US2963784A - Base stock for vitreous enamel coatings - Google Patents

Description

tem. .1

BASE STOCK FOR VITREOUS ENAMEL COATINGS Allan E. Chester, Highland Park, 111., assignor to Poor 8: Company, Chicago, 111., a corporation of Delaware No Drawing. Original application June 14, 1951, Ser. No. 231,646. Divided and this application Sept. 7, 1955, Ser. No. 533,030

2 Claims. (Cl. 29-194) This invention relates to vietreous enameling, more particularly to new and improved base stocks for vitreous enamels providing a new and improved method of controlling the adherence of vitreous enamels to ferrous metals.

This application is a division of my application Serial No. 231,646 filed June 14, 1951, now US. Patent No. 2,768,904, which application is a continuation-impart of my applications Serial No. 753,805, filed June 10, 1947, now abandoned, and Serial No. 87,375, filed April 13, 1949, now US. Patent 2,639,264, the former being a continuation-in-part of my application Serial No. 689,524, filed August 9, 1946, now abandoned.

One of the principal problems in the art of vitreous enameling is to secure proper adherence between the vitreous enamel coating and the base stock to which the enamel is applied without the use of a ground or grip coat of vitreous enamel containing cobalt, manganese and usually nickel. It has been recognized that it would be desirable to employ as the enamel base stock ordinary mild carbon steels (also called low-alloy or mild alloy steels) containing the usual percentages of carbon, as found, for example, in SAE 1010 steels, SAE 1020 steels, SAE 1030 steels, cold rolled commercial quality sheets and cold rolled drawing quality sheets. (See A.I.S.I. Steel Products Manual, section 11, October 1948 issue, pages 34 and 36.) Thus, SAE 1010 steel, as defined by Metals Handbook, 1948 edition, published by the American Society for Metals, page 307, has an average carbon content between 0.08% and 0.13% by weight; SAE 1020 steel has an average carbon content between 0.18 and 0.23% by weight; SAE 1030 has an average carbon content from 0.28 to 0.34% by weight. The attempts which have been made heretofore to use such mild carbon steels as a vitreous enamel base stock have met with little success due to warpage and distortion of the object being enameled. Warping and distortion have been accelerated, moreover, because of the necessity of applying more than one coat of enamel in order to secure adherence.

Many efforts have been made both by enamelers and steel manufacturers to overcome the handicap of using a ground coat of vitreous enamel followed by a second or even a third coat of enamel. Special steels have been specifically produced and designed for enamel purposes. The most satisfactory vitreous enamel base stocks have been specially prepared from low carbon steels (containing say, 0.01% to 0.02% by weight of carbon) which are more expensive than many of the ordinary steels.

An object of this invention is to provide a new and improved vitreous enamel base stock.

A still further object of the invention is to produce a satisfactory vitreous enamel article in white and pastel colors with but one coat of enamel. Other objects will appear hereinafter.

In accomplishing these objects in accordance with this invention, it has been found that improved results in the adherence of vitreous enamels to ferrous metal base stocks, especially mild alloy steels, can be obtained in a "atent v '9 relatively simple manner by: (1) surface etching the ferrous metal base stock, preferably with a fine grained etch that might be described as a pin point etch and which in the case of a cold reduced steel should preferably extend just below the surf-ace and not below the rolling skin, the said etch being effected in the presence of an oxidizing compound of manganese, preferably a compound which is at least partially soluble in the etching bath; and (2) applying to the etched surface of said base stock, preferably after rinsing but before drying, a layer of a metal, or a compound of a metal reducible to the metal, which is reducing with respect to ferrous iron under the firing conditions (and hence is below ferrous iron in the electrochemical series) and does not flash off or volatilize rapidly at vitreous enamel firing temperatures.

In the practice of the invention, especially good results have been obtained by etching the ferrou metal base stock with an acid etching solution to which has been added an oxidizing manganese compound, e.g., manganese nitrate, manganese sulfate and/ or manganese dioxide. The etching should be carried out under conditions such that the etched base stock contains a substantially uniform pin point surface etch free from smudge and which does not have to be scrubbed.

In the second step of the process, excellent results have been obtained by immersing the etched base stock from the first step in an aqueous bath containing dissolved nickel salts and a relatively high acidity, for example, a pH of 1.8 to 2, preferably at a temperature around 180 F.l F. for a period of 5 to 15 minutes followed by rinsing, scrubbing with water and drying prior to the firing of a vitreous enamel thereon. If the base stock is to be shipped or stored it may be oiled in which case, however, the oil should be removed prior to vitreous enameling.

