IE47111B1 - Protective coating process for aluminum and aluminum alloys - Google Patents

Abstract

A process is described for sealing anodic coatings on aluminum and the alloys of aluminum while simultaneously impregnating the anodic coating with organic resin during the sealing operation, thereby providing a protective coating to the anodized aluminum which, among other things has superior corrosion resistant characteristics. A specified water-borne resin coating material possessing excellent stability at temperatures in excess of 170.degree.F is used to convert the unsealed anodic coating to the monohydrate/trihydrate form of aluminum oxide during the sealing step of an otherwise conventional aluminum anodizing process. Subsequently, the sealed anodic film is cured at temperatures up to 500.degree.F. This process provides a total protection system having characteristics superior to separately sealed and organically primed aluminum obtained through conventional processing.

Description

This invention relates generally tu processes i'or providing protective coatings to metals and more particularly to sucli a process whereby a substantially superior coating than heretofore possible, notably with respect to protection against corrosion,is applied to aluminum and aluminum alloys and thereby producing an ultimate product of great longevity.

Current methods of applying protective coatings, lor example so called corrosion-proof coatings or primers, on aluminum substrates rely on the application of a primer subsequent to the chemical surface treatment process. Thus, multiple separate and distinct processes are required whereby an aluminum substrate or workpiece is first anodized, then sealed and finally the primer is applied. In other words, a build-up of layers results in tlie ultimate coating witb tlie eliemieal/meclianical ^bonding of the layers being critical to tlie durability of tlie protective coating and ·therefore to tlie service life of tlie aluminum product. This entire operation in producing tlie layered coating involves appreciable time and labour.

Tlie present invention is directed to improvements in this art ’and to that end proposes a new process whereby the required sealing and primer applying actions are simultaneous and a novel material formulation to accomplish such simultaneous actions.

According to the present invention there is provided a protective coating process ior an aluminum or aluminum alloy component comprising anodizing the component, applying a coating composition comprising a water-soluble acrylic polymer or copolymer to the anodized component, said composition sealing the anodic coating through hydration and simultaneuously priming said anodic coating, and curing the sealed-and primed ccnpcnent at a temperature range of 200¾ to 5oo°F.

The process herein proposed permits a substantial reduction in time and labour for the complete operation while at the same time producing a substantially superior coating than heretofore possible, effected by an integral bond of the coating and the oxide film.

This is believed to be due to the introduction of the water soluble resin into the pores of the oxide film during the sealing operation. Subsequently, the thermosetting resin is cross-linked during the curing operation providing a durable and protective coating.

The material herein proposed simultaneously seals the anodic coating to its monohydrate/triliydrate form and impregnates it with a water-borne organic resin.

The water-borne organic resin material, when applied to an unsealed anodic coating under controlled time/temperature conditions, provides a resin impregnated, sealed anodic coating capable of withstanding sevci'e corrosion environments. Such environments are exemplified by coastal regions subject to heavy moisture and often salt-laden atmosphere such as that encountered in the aviation, marine, architectural and auto industries. in The coating composition usec/this invention comprises a water soluble acrylic polymer or copolymer, which in turn may he comprised of various acrylic containing materials such as acrylonitrile, mcthacrylonitrile, •acrylamide, methacrylamide, methacrylic acid, acrylic acid, methacrylic esters and acrylic esters. Most preferably, the coating composition comprises polymerized or eopolymerized monomers selected from the following formula: CH„ = CH (A) - C(0) - 0 - R wherein A is a hydrogen atom, or a methyl or an ethyl group; and R is a- hydrogen atom; an alkyl group having from 1 to 20 carbon atoms, preferably from 2 to 8 carbon atoms; a hydroxysubstituted alkyl group having from 1 to 20 carbon atoms, preferably 2 to-8 carbon atoms, and even more preferably from 2 to 5 carbon atoms; a primary or secondary arainosubstituted alkyl group having from 1 to 20 carbon atoms, preferably 2 to 8 carbon atoms, wherein tho optimal substituent on the amino group is an alkyl group or a bydroxyallcyl group having from 1 to 6 carbon atoms, a morcaptosubstituted alkyl group having from 1 to 20 carbon atoms, preferably 2 to 8 carbon atoms; an alkylthioalkylene group having ui) to 20 carbon atoms, preferably up to 8 carbon atoms; or a group of up to 8 carbon atoms containing an oxirane ring, most preferably 3 carbon atoms as glyeidyl.

