US3418218A - Sealing of aluminum surfaces - Google Patents

Description

United States Patent Oflice 3,418,218 Patented Dec. 24, 1968 3,418,218 SEALING F ALUMINUM SURFACES Gert G. Levy, Detroit, Mich., assignor to Chrysler Corporation, Highland Park, Mich., a corporation of Delaware No Drawing. Filed Mar. 11, 1966, Ser. No. 533,410 3 Claims. (Cl. 204--35) ABSTRACT OF THE DISCLOSURE A process for sealing aluminum oxide coatings which comprises contacting the coating with an aqueous solution of a polyoxyalkylene glycol. This solution should have a minimum temperature of about 175 F. and contain at least about 0.1 weight percent of a polyoxyalkylene glycol having a molecular weight in the range of about 100 to 20,000.

This invention relates to the treatment of metallic surfaces to produce thereon coatings which have improved resistance to corrosion and discoloration. More specifical- 1y, it relates to sealing an inorganic oxide protective coating which is formed on a metallic surface by a chemical reaction therewith. In particular, this invention pertains to a composition for sealing anodized aluminum surfaces.

It has long been known that the resistance of aluminum to abrasion, corrosion, chemical action, discoloration and the like, can be increased by forming an artificial oxide coating or film on the surface thereof. Such a protective oxide coating is usually formed by anodic oxidation, generally referred to as anodizing wherein the aluminum is made the anode in an electrolytic acid solution, or by treating the aluminum surface with an alkaline composition such as an aqueous solution of sodium carbonate. However, while the oxide coating as formed is harder and more resistant to abrasion than the metal on which it is formed, it still does not provide significant stain and corrosion protection since the coating is inherently porous and absorbent. Accordingly, it is customary to attempt to close or seal the pores of the oxide film and thereby provide a coating having good corrosion and stain resistance. Thus, one of the best knOWn and widely used aluminum oxide sealing techniques entails contacting the oxide coating with hot water. The success of this approach apparently stems from the fact that the water reacts with the anodic film at a temperature of above about 167 F. to form a non-porous coating of Boehmite (Al O .H O) which is corrosion and stain resistant. This same hot water technique is also used in sealing dyed anodized aluminum which is produced by first immersing the anodized surface in a hot aqueous dye solution of the desired color. The dye enters the pores of the anodic coating and is retained therein when the pores are sealed.

Another technique used to seal anodic coatings involves precipitating a material such as nickel hydroxide on the porous coating. However, this technique is frequently undesirable since the precipitate usually appears as a smut on the anodic coating. Etforts to overcome the smut problem have not been entirely successful since most antismut additives are costly and frequently reduce the corrosion resistance of the anodic film.

Although it is known that the corrosion resistance of anodized aluminum is primarily determined by the effectiveness of the sealing process, it will be recognized that the most widely used sealing techniques as mentioned above are simply attempts to close or plug the pores of the anodic coating. Likewise, the efiectiveness of such techniques is ditficult to control since good corrosion resistance cannot be attained if even a small percent of the pores remain unsealed. Furthermore, it is essentially impossible at any time during the sealing process to determine whether an anodic coating is properly sealed. Moreover, the quality of the seal is affected, as for example in hot water sealing, by such variables as the pH and purity of the sealing water, time of sealing and thickness of the anodic coating. Thus, low temperature, pH or time may result in an inadequate seal while high pH or sealing time may result in a chalky appearance. Accordingly, it is diflicult to consistently produce anodic coatings having good corrosion resistance by the conventional hot water sealing technique. This is especially true in high volume shop production where constant monitoring and control of numerous critical factors are ditficult to achieve.

Another problem encountered with anodized aluminum is that the anodic coating itself is dull and interfers with the attainment of a polished or chrome-like appearance in the end item. Therefore, the thickness of an anodic coating applied to decorative parts is generally less than 0.4 mil. However, experience has shown that such thin coatings frequently do not provide the desired degree of protection to parts such as automobile trim which are subject to severe corrosion conditions. Such failure, however, is due primarily to inadequate sealing rather than inadequate coating thickness.

Accordingly, it is an object of this invention to improve the corrosion resistance of anodically formed aluminum oxide coatings.

A further object is to provide a composition which will impart increased corrosion resistance to anodic aluminum oxide coatings.

A still further object is to provide an improved process for sealing anodized aluminum which is easy to employ, inexpensive and which produces an anodic coating having excellent corrosion and stain resistance.

