Hard Anodic Coatings on Aluminum Alloys: Evaluation ... - infoHouse

<strong>Hard</strong> <strong>Anodic</strong> <strong>Coatings</strong> on Aluminum Alloys
Evaluation and Control of Porosity t.Df
by Leah Markowitz
HR Textron, Los Angeles
listered hard anodic coating has
B been a recurring processing problem
with aluminum hydraulic cylinders
and pistons. The blister's sharp
edges result in excessive seal wear
causing leakage and degrading the
hydraulic actuator's performance. The
pitlike blisters can always be traced to
either the preanodizing surface preparation
or the anodizing process itself.
This study determined the effect of
the following variables on the resultant
hard anodized coating quality:
Treatments introducing compressive
stresses into the surface to be ano-
dized (shot peening and roller bur-
nishing)
Coating thickness (in the 0.010 to
0.035 in. range)
F'rehard anodizing cleaning methods
during the surface finishing stage
Evaluation was conducted based on
the following: Taber abrasion resis-
tance test, microscopic visual evalua-
tion of hard anodized cylinders' cross-
sections, and quantitative image analy-
sis of the latter. Results indicate that
susceptibility to voids (blisters) in the
hard anodic coating increases with the
coating thickness. Abrasion resistance
deteriorates with increased blister con-
tent. Shot peening has no adverse
effect on the hard anodic coating
quality as compared with roller bur-
nishing. provided all shot residue is
removed.
Etching prior to hard anodizing
should be avoided. Mechanical clean-
ing and chromated deoxidizers were
found to be the best preanodizing
cleaning practices.
<strong>Hard</strong> anodizing is a common surface
finish to provide wear-resistant coat-
ings on hydraulic actuator intemal
diameters (ID). Pitlike defects (referred
to as voids or blisters) may show on the
as-anodized cylinder bore (see Fig. 1)
or may be exposed by the postano-
dizing honing operation. The blister
sharp edges may cause excessive wear
of the seals, resulting in unacceptable
DECEMBER 1992
Fig. 1. Pitfed hard anodic coating (0.0025 in.
thick on 7075-T73 aluminum CylindersJ
leakage and degraded actuator per-
formance.
Stripping and reapplying hard ano-
dizing involves a need to increase the
coating thickness, due to the penetrat-
ing nature of the coating.' The coat-
ing's reapplication may further pro-
mote porosity/blistering.2.3 Close toler-
ance areas, such as threads or tapped
holes, may not be stripped and reano-
dized. In view of the high cost involved
when hard anodic coatings blister,
attempts have been made to eliminate
the phenomenon. Different coating
thicknesses were tried, utilizing vari-
ous preanodizing cleaning methods.
The effects of coating thickness and
mechanically introduced compressive
stresses (by shot peeninglroller bur-
nishing) were evaluated as well.
POSSIBLE CAUSES OF
POROSITY IN HARD ANODIC
COATINGS
Analyses of blistered hard anodized
parts at HR Textron could always trace
the defective coating to either preano-
dizing manufacturing operations per-
taining to surface conditions or to the
hard anodize process variables.
Manufacturing operations may con-
taminate the surface to be anodized
with either nonaluminum contami-
nants or with aluminum corrosive
products that may result in a nonuni-
form anodizing rate and a high local
current density leading to pin hole
RFK64
2-pPLp
(void) formation at the contaminated
site.* Possible surface contaminating
sources identified at HR as well as
other aerospace manufacturers 4-'2 in-
clude (1) contaminated nonfiltered air
used for blowing the part surfaces, (2)
nondedicated production tools for alu-
minum parts, (3) shot-peening residue
not removed by sufficient subsequent
etching, (4) moisture and/or chemical
residue from the penetrant inspection
stage, (5) contaminants introduced
through the final surface- honing oper-
ation, (materials and handling) and (6)
improper oil protection of the honed
part to be anodized.
The anodizing process itself may
promote blisters in the hard anodic
coating through the cleaning, etching,
anodizing or handling stage.
CLEANING STAGE
Regular water-based soap solutions
are common for cleaning aluminum
parts prior to anodizing. Operating at a
temperature range of 130 to 190"F,
regular tap water needs to he added
constantly to compensate for evapora-
tion and to maintain the solution level
in the tank. Increasing chloride Level is
the indirect result of such additions,
which eventually may cause pitting of
the cleaned aluminum parts. Alumi-
num forgings are especially suscepti-
ble to this kind of attack in view of
iigh grain boundary area exposed to
:he outer surface.13 These pits will
2ecome enlarged during anodizing and
t blistered anodic coating may result.