Of the above identified monomers, the most preferred, are the acrylics and methacrylies.

Suitable examples of an acid are acrylic acid and methyacrylie acid. Suitable examples of alkyl esters are those of acrylic or methacrylic acid where the alkyl group is methyl, ethyl, propyl, isopropyl, butyl, tertiary-butyl, pentyl, hexyl, cthyihcxyl, octyl, decyl and dodecyl, suitable examples ol' a hydroxysubstituted alkyl group are liydroxyethyl, hydroxypropyl, hydroxybutyl, hydroxypcntyl, hydroxyhexyl, hydroxyoctyl, hydroxydecyl, and hydroxydodocyl, suitable examples of an aininoalkyl group are aminomethyi, aminoethyl, aminopropyl, aminobutyl, aminopontyl, aininolicxyl, aminooctyl, aminodecyl and aminododecyl, wherein the amino group is a primary or a secondary amino group and the optimal substituent on the amino group is an alkyl group of from 1 to 6 carbon atoms, preferably 1 to h carbon atoms; suitable examples of a inercaptoaikyl group are mothylincrcapto, etliylBiercapto, propylmereapto, butylmercapto, pentylmercapto, hexylmercapto, oetylinercapto, decylmercapto, and dodecylmercapto; in like manner, the alkoxyalkyi and the alkylthioalkyl groups may lie methoxymethyl, etlioxymc thyl, ethoxyetliyl, methylthiomethyl, and ethylthioethyl.

It is to he appreciated that while the polymerized resin may he a homopolymer, it is most desirable to utilize a mixture oi monomers to give various properties to the ultimate coating material such as good corrosion resistance, chip resistance, adherence, gloss, flexibility, durability, hardness, flow and solvent resistance.

The polymer may he prepared hy known polymerization techniques such as with an azo catalyst in an organic solvent in emulsion ox- solution polymerization.

A preferred polymerizable monomer mixture is one having a major amount of acrylic or methacrylic esters r (R in the above formula is an alkyl group) or from 15 approximately 75j£ to 99# by weight and a minor amount of an acid (R is hydrogen) in an amount from approximately 1# to approximately 25# hy weight totalling 100# hy weight.

In addition, other copolymerizahle monomers, having an ethylenically unsaturated group, may he added to the mixture of monomers recited above in minor amounts of from 0.1# to approximately 15# by weight such as aliphatic vinyls as vinyl acetate and vinyl chloride, or aromatic vinyls such as styrene, vinyl styrene, 3,5-dimetliyl styrene, para-tert-butyl styrene and alpha methyl styrene.

The coating material is preferably one having a basic pH and even more preferably a pH of 8 to 9, and preferably contains a water solubilizing agent, the amount of which can vary depending upon the polymeric material. Preferably, the water solubilizing agent is used in an amount ranging from approximately 1 part to 20 to 40 parts of resin. It is to be appreciated that any solubilizing agent may be used, such as an amine, ammonia or an alkali metal; due to the high processing temperature a high boiling material is used.

Most preferably the water solubilizing agent is a nitrogen containing material such as an alcoholic amine such as diethyl ethanol amine; 2 amino - 2 methyl - 1 propanol; diethanol amine, trietlianol amine, dimethylpropanolamine, dimethylisopropanolamine, dimethylbutanolamine, N-benzylethanolamino, 2-[2(dimethylamino) ethoxy] ethanol; and 1-[2-(dimethylamino) ethoxy ]-2-propanol.