In accordance with the present invention, the above objects and other objects and features of the invention which will be apparent from the following detailed description thereof are accomplished by sealing an aluminum anodic coating with a composition which contains a corrosion inhibiting amount of a polyoxyalkylene glycol.

The sealing composition of this invention comprises a solution of water and a polyoxyalkylene glycol. The polyoxyalkylene glycol should have a molecular weight of at least about and the solution should contain a minimum of about 0.1 weight percent of the polyoxyalkylene glycol based on the total weight of the solution.

The term polyoxyalkylene glycol as. used throughout the present specification and claims refers to polyether compounds having the formula: HO(RO) H, in which R stands for an alkylene radial such as methylene, ethylene, propylene, etc., and n is an integer. It is to be understood that not all of the alkylene radicals present need be the same as for example the glycol derived from dioxolane having the formula HO(CH OC H O),,H, and that the alkylene radicals can be straight or branched chain. Any polyoxyalkylene glycol can be used in the sealing composition of this invention provided it is sufiiciently soluble to form an aqueous solution containing at least about 0.1 weight percent of the glycol. However, due to its availability and low cost, the preferred glycol for use in this invention is polyoxyethylene glycol. Other suitable polyoxyalkylene glycols include polyoxypropylene glycol, polyoxybutylene glycol and mixed polyoxyethylene-polyoxypropylene glycols such as those having the structure HO{C H O) (C H O) (C H O-} H wherein b has a value such that the molecular weight of the oxypropylene group is at least 900 and a and c are integers each having a value such that the combined molecular weight of the two oxyethylene groups constitutes from about to 90 Weight percent of the total Weight of the compound. Such block polymers are commercially available under the trademark Pluronic. In addition, block polymers, such as are disclosed in U.S. 3,022,335, having blocks composed of a mixture of propylene oxide and ethylene oxide instead of a single oxide as shown in the above formula can also be used.

The aqeuous sealing composition of this invention should contain at least 0.1 weight percent of the polyoxyalkylene glycol in order to obtain an anodic coating having good corrosion resistance. The preferred amount of glycol is in the range of about 1 to 10 weight percent. Concentrations of glycol greater than about 10 weight percent can be used, however, such concentrations do not appear to increase the corrosion resistance of the anodic coating.

It has been found that good sealing will be achieved if the glycol compound has a molecular weight of at least about 100. Surprisingly, however, the upper limit for the molecular weight is determined only by the requirement of solubility of the glycol in water, and good results have been obtained with polyoxyalkylene glycols having molecular weights of about 20,000. The preferred molecular weight range is from about 300 to 12,000.

The sealing composition of this invention is employed in a process in which the aqueous polyoxyalkylene solution is brought into contact with the anodic coating to be sealed. The temperature of the solution should be in the range of about 175 F. to the boiling point of the solution. Care should be taken to prevent the temperature from falling below this range since water reacts with the anodic film at temperatures less than 175 F. to form an undesirable loose white substance called Bayerite. Similarly, the most complete sealing will be obtained if the pH of the sealing bath or solution is in the range where 1 Boehmite formation occurs, namely a pH of 4.0 to 8.5. The preferred pH range is about 5 to 7 and excellent results have been obtained when the anodized aluminum surface to be sealed is dipped or immersed in the sealing solution of this invention for a period in the range of about 5 to 25 minutes.

It is generally preferable to use distilled or demineralized water in the sealing solution of this invention since certain ions or combinations of ions interfere with proper sealing. Thus, it is known that magnesium and calcium salts such as the chloride and sulfate are not particularly harmful in and of themselves. However, such salts in the presence of heavy metal ions result in poor sealing. Likewise, copper sulfate and fluoride appear to cause very poor sealing even when present in concentrations as low as parts per million. Accordingly, due to the variations which occur in the chemical make-up of water obtained from even a single source, and in light of the generally unpredictability of ion combinations which will interfere with proper sealing, it is desirable to use deionized water.

The examples which follow are set forth to more clearly illustrate the principle and practice of this invention and to demonstrate the excellent corrosion and stain resistance of anodic aluminum coatings sealed in accordance with this invention.

The aluminum panels which were used in the tests described in the following examples were made of aluminum supplied by Kaiser Aluminum and Chemical Sales Inc. which conformed to the requirements of the aluminum alloy specification SAE AA 5252. The composition of this aluminum was:

Silicon percent 0.04 Copper do 0.06 Magnesium do 2.30 Iron do 0.05 Manganese do 0.07 Nickel do 0.03 Zinc do 0.03 Titanium do 0.01 Chromium do 0.01 Aluminum Remainder Test panels 3" x 6" were cut from the above metal, stamped for identification and buffed to remove any scratches. The panels were then cleaned and electrolytically anodized as follows:

(1) Cleaned in a vapor degreaser of trichloroethylene to remove bufiing compound.