ETCHING (DEOXIDIZING) STAGE
2xxx and 7xxx series aluminum
alloys contain intermetallic com-
pounds (precipitates), which account
for each alloy's mechanical properties.
Exposed to either caustic etch or a
deoxidizer containing active acids such
as hydrochloric or hydrofluoric acid
(e&, nonchromated deoxidizers), in-
termetallic compounds would etch at a
different rate than the aluminum ma-
trix, creating surface protrusions or
19

Table 1. Processing Sequence for Test Cylinders
Test Cylinder
Pmcesing Group (I) Group (I/)
Machining 16 microinch finish 16 microinch finish
Compressively Roller burnishing
stressed by:
Hone ID to
8 microinch finish
Oil Using water displacement oil
Penetrant inspect Yes
Shot peening 170-230 cast iron shot,
0.008 to 0.011 Ahen intensity
Etch inner diameter to remove shot residue
per MIL-S-13165
8 microinch finish
Using water displacement oil
Anodize Type I chromic (MIL-A-8625) on OD;
Type I chromic (MIL-A-8625) on OD;
Type 111 (hard anodize) on ID
Type 111 (hard anodize) on ID
Coating Thickness: Coaling Thickness:
0,001 to 0.002 in. or 0.0025 to 0.0035 in.
0.001 io 0.002 in. or 0.0025 io 0.0035 in.
Preanodizing Cleaning: Preanodizing Cleaning:
Mechanical, chromated deoxidizer or
Mechanical, chromated deoxidizer or
nonchromated deoxidizer. nonchromated deoxidizer.
00 -0utsae Diameter: ID - inside Diameter
pits. These surface imperfections
would result in voids during the subse-
quent hard anodizing.
ANODlZING STAGE
Anodizing conditions, which gener-
ate local electrical heating, would
promote rapid dissolution and creation
of void^.^,^^.'^ High sulfuric acid con-
centration, high anodizing temperature
(more than 4OoF), poor agitation, and
nongradual increase of current density
from zero to operating range are exam-
ples of such undesirable anodizing
conditions.
HANDLING
Parts undergoing two different ano-
dizing processes on their outer and
inner diameter go through masking
operations. Poor planning may result in
an unnecessarily long exposure of
moiled aluminum surfaces to the ele-
ments. A typical finish of less than 8
microinches makes it highly suscepti-
ble to corrosion attack.'6 Corrosion
products (pits) will result in voids
during hard anodizing.
HR Textron has addressed the afore-
mentioned manufacturing operations
to prevent contamination of the surface
to be anodized. Yet at times blistered
hard anodic coatings were obtained.
This study looked at three variables
which seemed to affect the defective
coating occurrence:
Preanodizing cleaning methodr.
Differences in hard anodize quality for
20
Yes
the same pan@) were encountered
when hard anodizing was performed at
different anodizing facilities. While all
shops observed the hard anodizing
process variables in such a manner as
to minimize or prevent local electric
heating and did not use any caustic
etch, different cleaning methods were
applied to the aluminum parts. This
study compared mechanical cleaning
to chemical deoxidizing (chromated
and nonchromated agents).
Coating thickness. <strong>Hard</strong> anodic
coating of 0.0005 to 0.001 in. thickness
suffices to provide the sought-after
abrasion resistance and corrosion pmtection.15
However, for hydraulic components
characterized by tight concentricity
and roundness tolerances (Le.,
H.0005 in.), coating thickness of
0.002-0.0035 in. is common to maintain
tolerance along the bore's length.
This study examined our ability to
affect porosity in hard anodic coatings
of various thicknesses using the aforementioned
cleaning methods.
Processes that introduce residual
compressive stresses. Shot peening is a
common practice to enhance fatigue
performance of hard anodized
The necessity to clean the shot residue
prior to anodi~ing'9,~~ may contradict
the prohibitionz1 to remove more than
ten percent of the Almen intensity,
especially for high peening intensities
when a fine surface finish (8 microinches)
is called out. This study examined
porosity occurrence when roller
bumishing was substituted for shot
peening.I7
EXPERIMENTAL
All experiments were conducted
using 7075.T6 cylinders (5 in. long, 2
in. outside diameter, 0.25 in. wall
thickness), machined from bar stock.
The cylinders were processed follow-
ing common practices for hydraulic
cylinders except for differences out-
lined in Table I.
Honing removed 0.008 to 0.01 1 in.
of roller-bumished cylinders and
0.0010 to 0.0017 of shot-peened cylin-
ders.