The coating material also contains an effective nitrogenous cross-linking agent such as a urea type, melamine type, or benzoguanamine, preferably a melamine type. While a variety of melamines may be employed, tho most preferred arc the substituted melamines such as a polyalkyl ether of a polymethylol melamine wherein each alkyl group contains from 1 to 'i carbon atoms, most preferably liexame thoxy methyl melamine. 7111 The basis for selecting a good cross-linking agent is one that has stability in the high temperatures to which the coating material will be subjected, as well as being reasonably stable to corrosion resistant pigments that are added to the material, and being capable o£ supplying good cross-linking properties to the acrylic resin coating material.

Bie amount of cross-linking agent aiployed which should be sufficient to pranote cross-linking of the resin is canparable to the number of carboxyl groups present in the -resin so that the number of functional groups in the cross-linking agent is substantially equivalent to the number of functional groups present in the acrylic resin.

In order to achieve a low cross-linking temperature, it may be desirable to add a thermosetting catalyst to the acrylic resin coating material. Preferred catalysts are Froidel-Crafts acid catalysts added in an amount from approximately 10 to approximately 150 based on the nitrogenous cross-linking agent. These catalysts induce the condensation reaction between the resin and the cross-linking agent. Examples of suitable aeid catalysts that may be employed are para toluene sulfonic acid, dodecyl succinic acid, n-butyl acid phosphate and phosphoric aeid.

It has also been found desirable to add free radical 471H inhibitors which inhibit hydrolysis or other reaction of the resin that may cause instability. These inhibitors may be added in amounts of from approximately 1# to approximately 10# by weight of the coating material (acrylic resin and the cross-linking agent combined). Examples of suitable inhibitors are guiacol and hydroquinone.

In addition the coating composition may contain various materials which impart substantially improved corrosion resistance to tbe ultimate coating. These materials are water soluble metallic salts of Group VX-B of the periodic table. Examples of suitable corrosion resistant materials are chromates, dichromates, molybdates and tungstates, such as strontium cliromatc, calcium chromate, zinc chromate, potassium chromate, sodium chromate, sodium dicliromate, potassium tungstate, sodium molybdate, lead silico chromate and ammonium chromate and bichromate. In addition, complexes of inolybdic and tungstic acid and the water solubilizing amines described above such as dimethyl ethanol amine may be employed.

It is also preferred that the acrylic resin be a low molecular resin, i.e. one having a molecular weight (number average) less than about 10,000 and even more preferably less than about 5,000 with an acid number of loss than 150, preferably less than 100.

A preferred formulation is one comprised as follows: Formulation A Component Parts by weight Acrylic resin (70/ hy wt.) of a polymer of ethyl acrylate (71.4/) Acrylic acid (12.9/) and methyl methacrylate (15.7/) in an acetone, isopropanoi solvent .................................. 1.25 Ilexa methoxy methyl melamine ............. 1 Water solubilizing agent (dimethyl ethanol amine) ................. 1.1 Strontian chranate........................... 2.9 Para toluene sulfonic acid (25/ in water) ........................— 0.24 Water .................................... 9.5 It is to he appreciated that other substances may he added to the coating material sucJi as fillers, pigments, dyestuffs, colouring agents, and levelling agents. These substances or components may ho added depending upon the use to which the coated product is to he employed.

The process of simultaneous sealing and impregnating 471H the unsealed anodic coating is carried out through immersion of a freshly anodized aluminum substrate surface, in the unsealed condition, in a process vessel containing one of the above described materials. The water-borne resin materials herein proposed arc useful in a range of concentrations from less than 1$ to in excess of 70#.