(2) Cleaned in an alkaline cleaner at a concentration of 6 ounces per gallon for 5 minutes at 140 F. An example of a suitable alkaline cleaner is one composed of 40 grams per liter of sodium carbonate, 20 grams per liter of sodium phosphate, 5 grams per liter of sodium metasilicate, balance water.

(3) Rinsed in warm water (120 F.) and then in cold water.

(4) Immersed for 15 seconds in a room temperature, by weight nitric acid solution.

(5) Rinsed in warm water (120 F.) and then in cold water.

(6) Immersed for two minutes in an aqueous solution containing 1350 grams per liter of phosphoric acid and 50 grams per liter of nitric acid. This solution had a specific gravity between 1.660 and 1.700 and was maintained at a temperature of 220 F.

(7) Immersed in a room temperature, 50% by weight nitric acid solution for 15 seconds.

(8) Cold water rinsed and again immersed in a room temperature, 50% by weight nitric acid solution for 15 seconds.

(9) Rinsed in warm water (120 F.) and then in cold water.

(10) Anodized in a 16.1 weight percent solution of sulfuric acid at F. for 20 minutes at 12 volts DC.

(11) Rinsed in cold water.

(12) Immersed for 1 minute in a room temperature, 5% by weight sodium-bicarbonate solution.

(13) Rinsed in cold water.

After cleaning and anodizing according to the above set forth procedure, all of the test panels had an anodic oxide coating of a thickness in the range of 0.29 to 0.32 mil.

Since at the present time, there is no single accelerated corrosion test for anodized aluminum which is accepted by aluminum producers, processors and users, each of the test panels which were sealed as described in the following examples were subjected to four distinct corrosion tests in order to fully evaluate the effectiveness of the seal. The four tests which were used are set forth by the American Society for Testing Materials (A.S.T.M.) and were the (l) Corrodkote TestASTM Standard B-380; (2) Salt Spray Test 250 hoursASTM Standard B-117; (3) Cass Test 6 hoursASTM Standard B 36862T; and (4) Humidity Test 250 hours. Each of the above first three tests are well known and fully described in the appropriate ASTM Standard and, hence, no description of these tests is needed. The fourth test or Humidity Test was conducted by placing the sealed test panels in a chamber in which the temperature was maintained at F. :2 F. while the humidity was maintained at 100% with condensation.

8,418,218 6 EXAMPLE I 000. The probable reason for this is that the high molecu- This example illustrates that highly effective sealing tg fi g gx g to penetrate the Smaller pores of the TABLE I Corrodkote Test Salt Spray Test Cass Humidity Percent Av. Rating Number Rating Number Test Test Sum of Performance Bath Panel Panel Rating Rating Rating Sum of Number Number Number Numbers Rating 1 2 1 2 NumbersXL67 baths are obtained from aqueous solutions of polyoxy- EXAMPLE H alkylene glycols of Varying molecular ght. This example shows that the aqueous polyoxyalkylene Six sealing baths were made up as follows: glycol sealing solutions of this invention do not have to Molecular Wt. Wt. Bath No. Water Glycol Wt. Glycol Percent Percent Glycol Water 1-1 Deionized Polyoxyethylene glycolm- 300 2.0 98.0 1-2 do do 600 2. 0 98. 0 1-2 do do 1, 000 2. 0 98. 0 4,000 2.0 98.0 6,000 2.0 98.0 1-6 do do 20,000 2. 0 98. 0

Six anodized test panels, prepared as described above, be made up with deionized water. The following sealing were then sealed in each of the above baths by immersing baths were prepared as follows:

Bath No. Water Glycol Glycol, Glycol, Water,

Molecular Wt. Wt. Percent Wt. Percent 1 Detroit, Mich. tap water having the following approximate analysis in parts per million (p.p.m.): 3.5 Silica, 0.07 iron, 26 calcium, 8 magnesium, 4.2 sodium, 92 bicarbonate, sulfate, 8 chloride, 126 total solids, pH 7.6.

