Method for removing chromium containing coatings from aluminum substrates

The invention is a process for removing a chromium containing coating from an aluminum article. The chromium containing coating is removed from the aluminum article by contacting the aluminum article having the chromium containing coating with an alkaline treating bath to form an ultrasonically removable chromium containing coating and treating the aluminum article with the ultrasonically removable coating with ultrasonic vibrations to remove the chromium containing coating.

FIELD OF THE INVENTION 
The invention is a method for removing chromium containing conversion 
coatings from aluminum substrates. 
BACKGROUND OF THE INVENTION 
Chromium containing coatings are applied to aluminum substrates to provide 
corrosion protection and to improve the adhesion of organic coatings to 
the aluminum substrate. 
The methods for applying chromium containing coatings to aluminum 
substrates were developed and brought to commercialization in the early 
forties. The early processes are disclosed in publications such as U.S. 
Pat. No. 2,438,877, U.S. Pat. No. 2,494,910, U.S. Pat. No. 2,678,291, and 
U.S. Pat. No. 2,859,147. These processes are still used today as can be 
seen from U.S. Pat. No. 4,668,305 which is an improvement in the earlier 
processes. The earliest processes were concerned with applying a chromium 
phosphate containing coating to an aluminum substrate. 
Another commercial process for applying a chromium containing coating to an 
aluminum substrate was developed in the early 1950's and did not require 
the presence of a phosphate ion in the coating bath. Processes of this 
type are disclosed in U.S. Pat. No. 2,796,370, U.S. Pat. No. 2,796,371, 
U.S. Pat. No. 2,843,513, and U.S. Pat. No. 2,859,144. An improvement in 
these process was patented as late as 1979 as evidence by U.S. Pat. No. 
4,146,410. The disclosures of the chromium phosphate containing coating 
and chromium oxide containing coating process patents are incorporated 
herein by reference. 
All of these processes are known to provide coatings which improve the 
corrosion resistance and improve the organic coating adhesion of aluminum 
substrates. 
RELATED ART 
A chromium containing coating on an aluminum article improves the corrosion 
resistance of the aluminum article. To provide enhanced corrosion 
resistance and organic coating adhesion, the chromium containing coating 
must be applied uniformly to the surface of the aluminum article. However, 
in commercial coating operations due to difficulties with the precleaning 
operation or with the coating operation, the chromium containing coatings 
are not always uniform and the corrosion protection and organic coating 
adhesion is not uniform over the surface of the aluminum article. 
The nonuniformity of the coating which can occur during commercial 
production, provides aluminum articles with improved corrosion resistance 
and improved organic coating adhesion only over a portion of the surface 
of the aluminum article, and the organic coating has a variable adhesion 
to the aluminum substrate. The aluminum articles with the uneven chromium 
containing coating are generally not commercially acceptable. Until this 
time, there was no commercially acceptable method for removing the 
chromium containing coating or the chromium containing coating which had 
been overcoated with an organic coating from the surface of an aluminum 
article. 
It is known that if a chromium containing coating has been recently applied 
(less than 24 hours), the coating can be removed by contact with a nitric 
acid solution containing about 30 to 35% nitric acid. However, if the 
chromium containing coating is applied more than about 24 hours before 
attempted removal, a 30 to 35% nitric acid solution is not capable of 
satisfactorily removing the chromium containing coating from the aluminum 
substrate. Under these circumstances the chromium containing coating can 
be removed from an aluminum substrate by contacting the coated substrate 
with a molten metal salt such sodium nitrite. However, this removal 
process is difficult, expensive, and is relegated to laboratory methods 
for measuring the amount of chromium which is deposited on a substrate by 
the various processes. 