The invention will be further illustrated but is not limited by the following examples in which the quantities are stated in parts by weight unless otherwise indicated:

EXAMPLE I (a) Sheets of SAE 1020 and SAE 1010 cold reduced steel of varying gauges (18, 20, 24, and 26 gauge) were etched in a bath of 4% sulfuric acid by weight containing 29 cc. of 50% manganese nitrate solution (Mn(NO for each gallon of bath. The steel sheets were immersed in the bath at a temperature of 150 F., for periods within the range of 310 minutes. Additional baths were prepared with varying quantities of manganese nitrate within the range from about 5 to about 50 cc. of 50% manganese nitrate for each gallon of bath. The treated sheets were uniformly etched and free from smudge. Especially good results were obtained in the range of 10-20 cc. of 50% manganese nitrate per 1300 cc. of 4% by weight of water of sulfuric acid. Five (5) cc. solution of 50% Mn(NO contains 0.75 gram of manganese and 1.73 grams of nitrate (N0 while 50 cc. of the same solution contains 7.5 grams of manganese and about 17.30 grams of nitrate. Since the 1300 cc. of 4% sulfuric acid corresponds to about 1336 grams the percentage of manganese in the baths employed was within the range of about 0.05% to about 0.5% and the percentage of nitrate in the bath was within the range of about 0.1% to about 1.0% by weight of the bath.

([1) The steel sheets prepared as in (a) were rinsed with water and without drying were immersed in baths prepared by mixing the following ingredients in the proportions indicated:

and making up to one gallon of bath with water, followed Patented Dec. 13, 1960 by the addition of 3.8 to 38 grams per gallon of ZnSO or CdSO with or without the addition of 3.8 grams per gallon of lauryl sulfate, and adjusting to a pH of 1.8 to 2 by adding 96% sulfuric acid.

The baths were maintained at a temperature of. 180- 190 F; and the steel sheets (treated as in (a)) were immersed therein for periods within the range of 5 to minutes, after which they were removed from the-bath, rinsed and scrubbed with water and then dried.

(0) Steel sheets which have previously been treatedv as described in (a) and (b) were then sprayed with a suitable vitreous enamel composition, dried and fired.

As an example of a suitable vitreous enamelcomposition there may be mentioned a molybdenum bearing.

The finishing and pouring temperature of this batch after smelting is approximately 2100 F.

For coats applied by spraying the following mill batch may be used.

Frit parts by weight 100 Clay parts by frit weight 4 Sodium nitrite do 0.5 Antimony opacifier do 4 Water d0 40 This is ground or milled to a fineness of 0 to 25% residue on a 200 mesh screen, preferably 4 to 8% residue. The resultant composition is then applied at the rate of 15-30 grams per square foot to the base stock prepared as previously described, dried and fired at a temperature of 1350-1500 F.

Good results are also obtained with an enamel containing approximately 60% of a non-feldspathic glass and 40% of a feldspathic glass leadless cast iron vitreous enamel.

A typical non-feldspathic frit is made from the following ingredients:

Parts by weight 510 10.9 ZnO 10.5 H 30 21.9 BaCO 35.0 Na SiF 22.6

A typical feldspathic frit is made from the following ingredients:

Parts by weight Feldspar 21.7 Borax 31.0 Quartz 7.0 Soda ash 2.4 Barium carbonate 8.6 Sodium nitrate 5.7 Zinc oxide 7.0 Fluorspar 7.6 Whiting 4.5 Sodium zirconium silicate 4.5

The foregoing frits are made in the usual manner by mixing, smelting, hitting and drying the compositions.

The hits are then mixed together in the 60:40 ratio and the following mill additions are made:

Frit parts by weight White clay parts by frit weight 7 Color oxide or opacifier "do-.." 0-20 Bentonite do A; Barium carbonate do 141, Water do 43 173 cc. of 96% sulfuric acid 36.5 grams of MnSO .4H O, and 20.4 grams of HNO made up to two gallons with water. The proportions of manganese and nitrate present in this bath correspond approximately to 10 cc. of 50% solution of Mn(NO per 1300 cc. of bath.

Excellent results were also obtained in the use of a bath of this type in combination with the other steps recited in (b) and (c) of Example 1.