2 Prepared by adding phosphoric acid and sodium metasilicate to Detroit, Mich. tap water so as to produce a concentration of 20 ppm. 0! P0 and 20 p.p.m. of SiOz.

the panels in the bath for a period of 15 minutes. Each Each of the above baths was used to seal six anodized bath had been adjusted to a pH of 6.0 with either sodium aluminum test panels which were prepared as described hydroxide or acetic acid and each bath was maintained at above. The sealing was accomplished, as in Example I, by a temperature in the range of 208 F. to the boiling point immersing the panels for a 15 minute period in the sealof the bath (approx. 212 F.) during the sealing period. ing bath which was adjusted to a pH of 6.0 and main- After being sealed, the corrosion and stain resistance of tained at a temperature in the range of 208 F. tothe boilthe test panels were evaluated by subjecting two panels ing point of the bath (approx. 212 F.). from each bath to the Corrodkote Test, two panels from In order to evaluate the corrosion and stain resistance each bath to the ZSO-hour Salt Spray Test, 1 panel from of the sealed anodic coatings, the test panels were subeach bath to the Cass Test (6 hours) and 1 panel from jected to the four ASTM tests described above. The reeach bath to the 250-hour Humidity Test. Due to the sults from these tests are shown below.

TAB LE 2 Corrodkote Test Salt Spray Test Rating Number Rating Number Cass Test Humidity Sum 01 Percent Av. Per- Bath Number Panel Panel Rating Test Rating Rating tormance Sum of a Number Number Nos. Rating Nos. l67

nature of the Corrodkote and Salt Spray tests two panels It is apparent from the above data that the purity of re d t i ur accuracy of the results the water used in the sealing composition of this invention The results of the above tests are expressed in terms is not especially Critical and that p of drinking Water is of the rating system developed by the ASTM Committee adequate for 1156 therein- B-8. A discussion of this rating system can be found in EXAMPLE 111 ASTM Proceedings, vol. 53, p. 267, 1953. The rating numbers run from 0 to 10 on a logarithmic scale with 0 being the poorest coating and 10 the best. Tabulated below in Table 1 are the results of the above mentioned tests.

This example demonstrates the ability of the sealing composition of this invention to effectively seal an anodic coating when the sealing bath is at a temperature as low as about 175 F. As in the previous two examples, 6 test It W111 bfi pp from the data m Table 1 that an panels prepared as described above were immersed for the P Y Y Y glycol Solutions were highly effective 15 minutes in each of the three sealing baths described in Producing a Sealed anodic coating of good corl'oslon below in Table 3. Subsequent to sealing, the effectiveness and stain resistance. It was observed, however, that the of th l was lu t d b means of th fo r ASTM eflectiveness of sealing lessened somewhat with the use of tests used in Examples 1 and 2 and the results of these glycols having molecular weights in excess of about 12,- 7 tests are shown in Table 4.

TABLE 3 Bath No. Water Wt. percent Glycol Wt. percent Molecular pH, Bath Temp. of I Water Glycol Wt. Glycol Bath, F.

98 Polyoxyethylene glycol. 2 1, 000 6. 175 98 do 2 4, 000 6. 0 180 98 d0 2 6,000 6. 0 175 TABLE 4 Corrodkote Test Salt Spray Test Percent Av. Rating Number Rating Number Cass Test Humidity Sum of Performance Bath No. Panel Panel Rating Test Rating Rating Sum of Rating 1 Number Number Numbers NumbersXLfi? 9 9 10 8 '56 93 .9 V 9 9 a 10 7 a 10 8 55 91 9 9 10 10 9 7 '54 90 r From the data presented in Table 4 it is seen that the sealing composition of this invention produces an excellent anodic seal at even the lowest sealing temperature at which it is practical to form Boehmite.

EXAMPLE IV This example demonstrates the ability of the sealing composition of this invention to provide excellent anodic sealing at various polyoxyalkylene glycol concentrations.

Five sealing baths were made-up which were identical in all respects except for the concentration of the glycol component dissolved in the water. Six test panels, cleaned and anodized as described above were then sealed by immersion in each of the sealing baths for a minute period and were then subjected to the four ASTM tests described above, The sealing compositions and test results are shown below in Tables 5 and 6, respectively.

Corrodkote Test Salt Spray Test Cass Test Humidity Test Sum of sealed anodic coating on each of the test panels was then tested by means of the four ASTM tests described above. From the data shown below in Table 7 it is seen that the aqueous solution of the polyoxypropylenepolyoxyethyle'ne glycol compound produced an excellent seal.

