US3730856A - Electrolytic preparation of valve group metal equipment for use in chemical plants - Google Patents

Abstract

EQUIPMENT WHICH IS INTENDED FOR USE IN CHEMICAL PLANT AND WHICH IS MADE OF TITANIUM, ZIRCONIUM, NIOBIUM, TANTALUM OR ANY ALLOY OF ONE OR MORE OF THESE METALS, IS EXPOSED TO AN ANODIC TREATMENT TO REMOVE HEAVY METALS FROM THE SURFACES TO BE EXPOSED TO CHEMICALS TO THUS PROTECT THE SURFACES AGAINST EMBRITTLEMENT CAUSED BY HYDROGEN WHICH IS EITHER PRESENT IN THE ENVIRONMENT TO WHICH THE EQUIPMENT IS EXPOSED OR IS FORMED AS A RESULT OF CORROSIVE ACTION AT THE SURFACE OF THE METAL.

Description

United States Patent O US. Cl. 20456 R 8 Claims ABSTRACT OF THE DISCLOSURE Equipment which is intended for use in chemical plant and which is made of titanium, zirconium, niobium, tantalum or any alloy of one or more of these metals, is exposed to an anodic treatment to remove heavy metals from the surfaces to be exposed to chemicals to thus protect the surfaces against embrittlement caused by hydrogen which is either present in the environment to which the equipment is exposed or is formed as a result of corrosive action at the surface of the metal.

CROSS-REFERENCE TO ANOTHER APPLICATION This application is a continuation-in-part of the application of Hines et al., Ser. No. 630,850, filed Apr. 14, 196-7 now abandoned.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to the protection against deterioration and especially deterioration by cracking of certain metals intended for use in chemical plants.

(2) Description of the prior art The use of the more common structural metals in chemical plants is limited by their comparatively poor corrosion resistance, and hence equipment which is exposed to a highly corrosive environment is often made of a more corrosion resistant metal. One such metal is titanium, which may be used either in its pure form or as part of a suitable alloy; and equipment made of titanium or the like has been shown to have good resistance to corrosion in a wide range of corrosive environments. However, the use of this metal has suffered from a serious disadvantage. After use for some time, in either moderately oxidizing or non-oxidizing environments, the metal has become brittle and liable to crack; and the incidence of this defect has often been greater in the vicinity of welds. Moreover, it has been found that if a crack develops it is diflicult to repair the metal by welding. Why this embrittlement occurs has hitherto not been understood.

We believe that we have now discovered at least one of the causes of this defect; and we have devised a method for avoiding or at least reducing the risk of embrittlement of equipment made of titanium or the like when used in chemical plant. Titanium usually contains some iron and other base metals and we have observed that the iron and some of the other base metals accumulate in the B phase of the titanium, which results in the stabilisation of an iron rich phase near welds. Also, the surface of titanium is usually contaminated by iron and possibly other base metals picked up from a variety of sources during fabrication of equipment from the titanium. In the presence of an electrolyte galvanic couples are formed by the titanium and the iron. These produce an electric current and cause the iron to dissolve with a reduction reaction at the titanium surface. Hydrogen is liberated,

and generally this is in a greater amount than the equivalent amount of iron which dissolves. The reason for this is that coupling to iron tends to depress the potential of the titanium to a level at which it reacts itself and disolves to liberate more hydrogen, i.e. the titanium begins to corrode. This slight corrosion of titanium with liberation of extra hydrogen, would not occur if there were not sufiicient iron present to initiate the corrosion reaction. However, only a very small amount of corrosion is necessary to produce hydrogen and we believe that the hydrogen in atomic form attacks the titanium and forms titanium hydride which renders the metal brittle.

We have also found that surface iron contamination can lead to hydriding of titanium and embrittlement of the metal in the total absence of electrolytic action but in the presence of hydrogen gas.

At high temperature and hydrogen pressure hydrogen can penetrate the surface oxide film on titanium and enter the metal. The temperature at which this can occur at a significant rate depends on the hydrogen partial pressure, the other constituents of the environment and the initial condition of the surface oxide film; but typical limiting temperatures are 500 C. at 15 p.s.i. hydrogen partial pressure and 350 C. at 1000 p.s.i. hydrogen partial pressure. However, in the presence of surface iron contamination we have found that hydrogen can enter the titanium at temperatures and hydrogen partial pressures below those at which hydriding can occur in the absence of surface iron contamination.

