Note: Descriptions are shown in the official language in which they were submitted.
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1.
TT~10-STEP ELECTROCHEMICAL PROCESS FOR COATTNG MAGNESIUM
Field of the 7~nvention
The invention re:Lc~l~es t:o 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 compri~~ing a hydroxide, a fluoride source and a
silicate.
-~0 Back~.round of the Invention
The use of magnesium in structural applications is
growing rapidly. Magnesium is generally allayed with any
of aluminum, manganese, thorium, lithium, tin, zirconium,
zinc and rare earth metals or other alloys oz: combinations
of these to increase its structural ability. Such
magnesium a'.loys are' often used where a high strength to
weight ratio is required. The appropriate magnesium alloy
can also of~°er the highest strength to weight ratio of the
ultra light metals at elevated temperatures. Further,
20 alloys with rare eai°th or thorium can retain significant
strength up to temperatures of 315°C and higher.
Structural magnesium alloys may be assembled in many of the
conventiona:L manners including riveting and bolting, arc
and electric resistance welding,, braising, soldering and
adhesive bonding. ~L'he magnesium-containing articles have
been used in the aircraft and aerospace industries,
military equipment, electronics, a.utomot:ive bodies and
parts inclv~.dimg, fo:r example, e:nc~ine part: such as pistons
and connecting rods, hand tools> and in materials handling.
While magnesium and its all.oya exhibit good stability in
30 the presence of a number of chemical substances, there is a
need to further protec::t
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the metal, especially in acidic environments and in salt
water conditions. 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
magnesium which have been developed and used. The most
common coatings are chemical treatments or conversion
coatings which are used as a paint base and provide some
corrosion protection. Both chemical and electrochemical
methods are used for the conversion of magnesium surfaces.
Chromate films are the most commonly used surface
treatments for magnesium alloys. These films of hydrated,
gel-like structures of polychromates 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 coating.
At least two low voltage anodic processes, Dow 17 and HAE,
have been commercially employed. However, the corrosion
protection provided by these treatments remains limited.
The Dow 17 process utilizes potassium dichromate, a
chromium (VI) compound, which is acutely toxic and strictly
regulated. Although the key ingredient in the HAE anodic
process is potassium 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 (VI) means that waste disposal from these
processes is a significant problem.
More recently, metallic and ceramic-like coatings have
been developed. These coatings may be formed by
electroless and electrochemical processes. The electroless
deposition of nickel on magnesium and magnesium alloys
using chemical reducing agents in coating formulation is
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' 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 processes 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. Patent 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/cmz 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. Patent No. 4,620,904, discloses a one-step
method of coating articles of magnesium using an
electrolytic bath comprising an alkali metal silicate, an
alkali metal hydroxide and a fluoride. The bath is
maintained at a temperature of about 5-70C and a pH of
about 12-14. 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.
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Based o:n the teac:h.ings of the prior art, a process for
the coating of magnesium-containing articles is needed
which results in a uniform coating with increased corrosion
resistance. Further, a more economical coating process is
needed which has reduced apparatus demands 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-r.:c>ntaining article. The article is
first immersed in an aqueous electrolytic solution
comprising about 3 tr.~ 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/cm2, an
increasing voltage di.f:ferential is established between an
anode comprising the pretreated article and a cathode also
in contact with the electrolytic solution. This
pretreatment step cleans the article and creates a base
layer comprising magnesium oxide, magnesium fluoride,
magnesium ox:ofluoride, or a mixture thereof at the surface
of the article. Next, the article is immersed in an
Zp aqueous electrolytic solution having a pH of at least about
11 and vahich so:l_ut:ion i s prepared from components
comprising a water soluble hydroxide, a water soluble
fluoride source and ~~ water soluble silicate in amounts to
result in a:n addition of about ? to 15 g of a hydroxide per
liter of solution, a~lo>ut. a? to 14 g of a fluoride per liter
of solution and abc~.zi=. 5 to 40 c~ of a silicate per liter of
solution. Again by ~:::omtrolling the current density to about
to 100 rr~~/cmZ, aw increasing voltage differential of at
least abouv 150 volts is established between an anode
30 comprising the pre treated ar_-ti..c-'~e and a cathode also in
contact with the e.ectrolytic solution to produce a spark
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discharge. Through tfais process, a silicon oxide-containing
coating is formed on the base layer.
In one ~_:~referred embodiment, a full wave
rectified alternatinc:~ current power source i.s used.
Th~~ term "magnesium-cont.a=fining art isle" , as used
in the specification and the claims, includes magnesium
metal and alloys comprising a major proportion of
magnesium.
