Process for treating the surface of aluminum or aluminum alloy

A process for treating the surface of aluminum or aluminum alloy comprising the steps of contacting aluminum or aluminum alloy with hot water or steam to form an aluminum oxide layer thereon and conducting electrolysis using the resulting aluminum or aluminum alloy as an anode by applying direct current in an aqueous solution consisting essentially of a water-soluble salt of at least one oxyacid selected from the group consisting of silicic acid, phosphoric acid, molybdic acid, vanadic acid, permanganic acid, stannic acid and tungstic acid.

This invention relates to a process for treating the surface of aluminum or 
aluminum alloy, more particularly to a process for treating the surface of 
aluminum or aluminum alloy having a boehmite layer formed thereon. 
The surface of aluminum or aluminum alloy is chemically active and 
susceptible to corrosion by acids and alkalis. Accordingly, various 
methods have heretofore been proposed for reducing the activity of the 
surface of aluminum or aluminum alloy to improve the corrosion resistance 
thereof. One of such methods is known as so-called boehmite treatment by 
which aluminum or aluminum alloy is brought into contact with hot water or 
steam containing or not containing ammonia or amines so as to form on the 
surface of aluminum or aluminum alloy an aluminum oxide layer 
predominantly consisting of Al.sub.2 O.sub.3.nH.sub.2 O wherein n is 
usually an integer of 1 to 3. Unlike other methods such as the anodic 
oxidation method wherein an acid such as sulphuric acid is used to form an 
aluminum oxide film, the boehmite treating method which does not employ 
acid is very advantageous for industrial operation since the use of acid 
causes corrosion in the apparatus during the anodic oxidation process, the 
resulting effluent involves pollution problems and removal of pollutant 
requires further treatment which needs high cost. Although the boehmite 
treating method is entirely free of these drawbacks, the thickness of 
aluminum oxide layer formed by this method on aluminum or aluminum alloy 
surface is limited up to about 1.0 .mu., and the aluminum oxide layer is 
not satisfactory in hardness and texture. Thus the boehmite treatment is 
inferior to other methods using acid in its ability to impart excellent 
corrosion resistance to aluminum or aluminum alloy. 
An object of this invention is to eliminate the foregoing drawbacks of the 
conventional boehmite treating method. 
Another object of this invention is to provide a process for treating the 
surface of aluminum or aluminum alloy which improves the corrosion 
resistance of aluminum or aluminum alloy having a boehmite layer formed 
thereon. 
Other objects of this invention will become apparent from the following 
description. 
The objects of this invention can be fulfilled by a process comprising the 
steps of contacting aluminum or aluminum alloy with hot water or steam 
containing or not containing ammonia or amines to form an aluminum oxide 
layer thereon and conducting electrolysis using the resulting aluminum or 
aluminum alloy as the electrode in an aqueous solution of a water-soluble 
salt of at least one oxyacid selected from the group consisting of silicic 
acid, phosphoric acid, molybdic acid, vanadic acid, permanganic acid, 
stannic acid and tungstic acid. 
Our researches have revealed the following results: 
(1) When aluminum or aluminum alloy is subjected to boehmite treatment, 
followed by electrolysis using the resulting aluminum or aluminum alloy as 
the electrode in an aqueous solution of water-soluble salt of at least one 
of the above-specified oxyacids, the oxyacid anions resulting from the 
dissociation of the oxyacid salt in the aqueous solution are adsorbed by 
the surface of the aluminum or aluminum alloy, whereupon they release 
their charges to react with the aluminum oxide layer, thereby forming a 
new layer. 
(2) As compared with the aluminum oxide layer produced only by the boehmite 
treatment, the new layer obtained as above has a considerably larger 
thickness, improved toughness and fine texture and is therefore much more 
resistant to corrosion than the aluminum oxide layer alone. 
(3) In the present invention in which only a water-soluble salt of the 
above-specified oxyacids is used, the salt hydrolyzes to produce a free 
base as well as an acid. Consequently, the solution containing the 
above-specified oxyacid salt but containing no free acid has a pH in the 
range of neutrality to alkalinity and acts effectively to form an 
inorganic composite boehmite-anodized film. The specified steps of this 
invention are essentially distinct from the process disclosed in U.S. Pat. 
