US2738897A - Tin plate baking pan - Google Patents

Description

March 20, 1956 .1. J. RUSSELL ET A1.

TIN PLATE BAKING PAN 2 Sheets-Sheet l Filed April 18, 1950 March 20, 1956 .1. .1. RUSSELL ET As. 2,738,897

TIN PLATE BAKING PAN Filed April 18, 1950 2 Sheets-Sheet 2 United States Patent O and lack Kollman, Chicago, Ill., assignors to Ekco Products Company, Chicago, Ill., a corporation of illinois Application April 18, 1950, Serial No. 156,671

The portion of the term of the patent subsequent to November 22, 1972, has been disclaimed 2 Claims. (Cl. 220-64) This invention relates to a composite metallic structure which may be fabricated into shaped metallic articles having outer surfaces rendered highly absorptive to radiant energy while retaining a relative high resistance to corrosion and inner surfaces which are substantially reflective to radiant energy; and this application is related to the co-pending application Serial No. 156,672 tiled April 18, 1950, now Patent No. 2,724,526, issued November 22, i955.

An object of our invention is the provision of a simple, direct and thoroughly practical process for chemically treating tin and tin alloys by anodic chemical treatment in various electrolytes so that the outer surfaces of the tin and tin alloys exhibit excellent heat absorption.

Another object of our invention is to provide a fabricated article which has been anodically treated wherein the outer surfaces of said article shall be characterized by excellent heat absorption, high abrasion resistance, and continuity and uniformity of the treated tin or tin alloy surfaces.

Another object of our invention is the provision that the heat absorbing outer coating produced by the chemical process for treating the surfaces of tin or tin alloys shall be adherent and suthciently ductile in order that articles may be fabricated by'mechanical drawing, forming, and/ or bending operations.

Another object of our invention is the provision that a shaped composite sheet metal fabricated article, such as a baking pan, cooking utensil or a process tray, possess outer surfaces having relatively high heat absorbing properties, and inner surfaces which are substantially transparent to and which will allow the transmission of reflected radiant energy from the co-extensive tin layer.l

Other objects and advantages of this invention will be made more apparent as this description proceeds particularly when considered in connection with the accompanying drawings in which:

Fig. l is a plan view of a baking pan embodying the invention;

Fig. 2 is a fragmentary cross sectional view taken along the lines 2 2 in Fig. l, showing a schematic cross section of the transparent oxide coating, the opaque coating alloy layers, tin layers, and the steel base;

Fig. 3 is a photornicrographic cross sectional view of a portion of a tin plate area showing the transparent oxide coating, tin layer, alloy layer, and steel base (2500)( magnification) Fig. 4 is a photomicrographic cross sectional view showing the relative thickness of the opaque oxide coating wherein the time of chemical treatment approaches about one minute (2500 magnification), and

Fig. 5 is a photomicrographic cross sectional view showing the relative thickness of the opaque oxide coating which is substantially greater than the tin layer wherein the time of chemical treatment approaches about a few minutes (2500)( magnification).

Referring now to the illustrated embodiments of this invention, the metallic shaped article, such as a baking pan, is designated generally by reference numeral 2, its lower transparent and reflecting inner surface, being shown by 4, and its inner transparent and reflecting side surfaces by numeral 6. The outer opaque side surfaces are designated by numeral S, and the outer opaque bottom surface thereof by numeral l0. l

The shaped article 2 as shown, is of the conventional folded end type baking panpalthough this type is being used for illustration, the invention shall not bey limited to such construction. The shaped article 2 is first formed from a flat composite sheet metal structure by the process of blanking into a desire dimension and then forming the blank into a conventional folded end baking pan in a manner well known in the art.