It was also known that the chromium conversion coating or the chromium 
conversion coating overcoated with an organic coating can be removed from 
the aluminum article by aggressive treatment of the coated aluminum 
article with a strong alkali metal hydroxide solution at an elevated 
temperature. However, the combination of a strong alkali metal hydroxide 
solution and an elevated temperature results in substantial attack on the 
aluminum, causing pitting, etching, embrittlement, smutting and staining. 
Aluminum articles treated with strong alkali metal hydroxides at elevated 
temperatures are generally not satisfactory for reprocessing in the 
chromium containing coating operation and subsequent overcoating with an 
organic coating material. In particular, the aggressive treatment of the 
aluminum article results in etching, pitting, embrittlement, smutting and 
staining with dissolution of large amounts of aluminum which is 
undesirable for providing an attractive surface to the aluminum article. 
The inability to recover and recycle unsatisfactorily chromium containing 
coated aluminum articles and chromium containing coated aluminum articles 
with an organic overcoating results in a large expense to a manufacturer 
of aluminum articles which must be corrosion protected and coated with an 
organic coating material. 
BRIEF DESCRIPTION OF THE INVENTION 
According to the present invention a chromium containing coating can be 
removed from an aluminum article by a process which comprises: contacting 
an aluminum article, having a chromium containing coating, with an aqueous 
alkaline treating solution at a pH of at least 10 preferably an alkali 
metal hydroxide or alkali metal salt of a weak acid for a sufficient 
length of time to form an aluminum article with an ultrasonically 
removable chromium containing coating; and treating the aluminum article 
with the ultrasonically removable coating, with ultrasonic vibrations to 
remove the chromium containing coating as particulate matter. 
The process can be carried out in a single zone in which ultrasonic 
vibrations are applied to the coated aluminum article in the zone in which 
the aluminum article with the chromium containing coating is contacted 
with an alkaline treating solution. Alternately, the aluminum article with 
the ultrasonically removable coating is treated in an ultrasonic treating 
zone separate from the alkaline solution treating zone. 
In an alternate embodiment an aluminum article with an ultrasonically 
removable coating can be treated to remove the coating and desmutted in an 
ultrasonic treating zone containing a desmutting solution. 
The process of the present invention is particularly useful in that the 
chromium containing coating need not be dissolved in the reagent and the 
particulate material containing the chromium containing coating can be 
readily recovered by a physical separation method. The process does not 
require large volumes of aqueous solution which require extensive 
treatment before disposal to remove contamination with heavy metals. 
The concentration and pH of the alkaline treating solution is adjusted in 
accordance with the temperature of the solution to provide an 
ultrasonically removable chromium containing coating on the aluminum 
article in a reasonable time without aggressively attacking the aluminum 
article. Higher concentrations of reagents can be utilized at lower 
temperatures to prevent aggressive attack on the aluminum article. 
The method is particularly useful for removing a chromium containing 
coating from delicate or complex parts or assemblies which are not 
suitable for aggressive chemical treatment or rough physical treatment. 
DETAIL DESCRIPTION OF THE INVENTION 
The process of the present invention is particularly useful for removing a 
chromium containing conversion coating or chromium containing conversion 
coating having applied thereto an organic coating, from an aluminum 
article. The process is particularly useful for reclaiming aluminum 
articles whose coating is not commercially useful. That is, a chromium 
containing coating which does not adequately cover the aluminum article to 
provide uniform improved corrosion resistance or uniform improved organic 
coating adhesion. A chromium containing coating can be too thin, or can be 
incomplete and only cover portions of the surface of the aluminum article 
such as islands in a sea. The process of the present invention can also be 
utilized to remove the organic coating and the chromium containing coating 
from aluminum articles which have had a organic coating applied over the 
chromium containing coating which organic coating is not satisfactory. 
The process of the present invention is useful in that a chromium 
containing coating can be removed from the aluminum article but also a 
chromium containing coating which has been coated with an organic coating 
can also be readily removed from an aluminum article. The process of the 
present invention can be utilized to remove chromium containing coatings 
from aluminum articles which chromium containing coatings have been in 
place on the aluminum articles for more than 24 hours. The process of the 
present invention can remove the chromium containing coatings from 
aluminum substrates which coatings have been in place for extended 
periods. 
