CA1071984A - Protective coating for aluminum products - Google Patents
Protective coating for aluminum productsInfo
- Publication number
- CA1071984A CA1071984A CA266,761A CA266761A CA1071984A CA 1071984 A CA1071984 A CA 1071984A CA 266761 A CA266761 A CA 266761A CA 1071984 A CA1071984 A CA 1071984A
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- coating
- alkaline
- long chain
- compound
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Abstract
ABSTRACT OF THE DISCLOSURE
This invention relates to the formation of a hydrophobic and usually oleophilic coating on an aluminium surface by treating the aluminium surface with an aqueous alkaline solution containing long chain aliphatic carboxylic acids, alkali metal salts or ammonium salts of such acids or other long chain aliphatic compounds which generate a long chain carboxylate anion in an alkaline solution.
This invention relates to the formation of a hydrophobic and usually oleophilic coating on an aluminium surface by treating the aluminium surface with an aqueous alkaline solution containing long chain aliphatic carboxylic acids, alkali metal salts or ammonium salts of such acids or other long chain aliphatic compounds which generate a long chain carboxylate anion in an alkaline solution.
Description
~7~g8~
1 This invention is directed to a simple method of forming a hydrophobic coating on an aluminium surface.
Many processes are presently available for coating aluminium surfaces, such as anodizing, plating, chemical conversion coatings, painting and the like. The coatings, although designed for long life, require extensive .
surface pretreatments and are quite expensive. However, frequently, only short-term protection is needed or desired, for example, in shipping or storing semi-fabricated aluminTum products, such as coiled sheet and the like, to prevent the formation of water staln or other oxidatlon products. On other occasions, it is desirable to prevent the gradual buildup of natural oxide on the aluminium surface, for example, in welding applications and adhesive bonding applications because the buildup of natural oxide can interfere with these types of operations. How-ever, no simple and inexpensive process is presently known whtch will give short-term protection without interfering ~ -wlth subsequent fabrication or surface treatments, partlcularly when lubrlcants must be applied to the surface.
A simple, inexpensive process for coating aluminium is described by Wittrock et al in U.S. ~,726,721, assigned ~-to the present assignee, but the friable coating formed by thts process can interfere with subsequent forming or coating operations.
Kubie in U.S. 2,963,391 describes a process for forming a coating designed as an extrusion lubricant wherein the alum;n;umsurface is first treated with an 1 ammonia-laden alkaline solution containing a fatty acid (or equivalent salt or ester thereof) and then baked at about 400F to form a coating having unknown properties except for lubrication.
~arosi in U.S. 3,8499207 describes and claims a process for treating aluminium and other metals wherein the surface is treated with an a1kaline sodium formate solution and then coated with a clear resinous film to form a sepia-colored coating. The nature of the coattng formed during treatment in the alkaline sodium formate solution is not describe~ in the reference. However, it has been found that the aluminium surface underlyTng such a coatlng 7s susceptible to water stain or other oxldatlon in much the same manner as untreated alumlnium because the coating is readily wet and penetrated by water or aqueous solutions.
A simple, inexpensive method to temporarily prevent extensive oxidation of an aluminlum surface by water or other media particularly in coiled sheet has been needed for many years, but has been theretofore unavallable.
It is against thts background that the present tnvention was developed. ' The invention generally relates to the 25 coating of an aluminium surface and particularly to th~ ~ ' treatment of an aluminium surface to form a tenacious hydrophobic coating which protects the underlying aluminium surface from oxidation and also'facilitates ' the application of lubricant for subsequent fabrication.
As used herein, alumimium refers to pure alumlnium, ~7~L9~34 1 commercjally pure aluminium and aluminium alloys.
In accordance with the invention, an aluminium surface is treated with an aqueous alkaline solution containing a long chain aliphatic carboxylic acid, an equivalent alkali metal salt thereof or a compound which generates a long chain aliphatic carboxylate anion in an alkaline solution at elevated temperatures greater than 60C. Treatment times usually will be about one second for a clean surface, but extended treatment times do not seem to detrimentally affect the coating. The surface coating is hyarophobic and usually highly oleophil7c. Moreover, the coating Ts not usually affected by mineral acids, such as nitric acld, hydrochlorlc acid or sulfuric acid or by common polar solvents, such as acetone or ethyl alcohol. The coating formed is very difficul~ to analyze because under most circumstances, it appears to be a monomolecular layer on the order of lOO A thick. The carboxylate anion generating compound Tn the alkaline solution is apparently either reacting wtth the aluminium surface to form a type o~ aluminium soap or at least strongly associating with the alumlnTum sur~ace.
The pH of the alkaline treating solutlon must be from about 8.5 to about 10.0, preferably about 9-10.
At a pH much above 10, no coating occurs, only etchTng.