As conducive to a clearer understanding of certain features of the invention, it may be noted at this point that the articles 2, having surfaces of tin or tin alloys, shall also include tinplate; although this invention may be adapted to other types of tin coated shaped articles formed of other base materials such as copper, brass and the like. 'Y

Hot dipped tin plate is generally considered to be steel base metal coated on its exterior surface with metallic tin, wherein intermediate to the tin and steel interface, a tin-iron alloy composition layer is formed. Tin plate may be formed byl either the conventional method of hot dipping or by electrolytically depositing the tin on the surface. The coating weight of the tin is usually specified in pounds of tin per basis box, or in grams of tin per square metre of tin. The conversion of weight per basis box to linear thickness depends on an assumed density which is compensated by the fact that in the hot dipped process, there is a greater proportional thickness of the alloy layer formed than in the electrodeposited process.

It is generally accepted that one pound per basis box is equivalent to 0.0000606 inch thick of tin on each surface of the tin plate. One and one half pound tin plate is generally assumed to be about 0.0000909 inch thick. The proportional ratio between thickness and total weight of a tin plate can be approximated from the relative proportion above mentioned for any weight of hot dipped tin plate.

In the case of the electrolytic process, tin plate thickness coatings of 8 ounces to l0 ounces per basis box is generally accepted, although some applications of tin plate may use less than 1% pound but generally not greater than lVz pound per basis box.

In the present invention, the anodic chemical treatment of tin plate is primarily directed toward tin plate having the tin content greater than about 11.41 pound per basis box which would primarily be adaptable to hot dipped tin plate, although the scope of the invention is also applicable to electrolytic tin plated articles.

The formation of oxide coatings of tin, as heretofore known may be produced by converting the tin into the oxide by subjecting the tin layer under oxidizing conditions, such as air, at elevated temperatures. This process of subjecting the tin to oxidizing conditionsat elevated temperatures, has been conventionally used by the baking industry in converting the tin layers to oxides of tinby placing formedY articles of tin plate in baking ovens, at an elevated temperature of approximately 400 to 425 for a period in excess of about 4 to l2 hours. This has been conventionally called the burning in or burning out process, wherein shaped articles 2, baking pans, acquire colors ranging from the interference films of light iridescent hues, such as yellow, blue and light greens.

It has been found that these oxide films possess wide variations in color characteristics; and it is highly desirable to eliminate these wide variations, in order to produce uniform and consistent bread crust color. in addition the tin-iron alloy layer intermediate to the tin layer and the steel base is brittle and less corrosion resistant than the tin layer. The tin-iron alloy layer is substantially increased with respect to the available metallic tin layer by these lengthy burning in operations; and therefore, it is important to reduce the burning-in time to about 30 minutes to one hour. The baking industry finds it economically undesirable to subject the bread pan to long periods of burning in because it ties up the baking ovens and baking pans, as well as personnel in a non-productive operation and also produces variable results.

In addition, the temperature controls of the oven may vary considerably. It has been found that oven temperatures generally will exceed the melting point of the tin layer on the surface; and as a result, the tin-iron layer will be substantially increased in thickness and the baking pans will be subsequently destroyed by the evaporation and decomposition of tin surface layer. Also uniformity of crust color of breadis a definite sales factor, and a baker who can produce a uniform loaf of bread in the initial baking operation without the conventional burning in of a new pan set will increase his productive capacity.

In addition, the adherence of oxide of tin, by the conventional burning in method has been found to be very poor. Chemical factors as well as atmospheric factors which control the adherence of oxides of tin are not well defined, and it is known that variations of the chemical and physical composition of the surface layers of the tin will produce variations in adherence of the oxide coatings. Therefore, it is desirous to secure an oxide coating formation having heat absorption which is uniform and reproducible and at the same time to have a surface oxide coating which will not tend to reect heat energy that impinges upon the surface of objects made of tin plate.

An outstanding object of the invention accordingly is the provision of an economical and an industrially practical process for anodically treating tin plated articles wherein Work of widely varying quality with respect to chemical and physical tin surfaces composition, as well as shapes of manufactured articles, may be employed. In addition, the condition of the surface of the tin plate may be physically varied by mechanical operation, and under these conditions a uniform oxide coating having excellent adherence and continuity showing substantial reduction of spangling due to the breakdown of large crystal growth on the tin surfaces, may be produced within a predetermined range of wholly practical conditions.

