U.S. patent application number 10/768825 was filed with the patent office on 2005-08-04 for pretreatment of aluminum surfaces.
This patent application is currently assigned to STAR FINISHES, INC.. Invention is credited to Barnard, Michael D., Hagen, Douglas W..
Application Number | 20050167005 10/768825 |
Document ID | / |
Family ID | 34807974 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050167005 |
Kind Code |
A1 |
Barnard, Michael D. ; et
al. |
August 4, 2005 |
Pretreatment of aluminum surfaces
Abstract
A process for preparing the surface of aluminum products to
receive colored coatings comprises a substantially pollution free,
non-chromate pretreatment and aluminum conversion coating process.
The process provides improved coating quality, increased process
stability, increased throughput and extended operating life of the
processing baths by controlling the electrical resistance of the
deionized water used for washing the aluminum product being
processed to at least about 50 k ohms and controlling the manner of
washing between each step of the process.
Inventors: |
Barnard, Michael D.;
(Fresno, CA) ; Hagen, Douglas W.; (Coarsegold,
CA) |
Correspondence
Address: |
KOPPEL, JACOBS, PATRICK & HEYBL
555 ST. CHARLES DRIVE
SUITE 107
THOUSAND OAKS
CA
91360
US
|
Assignee: |
STAR FINISHES, INC.
|
Family ID: |
34807974 |
Appl. No.: |
10/768825 |
Filed: |
January 30, 2004 |
Current U.S.
Class: |
148/275 |
Current CPC
Class: |
C23C 22/66 20130101;
C23C 22/73 20130101 |
Class at
Publication: |
148/275 |
International
Class: |
C23C 022/56; C23C
022/82 |
Claims
We claim:
1. An improved process for the conversion coating of aluminum
products comprising the steps of: a) immersing an aluminum product
in a solution for removal of dirt and oil, followed by b) immersion
in an oxidizing solution, and then followed by c) immersion in a
solution of a permanganate based conversion composition, the
improvement comprising, following each of step a), b) and c) above,
subjecting the aluminum product, upon removal from immersion in
said solution, to at least one spray wash with deionized water and
at least one immersion in deionized water, the deionized water
being processed or continuously reprocessed through a series of
filters and ion beds to maintain an electrical resistance of at
least about 50 k ohms.
2. The process of claim 1 wherein the aluminum product, after each
of steps a), b) and c) is exposed to i. a first spray wash with
deionized water, ii. followed by immersion in deionized water,
followed iii. by a second spray wash with deionized water,
3. The process of claim 1 wherein the deionized water is maintained
at an electrical resistance of at least about 50 k ohms by passing
it through at least a 0.5 micron filter, a carbon filter, a cation
bed, an anion bed and a DI mixed bed.
4. The process of claim 1 wherein the deionized water is processed
serial through a 0.5.mu. filter, a carbon filter, a cation bed, two
anion beds, a DI mixed bed and a 0.5.mu. filter.
5. The process of claim 1, wherein at least about 270,000 ft.sup.2
of exposed surface of aluminum parts is processed before streaking,
spotting, peeling or discoloration of the surface appears.
6. The process of claim 1, wherein aluminum parts can be processed
for at least 480 hours before unacceptable streaking, spotting,
peeling or discoloration of the surface appears.
7. An improved method of applying a conversion coaching to aluminum
parts, said aluminum parts being exposed to treatment baths for a
cleaning the parts, b) oxidizing the exposed surface of the parts
and c) conversion coating the parts, wherein the parts are spray
washed and immersed in deionized water between each treatment bath
exposure, the deionized water having an electrical resistance equal
to or greater than about 50 k ohms.
8. The improved method of claim 7 wherein the deionized water is
maintained at a resistance of at least about 50 k ohms by
continuously recirculating the water through at least a 0.5.mu.
filter, a carbon filter, a cation bed, an anion bed and a mixed bed
at a flow rate from about 3 gal/min to about 10 gal/min.
Description
[0001] The invention is directed to a process for preparing the
surface of aluminum to receive colored coatings. More particularly
it is directed to a substantially pollution free, non-chromate
pretreatment and aluminum conversion coating process which provides
improved coating quality, increased process stability and extended
operating life of the processing baths.
