[0001] This invention was made with Government support under contract number W911W6-12-2-0003
awarded by the United States Army. The Government has certain rights in the invention.
BACKGROUND
[0002] Metal plating such as cadmium plating has been widely used on various materials,
including but not limited to high-strength steel, aircraft components, fasteners,
electrical connectors, and numerous others. Metal plating can be used to promote properties
such as corrosion resistance, lubricity, and electrical conductivity. Cadmium has
been used as a metal plating material for various reasons such as the cost of plating
and its anti-galling performance (no adhesive wear on threaded surfaces). However,
continued commercial uses of cadmium have been facing pressure due to health concerns
in recent years, including being listed as a substance of very high concern (SVHC)
in 2012 by the European Union environment & safety regulatory agency REACH.
[0003] Various metal plating technologies have been developed and evaluated over the years.
For example, aluminum has been proposed as a metal plating material, and has been
approved for corrosion protection of high-strength structural steels in aircraft as
set forth in specifications such as MIL-DTL-83488. However, various technical issues
continue to present challenges. Commercially available aluminum metal plating processes
utilize toxic or environmentally unfriendly organic solvents and pyrophoric alkylaluminum
compounds that require extra measures for operation under inert conditions. Therefore,
there continues to be a need for further developments regarding aluminum metal coatings.
[0004] High strength steels (HSS) are used throughout aerospace industries for components
such as aircraft landing gear, bolts, aircraft tail hooks on aircraft carriers etc.
High strength steels are protected by sacrificial coatings such as electroplated cadmium,
zinc-nickel and non-electrolytic aluminum-based SermeTel (Praxair Surface Technologies)
coatings. It has been known that HSS are prone to hydrogen embrittlement representing
brittle mechanical failures under stress that is manifested by brittle intergranular
fracture and trans-granular cleavage. Incorporation of hydrogen during either the
electroplating process or the corrosion during service promotes the increased risk
of failure arising from hydrogen embrittlement. Porous electroplated coatings can
allow hydrogen to be released from the steel substrates via a post-plating baking
process. However, substrates coated with porous coatings can become susceptible to
re-embrittlement due to galvanic cells formed on exposed area. Dense coatings of lower
porosity can mitigate the risk of hydrogen re-embrittlement; however, dense coatings
can prevent hydrogen from being released. Hydrogen embrittlement can also be caused
by pre-treatment processes for of HSS, such as treatment with acids or electrolytic
cathodic cleaning which can generate hydrogen.
BRIEF DESCRIPTION
[0005] According to some embodiments, a method of coating a metal substrate comprises disposing
the metal substrate as an anode in an electrolytic circuit comprising an aqueous alkaline
liquid electrolyte. An electrical voltage differential is applied to the metal substrate
and a cathode. The treated metal substrate is disposed as a cathode in an electrolytic
circuit comprising a liquid electrolyte that comprises an ionic liquid and an aluminum
salt. An electrical voltage differential is applied to the metal substrate and an
anode to form a layer comprising aluminum on the metal substrate.
[0006] According to some embodiments, an article comprises a metal substrate and a layer
thereon comprising aluminum. The layer comprising aluminum has a volumetric density
greater than 99%, and the coated article meets or exceeds hydrogen embrittlement test
requirements of ASTM F519-10.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Subject matter of this disclosure is particularly pointed out and distinctly claimed
in the claims at the conclusion of the specification. The foregoing and other features,
and advantages of the present disclosure are apparent from the following detailed
description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic depiction of a cross-sectional view of a coated article as described
herein;
FIG. 2 is a schematic depiction of an example embodiment of a method as described
herein; and
FIG. 3 is a schematic depiction of another example embodiment of a method as described
herein.
DETAILED DESCRIPTION
[0008] With reference now to the Figures, FIG. 1 schematically depicts a cross-sectional
view of a coated article 10 comprising a substrate 12 having thereon a surface layer
14 comprising aluminum. The substrate 12 can comprise high strength steels that are
particularly prone to hydrogen embrittlement due to lack of corrosion resistance and
facile hydrogen diffusion in the steel lattice. Further, the substrate 12 comprises
a metal or metal alloy having a first oxidation-reduction equilibrium (corrosion)
potential more noble than aluminum and aluminum alloys. This difference in potential
can allow the layer to provide sacrificial corrosion protection in an electrolyte
that is experienced in the service environment where the parts are used. In some embodiments,
the metal or metal alloy of substrate 12 has an electrode potential of -1.0 V to 0
V with respect to saturated calomel electrode (SCE) in 3.5% wt. sodium chloride solution
or sea water solution or synthetic sea water solution, more specifically -0.6 to -0.1,
and even more specifically from -0.4 V to -0.2 V. Examples of metals or metal alloys
for the substrate 12 include but are not limited to high-strength steels (e.g., D6AC,
300M, M-50, AERMET 100, 4330, 4340, 52100). In some embodiments, the substrate 12
comprises a high strength low alloy (HSLA) steel as that term is defined by SAE (Society
of Automotive Engineers) standards.