Metallic iron is permeable to hydrogen at moderate temperatures and hydrogen partial pressures, and we believe that the hydriding of titanium contaminated by iron in the absence of electrolytic action occurs because the iron contaminant particles provide a path whereby hydrogen can enter the metal under conditions such that hydrogen would not be able to penetrate the surface oxide film on titanium. Other metals which are permeable to hydrogen can have a similar effect, but most common metals, such as for example nickel, copper, aluminium and stainless steels, are less permeable to hydrogen than is iron and are therefore less likely to lead to hydriding.

Certain less common metals or noble metals such as for example platinum and palladium are permeable to hydrogen and can lead to serious hydriding. These metals can also be dangerous in the presence of a non-oxidising electrolyte when there is mild corrosion of the titanium which evolves hydrogen. In the absence of noble metal corrosion may still occur but hydrogen is formed on the surface of t the oxide film and does not penetrate. the metal at a significant rate. If a noble metal is present, however, hydrogen evolution occurs preferentially at the surface of the noble metal and hydrogen may penetrate into the titanium. The presence of the noble metal usually reduces the corrosion rate by raising the potential of the titanium and so assisting the formation of a resistant passive film, but if complete passivity is not achieved there may be hydriding even though the corrosion rate, and consequently the amount of hydrogen produced, has been reduced to a nearly negligible amount.

The problem is more acute near welds, for three reasons. Firstly, although the average iron content near welds may be no greater than elsewhere, the heat cycle during welding produces an iron rich B phase. Secondly, if there is iron contamination the heat generated during welding tends to diffuse the iron into the titanium and produce a more iron rich titanium. This iron is particularly deleterious because it cannot be removed as easily as can undifused surface contamination. Thirdly, welds are commonly at the edges of a section of material, and the regions near welds are therefore more likely to be accidentally contaminated during handling than other parts of the surface of the material, and are also liable to contamination during operations such as machining of welds, preparation of metal for welding and removal of oxide prior to welding.

We believe that if some base metals other than iron are present as contaminants they give rise to a similar effect; and the problem also arises when some metals similar to titanium are used. However, we have discovered that the problem can be avoided, or at least diminished, if the titanium or the like is exposed to an anodic treatment to dissolve the contaminant from the titanium surface.

Accordingly, the present invention comprises equipment for use in chemical plants, which is made of titanium, zirconium, niobium, tantalum, or an alloy of one or more of these metals having corrosion resistance characteristics comparable with those of the metals themselves, characterised in that at least part of the said equipment is exposed to an anodic treatment.

Now, the process of anodizing titanium to improve its corrosion resistance to certain electrolytes is well known. However, the prior art as described, for example, in the patent to Cotton, 3,102,086 issued Aug. 27, 1963 only used the anodizing process where the corrosion rate was so excessive that the equipment life was imperiled thereby. As an example shown in the table of Example 3 of the Cotton patent an unanodized specimen of titanium exposed to concentrated hydrochloric acid corroded at a rate of 3.34 in./year while anodized titanium corroded at a rate of .005 in./year. Accordingly, a reduction in corrosion rate of approximately six hundred sixty-eight times was realized through the anodizing process which, therefore, made the anodizing process economically attractive. Obviously, in this range the cost of the anodizing treatment was more than offset by the increased equipment life.

In contradistinction, the following Table I shows that for low concentrations of hydrochloric acid the corrosion rate of unanodized titanium is relatively small and the anodizing of the titanium provides only a marginal improvement. In viewing this table it must be noted that the corrosion rates given therein are in mm./year not inches peryear. Thus, for the sake of comparison, taking the most extreme example which is shown in the lower right-hand square of Table I, the unanodized corrosion rate is 4.26 mm./year and the anodized corrosion rate is 3.82 mm./year which translates to about .17 in./year for the unanodized titanium and about .15 in./year for the anodized titanium. Thus, it is evident from Table I that it would be highly uneconomical to anodize titanium for the purpose of corrosion resistance when it is exposed to a less corrosive medium because the unanodized corrosion rate is small and the corrosion rate reduction factor is also small.

TABLE I Corrosion rate, mm. per year Condition 1% H01 2% H01 3% H01 7% H01 Unanodized 3. 66 (1) Boiling Nil Nil Nil 3. 93(2) Anodized 10 v. for 30 minutes..- 004 0. 002 0. 002 3. 78 B il' 0.002 0.001 0.005 3. 87 0. 004 0. 005 4. 11

Boiling 0. 003 0. 002 0. 004 3. 82

that at low concentrations the provision of an anodic film on titanium fails to significantly retard its corrosion rate and thus has not been used.