BACKGROUND
[0002] It is well known that it is particularly difficult to apply
colored finishes to aluminum surfaces because of the oxide coating
which naturally forms on the aluminum surfaces soon after it is
formed. Accordingly, aluminum or aluminum alloy surfaces are
routinely treated by applying an intermediate corrosion resistant
conversion coating to the surface. A broad range of subsequent
coatings can then be readily applied to the conversation coating to
produce an acceptable, blemish free new surface. A common technique
is to clean the aluminum surface and then apply a acid based
hexavalent chromium composition to that clean surface. The chromium
conversion coatings have been readily accepted because they are
generally corrosion resistant and provide excellent retention of
subsequent coatings. However, because of the extreme toxicity of
chromium compounds and ecological problems resulting from waste
disposal from aluminum treatment plants, the industry has replaced
chromium conversion coating with chromium free systems.
[0003] A significant advance has been the use of potassium
permanganate based conversion coating systems, such as developed by
SanChem. Representative of such systems is the use of SanChem
CC3400, a KMnO.sub.4 based compound which is chromium, cyanide and
fluoride free. A typical SanChem treatment composition comprises
immersion of the aluminum product in a 10% solution of SanChem
CC3400+1.7 oz activator @ 135-140.degree. F. for 1.5-2 min to
produce a gold color on the aluminum; longer time provides a
thicker coating with a darker yellow color. A preliminary step is
cleaning the surface prior to the permanganate dips. If an alkaline
cleaner is used it is usually followed by a deoxidizer or a mild,
nitric acid based solution. Generally speaking the SanChem process
comprises degreasing, an alkaline wash at 160.degree. F. (such as
SanChem 500,--a mildly alkaline phosphate cleaner), deoxidizing at
125.degree. F. using SanChem 1000 which is a nitric acid based
deoxidizer free of fluorides, chromates and heavy metals, forming
an oxide film by immersion in DI Water @ 210.degree. F. and a three
step sealing process (SanChem 2000+SanChem 3000+SanChem 4000).
[0004] SanChem has several patents directed to permanganate
conversion coatings of which the following are representative:
[0005] U.S. Pat. No. 4,711,667 describes degreasing the aluminum
surface using mineral spirits followed by an alkaline wash (NaOH,
alkaline NaNO.sub.3, HF, Na.sub.2CO.sub.3 or borax). This is
followed by a water wash and then immersion in a bath of KMnO.sub.4
with borax, sodium benzoate or sodium carbonate, typically for 1
minute at 155.degree. F., followed by a water rinse. All solutions
were preferably silicate free.
[0006] U.S. Pat. No. 4,988,396 is directed to aluminum alloys with
1% Cu. After the surface is degreased it is deoxidized using a 10%
nitric acid solution (85.degree. C., for 20 min.) followed by a
deionized (DI) water rinse and immersion in 195.degree.-212.degree.
F. DI water for 5 minutes to form a boehmite layer followed by the
permanganate treatment.
[0007] U.S. Pat. No. 5,437,740 has several examples for anodizing
aluminum and conversion coating of aluminum castings. Room
temperature DI water washes after several of the treatment stages
are utilized. Deoxidizing is performed using 70% HNO.sub.3, 2.4%
HF, 27.5% water.
[0008] U.S. Pat. No. 5,707,465 uses a permanganate solution
containing hexavalent chromium. The aluminum material is degreased
(using an organic solvent and a non-ionic detergent), rinsed with
DI water, deoxidized using 10% HNO.sub.3 at 70.degree. F. for 1
minute and then rinsed with DI water. It was then treated with
various different permanganate solutions.
[0009] U.S. Pat. No. 6,087,017 is directed to a corrosion resistant
wax polyester film for aluminum. However, the pretreatment process
is relevant. The aluminum panel is cleaned using a mild alkaline
cleaner at 150-160.degree. F. for 3 minute, rinsed in DI water,
deoxidized in 10% HNO.sub.3 and 3% sodium bromate at 120.degree. F.
for 5 minutes followed by a DI water rinse. This can then be
followed by the application of the permanganate conversion coating
at 150.degree. F. for 1 minute.
[0010] U.S. Pat. No. 4,883,541 discusses the prior use of acidic
deoxidizers in combination with HF. The claimed invention is then
directed to acidic deoxidizers, particularly 10% HNO.sub.3 with 3%
sodium bromate or iodate.