[0009] The thickness of layer 14 can be specified to meet target specifications. In some
embodiments, the layer 14 has a thickness in a range having a low end of 2.5 µm (0.0001
inches), more specifically 7.5 µm (0.0003 inches), and even more specifically 12.5
µm (0.0005 inches), and an upper end of 17.5 µm (0.0007 inches), more specifically
25 µm (0.001 inches), and even more specifically 250 µm (0.010 inches). The above
range endpoints can be independently combined to serve as a disclosure of a number
of different ranges, which are hereby expressly disclosed. In some embodiments, the
layer 14 can be utilized to promote resistance to corrosion. In some embodiments,
the layer 14 can be utilized to promote resistance to galling along contact portions
of the article. In some embodiments, the layer 14 can be disposed on surface to be
subjected to sliding contact with another article or component. Such articles can
include, but are not limited to, threaded fasteners, press-fit connections, propeller
barrels, electrical connectors, press-fit high strength steel bolts used in turboprop
propellers, and other various fasteners or connectors.
[0010] In some example embodiments, articles such as shown in FIG. 1 can be prepared by
methods shown in FIGS. 2 and 3. In some embodiments, the surface of the substrate
12 can optionally be subjected to physical surface preparation such as degreasing,
grit blasting, or both degreasing and grit blasting as shown in FIGS. 2 and 3. Examples
of degreasing agents include organic solvents such as acetone or methyl ethyl ketone,
alkaline degreasers such as aqueous solutions of sodium hydroxide, potassium hydroxide,
or ammonia, or aqueous surfactants with degreasing capability. After degreasing, the
substrate can optionally be dried such as by radiant heat or blowing with air. Grit
blasting can be performed by directing abrasive particles at the substrate at velocities
of 50 to 100 in/min. Examples of materials for grit blasting include alumina grit,
silicon carbide grit, steel grit, or glass particles. The particles for grit blasting
can have particle sizes expressed as mean diameter and distribution as described in
ANSI B74.12-2001.
[0011] As further shown in FIGS. 2 and 3, after optional degreasing and grit blasting, the
substrate is subjected to anodic alkaline electrolytic treatment. This treatment involves
disposing the metal substrate as an anode in an electrolytic circuit comprising an
aqueous alkaline liquid electrolyte, and applying an electrical voltage differential
between the metal substrate anode and a cathode. Conditions for anodic alkaline electrolytic
treatment can be controlled to promote formation of oxygen at the metal substrate
surface according to the reaction 40H
- → 2H
2O + O
2 and to minimize formation of metal oxide at the metal surface substrate. In some
embodiments, the formation of oxygen at the metal substrate surface can promote cleaning
of the metal substrate surface. The alkaline electrolyte can comprise aqueous solutions
of alkaline compounds such as sodium hydroxide, potassium hydroxide, and their weak
acid salts such as Na
2CO3, Na
3PO
4, Na
2SiO
4 at concentrations in a range having a lower end of 15 wt.%, 10 wt.%, or 5 wt.%, and
an upper end of 50 wt.%, 75 wt.%, or 100 wt.%. Commercial alkaline cleaning products
such as Henkel Turco 4181 can be used as well. The above range endpoints can be independently
combined to serve as a disclosure of a number of different ranges, each of which is
hereby expressly disclosed. The alkaline electrolyte can have a pH in a range having
a lower end of 9, 8, or 7.5, and an upper end of 14. The above range endpoints can
be independently combined to serve as a disclosure of a number of different ranges,
each of which is hereby expressly disclosed. The alkaline electrolyte can be at a
temperature in a range having a lower end of 25°C and an upper end of 50°C. In some
embodiments, the voltage differential applied between the cathode and the metal substrate
anode can provide an electrical current density (based on anode surface area) between
the cathode and anode in a range having a lower end of 5 mA/cm
2, 10 mA/cm
2, or 15 mA/cm
2, and an upper end of 25 mA/cm
2, 50 mA/cm
2, or 75 mA/cm
2, and can be applied for a duration in a range having a lower end of 15 seconds, 30
seconds, or 60 seconds, and an upper end of 120 seconds, 180 seconds, or 300 seconds.
