US2390791A - Protective transparent coatings - Google Patents

Description

Dec. 11, 1945. G. w. JERNSTEDT PROTECTIVE TRANSPARENT COATING Filed Oct. 16, 1940 Heat Treatment Electra-Depositing I Cell Cleaning INVENTOR W. Jernsted t.

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George Patented Dec. 11, 1945 PROTECTIVE TRANSPARENT COATINGS George W. Jernstedt, Bloomfield, N. J assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application October 16, 1940, Serial No. 361,353

2 Claims.

' This invention relates to protective coatings for surfaces subject to corrosion and tarnishing.

It has long been desirable to apply corrosion and tarnish-resisting coatings, par icularly transparent protective coatings; to met lic surfaces. In'particular, metals such, for example, as silver and copper and their alloys, are particularly susceptible to oxidation and corrosion by sulphide gases. Loss of metal as well as the care required to keep surfaces in good condition are factors making it desirable to reduce corrosion and tarnish.

The object of. this invention is to provide for protecting Surfaces. A further object of the invention is to provide for applying a transparent protective coating upon surfaces subject to tarnish and corrosion.

Other objects of the invention will, in part, be obvious, and will, in part, appear hereinafter.

The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others and the article possessing the feature, properties and the relation of elements which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed. description, taken in ccininection with the accompanying drawing, in w ch:

Fig. 1 is a schematic diagram of a process layout;

. Fig. 2 is a view in section of a metallic surface; and v Fig. 3 is a view in section of a coating on a metallic layer upon a piece of glass.

According to this invention, it is proposed to apply an electro-deposited coating of a very micoating on a .nute thickness, which is substantially transparent and which will protect a base material, such as copper and silver and their alloys, against corrosion and tarnishing. The protective coating is applicable to metals and alloys in general.

This invention relates to a process whereby a protective coating comprising wholly or in part 'of the oxideof beryllium or aluminum may be Initially the member. to be coated is treated to provide a surface with the finish which it is desired to have in the final product. Such treat ment may comprise buffing, polishing, etching, sandblasting or the like. The protective coating is transparent and the underlying surface shows up in full detail.

Referring to Fig. 1 of the drawing, there is illustrated a typical process diagram for the for mation of the protective coating upon the finished surface of the member. The member is first placed in a cleaning bath. Metallic surfaces may be cleaned expeditiously in a solvent. Other cleansing methods such as electro-cleaning or acid dipping which provide a chemically clean surface may be employed.

The cleaned member is then made the cathode in an electro-depositing cell. The anode in the cell consists of some inert metal. Suitable anode materials are lead, lead oxide, alloys of lead, and

noble metals. A lead-tin alloy having 7% tin is a satisfactory anode material.

The bath in the electro-depositing cell is composed of an aqueous solution containing a soluble salt of either aluminum or beryllium depending on the coating desired. The sulphates, chlorides and nitrates are typical soluble salts of these metals. The concentration of the metallic salt should correspond to a molar concentration of from 1 to 6 grams of BeSO4.4H2O per liter for best results. The optimum salt concentration is from 3 to 6 grams. Variations from these amounts of salt may be utilized.

Plated copper test specimens exposed fortwo minutes to the fumes of centrated ammonium polysulfide gas gave an indication of'the quality Table I Concentration Amount BeSO.4l I:0, of plate grams/liter tarnished Per cent The preferable beryllium sulfate concentration appears to be over 3 grams per liter for this current density.

The hydrogen-ion concentration in the bath containing the metallic salt is adjusted by adding ammonium hydroxide until a hydrogen-ion concentration of 5.5.to 5.9 has been reached. .At this hydrogen-ion concentration a portion of the posited upon the member.

beryllium or aluminum salt is precipitated. It is believed that a major proportion of the remainder of the salt remains as a colloidal dispersion in th bath.

In order to improve the electro-deposition of the colloid, it is desirable to add a bufier to the bath. A suitable bufier is boric acid. Boric acid in quantities of from 1 to 7.5 grams per liter of bath solution has given good results. The use of other boron compounds yielding boric acid in water will accomplish the same result and they may be employed in some cases.

The action of the boric acid or its equivalent in the plating bath is believed to result in a more uniform colloidal dispersion. Without the boric acid, the electro-deposited coating is iridescent due to interference fringes. It is thought that the coating is electro-deposited in extremely irregular thickness from baths lacking boric acid. Th addition of the boric acid is primarily to reduce the interference color effect, but other beneficial results, such as increased tarnish resistance, also follow its use.