[0011] U.S. Pat. No. 5,192,374 is directed to a treatment following
the deoxidation step. The initial cleaning steps use an alkaline
cleaner (Chemidize 740) at 71.degree. C. for 3 min. followed by a
rinse for 1 min in DI water, deoxidizing at 30.degree. C. using 10%
HNO.sub.3+3% sodium bromate, rinsing with DI water for 1 min. and
then immersion in boiling DI water for 5 minutes.
[0012] U.S. Pat. No. 5,417,819 is directed to compositions for
desmutting aluminum surfaces comprising the use of 10-100% nitric
acid and 15 gr/liter of a fluoride, the balance being water with
the possible addition of H.sub.2SO.sub.4 and/or H.sub.3PO.sub.4.
The preferred source of fluoride ion is ammonium bifluoride. The
surface is first treated with an alkaline cleaner solution (A31K)
at 140.degree. F. It is then immediately rinsed with water,
preferably DI water. The surface is then subjected to
electro-brightening followed by a DI water rinse. Desmutting is
then performed for 0.5-2 min @ 60-110.degree. F. followed by a
water rinse.
[0013] U.S. Pat. No. 6,123,782 is directed to a process for forming
a coated aluminum product comprising cleaning the surface using a
nonchromated, nonsilicated alkaline cleaner followed by a hot water
rinse, deoxidizing at up to 120.degree. F. using a nonchromated
deoxidizer such as SanChem 1000 (10% nitric acid, 3% sodium
bromate) and then immersion in boiling deionized water for 5-10
minutes.
[0014] Prior available processes still suffer from several
recurring problems which are not addressed by a mere optimization
of the process variables or use of alternative compositions.
Initial operation of the processes set forth in each of the above
references gives generally acceptable results. However, after a
short period of continuous. operation the quality of the surface
produced, or subsequently applied surface, becomes unacceptable due
to streaks, non-uniform appearance and subsequent fading. The
treatment baths have to be dumped frequently and replaced with
fresh baths in order to maintain acceptable coating properties.
Prior techniques and/or solutions appear to include the existence
of unrecognized and unaddressed variables which contribute to a
rapid decrease in product quality. Improving the process quality is
not a mere matter of optimizing the operating conditions. Applicant
has discovered that there is a necessity to scrupulously process,
reprocess and filter the DI water to remove interfering ions which
may be in the feed and rinse water feed streams, a need to limit
the time and temperature of exposure of the aluminum to DI water
and the importance of subsequent use of both spray and immersion
treatments. Pietschmann, J. and Jehn, Hermann (Powder Coatings,
Pre-Treatment and Quality Control of Aluminum for Architectural
Applications, (Galvanotechnik, D-88348 Saulgau, 91, (2000) Nr. 9))
addressed the various steps of the conversion coating process,
namely degreasing, pickling/etching, acid dipping if alkaline
pickling is used, conversion coating and drying. The authors states
that:
[0015] "Between the individual steps, sufficient rinsing is
necessary. Rinsing is a diffusion controlled procedure. Not only
the amount of water, its purity and the temperature are decisive,
but also the duration of rinsing is important. The type of rinsing
technique, dipping or spraying, also influences the result"
[0016] However, the criticality of the washing steps, scrupulous
reprocessing of the water and the use of DI water at ambient
temperatures was not appreciated.
DETAILED DISCUSSION
[0017] A preferred process, incorporating features of the
invention, comprises:
[0018] a) a cleaning step using a commercially available alkaline
composition in the manner and under conditions generally suggested
by the manufacturer,
[0019] b) use of 10% or greater HNO.sub.3 as a deoxidizer with a 3%
fluoride solution (KF preferred) at room temperature, and
[0020] c) application of a SanChem conversion coating (SanChem
3400) under conditions and in a manner substantially the same as
recommended by the manufacturer.
[0021] The improvement comprises the use of extensive DI water
washes between each step of the process. These water washes include
a combination of spray and immersion washes that appear to be
critical to decontaminating the treated surface between each step
to eliminate trace amounts of prior applied materials. The DI water
is preferably continuously reprocessed and filtered to remove
undesirable ions and maintain the resistively within defined
limits. Care is also taken to not use DI water at elevated
temperatures so as not to react with the surface (i.e. oxidize the
surface) to form boehmite.
[0022] More specifically a process incorporating features of the
applicant's invention comprises processing through a series of wash
tanks as follows:
[0023] 1) The aluminum part is immersed (agitation optional but
preferred) in a first water bath containing a non-silicated,
non-caustic, alkaline cleaner (such as US Specialty Color Corp
Specialty 740), under conditions recommended by the supplier,
namely 6-8 oz of the cleaner/gal at 140-165.degree. F. (preferably
155.degree. F.) for 3-7 min.