As used herein the term "chromium containing coatings" refers to chromium 
conversion coatings which can be of the chromium phosphate type, chromium 
oxide type or the variations which are applied according to the chromium 
conversion coating art. The term "chromium containing coatings" also 
encompasses chromium conversion coatings which have been overcoated with 
an organic coating. 
As is well understood in the art, chromium conversion coatings are 
extremely difficult to remove from substrates, particularly aluminum 
substrates, to which they have been applied without undue damage to the 
aluminum substrate. 
The first step of the process of the present invention is contacting the 
aluminum article, which is coated with the chromium containing coating to 
be removed, with an alkaline treating solution at a pH above about 10. The 
temperature of the solution is generally in the range from about 
15.degree. C. to about 90.degree. C., preferably 20.degree. C. to about 
80.degree. C. and more preferably 25.degree. C. to about 80.degree. C. The 
temperature and the concentration of the solution are adjusted to achieve 
formation of an ultrasonically removable coating in a reasonable time. The 
concentration and temperature of the solution must be in a range that the 
solution does not aggressively attack the aluminum article to cause 
pitting, etching, embrittlement, smutting or staining. 
The alkaline treating solution preferably comprises an alkali metal 
hydroxide and/or an alkali metal salt of a weak acid. The concentration of 
the alkaline material in the solution is generally from about 0.2% to 
about 20% by weight, preferably from about 0.2% to about 10% and most 
preferably from about 0.5% to about 7%. The pH of the solution is greater 
than about 10 preferably above about 11 and more preferably above about 
12. To use a solution with a low concentration of an alkali metal 
hydroxide or alkali metal salt of a weak acid of about 0.2% to about 2% by 
weight, the temperature of the solution should be in the upper portion of 
the useful temperature range to achieve formation of an ultrasonically 
removable coating in a reasonable time period. The preferred alkali metal 
hydroxides are sodium hydroxide and potassium hydroxide. As used herein an 
alkali metal salt of a weak acid refers to alkali metal carbonates, 
phosphates, pyrophosphate, borates, acetates, propionates and the like. 
The preferred alkaline treating solutions useful in the practice of the 
invention are solutions containing alkali metal hydroxides, alkali metal 
salts of weak acids which provide a sufficiently high pH to provide the 
ultrasonically removable coating when the chromium conversion coated 
aluminum article is contacted with the solution. However, other alkaline 
materials such as amines and quaternary ammonium compounds can also be 
used in the process to form the alkaline treating solution as long as they 
can provide a suitable pH and do not produce undesirable or excessive 
pitting and etching of the aluminum surface. Preferably the alkaline 
treating solution does not solubilize the chromium containing coating. 
Sodium silicate solutions can be used to loosen the chromium containing 
coating but is not preferred since it solubilizes the chromium containing 
coating. 
The alkali metal hydroxide or salt of a weak acid solution can also contain 
agents for buffering the pH of the solution. Surfactants which are stable 
in alkaline solutions can be present to increase the wetting ability of 
the solution to permit more rapid penetration of an organic coating and 
attack on the bond between the chromium containing coating and the 
aluminum substrate to more rapidly form the ultrasonically removable 
coating. The preferred alkali metal hydroxide or alkali metal salt of a 
weak acid treating solution can also contain chelating agents to aid in 
sequestering contaminating metal ions which could make the treatment 
ineffective and require frequent changes of the solutions in the treating 
bath. 
The aluminum substrate with the chromium containing coating is generally 
contacted with the alkaline treating solution for from about 15 seconds to 
about 30 minutes. The longer treatments are required when the solution is 
of a low concentration and the pH and temperature are relatively low. The 
concentration and temperature of the alkaline treating solution can be 
adjusted to achieve rapid formation of the ultrasonically removable 
chromium containing coating in a reasonable time period without aggressive 
attack on the aluminum article with extensive etching, pitting, 
embrittlement, staining and smutting. 