No. 2,868,702 or U.S. Pat. No. 2,981,647 wherein aluminum or aluminum 
alloy is subjected to boehmite treatment and then subjected to anodic 
oxidation in an aqueous solution containing boric acid and borate, 
according to which process the addition of an acid makes the solution 
acidic and promotes the formation of alumite film by anodic oxidation. 
According to the present invention, it is essential to conduct electrolysis 
using boehmite layer bearing aluminum or aluminum alloy as an anode by 
applying direct current in an aqueous solution of water-soluble salt of at 
least one oxyacid selected from the group consisting of silicic acid, 
phosphoric acid, permanganic acid, vanadic acid, tungstic acid, molybdic 
acid and stannic acid. The water-soluble oxyacid salts to be used include 
various salts of the above oxyacids with monovalent to trivalent metals, 
ammonia or organic amines. The silicates include orthosilicates, 
metasilicates and disilicates and like polysilicates. Examples thereof are 
sodium orthosilicate, potassium orthosilicate, lithium orthosilicate, 
sodium metasilicate, potassium metasilicate, lithium metasilicate, lithium 
pentasilicate, barium silicate, ammonium silicate, tetramethanol ammonium 
silicate, triethanol ammonium silicate, etc. 
The phosphates include orthophosphates, pyrophosphates and 
polymetaphosphates. Examples are potassium monobasic phosphate (KH.sub.2 
PO.sub.4), sodium pyrophosphate (Na.sub.4 P.sub.2 O.sub.7), sodium 
metaphosphate (NaPO.sub.3), aluminum hydrophosphate [Al(H.sub.2 
PO.sub.4).sub.3 ], etc. The vanadates include orthovanadates, 
metavanadates and pyrovanadates. Examples are lithium orthovanadate 
(Li.sub.3 VO.sub.4), sodium orthovanadate (Na.sub.3 VO.sub.4), lithium 
metavanadate (LiVO.sub.3.2H.sub.2 O), sodium metavanadate (NaVO.sub.3), 
potassium metavanadate (KVO.sub.3), ammonium metavanadate (NH.sub.4 
VO.sub.3) or [(NH.sub.4).sub.4 V.sub.4 O.sub.12 ], sodium pyrovanadate 
(Na.sub.2 V.sub.2 O.sub.7), etc. The tungstates include orthotungstates, 
metatungstates, paratungstates, pentatungstates and heptatungstates. Also 
employable are phosphorus wolframates, borotungstates and like complex 
salts. Examples are lithium tungstate (Li.sub.2 WO.sub.4), sodium 
tungstate (NaWO.sub.4.2H.sub.2 O), potassium tungstate (K.sub.2 WO.sub.4), 
barium tungstate (BaWO.sub.4), calcium tungstate (CaWO.sub.4), strontium 
tungstate (SrWO.sub.4), sodium metatungstate (Na.sub.2 W.sub.4 O.sub.13), 
potassium metatungstate (K.sub.2 W.sub.4 O.sub.13.8H.sub.2 O), sodium 
paratungstate (Na.sub.6 W.sub.7 O.sub.24), ammonium pentatungstate 
[(NH.sub.4).sub.4 W.sub.5 O.sub.17.5H.sub.2 O], ammonium heptatungstate 
[(NH.sub.4).sub.6 W.sub.7 O.sub.24.6H.sub.2 O], sodium phosphowolframate 
(2Na.sub.2 O.P.sub.2 O.sub.5.12WO.sub.3.18H.sub.2 O), barium borotungstate 
(2BaO.B.sub.2 O.sub.3.9WO.sub.3.18H.sub.2 O), etc. Examples of 
permanganates are lithium permanganate (LiMnO.sub.4), sodium permanganate 
(NaMnO.sub.4.3H.sub.2 O), potassium permanganate (KMnO.sub.4), ammonium 
permanganate [(NH.sub.4)MnO.sub.4 ], calcium permanganate 
[Ca(MnO.sub.4).sub.2.4H.sub.2 O], barium permanganate [Ba(MnO.sub.4).sub.2 
], magnesium permanganate [ Mg(MnO.sub.4).sub.2.6H.sub.2 O], strontium 
permanganate [Sr(MnO.sub.4).sub.2.3H.sub.2 O], etc. The stannates include 
orthostannates and metastannates. Examples are potassium orthostannate 
(K.sub.2 SnO.sub.3.3H.sub.2 O), lithium orthostannate (Li.sub.2 
SnO.sub.3.3H.sub.2 O), sodium orthostannate (Na.sub.2 SnO.sub.3.3H.sub.2 