The bath may be maintained at 170°F to 200°F indefinitely with no gellation or resin precipitation or separation. Simple pH adjustments maintain excellent stability by adjusting the pH to a basic pH, preferably approximately 7.5 to approximately 9.0, even more preferably approximately 8.2. Ilie .sealing and priming is accanplished for a period ranging frcm about 30 minutes to about 60 minutes, the time and temperature being inversely related. Hie anodic coating is converted frcm the unsealed condition to the sealed condition through hydraticn of the oxide and having the structural characteristics of metal oxide manohydrate and metal oxide trihydrate in accordance with the following reactions.· (ι) ai2o3 + h2o -=>2 aio(oh)->ai2o . h2o (2) 2A10(0H) + 2H„0->AI203 . 5Ho0 Following sealing/impregnation of tho anodic coating, the anodized metal is rinsed in water followed by exposure in a drying oven controlled at preferably 200°F to 5θθ°Ε for the purpose of curing. This requires in the order of 1 to 60 minutes, the time and temperature being inversely related. When the curing is of a component for the aircraft industry, temperatures greater than j00°F are not normally employed. On occasion, it may he desirable to allow the component to dry prior to the curing operation, thereby eliminating the rinsing step alter the simultaneous seal/impregnation step.

The foregoing describes a.typical processing sequence which follows the conventional steps of preparing and anodizing the aluminum substrate. Such preparation for anodizing includes (a) degreasing, (b) alkaline cleaning, and (c) deoxidizing with intermediate water rinsing after each of the opei-ations, (a), (b) and (c). Anodizing may be accomplished using the electrolytes and process control parameters necessary to develop anodic coatings of, although not limited to, the chromic, sulphuric, and modified sulphuric acid types followed by immediate water rinse. For a discussion of cleaning and finishing aluminum and aluminum alloys, see Metals Handbook, 8th Ed. (1964), Vol. 2, published by American Society I'or Metals, pp. 611-634, which is hereby incorporated by reference.

Specific examples of applying a corrosion inhibitive coating to aluminum and aluminum alloy components, specifically 7O75-T6 in accordance with the present invention arc set forth as follows: EXAMPLE 1 1. The component was vapoui* degreased using a trichloroethylene or 1-1-1 trichioroothane material and then loft in the degreaser until dry. 2. The degreased component was cleaned for approximately 18 minutes in an inhibited alkaline cleaner of pH 11.8 to 13, active alkalinity of 20# to 25# by weight concentration of approximately 5 02. per U.S. gallon and maintained at approximately 170°F. ' 3. The component was then rinsed in ambient water (approximately 70°F) for approximately 90 seconds. 4. The component was deoxidized for approximately 8 minutes in an aluminum deoxidizer compounded from 17 to 23 oz/gal (wt.) 66° Be sulfuric acid, 3 to 5 oz/gal (wt.) sodium dichromate, and 0.6 to 0.8 oz/gal (wt.) ammonium hil'luoride maintained at room temperature (approximately 70°F).

. The component was then rinsed in ambient water for approximately 2 minutes. 6. The component was then anodized for approximately minutes in 15# 66° Be sulphuric acid to 6 to 24 Volts (DC), 12 to 15 amps/ft, and maintained at approximately 70°F. 7. The anodized component was rinsed in ambient water for approximately 2 minutes. 8. The component was then immersed for approximately one hour in the water borne organic resin coating material, Formulation A, diluted two parts water to 1 part Fonnulation A, and. maintained at 1.75° + &°F. - 47111 9. Following this hath whereby the component was concurrently sealed and primed, it was air dried at ambient temperatures ior 60 + 5 minutes.

. Finally, the component was cured in a dry air oven for 15 minutes at 295° + 5°F.

EXAMPLE 2 The component to be coated was prepared following the steps 1 to 7 of Example 1. 8. The component was then scaled and primed by a bath for approximately 30 minutes in tho resin-contained water borne composition described above in Example 1 and maintained at 180°F + 5°F· 9. Following this bath the component was air dried and cured as in steps 9 and 10 of Example 1; however curing was effected for approximately 30 minutes at 255nF + 5°F in this case.

EXAMPLE 3 The component to be coated, was prepared following the steps 1 to 5 of Example 1. 6. The component was then anodized for approximately 68 minutes in 6 to 7 oz/gal (wt.) chromic acid operated at approximately 68 volts (i)C), one amp/ffc.“ (minimum) and maintained at approximately 100°F.

