US 3854961 A
An improved method for manufacturing shell molds which comprises coating a pattern with a binder composition containing a refractory metal and a zirconium compound, gelling the coated pattern and thereafter contacting the gelled coating with an acidified volatile organic solvent before applying subsequent coats. Latter backup coats which are made from a slurry containing hydrolyzed or partially hydrolyzed orthosilicates and refractory materials, may be gelled and treated with an acidified organic solvent.
Description (OCR text may contain errors)
United States Pa t gent A [1 1 Flasch Dec. 17, 1974  PREPARATION OF HIGH TEMPERATURE 3,392,036 7/1968 McLeod l06/38.35
SHELL MOLDS 1 3,455,368 7/ 1969 Shepherd 3,537,949 11/1970 Brown et al. 106/383 Inventor: John Robert Flasch, Adria Mich. 3,616,840 .11/1971 Dunlop 106/383 Assigneez Stauffer Chemical p y, 3,752,681 8/1973 Moore 106/383 Westport, Conn.  Filed: Apr. 9, 1973 Primary ExammerLorenzo Hayes  Appl. No.: 349,148
Related US. Application Data  ABSTRACT  Continuation-impart of Ser, No, 102907, e 30, An improved method for manufacturing shell molds 1970. which comprises coating a pattern with a binder composition containing a refractory metal and a zirconium  US. Cl 106/38.3, 106/3835, 106/389, compound, gelling the coated pattern and thereafter 164/20 contacting the gelled coating with an acidified volatile  Int. Cl B28b 7/34 organic solvent before applying subsequent coats. Lat-  Field of Search 117/5 .1, 5.2; 106/383, ter backup coats which are made from a slurry con- 106/3835, 38.9; 164/20 taining hydrolyzed or partially hydrolyzed orthosilicates and refractory materials, may be gelled and  References Cited treated with an acidified organic solvent.
UNITED STATES PATENTS 9 Cl N D 2,945,273 7/1960 Herzmark et al. 106/383 rawmgs PREPARATION OF HIGH TEMPERATURE SHELL MOLDS This application is a continuation-in-part of application Ser. No. 102,907, filed Dec. 30, 1970 now abandoned.
This invention relates to the manufacture of high temperature shell molds and more particularly to high temperature moldsfor titanium casting.
Precision castings of metal and other types of material cast in the molten state are used in many industries and, generally, such castings are made in expendable molds. There are three general types of processes for making the expendable molds and these may be classified as the lost wax, the single investment and the double investment processes. These all have one thing in common; they are one-use molds in which the mold is generally destroyed in removing the casting therefrom. To provide an economical process, a master mold or pattern is initially prepared, from which refractory molds are made by one of the above processes. Such molds generally include a refractory and a binder. The art of preparing the materials for molds has heretofore required precision techniques and highly trained personnel in the art of mixing the materials and preparing the expendable molds.
Generally, the preparation of the binder was the critical part of the process, as it had to be carefully and critically prepared according to a most precise recipe. One commonly-used type of binder included a mixture of an alcohol, an acid and an organic silicate, sometimes with additional ingredients added.
Although there are many types of metal casting molds, few of them are applicable to the high integrity precision casting of such metals as chromium, hafnium, molybdenum, niobium, tantalum, titanium and vanadium.
Heretofore, in the manufacture of precision castings by investment shell casting techniques, a disposable pattern was made by injecting a pattern material into a die and thereafter gating the pattern to a central sprue to form a pattern cluster.
One method for treating the pattern cluster is described in US. Pat. No. 3,537,949 to Brown et al. In the Brown et al. process, the pattern cluster was dipped into an agitated slurry of molding material, drained, stuccoed while still wet with'particulate mold material and dried, preferably to a solvent content of less than percent by volume. The dipping, draining, stuccoing and drying sequence was repeated the desired number of times to produce a laminated investment shell mold having the desired thickness and strength.
Thereafter the disposable pattern is removed by methods such as melting or solvent treatment and the mold cured by firing at a temperature sufficient to remove the volatiles and provide adequate bonding. The molds are then heated and filled with molten metal and after cooling, the castings are removed from the sprue and finished in the usual manner.