O), magnesium stannate, calcium stannate, lead stannate, ammonium 
stannate, potassium metastannate (K.sub.2 O.5SnO.sub.2.4H.sub.2 O), sodium 
metastannate (Na.sub.2 O.5SnO.sub.2.8H.sub.2 O), etc. Examples of 
molybdates are orthomolybdates, metamolybdates and paramolybdates. More 
specific examples are lithium molybdate (Li.sub.2 MoO.sub.4), sodium 
molybdate (Na.sub.2 MoO.sub.4), potassium molybdate (K.sub.2 MoO.sub.4), 
ammonium molybdate [(NH.sub.4).sub.6 Mo.sub.7 O.sub.24 4H.sub.2 O], 
triethylamine molydate, etc. 
Preferable among these oxyacid salts are those of alkali metals which 
generally have high water solubilities. Among the oxyacid salts enumerated 
above, silicates are preferable to use because they are economical and 
readily available. According to this invention these oxyacid salts are 
used singly or in admixture with one another. 
The concentration of such oxyacid salt in its aqueous solution is usually 
about 0.1% by weight to saturation, preferably about 1.0% by weight to 
saturation, although variable with the kind of the oxyacid salt. 
In the present invention, water-soluble salts of chromic acid can be used 
together with the abovementioned oxyacid salts, whereby the anti-corrosive 
property of the resulting coating is further improved. Such chromate is 
used in an amount of about 0 to 50 weight percent based on the oxyacid 
salt. Examples of the chromates are lithium chromate (Li.sub.2 
CrO.sub.4.2H.sub.2 O), sodium chromate (Na.sub.2 CrO.sub.4.10H.sub.2 O), 
potassium chromate (K.sub.2 CrO.sub.4), ammonium chromate 
[(NH.sub.4).sub.2 CrO.sub.4 ], calcium chromate (CaCrO.sub.4.2H.sub.2 O) 
and strontium chromate (SrCrO.sub.4). 
Alminum alloys to be coated by the process of this invention include, for 
example, Al-Si, Al-Mg, Al-Mn or Al-Si-Mg. In the present invention the 
aluminum and aluminum alloys can usually be used as substrates in various 
shaped forms. 
To practice the present process, the aluminum or aluminum alloy serving as 
a substrate is subjected to degreasing and etching procedures. The 
degreasing is conducted by conventional methods, for examples, by 
immersing the aluminum or aluminum alloy in acid, such as nitric acid, 
sulfuric acid, at room temperature for 5 to 60 minutes. In the etching 
procedure, the defacement and spontaneously formed oxide film are removed 
from the aluminum or aluminum alloy by conventional methods, for example, 
by immersing the aluminum or aluminum alloy in alkali solution. 
The aluminum or aluminum alloy thus pretreated is then subjected to 
boehmite treatment in conventional manner. The boehmite treatment is 
usually conducted by contacting the aluminum or aluminum alloy with hot 
water or steam containing or not containing ammonia or amines. Examples of 
the amines usable are monoethanolamine, diethanolamine, triethanolamine, 
dimethylethanolamine and like water-soluble amines. Generally, about 0.1 
to 5 parts by weight of amine or ammonia are used per 100 parts by weight 
of water. The aluminum or aluminum alloy is kept in contact with hot water 
or steam usually for about 5 to 60 minutes under atomospheric pressure or 
elevated pressure. The temperature of hot water to be used is usually in 
the range of 60.degree. C to boiling point, preferably boiling point and 
that of steam in the range of 100.degree. to 200.degree. C, preferably 
120.degree. to 180.degree. C. Such contact is effected by methods 
heretofore employed, for example, by immersion or spraying. 