Table 7 Corrodkote Test rating number (2 panels) 9 and 10 Salt spray rating number (2 panels) l0 and 10 Cass Test rating number 10 Humidity Test rating number 8 Sum of rating number 57 Average performance (percent) 57(1.67)=95 EXAMPLE VI This example records the sealing of a dyed anodic aluminum oxide coating with the composition of this invention. Six test panels were cleaned and anodized as described above. They were then immersed in a dye solution having a pH of 6.0 which was at a temperature of about 150 F. for a period of 10 minutes. The dye solution was prepared by dissolving 2 grams per liter of Sandox Red-RN dye (a product of Sandox Chemical Works, Inc.) in deionized water.

The dyed test panels were then sealed by immersing them for 15 minutes in an aqueous solution composed of approximately 98 weight percent deionized water and Percent Av. Per- Bath No. Rating No. Panel Rating No. Panel Rating ating Rating fo'rmance Sum Number Number Numbers of Rating Num- 1 2 1 2 bersX1.67

The results shown in Table 6 reveal that good sealing is achieved even with low glycol concentration in the sealing bath.

EXAMPLE V This example demonstrates the use of a polyoxypropylenepolyoxyethylene compound in the sealing bath of this invention. A sealing bath was prepared by forming an aqueous solution composed of approximately 98 weight percent of deionized water and 2 weight percent of a glycol having the formula 2 percent by weight of a 600 molecular weight polyoxyethylene glycol. The sealing solution had a pH of about 6.0 and was at a temperature of about 212 F. The eifectiveness of the seal was then evaluated by subjecting the test panels to the four ASTM tests previously described. From the test results shown in Table 8, it is apparent that the sealing composition of this invention provides an excellent seal of dyed anodized aluminum.

Table 8 Corrodkote Test rating number (2 panels) 9 and 9 Salt SprayTest rating number (2 panels) 9 and 9 Cass Test rating number 10 Humidity Test rating number 9 Sum of rating numbers 55 Average performance (percent) 55 (1.67)=91 EXAMPLE VII The following sealing baths will seal aluminum anodic coatings and thereby provide a protective film which is highly resistant to staining and corrosion.

1 Marketed as Pluracol P 1310. A mixture of 20 wt. percent polyoxypropylene glycol of 1,300 mol. wt. and 80 wt. percent polyoxyethylene glycol of 2,000 mol. wt.

It will be apparent from the foregoing description and examples that the objects of this invention have been obtained. A sealing composition is hereby provided which is easy to prepare and use, which is convenient to store due to its excellent stability and which is economical. Furthermore, it should be understood that other agents conventionally employed in aluminum sealingbaths, such as wetting additives, can be incorporated into the sealing bath of this invention without harmful consequences. It will also be understood that the term aluminum as used herein means high purity aluminum, commercially pure aluminum and aluminum based alloys containing at least 50% aluminum by weight.

I claim:

1. A process for sealing an anodically formed aluminum oxide coating which comprises immersing said coating in a bath comprising a solution of water and at least about 0.1 weight percent of polyoxyalkylene glycol, said bath having a pH in the range of about 4.0 to 8.5 and a temperature of about 175 F. to the boiling point of the solution, and said polyoxyalkylene glycol having a molecular weight in the range of about 100 to 20,000 and corresponding to the formula HO(RO) H wherein R represents an alkylene radical selected from the group consisting of methylene, ethylene, propylene, butylene and mixtures of such radicals.

2. The process of claim 1 wherein the .bath comprises a solution of water and a polyoxyalkylene glycol having a molecular weight of from about 300 to 12,000 selected from the group consisting of polyoxyethylene glycol, polyoxypropylene glycol, polyoxypropylene-polyoxyethylene compounds having the formula wherein b has a value such that the molecular Weight of the oxypropylene group is at least about 900 and a and c are integers of a value such that the oxyethylene groups constitute about 10 to weight percent of the total weight of the compound, and mixtures of the foregoing.

3. The process of claim 1 wherein the bath comprises a solution of water and polyoxyethylene glycol.

References Cited UNITED STATES PATENTS 2,674,619 4/1954 Lundsted 260-485 2,799,649 7/1957 Caldwell et al. 252396 XR 3,022,335 2/1962 Lundsted 260-485 3,202,523 8/1965 Steeg 106-271 3,301,680 1/1967 Nycander et al. 10 6-311 XR OTHER REFERENCES C. P. McClelland et al.: Technology of the Polyethylene Glycols and Carbowax Compounds, Chemical and Engineering News, vol. 23, No. 3, February 1945, pp. 247-25l.

JULIUS FROME, Primary Examiner.

LORENZO HAYES, Assistant Examiner.

US. Cl. X.R. 10614, 271; 117-135; 252-396