The above range endpoints can be independently combined to serve as a disclosure of
a number of different ranges, each of which is hereby expressly disclosed. In some
embodiments, the current can be pulsed or varied. Other conditions (e.g., temperature)
can also be varied during anodic alkaline electrolytic treatment. After anodic alkaline
electrolytic treatment, the metal substrate can be rinsed with deionized water and
dried in nitrogen or air.
[0012] With reference now to FIG. 3, the anodic alkaline electrolytically-treated metal
substrate can be further subjected to an anodic ionic liquid electrolytic treatment.
This treatment involves disposing the metal substrate as an anode in an electrolytic
circuit comprising a liquid electrolyte that comprises an ionic liquid, and applying
an electrical voltage differential between the metal substrate anode and a cathode
to induce current. In some embodiments, this treatment can etch the surface of the
metal substrate, which can promote release of metal oxide from the surface. The ionic
liquid electrolyte can be at a temperature in a range having a lower end of 25°C,
40°C, or 55°C, and an upper end of 60°C, 70°C, or 80°C. The above range endpoints
can be independently combined to serve as a disclosure of a number of different ranges,
each of which is hereby expressly disclosed. In some embodiments, the voltage differential
applied between the cathode and the metal substrate anode can be set to produce a
current density (based on anode surface area) in a range having a lower end of 5 mA/cm
2, 10 mA/cm
2, or 15 mA/cm
2, and an upper end of 20 mA/cm
2, 30 mA/cm
2, or 40 mA/cm
2, and can be applied for a duration in a range having a lower end of 15 seconds, 30
seconds, or 45 seconds, and an upper end of 60 seconds, 120 seconds, or 300 seconds.
The above range endpoints can be independently combined to serve as a disclosure of
a number of different ranges, each of which is hereby expressly disclosed. In some
embodiments, the voltage can be pulsed or varied. Other conditions (e.g., temperature)
can also be varied during anodic ionic liquid electrolytic treatment. After anodic
ionic liquid electrolytic treatment, the metal substrate can be rinsed (e.g., with
methanol, ethanol, deionized water, or a combination of rinsing agents) and dried
(e.g., with heat or blown air), or can remain in the ionic liquid electrolyte for
further processing.
[0013] As further shown in FIG. 3, the anodic ionic liquid electrolytically-treated metal
substrate can be subjected to sonicating treatment in the same or a different electrolyte
bath. In some embodiments, anodic etching treatment of steel substrates in ionic liquids
can produce smut on the substrate, which can interfere with the subsequent aluminum
plating process, leading to poor coating quality. Sonicating the parts can be conducted
in ionic liquids or in water-free organic solvents such as ethanol, toluene, acetone,
etc. In some embodiments, sonic treatment can promote the removal of smut or other
substances that may have been formed on the surface by the treatment process or are
otherwise disposed on the surface. In some embodiments, sonicating treatment can involve
exposure of the metal substrate to sonic energy in a frequency range having a lower
end of 10 kHz and an upper end of 200 kHz. In some embodiments, the sonic treatment
can be applied for a duration in a range having a lower end of 30 seconds and an upper
end of 300 seconds. In some embodiments, the sonic energy can be pulsed or varied.
[0014] As further shown in FIGS. 2 and 3, a layer comprising aluminum can be deposited onto
the treated metal substrate by disposing the metal substrate as a cathode in an electrolytic
circuit comprising a liquid electrolyte that comprises an ionic liquid and an aluminum
salt, and applying an electrical voltage differential between the metal substrate
and an anode. As used herein, the term "ionic liquid" means a salt having a melting
point below the processing temperature (e.g., below 100°C). In some embodiments, the
ionic liquid is non-volatile or of low volatility at the process temperature. Cations
for the ionic liquid used as electrolyte for cathodic electrodeposition of aluminum
shown in FIGS. 2 and 3, as well as for the anodic electrolytic treatment shown in
FIG. 3, can include, but are not limited to imidazolium (e.g., 1-ethyl-3-methylimidazolium,
1-ethyl-2,3-dimethylimidazolium, 1-butyl-3-methylimidazolium ("BMI"), 1-hexyl-3-methyl-imidazolium
("HMI"), pyridinium (e.g., N-methylpyridinium), tetraalkylammonium, pyrrolidinium
(e.g., 1-butyl-1-methyl-pyrrolidinium ("BMPyr"), trialkylsulfonium (e.g., triethylsulfonium),
pyrazolium, triazolium, thiazolium, oxazolium, pyridazinium, pyrimidinium, pyrazinium.