The bath may be operated with satisfactory results within a hydrogen-ion concentration range of 4.5 to 5.9. Additions of ammonium hydroxide or metallic salt may be made to adjust the hydrogen-ion concentration to this range.

The colloidal particles in the bath are sensitive to elevated temperatures and for best results the bath should be kept close to room temperature. The stability of the colloidal salt particles decreases rapidly as the temperature rises above 50 C. Agitation of the bath is not advisable since this also tends to break down the colloid The deposit of colloidal particles has a high electrical resistance and the-throwing power of the solution is high. Accordingly, a fairly uniform coating which even reaches recessed areas is produced.

When an electrical current is passed in series through the member being coated or plated, the bath and the inert metal anode, a layer of colloidal metallic oxide and hydroxide will be de- The current density for practical purposes will range from 20 to 200 milliamperes per square 'foot of surface on the cathode. The coating is electrically resistant and the initial voltage may rise 500% or more, as the termination of the electro-depositing process is being reached.

The current density which yields the best protective coatings on copper plates varies with the concentration of the salt. Employing the two minute test exposure of coated copper plates to fumes of ammonium polysulfide, the following tables indicate the relation of current density to salt concentration:

Table III [Bath 3.4 gin/ liter BQSOAAHgO and 5 gr./liter H;BO;-pH 4.9]

Current Amount of density, plate malsq. it. tarnished Per cent 20 o 50 0 0 15 240 25 The time of electro-depositing may vary from 1 to 45 minutes. -A commercially satisfactory time period would be from 8 to 16 minutes. If the member is removed from the bath at an earlier time, the coating is thinner. If the plating time is increased beyondl5 minutes, the amount of coating deposited is not increased in proportion to the longer time period.

Various electro-deposits upon copper and silver have been made in this manner. An investigation of the thickness of the deposit by means of a Michelson interferometer disclosed the following: The thickness of a coating electro-deposited in 4 minutes was 750 angstroms. This corresponds to an average coating thickness of about 0000002 to 0.000004 of an inch. The film deposited was quite transparent.

The coating applied from the electro-depositing bath is soft but may be improved by a heat treatment. It is believed that the coating comprises a layer of colloidal beryllium oxide and beryllium' hydroxide from a bath containing the coating deposited by this process on the members simply as the oxide and hydroxide of the metal throughout the specification and claims, even if the chemicalnature'of the coating may not be entirely accurately described thereby.

It has been found that by heat treating at a temperature of from 250 C. to 300 C. for a period of 1 to 30 minutes, the soft electro-deposited coating changes its nature in that it becomes much more durable and is harder. The coating also appears to be somewhat more uniformly distributed by such treatment. It is believed that the coatin fuses to a certain extent during the heat treatment. It is suspected that the coating is in the nature of a submicroscopic glaze. Examination under the microscope at a magnification of 4000K reveals a granular structure after scratching and heating.

In some instances heat treatment of the soft electro-deposited coating at temperatures above 300 C. for shorter periods of time, of the order of a few seconds to a minute, will produce the durable and hard protective coating.

The heat-treated coating forms an exceedingly durable protective layer upon the surface of metals, such as copper and silver and their alloys, which are particularly subject to corrosionand tarnishing on exposure to the atmosphere. Metals and alloys in general will resist oxidation and tarnishing when protected by the heat-treated coating. silver and copper surfaces from the action of bid. dizing gases and sulphide fumes at temperatures of up to 300 C.

The heat treatment of the electro-deposited coating results in a more effective protective layer when conducted for periods of time greater than In particular, the coating will protect' the minimum times above indicated. Copper plates with an electro-deposited coating heat treated for 2 minutes at 300 C. are satisfactory for withstanding atmospheric corrosion and tarnishing. However, if the heat treatment is continued for 10 minutes at 300 C., the copper plates will be protected from the action of most acids and it withstands even the action of concentrated nitric and sulphuric acids. Accordingly for the most durable and resistant coatings, prolonged heat treatment for 10 minutes ormore at 300 C. is advisable.