After boehmite treatment, electrolysis is conducted as follows: 
The aluminum or aluminum alloy and another electroconductive material used 
as anode and cathode respectively are immersed in aqueous solution of the 
above-specified oxyacid salt, and electric current is applied between the 
electrodes. The electric current may be either direct current or 
alternating current. When direct current is used, the aluminum or aluminum 
alloy is to be the anode and when alternating current is used, the 
aluminum or aluminum alloy can be used either as anode or as cathode. The 
advantageous range for the electric voltage is from 5 to 300 volts for 
direct current, or from 5 to 200 volts for alternating current. The 
electric current is applied for more than 5 seconds. The temperature of 
the electrolytic solution is usually in the range between the separating 
point of the salt of the oxyacid from the solution and the boiling point 
of the solution, preferably in the range of 20.degree. to 60.degree. C. 
According to this invention, the electrolytic operation can be conducted 
repeatedly two or more times with an aqueous solution of the same oxyacid 
salt or with aqueous solutions of different oxyacid salts. For example, 
electrolysis is conducted with an aqueous solution of silicate and then 
with the same aqueous solution of silicate, or first with an aqueous 
solution of silicate and subsequently with an aqueous solution of another 
oxyacid salt. When repeatedly carried out, the electrolysis gives the 
resulting aluminum or aluminum alloy product higher corrosion resistance 
than when it is conducted only once. Moreover, the electrolysis causes 
some water to undergo electrolysis to give off hydrogen gas in the form of 
bubbles. Consequently, the bubbling lowers the efficiency of the 
electrolytic operation. However, if the electrolysis is conducted 
repeatedly, the evolution of hydrogen gas is noticeably reduced as 
compared with the case wherein the electrolytic operation is conducted 
only once, assuring improved efficiency. 
After the electrolysis, the aluminum or aluminum alloy is rinsed with water 
and dried, whereby a thick coating of higher hardness and finer texture is 
formed. According to this invention, the dried product may further be 
heated at a temperature of about 150.degree. to 250.degree. C when desired 
to thereby increase the hardness of the coating. 
The process of this invention will be described below in greater detail 
with reference to examples and comparison examples, in which the 
percentages and parts are all by weight unless otherwise specified. In the 
examples aluminum panels serving as substrates were prepared and 
electrolytic operation was conducted according to the procedures stated 
below. 
Preparation of Substrate 
A substrate was prepared by degreasing and etching an aluminum alloy panel 
measuring 70 mm in width, 150 mm in length and 2 mm in thickness 
(consisting of 98.0% aluminum, 0.45% Si, 0.55% Mg and 1% others; JIS H 
4100) according to the procedure given below: 
(a) Immersion in 10% aqueous solution of nitric acid at room temperature 
for 5 minutes. 
(b) Rinsing in water. 
(c) Immersion in 5% aqueous solution of caustic soda at 50.degree. C for 5 
minutes. 
(d) Rinsing in water. 
(e) Immersion in 10% aqueous solution of nitric acid at room temperature 
for 1 minute. 
(f) Rinsing in water. 
Electrolytic operation 
Into a plastic container measuring 10 cm in width, 20 cm in length and 15 
cm in depth was placed 2,000 cc of an solution of an oxyacid salt and the 
substrate serving as the anode and a mild steel plate serving as the 
cathode were immersed in the solution as spaced apart from each other by 
15 cm. Electrolytic operation was conducted at a liquid temperature of 
25.degree. C by applying a specified voltage. The substrate was thereafter 
washed with water and dried. 
In the examples and comparison examples to follow, acid resistance was 
determined by CASS test according to JIS H 8601. Alkali resistance was 
expressed in terms of time (in seconds) taken for bubbling to occur when 1 
N aqueous solution of caustic soda was applied dropwise to the treated 
sample.

