US2594124A - Electrolytic polishing of metals - Google Patents

Description

Patented Apr. 22, 1952 2,594,124 ELECTROLYTIC POLISHING OF METALS Percy Allan Charlesworth, Adlington, near Macclesfield, England No Drawing Application September 2, 1947, Se-

rial No. 771,591. 12, 1946 1 Claim. (01. 204-1405) This invention relates to improvements in the electrolytic polishing of metal surfaces wherein an electrolyte of sulphuric acid or of a mixture of sulphuric acid and phosphoric acid, the electrolyte being of high acid concentration, is employed and the. article to be polished is made the anode.

The generally accepted theory is that an anodic film is formed on the surface of the metal being polished, through which film the high spots project and are dissolved away resulting in the formation of a smooth surface.

The method of polishing metals and metal alloys byimmersing them in an electrolyte and passing an electric current therethrough, utilising the article to be polished as the anode and employing as the cathode either the tank containing the electrolyte or an independent or conforming cathode, has in general, as is wellknown, many advantages over the mechanical methods of polishing. It utilises non-skilled labour, is quicker and cleaner than the mechanical method, articles once they have been introduced into the solution can be left unattended until it is time to withdraw them from the electrolyte, in most cases one electrolytic treatment is suflicient to obtain the required degree of polish and the process lends itself to the mass production of polishing of large outputs.

These advantages are particularly great when an electrolytic polishing process is applied to the polishing of stainless steels since these metal alloys are in most cases very difficult to polish mechanicallyand for their successful polishing require the use of skilled labour and long and arduous abrasive treatment with difierent sizes of grits and finally mopping. Even when a bright surface is obtained thesurface layerhas been greatly worked,"piled, heated, to form an amorphous or pseudo amorphous layer. Moreover particles of abrasive are often driven into the metal and are subsequently responsible for pit corrosion. 7 Q

The electrolytic method of polishing has been described in the technical literature and the specifications of a number of patents describe various electrolytes for use in the method but so far as the applicant is aware none of these various electrolytes are suitable or economical for electropolishing large articles on a commercial scale.

Among prior specifications relating to electrolytes for the electrolytic polishing of metals is the specification of EnglishfPatent l \l o.-526,854

which describes an electrolytic polishing method wherein the metal to be polished is made the In Great Britain September anode and the electrolyte contains sulphuric acid and phosphoric acid of a combined concentration of at least 50% by weight of the electrolyte and an electric current is passed therethrough of suflicient density and for a sufficient length of time to produce a polished surface of the metal. The composition of the electrolyte described and claimed in the specifiction lies within the wide range of 15% to sulphuric acid and 15% to phosphoric acid. 7

Experiments carried out by the applicant show that the aforesaid range covers mixtures of sulphuric acid and phosphoric acid which do not give a satisfactory polish within reasonable limits of current density and temperature, i. e. a current density below 2 amps. per square inch and a temperature not exceeding about 50 C. and in some cases etching occurs over the whole surface immersed in the electrolyte. Speaking generally unsatisfactory results are obtained when the ratio of sulphuric acid to phosphoric acid exceeds 5 3. r

The aforesaid specification No. 526,854 infers that the best results are obtained when the relative proportions of sulphuric acid and phosphoric acid are 15% to 20% sulphuric acid and 63% to 67% phosphoric acid but it is found that when the proportion of phosphoric acid is high the resistance of the solution rapidly increases with use and driving potentials in the region of 50 volts are necessary to obtain a reasonable life from the electrolyte.

The use of an electrolyte based on sulphuric acid alone is not a commercial proposition since although there is a tendency to polish in an aqueous solution of 60% by weight of sulphuric acid with high current densities in the range of 3 amps. per sq. inch and upwards, much spray is thrown off by the solution which is an inconvenience in practice. At higher concentrations of acid less spray is evolved but the tendency of salts to crystallize out on the metal is great and spoils the polish. When high current densities are used for polishing on a large scale very eflicient cooling is required.

Other baths have been suggested making use of anaqueous solution of phosphoric acid and containing additions of glycerine (20% or more).

These solutions are expensive and unless the metal undergoing polish has a fairly smooth finish to start with, treatment times require to be prolonged. Treatment times can be shortened by -increasing operating temperatures e. g. C.

' The solutions tend to absorb or over but this introduces practical di filculti es.

moisture very'freely from the air and thus become unbalancedpar ticularly when not in use. The solutions are also viscous thereby increasing drag-out losses and introducing other objections inherent in such solutions.

As the solutions are worked resistance increases and the working voltage to obtain reasonable life of the solution may have to be increased to as much as 50 volts. High voltages of course, introduce many difficulties. Temperatures due to working become excessive and external cooling has to be very efiicient and thus expensive, the solutions eventually become saturated with salts and no longer capable of polishing and necessitating expensive and elaborate generating apparatus and electrical gear.

Other electrolytes based on sulphuric acid and carboxylic acids e. g. citric acid) have been suggested but such are relatively expensive and unless the solution is kept warm the organic acid tends to be thrown out of the solution.

