1.)
`
`60
`
`65
`
`Fluoride
`Example Hydroxide
`450 g KF
`11
`130 g KOH
`III
`l20 g NaOH 310 g NaF
`IV
`150 g KOH
`500 g KF
`v
`90 g 1.1011
`500 g x1:
`V1
`180 g KOH
`560 g KF
`VII
`135 g NaOH 250 g LiF
`Vin
`150 g KOH
`550 g KF
`
`pH
`12.8
`12.7
`127
`12.6
`12.8
`12.8
`12.7
`
`Current
`Density
`(mA/cmzl
`50
`60
`80
`70
`80
`70
`80
`
`Time
`(min)
`2
`1.5
`2
`1.5
`1
`2
`1.5
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`10
`
`9
`
`TABLE VI
`
`5,266,412
`
`Electrochemical Bath #2 [30 1.}
`Current
`Potassium
`Silicate
`Density
`
`Example
`Hydroxide
`Concentrate‘
`Fluoride
`pH
`(mA/cm2)
`Time (min)
`11
`180 g KOH
`600 mL
`250 g KF
`12.8
`30
`30
`III
`150 g KOH
`700 m1.
`300 g KF
`12.7
`40
`20
`IV
`120 g NaOH
`600 mL
`300 g KF
`12.7
`30
`25
`V
`80 g LiOH
`500 m1.
`250 g KF
`12.6
`20
`25
`VI
`150 g KOH
`600 mL
`200 g NaF
`12.7
`30
`20
`VII
`180 g KOH
`81X) ml.
`350 g KF
`12.8
`30
`30
`140 g NaOHVIII 20 600 ml. 250 g NaF 12.8 40
`
`
`
`
`
`
`'20% $02 (w/w) in water. In other words. the concentration can be characterized as the equivalent of 20
`wt % $101 in water.
`
`ing a magnesium-containing substrate, a first, silicate-
`free base layer comprising a magnesium fluoride, a mag-
`nesium oxide, and a magnesium oxofluoride, and a sec-
`ond, outer layer comprising silicon oxide and magne-
`sium oxide.
`
`2. The article of claim 1 further comprising a third,
`sealing layer disposed upon the second, outer layer.
`3. The article of claim 1 which is substantially free of
`chromium (V1).
`4, A magnesium-containing article offering improved
`corrosion and abrasion resistance, the article compris-
`ing a magnesium-containing substrate, a first, substan-
`tially continuous, silicate-free base layer comprising a
`magnesium fluoride, a magnesium oxide, and a magne-
`sium oxofluoride, and a second, outer comprising sili-
`con oxide, 3 fluoride, and magnesium oxide.
`*
`t
`t
`t
`t
`
`Wear resistance or abrasion testing (Federal Method,
`141C) of these panels resulted Taber Wear Index (TWI)
`of less than 15 and in wear cycles of at least about 2000
`cycles before the appearance of the metal substrate
`using a 1.0 kg load on CS-l7 abrading wheels.
`EXAMPLE IX
`
`A magnesium test panel was coated as in Example 1.
`Upon drying an optional coating was applied in the
`following manner. The panel was immersed in a 20%
`(v/v) solution of potassium silicate (20% 5102, (w/w))
`for 5 minutes at 60° C. The panel was rinsed and dried
`and subjected to salt fog ASTM 13117 testing. The panel
`achieved a rating of 10 (ASTM D1654) after 700 hours
`in the salt fog.
`What is claimed is:
`1. A magnesium-containing article offering improved
`corrosion and abrasion resistance, the article compris-
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`TSMC1014
`
`IPR of U.S. Pat. No. 7,335,996
`
`TSMC1014
`IPR of U.S. Pat. No. 7,335,996
`
`

`

`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`
`PATENTND.
`
`DATED
`
`:
`
`:
`
`INVENTBNS) :
`
`5,266,412
`
`Page 1 of 4
`
`November 30, 1993
`
`Duane E. Bartak et a1.
`
`It is certified that war appears in the above'idemified patent and that said Letters Patent is hereby
`contacted as shown below:
`
`The title page showing the illustrative figure should be deleted
`to be replaced with the attached title page.
`
`in the drawings, sheet 1 of 2, consisting of figures 1-3, should be
`deleted to appear as shown on the attached p