Variation in the amount of metal
removed by honing in any given group
of cylinders (three for each coating
thickness and preanodizing cleaning
method) was 0.0002 to 0.0003 in.
Each preanodizing cleaning method
was performed at a different plating
shop to be referred to as facility A, B, or
C, corresponding to mechanical clean-
ing, chromated deoxidizer and nonchro-
mated deoxidizer, respectively.
The test matrix is comprised of three
test cylinders per cleaning method per
coating thickness range for each group
(I and 11).
The quality of the resultant hard
anodic coatings was evaluated using
the following tests: (1) abrasion resis-
tance based on Taber test of (sheet)
panels accompanying the anodized
cylinders, (2) porosity evaluation
based on qualitative optical micros-
copy performed on cross-sectioned
METAL FINISHING

Table II. Taber Abrasion Test Results
Cleaning Method Weight Loss (mg)
Mechanical (A) 9.0, 12.0, 13.0
Chromated Deoxidizer (6) 14.8, 16.5, 11.4
Nonchromated Deoxidizer (C) 15.0, 27.0, 15.0
CS-17 wheels, 1,000 grams load, 10.000 cycles, coating thickness 0.0025 in.
hard anodized cylinders, and (3) poros-
ity evaluation of the samples in (2)
using an image-analysis technique.
The preanodizing cleaning methods
were the following:
Mechanical cleaning. Regular
water-soap solution with Scotch-
Bright cleaning as a substitute for a
deoxidizer, followed by soap solution
rinse and deionized water rinse
Chromated deoxidizer. Regular
water-soap solution with chromated
deoxidizer (containing nitric acid) fol-
lowed by a regular water rinse
Nonchromated deoxidizer. Regular
water-soap solution with nonchro-
mated deoxidizer (containing nitric/
hydrofluoric acid) followed by a deion-
ized water rinse.
Caustic etch was not used in either
facility. Facilities (A) and (B) do not
monitor chloride levels in the soap
solution. Facility (C) limits the chlo-
ride level to 300 ppm.
RESULTS
ABRASION RESISTANCE
Abrasion resistance of the hard ano-
dized cylinders was evaluated using a
Taber test. Flat aluminum sheet panels
accompanying each of the cylinder
subgroups were tested.23 The maxi-
mum weight loss for 7075-T6 alumi-
num should not exceed 15.0 milli-
gramzz The panels were hard ano-
dized in facilities (A), (B) and (C)
without prior shot peening or roller
burnishing and without a caustic etch.
Any performance difference, therefore,
would reflect different hard anodize
quality due to different preanodize
cleaning methods only (all other hard
anodizing variables being equal). The
results are presented in Table 11.
A lower weight loss in the Taber test
reflects better hard anodic coating
quality. According to these results, the
cleaning methods ranging from best to
worst are: mechanical, chromated de-
oxidizer, and nonchromated deoxi-
dizer.
QUALITATIVE COATING
EVALUATION
Metallographic samples of the hard
anodized cylinders were inspected
through an optical microscope at
200 x magnification. Coating thick-
ness could be accurately determined.
The worst field in terms of porosity
was photographed (see Figs. 2, 3, 4).
Results are presented herein for each
cleaning method (Le., for each anodiz-
ing facility) separately, for the roller-
bumished as well as shot-peened cylin-
ders, and for thin (O.OOl4.OOl5 in.) as
well as thick (0.00254.0035 in.) hard
anodic coatings.
Since similar results were obtained
for each test group (three cylinders)
corresponding to a given set of proc-
essing conditions, (i.e., surface prepa-
ration, preanodizing cleaning, coating
thickness), only one metallographic
specimen is displayed. Each picture
depicts the base aluminum (white
zone), the bard anodic coating on top
A
of it (gray zone), and the epoxy resin
@lack zone) used for mounting the
cross-sectioned samples. Porosities or
pits in the hard anodic coating show as
black spots.
Based on the metallographic sam-
ples, several conclusions were reached.
Thinner coating (O.OOlC-O.OOl5 in.)
is associated with less porosity: fewer
pores (blisters) appear in the hard coat,
and their size is smaller. Thicker
coatings are associated with more
blisters being of a larger size. This
trend corroborates other report~.~,~J'J~
The amount of metal removed dur-
ing honing seems to have no effect on
the hard anodic coating quality.
No marked difference could be
noted between shot peening and roller
burnishing in their effect on the poros-
ity (frequency and size) in the hard
anodic coating. This seems to be the
case for each method of preanodizing
cleaning.