Tho component was then treated following the steps 7 to 10 of Example 1.

EXAMPLE h The component to he coated was prepared following the steps 1 to 6 of Example 3· Following a rinse in ambient water for approximately 2 minutes, the component was then treated following the steps 8 to 10 of Example 2.

Environmental salt spray tests were conducted with tho specimen of Example 2 in 5# salt fog operated at 95°F within a pH range of 6.5 to 7.2. After 9,000 hours of exposure of these specimens no signs of corrosion attack were observed. Comparable results were obtained in other examples tested.

From all of these tests it was concluded that maximum protection against corrosion attack is achieved through scaling at higher temperatures of the specified range.

The coated aluminum component in each of the above cases demonstrated that superior corrosion protection can be imparted to aluminum and aluminum alloys through the above described process than heretofore obtained from tbe separate processes of sealing and organic priming. This was determined by comparative tests between components conventionally anodized, scaled and zinc chromate and epoxy primed in successive steps and those treated as herein described both subjected to the same corrosion environment ior equal·, prolonged periods. Upon inspection oi'test’ components after 5900 hours of exposure in 5/ salt spray, all of the prior art (iomponents demonstrated corrosion failures whereas none of the components treated as herein .described exhibited any' signs of corrosion.

Further, the cost of performing the anodic coating scaling and priming operations simultaneously was found to he considerably less than performing separate operations of scaling and priming. This is duo principally to the absence of manual labour input normally required during the primer application operation. The process of the present invention is useful l'or aluminum and aluminum alloys wherein the alloys comprise- aluminum and other elements of the Periodic Chart, sec Metals Handbook, Sth Ed. (1961) published by American Society for Metals, page 917 which is hereby incorporated by reference.

Claims (11)

CLAIMS:

1. A protective coating process for an aluminum or aluminum alloy component comprising anodizing the component, applying a coating composition ocnprising a water soluble acrylic

2. A process according to claim 1 wherein said sealing and 10 priming is accomplished at a temperature range of 170°F to 200°F for a period ranging from about 30 to 60 minutes, the time and temperature being inversely related.

3. A process according to claim 1 or 2,wherein said application of the coating composition is accomplished by immersion 15 of said component in the composition for about one hour and maintaining the temperature at approximately 175°F.

4. A process according to claim 1 or 2, wherein said application of the coating composition is accomplished by immersion of said component in the composition for about 30 2o minutes and maintaining the temperature at approximately 180°F. 5. Agent is hexamethoxy methyl melamine.

5. A process according to any preceding claim, wherein the coating composition includes a stable nitrogenous crosslinking agent in an amount sufficient to promote cross-linking of the resin. -1747111 5 polymer or copolymer to the anodized component, said composition sealing the anodic coating through hydration and simultaneously priming said anodic coating, and curing the sealed and primed component at a temperature range of 200°F to 500°F.

6. A process according to claim 5, wherein the crosslinking agent is polyalkyl ether of a polymethylol melamine wherein each alkyl group contains from 1 to 4 carbon atoms.

7. A process according to claim 5 wherein the cross-linking

8. A process according to claim 5, wherein said curing is effected by adding to said coating composition a nitrogenous cross-linking agent and a Friedel-Crafts catalyst in an amount sifficient to promote thorough curing when exposed to 10 temperatures of 200°F to 500°F for approximately 1 to 60 minutes, the time and temperature being inversely related, . followed by curing under the aforesaid conditions.

9. A process according to any preceding claim, wherein the coating composition includes a corrosion inhibitor containing 15 water soluble metallic salts of chromium, molybdenum and tungsten.

10. A protective coating process for an aluminum or aluminum alloy component substantially as herein described with reference to the Examples. 2o

11. An aluminum or aluminum alloy component having a protective coating as prepared by a process as claimed In any preceding claim.