The temperature at the interface between the metal and the bath should exceed 60C and preferably should be from about 75C to the boiling point of the solutTon. For optimum results, the temperature is maintained at about 85C ~ 5C. Although generally it ~ill be most convenient to treat the aluminium surface with a solutlon .
- ~719~3~
1 maintained at the prescribed tempe-ratures, it is con-templated to treat an aluminium workpiece heated well above the prescribed temperature with a so1ution at 1ess than the prescribed temperature to e~fect ~he required interface tempera~ures. No significant coating formation is found at interface temperatures much below 60C.
The long chain aliphatic compound in the treat-ment bath ne~d only be present in smal1 quanti~ies, 10 usually greater than 1 part per millicn by weight. How- -ever, it is preferred to maintaTn the compound in a slTght excess of saturation to form an emulsion for the convenlence of compositlon con~rol. The compound can be a long chain allphatlc carboxyltc acld ~a fatty acid), an alIsall metal salt or ammonium salt thereof or other compounds whlch generate a long chain carboxylate anion in an alkafine solution. The long chain carboxylate anion should have from 10-20 carbon atoms, preferably 12-18. Below 10 carbon atoms, the coatings are not sufficiently hydrophobic to be of any value In preventing the wetting of the coating and the penetration thereof by water or other aqueous solutions which leads to water stain. Compounds wlth more than 20 carbon atoms in the chain usually Just coat the aluminiumsurface and neither react with nor strongly associate with the alumin~m substrate. This latter feature is readily shown by removing the coating with polar solvents, such as acetone or ethyl alcohol.
Addtt7Onally, excessively long chain carboxylate components are usually too difficult to maintain as an emulsion to b~ effective for treating the sur~ace-_ 5 _ ~, ~7~8~L
1 Suitable long chain aliphatic carboxylic acids include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid, arachidic acid and the like. Preferably, alkali metal salts of the above acids are used. Although the sodium and potassiumsalts are most desirable in the present invention, lithium, cesium and ammonium salts are functional. Other compounds which generate the appropriate carboxylate anion in an alkaline solution can also be used. The aliphatic component of the carboxylate generating compound can be saturated or unsaturated; however, unsaturated compounds tend to be less effective than saturated compounds. SubstitutTons on the aliphatic carbon chaln can be made provided they do not prevent the carboxylate compound from forming the desired coatlng-If desired, ~etting agents, such as Emsorb 6903(sold by Emery Industries, Inc.), Tween 85 tsold by ICI
American, Inc.) and Ultrawet (sold by the Armour and Company) can be added to ths solution in amounts up to 3~ by weight to facilitate the wetting of the alumi~um surface by the alkalIne solutlon durlng treatment. Use of wettlng agents tends to render the coatTng more ol'eophillc. Other components, such as emulsifTers and the like, can be added up to 3% by weight to render the fatty acid component 2S mlscible or at least emulsifiable with the aqueous solution. Mechanical dispersions can also be employed, for example, when the emulsification of ~he carboxylate generat-lng componsnt in the alkaline solution; is difficulto The treatment solution may be rendered alkaline by the addTtion of appropriate sapon7fyin~ agents, such as ~c~cl~ ~lork - 6 -:
~07~9~
1 an alkali ~etal hydroxide or ammonium hydroxide. However, in many instances, such as when utilizing sodium stearate or sodium palmitate, the solution will be sufficiently alkaline so that additional hydroxide additions become unnecessary. Although ammonium salts and ammonium hydroxide can be employed in the alkaline treatment solution, these compounds are not particularly desirable because at the required elevated interface temperatures, ammonia is driven from the solution rendering the maintenance of a suitably alkaline pH at the~interface very difficult.
When the aluminjum surface is flrst contacted by the alkalIne solution of the inventlon, an Initial, very short burst of ef~ervescence occurs Indicatlng that the alumlnlum substrate is belng etched. The effervescence qùickly subsides, however, and the desired hydrophobic coating forms, usually within a few seconds. A heavy oxide layer can retard coating formation and apparently most, if not all, of oxide coating must be etched away ~efore any reaction or strong association can occur 20'~between the carboxylate anion and the aluminum substrate.
The etchlng whlch occurs inltlally may leave or generate a very thin layer oF oxide on the metal surface, and in all likelihood, the carboxylate anion may be reactlng with basic sites on this thin oxide coating to form the soap.
The surface coating wh1ch forms ;n the Inven~ion Ts neither readily wet nor penetrated by water or other nonalkaline aqueous solutions. The surface 7s usually oleophilic and is generally compatible with most~ Jf not all, m~tal-working lubricant~ includ7ng water-based :, ~L~7~9~34 1 emulsions~ This compatibility of the coating with lubricants is enhanced considerably by the use of wetting agents in the alkaline treating solution and particularly by treating the formed hydrophobic coating with a hot (greater than 60C) alkaline solution (pH 8.5-10, pre-ferably 9.0-10.0) of a wetting agent. Nonionic and cationic wetting agents are preferred.