By chemical plant is meant any plant which is primarily intended for the purpose of manufacturing a chemical substance.

The titanium or the like may be exposed to anodic treatment before fabrication but more conveniently the equipment is exposed to an anodic treatment after it has been fabricated into equipment for use in chemical plant. Hence the present invention also comprises a method of protection against deterioration of equipment which is made of titanium, zirconium, niobium, tantalum, or an alloy of one or more of these metals having corrosion resistance characteristics comparable with those of the metals themselves, and which is intended for use in chemical plants, which comprises exposing at least part of the said equipment to an anodic treatment.

The invention applies to any type of chemical plant equipmentfor example tanks, chemical reaction vessels or liners for such vessels, boilers, distillation columns, condensers, heating vessels and connecting pipe-work which is exposed to a hydrogen containing or producing environment-either as a steady state condition or as a temporary condition. Preferably the whole surface of the equipment to be exposed to the hydrogen containing or producing environment is exposed to anodic treatment.

The invention provides protection against the hydriding which, as mentioned above, could occur in certain circumstances in the presence of hydrogen without electrolytic action. The invention also gives protection against hydrogen produced by an environment comprising any materials which are capable of giving rise to hydrogen at the surface of the unprotected metal equipment, and may involve either moderately oxidising or non-oxidising conditions. Normally hydrogen cannot be produced in more than very mildly oxidising conditions. However, if the metal is grossly contaminated by iron or other base metal the corrosion of this may be so extensive that the potential of the Whole metal is reduced to below the hydrogen potential, in which circumstances hydrogen is evolved, even though there is a moderately oxidising environment. Moreover, even when the nominal conditions are moderately oxldising it is probable that the conditions will be temporarily non-oxidising for some periods and then hydrogen can be produced. As the amount of hydrogen required to cause appreciable embrittlement is very small (a concentration of a few hundred parts per million) short periods of non-oxidising conditions could have serious consequences if our invention were not used to provide protection.

The invention is applicable, for example, to equipment submitted to a mildly corrosive environment in plants for the productioin of for instance urea, vinyl acetate, phenol or terephthalic acid and plants involving the presence of such corrodants as sulphuric, hydrochloric, nitric, phos- PhOIlC, formic, acetic or oxalic acid or phthalic acids in low concentration or, for instance, chlorine, hypochlorites or chlorine dioxide. In higher concentrations at least sulphuric, hydrochloric and oxalic acids produce considerable corrosion and thus the prior art already used anodic films to protect titanium from corrosion, but in mildly corrosive situations, as stated above, the prior art taught away from the use of an anodic film.

The invention is also very applicable to certain situations where the environment is completely non-corrosive to titanium. For example, in the synthesis of urea from ammonia and carbon dioxide some oxygen is usually fed into the synthesis converter along with the carbon dioxide, and this may normally be adequate to prevent the formation of hydrogen. But if there is temporarily a low level of oxygen some hydrogen may be formed. In the production of vinyl acetate, either from acetylene or by means of the more recently developed process in which ethylene is the hydrocarbon starting material, there are parts of the plant, for example distillation columns for separating the product, which are exposed to acetic acid under non-oxidising conditions. Our invention is particularly important at temperatures above ambient, and especially above 100 C. At ambient temperature a surface layer of hydride may be formed in a hydrogen containing or producing atmosphere; but the effect of this is not serious because the rate of diffusion of hydrogen into the interior of the metal is small. However, as the temperature rises the rate of diffusion increases and bulk embrittlement occurs more readily.

The term anodic treatment is used throughout this specification instead of the term anodizing for three reasons. Anodizing is a known technque for protecting a metal by means of an adherent surface film of a comparatively unreactive material, usually the oxide, which has been formed by an electrolytic process in which the metal serves as the anode. It involves passing an electric current tnrough an electrolyte cell which is formed by a cathode, a suitable electrolyte (e.g. dilute sulphuric, nitric or phosphoric acid of say 3% concentration) and an anode which is constituted by the metal to be protected, the current being passed for a time sufficient to produce a film of the desired thickness. The primary purpose of our anodic treatment, however, is to dissolve iron and other base metals clinging to the surface of the titanium to remove the avenues througn which hydrogen passes into the titanium and therefore to reduce the hydrogen content of the titanium or like metal. The following Table II shows typical reductions of surface hydrogen content following our anodic treatment. Only as secondary consequence does our anodic treatment produce an anodic film which helps to seal the titanium from additional hydrogen attack.