Although, gaseous ammonia has been used to gel shell binders and thereby accelerate the preparation of these shell molds, it was found that residual ammonia remaining on the pattern cluster causes spot-gelling of the slurry when the dipping step is repeated.
Contrary to the heretofore described procedure, the applicant has-found that high temperature shell moldsmay be prepared without removing the solvent after each application of stucco material. In addition, the applicant has found that the shell mold process may be accelerated by the use of gelling agents such as ammonia and amines without effecting the pot-life of the binder slurry.
Therefore, it is an object of this invention to provide shell molds for precision refractory castings.
Another object of this invention is to provide an improved method for the production of high temperature shell molds.
More specifically, it is an object of this invention to provide a new and improved method for making high temperature shell molds, which while embodying some of the desirable features of the investment casting shell process, eliminates some of the undesirable characteristics inherent therein. In addition the improved method substantially reduces the time required to produce a desirable shell mold.
The foregoing objects and others which will become apparent from the following description are accomplished, generally speaking, by a sequence of novel processing steps in which a previously cleaned and etched disposable pattern is coated with a precoat slurry containing a zirconium compound,'a refractory metal powder and an organic solvent, the excess slurry is removed and the coated pattern stuccoed with a coarse refractory metal. The precoated pattern is gelled in the presence of a basic material, i.e., ammonia, or an amine and thereafter contacted with an acidified volatile organic solvent. The sequence 'of steps is repeated until the desired precoat thickness is achieved.
The precoated pattern is subsequently coated with a backup coat containing a hydrolyzed, or partially hydrolyzed orthosilicate binder and a ceramic material diluted with an organic solvent, stuccoed with a coarse ceramic material and thereafter air dried before applying subsequent coats. As a final step, a sealing coat is frequently applied, this being achieved by dipping the shell into the refractory-binder slurry.
After the final coat 'has been applied, the shell is heated to fluidize and remove the disposable pattern from the shell mold with which the pattern has been invested. The resulting mold is cured by heating the mold to a temperature of between and 450C. to remove most of the volatiles and thereafter fired in a furnace at temperatures from 800 to 3000C in the presence of a nonoxidizing atmosphere which is nonreactive toward the metal present in the mold.
The first coating applied about the pattern is referred to as the precoat" and as above indicated, it has'been found that the drying step can be omitted during the precoat application by gelling each successive coat with a basic compound and thereafter subjecting the gelled coating to an-acidified volatile organic solvent. Treatment of each successive coating with the basic compound avoids the drying step conventionally used in the industry and reduces the amount of time necessary for preparing the shell molds.
Prewetting of the shell with an acidified solvent, neutralizes the basic residue remaining on the gelled surface and also enhances the penetration of a successive coating into the pores of the prior coating, thereby resulting in the formation of shell molds having superior quality. In addition, treatment of the shell with the acidified solvent prevents spot-gelling on the coated surface when a subsequent coating is applied, thus providing for a more uniform shell coating. A shell mold thus produced is possessed of high permeability and excellent crack resistance. Also treatment of the shell with the acidified solvent prevents premature gelling of the slurry in subsequent coating steps, thereby extending the pot-life of the slurry;
Suitable examples of acidified organic solvents which may be used totreat the gelled coating are aliphatic and aromatic hydrocarbons such as hexane, heptane, octane, benzene, toluene, xylene; chlorinated hydrocarbons such as chloroethylene, chlorobenzene and the like; aliphatic and aromatic alcohols such as methanol, ethanol, propanol, butanol, hexanol, octanol, benzyl alcohol and the like; ketones such as acetone, 2- butanone, Z-pentanone, 3-pentanone, 4-heptanone, 2,4-pentainedione and the -pentanedione esters such as ethyl formate, butyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, amyl acetate, ethyl propionate, ethyl valerate; ethers such as ethyl ether, n-propyl ether, n-butyl ether, methyl-n-propyl ether, methyl-n-butyl ether, 1,4-dioxane; glycols such as ethylene glycol, propylene glycol, l,2-butanediol, 2,3-
- butanediol and glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol col monobenzyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol monobenzyl-ether and diethylene glycol mono-ppropylphenyl ether. it is preferred that the organic solvent have a boiling point below about 250C. Although, it is not essential, it is preferred that the solvent used to prewet the shell be the same as or of a similar class as the solvent employed in the slurry.