[0024] 2) The rinse is followed by a 20-30 sec atomized spray rinse
using DI water. The aluminum part is then immersed in DI water at
ambient temperature for 45-60 sec, agitation optional. The rinse is
followed by a 20-30 sec atomized spray rinse using DI water.
[0025] 3) The aluminum part is immersed in DI water/Nitric acid
(30% by Vol)/KF1 (6-8 oz/gal) at ambient temperature for 15-60 sec.
The rinse is followed by a 20-30 sec atomized spray rinse using DI
water which drains into that dip tank.
[0026] 4) The piece is then exposed to a 45-60 sec atomized spray
rinse using ambient DI water, a 45-60 sec immersion in ambient DI
water and a 20-30 sec atomized spray rinse using DI water.
[0027] 5) The aluminum piece is then immersed in DI water/SanChem
CC3400 (10% by vol)+SanChem CC3400 Activator (1.7 oz/gal) at
140-165.degree. F. (preferably 155.degree. F.) for 2.5-4 min.
Agitation optional. The appearance of the product is light
yellow.
[0028] 6) The immersion is followed by 20-30 sec atomized spray
rinse using DI water, a 30-45 sec immersion in DI water and then a
20-30 sec atomized spray rinse using DI water.
[0029] It is preferred that all process equipment and contact
surfaces (tanks, racks, baskets, hangers, etc.) are non-conductive,
non-reactive material such as polypropylene, polyethylene or Teflon
coated surfaces so that none of the baths or wash solutions can
leach any contaminants from the processing equipment. All of the DI
water feed to the system as well as the DI water spray and dip tank
contents used in each wash stage may begin with city and/or well
water which is filtered through a initial water purification system
consisting of a) 0.5 micron particulate filter, b) a D.I mixed-bed
canister and c) a 0.5 micron particulate post filtration at a rate
of 10. gals/min upon demand. Once the DI water has been processed
through the initial water purification system and enters the
aluminum treatment system it is preferably collected and
continuously filtered serially through a) a 0.5 micron particulate
filter, b) a carbon filtration canister, c) a cation bed, d) two
anion beds, e) a DI mixed bed canister and f) a final 0.5 micron
particulate filter at a rate of from about 3 to about 10 gal/min to
maintain a water quality of at least about 50 k ohms-cm. A typical
DI immersion tank contains 320 gal of DI water so that the content
is recycled approximately at least every 1.75 hours. Alternatively,
a typical conversion coating system could treat the rinse water
simply as a waste steam. Using a three stage cascading method of
rinsing, the impurities from the cleanest rinse overflow to the
medium purity rinse and from that into the dirtiest rinse. The
dirtiest rinse is treated by adjusting the pH and ORP but is
discharged to the sewer on a daily basis. In any event, the
processed DI water entering each washing stage is maintained at a
resistance of at least about 50 k ohms and at ambient temperature
to minimize oxidation which can occur when clean aluminum surfaces
are contacted with DI water at elevated temperatures.
[0030] Besides controlling the quality of the DI water used
throughout the system, the chemical makeup of the water feed to the
system and subjected to deionization can also have an influence on
the quality of the conversion coated product. A typical analysis of
the feed water entering the system is given in Tables 1 and 2.
Table 3 lists the analysis of that feed water returned to the
system following filtering and RO treatment. Silica, an ion known
to present aluminum conversion coating processing problems, while
present in the feed water, has been eliminated by the filter and
deionization process, and as such the water treatment process
contributes significantly to the production of quality
coatings.
[0031] It has been found that operation of a process incorporating
features of the invention results in significantly increased
quantities of acceptable finished product which is free of
streaking, spotting, peeling and discoloration. Additionally, the
life of the various solution baths (the time before the quality of
the treated product is unacceptable) is increased to about 480
hours of production, resulting in a significant increase in process
throughput and a commensurate reduction in the down time for
cleanup and bath replacement as well as a reduction in the cost for
replacement bath solutions. Still further, the frequency of
discharge of bath contents, which contain EPA regulated pollutants,
into the surrounding environment is significantly reduced.