The hydrophobic coating has an electrical resistance initially of about 15 micro-ohms per cm2 which remains relatively stable for at least 2-3 weeks. This indicates that essentially no oxidation of the underlytng alum7rium surface is occurr7ng. These resistance levels are to be compared w7th a natural oxide coating wh7ch has an inftial reslstance of about 30 micro ohms per cm2 and which can gradually tncrease to well over 1000 micro-ohms per cm2 Tn a matter of days. The relatively stable re-slstance exhibited by the coating of the invention is a very advantageous feature. For example, in welding alumTn~m, the oxide coating is not desirable because it Tnterferes with the welding operation, particularly spot reststance welding. By maintain1ng the reslstance at a relatively constant level, there is no need to clean the surface pr70r to welding.
Moreover, in manufacturlng facilities utilizing the adhesive bonding of 21uminjum components, the coating of the 7nven-tTon prevents oxidation during the various ~ '~
fabrication or assembly procedures, ye~ it provides an excellent foundation for adhesion between the alumin;um suhstrate and o~her materials.
The coating of the invention can be readily . ..' .
~C~71~4 1 removed by treatment with alkaline cleaning solutions normally employed to industrially c1ean aluminium surfaces prior to various surface treatments, such as anodizing, painting and the like.
It has also been found that the process of the invention can be employed to treat aluminium surfaces which are oxidized or otherwise contaminated with oxidized pro-ducts prior to subsequent fabrication. The etchlng whlch`
occurs in the process removes the contaminated oxide surface and the coating formed is readily compatible with and wet by metal-working lubricants. For example, the surface of aluminium alloy sheet used for maklng drawn and Troned can bod7es is frequently contamTnated with water staln (oxiciation product from the condensatlon of water on the surface during transporting or storing) or pick-up (highly oxidized metal particles embedded in the surface during rolling), which interferes with the drawing and ironing operations. By treating such sheet in accordance with the Tnvention9 the etching removes substantially all of the oxidized surface contaminants. The coating formed readily accepts the draw and Iron lubricant, part7cularly when subsequently treated w7th a hot alkaline solution containing a wetting agent. Treatment times with heavy natural oxide coatings which have been agad ~end to be in the order of several minutes rather than a few seconds as when a fresh natural oxide-coated alumi~7u~ surface is - treated.
The following examples are given to fur~her illustrate the invention.
In each of the Examples 1-7, a clean 3004-Hl9 _ g _ 1 aluminium alloy sheet was treated. The treating solutions, which were maintained at 85 + 5C~ were prepared by adding l.O gram/liter of the noted acid to deionized or dis~illed water and then adjusting the pH to 9.0 ~ Q.l with NaOH if needed. Treatment time in each case was 30 seconds. Each treated specimen was checked for water wettability after treatment in the alkaline solution, after a 30-second dlp in a 35% (by weight) nitric acid solution and then after an acetone-ethyl alcohol rinse.
Example l Lauric acid (C-12) formed a hydrophobtc, oleo-philic surface which remained hydrophobic a~er a 30-second d1p in the nitric acid. However, aPter the nitric acld treatment, the acetone-ethyl alcohol rinse apparently lS removed the hydrophobic coat1ng because the treated surface could then be wet with water.
Example 2 Myristlc acid (C-14) formed a hydrophobic, oleo-philic surface which remained hydrophobic after a 30-second dip in the nitrTc acid. However, after the nttrtc acid treatment, the acetone-ethyl alcohol solution apparently removed the hydrophobic coatlng because the treated surface could then be wet with water.
Example 3 ~i~
Palmitic acid (C-16) formed a hydrophobic, oteophilic surface which remained so after both ~he nitric acid dip and the acetone-ethyl alcohol rinse. ;
Exampl~ 4 Stearic acid (C-18) formed a hydrophobic, oleo- I
phTlic surface whtch remained so after both the nitric -- 1 0 -- , . .
1' .. . , . ':
~i7~34 1 acid dip and the acetone-ethyl alcohol rinse.
Example 5 Oleic acid (unsaturated C-18) formed a hydrophobic, oleophilic surface which remained hydrophobic after a 30-second dip in the nitric acid. However, after the nitricacid treatment, the acetone-ethyl alcohol solution apparently removed the hydrophobic coating because the treated surface could then be wet with water.
Example 6 Linoleic acid (unsaturated C-18) formed a hydro-phobic, oleophilic surface which remained hydrophobTc after a 30-second dip in the nTtric acid. However, after the nitrtc acid treatment, the acetone ethyl alcohol solution apparently removed the coating because the treated surface could then be wet with water.