Referring now more particularly to the practice of this invention, the tin plate will normally have wide variations in the amount of tin on the surface, and will vary widely with respect to surface conditions, such as chemical composition, the presence of embedded organic foreign matter, porosity of the tin layer, and the crystalline structure of the tin surface. It is undesirable to have a heat reiiecting exterior surface in a bread baking operation, and the conventional burning in process will reproduce a coating corresponding to the initial surface condition which is generally highly reflective.

The anodic chemical process of the present invention for treating tin surface layers will substantially reduce the heretofore mentioned objections; and in addition, will substantially remove objectionable carbonaceous deposits within the porous tin plate, such as grease and oil. In addition the anodic chemical process will tend to seal the pores of the tin plate, thus reducing the tendency toward corrosion of the tin plate. In the chemical anodic treatment of tin plate articles 2 or products of various shapes and configurations, the tin plate layer may be chemically treated by using one or more of the articles or products as the anodes in an electrolytic bath.

The electrolytic bath may contain at least one or more or combinations thereof of a complexing reagent consisting of substantial amounts of (l) polybasic organic acids,

such as citric acid, picric acid, tartaric acid, oxalic acid, malic acid, maleic aid, and succinic acid; and (2) monobasic organic acids, such as acetic acid, lactic acid, propionic acid, benzene sulphonic acid, trichloroacetic acid and salicylic acid; and (3) non-oxidizing inorganic acids, such as phosphoric, boric, molybdic, tungstic, and hydrouoric acids; and (4) aqueous soluble salts, such as the alkali metal and/or alkaline earth salts of the above mentioned organic acids, or inorganic acids and other metallic salt compositions. It has been found that combinations of tde coinplexing reagents in aqueous solution may be employed such as phosphoric acid combined with citric acid, phosphoric acid combined with sodium phosphate; phosphoric acid combined with oxalic acid, acetic acid combined with sodium citrate, and sodium phosphate combined with sodium citrate. The scope of this invention shall not be limited to the specific chemical composition of the complexing reagent.

In the anodic treatment of tin plated articles 2, and products thereof, in the electrolyte, it has been found advantageous to maintain current densities ranging from about 4 amps. per square foot to about 60 amps. per square foot of tin surface layer undergoing treatment together with a solution temperature of at least 50 C. and usually more, up to the maximum temperature which falls below the boiling point of the solution so as not to cause excessive evaporation. Under the chemical conditions specified together with time of immersion, it is possible to obtain a uniform meta stable tin oxide coating on the tin layer having a thickness ranging from about 3 to 5 micro inches to almost complete conversion of the free tin layer.

It is preferred to employ an electrolyte which by weight consists of at least 0.5% up to about 30% of the complexing reagent wherein the pH of the solution may be adjusted if necessary, with the corresponding acid or salts or combinations thereof so that the pH range may vary from about a pH of 2 to a pH of about 8; and the remaining parts needed to form 100% by weight being substantially of water. It has been found that the anionic stanno complexes of tin are more readily formed when the pH is within the range of about two and one-half (2l/z) to ive (5).

One or more other tin plated articles 2, such as tin formed baking pans, may be used as the anodes of the electrolytic solution, and are then subjected to the anodic treatment while maintaining a preferred solution bath temperature of about to 98 C. and a current density of about 20 to 40 amperes per square foot. The cathode may consist of shaped metallic electrodes, such as stainless steel or monel, or the lead lined tank may even be used as the cathode. Under such temperature conditions, the solution has been found to remain stable to the extent that the complexing reagent is substantially retained in the bath with respect to the initial weight percentage. The complexing reagent had not been substantially chemically removed from the solution.