A sufficient amount of acid should be present in the volatile organic solvent to neutralize the residual gelling agent pre sent on the shell surface. Although, the amount of acid present in the solvent is not critical, it is preferred that the available hydrogen ion concentration be from about 0.1 mole per liter to about 0.000] mole per liter of solvent solution. In other words the titrated organic solvent acidity should be from about 0.001 to about 5 percent and more preferably from about 0.01 to about 3.0 percent by weight calculated as hydrochloric acid.
Suitable examples of acids which may be employed are inorganic mineral acids such as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, phosphoric acid, fluoroboric acid and the like. Suitable organic acids which may be used are carboxylic acids such as formic acid, acetic acid, propionic acid, benzoic acid, phthalic acid and mandelic acid; substituted carboxylic acids such as chloroacetic acid; dicarboxylic acids such as oxalic acid, malonic and maleic acids; hydroxy carboxylic acids such as citric and glycolic acids. Other acids which may be used as sulfonic acids such as toluene sulfonic acid and chlorosulfonic acid and alkyl phosphoric acids.
The shell molds of the present invention comprise a facing portion including a major proportion of at least 50 percent by weight of finely divided metal particles of columbium, molybdenum, tantalum, titanium or tungsten, all bonded together with a suitable zirconium binder and a backup reinforcing portion composed of finely divided particles of shell mold backup material including ceramic mold materials and refractory metal oxide binders, all the mold components being integrally bonded together to form a strong structure having an inner face portion comprised predominately of one of the indicated metals.
in accordance with the invention there is used as the binder material for the refractory particles in the precoat slurry a zirconium compound, preferably a hydrolyzed, or partially hydrolyzed-zirconium compound, in an organic solvent solution and following the application of at least the first coat on the pattern, the binder is caused to gel through the use of a suitable base, preferably ammonia, or an amine. In the backup coating, the binder material for the refractory particles is an acid-hydrolyzed orthosilicate solution.
Refractories which may be employed in the precoat slurry are many and varied. Generally, such wellknown metals as zirconium molybdenum, tungsten, tantalum, columbium, hafnium, titanium and vanadium are applicable. in addition, a minor amount of oxides of the above metals may also be included in this slurry.
As indicated, the'refractories may be employed severally, or in any combination deemed desirable with relation to the particular casting operation contemplated. Also, the particle size of the refractory is selected in relation to the specific purpose for which the mold is being fabricated. in some cases, it is desirable to employ a mixture of relatively coarse particles and relatively fine particles. The stuccoing referred to above is normally carried out using refractory particles which are coarser than the particles used in the slurry.
A shell mold conforming to the invention is produced without the use of any binder other than the zirconium compound as a precoat followed by a backup coat of a hydrolyzed orthosilic'ate. It is particularly important that the precoat slurry contain the zirconium binder, thereby obviating the difficulties previously experienced with collodial silica and hydrolyzed orthosilicates, for example, which tend to react with metals to cause surface defects in the finished casting.
The zirconium binder may be prepared by hydrolyzing a zirconium compound in the presence of an inorganic acid, such as hydrochloric acid, sulfuric acid. nitric acid and the like. Examples of suitable zirconium compounds are zirconium alkoxides, such as zirconium methoxide, zirconium ethoxide, zirconium isopropoxide, zirconium butoxide, zirconium hexyloxide, zirconium heptyloxide, zirconium octyloxide, zirconium decyloxide and the like. Other zirconium compounds which may be employed are partially hydrolyzed zirconium alkoxides such as (C,l-l 0) Zr O, (C,,H,,0) Zr o (C H Oho 2110;, and (C H O) Zr O zirconium carboxylates such as zirconium acetate, zirconium propionate, zirconium butyrate', halogenated carboxylates such 'as zirconium tetra-(trifluoroacetate); zirconium hydroxycarboxylates such as trisglycolatozirconic acid, tris-lactatozirconic acid, tetramandelatozirconic acid, zirconium trihydroxyglutarate; zirconium ketonates such as zirconium acetylacetonates; zirconium polyolat'es such as those obtained from polyols, e.g. glycols, glycerol and pentaerythritol.