Exemplary anions for ionic liquids used in the embodiments described herein include,
but are not limited to, chloroaluminate (Al
2Cl
7- ), tetrafluoroborate (BF
4), hexafluorophosphate (PF
6), trifluoromethanesulfonate (CF
3SO
3), bis(trifluoromethylsulfonyl)imide, trifluoroethanoate, nitrate, SCN, HSO
4, HCO
3, CH
3SO
3, CH
3CH
2SO
4, (CH
3(CH
2)
3O)
2POO, (CF
3SO
2)
2N, dicyanamide, (CF
3CF
2SO
2)
2N, L-(+)-lactate, CH
3SO
4, and CH
3COO, and the like. In some embodiments, the ionic liquid has a cation that is an imidazolium,
and more specifically the ionic liquid has the formula:
wherein, R and R
1 are independently selected from H, an unsubstituted or substituted alkyl group having
1 to 30 carbon atoms, or an unsubstituted or substituted aryl group having 6 to 30
carbon atoms. X is an anionic group, as described hereinabove, that associates with
imidazolium to form an ionic-liquid cation/anion pair.
[0015] In addition to the cation (or mixtures of cations) and anion (or mixtures of anions)
of the ionic liquid, the liquid electrodeposition composition for depositing the layer
comprising aluminum also comprises an aluminum salt. Aluminum salt can be introduced
to the composition in the form of aluminum chloride (AlCl
3), but will tend to form different aluminum-containing ions in the ionic liquid composition,
including but not limited to AlCl
4- (tetrachloroaluminate) or Al
2Cl
7- (heptachlorodialuminate). Aluminum-containing anions can also be introduced electrolytically
by electrochemical reaction of metallic aluminum in the anode(s) of an electrochemical
cell of which the electrodeposition forms a part. The electrodeposition composition
can also include additives to improve the integrity of the aluminum coating such as
a nucleation aid like a surfactant. Other additives are known in the art, see, e.g.,
US 2012/0006688 A1, and can be included as well. Organic solvents can also be present in amounts up
to 30 wt. %, such as toluene, chlorobenzene, dichlorobenzenes, xylene, cyclohexane,
heptane, and others.
[0016] In some embodiments, process conditions for deposition of the layer comprising aluminum
can be managed to promote targeted results such as density of the aluminum layer as
described in more detail below. In practice, an electrical current is provided by
a power source that is sufficient to provide an electric current density (current
per effective electrode area) of at least 5 mA/cm
2, more specifically of at least 10 mA/m
2, even more specifically at least 20mA/cm
2, and even more specifically at least 30 mA/cm
2. Current is applied until the desired aluminum coating layer thickness is achieved
(e.g., 5 µm to 50 µm). The electrodeposition method can be carried out at temperatures
ranging from 20°C. to 100° C., more specifically from 20° C. to 80° C., and even more
specifically from 60° C. to 70° C. The above range endpoints can be independently
combined to serve as a disclosure of a number of different ranges, each of which is
hereby expressly disclosed.
[0017] In some embodiments, the above-described methods can avoid conditions that can contribute
to hydrogen embrittlement of the substrate 12. For example, the above-described electrolytic
treatment of the substrate 12 as an anode in aqueous alkaline electrolyte can evolve
oxygen at the surface of the metal substrate that can promote cleaning of the surface
before electrolytic deposition of aluminum in an ionic liquid electrolyte. Cathodic
electrocleaning in an acidic electrolyte, on the other hand, would evolve hydrogen
at the surface of the metal substrate that could contribute to the promotion of hydrogen
embrittlement of the substrate 12. Hydrogen embrittlement of the substrate 12 can
be managed by post-fabrication heat treatment to drive hydrogen out of the substrate,
but the effectiveness of such post-fabrication heat treatment can be limited in the
case of dense aluminum overlayers, which can inhibit removal of hydrogen from the
substrate. In some embodiments, the above-described methods can avoid conditions that
contribute to hydrogen embrittlement, thereby allowing for deposition of higher density
aluminum layers (i.e., volumetric density greater than 99%). Unlike cadmium and zinc-nickel
plating on high strength steels which would require a baking process at an elevated
temperature (ca. 200 °C) to liberate hydrogen absorbed in the metal lattice, the methods
described herein don't require baking after the coating is applied to the substrate.