A uniform meta stable oxide of tin may be imparted to the tin surface in a short interval of time. The time of immersion together with solution conditions are important factors with regard to the amount of meta stable tin oxide formed by the anodic treatment. It has been found that interference films of about 2 to 6 micro inches thick may be formed by anodic treatment in the electrolyte for an interval of time corresponding to a few seconds. In addition it has been found that in a matter of minutes the entire tin layer can be converted into a meta stable oxide of tin.

As examples of other exceptionally stable, conductive and highly effective electrolyte solutions, and the related operating conditions, which are employed for rapidly obtaining a uniform continuous meta stable oxide of tin, mentionis made of the following:

Treatment A v Treatment E Percent Percent Eleetrolyte W'lof Electrolytc WeTiglof Bath Bath Disodium hydrogen phosphate, NazHPO4 3-15 Dsodium hydrogen phosphate NazHPO4 3-30 Ortho Phosphoric Acid HaP04(85%) 2-15 Any remaining parts needed vwith the 'above to total 100 percent by weight, being substantially water.

Bath temperature f 90-100 C. Y Minimum current density amperes per sq. ft. Time of immersion 30 to 120 seconds Treatment B Percent Eloctrolyte Wgglo Bath Potassium Tartrate K204H40i--.. 5-25 Tartaric Acid H2C4H4O 1-3 Any remaining parts needed With the above to total 100 percent by Weight, being substantially water.

Bath temperature 80-l00 C. Minimum current density 15 amperes per sq. ft. Time of immersion 30 to 120 seconds Treatment C Percent Electrolyte Wggltlof Bath citric Acid COOH-CHZCwH)(C00H)CH2C0OH 5-20 Phosphoric Acid H3PO4(85%) 2-4 Any remaining parts needed with the above to total 100 percent by Weight, being substantially water.

Bath temperature 80-100 C. Minimum current density 15 amperes per sq. ft. Time of immersion 30 to 120 seconds y Treatment D Percent Elcctrolyte Wglo Bath 'rartaric Acid COOH-(OHOH)2-CO0H 5-50 Disodium Hydrogen Phosphate NarHPOi.. 5-30 Any remaining parts needed with the above to total 100 percent by weight, being substantially water.

Any remaining parts needed with the above to total 100 percent by weight, being substantially Water.

Bath temperature 90-100 C. Minimum current density 15 amperes per sq. ft. Time of immersion 15 c0120 seconds Any remaining parts needed with the above to total Any remaining parts needed with the above to total percent by Weight, being substantially Water.

Bath temperature 70-100" C. Minimum current density 10 amperes per sq. ft. Time of immersion 60 to 25() seconds Treatment H Percent Eleotrolyte Wgiglof Bath Magnesium Citrate MgMCHoOnz-lt H2O 3-10 Oitric Acid 3-5 Any remaining parts needed with the above to total 1G() percent by weight, being substantially water.

Bath temperature 90100 C. Minimum current density 2O amperes per sq. ft. Time of immersion 60 to 150 seconds An excellent meta stable oxide layer of tin was obtained on the tin layers in the instance of Treatment A through Treatment H, inclusive. The scope of the invention should not be bound by any quality of chemical composition nor by the specific proportions of acids, salts and water given in the several illustrative examples of the treatment.

The treated tin plate articles are then removed from the electrolyte bath and rinsed with water in order to remove any occluded salts, then dried at slightly elevated temperatures in order to remove the adherent water. The treated tin plate articles upon which the meta stable oxide of tin is deposited thereon are then subjected to a conversion oxide temperature step whereby the meta stable oxide of tin is then converted at elevated temperatures to form the stable oxide of tin coating primarily stannous oxide. The elevated temperature conversion range shall include approximately the melting point of tin although the prefer-red temperature range is from about 198 C. to about 231 C. This step in the process may be obtained by direct heat application to formulate the oxide conversion, although exposure to air 0r to other oxidizing conditions will somewhat accelerate the oxide conversion at elevated temperatures.