EXAMPLE 111 The procedure was the same as in Example I except that the baths in (a) were prepared by dissolving 22.3 grams of MnSO .4H O in 200 cc. of water, and adding the resultant manganese sulfate solution to a solution of 10.6 grams of N-a CO in 100 cc. of Water. The resultant mixture was boiled to precipitate the manganese carbonate which was filtered, washed and dissolved in a solution of 12.6 grams or HNO in 50 cc. of water.

The resultant solution was added to 4% H SO in an amount sufiicient to give two gallons of bath. The concentration of this bath corresponds to approximately 6 cc. of 50% solution of Mn(NO in 1300 cc. of 4% H 80 Very good adherence of the vitreous enamel to the steel was obtained when baths of this type were employed in conjunction with steps (b) and (c) of Example 1.

EXAMPLE IV Sheets of SAE 1010 and SAE 1020 cold reduced steel of varying gauges (18, 20, 24 and 26 gauge) are etched in a bath composed of the following ingredients:

35 grams ferrous sulfate (FeSO JH O) 6 grams zinc sulfatetZnsO jl-l O) 0.8 gram manganese dioxide 9.2 grams sodium bisulfate (NaHSO l-I O) 35 cc. commercial sulfuric acid 66 B.

1 gram isopropylnaphthalene sodium sulfonate (Santomerse No. 1)

made up to 2 gallons with water.

The steel sheets are immersed in the bath at temperatures of F. to 170 F, preferably at F. to F.

The treatment in this first bath is preferably carried out for about 15 minutes. Although the time of treatment may be vary over a wider range, it is preferably within the range of 12 to 25 minutes.

The pH of the surface etching bath is preferably about 5.0 to 5.5.

The treated sheets are withdrawn from the first bathand immersed in a rinsing bath.

The metal sheets are taken from the second bath and,

without drying, are immersed in a third bath having the following composition:

170 grams nickel sulfate (NiSO .6I-I O) 40 grams nickel chloride (NiCl .6H O) 17 grams boric acid (H 30 3.8 grams citric acid 7.0 grams zinc sulfate (ZnSO JI-I O) 0.8 gram isoprc-pyl-naphthalene sodium sulfonate (Aerosol OS) made up to 2 gallons with Water.

The pH of this bath is maintained around 1.8 to 3.5, preferably around 2.5 to 3.5. The steel sheets are immersed in this bath for periods within the range from 5 to 15 minutes.

The temperature of this bath is preferably within the range of 180 F. to 200 F. They are then removed from the bath, rinsed, scrubbed with water and dried, and are ready to be sprayed with suitable vitreous enamel compositions, dried and fired. it is preferable to employ vitreous enamel compositions which can be fired at temperatures not higher than about 1500 F.

EMMPLE V Steel sheets (22 gauge) were treated on a large scale as described in Example 1V using a lviesta pickler. The raw steel was loaded and then immersed in the successive baths. Each bath consisted of 4000 gallons of solution made up in the same relative proportions given in Example 1V. After the treatment in the third bath, the steel was unloaded, scrubbed, dried and stacked.

in order to compensate for losses of the chemical addition agents of the bath due to their adherence or reaction with the metal and drag-out losses, additional quantities were added by increment to the first bath at the rate of 0.8 pound to 1.74 pounds per ton of steel produced of a composition made by fusing together:

7.5 parts ZnSO .7H O 1.0 part MnO 11.5 parts IJaHSO .H O

In the third bath, additional zinc sulfate (ZnSO .7I-I O) was added in increments at the rate of .44 to .51 pound per ton of steel produced. The citric acid content was maintained at about 1 gram per liter. Nickel chloride (NiCl .6H O) was added at the rate of .17 to .51 pound per ton of steel produced. Nickel sulfate (NiSO .6l-1 O) was added at the rate of 8.3 to 9.9 pounds per ton of steel produced. Boric acid was added at the rate of .11 pound to .4 pound per ton of steel produced. Aerosol OS was added at the rate of .033 to .042 pound per ton of steel produced.