The preanodizing cleaning method
seems to affect the amount of porosity
and pore size for the thick hard anodic
coating, other processing conditions
being equal. Mechanical cleaning (fa-
cility A) appears to produce the best
hard anodic coating. Nonchromated
deoxidizing (facility C) seems to pro-
duce the worst hard anodic coating.
Chromated deoxidizing (facility B)
Fig. 2. Mechanically cleaned sampler from faciliry A (200 x ); (A). B (0.0017 in. rhick) roller
burnished; (E), B (0.0017in. thick) shorpeened; (C). B 005 (0.0031 in. rhick) roller burnished:
ID), R (0.0032in. thick), shorpeened.
DECEMBER 1992 21

a B
C
Fig. 3. Chromafed deoxidizer cleaned samplesfrom facility B (200 x J; (A). B 007 (0.0015 in. thick)
roller burni,vhed; (B), C (0.0016 in. thick), shot peened; (C), B 014 (0.0031 in. thick) roller
burnished; (D), L (0.0027in. thick) shot peened.
produces intermediate quality of hard
anodize, in terms of blister size and
content.
QUANTITATIVE MICROSCOPIC
ANALYSIS
An image analysis method was used
to evaluate the two groups of samples
for quality of the hard anodic coating
(see Table I). This method involves
digitizing an optical microscope field
and analyzing a certain feature (or
component) in terms of relative area. In
this case, the total voidblister area
divided by the total hard anodic coat-
ing area yielded the porosity percent-
age. About twenty optical fields of the
coating (100 x magnifying power)
were analyzed for each sample. The
hard anodic coating boundaries were
defined as the outer surface of the
coating on one side and its lowest edge
including the aluminum bumps on the
other side.
The image analysis results are given
in Table III. The samples analyzed
were those examined through optical
microscopy. Except for one of the
mechanically cleaned samples, which
seems out of place, all other results
point to some basic trends: (1) compa-
rable coating quality for the two me-
chanical surface treatments-shot
D
peening and roller burnishing; (2) an
increase in the percentage porosity
with increase of hard anodic coating
thickness; and (3) higher susceptibility
of thick anodic to the preanodizing
cleaning method compared with the
a B
C
thin anodic coating.
In order to better compare the three
preanodizing cleaning methods, the
void distribution and relative size were
compared. In other words, which pre-
anodizing treatment leads to the largest
blisters and what is the most frequent
void size? Results are given in Table
IVB for the same samples presented in
Table 111.
Based on these results, the following
conclusions can he made:
For thin hard anodize (up to 0.002
in.), the three preanodizing cleaning
procedures seem to produce compara-
ble quality of hard anodic coating.
For thick hard anodize (0.002 to
0.0035 in.), the mechanical cleaning
and chromated deoxidizer produce
comparable hard anodize while the
nonchromated deoxidizer seems to
have an adverse effect on the coating
quality.
Evidence of the above is the de-
crease in percentage of the small size,
the most frequent void, accompanied
by an increase in the mean void size
and appearance of larger blisters. A
possible explanation for this detrimen-
tal effect is the active acid (hy-
drofluoric) present in the nonchro-
mated deoxidizer.= (Hydrochloric acid
may also be present in some nonchro-
mated deoxidizers.) The detrimental
Fig. 4. Nonchromateddeoxidiiercleanedsamplesfromfncility C (200 x 1; (A), B 008 (0.0009in. thick)
roller burnished; (E), E (0.0008 in. thick), shot peened; (C). B 006 (0.0028 in. thick) roller
burnished; (D). Q (0.0027in. thick) shot peened.
22 METAL FINISHING

.. ’
Table 111. image Analysis of <strong>Hard</strong> <strong>Anodic</strong> Coating Samples
Sample-Mechanical <strong>Hard</strong> Anodize Total Porosity
Preanodzing Cleaning Treatment Thickness (in.) Remarks
Mechanical Shot peened 0.0021 0.432 Thin hard anodize
Roller burnished 0.0017 0.2590
Chromated Deoxidizer Shot peened
RO ler ournisned
Shot peened 0.0031 1.0676 Thick hard anodize
Roller burnished 0.0032 3.5656”
Shot peened
Roller burnished
0.0013
0.0015
0.0033
0.0025
0.6849
1.5626
1.481 7
1.0769
Thin hard anodize
Thick hard anodize
Nanchromated Deoxidizer Shot peened 0.0008 0.7530 Thin hard anodize
Roller burnished o.oot0 1.3232
*Except for this measurement, all other results fallow a basic trend
etching of the preanodized surface by
that acid may account for the large
voids and their relative high content in
the hard anodic coating.