Example 7 Arachidic acid (C-20) formed a hydrophobic, oleophilic surface but after the nltric acid dip, the surface became hydrophilic.
Example 8 A clean 3004-H32 alumin1um alloy sheet was treated for 5 seconds In an aqueous alkaline solution main-tained at 80OC which contained 1.0 gram~liter sodium stearate. The pH of the solution was 9.3. Initially, a burst of effervescence occurred but the effervescence quickly subsided and the desired hydrophobic, oleophilic coating formed. The treated sheet withstood 20 hours of I' continuous water-fog exposure with no evidence of water stain or other surface defects. The coating was $u11y compatible with varîous metal-working lubricants, such as ~ 1 1 - , , 7~ 4 1 are used in rolling, forging, drawing and ironing, shap-ing, stamping and the like. Initially, the treated sur- -face had an electrical resistance of 16 micro-ohms/cm~
and after 6 weeks of laboratory exposure (23C and 70%
humidity) had an- electrical resistance of only 30 micro-ohms/cm2 .
Example 9 A steel sheet was treated in accordance with the conditions set forth above for E~amples 1-7. However, no significant coating formation was noted. The surface of the ferrous product was readily wet by water after treatment. There was some evidence (dTscoloration of the solutlon) that the solutlon was merely dissolvin~
the surface.
Example 10 A plurality of clean, closely packed 3004-H32 aluminilm sheets were treated in the manner set forth above In Example 8 except that 2.0 grams/liter of a polyoxy-ethylene sorbltan trioleate soid under the brand name Tween 85 was added to the solution as a wetting agent.
The coatlng formed was fully equivalent to the coatlng formed in Example 8. The wetting agent allowed the sclution to penetrate in between closely packed aluminium sheet and react with the surfaces thereof.
Example 11 A 3004-H32 aluminium alloy sheet was treated with a solution containing a polyoxyethylene sorbitan trioleate alone (Tween 85) as the source for carboxylate anions. The solution which contained 2 grams/liter of the trioleate (Tween 85) was at a pH of 9.5 and a temperature of 80C.
1,'` .
- 12 ~
. ~
~7~9~34 1 The coating formed was hydrophobic and oleophi1ic.
Example 12 Clean 3004-H32 aluminium alloy sheets were treated for 30 seconds in a hot aqueous alkaline solution containing 1 gram/liter of sodium stearate. The pH of the so1ution was 9.5 and the temperature was 80C. After treatment2 the sheets were rinsed and ~hen separate sheets were treated for 5, 15, 25 and 35 seconds in a second hot, aqueous a1kaline solution containing 2 grams/liter of a polyoxyethylene sorbitan trioleate (Tween 85). The pH
and temperature of the second solution were also 9.5 and 80C, respectively. The coatings formed were hydrophobic and hlghly oleophilTc. Treated sheets were then evaluated for compatibility wTth mineral oil. The evaluation was conducted by placing a drop of mineral oTl on a treated surface inclined about 70C from the horizontal and then determining the time required for the drop of oil to travel 3 inches on the inclined surface. Longer times indicate greater wettability and thus greater compatibility with the lubricant. The results are as follows:
Duration of Second Treatment Time2 Sec ` 60 3~ 90 Similar results were obtained with a drop of a 30~ by volume oil-in-water emulsion of Texaca 591, a common alùminium metal-working lubricant.
In other tests, C-8 acids, such as caprylic .
` - 13 .
1 acid, and C-22 acids, such as behenic acid, were found to form hydrophilic coating.
It is obvious that various modifications and improvements can be made to the invention described herein without departing from the spirit thereoF and the scope of the appended claims.
':
. ~ .
. .,, ~., .
20'
1 This invention is directed to a simple method of forming a hydrophobic coating on an aluminium surface.
Many processes are presently available for coating aluminium surfaces, such as anodizing, plating, chemical conversion coatings, painting and the like. The coatings, although designed for long life, require extensive .
surface pretreatments and are quite expensive. However, frequently, only short-term protection is needed or desired, for example, in shipping or storing semi-fabricated aluminTum products, such as coiled sheet and the like, to prevent the formation of water staln or other oxidatlon products. On other occasions, it is desirable to prevent the gradual buildup of natural oxide on the aluminium surface, for example, in welding applications and adhesive bonding applications because the buildup of natural oxide can interfere with these types of operations. How-ever, no simple and inexpensive process is presently known whtch will give short-term protection without interfering ~ -wlth subsequent fabrication or surface treatments, partlcularly when lubrlcants must be applied to the surface.
A simple, inexpensive process for coating aluminium is described by Wittrock et al in U.S. ~,726,721, assigned ~-to the present assignee, but the friable coating formed by thts process can interfere with subsequent forming or coating operations.