The formation of the anionic stanno-complex of tin under elcctrolytic treatment will vary depending upon solution compositions and conditions. Since tin exhibits ampheteric properties, it has been noted that the meta stable oxide of tin may be formed under alkaline con ditions, although the electrolytic step in the process is preferred under acid conditions (pH from 2%/2 to 5), successful results have been obtained under alkaline condi-V tions. In addition, it has been noted that the formation of the meta stable oxide of tin may be formed first by immersion in an alkaline media and then subsequently into an acid media or combination thereof.

The complexing reagent forms the anionic stannocomplex which in turn is converted into the hydrate of stannous oxide. The hydrate is unstable in form and is then partially converted into the meta stable form of stannous oxide. The meta stable stannous oxide exhibits the interference colorson the surface of tin, and when the thickness of the meta stable stannous oxide appears to approach about seven (7) micro inches, opacity of the film begins to occur. By varying'the time of immersion in the electrolytic bath, the `blue-black meta stable stannous oxide is formed and at about 2 to l0 minutes immersion time under the aforementioned conditions, it appears that the tin layer may be converted totally into the meta stable stannous oxide. Therefore, it is desirable to reduce the time of electrolytic immersion to about fifteen to ninety seconds in order to retain a relatively high percentage of free tin.

The time interval for electrolytic immersion will vary depending upon solution conditions. By reducing the current density and/or the temperature as well as the concentration of the complexing reagent, it is possible to increase substantially the time interval of immersion of the tin layer in the electrolytic bath. Therefore, the scope of the invention shall not be bound by the specific time interval of treatment specifically aforementioned.

The meta stable stannous oxide exhibits various transitional interference colors approaching the blue-black. TheAblue-black is then considered the end point of opacity. audit has been found that the interference colors of red, purple, green, and blue-green will upon subjecting the then treated meta stable stanneus oxide coating to conversion conditions at elevated temperature, form the stable stannous oxide coating, exhibiting a uniform olive green color.

Referring to Figure 2 of the drawings7 the schematic fragmentary cross-sectional view represents a composite structure section lof a processed shaped article 2, wherein the diagram illustrates schematically the relative proportional thickness of therelatively transparent oxide of tin coating 22a, the tin layers 24 (a and c), the tin-iron alloy layers 26 (a and c), and the steel base 28 (a and c).

It shall .be noted that the transparent oxide of tin coating 22a which is .co-extensive with the inner surfaces 4 and 6, is substantially less in thickness as compared with the opaque heat `absorbing oxide of tin coating 22e, which is cci-extensive with the outer surfaces S and 10.

Fig. 3 is a photomicrograph .of a section of composite tin plate structure, suchas a processed sheet of tin plate or a processed inner surface 4 of a section of a shaped article 2, wherein the photomicrograph exhibits a magnitication of about 2500.

It shall be noted that the photomicrographic crosssection as shown in Fig. 3 shows the copper mounting 20a which is used as a metallurgical polishing backing in order to secure a defined edge demarcation of the transparent oxide coating 22a.

The transparent oxide coating 22a has a thickness which may vary from about 3 micro inches to about 5 micro inches depending upon the chemical oxidation treatment to which the initial tin plate layer 24a has been subjected. The tin layer 24a represents the remaining portion of free tin after the formation of the oxide coating 22a.

In Figs. and 4, the steel base is identitied by numerals 28b and 28e and is substantially the same as 28a. Figs. 4 and 5 represent photomicrographs of composite tin plate structures of theouter surfaces 8 and 19, showing the relative proportions of the opaque heat absorbing oxide coating 22 (aand b),the respective tin layers 24 (b tand c),rand the respective tin-iron alloy layers 26 (,b.and..c). .The respectivesteel :bases 2S (b and c) are the same, as well as theV QQPPQ! .IOUUDSS 20 (b and C) in each 0f there photomicrosfraphs,