In addition other zirconium compounds having the general formula:
in which R is a monovalent hydrocarbon radical having up to 10 carbon atoms or hydrogen, R is a divalent hydrocarbon radical having up to 10 carbon atoms and n is either 1 or 2 may be employed. These zirconium compounds can be prepared, for example, by an ester interchange reaction in which a mixture containing a zirconium alkoxide, such as zirconium ethoxide and an alkylene glycol monoalkyl ether, is heated to an elevated temperature at a pH of 1.4 to 4.0, while distilling off the lower boiling alcohol thus produced. Such zirconium compounds may be hydrolyzed to form the zirconium binders of this invention.
The binder used in the aforementioned precoat slurry may contain from 5 to about 50 percent zirconium and more preferably from about to 30 percent zirconium based on zirconium dioxide.
Where the binder used in the precoat is a hydrolyzed zirconium alkoxide, the acid catalyzed hydrolysis is so carried out that from 30 to 100 percent, more preferably 45 to 70 percent, of the zirconium alkoxide is hydrolyzed while the solvent is employed in such quantity that the ZrO content of the binder is of the order of 10 to 30 percent by weight. As previously indicated, hydrochloric acid is normally used as the catalyst, preferably in l to 5 percent aqueous solution. The binder should have a pH of from 1.0 to about 3.2 and should be allowed to age for a day or so before use.
The backup coat contains hydrolyzed and partially hydrolyzed orthosilicates of the formula:
in which each R' is a monovalent hydrocarbon group having up to 10 carbon atoms free of aliphatic unsaturated, or an R'OR group in which R and R are the same as above. The R and R' radicals can be the same, or different and are selected from alkyl, aryl, aralkyl and alkaryl groups. Suitable alkyl groups are methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, isoamyl, neopentyl, hexyl, heptyl and octyl groups. Suitable aryl, aralkyl and alkaryl groups are phenyl, benzyl, ethylphenyl, isopropylphenyl and butylphenyl groups. The preferred silicates are those where the R groups are alkyl radicals containing one to six carbon atoms. Divalent hydrocarbon radicals represented by R are aliphatic hydrocarbon radicals such as ethylene, trimethylene, pentamethylene, hexamethylene, octamethylene and aromatic radicals such as phenyl.
It is preferred that these hydrolyzed orthosilicates be a prehydrolyzed type commercially available, or prepared by acid hydrolysis, such as commercially available forms of ethyl silicate, including monomeric tetraethyl orthosilicate, condensed" ethyl silicate and ethyl silicate 40. Condensed ethyl silicate consists of monomeric tetraethyl orthosilicate plus up to 10 percent polysilicate, while ethyl silicate 40 consists of monomeric tetraethyl orthosilicate and mixed polysilicates having an average of 5 silicon atoms per molecule.
Where the backup coatings are gelled with the aid of a base, a partially hydrolyzed ethyl silicate may be employed as the binder. However, where the subsequent coatings are not gelled, it is preferred that the ethyl silicate in the binder be 100 percent hydrolyzed and contain from 5 to percent SiO by weight as determined by the quantity of solvent used.
As mentioned, heretofore, gelation of the coatings is preferably effected using ammonia, but other bases, particularly amines, may be substituted if desired. Suitable amines are aliphatic, aromatic and heterocyclic amines having up to 10 carbon atoms such as methylamine, monoand triethanolamines, n-butylamine,
propylamine, cyclohexylamine, hydrazine, piperidine and the like.
To prevent premature gelation of the backup slurry, a small quantity of an acid having an acidity greater than that of the hydrolyzed ethyl silicate or an acidic buffering agent can be added to the binder solution. The pH of the slurry will thereby be maintained in the range from about 1.0 to about 3.2. Suitable acids and buffering agents include hydrochloric acid, phosphoric acid, sulfuric acid and ammonium nitrate. The quantity of acid or buffering agent employed will vary depending upon the acid or buffering agent used and the refractory material present in the slurry. Typically, when 5000 grams of fused silica is employed as the refractory material, from about 1.0 to about 3.0 milliliters of concentrated sulfuric acid can be used.
Refractories which may be employed in the backup coat slurry are many and varied. Generally, such wellknown refractories as the oxides and silicates of silicon, aluminum, zirconium, zinc, tin, magnesium, chromium and titanium are applicable. These include the acidic minerals, the neutral or amphoteric minerals, especially those whose primary elements are found in Group II of the Periodic Table, the gel acid minerals, e.g., allophane and the basic minerals, e.g., zirkelite. Also, there should be mentioned the many synthetic materials including the zeolites, molecular sieves, ion exchange resins and similar substances.