1TABLE 1 FEED WATER ANALYSIS Turbidity (Method 180.1) 0.1 NTU
Turbidity after N.M filtration Conductivity (Method 151.0 MMHOS/CM
Fst TDS by 143.7 Color (Method 2120C) 0.0 Color after N.M
Acidification pH(Method 150.1) 7.7 Tannins N.D. (Concentrations
reported as mg/L (PPM) unless otherwise reported) CATIONS (Method
200.7) ANIONS (Method 300.0) As As As Element CaCo3 As Ion CaCo3
Calcium (Ca) 9 22.5 Chloride (CI 7.9 11.1 Magnesium (Mg) 4.4 18.1
Nitrate/Nitrite 1.2 4.3 Sodium (Na) 13.9 30.3 Sulfate (SO4) 4 4.2
Potassium(K) 3.7 0.1 Bi-carbonate 55.3 45.3 Barium (Ba) 0.3944
Fluoride (F) 0.2 0.50 Iron (Fe) ND Silica (SiO.sub.2) 53.1
Strontium (Sr) 0.1 0.1 Manganese (Mn) ND Copper (Cu) 0.116 Zinc
(Zn) 0.11 Mg/L GPG Mg/L GPG Mg/L GPG Cations 75.7 4.42 Anions 69.7
4.08 Hardness 40 2.4 Additional (CaCO3) Aluminum by ICP N.D.
.mu.g/L
[0032]
2TABLE 2 FEED WATER ANALYSIS Inorganics Analyte Method Result Units
PQL Dilution DLR Antimony (Sb) EPA200.8 ND .mu.g/L 2 1 2 Arsenic
(As) EPA200.8 2.0 .mu.g/L 2 1 2 Barium (Ba) EPA200.8 34 .mu.g/L 5 1
5 Beryllium (Be) EPA200.8 ND .mu.g/L 1 1 1 Cadmium (Cd) EPA200.8 ND
.mu.g/L 1 1 1 Chromium - EPA200.8 1.0 .mu.g/L 1 1 1 Total (Cr)
Cobalt (Co) EPA200.8 ND .mu.g/L 50 1 50 Copper (Cu) EPA200.8 ND
.mu.g/L 5 1 5 Lead (Pb) EPA200.8 ND .mu.g/L 5 1 5 Mercury (Hg)
EPA200.8 ND .mu.g/L 10 1 10 Nickel (Ni) EPA200.8 ND .mu.g/L 10 1 10
Nitrate (NO3) EPA200.8 4.0 mg/L 1 1 1 Selenium (Se)- EPA200.8 ND
.mu.g/L 2 1 2 (Total) Silver (Ag) EPA200.8 ND .mu.g/L 10 1 10
Thallium (Tl) EPA200.8 ND .mu.g/L 1 1 1 Vanadium (V) EPA200.8 ND
.mu.g/L 10 1 10 Zinc (Zn) EPA200.8 68 .mu.g/L 50 1 50 .mu.g/L =
micrograms/liter (ppb) PQL: Practical Quantitation Limi DLR:
Detection Limit for Reporting PQL .times. Dilution ND: None
Detected at DLR
[0033]
3TABLE 3 POST RO TREATMENT Ppm Ppm Ion CaCO.sub.3 Cations Aluminum
0.21 1.18 Barium BDL NA Cadmium BDL NA Calcium 0.19 0.5
Chromium(+3) BDL NA Copper BDL NA Iron BDL NA Lead BDL NA Magnesium
0.05 0.21 Manganese 0.08 0.15 Nickel BDL NA Potassium 0.48 0.61
Sodium 0.16 0.35 Zinc BDL NA TOTAL 2.96 CATIONS Strong Anions
Chloride 02.1 0.30 Chromium(+6) BDL NA Fluoride BDL NA Hydroxide
BDL NA Nitrate 11.2 9.07 Phosphate BDL NA Sulfate BDL NA SUB.TOTAL
9.37 Weak Anions Bicarbonate BDL NA Carbonate BDL NA Cyanide BDL NA
SUB-TOTAL 0.00 TOTAL 9.37 ANIONS pH 4.21 Units (by meter) Silica
BDL mg/L SiO.sub.2 TOC BDL mg/L COD NA mg/L Carbon NA Ml/gram
Isotherm Conductivity 54.0 .mu.mhos/cm Color clear Process
Information: System Flowrate (gpm) 6 Operating Temp. (.degree. F.)