EXAMPLE 1 
An aluminum substrate prepared as described above was immersed in boiling 
deionized water for 5 minutes for boehmite treatment, then rinsed with 
water and subsequently immersed in 20% aqueous solution of sodium silicate 
(Na.sub.2 0.2SiO.sub.2) to conduct electrolysis at the specified voltage 
(d.c.) for the specified period of time as listed in Table 1 below. The 
aluminum substrate was then rinsed with water and dried at room 
temperature. The corrosion resistance of the aluminum substrate thus 
treated was measured with the result given in Table 1. 
EXAMPLES 2 to 4 
Aluminum substrates were treated in the same manner as in Example 1 except 
that electrolysis was conducted at the voltages and for periods of time 
listed in Table 1. The corrosion resistance of each of the aluminum 
substrates thus treated was measured with the result shown in Table 1. 
COMISON EXAMPLE 1 
An aluminum substrate prepared as above was immersed in boiling deionized 
water for 5 minutes for boehmite treatment, followed by rinsing with water 
and drying. The corrosion resistance of the treated substrate is listed in 
Table 1. 
COMISON EXAMPLES 2 and 3 
Aluminum substrates prepared as above were immersed in 20% aqueous solution 
of sodium silicate (Na.sub.2 O.2SiO.sub.2) without conducting boehmite 
treatment, and electrolysis was carried out under the conditions listed in 
Table 1, followed by rinsing with water and drying. The corrosion 
resistance of each of the treated substrates was measured with the result 
given in Table 1. 
EXAMPLES 5 and 6 
After conducting electrolysis in the same manner as in Example 1 except 
that the conditions were otherwise specified as listed below, aluminum 
substrates were rinsed with water and then dried. Subsequently, the 
substrates were further heated to 200.degree. C. The corrosion resistance 
of each of the substrates thus treated is shown in Table 1. 
COMISON EXAMPLE 4 
An aluminum substrate prepared as above was immersed in boiling deionized 
water for 5 minutes for boehmite treatment, followed by rinsing with 
water, drying and then heating at 200.degree. C. The corrosion resistance 
of the treated substrate is listed in Table 1. 
COMISON EXAMPLES 5 and 6 
Aluminum substrates prepared as described above were immersed in 20% 
aqueous solution of sodium silicate (Na.sub.2 O.2SiO.sub.2) to conduct 
electrolysis under the conditions listed in Table 1 below. The aluminum 
substrates were then rinsed with water, dried and then heated at 
200.degree. C for 1 hour. The corrosion resistance of each of the aluminum 
substrates thus treated was measured with the result given in Table 1. 
Table 1 
__________________________________________________________________________ 
Alkali 
Electrolysis conditions 
Acid resistance (Rating Number) 
resistance 
Voltage (V) 
Time (sec) 
4 (hours) 
8 (hours) 
(sec) 
__________________________________________________________________________ 
Example 
1 40 120 9.5 9 95 
2 40 600 9.5 9 110 
3 80 120 9.5 9.5 132 
4 80 600 9.5 9.5 150 
Comparison 
Example 
1 -- -- 8 6 12 
2 40 120 8 6 21 
3 80 600 8 6 25 
Example 
5 40 120 9.5 9 115 
6 80 120 9.8 9.5 160 
Comparison 
Example 
4 -- -- 8 6 11 
5 40 120 8 6 20 
6 80 120 8 7 26 
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EXAMPLES 7 and 8 
Aluminum substrates prepared as described above were immersed in a boiling 
solution consisting of 0.5 part of diethanol amine and 100 parts of 
deionized water for 15 minutes, then rinsed with water and subsequently 
immersed in 20% aqueous solution of potassium silicate (K.sub.2 
O.3SiO.sub.2) to conduct electrolysis under the conditions listed in Table 
2 below. The aluminum substrates were then rinsed with water and dried at 
room temperature. The acid resistance of each of the aluminum substrates 
thus treated was measured with the result given in Table 2. 