CONCLUSIONS
Thin hard anodic coatings (0.0010 to
0.0020 in.) are less susceptible to voids
(blisters, pores). Blister occurrence and
size become more pronounced in thick
hard anodic coatings (0.0025 to 0.0035
in.), resulting in lower abrasion resis-
tance.
There is no apparent difference
among the effects of preanodizing,
shot peening, and roller burnishing (as
methods to introduce compressive
Shot peened 0.0027 4.3843 Thick hard anodize
Roller burnished 0.0029 8.5498
stresses) on the hard anodic coating
quality, provided all shot contamination
is removed by thorough etching
prior to anodizing. In view of these
findings, the better-controlled shotpeening
process is preferred for improving
fatigue performance of hard
anodized parts. Blisters or porosity in
thick hard anodic coatings may he
minimized by using mechanical preanodize
cleaning in lieu of chemical
deoxidizing. Thin hard anodic coating
is less sensitive to the nature of the
preanodize cleaning. MF
* * * * *
The author wishes to thank Stuart
Myron and Marietta Molina of the
Metallurgical Lab at HR for their
Table IV. Void Distribution in the <strong>Hard</strong> <strong>Anodic</strong> Coating*
support in the microscopic analysis of
the anodized samples.
Also the author wishes to thank Bernie
Kooper, Anaplex Corp., Paramount,
for his assistance regarding the me-
chanical cleaning approach.
Biography
Leah Markowitz is a graduate of
the Chemistry Faculty of the Israel
Institute of Technology (I.I.T.) and
holds a Ph.D. from the Materials
Department of I.I.T.
She is currently a senior materials
engineer at HR Textron, Los Angeles,
where she is involved in research and
development of advanced materials
and provides support to manufactunng
Sample-Mechanical Mean Void Size“ Most Frequent Void Size RangeC Largest Void Size Rangec
Preanodizino Cieanino Treatment /umlz+ SM Dnv lump lump
Mechanical Cleaning Shot peened 10.1 -t 23.3
Roller burnished 7.4 f 7.5
1 .o to 2.0 (31 .O%)
2.0 to 4.0 (41.4%)
128 to 256 (I=)
32 to 64 (1 .E%)
Shot peened 34.8 f 69.4 2.0 to 4.0 (18.3%) 128 to 256 (4.7%)
Roller burnished 65.5f191.4 8.0 to 16.0 (18.4%) 51210 1024(1.8%)
Chromated Deoxidizer Shot peened 7.2-tt0.4 2.0 to 4.0 (30.0%) 32 to 64 (2.9%)
Roller burnished 10.2-tZO.7 1 .O to 2.0 (30.8%) 64 to 128 (3.1%)
Shot peened 38.7 f 80.9 4.0 to 8.0 (20.3%) 256 lo 512 (3.9%)
Roller burnished 19.1 -t 38.1 4.0 to 8.0 (24.5%) 128to256(3.1%)
Nonchromated Deoxidizer Shot peened 14.0 -t 20.6 4.0 to 8.0 (22.4%) 64 to 128 (3.4%)
Roller burnished 9.8-tt9.0 1 .o to 2.0 (38.9%) 64 to 128 (1.6%)
Shot Peened 98.3 -t 220.4 2.0 to 4.0 (15.4%) 1024 to 2048 (1.4%)
Roller burnished 175.6 -t 475.2 2.0 to 4.0 (20.4%) 2048t04096 (1.1%)
‘The first two entries for each cleaning method represent thin anodic coatings, followed by the two comparable values for thick anodic coatings.
b($m)2 =square micrometers.
OThe percsnlage in parentheses indicates the percentage of this area range of the total hard anodized inspected area.
DECEMBER 1992 23

and maintenance of flight control ser-
voactuators.
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Anodized Surface PIN 3001416,” HR Dacument
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<strong>Anodic</strong> Coating of Aluminum Alloys,” HR
Report MLR #1363. Jan. 1988
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16. Grunberg, L., Properries of Mclallic Sur-
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17. Campbell, J.E., “Shot Peening for Improved
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19. MIL-P-81985 “Peening of Metals,” para-
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20. MIL-S-13165, “Shot Peening of Metal
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1989
21. MIL-S-13165, “Shot Peening of Metal
Parts,” paragraph 4.3.2.2; 1989
22. MIL-A-8625, “<strong>Anodic</strong> <strong>Coatings</strong> for Alumi-
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23. Fed. Std. 141, Method 6192, Taber Abrasion
Test
24. Detailed Image Analysis results available
from the author upon request
25. Nonchromated deoxidizer4Aite LNC
was applied in this study
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