120 Hours Operated/Day 16 Days Operated/Week 5 Cleaner/System Type
aqueous Water Source DI Water Quality 2 M.OMEGA.-cm (Required water
quality - 50 Kohms-cm)
[0034] Besides providing improved quality conversion coated
aluminum products, the process incorporating features of the
invention has also been found to generate less waste products, and
the waste products generated are easier to handle.
[0035] Typical waste streams generated by the conversion coating
process and DI recycling procedure are:
[0036] 1) depleted coating/processing tank contents,
[0037] 2) processing tank sludge (solids or slurry deposited in the
bottom of processing tanks),
[0038] 3) materials filtered from the DI water, and
[0039] 4) discharge generated by reactivating the deionizing
filters.
[0040] By employing the DI reprocessing described herein, it has
been discovered that
[0041] 1) The processing tank contents have an extended processing
life,
[0042] 2) The amount of sludge generated is decreased, and can be
readily and safely collected, dried and disposed of as a safe solid
waste, and
[0043] 3) Ionic materials and organic materials discharged from the
filtration system can be processed in a safe manner and the
particulate filters can be dried and discarded as safe solid
waste.
[0044] For comparison purposes, set forth below are four examples
of procedures evaluated for applying a non-chromate conversion
coating. These examples, while not using applicant's washing and DI
water cleaning and recycling procedure, were less effective
experimental attempts to obtain a suitable end product. As a result
they all suffer from operating deficiencies including varying
amounts of streaking, spotting, peeling and discoloration to the
finished (treated) product and limitations on bath operating life.
Typical prior baths and treatment procedures was found to be
capable of processing only about 60,000 ft.sup.2 of acceptable
product before the treatment baths had to be discarded and new
solutions prepared.
EXAMPLE 1
[0045] A four (4) stage system for cleaning and conversation
coating aluminum alloy was utilized comprising:
[0046] Stage 1--Sanchem 560 Mild acid cleaner @ 150.degree. F. for
10 min
[0047] Stage 2--Hot D.I. Rinse @ 140.degree. F. for 30-60 sec.
[0048] Stage 3--Sanchem CC3400 conversion coating @ 160.degree. F.
for 3 min
[0049] Stage 4--Hot D.I. Rinse @ 140.degree. F. for 30-60 sec.
[0050] All process equipment, (hooks, baskets to tanks) was
stainless steel. Appearance of the processed aluminum parts was
poor, coverage was splotchy and streaky, film build was irregular
and was found to rub off. The process was not adequate to remove
oils and dirt on the surfaces.
EXAMPLE 2
[0051] An alkaline wash was added to the front end of the process
to better remove oils and dirt. The system includes six (6) process
steps.
[0052] Stage 1--Sanchem 8104 silicated alkaline cleaner @
120-130.degree. F. for 10 min.
[0053] Stage 2--D.I. Rinse @ 120-130.degree. F. for 45-60 sec.
[0054] Stage 3--Sanchem 560 Mild acid cleaner @ 150.degree. F. for
1-2 min.
[0055] Stage 4--Hot D.I. Rinse @ 140.degree. F. for 45-60 sec.
[0056] Stage 5--Sanchem CC3400 conversion coating @ 160.degree. F.
for 3.0-3.5 min.
[0057] Stage 6--Hot D.I. Rinse @ 140.degree. F. for 45-60 sec.
[0058] The processed aluminum alloy parts were slightly improved in
appearance for a short period of time (about 3 hours), but the life
of bath solutions was not adequate (about 5 hours) due to poor
product surface quality. Coating voids occurred at hang points on
work pieces; streakiness and rub off still existed.
EXAMPLE 3
[0059] The make-up of stage 3 solution was changed in an attempt to
improve cleaning.
[0060] Stage 1--Sanchem 8104 silicated alkaline cleaner @
120-130.degree. F. for 10 min.
[0061] Stage 2--D.I. Rinse @ 120-130.degree. F. for 45-60 sec.
[0062] Stage 3--Sanchem 6500 acid cleaner @ 150.degree. F. for 1-2
min
[0063] Stage 4--Hot D.I. Rinse @ 140.degree. F. for 45-60 sec.
[0064] Stage 5--Sanchem CC3400 conversion coating @ 160.degree. F.
for 3.0-3.5 min
[0065] Stage 6--Hot D.I. Rinse @ 140.degree. F. for 45-60 sec.