Kubie in U.S. 2,963,391 describes a process for forming a coating designed as an extrusion lubricant wherein the alum;n;umsurface is first treated with an 1 ammonia-laden alkaline solution containing a fatty acid (or equivalent salt or ester thereof) and then baked at about 400F to form a coating having unknown properties except for lubrication.
~arosi in U.S. 3,8499207 describes and claims a process for treating aluminium and other metals wherein the surface is treated with an a1kaline sodium formate solution and then coated with a clear resinous film to form a sepia-colored coating. The nature of the coattng formed during treatment in the alkaline sodium formate solution is not describe~ in the reference. However, it has been found that the aluminium surface underlyTng such a coatlng 7s susceptible to water stain or other oxldatlon in much the same manner as untreated alumlnium because the coating is readily wet and penetrated by water or aqueous solutions.
A simple, inexpensive method to temporarily prevent extensive oxidation of an aluminlum surface by water or other media particularly in coiled sheet has been needed for many years, but has been theretofore unavallable.
It is against thts background that the present tnvention was developed. ' The invention generally relates to the 25 coating of an aluminium surface and particularly to th~ ~ ' treatment of an aluminium surface to form a tenacious hydrophobic coating which protects the underlying aluminium surface from oxidation and also'facilitates ' the application of lubricant for subsequent fabrication.
As used herein, alumimium refers to pure alumlnium, ~7~L9~34 1 commercjally pure aluminium and aluminium alloys.
In accordance with the invention, an aluminium surface is treated with an aqueous alkaline solution containing a long chain aliphatic carboxylic acid, an equivalent alkali metal salt thereof or a compound which generates a long chain aliphatic carboxylate anion in an alkaline solution at elevated temperatures greater than 60C. Treatment times usually will be about one second for a clean surface, but extended treatment times do not seem to detrimentally affect the coating. The surface coating is hyarophobic and usually highly oleophil7c. Moreover, the coating Ts not usually affected by mineral acids, such as nitric acld, hydrochlorlc acid or sulfuric acid or by common polar solvents, such as acetone or ethyl alcohol. The coating formed is very difficul~ to analyze because under most circumstances, it appears to be a monomolecular layer on the order of lOO A thick. The carboxylate anion generating compound Tn the alkaline solution is apparently either reacting wtth the aluminium surface to form a type o~ aluminium soap or at least strongly associating with the alumlnTum sur~ace.
The pH of the alkaline treating solutlon must be from about 8.5 to about 10.0, preferably about 9-10.
At a pH much above 10, no coating occurs, only etchTng.
The temperature at the interface between the metal and the bath should exceed 60C and preferably should be from about 75C to the boiling point of the solutTon. For optimum results, the temperature is maintained at about 85C ~ 5C. Although generally it ~ill be most convenient to treat the aluminium surface with a solutlon .
- ~719~3~
1 maintained at the prescribed tempe-ratures, it is con-templated to treat an aluminium workpiece heated well above the prescribed temperature with a so1ution at 1ess than the prescribed temperature to e~fect ~he required interface tempera~ures. No significant coating formation is found at interface temperatures much below 60C.
The long chain aliphatic compound in the treat-ment bath ne~d only be present in smal1 quanti~ies, 10 usually greater than 1 part per millicn by weight. How- -ever, it is preferred to maintaTn the compound in a slTght excess of saturation to form an emulsion for the convenlence of compositlon con~rol. The compound can be a long chain allphatlc carboxyltc acld ~a fatty acid), an alIsall metal salt or ammonium salt thereof or other compounds whlch generate a long chain carboxylate anion in an alkafine solution. The long chain carboxylate anion should have from 10-20 carbon atoms, preferably 12-18. Below 10 carbon atoms, the coatings are not sufficiently hydrophobic to be of any value In preventing the wetting of the coating and the penetration thereof by water or other aqueous solutions which leads to water stain. Compounds wlth more than 20 carbon atoms in the chain usually Just coat the aluminiumsurface and neither react with nor strongly associate with the alumin~m substrate. This latter feature is readily shown by removing the coating with polar solvents, such as acetone or ethyl alcohol.