Apart from the foregoing, grog, calcined clay, silica flour or sand and fused silica have been determined as suitable.
Acidic, weakly acidic or neutral amphoteric minerals are preferred for use in the practice of the invention. As examples there may be cited particularly: zircon, sillimanite, zirconite which is a mixture of silicates and oxides of zircon and mullite which is a synthetic mixture of alumina and sand.
Zircon and fused silica are particularly preferred for use as the refractory in the backup coat.
As indicated, the refractories may be employed severally or in any combination deemed desirable with relation to the particular casting operation contemplated. Also, the particle size of the refractory is selected in relation to the specific purpose for which the mold is being fabricated. Generally, the refractory particles used in stuccoing are coarser than the particles used in the slurry.
Suitable solvents for the precoat and backup coats are for example, alcohols, such as methanol, ethanol, propanol, butanol, hexanol and the like; ketones such as acetone, methyl ethyl ketone and the like; ethers such as monoalkylene glycol monoalkyl ethers, dialkylene glycol monoalkyl ethers, monoalkylene glycol dialkyl ethers and dialkylene glycol dialkyl ethers. Excellent results have been obtained using ethylene glycol monoethyl ether and this solvent accordingly is preferred. Other solvents which have been employed are ethylene glycol, glycerol and the like.
The amount of binder used in the precoat slurry is substantially determined by the particle size of the refractory since the finer the refractory the more readily it absorbs the binder, hence the more binder required. In some cases where the particle size of the refractory is coarse, the amount of binder required may be such that for every 100 parts of refractory there need be used only that amount of binder required to provide 1 part ZrO Conversely, where the refractory is extremely fine, it may be necessary to use with each 4 parts of refractory that amount of binder needed to provide 1 part of ZrO However, the viscosity of the precoat slurry should be in the range from about to about 35 seconds on a No. 4 Zahn Cup.
As in the precoat slurry, the amount of binder used in the backup slurry is substantially determined by the particle size of the refractory. Thus, the amount of binder required may be such that for every 4 to 100 parts of refractory there need be used only that amount of binder required to provide 1 part SiO However, the viscosity of the backup coat slurry should be in the range of from about 7 to about seconds on a No. 4 Zahn Cup.
After the pattern has been covered with the predetermined number of dips and stucco coats, it is dried at a temperature which may vary from about C. up to the fluidizing temperature of the disposable pattern. The temperature may then be increased in a nonoxidizing atmosphere to fluidize and remove the disposable pattern therefrom. The resulting shell mold is subsequently fired by exposure to a'temperature of from about 800 to 3000C. in a nonreactive, nonoxidizing atmosphere for from 1 to 12 hours. Examples of gases which are nonreactive with the metals present in the face coatings are hydrogen, the inert gases and dissociated ammonia.
Various aspects of the invention are illustrated by the following examples which are not to be taken as in any way limiting the scope of the invention. All quantities are given in parts by weight unless otherwise specified.
EXAMPLE 1 PRECOAT SLURRY PARTS Zirconium isopropoxide 500 isopropanol 334 Molybdenum metal 4000 (3-5 microns) The above mixture was stirred for three hours prior to use.
The backup slurry was prepared in the following manner:
BACKUP SLURRY PARTS Water 0.6 Silbond H-4. (a partially 15.8
hydrolyzed ethyl silicate binder available from Staufier Chemical Co.)
Continued BACKUP SLURRY PARTS lsopropyl alcohol 14.0 Fused Silica flour P2 46.4
(available from Nalco Chemical Co.) Fused Silica flour PlW (available 23.2
from Nalco Chemical Co.)
Water was added to the Silbond H-4 and the mixture allowed to stand overnight before the alcohol and fused silica were added. To the backup slurry, sufficient sulfuric acid was added slowly to reduce the pH to about 2.0.