Also, as stated above, our anodic treatment is used in situations where the prior art did not use anodizing, namely, situations where the titanium or like metal is exposed to a low or non-corrosive environment.

Finally, since it is the main purpose of our anodic treatment to dissolve iron and other base metals upon the surface of the titanium rather than to form an anodic film thereupon, electrolytes are chosen which best dissolve the base metals.

TABLE II The anodizing methods commonly used in the prior art were designed to produce the thinnest possible film having maximum dielectric strength and minimum leakage current at high breakdown potentials. For this reason highly reactive electrolytes were employed. In addition, the use of the highly corrosive medium itself as the electrolyte is taught by the Cotton patent cited above. In the present inventioln it is preferable, in order to more readily dissolve the base metal, to use solutions of neutral or acid ammonium or alkali metal salts other than halides, and the solutions may also include free acids other than hydrohalogen acids (i.e. excluding, for example, hydrochloric acid, but not excluding, for example, perchloric acid). An aqueous solution of ammonium sulphate has been found to be particularly convenient. The concentration of the electrolyte is not critical so long as the concentration does not greatly exceed 20% by weight, and preferably can be in the range of 520% by weight, but 35% concentrations have been found to be convenient since they give a small potential drop in the electrolyte and hence a small heat liberation.

Thus, having shown the difference between our anodic treatment and anodizing, various types of anodic treatments will now be discussed.

In the anodic treatment the cathode may consist of any convenient conductor and need not conform accurately to the contours of the article being exposed to the treatment. For treating the interior of a hollow vessel the cathode may conveniently comprise a stainless steel tube dipping concentrically through the vessl; and for treating the outside surface of a tube bundle a mild steel strap around the circumference of the containing vessel may conveniently be used as the cathode.

The applied potential difference between the anode and the cathode may suitably be 10-40 volts. At an applied potential difference of 25 volts an initial pulse of current as high as 10 amps/sq. ft. may pass. During the first minute of the electrolytic process the current may fall to 1.1.5 amps/sq. ft., and in the absence of excessive surface contamination by iron and the like may remain at that level for about 3 hours and then fall to a minimum of about OJ-0.2 amps/sq. ft., after 7283 hours. At this stage the metal is covered with an anodic film of suitable thickness. If excessive iron is present as a surface contaminant, the current, after falling from the initial pulse, may rise again to 5-10 amps/sq. ft. and will remain high until all the iron has dissolved, when it will progressively fall to the value which would have been obtained had no iron been present. If the initial current is limited to less than 20 amps/sq. ft. by the characteristics of the source, the early stages will be prolonged, but the anodic film finally obtained will be the same when the current has fallen to 0.1-0.2 amps/sq. ft. at a measured anode to cathode potential of 25 volts.

Not only does the anodic treatment protect the metal equipment because the surface film shields the metal from hydrogen or from the mildly corrosive environment which by a corrosive action would generate hydrogen at the surface of the metal and thereby render the metal brittle, the anodic treatment more importantly, results in the dissolving of at least some of the iron and other base metals and therefore removes some of the contaminant, the presence of which, we believe, is responsible for the corrosion and generation of hydrogen which causes the embrittlement of the titanium or the like. The anodic treatment substantially removes all the iron, or other base metal, which has not diffused arid is on the surface, and also may remove some iron held in the B phase of the titanium. The remaining exposed B phase is protected by the anodic film which is built up.

As mentioned above, iron is usually the contaminant which gives rise to hydriding of the titanium or the like, but other base metals, such as, for example, magnesium, aluminum or zinc which are capable of evolving hydrogen when in contact with the specific environment to which the titanium or the like is to be exposed, also may be present and if so will also give rise to hydriding. Even metals which are not normally classified as base metals can produce hydrogen under special condition, for example, when certain complexing agents are present.

In a preferred form of the invention, the titanium, zirconium, niobium or tantalum metal or alloy used contains as little as possible of such base metals or iron, preferably less than 0.1% and more preferably, less than 0.05% by weight. Not only is it desirable to fabricate the equipment from a metal containing as little of the harmful contaminants as possible, but also it is desirable to avoid as far as possible any surface contamination caused during fabrication by the metal picking up traces of iron or other base metals. As previously indicated, the anodic treatment itself may effectively remove substantially all the surface contamination; but it does not appreciably reduce the content of the contaminant in the interior of the metal, and a substantial reduction in the amount of such contamination can be successfully achieved only by purifying the metal by a suitable metallurgical technique during its preparation. Commercially available titanium and the like frequently contain more iron and other base metals than is desirable in the present invention, so that it is preferable to use a specially purified material to obtain more benefit from the invention. Cleanliness during preparation of the metal and its fabrication is important, because it is desirable to minimize contamination; and this is especially so where surfaces have to be heated, as in heat treatment or welding. If contamination is present on such surfaces it will diffuse inwardly, and then cannot readily be removed by any easily cleaning process.