Fig. 4 exhibits an opaque heat absorbing oxide tin coating 22b which has a thickness of approximately 20 to 30 micro inches. The oxide coating 22b was formed on ll/z pound tin plate using the aforementioned Treatment A having a time of immersion of approximately 30 to 50 seconds'. lt shall be noted that the oxide coating 22b is substantially greater than the alloy layer 2Gb. The total thickness of the oxide layer 2 2b is less than the tin layer Zib. Thisphotomicrograph illustrates the thickness of the'opaque oxide coating 22b wherein a substantial portion ofthe tin layers 24b remains. We have found that. the corrosion resistance ofthe tin plate has been somewhat enhanced due to the sealing of the pores of the tin layer 24h, and the heat absorption properties of the tin layer 24h have been substantially increased.

Fig. 5 is an illustrative photomicrograph cross-section of ll/z pound tin plate, wherein the opaque oxide coating 22e is substantially greater than the tin layer 24C. The oxide coating 22e was formed by the electrolytic treatment of ll/z pound tin plate using Treatment B, as previously described. The stable -oxide coating 22C has a thickness of approximately A60 micro inches as compared to the tin layer 24e which is approximately 20 micro inches thick. l

ln the accompanying Figures 4 and 5. the relative thicknesses of the stable form of the opaque oxide of tin coatings 22b and 22C `are proportional -to the thickness of the combined composition layers 24 (b and c) and the alloy layers 26 (b and c) respectively. As the thicknesses of the oxide coatings 2 2 (b and c) increase, the relative thicknesses of the combined tin layers 24 (b and c) and the alloy layers 26 (b and c) `decrease respectively.

in examining the photomicrograph cross-section of the composite structure, that has been subjected to the anodic treatment and the temperature conversion steps, as shown inFigs. 4and 5, we have found that the opaque stannous oxide coating 22 may be varied in thickness ranging from a few micro inches to a thickness equivalent to the entire `tin layer 24 (b or c) which represent the available free tin. Photomicrographic cross-section examination of ll/z pound per basis box of untreated tin `plate shows that the lthickness of the combined free tin layer 24 and the tin-iron alloy layer 26 approximates about 90 micro inches. The tin-iron alloy layer 26 on hot dipped tin plate will b e approximately equivalent to about l0 to 15 micro inches thick.

Allowing a sufficient time interval in the electrolytic Aimmersion step, the tin layer 24 (b and c) can be substantially converted into the meta stable form of the stannous oxide. We have found that substantially all of the tin layer 24 may be converted into the meta stable form of the Voxide of tin. Since free tin exhibits defined corrosion resistance, it-is desirable to limit the amount of the oxide of tin coating ,22 that may be formed. We found that a tin oxide coating 22 (b and c) of approximately 7 to about 40 ,micro inches thick will exhibit the preferred heat absorption on the outer surfaces ofthe tin plate. Experimentally, it has been found that excellent bread baking was produced by converting up to about of the free-tin Vlayer 24 to the tin oxide coating 22 (b and c) without substantially decreasing the corrosion resistance of the tin plate.

The photomicrographc cross-secton, as shown in Figure 3, represents a section of the inner surface 4 or 6 of a formed baking pan article 2. The photomicrographic cross-sections as shown in Figures 4 and 5, represent sections oftheouter .surfaces 8 or 10. In order to illustrate the schematic difference in thickness between the transparent oxide of tin coating22a and the-opaque oxide of tin coatings 22 (band c), the sections shown `in Figures 4 and 5 should be superimposed on the section illustrated in Figure 3. The steel.base.22ais ,substantially the same as the .st,eelrbase.22 (b or ,c). By-combining the-photomierogranhiinfliigwith the photomicrqsraphiinlis 4 or 5, the composite structure may be obtained. The composite structure represents a sheet of tin plate which has been subjected to the anodic chemical treatment wherein the inner surfaces 4 and 6 have been substantially masked from the electrolytic treatment and the outer surfaces 8 and 10 have been subjected to anodic chemical treatment. Y