The wax pattern tree was dipped into the precoat slurry, removed, drained and stuccoed with molybdenum metal (-30 to +200 mesh). The stuccoed coating was placed in a cabinet filled with ammonia gas to gel for 1 minute. After gelling, the shell was dipped in an acidified solution of isopropanol, removed and immediately dipped into the precoat slurry. The cycle of dipping in the precoat slurry, removing, draining, stuccoing, gelling and dipping in acidified isopropanol was repeated twice. The coated pattern was next dipped into the backup slurry, removed, drained and stuccoed with Nalcast S-2 (available from Nalco Chemical Co.). The shell was dried with forced air for about 35 minutes. The cycle of dipping in the backup slurry, removing, draining, stuccoing, with Nalcast 8-2 and drying was repeated three times. The shell was allowed to dry overnight in a well-ventilated area. Dewaxing was achieved by means of a high-pressure steam autoclave and thereafter the shell was dried at a temperature of about 350C. in a nonoxidizing atmosphere to remove all wax and carbon. The shell was then fired at 900C. in a reducing atmosphere to convert the precoat to a zirconium oxide-bonded shell coating and remove traces of volatiles from the shell. While the shell was still hot, molten titanium was poured into it. After the casting had cooled, the friable shell was broken off by vibration to provide a casting which showed no appreciable scaling or imperfections.
EXAMPLE 2 The process of Example 1 was repeated except that the precoat slurry contained the following ingredients:
Tungsten metal (30 to 200 mesh) was substituted for molybdenum as the stucco coating.
EXAMPLE 3 The process of Example 2 was repeated except that the precoat slurry contained the following ingredients:
INGREDIENTS PARTS Zirconium butoxide Ethylene glycol monoethyl Continued INGREDIENTS PARTS ether 380.0 Tungsten metal (3-5 microns) 6490.6 Water 36.0
EXAMPLE 4 The process of Example 2 was repeated except that columbium was substituted for tungsten metal.
EXAMPLE 5 The process of Example 2 was repeated except that tantalum was substituted for tungsten metal.
EXAMPLE 6 The process of Example 1 was repeated except that the precoat was gelled with trimethylamine.
EXAMPLE 7 The process of Example 1 was repeated except that the precoat was gelled with gaseous dicyclohexylamine.
EXAMPLE 8 The process of Example 2 was repeated except that a product obtained from the ester interchange of zirconium ethoxide and ethylene glycol monoethyl ether was substituted for the zirconium ethoxide.
Although specific examples of the invention have been described herein, it is not intended to limit the invention solely thereto, but to include all variations and modifications falling within the spirit and scope of the appended claims.
What is claimed is:
1. In a method for preparing shell molds for precision casting which comprises precoating an expendable pattern with a slurry containing a refractory material, a zirconium compound and an organic solvent, stuccoing the coated pattern, contacting the coated pattern with a gelling agent selected from the class consisting of ammonia and amines having up to 10 carbon atoms and thereafter applying a backup coat comprising a refractory material, an acid hydrolyzed orthosilicate binder and an organic solvent to said precoat, the improvement which comprises contacing said gelled precoat with an organic solvent having a boiling point below about 250C. and containing sufficient acid to provide a hydrogen ion concentration of from 0.001 mole to 0.1 mole per liter of solvent, said solvent is selected from the class consisting of aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons, alcohols, ketones, esters, ethers, glycols and glycol ethers.
2. The method of claim 1 wherein ammonia is used as the gelling agent.
3. The method of claim 1 wherein the precoat organic solvent is an alkylene glycol monoalkyl ether.
4. The method of claim 1 wherein the hydrolyzed orthosilicate is an alkyl silicate.
5. The method of claim 1 wherein the backup coat is contacted with a gelling agent and thereafter the gelled coating is treated with an organic solvent having a boil ing point below about 250C. and containing sufficient acid to provide a hydrogen ion concentration of from 0.0001 to 0.1 mole per liter of solvent, said solvent is selected from the class consisting of aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons, alcohols, ketones, esters, ethers, glycols and glycol ethers.
6. The method of claim 1 wherein the zirconium compound is a hydrolyzed zirconium compound.
7. The method of claim 1 wherein the zirconium compound is a partially hydrolyzed zirconium compound.
8. The method of claim 1 wherein the precoat organic solvent is isopropanol.
9. The method of claim 1 wherein the sequence of steps is repeated until the desired precoat thickness is achieved and then the backup coat is applied over the