Micro-Finish Hard Anodized Coatings on Aluminum

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Micro-Flnish Hard Anodized Coatings on Aluminum
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Materials Fabrication Division
Lawrence Livermore National Laboratory
For AESF SUR/FIN 92
Atlanta, GA
June 22-25,1992
ABSTRACT
The production of thin hard anodized coatings on Single Point Diamond Turned
(SPDT) 6061-T6 aluminum has been studied. The investigation centered on
producing a surface finish of less than 10 microinch after anodizing. By
starting with a 2 microinch (AA) surface finish and controlling time,
temperature, current density and solution chemistry, coatings with surface
finishes of 8 microinch and a thickness of .0003”,are obtained. Surface
roughness from several anodizing solutions is compared. The operational life
of a PTFE sliding seal against a coated cylinder bore is used as verification of
I
‘ h i s work was performed under the auspices of the U. S.Department of Energy by Lawrence Livermore
National Laboratory under contract No. W-7405-Eng-48.
123
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DVTRODUCTION
Aluminum alloys are used extensively for various aerospace assemblies and
components because of a combination of high strength and low density. In
addition i t has the ability to be machined or formed to a wide range of
configurations. Some of the assemblies made from aluminum include
reciprocating pumps used to transfer liquid or gaseous materials.
During pump use, friction occurs from a combination of surface contact and
the forces required t o maintain a seal between piston and cylinder wall. This
results i n piston and/or cylinder wall wear and eventually component failure.
Because aluminum sliding against aluminum has a high coefficient of
frictionl. lubricants and coatings are used t o minimize this wear.
To further reduce friction and improve the seal between piston and cylinder
wall, rings, seals and wipers are used. These may be composed of various
materials that include elastomers, polymers, metals, or intermetallic
compounds such as oxides or carbides. There are several problems associated
with the use of friction reducing seals and coatings. Some applications require
ultra clean operation which prohibit most lubricants. Certain types of seals can
produce compatibility problems, i.e., corrosion o r galling.
One technique for increasing the wear and corrosion resistance of aluminum,
is the application of an inert oxide coating, This surface treatment combines
oxygen and aluminum to form one of the hardest substances known: A l 2 . 0 3 .
This oxide coating has many advantages, including resistance t o heat, erosion,
and moderate chemical solutions2. It also has a very low coefficient of friction.
These anodic coatings are formed by electrolytic oxidation of an aluminum
surface while immersed in a suitable electrolyte. Many formulations have been
used, but the most prevalent are the sulfuric, sulfuric-oxalic, and the oxalic. In
some instances electrical current manipulation is used t o increase the density,
thickness and/or hardness of the coating3. The surface finish of these coatings
depend on the condition of the surface before anodizing and the processing
conditions*. One method of reducing surface finish after anodizing is t o lap or
hone the coating. This has produced finishes down t o a less than 2 microinch.
An aluminum pistmi conventionally machined to a finish of 32 microinch was
fitted with PTFE sealing ring@. This piston was then placed into a matching
aluminum cylinder, as part of a reciprocating pump. The assembly was then
leak checked using a vacuum pump equipped with a helium mass
spectrometer. The leak rate was initially excessive (>80 Vmin) and grew
successively worse with continued piston travel.
2
I 24
To reduce the leak rate, a smoother finish was machined into the internal
cylinder bore using a lathe fitted with a diamond tool. This reduced the surface
finish to 1-2 microinch, which was well below the seal manufacturers
recommendations. A pump assembly was then tested. The results showed an
acceptable initial leak rate, but after one piston pass leakage became excessive.
At this point the surface of the cylinder was visually inspected and was found to
have numerous scratches. The leading edge of the PTFE seal was also
inspected and found to contain minute particles of aluminum. The source of
the particles was thought to be a residue from the machining process. A new
cylinder was machined and then cleaned using an ultrasonic tank and
aqueous cleaning agents. The piston was fitted with new seals and installed in
the cylinder. Leak testing this cleaned assembly still showed excessive leak
rates after one piston cycle.
An microscopic evaluation of the diamond turned surface showed scratches
from contact with PTFE sealing surface. These scratches, running horizontally
down the cylinder bore, provided leak paths. To make the surface less
susceptible to damage from the sealing surface, a coating of some type was
envisioned.