Addtt7Onally, excessively long chain carboxylate components are usually too difficult to maintain as an emulsion to b~ effective for treating the sur~ace-_ 5 _ ~, ~7~8~L
1 Suitable long chain aliphatic carboxylic acids include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid, arachidic acid and the like. Preferably, alkali metal salts of the above acids are used. Although the sodium and potassiumsalts are most desirable in the present invention, lithium, cesium and ammonium salts are functional. Other compounds which generate the appropriate carboxylate anion in an alkaline solution can also be used. The aliphatic component of the carboxylate generating compound can be saturated or unsaturated; however, unsaturated compounds tend to be less effective than saturated compounds. SubstitutTons on the aliphatic carbon chaln can be made provided they do not prevent the carboxylate compound from forming the desired coatlng-If desired, ~etting agents, such as Emsorb 6903(sold by Emery Industries, Inc.), Tween 85 tsold by ICI
American, Inc.) and Ultrawet (sold by the Armour and Company) can be added to ths solution in amounts up to 3~ by weight to facilitate the wetting of the alumi~um surface by the alkalIne solutlon durlng treatment. Use of wettlng agents tends to render the coatTng more ol'eophillc. Other components, such as emulsifTers and the like, can be added up to 3% by weight to render the fatty acid component 2S mlscible or at least emulsifiable with the aqueous solution. Mechanical dispersions can also be employed, for example, when the emulsification of ~he carboxylate generat-lng componsnt in the alkaline solution; is difficulto The treatment solution may be rendered alkaline by the addTtion of appropriate sapon7fyin~ agents, such as ~c~cl~ ~lork - 6 -:
~07~9~
1 an alkali ~etal hydroxide or ammonium hydroxide. However, in many instances, such as when utilizing sodium stearate or sodium palmitate, the solution will be sufficiently alkaline so that additional hydroxide additions become unnecessary. Although ammonium salts and ammonium hydroxide can be employed in the alkaline treatment solution, these compounds are not particularly desirable because at the required elevated interface temperatures, ammonia is driven from the solution rendering the maintenance of a suitably alkaline pH at the~interface very difficult.
When the aluminjum surface is flrst contacted by the alkalIne solution of the inventlon, an Initial, very short burst of ef~ervescence occurs Indicatlng that the alumlnlum substrate is belng etched. The effervescence qùickly subsides, however, and the desired hydrophobic coating forms, usually within a few seconds. A heavy oxide layer can retard coating formation and apparently most, if not all, of oxide coating must be etched away ~efore any reaction or strong association can occur 20'~between the carboxylate anion and the aluminum substrate.
The etchlng whlch occurs inltlally may leave or generate a very thin layer oF oxide on the metal surface, and in all likelihood, the carboxylate anion may be reactlng with basic sites on this thin oxide coating to form the soap.
The surface coating wh1ch forms ;n the Inven~ion Ts neither readily wet nor penetrated by water or other nonalkaline aqueous solutions. The surface 7s usually oleophilic and is generally compatible with most~ Jf not all, m~tal-working lubricant~ includ7ng water-based :, ~L~7~9~34 1 emulsions~ This compatibility of the coating with lubricants is enhanced considerably by the use of wetting agents in the alkaline treating solution and particularly by treating the formed hydrophobic coating with a hot (greater than 60C) alkaline solution (pH 8.5-10, pre-ferably 9.0-10.0) of a wetting agent. Nonionic and cationic wetting agents are preferred.
The hydrophobic coating has an electrical resistance initially of about 15 micro-ohms per cm2 which remains relatively stable for at least 2-3 weeks. This indicates that essentially no oxidation of the underlytng alum7rium surface is occurr7ng. These resistance levels are to be compared w7th a natural oxide coating wh7ch has an inftial reslstance of about 30 micro ohms per cm2 and which can gradually tncrease to well over 1000 micro-ohms per cm2 Tn a matter of days. The relatively stable re-slstance exhibited by the coating of the invention is a very advantageous feature. For example, in welding alumTn~m, the oxide coating is not desirable because it Tnterferes with the welding operation, particularly spot reststance welding. By maintain1ng the reslstance at a relatively constant level, there is no need to clean the surface pr70r to welding.
Moreover, in manufacturlng facilities utilizing the adhesive bonding of 21uminjum components, the coating of the 7nven-tTon prevents oxidation during the various ~ '~
fabrication or assembly procedures, ye~ it provides an excellent foundation for adhesion between the alumin;um suhstrate and o~her materials.
The coating of the invention can be readily . ..' .
~C~71~4 1 removed by treatment with alkaline cleaning solutions normally employed to industrially c1ean aluminium surfaces prior to various surface treatments, such as anodizing, painting and the like.
It has also been found that the process of the invention can be employed to treat aluminium surfaces which are oxidized or otherwise contaminated with oxidized pro-ducts prior to subsequent fabrication. The etchlng whlch`
occurs in the process removes the contaminated oxide surface and the coating formed is readily compatible with and wet by metal-working lubricants. For example, the surface of aluminium alloy sheet used for maklng drawn and Troned can bod7es is frequently contamTnated with water staln (oxiciation product from the condensatlon of water on the surface during transporting or storing) or pick-up (highly oxidized metal particles embedded in the surface during rolling), which interferes with the drawing and ironing operations. By treating such sheet in accordance with the Tnvention9 the etching removes substantially all of the oxidized surface contaminants. The coating formed readily accepts the draw and Iron lubricant, part7cularly when subsequently treated w7th a hot alkaline solution containing a wetting agent. Treatment times with heavy natural oxide coatings which have been agad ~end to be in the order of several minutes rather than a few seconds as when a fresh natural oxide-coated alumi~7u~ surface is - treated.