The protection against hydriding which is provided by our anodic treatment is illustrated by the following examples:

EXAMPLE 1 A sample of commercial quality titanium was contaminated with iron by scratching with a carbon steel rod and introduced into a titanium distillation column handling a mixture of acetic acid, water, and esters which contained small quantities of inorganic and organic chlorides. After exposure to the process stream at 115 C. for 20 days the hydrogen content was found to have risen to 180 p.p.m. A similar sample cleaned in an oxidizing acid and not contaminated with iron was found to contain 35 p.p.m. hydrogen after the same test period, and a sample contaminated with iron and exposed to anodic treatment in wt. percent ammonium sulphate solution at room temperature was found to contain 11 p.p.m. hydrogen. The hydrogen content of the titanium from which these samples were taken was 10 p.p.m.

The following examples illustrate the anodizing of titanium equipment for use in chemical plant:

EXAMPLE 2 A titanium lined vessel in the form of a vertical cylinder with domed ends and approximately 60 feet long and 10 feet in diameter was prepared for anodic treatment by bolting titanium lined blank flanges onto the bottom branches or nozzles of the vessel and a plastic weir onto the top of the vessel. A stainless steel tube insulated by a plastics thimble at the bottom end and by a plastics sleeve over the part passing through the top part of the vessel was supported vertically down the middle of the vessel.

The vessel was filled with a solution'comprising 10% by weight of fertilizer grade ammonium sulphate dissolved in steam condensate. The stainless steel tube was electrically connected to the negative terminal of a 200 amps DC welding generator and the positive terminal of the generator was connected to the vessel wall through three distributed leads. The generator was set to deliver -25 volts and switched on. The maximum current which passed initially soon fell to about 70 amps and then remained constant for a considerable period before falling again. The current was continued for 72 hours, after which time the current had fallen to about 10 amps. The vessel was then washed out with steam condensate and the anodic film formed on the blank flanges was examined metallographically.

The film, which was purple-grey in colour, was found to have a thickness of about 3000 A. and satisfactorily to cover the metal surface.

EXAMPLE 3 A titanium lined pressure vessel similar to that used in Example 2 was used as a reactor handling an oxidizing acidic catalyst solution under conditions such that the upper part of the vessel was exposed to non-corrosive condensate under come conditions of operation. The ves sel was made from titanium containing 0.12-0.15 iron, and no special precautions were taken to avoid iron contamination during fabrication, and the vessel was not anodized before being put into service. After 6 months operation cracks were found near welds in the upper part of the vessel, and examination of a sample removed from a cracked area showed the presence of titanium hydride, the hydrogen content being 130 p.p.m. No cracks occurred in the part of the vessel only exposed to oxidizing conditions. After repair of the cracks the vessel was exposed to anodic treatment in 10 wt. percent ammonium sulphate solution and has operated satisfactorily.

EXAMPLE 4 A titanium lined pressure vessel similar to that used in Example 2 was exposed to anodic treatment before being placed in service as a reaction vessel where the hydriding tendency was similar to that in Example 3, and the vessel was not cracked when it was examined after four months service.

EXAMPLE 5 Lengths of titanium pipe which included bends and branches and which, from their shape, could not be exposed to anodic treatment by immersion in a separate bath were treated internally by the following method:

Plastics extension pieces were fitted to the open ends of the pipe and its branches and arranged so that the 'pipe could be filled with 10 wt. percent ammonium sulphate solution to a level above any titanium surface. Where branches were in a downwards position so that gas would not be trapped in them they were closed by plastics blank flanges. A cathode made from A; in. steel wire covered by a perforated plastics tube to prevent short circuits developing between cathode and pipe, but to permit current to flow from all parts of the cathode was inserted through each pipe and short subsidiary cathodes introduced into the branches. Several lengths of pipe were then connected together and to the positive terminal of a DC generator, and the electrodes were connected together and to the negative terminal of the generator. The pipes were then anodized at 25 volts for 24 hours. The current was initially more than 100 amps but fell in a few hours to about 8 amps, which current was maintained steady for the remainder of the anodizing period. 100 ft. of 6 in. diameter piping was anodized simultaneously. At the end of this treatment the colour of all surfaces exposed to anodic treatment which were visible was purple-grey.