The soluble chromium ion content of the alkaline treating solution remains 
low throughout the life of the solution since the chromium containing 
coating is preferably not soluble in the alkaline treating solution at the 
concentration and temperature utilized in the process. 
As the alkaline treating solution is contacted with the aluminum articles, 
the amount of soluble aluminum in the solution increases. It is believed 
that the increase in the aluminum in solution is due to a reaction of an 
aluminum containing interface composition between the chromium containing 
coating and the aluminum substrate. Although the concentration of aluminum 
in the alkaline treating solution increases during use, the aluminum 
articles do not show any extensive etching, pitting, embrittlement, 
smutting or staining due to contact with the solution. 
The aluminum article with the chromium containing coating can be contacted 
with the alkaline treating solution by dipping, spraying, flowing, 
brushing or any other means for contacting a solid with a liquid. 
Preferably the aluminum article is contacted with the alkaline solution by 
dipping or spraying. 
Any surfactant present in the alkaline treating solution must be compatible 
with and stable in the solution. Anionic, amphoteric, ampholytic and 
nonionic surfactants are generally useful for inclusion in the alkaline 
treating solution. Surfactants such as organic phosphate and phosphonate 
anionic surfactants, fluorinated anionic surfactants, organic sulfonate 
and sulfate surfactants can also be used provided they are stable and 
compatible with the alkaline solution. Amphoteric and ampholytic 
surfactants can also be useful in the practice of the invention. Nonionic 
surfactants can also be useful in the practice of the invention. Any 
surfactant which is stable in the alkaline solution and is not harmful to 
the aluminum substrate can be used in the practice of the invention. It is 
preferred that the surfactant be a low foaming or moderate foaming 
surfactant. As used herein a low foaming surfactant has a foam height of 
60/30 mm and a moderate foaming surfactant has a foam height of 100/60 mm 
according to the Shaking Test Method. 
Shaking Test Method 
A glass stoppered 250 milliliter measuring cylinder, about 30 millimeters 
in diameter, is filled to the 150 milliliter mark with a 0.1% by weight 
solution of the surfactant in water at room temperature. The measuring 
cylinder is stoppered then vigorously shaken for 30 seconds. The foam 
height immediately after shaking is measured, and the foam height is 
measured again 30 seconds after shaking is completed. A foam height of 60 
millimeters or less after shaking or 30 millimeters or less 30 seconds 
after shaking is considered a low foaming surfactant. A foam height of 
60-100 millimeters immediately after shaking or 30-60 millimeters 30 
seconds after shaking is considered a moderate foaming surfactant. 
Anionic surfactants such as TRITON.RTM.DF-20, TRITON.RTM.DF- 16, 
TRITON.RTM.H-66, TRITON.RTM.QS-44, MONATROPE 1296, and RHODAFAC.RTM.RM 710 
can be useful. Amphoteric surfactants such as MONATERIC.RTM.LF-100, 
MONATERIC.RTM.CEM-38%, and MONATERIC.RTM.CyNa 50% can be useful. Nonionic 
surfactants such as PLURONIC.RTM.L-61, TRITON.RTM.X-102, TRITON.RTM.X-100 
and PLURAFAC.RTM.D-25 can be used. Surfactants such as 
alkylpolyglycosides, alkylethoxylates, fatty alcohol ethoxylates, fatty 
acid amidoethoxylates and the like can be useful in the practice of the 
invention. 
The addition of the surfactants to the alkaline treating solution aids in 
penetration of any organic coating which is formed over the chromium 
containing coating and in penetration of the chromium containing coating. 