The ferrous sulfate will tend to build up in the first and third baths. In the first bath, a small amount of ferrous sulfate is desirable but more than 2 pounds per gallon interferes with the activity of the bath. If the amount of ferrous iron becomes too high in the third bath, a spongy deposit of iron is formed on the sheet which interferes with the bonding power of the subsequently applied vitreous enamel. The citric acid permits the operation of this bath over a longer period of time by preventing the precipitation of ferrous sulfate. Where no citric acid was used, the practical limit of ferrous iron concentration was grams per liter, while with the employment of citric acid, this limit may be increased to 30 grams per liter or even higher in some cases to 45 grams per liter, but care must be exercised to insure that the concentration is below the point' at which a spongy deposit of iron occurs.

The surface tension :of the third bath is preferably maintained at 30 drops per minute or greater as measured on a Stalagometer.

The pH of this bath may be around 2.0 to 2.5 but may vary over a wider range of 1.8 to 3.5. At a pH around 6 4.0 there is a very sharp drop in nickel deposition. At a pH of 4.5 to 5.5, however, excellent results are obtained with proper buffers.

The quantity of citric acid employed should not be any more than is employed to keep the iron in solution or to prevent precipitation of the ferrous sulfate in the above mentioned operating range. One gram of citric acid per liter of solution will usually be sufficient. Too much citric acid interferes with nickel deposition where the nickel is being deposited by over-voltage as in the present case. Formic acid and other substances which have a sequestering or an inhibiting effect on the precipitation of ferrous sulfate may be employed instead of the citric acid.

By a procedure of the type described in this example, the steel is given a sunface etch and manganese salts are deposited thereon and sealed with a nickel plate of approximately 0.01174 to 0.01267 ounce per square foot. The thickness of this nickel plate in terms of millionths of an inch is of the order of 8.0 to 8.6. The sheet containing the nickel coating has a bright metallic finish. The nickel coating may be removed readily with nitric acid solution and after this has been stripped, the sheet exhibits a dark grain etch.

EXAMPLE VI This example illustrates the process of the invention Where a nickel-cobalt alloy is plated by over-voltage in the presence of a cathode depolarizcr.

Ferrous metal base sheets similar to those used in Examples IV and V, but in the form of test plates 1 /2" by 4 sq. ft.) were immersed in successive baths in three stages.

The first bath had the same composition as the first bath described in Example 1V except that it contained 205 cc. of 60 Baume' commercial sulfuric acid. Fifty milligrams of ferrous sulfide were added. The ferrous sulfide decomposes, libenates hydrogen sulfide which assists in activating the sheet. Instead of ferrous sulfide, sodium polysulfide may be used. The temperature of the bath was maintained at to F. The sheets were treated in 15 to 20 minute cycles, that is, they were immersed for that length of time.

After the treatment in the first bath each sheet was rinsed and immersed (in water containing 0.3% H SO The surface of the sheet may be regarded as containing myriads of couples with the subsequent deposition in the third bath occurring only on those areas of the proper polarity. The same time period was employed in the second bath, that is, 15 to 20 minutes.

The third bath consisted of the following ingredients:

37.5 grams CoSO .6H O

20 grams boric acid 3.8 grams citric acid 3.75 grams dextrose 25 cc. of a product made by dissolving 40 pounds of sodium hydroxide in 8 /2 gallons of water, adding chromic acid (CrO and then mixing with 37 gallons of 50% gluconic acid. (See my US. Patent 2,428,356.)

50 mgs. sodium bisulfiite (NaHSO made up to two gallons with water.

The pH of this bath was maintained at 4.9 to 5.1 and the treatment again was in a 15 to 20 minute cycle. In this bath the use of the dextrose is optional. The compound derived by the reaction of the sodium hydroxide, chromic acid and gluconic acid acts as a depolarizer. The sodium 'bisulfite functions as a reducing agent. This bath deposits alloys of nickel and cobalt by over-voltage onto the ferrous base stock previously treated in the first and second baths. For the purpose of this invention, excellent results have been obtained where the alloy plated consisted essentially of 90% nickel and 10% cobalt or 92.8% nickel and 7.2% cobalt.

In the foregoing examples, cold rolled commercial quality and drawing quality sheets may be used with .excellent results. One such steel is specified as .08 carbon max. and .32 to .42 manganese, as described in A.I.S.l. Steel Products Manual, sec. 11, October 1948, pp. 34 and 36.