Referring to Fig. 2 of the drawing, there is i1- lustrated in cross-section, a metal base member ill, of copper, for example, whose surface it is desired to protect. The metal base member l0, after cleaning and being made the cathode in the electro-depositing cell of Fig. 1, had ,a layer [2 of beryllium oxide and hydroxide deposited thereon. After heat treatment at a temperature of 250 to 300 C., the coating [2 became quite uniform and durable. It will Withstand considerable handling and usage. The coating will protect the surface of the base member I directly underneath the coating from oxidation and tarnishing. The copper below is visible in itsoriginal condition just as it existed prior to the treatment.

Where some of the surfaces of the metal base member I0 are to be kept free from the coating, a suitable insulating paint or substance is applied to the selected surface before placing the member in the electro-depositing bath. The coating 12 can thus be placed on specific, unpainted portions only, or about the entire article if it were so desired.

Referring to Fig. 3 of the drawing, there is illustrated a particular application'for the protective coating. A plate glass mirror 20 with a layer 2201? silver reflecting metal on the back thereof has a protective beryllium oxide coating 24 applied. The protective coatin 24 may be applied to the silver to prevent its tarnishing and oxidation by placing the mirror in the electro-deposit-.

In the case of optical instrument employing silvered prisms, mirrors, and the like, the coating 24 may be placed directly on the reflecting side of the silver surface. Allowance, in some instances, may have to be made for the thickness of the coating 24 but the coating 24 is so exceedingly thin that in most cases, it is negligible in its effect on the instrument.

It will be readily appreciated that a corrosion and tarnishing-resisting coating of this type will eliminate the need for periodic resilvering of various instruments, such as is common at'the present time. It is well known that a freshly prepared silver reflecting surface has about the best reflecting characteristics of any metal known. Due to the effect of tarnish and oxidation, this valuable property of silver is rapidly deteriorated. A heat-treated protective coating of beryllium or aluminum oxide will keep the specular silver. atits optimum reflecting state and eliminate the necessity for frequent resilvering, as at present necessary.

' coating on mirror will be prolonged considerably.

Silver and silver plated articles, such as household silverware and commercial and scientific apparatus, may be readily protected by employing the coating such as disclosed hereinbefore. The necessity of frequent cleaning due to tarnishing will be eliminated. It will be appreciated that, due to the small thickness of the coating, it will not stand up under abrasive treatment. However, it will withstand the usual usage and cleansing in water with ordinary soaps and non-abrasive detergents.

Copper may also be protected in the same way as silver. Electrical apparatus in which the state of the copper surface is important, such, for example as contact members, wherein contact drop is an important surface phenomenon, may be coated and kept in the desired state of electrical and mechanical efliciency. The extreme thinness of the electro-deposited coating of oxide and hydroxide of beryllium or aluminum does not add appreciably to the electrical resistance, and the protection against deep seated corrosion does result in better continuous electrical performance.

Copper coated with the protective coating of the invention has a pleasing, bright surface. Domestic and commercial appliances with this surface are attractive in appearance.

Since certain changes may be made in carrying out the above process and in the product and modifications effected in the apparatus for practicing the principles thereof, without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. An electroplating bath for coating surfaces with a durable, oxidation and corrosion-resisting coating of oxides and hydroxides 0f beryllium and aluminum, comprising, in combination, a water soluble salt of the metals of the group beryllium and aluminum, the concentration of the metallic salt corresponding to the molar equivalent of l to 6 grams of BBSO4.4H2O per liter, ammonium hydroxide to effect an initial hydrogen ion concentration of 5.5 to 5.9, and to provide for precipitation of a portion of the metallic salt, and boric acid to provide for a uniform coating.

2. The method of applying a protective corrosion and tarnish-resisting coating to surfaces of members, which comprises, in combination, making the member to be protected the cathode in an electroplating bath, the electroplating bath containing salts of the metals selected from the group beryllium and aluminum, the concentration of the metallic salts corresponding to the molar equivalent of 1 to 6 grams of BGSOAHZO per liter, ammonium hydroxide to effect precipitation of a portion of the metal salts and to provide for an initial hydrogen ion concentration of 5.5 to 5.9 and from about 1 to 7.5 grams per liter of boric acid to provide for a uniform coating, passing a current of from 20 to 200 milliamps per square foot of surface of the members, to effect the electro-deposition of a layer of material from the bath, and thereafter heattreating the member with the electro-deposited layer in air at a temperature of about 250 C. to 400 C. to provide for a uniform and durable protective coating.

GEORGE W. JERNSTED'I.

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