The addition of the surfactants aids in reducing the required contact time 
between chromium containing coated aluminum article and the alkaline 
treating solution and in addition assures that the ultrasonically 
removable coating has been formed more evenly over the surface in a 
shorter time. 
The alkaline treating solution can also contain metal chelating agents 
which can reduce contamination of the treating solution with unwanted 
metal precipitates. The chelating agent should be stable in the alkaline 
solution and in addition be able to chelate the ions of the metal which is 
required to be chelated or sequestered. Chelating or sequestering 
materials such as sodium gluconate, polyphosphates, pyrophosphates, 
phosphate esters, NTA and EDTA can be useful in the practice of the 
present invention. The nature of chelating agent or sequestering agent is 
not critical as long as it is stable in the alkaline solution, does not 
interfere with operation of the alkaline treating solution and adequately 
sequesters the desired metal ions. The chelating or sequestering agents 
are sometimes necessary when treating aluminum articles which are alloyed 
with certain metals which tend to collect in the alkaline treating 
solution. If the metals are not soluble in the alkaline treating solution, 
the metals need not be chelated or sequestered but can be eliminated from 
the treating bath by means known for separating solid particulate material 
from aqueous solutions. 
After the aluminum article coated with the chromium containing coating 
and/or organic coating over the chromium containing coating is treated to 
form the ultrasonically removable coating, the aluminum article is then 
passed to an ultrasonic treating zone. Optionally, the treated article can 
be rinsed before it is subjected to the ultrasonic vibrations in the 
ultrasonic treating zone. 
The ultrasonic treating zone comprises means for applying ultrasonic 
vibrations, to the aluminum article with the ultrasonically removable 
coating. The ultrasonic vibrations are generally applied at a frequency in 
the range of from about 20 kilohertz to about 100 kilohertz. Preferably 
from about 20 kilohertz to about 50 kilohertz. The most effective 
frequency range can be determined by subjecting articles, with an 
ultrasonically removable coating, to ultrasonic vibrations of different 
frequencies and observing the frequency or frequency range which is most 
effective for removing the coating. 
In an alternative embodiment, the aluminum article with the chromium 
containing coating can be contacted with the alkaline treating solution 
and simultaneously subjected to ultrasonic vibrations in a single treating 
zone. 
The ultrasonic treatment comprises subjecting the aluminum article with the 
ultrasonically removable coating to ultrasonic vibrations. Preferably, the 
aluminum article is immersed in an aqueous bath and subjected to the 
ultrasonic vibrations while it is submerged in the bath. Preferably, the 
aqueous bath comprises only water. However, minor amounts of surfactants 
and sequestering agents can be included in the ultrasonic treating bath. 
Preferably, the ultrasonic treating bath is a water bath and the chromium 
containing coating is removed from the aluminum article by the ultrasonic 
vibrations as fine particulate matter. The chromium containing coating is 
not soluble in the aqueous treating bath and can be readily filtered from 
the treating bath and recovered or disposed of as required. 
In another preferred embodiment the aqueous ultrasonic treating bath 
contains sufficient alkaline material to comprise the alkaline treating 
solution and formation of the ultrasonically removable coating and removal 
of the coating occurs in one treating zone. If the initial treatment with 
the alkaline treating solution to form the ultrasonically removable 
chromium containing coating, forms a smut on the surface of the aluminum 
article (generally alloys of aluminum containing metals such as copper, 
manganese, chromium and zinc), the article can be further treated by known 
deoxidizers or chromium containing baths to remove the smut. However, a 
major portion of the smut is removed by the ultrasonic treatment. 
In an optional embodiment of the invention, the ultrasonic treating bath 
comprises an aqueous smut removing solution. These solutions are 
well-known in the art and comprise a combination of acids and deoxidizers 
such as nitric, sulfuric, phosphoric and hydrofluoric acid with nitrites, 
persulfates, hydrogen peroxide and chromates. The combination of removing 
the chromium containing coating and desmutting in one treating zone is 
advantageous. The use of a desmutting solution as the liquid in the 
ultrasonic treating zone is an advance in the art since the aluminum 
article is removed from the process and after rinsing is ready for 
application of a chromium containing coating and optionally overcoating 
with an organic coating material. 