[0066] The Sanchem 6500 solution contained approximately 3% nitric
acid in addition to CC6500 surfactant. This was an improvement over
the process of Example 2. However the appearance of product was
still unacceptable. The stainless hangers and baskets were reacting
at a different rate than the aluminum alloys thereby causing voids
at contact points. Oil was still not adequately removed.
EXAMPLE 4
[0067] The silicated alkaline cleaner (stage 1) was changed to a
non-silicated alkaline cleaner. All baskets and racks were changed
from stainless steel to aluminum. The steps were as follows:
[0068] Stage 1--Specialty 740 non-silicated, non-caustic alkaline
cleaner @ 150.degree. F. for 3-7 min.
[0069] Stage 2--D.I. Rinse @ ambient for 45-60 sec.
[0070] Stage 3--De-Ox 30% Nitric Acid w/Potassium Fluoride @
ambient for 15-60 sec.
[0071] Stage 4--Spray rinse w/D.I. @ ambient for 45-60 sec.
[0072] Stage 5--D.I. Rinse @ ambient for 45-60 sec.
[0073] Stage 6--Sanchem CC3400 conversion coating @ 160.degree. F.
for 2.5-3.0 min.
[0074] Stage 7--D.I. Rinse @ ambient for 45-60 sec.
[0075] These changes resulted in a significant improvement in the
quality of the surface of conversion coated products over the
methods, of Example 1-3. The products were consistently finishing
without the splotchy, streaky appearance and the coating was no
longer thickening excessively, thereby eliminating flaking and rub
off of the surface coating. Switching to aluminum hangers and
baskets also eliminated witness marks. However, the bath life of
the chemical solutions was not acceptable. After about 72 hours (or
processing about 76,000 ft.sup.2 of products) streaking re-appeared
and filtration cost increased significantly. The system baths had
to be changed ever 120 hours (or after processing only 127,000
ft.sup.2 of aluminum parts) as product quality was no longer
acceptable.
[0076] The filtration system was then modified to increase its
ability to handle the ionic loading, pump sizes were increased to
improve flow through the recycle system, the efficiency and
filtering capacity of the filters were increased, inert liquid
contacting surfaces (tanks, hangers, baskets, etc.) were installed
and instrumentation was installed to monitor and control the
process, it was discovered that the overall quality of the coated
products was significantly increased, by these changes, the amount
of treatment materials required was reduced and the cost of
operating the conversion process was reduced. By improving the
filtration as set forth herein, monitoring the flow rate and
maintaining resistance of the wash solution at equal to or greater
than about 50 K-ohms, it was discovered that the bath life could be
increased to 256 hours, allowing processing of about 270,352
ft.sup.2 of aluminum before product quality started to degrade. The
wash solutions could still be used for an additional period,
although with decreasing product quality, and had to be discarded
and the treating system baths cleaned out at around 480 hours of
processing time. Also, wash water flow rates were increased to 10
gpm without degrading the quality of the end product. It was also
discovered, because the water processing reduced the corrosive
nature of the wash solution, that CPVC plumbing materials were
adequate for the purpose of transporting process water through the
filtration system. However, PVDF or polypropylene can be used.
[0077] De-ionized water is naturally corrosive. While heating the
rinses was expected to improve the process, because an elevated
temperature would allow the parts to dry more rapidly, it was found
that components retained water in cupped areas, and the remaining
water would remove the finish within a couple of minutes.
Accordingly, it was also unexpectedly discovered that the
elimination of heated rinses significantly improved the product
produced. This improvement was not offset by a slight increase in
drying times which were required in the end stages of the
process.
[0078] It is evident from the foregoing that there are many
additional embodiments of the present invention which, while not
expressly described herein, are within the scope of this invention
and may suggest themselves to one of ordinary skill in the art. For
example, the invention may be further improved by adding an
additional chemical bath including a sealing rinse to add further
corrosion resistance and improve paint and powder coat adhesion or
apply, inline, a functional coating to be used in place of paint or
powder coating. Further, it is not intended that processing of
aluminum incorporating features of the invention be limited to the
chemical compounds specifically identified by trade names.
Alternatives or competitive compositions may be used. However,
critical to the quality of the end product is the washing steps
taken, the extensive cleaning and filtering of the DI wash
solutions, and maintaining the electrical resistance of the wash
solutions.
[0079] It is therefore intended that the invention be limited
solely by the appended claims.
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