The present invention also provides a process
1.0 which is free of chromium (VI1 for forming an improved
corrosion resistant coating on a magnesium-containing
article, which process's comprises:
(a) placinug the article into a first aqueous
electrolytic solution having a pH of 13 and a temperature
of 20°C which comprises :
ti) 6 g/L of a hydroxide; and
(ii) 13 g/L of a fluoride;
(b) r:onne::t.ing a first anode comprising the
article ano: a first cathode to a full. wave rectified power
20 source;
(c) establishing a current densit~~ of 50 mA/cm2,
to produce an incre~~~sing voltage differential up to 180 V
between a f first anc:~de r:ornprising an arti~Jle and a first
cathode in an electrolytic solution to result in a first
layer at the surfacE-~ of the article, which layer comprises
a fluoride, an oxide, an oxofluoride or a mixture thereof,
to form a f>r~etreatec~. ~~:rtic Le;
(d) placiric:~ the pretreated arti_cle~ into a second
aqueous electrolytic solution having a pH of 13 and a
30 temperature of 20'~~, which solution is prepared from
components comprising:
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5a
(i) 5 g/L of a hydroxide;
(ii) 10 g/L of a fluoride source; and
(iii) 15 g/:L of a. silicate;
(c=) connec:~ting a second anodes comprising the
pretreated article rind a second. cathode to an additional
full wave rectified power source;
(:E) establishing a current density of 30 mA/cm2
to create a voltage differential of at least 150 V between
a second anode comprising the pretreated article and a
7.0 second catrrode in t:he second electrolytic solution under
conditions producing a spark discharge;
wherein a silicon oxide-containing coating is formed on the
article.
In accordance with a further aspect, the
invention F>rovides <~ process for forming an improved
corrosion resistant: coating on a magnesium-containing
article, which proces:~ comprises:
(a) placi.zc~ the article into a first aqueous
electrolytic solutic:~ru having a pH of at Least 11 which
;?0 comprises:
(i) 3 to 10 g/L of hydroxide;
(ii) 5 to 30 g/~~ of fluoride;
estab:L:i_shinc~ a current denss_ty of 10 to 200
mA/cm2, to produce ~i.n increasing voltage differential up to
180 V between a first anode r_cmprising the article and a
first cathode in the electrolytic solution to result in a
first layer at the surface of the article, which layer
comprises a fluoride::, an oxide, an oxofluoride or a mixture
thereof, tc> form a ~;~r~~treated article;
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5b
(c) placing the pretreated article into a second
aqueous electrolytic :aolution having a pH of at least 11,
which solution is prepared from components comprising:
(s) l to 15 g/L of a hydroxide;
(ii) 2 to 40 g/L of a fluorosilicate;
(d) estab_L.ishing a current density of 5 to 100
mA/cm2 to create a r,TC~itage differential of at least 150 V
between a second ar~ode comprising the pretreated article
and a second cathode in the second electrolytic solution
7_0 under conditions producing a spark discharge;
wherein a silicon oxide-containing coating is formed on the
article.
Brief Description of the Drawings
Figure 1 illustrates a cross-section of the coated
magnesium-containing article of the present invention.
Figure 2 is a b:L~~c~k diagram of the present invention.
Figure 3 is a diagram of the electrochemical process of
the present invention.
Figure 4 is a scanning electron photomicrograph of a
?0 cross-section through the magnesium-containing substrate
anc~ a coating according to the invention.
Detailed Description of the Preferred Embodiment
Figure 1 illust:.rates a cross-section of the surface of
a magnesiurn-containing article having been coated using the
process of the present invention. The magnesium-containing
article 10 is shows: with a first inorganic layer 12
comprising magnesium oxide, magnesium fluoride, magnesium
oxofluoride, or a mixture thereof and a second inorganic
layer 14 c~amprisin~::1 s.ilican oxide . The layers 12 and 14
30 combine to form a ~:::c~rrosion resistant coating on the
surface of the magmeaium-containing article.
Figure 2 illustrates the steps used to produce these
coated articles. .Ar1 untreated article 20 is first treated
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5c
in a first electrochemical bath 22 which cleans and forms a
layer comprising magnesium oxide, magnesium fluoride,
magnesium oxofluoride, or a mixture thereof on the article.
Next, the article is treated in a second electrochemical
bath 24 resulting in the production of a coated article 26.