COMISON EXAMPLE 7 
An aluminum substrate prepared as above was immersed in a boiling solution 
consisting of 0.5 part of diethanol amine and 100 parts of deionized water 
for 15 minutes, followed by rinsing with water and drying at room 
temperature. The acid resistance of the treated substrate is listed in 
Table 2. 
Table 2 
__________________________________________________________________________ 
Electrolysis conditions 
Acid resistance (Rating Number) 
Voltage (V) 
Time (sec) 
4 (hours) 
8 (hours) 
__________________________________________________________________________ 
Example 
7 40 (d.c.) 
120 9.8 9.5 
8 80 (d.c.) 
120 10 9.5 
Comparison 
Example 
7 -- -- 8.5 8 
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EXAMPLE 9 to 14 
Aluminum substrates were treated in the same manner as in Example 1 except 
that oxyacid salts were used in the amount given in Table 3 in place of 
sodium silicate. The acid resistance of each of the substrates thus 
treated was determined with the result shown in Table 3. 
Table 3 
__________________________________________________________________________ 
Concen- Acid resistance (Rating 
Ex. tration 
Electrolysis conditions 
number) 
No. 
Kind of oxyacid salt 
(%) Voltage (V) 
Time (sec) 
4 (hours) 
8 (hours) 
__________________________________________________________________________ 
9 Sodium metaphosphate 
3.0 60 120 9.5 9.5 
10 Potassium permanganate 
2.0 60 120 9.5 9.5 
11 Ammonium metavanadate 
1.5 60 120 9.5 9.0 
12 Potassium tungstate 
3.0 60 120 9.5 9.0 
13 Potassium molybdate 
3.0 60 120 9.5 9.3 
14 Potassium orthostannate 
2.0 60 120 9.5 9.0 
__________________________________________________________________________ 
EXAMPLE 15 
An aluminum substrate prepared as described previously was immersed in 
boiling deionized water for 10 minutes for boehmite treatment, then rinsed 
with water and subsequently immersed in a solution prepared by adding 10 
parts of 55% aqueous solution of sodium silicate (Na.sub.2 O.2SiO.sub.2) 
and 3 parts of potassium orthomolybdate (K.sub.2 MoO.sub.4) to 100 parts 
of deionized water to conduct electrolysis by applying direct current at 
50 volts for 60 seconds. The substrate was then taken out of the solution, 
rinsed with water and then dried at room temperature. 
EXAMPLE 16 
An aluminum substrate prepared as described previously was immersed in a 
boiling solution of 0.3 part of diethanolamine in 100 parts of deionized 
water for 10 minutes, then rinsed with water and subsequently immersed in 
an aqueous solution prepared by adding 10 parts of 55% aqueous solution of 
sodium silicate and 0.5 part of potassium metavanadate (KVO.sub.3) to 150 
parts of deionized water to conduct electrolysis by applying direct 
current at 30 volts for 3 minutes. The substrate was then taken out of the 
solution and rinsed with water. After drying, the substrate was heated at 
180.degree. C for 30 minutes. 
EXAMPLE 17 
An aluminum substrate subjected to boehmite treatment in the same manner as 
in Example 15 was immersed in a solution prepared by adding 15 parts of 
40% aqueous solution of sodium silicate (Na.sub.2 O.2SiO.sub.2) and 2 
parts of potassium stannate (K.sub.2 SnO.sub.3.3H.sub.2 O) to 150 parts of 
deionized water to conduct electrolysis by applying direct current at 30 
volts for 30 minutes. The substrate was then taken out of the solution and 
rinsed with water. After drying, the substrate was heated at 180.degree. C 
for 30 minutes. 
The acid resistance of each of the treated aluminum substrates obtained in 
Examples 15 to 17 was measured with the result listed in Table 4 below. 
Table 4 
______________________________________ 
Acid resistance (Rating Number) 
4 (hours) 8 (hours) 
______________________________________ 
Example 15 9.5 9 
16 9.5 9 
17 9.5 9 
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EXAMPLE 18 
Aluminum substrates were treated in the same manner as in Example 15 except 
that oxyacid salts indicated in Table 5 were used in place of sodium 
silicate and potassium orthomolybdate. The acid resistance of the 
substrates thus treated was determined with the result shown in Table 5. 