The following examples are given to fur~her illustrate the invention.
In each of the Examples 1-7, a clean 3004-Hl9 _ g _ 1 aluminium alloy sheet was treated. The treating solutions, which were maintained at 85 + 5C~ were prepared by adding l.O gram/liter of the noted acid to deionized or dis~illed water and then adjusting the pH to 9.0 ~ Q.l with NaOH if needed. Treatment time in each case was 30 seconds. Each treated specimen was checked for water wettability after treatment in the alkaline solution, after a 30-second dlp in a 35% (by weight) nitric acid solution and then after an acetone-ethyl alcohol rinse.
Example l Lauric acid (C-12) formed a hydrophobtc, oleo-philic surface which remained hydrophobic a~er a 30-second d1p in the nitric acid. However, aPter the nitric acld treatment, the acetone-ethyl alcohol rinse apparently lS removed the hydrophobic coat1ng because the treated surface could then be wet with water.
Example 2 Myristlc acid (C-14) formed a hydrophobic, oleo-philic surface which remained hydrophobic after a 30-second dip in the nitrTc acid. However, after the nttrtc acid treatment, the acetone-ethyl alcohol solution apparently removed the hydrophobic coatlng because the treated surface could then be wet with water.
Example 3 ~i~
Palmitic acid (C-16) formed a hydrophobic, oteophilic surface which remained so after both ~he nitric acid dip and the acetone-ethyl alcohol rinse. ;
Exampl~ 4 Stearic acid (C-18) formed a hydrophobic, oleo- I
phTlic surface whtch remained so after both the nitric -- 1 0 -- , . .
1' .. . , . ':
~i7~34 1 acid dip and the acetone-ethyl alcohol rinse.
Example 5 Oleic acid (unsaturated C-18) formed a hydrophobic, oleophilic surface which remained hydrophobic after a 30-second dip in the nitric acid. However, after the nitricacid treatment, the acetone-ethyl alcohol solution apparently removed the hydrophobic coating because the treated surface could then be wet with water.
Example 6 Linoleic acid (unsaturated C-18) formed a hydro-phobic, oleophilic surface which remained hydrophobTc after a 30-second dip in the nTtric acid. However, after the nitrtc acid treatment, the acetone ethyl alcohol solution apparently removed the coating because the treated surface could then be wet with water.
Example 7 Arachidic acid (C-20) formed a hydrophobic, oleophilic surface but after the nltric acid dip, the surface became hydrophilic.
Example 8 A clean 3004-H32 alumin1um alloy sheet was treated for 5 seconds In an aqueous alkaline solution main-tained at 80OC which contained 1.0 gram~liter sodium stearate. The pH of the solution was 9.3. Initially, a burst of effervescence occurred but the effervescence quickly subsided and the desired hydrophobic, oleophilic coating formed. The treated sheet withstood 20 hours of I' continuous water-fog exposure with no evidence of water stain or other surface defects. The coating was $u11y compatible with varîous metal-working lubricants, such as ~ 1 1 - , , 7~ 4 1 are used in rolling, forging, drawing and ironing, shap-ing, stamping and the like. Initially, the treated sur- -face had an electrical resistance of 16 micro-ohms/cm~
and after 6 weeks of laboratory exposure (23C and 70%
humidity) had an- electrical resistance of only 30 micro-ohms/cm2 .
Example 9 A steel sheet was treated in accordance with the conditions set forth above for E~amples 1-7. However, no significant coating formation was noted. The surface of the ferrous product was readily wet by water after treatment. There was some evidence (dTscoloration of the solutlon) that the solutlon was merely dissolvin~
the surface.
Example 10 A plurality of clean, closely packed 3004-H32 aluminilm sheets were treated in the manner set forth above In Example 8 except that 2.0 grams/liter of a polyoxy-ethylene sorbltan trioleate soid under the brand name Tween 85 was added to the solution as a wetting agent.
The coatlng formed was fully equivalent to the coatlng formed in Example 8. The wetting agent allowed the sclution to penetrate in between closely packed aluminium sheet and react with the surfaces thereof.
Example 11 A 3004-H32 aluminium alloy sheet was treated with a solution containing a polyoxyethylene sorbitan trioleate alone (Tween 85) as the source for carboxylate anions. The solution which contained 2 grams/liter of the trioleate (Tween 85) was at a pH of 9.5 and a temperature of 80C.