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The article is subjected to a first electrochemical
coating process shown in Figure 3. In the first
electrochemical step, the first electrochemical bath 22 ,
comprises 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 hydroxide. More
preferably, the hydroxide is an alkali metal hydroxide, and
most preferably, the hydroxide is potassium hydroxide.
The soluble fluoride may be a fluoride such as an
alkali metal fluoride, ammonium fluoride, ammonium
bifluoride, and hydrogen fluoride. Preferably, the
fluoride comprises an alkali metal fluoride, hydrogen
fluoride or mixtures thereof. More preferably, the
fluoride comprises potassium fluoride.
Compositional ranges for the aqueous electrolytic
solution are shown below in Table I.
Table I
More Most
Component Preferred Preferred Preferred
Hydroxide (g/L) 3 to 10 5 to 8 5 to 6
Fluoride (g/L) 5 to 30 10 to 20 12 to 15
In both the first and second electrochemical
operations, 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
rectifier 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
microprocessor control 40 for purposes of controlling the
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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 > 11 12 to 13 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 b0~
( mA~cm2 )
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-containing articles. This results
in the production of a magnesium-containing article which
is coated with a first or base layer, comprising magnesium
oxide, magnesium fluoride, magnesium oxofluoride, or a
mixture thereof. Too brief a residence time in the
electrochemical bath results in an insufficient formation
of the first layer and/or insufficient 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 uniform in composition and
thickness across the surface of the article and provides an
excellent base upon which a second, inorganic layer may be
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_g_
deposited. Preferably, the thickness of the first layer is
about 0.05 to 0.2 microns.
Although we do not wish to be confined to a particular
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 substrate. The base
layer is compatible with the composition 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 mixture thereof which strongly
adheres to the metal substrate. It appears that the
compatibility of these compounds with those of the second
layer permits the deposition of a layer comprising silicon
oxide, in a uniform manner, without appreciable etching of
the metal substrate. 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 protection
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 adherent coating,
which has relatively poor corrosion-resistant properties,
will result.
Between the first and second electrochemical baths, 22
and 24 respectively, the pretreated article is preferably
thoroughly washed with water to remove any contaminants.
The article is then subjected to a second
electrochemical coating process as also depicted in Figure
3 and generally discussed above. The details of the second
electrochemical coating step follows. The second
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_ g _
electrochemical bath 24 comprises an aqueous electrolytic
solution comprising about 2 to 15 g/L of a soluble
hydroxide compound, about 2 to 14 g/L of a soluble fluoride
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 hydroxide.
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 comprises an alkali metal
fluoride, an alkali metal fluorosilicate, hydrogen fluoride
or mixtures thereof. Most preferably, the fluoride source
comprises an alkali metal fluoride. The most preferable
fluoride source is potassium 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 used. Of
course, the fluorosilicate may be used in conjunction with
other fluoride and silicate sources to provide the
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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 provide
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
Fluoride
Source (g/L) 2 to 14 6 to 12 7 to 9
Silicate (g~L) 5 to 40 10 to 25 15 to 20
The conditions of elect rochemical deposition
the
process are preferably illus trated below Table
as in I~T.
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
( ~~cm2 )
These reaction conditions allow the formation of an
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
preferred and can be obtained through a coating time of
about 10 to 30 minutes.
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In the second electrochemical bath, the coating
is formed through a spark discharge process. The current
density applied through the electrochemical solutions
establishes an increG.sing 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 reduced light
conditions, the spark discharge is visible to the eye. Of
course, as the coating increases in thickness, its
.LO resistance increases, and to maintain a given current
density, the volt~~ge must increase. Sirni.lar sparking
procedures ~~re discl.o;sed in Hradcovsky et al . , U. S . Patent
Nos. 3,834,999 and 3,~a55,080.
T:he seconc::l coating produced according to the
above-described process is ceramic-like and has excellent
corrosion and a:b~_asi.on resistance and hardness
characteristics. While not wishing to be held to this
mechanism, it appears that these properties are the result
of the morphology ar:~d adhesion of the base and the second
20 coating to the met: al substrate and the base coating,
respectively. It al~~o appears that the preferred second
coating comprises a mixture of fused siZ.icon oxide and
fluoride along with ~::~n. alkali metal oxide, most preferably,
this seconc. coatiri.g is predominantly silicon oxide.
"Silicon oxide" here includes any of the various forms of
silicon oxides.
The superior coating of the invention is produced
without a need for chromi urn ;PTT) vn the process solutions .
Therefore, l:;:here is ~:m~ Cleed to employ costly procedures to
30 remove this hazardou:_~ ~n eavy metal ~cont~amir~ant.-. from process
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12
waste. As a result, i:he preferred coatings are essentially
chromium ( V:L ) - f ree .