Table 5 
__________________________________________________________________________ 
Concen- 
tration 
(parts per Acid resistance 
Ex. 100 parts 
Electrolysis condition 
(Rating Number) 
No. 
Kind of oxyacid salts 
of water 
Voltage(V) 
Time(sec) 
4(hours) 
8(hours) 
__________________________________________________________________________ 
Ammonium metavanadate 
1.5 
18 60 120 9.5 9.3 
Ammonium tungstate 
1.5 
__________________________________________________________________________ 
EXAMPLE 19 
An aluminum substrate prepared as described previously was immersed in 
boiling deionized water for 10 minutes, then rinsed with water and 
subsequently immersed in 5% aqueous solution of sodium silicate (Na.sub.2 
O.2SiO.sub.2) to conduct electrolysis at 30 volts for 60 seconds. After 
rinsing with water, the substrate was immersed in 3% aqueous solution of 
ammonium paramolybdate [(NH.sub.4).sub.6 Mo.sub.7 O.sub.24 ] to conduct 
electrolysis at 60 volts for 60 seconds. The substrate was then rinsed 
with water, thereafter dried and heated at 160.degree. C for 30 minutes. 
Currents applied for electrolysis were direct current. 
EXAMPLES 20 and 21 
Aluminum substrates were treated in the same manner as in Example 19 except 
that oxyacid salts indicated in Table 6 were used in place of sodium 
silicate and ammonium paramolybdate. The corrosion resistance of each of 
the substrates thus treated in Examples 19 to 21 was determined with the 
result shown in Table 6. 
Table 6 
__________________________________________________________________________ 
Acid resistance 
(Rating Number) 
Ex. 
Kind of oxyacid acid salt 
4 8 Alkali resistance 
No. 
1st Electrolysis 
2nd Electrolysis 
(hours) 
(hours) 
(sec) 
__________________________________________________________________________ 
Sodium silicate 
Ammonium 
19 (Na.sub.2 O.multidot.2SiO.sub.2) 
paramolybdate 
9.8 9.5 120 
[(NH.sub.4).sub.6 Mo.sub.7 O.sub.24 ] 
Calcium Potassium 
20 permanganate 
stannate 10 9.8 360 
[Ca(MnO.sub.4).sub.2 .multidot.4H.sub.2 O] 
(K.sub.2 SnO.sub.3) 
Aluminum Potassium 
21 hydrophosphate 
metatungstate 
9.5 9.5 360 
[Al(H.sub.2 PO.sub.4).sub.3 ] 
(K.sub.2 W.sub.4 O.sub.13) 
__________________________________________________________________________ 
EXAMPLE 22 
An aluminum substrate prepared as described previously was immersed in 
boiling deionized water for 5 minutes for boehmite treatment, then rinsed 
with water and subsequently immersed in 3 wt.% aqueous solution of sodium 
metaphosphate to conduct electrolysis by applying direct current at 80 
volts for 120 seconds. 
COMISON EXAMPLE 8 
An aluminum substrate prepared as described previously was immersed in 
boiling deionized water for 5 minutes for boehmite treatment, then rinsed 
with water and subsequently immersed in an aqueous solution containing 6 
wt.% of phosphoric acid and 1.0 wt.% of sodium metaphosphate to conduct 
electrolysis by applying direct current at 80 volts for 120 seconds. 
COMISON EXAMPLE 9 
An aluminum substrate was treated in the same manner as in Comparison 
Example 8 except that 3 wt.% of sodium metaphosphate was used in place of 
1.0 wt.% of sodium metaphosphate. 
The corrosion resistance of each of the substrates thus treated in Example 
22 and Comparison Examples 8 and 9 was determined with the result shown in 
Table 7. 
Table 7 
______________________________________ 
Acid resistance 
(Rating No.) 
Alkali resistance 
4 hours 
8 hours (sec) 
______________________________________ 
Example 22 9.5 9.5 121 
Comparison 
Example 8 9.0 7.5 40 
9 9.0 7.5 50 
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