The object of the present invention is to provide a method for the electrolytic polishing of metals employing an electrolyte which will be less expensive than those which it has hitherto been found possible to work on a commercial scale, which can be regenerated, which will in general give faster working at given current densities and which can be employed within a temperature range of 45 C. to 60 C. A temperature within this range will not burn the operative and has many other practical advantages.

The electrolyte is primarily intended for the electrolytic polishing of stainless steels but they can be employed for polishing some other metals and alloys by suitable adjustment of the operating condition.

The electrolyte according to the invention consists of an aqueous solution of sulphuric acid and phosphoric acid with a relatively small amount of an additive consisting of aniline.

The acid concentration should be between 50% and 80% by weight of the solution and the ratio of sulphuric acid to phosphoric acid in the case of a. mixed solution should be greater than :3. Suitable ranges of sulphuric acid are 4.0% to 70% by weight and of phosphoric acid 0 to 30% by weight.

Aniline is effective at concentrations of from 0.3% to 3% by weight.

Suitable current densities when using an electrolyte according to the invention are between 0.5 amp. and 1.5 amps. per square inch of the surface of the metal forming theanode and the voltage in general lies between 3 volts and volts.. A satisfactory electrolyte working temperature is 50 C. to 60 C. which is a very convenient range from a commercial standpoint. The time of treatment varies according to different factors and the initial condition of the surface of the metal and may be between a-few minutes and half an hour.

The following are typical examples illustrating the invention. In these examples the sulphuric acid is commercial sulphuric acid (95% H2SO4 specific gravity 1.84) and the phosphoric acid is commercial ortho-phosphoric acid (85% H3PO4 specific gravity 1.7). It is of course possible to replace ortho-phosphoric acid by suitable proportions of meta-phosphoric acid or pyro-phosphoric acid. The percentage of the additive is the pure chemical.

4 In all the examples the parts are by weight.

Examplel Sulphuric acid 55 Phosphoric acid; 22 Water 23 Aniline 0.5

' Example 2 Sulphuric acid 62 Phosphoric acid 15 Water 23 Aniline 0.5

Example 3 Sulphuric acid '70 Water 30 Aniline 1.5

Y Example 4 Sulphuric acid 60 Water 40 Aniline 1.5

With baths based solely on sulphuric acid or where the proportion of this acid to phosphoric acid is high, it is advisable to provide a continuous filtration unit on the polishing tank and in some cases continual agitation is an advantage.

Other advantages of the use of aniline in addition to its relative cheapness are:

(a) It has a long life before its effects Wear off.

(2)) When polishing large areas a uniform degree of polishing is obtained over the whole surface.

(c) When polishing stainless'steel on passing the current a blue anodic film is formed on the surface of the metal which can be removed by washing. This blue colouration serves as an indicator of the extent to which the adhering drag-out electrolyte has been washed off.

Although the invention has been mainly described in its application for the electrolytic polishing of stainless steel, it can be used for polishing other ferrous base metals and alloys, and nickel and its alloys.

. The invention has the following advantages over other known processes for polishing stainless steel and other metals.

' 1. Relative cheapness of the electrolyte since the proportion of phosphoric acid can be kept low or in some cases phosphoric acid can be omitted.

2. When salts are formed due to dissolution of the metal undergoing treatment depending to some extent on the composition of the electrolyte, they begin to precipitate out after a time and can be filtered off.

3. Solutions will polish at relatively low current densities of 0.3 amp. per square inch and upwards. It is found that current densities in excess of 1 amp. per square inch are seldom necessary in solutions containing phosphoric acid. In solutions based on sulphuric acid alone higher current densities may be advisable but it is found that little improvement in polish is obtained with a current density over 2-3 amps. per square inch.

4. Owing to the high conductivity of aqueous solutions containing relatively large amounts of sulphuric acid, low driving potentials can be used e. g. 3 'to 10 volts are adequate forpractically all purposes. Moreover large increases of required driving potentials are not found to be necessary after the solution has been in use for long periods.

' 5. The electrolytic solution can be regenerated by filtration and fortification with fresh acid or .wate'r; f' l 6. The, solutions are of relatively low viscosity and the use of the additive does not rely for its bcneficial' feifect on increasing the viscosity of the body of the electrolyte although it induces desirablegcliaracteristics in the anodic layer.

7. The electrolyte can be used commercially in tanks holding up to 5 tons and articles with areas as great as 20 square feet can be satisfactorily polished.

I claim:

A method for the electrolytic polishing of.

metals employing an electrolyte consisting essentially of an aqueous solution of sulphuric acid and phosphoric acid, which comprises immersing the article to be polished as the anode in the acid solution having an acid concentration of between 50% and 80% by weight of the solution and a ratio of sulphuric acid to phosphoric acid greater 6 than 5 to 3, and which contains an additive or between 0.3% and 5% by weight of the solution and consisting of aniline.

PERCY ALLAN CHARLESWOERTH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,334,698 Faust Nov. 23, 1943 2,348,517 Beckwith May 9, 1944 2,411,410 Beckwith Nov. 19, 1946 OTHER REFERENCES "Journal of The Electrodepositors Technical Society, vol. 21 (194546), pages 256-258.