The term mild alloy steels as used herein is intended to cover steels having an average carbon content in excess of 0.02% and not more than 0.45%, preferably 0.07 to 0.20%, including the steels having the following specific compositions:

COIIZPOSZIZOHS O Mn Si P Ni Cr Cu M .09 Low .55 .095 .45 0. l0 0. 25 0. 75 0. l 0 75 0. 40 0.12 0.60 .08 0.40 0. 05 0. 35 0.95 I" 80 0.40 0. 0. 70 0. 17 C 50 0. 00 0.10 0.60 0.15 0 l2 1.10 0.14 0.80 0.15 0. 12 0.30 0.10 0.60 a 0.10 0 60 0.60 0. 10 0. 35 0. 0S 0. 35 0 l2 0. 0.13 0.90 0.15 i "h i... 0.20 0.10 0. '55 0. "i 10 0. 50 75 0. 50 0. 13 0. 70 0. 30 a. 0. 20 0. 60 0. 20 0.10 1.10 0.05 J12 i 0.3-5 0.12 0.75 0 75 1. 40 0.10 0.15 1.00 0. 50 0.25 0.10 0.08 0. 50 0.25 2.0 1.00 0. 07 0. Trace 0. 06 0. 75 0.

The metal which is to be treated may be a regular cold rolled box annealed steel sheet of the type commonly referred to as autobody sheets. The control of the composition of the metal, the annealing of the sheet and the temper rolling, if any, will be determined from the requirements of the end use of the sheet, especially as to the amount of drawing and firing which has to be done. Temper rolling of the sheet may be per-formed before'or after the processing herein described, whichever is more convenient in a particular plant.

Although the invention is not limited to any theory, it is believed that the minute cavities formed by the etching or pickling entrain a part of the pickle solution containing manganese nitrate or other manganese compound. The aforesaid entrained compounds are then sealed by the coating resulting from the deposition of a film of nickel or other metal below iron in the electrochemical series by the second step of the process. The entrained particles and the sealing layer are therefore both available to assist in controlling the oxidation and reduction reactions that occur when the vitreous enamel is fired on the ferrous metal sheet thereby increasing the adherence and making it possible to prepare a commercially usable sheet of steel covered with a single coat of vitreous enamel.

In the step of the process whereby there is applied a thin layer or coating of a metal which is reducing with respect to ferrous iron, other types of baths can be used to accomplish a similar purpose.

Among the preferred reducing metals which may be employed in the practice of the invention are antimony, arsenic, cobalt, nickel or an alloy of two or more of these metals. The thickness of the coating of reducing metal is usually around 0.000001 to 0.000010 inch although it may vary somewhat. The weight of metal per square foot will also vary depending upon the weight of the particular reducing metal used but in the case of nickel or nickelcobalt alloys will'usually be within the range of 21 to 210 milligrams per square foot.

The plating step of the process which has been illustrated by the examples as involving merely the immersion of the treated metal sheet in a bath represents the simplest way of carrying out this step of'the process.- It will be understood, however, that the coating of reducing metal can be applied by other methods, such as, for example, by electroplating, or by vaporizing the metal onto the sur face of the previously treated sheet where the natureof the metal lends itself to such a process.

The specific types of enamels given in Example 1(0) are illustrative of molybdic oxide-containing enamels and high fluorine enamels which have given especially good results in the practice of the invention but it will be understood that the invention in its broader aspects is not limited to these specific types of enamels.

It will be understood that in the examples some of the ingredients recited are optional. Thus, zinc sulfate or cadmium sulfate are employed in the plating step of the process to promote a smaller grain of the coating metal. The sulfonated lauryl alcohol is employed as a surface tension reducing agent to permit more penetration into the interstices of the etched surface. Boric acid increases the efficiency of the bath. Other substances, which are 'known in the art of electroplating as being efiective in reducing grain growth of electrodeposited metals can be employed in the immersion baths of the present invention and contribute a marked improvement in the result. The amount of metal deposited from an immersion bath, however, is not such that these substances function in the same Way, viz., as brighteners.

It will also be understood that the ingredients and concentrations of the various baths employed in the different steps of the processes herein described may be varied within the bounds of the invention previously set forth. In general, it is desirable that the etching or pickling bath employed in the first step of the process contain as little sulfuric acid as is required to accomplish the result and in any case the quantity of sulfuric acid present should be such as to avoid pickling blisters.