EXAMPLE 6 Two titanium tube/tube plate exchangers positioned vertically in a structure were exposed to anodic treatment together by the following method, the treatment being required only on the tube side of the exchangers.

Mild steel end chambers were fitted to the lower end of each exchanger, each end chamber being insulated from the exchanger, by plastics and rubber gaskets and washers and sleeves on the bolts. Side branches on the end chambers were connected by rubber tubing to the bottom of a reservoir vessel so placed that the liquid level could be maintained at that of the upper tube plates. A plastics header was attached to each upper tube plate, the two header tanks being connected together and to the reservoir vessel. A pump was arranged between reservoir and header tanks so that the anodizing solution could be circulated round the system, passing down the tubes of the exchangers. Mild steel cathodes were introduced into the header tanks, being held in plastics supports, and were connected to the mild steel end chambers which also functioned as cathodes. The cathodes were connected to the negative terminal of a DC generator and the two exchangers were connected to each other and to the positive terminal of the generator. The system was filled with 10 wt. percent ammonium sulphate solution and the generator set to provide a final voltage of 25-30 volts. The current was initially about amps but fell in a few hours to 10 amps which current was maintained for 24 hours.

The final colour of the tube plates was a dark straw, with blue tinges around the welds and in the mouths of the tubes.

What is claimed is:

1. A method of preventing cracking and hydrogen embrittlement of metal surfaces selected from the group consisting of titanium, zirconium, niobium, tantalum and alloys thereof, contaminated with small amounts of iron or other base metal, which metal surfaces are in contact with a hydrogen environment effecting metal hydride formation, comprising anodically treating said metal surfaces in an electrolyte solution to dissolve the contaminants into the solution, and rendering the metal surfaces resistant to hydrogen penetration by formation of a protective film thereon.

2. A method according to claim 1 in which the anodic treatment is effected at 10-40 volts in the presence of an electrolyte comprising a -20 weight percent aqueous solution of a salt selected from the group consisting of neutral and acid ammonium and alkali metal salts other than halides.

3. A method according to claim 2 in which the electrolyte is an aqueous solution of ammonium sulphate.

4. A method as in claim 1 wherein said metal is titanium.

5. The method of claim. 1, wherein the metal surface is titanium; the contaminants are metals selected from the group consisting of iron, magnesium, aluminum, zinc and mixtures thereof, and the electrolyte solution is an aqueous ammonium sulphate solution.

6. The method of claim 5, wherein the electrolyte solution is a 5-20% by weight aqueous ammonium sulphate solution.

7. A method of preventing cracking and hydrogen embrittlement of metal surfaces selected from the group consisting of titanium, zirconium, niobium, tantalum and alloys thereof, contaminated with small amounts of iron or other base metals, which metal surfaces are exposed to a hydrogen containing or producing environment, comprising anodically treating at least a portion of said metal surfaces in an electrolyte solution to dissolve the contaminants into the solution, and rendering the metal surfaces resistant to hydrogen penetration by formation of a protective film thereon.

8. The method of claim 7, wherein the metal surface is titanium; the contaminants are metals selected from the group consisting of iron, magnesium, aluminum, zinc and mixtures thereof, and the electrolyte solution is an aqueous ammonium sulphate solution.

References Cited UNITED STATES PATENTS 3,391,073 7/1968 Rusch et al. 204297 R 3,300,396 1/1967 Walker 204297 R 3,176,850 4/1965 Rosner 204297 R 3,108,058 10/ 1963 Mines et al 204-297 R 3,035,999 5/1962 Sharon et al 204297 R 2,949,411 8/1960 Beck 204-5'6 R JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 204141 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTEON Patent No. 3 5 Dated M 7 3 973 w fls) John Grahame Hines and Joseph Bernard Cotton It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

i Inthe heading; line 30 was omitted and should appear as follows: i Q 1.

[30] Foreign Application Priority Data May 2, 1966 Great Britain 19215/66 Signed and sealed this 27th day of Noire mber 1973.

AtteSt:

EbW DM- TP B RENE n. TEGTMEYER Attestlng Offlcer Acting Commissioner of Patents FORM P0-1050 HO-69) uscoMM-oc 0O37o-Pou