The easy removal of the chromium containing coating by the process of the 
invention is unexpected since the coating before alkaline solution 
treatment is resistant to removal by ultrasonic vibration. The 
ultrasonically removable coating cannot be adequately removed by 
immersion, rinsing, spraying or the like alone. The ultrasonic vibration 
treatment is necessary to remove all of the coating. The process is 
particularly useful for recycling delicate parts or assemblies which have 
been coated with a chromium containing conversion coating. Contact of the 
aluminum article with a dilute acid solution does not provide an 
ultrasonically removable coating. 
The advantages of the process of the present invention will become 
immediately apparent from the following examples.

EXAMPLE 1 
An aluminum heat exchanger assembly for an air conditioning system having a 
tube of 3003 aluminum alloy, fins of 3003 aluminum alloy and brazing parts 
of 4004 and 4015 aluminum alloy, was cleaned with a standard industrial 
aluminum cleaner, rinsed with tap water, and coated with a chromium oxide 
type conversion coating using a conversion coating solution of 
BONDERITE.RTM.713 (a product of Parker + Amchem). The chromium oxide 
conversion coating was applied according to the manufacturer's 
instruction. The chromium oxide conversion coating on the aluminum article 
was in the range of about 200 mg/ft.sup.2. 
The chromium oxide conversion coated aluminum article was rinsed, dried, 
then coated with a dispersion of a nylon polymer containing hydrophilic 
substituents to form a hydrophilic coating over the aluminum article. The 
coated aluminum article was aged for several days. 
The chromium containing coating was removed by immersing the heat exchanger 
assembly in 5% sodium hydroxide solution at ambient temperature for 2 
minutes, then immersing the article in an ultrasonic treating zone of 2 
liters volume containing tap water for 5 minutes. The ultrasonic vibration 
in the treating zone was at 25 kilohertz during the treatment at a power 
level of 40 watts. 
The chromium containing coating and the hydrophilic coating fell off the 
heat exchanger assembly in the ultrasonic bath and formed a readily 
filterable powder. 
EXAMPLE 2 
A heat exchanger assembly as in Example 1 was prepared as in Example 1 with 
a hydrophilic nylon coating over a chromium containing coating. An alkaline 
solution was prepared which contained 
______________________________________ 
Ingredients % by weight 
______________________________________ 
Sodium gluconate 0.07 
Tetra Potassium Pyrophosphate 
1.5 
Potassium hydroxide 1.5 
TRITON .RTM. DF-20 (anionic surfactant) 
0.22 
PLURONIC .RTM. L-61 (nonionic surfactant) 
0.07 
Water 96.64 
______________________________________ 
The pH of the solution was 12.9. The heat exchanger assembly was contacted 
with the alkaline solution by submerging the heat exchanger assembly in 
the solution with agitation for 5 minutes at 30.degree. C. The alkaline 
treating bath had a volume of 60 liters. The article was removed from the 
alkaline treating bath, rinsed and placed in the ultrasonic treating zone. 
The ultrasonic treating zone had a volume of 80 liters, was filled with tap 
water and the ultrasonic vibration was at 25 kilohertz at 1260 watts. After 
10 minutes all of the coating fell from the heat exchange assembly as a 
fine powder. 
After 600 ft.sup.2 of aluminum, which had been prepared as above, were 
treated by the process, the alkaline treating solution and the tap water 
in the ultrasonic treating zone were analyzed for their chromium and 
aluminum content. The results of the analysis are shown in Table 1. 