The masking operation of a sheet of tin plate may be accomplished by placing a plurality of sheets of tin plate back to back wherein the contacting surfaces would be chemically masked from the effect of the anodic treatment. For the purpose of illustration, one sheet of tin plate may be considered the masking form although materials of other composition, such as steel, brass, stainless steel, and plastic may be used in order to mask one surface. Other types of maskingforms may be used such as coatings, organic and inorganic oils, greases, paints, and the like. These masking forms may also be used in a manner to produce geometric patterns on the surface of the masked tin plate layer 24 so that preferred portions of the tin plate may exhibit an opaque oxide coating 22 leaving the balance of the tin layer 24 lwith a transparent oxide coating 22a.

In order to effectively produce a substantial opaque oxide of tin coating 22 (b or c) on the outer tin layer 24 (b or c) the current density in the electrolytic bath should not be less than about 9 amperes per square foot. By reducing the effective value of the current density relative to one surface, such as the inner surfaces 4 or 6, as varying the distance of the cathode or decreasing the voltage, a relative transparent oxide of tin coating 22a may be produced on the surfaces 4 and 6. The outer surfaces 8 and 10 may be coated with an opaque stable oxide of tin coating 22 (b or c) by keeping the current density and the spacing of the electrodes above the effective electrolytic bath requirements.

The novel chemical treatment described in this process may be further accomplished by using a fused salt, such as disodium hydrogen phosphate dodecahydrate or magnesium phosphate, as the electrolyte or combinations thereof. One may consider this to be substantially an aqueous solution when the temperature of the electrolyte bath is within 80 or 100 C. We have found that satisfactory results can be obtained by holding the bath temperature between 80 and 90 C. and operating at a current density of about 15 amperes for a square foot for a period of time of about 20 to 90 seconds. When the current density is held at about 30 amperes per square foot, it has been found that the voltage applied dropped to about 2 volts. In this invention, the term complexing reagent shall also include a fused salt as described above.

A further step in the process as herein described may be accomplished by passivating the masked surface of the tin layer 24a in order to form a transparent heat resistant oxide coating 22a on said surface. This transparent oxide 22a isrelatively inert to further oxidation under conditions employed invconventional baking operations, and this process constitutes the step of immersing the anodically treated composite structure in an oxidizing media wherein the tin layer 24a is converted into a complex tetravalent and divalent forms of the oxide of tin which may be identified as the stannous-stannate complex of tin without substantially altering the meta stable form of the oxide of tin 22 (b or c) deposited thereon.

It does not appear that the oxidizing media must contain a chemical oxidizing constituent in the form of oxidation agents inasmuch as the entrapped oxygen in the media will convert a portion of the tin layer 24a into the stannous-stannate complex, although the time of immersion will be substantially longer. It has been found that by making the bath media alkaline, pH from about 7 to l2, the time of immersion may be substantially decreased. By adding oxidizing agents, such as oxygen,

i v v 10 peroxides, chromates or chromc'acid and perborates in an alkaline bath, the rate of formation of the stannousstannate complex may be increased. It appears that this complex is somewhat insoluble in water; and if the time of immersion is lengthy, the stannate of a contained anion may be formed which is soluble in water whereupon the entire tin layer 24a may be dissolved. Therefore, the time of immersion should be relatively short, such as a few seconds. We have found that a passivating media containing approximately 0.1% to 10% of sodium hydroxide, oxygen and/or an oxidizing agent, may be used at a temperature from about 40 to 100 degrees C., although the temperature range is not critical. The time of immersion depends somewhat on the concentration of the bath media as well as the temperature of the bath. We have found that the transparent oxide 22a which is formed on the surface of tin plate, is substantially impervious to oxidation and does not appear to oxidize up to the temperature approximating the melting point of tin under conditions encountered in the baking operation.