The first choice was to coat the internal surfaces with electroless nickel and
then SPDT. This has provided excellent surface finishes on a variety of
components6. After zincating, copper striking, and deposition of .002" of
electroless nickel, the can was SPDT. Leak test results looked excellent (<8
Umin).
At this time it was discovered that copper of any kind was incompatible with the
material that would be used in the pump. Without a copper strike, the only
alternative to provide adequate adhesion on this alloy was heat treatment'. The
heat treatment step could not be done because the 6061-T6 aluminum cylinder
had to remain in the T6 condition, to retain its mechanical properties.
The next choice was t o coat the internal surfaces of the cylinder by hard
anodizing. The first coating was applied t o a rough machined cylinder using a
chilled (32°F)sulfuric acid solution (15%weight) to a thickness of .002". This
increased the 32 microinch finish to 82.
Since this finish was too rough, a hand lapping operation followed. The time
required to decrease the surface finish in the cylinder bore t o an acceptable level
was over 4 hours. The remaining lapping compound and alumina particles
were removed by ultrasonic cleaning. It wzs decided t o cliscontinue this
approach because of the time required to produce the required finish. Another
WOW was the lapping residue, which if not completely removed, could clog the
small pump delivery lines during operation.
3
In an effort to reduce the surface roughness caused by the hard anodizing,
several test specimens were anodized for various lengths of time. As expected,
Figure 1 shows that shorter anodizing time produces much less surface
roughness and of course a thinner coating. The project designers felt that the
coating thickness should be no less than .0003”, which should withstand
moderate contact w e e . This thickness was obtained after 10 minutes a t 36
ASF. This produced a surface finish of about 21 microinch, twice the seal
manufacturers recommended value. In an attempt t o reduce this finish, other
chemistries were examined.
Because of the myriad of solutions that will electrolytically form oxide coatings
on aluminum, the next most widely used chemistry was chosen for
investigation. This is a sulfuridoxalic process that was developed
commerciallp. Several test specimens were hard coated in this bath for various
times. The surface finish results are shown in Figure 2. This process increased
the surface roughness from 2 to 14 microinch in 10 minutes.
With these results a new cylinder was prepared and run for 10 minutes. The
internal surface finish increased t o 15 microinch after anodizing. After
assembly the piston was able to make several passes before the leak rate became
excessive. Because of the success of this chemistry change, another process,
that had originated in Germany and Japan, was tried. This solution contains
(45 gm/l) oxalic acid and is operated a t 60°F and 25 ASF.
From this formula a new solution was prepared and test samples were
processed. Several differences were noted including higher starting voltage (45
volts vs. 20 volts) and a coating color of translucent yellow instead of grey. The
results (Figure 3) show that this produced a 12 microinch finish in 14 minutes.
With this data, a cylinder was SPDT and then anodized for 14 minutes. This
produced a coating that averaged .0002” thick which had a surface finish that
varied from 8 to 18 microinch. This variation was due to the coating being
thicker a t the open end of the cylinder.
The cathode up to this point had been a 1”diameter aluminum tube, which was
also the source of air agitation. To try and remedy the thickness disparity, a
“conical” cathode was fabricated. The conical shape was to increase the current
density where the coating was thin, and reduce the surface finish where it was
thick. After anodizing the surface was inspected and the conical cathode had
produced an elliptical coating. A new parallel cathode was fabricated allowing
only 1/2” of clearance between electrode surfaces. The results of using this
“parallel” cathode provided a uniform coating.
Variations in the surface finish of the cylinder were noted after the use of the 1”
cathode. These were attributed to current density variations. To find the
optimal current density, samples were run at current densities varying from 40
t o 10 ASF. To minimize any thickness disparity all samples were run for 360
amp min/ft2. The results, shown in Figure 4, show an optimal anode current
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density of 18 ASF. The .0003" thickness is reached in 18 minutes a t this current
density.
Solution temperature is known td have an effect on the porosity of anodized
coatingslo. To determine its impact, another set of samples was coated using
various solution temperatures ranging from 70 to 30"F, with a constant 360
amp min/ft2. The results in Figure 5, show a decrease in surface finish as the
solution is cooled to 32°F. The voltage required t o maintain a constant current
density also increased with reduced temperature. This and the fact that the
coating is less soluble at reduced temperature is most likely why the surface
finish is reduced".
The three anodizing methods are compared in Figure 6. The large reduction in
surface finish is easily seen on this chart. When the coating thickness is
factored in, the decrease in roughness between the suluric and the oxalic is
about 65%, and between the sulfuridoxalic and the oxalic 46%.