TABLE 1 
______________________________________ 
Metal Ions Concentration ppm 
______________________________________ 
Alkaline Solution 
Cr (Total) 30 
Cr (Soluble-filtered solution) 
2 
Al (Total) 750 
Al (Soluble-filtered solution) 
710 
Water in Ultrasonic Treating Zone 
Cr (Total) 28 
Cr (Soluble-filtered solution) 
&lt;1 
Al (Total) 7 
Al (Soluble-filtered solution) 
&lt;2 
______________________________________ 
The above results clearly show that the major portion of the chromium is 
not soluble in either the alkali treating solution or in the aqueous 
solution in the ultrasonic coating removal zone. This makes the process of 
the present invention particularly advantageous in that the chromium can be 
removed from the solutions involved in the process as a solid which 
substantially reduces the disposal problems for the treating solutions. In 
addition, the treating solutions can be utilized for longer periods of time 
before they need be discarded due to the high solid content in the 
solution. 
EXAMPLE 3 
Aluminum Panels were prepared from Aluminum Alloy 3003 (Mn), 5052 (Mg), 
2024 (Cu), 7075 (Zn) and 6061 (Mg+Si) (Major alloying elements shown in 
parenthesis adjacent the Alloy number). One panel of each alloy was 
treated with a chromium phosphate type conversion solution 
(BONDERITE.RTM.407-47 product of Parker + Amchem) according to 
manufacturers instructions. One panel of each alloy was treated with a 
chromium oxide type conversion coating solution (BONDERITE.RTM.713 a 
product of Parker + Amchem) according to manufacturers instructions. The 
coatings on the panels were in the range of 200 mg per square foot. After 
the conversion coating treatment, the panels were rinsed, dried and aged 
for more than 24 hours. 
Each panel was immersed in the agitated alkaline solution of Example 2 for 
2 minutes at 30.degree. C., water rinsed then immersed in an ultrasonic 
treating zone containing tap water at 38.degree. C. for 10 minutes. The 
ultrasonic treating zone had a volume of 2 liters and was operated at a 
frequency of 25 kilohertz and power of 40 watts. 
Both the chromium phosphate type conversion coating and the chromium oxide 
type coating were completely removed from the panels by the treatment. 
Alloy 2024 generated a substantial amount of black smut on its surface. 
About 80% of the black smut on the surface of alloy 2024 was removed in 
the ultrasonic treating zone. Small amounts of smut which formed on the 
surface of the other alloys was completely removed in the ultrasonic 
treating zone. 
EXAMPLE 4 
The 2 liter ultrasonic treating zone was filled with the alkaline treating 
solution of Example 2. Ten aluminum alloy panels of the same alloys as in 
Example 3 were coated with the chromium phosphate type and chromium oxide 
type conversion coatings as in Example 3. The coatings were rinsed, dried 
and aged for more than 24 hours as in Example 3. The coating was about 200 
mg. per square foot. 
Each coated panel was immersed in the alkaline solution in the ultrasonic 
treating zone for 5 minutes at 38.degree. C. After the ultrasonic 
treatment the panels were rinsed and dried. 
The conversion coatings were completely stripped from the 3003, 5052, 7075 
and 6061 alloys. The 7075 alloy showed significant smutting and etching 
which indicated that a less concentrated solution, lower temperature or 
shorter treating time is required. 
The 2024 alloy had 80% of the chromium oxide type conversion coating 
removed and 60% of the chromium phosphate type coating removed by the 
treatment which indicated that a longer treatment time, higher temperature 
or more concentrated solution is required to remove 100% of the coating. 
The treating bath was operated at a frequency of 25 kilohertz and the power 
was 40 watts. 
EXAMPLE 5 
Ten aluminum alloy panels coated and aged as in Example 3 were immersed in 
an agitated 5% by weight sodium carbonate solution at 50.degree. C. for 5 
minutes; followed by a warm water rinse and a ten minute immersion in an 
ultrasonic treating zone of 2 liters capacity (tap water) at 38.degree. C. 