Another embodiment of this invention comprises the coating of the transparent oxide of tin coating 22a with a stable high temperature organic bread releasing film. The organic releasing film may or may not be applied to the opaque oxide coating 22h and 22C. The organic bread releasing film may be applied prior to the final temperature conversion step wherein the meta stable stannous oxide 22 (b and c) may be coated with a stable high temperature organic bread releasing film, such as alkyl aryl siiicones, polytetraliuoroethylene, polytrifluoromonochloroethylene and other high polymer film forming organic substances and/or mixtures thereof. The film coated tin plated article 2 is then subjected to the temperature conversion step wherein independently but simultaneously the meta stable oxide coatings 22b and 22C are converted into opaque stable oxide of tin coatings 22h and 22e, and the organic film is cured.

Excellent adhesion of the organic bread releasing film to the transparent tin oxide coating 22a as well as to the opaque tin oxide coating 22 (b and c) has been obtained. It shall be noted that the organic film may be applied to a shaped article 2 which has been previously temperature converted into the stable oxide of tin coatings 22 (b and c), and then cured. By applying the organic bread releasing film to the transparent oxide of tin coating 22a and/or the meta stable form of the oxide of tin coatings 22 (b and c), then subjecting the combined meta stable oxide coatings 22 (b and c) and the uncured organic film as well as the transparent oxide 22a and the uncured organic film to the temperature conversion step, the steps of curing the resin as well as 'temperature conversion may be accomplished in a single operation.

As many possible embodiments may be made of our invention, and as many changes may be made in the embodiments hereinbefore set forth, it is to be understood that all matter described herein, is to be interpreted as illustrative and not as a limitation.

We claim:

1. A tin plate baking pan comprising a shaped sheet Vstructure including a steel supporting base having an outer surface and tin composition layers bonded coextensively to said surface, said tin composition layers comprising an inner portion consisting of an iron-tin alloy having a thickness of approximately 10 to l5 micro-inches, an intermediate portion consisting of a metallic free tin layer bonded coextensively to said inner portion, on the exterior of the pan an outer portion consisting of a uniform continuous heat absorbing tenaciously adherent opaque olivegreen oxide of tin bonded coextensively to said intermediate portion, said outer portion having a thickness of about 7 micro-inches to 30 micro-inches, on the interior of the pan a second outer portion consisting of a substantially transparent oxide of tin bonded coextensively to said other intermediate portion, and said second outer 11 @9911997111691111919199591? abmlt? miszmzinchs t0 6 @491.91199- 2- A .fn Plat baking 11911 sqmprising ,a 911912941911991 m1111119 including 02169191112119111116 bashavug anouter surfae and tin composition layers bonded cloextensively to said surface, said tinotnpvosition layers omprising an inner portionrconsisting of an livrontin alloy having a thikness of `applroximately 10 to 15 micro-insbes, an intermedia@ 119199990095996 Of .a met-211i? free fin layf bonded coextensivelyto said inner 41591":1011, saidv metallic free tin layer having a thiekness ofat least l2 micro-inches, on the exterior ofthe pan an outer portion consisting ot' a uniformontinuous heat absorbing tenaeiousiy adherent opaque olive-green `ozyide of tin bonded coextensively to said intermediate portion, said outer portion having a thickness of about 7 mirofinhes to 30 micro-inches, on the interior of the pan a ,second `outer portion consisting of a substantially ltransparelnt oxide of tin bonded coextensively-to said other intermediate portion and said seond opterportion having athiokness of about 2 micrm inches to 6 micro-inches.

References `Cited inthe 111e of this patent UNITEDV STATES PATENTS Ratzinger Sept. 15, Schutte Dec, 17, Romano Dec. 21, Sumner Sept. 17, Nelson .Tune 17, `Botcheller May 19, Rath Aug. 17, Ward Dec. 25, Debs Feb. 22, Pfeffer Nov. 7, Collings Aug. 12, Clark Aug. 12, Russell et al. Aug. 31,

FOREIGN PATENTS Great Britain Apr. 7, Great Britain Mar. 21, Great Britain Feb. 7, Great Britain Apr. 17,

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