The use of a metal which does not deposit on the ferrous iron but serves as a driver electromotive force to cause ion interchange between the iron and other metals is desirable in the practice of the invention. For example, in the surface etching bath where the manganese has been added as manganese dioxide, it actually exists in solution as manganese sulfate. The ferrous ion is oxidized to the ferric ion. Zinc in the form of its sulfate in this bath has a high over potential and acts as a driver electromotive force for ion interchange between the iron and the manganese. Again in the plating bath where, for example, nickel is to be plated, the zinc acts as a catalyst because it has a higher overvoltage than nickel in relation to iron. In a similar manner, cadmium, aluminum and/or magnesium in the form of their soluble salts, may be employed in place of zinc. The quantities used are preferably 0.1 to 1 gram per liter of zinc sulfate (ZnSO .7H O) or the chemically equivalent amount of the other metal salts mentioned.

The importance of the invention resides in the fact that it provides a very simple and economical method of increasing the adherence of vitreous enamel coatings to ferrous metal base stocks without the use of ground or grip coats of vitreous enamels containing cobalt, manganese and/or nickel. As a result, it is possible to prepare vitreous enamel coated articles containing only a single coat of vitreous enamel possessing excellent adherence to the base stock. The simplicity of the procedures involved in securing this adherence is such that the enameler is not required to depend upon the use of a special type of base stock. The base stock used can be similar to that now employed in making automobile bodies. Another important advantage of the invention is that it can be applied in the treatment of welded shapes.

The invention is hereby claimed as follows:

1. A base stock for vitreous enamel coatings comprising a cold reduced mild alloy steel having an average carbon content in the range of 0.07-0.20%, the surface of said steel being acid-etched to a fine grained pin point etch extending below the surface but not below the rolling skin, said etched surface containing an oxidizing compound of manganese selected from the group consisting of manganese nitrate, manganese sulfate, and manganese dioxide deposited on said acid etched surface from the acid-etching bath at a concentration in said bath equivalent to ODS-0.5% manganese by weight of a thin coating over said etched surface containing the deposited oxidizing compound of manganese of at least one metal below ferrous iron in the electrochemical series and selected from the group consisting of antimony, arsenic, cobalt and nickel.

2. A base stock on which vitreous enamel coatings can be fired directly which comprises an article of mild alloy steel having an average carbon content in the range of 0.02-0.45%, said article having a fine grained pin point acid etched surface containing an oxidizing compound of manganese selected from the group consisting of manganese nitrate, manganese sulfate, and manganese dioxide deposited on said acid-etched surface from the acid-etching bath at a concentration in said 10 bath equivalent to ODS-0.5% manganese by weight, and a thin coating over said etched surface containing said oxidizing compound of manganese of at least one metal below ferrous iron in the electromotive series and selected from the group consisting of antimony, arsenic, cobalt and nickel.

References Cited in the file of this patent UNITED STATES PATENTS 2,032,256 Canfield Feb. 25, 1936 2,101,950 McGowan Dec. 14, 1937 2,206,597 Canfield July 2, 1940 2,279,935 Belding Apr. 14, 1942 2,495,762 Porter Jan. 31, 1950 2,495,835 Comstock Jan. 31, 1950 2,495,837 Porter Ian. 31, 1950 2,639,264 Chester May 19, 1953 2,775,531 Montgomery Dec. 25, 1956

Claims (1)

1. A BASE STOCK FOR VITREOUS ENAMEL COATINGS COMPRISING A COLD REDUCED MILD ALLOY STEEL HAVING AN AVERAGE CARBON CONTENT IN THE RANGE OF 0.07-0.20%, THE SURFACE OF SAID STEEL BEING ACID-ETCHED TO A FINE GRAINED PIN POINT EACH EXTENDING BELOW THE SURFACE BUT NOT BELOW THE ROLLING SKIN, SAID ETCHED SURFACE CONTAINING AN OXIDIZING COMPOUND OF MANGANESE SELECTED FROM THE GROUP CONSISTING OF MANGANESE NITRATE, MANGANESE SULFATE, AND MANGANESE DIOXIDE DEPOSITED ON SAID ACIDETCHED SURFACE FROM THE ACID-ETCHING BATH AT A CONCENTRATION IN SAID BATH EQUIVALENT TO 0.05-0.5% MANGANESE BY WIEGHT OF A THIN COATING OVER SAID ETCHED SURFACE CONTAINING THE DEPOSITED OXIDIZING COMPOUND OF MANGANESE OF AT LEAST ONE METAL BELOW FERROUS IRON IN THE ELECTROCHEMICAL SERIES AND SELECTED FROM THE GROUP CONSISTING OF ANTIMONY, ARSENIC, COBALT AND NICKEL.