The ultrasonic frequency was 25 kilohertz and the power 40 watts. 
The chromium phosphate type coating was completely removed from alloys 
7075, 3003, and 6061, 85% removed from alloy 5052 and 20% from alloy 2024. 
The chromium oxide type coating was completely removed from the 7075 alloy. 
Fifty to 95% of the chromium oxide type coating remained on the other 
alloys. 
EXAMPLE 6 
Alloy test panels were prepared as in Example 3. The prepared test panels 
were immersed in the alkaline treating solution of Example 2 for 30 
seconds at 30.degree. C., rinsed and immersed in tap water in a 2 liter 
ultrasonic treating zone for 10 minutes at 38.degree. C. The ultrasonic 
treating zone was operated at 25 kilohertz and 40 watts. 
The chromium phosphate type coating was completely removed from alloys 
3.003, 6061 and 7075 and the chromium oxide type coating was completely 
removed form alloy 6061 and 7075. 
The coating removal from the other alloy panels ranged from 5 to 70%. 
EXAMPLE 7 
Example 6 was repeated with the addition of oxidizing agents to the 
alkaline treating solution. In separate tests, 0.2% by weight of sodium 
chlorate, hydrogen peroxide, or ammonium persulfate was added to the 
alkaline treating solution. The oxidizing agents did not significantly 
affect the removal of the coatings. 
EXAMPLE 8 
The aluminum alloy panels of the same types as in Example 3 were coated as 
in Example 3. Treating solutions of 5% by weight sulfuric acid, 5% by 
weight nitric acid and a solution of 5% by weight sulfuric acid and 0.25% 
by weight hydrofluoric acid were prepared. 
The acid solutions were evaluated for removing the coatings from the 
aluminum alloy panels. Panels treated by immersion in the 5% sulfuric acid 
solution or in the 5% nitric acid solutions for 5 minutes at 30.degree. C. 
retained more than 95% of the coating after the ultrasonic treatment for 
10 minutes at 38.degree. C. at a frequency of 25 kilohertz and 40 watts in 
a 2 liter ultrasonic treating zone. 
The panels treated for 5 minutes at 30.degree. C. with the solution of 
sulfuric and hydrofluoric acid had 40% to 90% of the coating removed in 
the ultrasonic treating zone at 25 kilohertz and 40 watts. However, the 
aluminum surface was severely etched and pitted. The ultrasonic treatment 
did not significantly assist in removing the coating. 
EXAMPLE 9 
Aluminum alloy panels prepared as in Example 3 were treated in the 2 liter 
ultrasonic treating zone (tap water at 38.degree. C.) for 30 minutes at 25 
kilohertz and 40 watts without previous contact with an alkaline or acid 
solution. 
The ultrasonic treatment did not remove any of the chromium phosphate type 
or chromium oxide type conversion coatings from any of the alloy panels. 
EXAMPLE 10 
Ten aluminum alloy panels as in Example 3 were coated, rinsed and dried and 
aged as in Example 3. The panels were immersed in a 5% by weight sodium 
silicate solution (Na.sub.2 O/SiO.sub.2 ratio 1:1) for 10 minutes at 
50.degree. C. followed by a warm water rinse and treatment in a 2 liter 
ultrasonic treating zone in tap water at 38.degree. C. The ultrasonic 
frequency was 25 kilohertz at 40 watts. 
Less than 10% of the chromium phosphate type conversion coating was removed 
from any of the aluminum alloy panels. 
About 80% to 90% of the chromium oxide type conversion coating was removed 
from all of the alloy panels. The chromium oxide type conversion coated 
panels were badly etched and smutted. The chromium oxide type coating 
dissolved in the sodium silicate solution. 
The above examples clearly show the effect of the process of the invention 
on the broad range of aluminum alloys with different types of chromium 
containing conversion coatings. The optimum condition for removing a 
coating from a particular aluminum alloy can be readily determined by 
modification of the above examples.