With these results a SPDT cylinder was coated using 45 gm/l oxalic acid
solution cooled to 32°F.The anode current density of 18 ASF with an anode to
cathode distance of U2". Air agitation was supplied to circulate the solution and
prevent localized overheating. After processing for 20 minutes the coating was
found to have a surface finish of 8 microinch. The thickness was .0003".
This cylinder was then assembled with a matching piston and subjected to the
cyclic pump test. The leak rate was very low and the pump cycled over 70 cycles
before any seal degradation. Most of the degradation was from flattening of the
seal face as opposed to scratching.
Microphotographs of the anodized surface, showed a visible reduction in the
surface roughness. This is attributed to the reduced solvent action of the cooled
oxalic process. To determine coating density, several coated samples were
weighed, stripped and reweighed. This mass, divided by the volume of the oxide
was used to determine density. A value of 3.2 gm/cms, is greater than that
obtained from a sulfuric acid solution12.
5
i
Thin oxide coatings on SPDT aluminum surfaces are produced in oxalic acid.
When produced on a SPDT surface, the finish obtained is much smoother that
from a sulfuric acid or sulfuridoxalic solution. The sliding life of a PTFE
sealing ring inside a coated cylinder can be greatly enhanced by this coating.
The optimal anodizing conditions are:
gmfl oxalic acid
18 ASF anode current density
Y2" anode to cathode distance (minimum cathode current density)
32 degree F. solution temperature
20 minute run time
45
\
6
128
90
-
50
-
30
-
20
-
10
-
0
-
40
I
./
I
5
0
10
15
20
25
30
35
40
45
Minutes
Fiaurs 2
Surface Flnish vs Anodizing Time
(12 % sulfuric Acid
+2
% Oxalic Acid- 36 ASF)
100 1
90
EO
70
-5
z
-ev
2
s
60
50
40
30
20
io
0
0
5
10
15
25
20
Minutes
7
30
35
40
45
/
Surface Finish vs Anodizing Tim.
(4.5% Oxalic Acid- 25 ASF-60 deg.
F.)
100 -,
90 -.
-
80
.-0
L
2
I
40
I
0
5
15
10
20
25
30
35
40
45
Minutes
Surface Finish vs Anode Current Density
(4.5 % Oxalic Acid-360 AmpMin.)
30
25
-
/
20
Q
J
5
0
,
5
10
15
I
1
20
25
ASF
8
130
i
30
35
40
45
EinuLs
Surface 'finish vs Solution Temperature
(4.5 % oxalic Acid25 ASF)
25
-
20
J
r
-2
-t
C
15
0
10
5
25
30
35
40
45
I
1
50
55
Degrees
60
F.
EkuhLfi
Surface Finish vs Anodizing Method
(360 Ampere Minutes)
Microinch
Minute8
2
9
65
70
E
75
References
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1 Handbook of Tables for Applied Engineering Science, 2nd Ed., R.E. Bolz and
G.L. Tuve, Ed., CRC Press, 1987.
2 Electroplating Engineering Handbook,3rdEdition, A.K.Graham Editor, (461464).
3 Sanford Process Co., US.Pat. 4,128,461 (1978),4,133,725 (1979).
4 Aluminum Company of America, Bulletin 14-A.
5 Ball Seal Company.
J.W. Dini, "Electroless Nickel: An Important Coating for Diamond Turning
Applications". Proceedings Electroless Nickel Conference II, Products
Finishing Magazine, Cincinnati, OH.
7 J.W. Dini, H.R. Johnson, "Quantitative Adhesion Data for Electroless
Nickel Deposited on Various Substrates" UCRL Pub.
9 ANSVASTM B 580
- 73, Anodic oxide coatings on aluminum.
9 ALUMALITE 225, Aluminum Company of America.
10 Miyata, A., Isawa, K., Furuichi, A. and Takamura, K., Rept. Inst. Phys.
Chen. Res., Tokyo, 1961,37, 142-48.
11 G. Bailey, G.C. Wood, "The Morphology of Anodic Oxide Films Formed on
Aluminum in Oxalic Acid.", Truns. Inst. Met. Fin., Vol. 52, (1974).
12 Spooner, R.C., "Anodic Treatment of Aluminum in Sulfuric Acid
Solutions"., J. Electrochemical SOC.,102, (4), 156-62, (1955).
10
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