As mentioned above, in some embodiments, the coated article meets or exceeds requirements
of the hydrogen embrittlement test as prescribed in ASTM F519-10. As used herein,
the coated article meets or exceeds these requirements if the article passes the sustained
load test of ASTM F519-10 of greater than 200 hours at 75% of the notch fracture strength.
[0018] In some embodiments, the layer 14 can be treated with a trivalent chromium passivation
process. Such a process can be carried out by treatment of the layered substrate (e.g.,
by dipping or application with a brush, sponge, spray, or other coating applicator)
with an aqueous solution or non-aqueous solution comprising trivalent chromium and
various anions. Exemplary anions include nitrate, sulfate, phosphate, and/or acetate.
Specific exemplary trivalent chromium salts can include Cr
2(SO
4)
3, (NH)
4Cr(SO
4)
2, KCr(SO
4)
2, CrF
3 Cr(NO3)
3, and mixtures comprising any of the foregoing. Embodiments of compositions and the
application thereof to substrates are described in
US Patent Nos. 5,304,257,
5,374,347,
6,375,726,
6,511,532,
6,521,029, and
6,511,532. Various additives and other materials can be included in the composition comprising
trivalent chromium as disclosed in the patent literature, and the trivalent chromium
salt composition can be selected from any of a number of known commercially-available
compositions.
[0019] While the present disclosure has been described in detail in connection with only
a limited number of embodiments, it should be readily understood that the present
disclosure is not limited to such disclosed embodiments. Rather, the present disclosure
can be modified to incorporate any number of variations, alterations, substitutions
or equivalent arrangements not heretofore described, but which are commensurate with
the scope of the present invention as defined by the claims. Additionally, while various
embodiments of the present disclosure have been described, it is to be understood
that aspects of the present disclosure may include only some of the described embodiments.
Accordingly, the present disclosure is not to be seen as limited by the foregoing
description, but is only limited by the scope of the appended claims.
1. A method of coating a metal substrate, comprising
disposing the metal substrate (12) as an anode in an electrolytic circuit comprising
an aqueous alkaline liquid electrolyte, and applying an electrical voltage differential
to the metal substrate and a cathode; and
disposing the metal substrate as a cathode in an electrolytic circuit comprising a
liquid electrolyte that comprises an ionic liquid and an aluminum salt, and applying
an electrical voltage differential to the metal substrate and an anode to form a layer
(14) comprising aluminum on the metal substrate.
2. The method of claim 1, further comprising degreasing the metal substrate prior to
disposing in the aqueous alkaline liquid electrolyte.
3. The method of claims 1 or 2, further comprising grit blasting the metal substrate
prior to disposing in the aqueous alkaline liquid electrolyte.
4. The method of any of claims 1-3, further comprising subjecting the metal substrate
to ultrasonic cleaning prior to forming the layer comprising aluminum on the metal
substrate.
5. The method of any of claims 1-4, wherein the formation of the layer comprising aluminum
on the metal substrate is performed under conditions to produce the layer comprising
aluminum, at a volumetric density greater than 99%.
6. The method of any of claims 1-5, wherein the metal substrate comprises steel.
7. The method of any of claims 1-6, wherein the electrolytic circuit comprising a liquid
electrolyte that comprises an ionic liquid and an aluminum salt includes an aluminum
anode.
8. The method of any of claims 1-7, further comprising, prior to forming the layer comprising
aluminum on the metal substrate, disposing the metal substrate as an anode in an electrolytic
circuit comprising a liquid electrolyte that comprises an ionic liquid, and applying
an electrical voltage differential to the metal substrate and a cathode.
9. An article, comprising:
a metal substrate (12); and
a layer (14) comprising aluminum over the metal substrate, at a volumetric density
greater than 99%, wherein said article meets or exceeds requirements hydrogen embrittlement
test requirements of ASTM F519-10.
10. The article of claim 9, wherein the metal substrate comprises steel.
11. The article of claim 10, wherein the metal substrate comprises high strength low alloy
steel.