1,'` .
- 12 ~
. ~
~7~9~34 1 The coating formed was hydrophobic and oleophi1ic.
Example 12 Clean 3004-H32 aluminium alloy sheets were treated for 30 seconds in a hot aqueous alkaline solution containing 1 gram/liter of sodium stearate. The pH of the so1ution was 9.5 and the temperature was 80C. After treatment2 the sheets were rinsed and ~hen separate sheets were treated for 5, 15, 25 and 35 seconds in a second hot, aqueous a1kaline solution containing 2 grams/liter of a polyoxyethylene sorbitan trioleate (Tween 85). The pH
and temperature of the second solution were also 9.5 and 80C, respectively. The coatings formed were hydrophobic and hlghly oleophilTc. Treated sheets were then evaluated for compatibility wTth mineral oil. The evaluation was conducted by placing a drop of mineral oTl on a treated surface inclined about 70C from the horizontal and then determining the time required for the drop of oil to travel 3 inches on the inclined surface. Longer times indicate greater wettability and thus greater compatibility with the lubricant. The results are as follows:
Duration of Second Treatment Time2 Sec ` 60 3~ 90 Similar results were obtained with a drop of a 30~ by volume oil-in-water emulsion of Texaca 591, a common alùminium metal-working lubricant.
In other tests, C-8 acids, such as caprylic .
` - 13 .
1 acid, and C-22 acids, such as behenic acid, were found to form hydrophilic coating.
It is obvious that various modifications and improvements can be made to the invention described herein without departing from the spirit thereoF and the scope of the appended claims.
':
. ~ .
. .,, ~., .
20'
Claims (18)
1. A method of forming a tenacious, hydrophobic coating on an aluminum surface which is resistant to polar organic solvents comprising treating the aluminum surface with an alkaline solution at a pH less than 10 containing a compound which generates therein a long chain aliphatic carboxylate anion having from 10-20 carbon atoms and maintaining the temperature at the surface-solution interface greater than 60°C, the solution initially etching the aluminum surface and then the solution forming a tenacious hydro-phobic coating on the aluminum surface which is resistant to polar organic solvents.
2. The method of claim 1 wherein the treatment solution is at a temperature from about 75°C to the boiling point of the solution.
3. The method of claim 1 wherein said compound is selected from the group consisting of long chain aliphatic carboxylic acids and alkali metal salts or ammonium salts of long chain aliphatic carboxylic acids.
4. The method of claim 1 wherein the treatment solution is at a temperature from about 80° to about 90°C.
5. The method of claim 1 wherein the pH of the solution is maintained greater than 8.5.
6. The method of claim 1 wherein the pH of the solution is from about 9-10.
7. The method of claim 1 wherein the solution contains more than 1 part per million of said carboxylate anion generating compound.
8. The method of claim 1 wherein the alkaline solution contains a wetting agent to facilitate the wetting of the untreated aluminum surface with the alkaline solution.
9. The method of claim 1 wherein said compound is sodium stearate.
10. The method of claim 3 wherein said alkali metal salts are selected from the group consisting of sodium and potassium salts.
11. The method of claim 1 wherein said solution is maintained alkaline by the addition of an alkali metal hydroxide.
12. The method of claim 1 wherein said coating is less than 100 angstrom units thick.
13. The method of claim 1 wherein the carboxylate anion generating compound has from 12-18 carbon atoms in the carbon chain.
14. An aluminum product having a hydrophobic coating formed by the process of claim 1.
15. The method of claim 1 wherein the hydrophobic coating is further treated with an alkaline solution containing a wetting agent at a temperature greater than 60°C.
16. The method of claim 15 wherein the pH of the solution containing the wetting agent ranges from about 8.5 - 10Ø
17. The method of claim 16 wherein the pH ranges from about 9.0 - 10Ø
18. The aluminum product having a coating formed by the process of claim 15.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA266,761A CA1071984A (en) | 1976-11-29 | 1976-11-29 | Protective coating for aluminum products |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA266,761A CA1071984A (en) | 1976-11-29 | 1976-11-29 | Protective coating for aluminum products |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1071984A true CA1071984A (en) | 1980-02-19 |
Family
ID=4107387
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA266,761A Expired CA1071984A (en) | 1976-11-29 | 1976-11-29 | Protective coating for aluminum products |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1071984A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113463105A (en) * | 2021-08-06 | 2021-10-01 | 立邦涂料（重庆）化工有限公司 | High-permeability aluminum fin environment-friendly cleaning agent |
-
1976
- 1976-11-29 CA CA266,761A patent/CA1071984A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113463105A (en) * | 2021-08-06 | 2021-10-01 | 立邦涂料（重庆）化工有限公司 | High-permeability aluminum fin environment-friendly cleaning agent |
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