The adhesion of the coating of the invention
appears to perforrr~ considerably better than any known
commercial coating. 'This v-s the result of coherent
interfaces between i=he metal substrate, base coating, and
second coat=.ing. A scanning electron photomicrograph cross-
section view of the coating on the metal substrate is shown
in Figure 4. The photomicrograph show that the metal
:LO substrate 50 has an irregular surface at high
magnification, and .--.~ coherent base layer 52 is formed at
the surface of tre substrate 50. The silicon oxide-
containing .Layer 54 vahich is formed on the base layer 52
shows excel___ent integrity, and both coating layers 52 and
54 therefore provide superior corrosion resistant and
abrasion resistant s~.zrface .
Abras~_on ~~esistance was measured according to
AS TM D4060--'a0. Prefc~rabl.y coat~.ngsproduced according to
the inventi~~n having thickness of 0.8 to 1.0 mil will
~:0 withstand at: least !.000 wear cyclesbefore the appearance
of bare metal substrate using a 1.0 kg load on CS-17
abrading wheels. '~9are preferably, the
coating will
withstand at least ?000 wear cycles before the appearance
of the metal substrate, and most preferably,
the coating
will withstand at least 3000 wear ~~ycles using a 1.0 kg
load on CS-:l_7 abradir:.g wheels.
Corrosion resistance was measured according to
ASTM standard methods. Salt fog test, ASTM B117, was
employed as the method for ~or_ros ion resistance testing
30 with ASTM D1654 , procedures A and used in the evaluation
B
of test sarnples. Prf~ferably, as measured according to
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:12 a
procedure E;, c_oati:rm on magnesium alloy AZ91D produced
according to the invention achieve a rating of at least 9
after 24 hours in salt fog. More preferably, 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 acco~=ding to the present process, they may be used
as is, offering very good corrosion resistant properties,
or they may be furth~°r sealed using an optional finish
~L 0
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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
5' decorative properties to the magnesium-containing articles.
Thus, the silicon oxide-containing coating provides an
excellent paint base having excellent corrosion resistance
and offering excellent adhesion under both wet and dry
conditions, for instance, the water immersion 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 coating 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 finish 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.
Examples
The following specific examples, which contain the best
mode, can be used to further illustrate the invention.
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These examples are merely illustrative of the invention and r
do not limit its scope.
r
Example I
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 bifluoride 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 hydroxide. 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 negative 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/fluoride layer was approximately one to two microns
thick. The panels were then taken out of the first
electrochemical bath, rinsed well with water, and placed
into the second electrochemical bath and connected to the
positive lead of a rectifier. The second electrochemical
bath was prepared by mixing together potassium silicate,
potassium fluoride, and potassium hydroxide. The second
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 SiOz) 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
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potassium silicate and about 5 g/L potassium fluoride. A
stainless steel panel served as the cathode 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/cmz. 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.
Table V. Electrochemical Bath #1 (30 L)
Current
Densit~ Time
Example Hydroxide Fluoride pH mA cm min.
II 180 g KOH 450 g KF 12.8 50 2
III 120 g NaOH 310 g NaF 12.7 60 1.5
IV 150 g KOH 500 g KF 12.7 80 2
V 90 g LiOH 500 g KF 12.6 70 1.5
VI 180 g KOH 560 g KF 12.8 80 1
VII 135 g NaOH 250 g LiF 12.8 70 2
VIII 150 g KOH 550 g KF 12.7 80 1.5
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Table VI. Electrochemical Bath #2 (30 L)
Potassium Current
Silicate Densit~ Time
Example Hydroxide Concentrate*Fluoride ~H mA cm min.
)
II 180 g KOH 600 mL 250 g KF 12.8 30 30
III 150 g KOH 700 mL 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 mL 250 g KF 12.6 20 25
VI 150 g KOH 600 mL 200 g NaF 12.7 30 20
VII 180 g KOH 800 mL 350 g KF 12.8 30 30
VIII 140 g NaOH 600 mL 250 g NaF 12.8 40 20
*20~ SiOZ (w/w) in water. In other words, the concentration
can be characterized as the equivalent of 20 wt-$ Si02 in water.
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-17 abrading wheels.
Example IX
A magnesium test panel was coated as in Example I.
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$ Si02, (w/w)) for 5
minutes at 60°C. The panel was rinsed and dried and
subjected to salt fog ASTM B117 testing. The panel
achieved a rating of 10 (ASTM D1654) after 700 hours in the
salt fog.
