US2681869A - Surface-modifying metal articles by action of an impregnating or alloying metal and composition therefor - Google Patents

Description

Patented June 22, 1954 UNITED STATS SURFACE -MODIFYING METAL ARTICLES BY ACTION OF AN IMPREGNA'EKNG R ALLOY- ING METAL AND @QMPOSEEIGN THERE- FOR Wallace C. Johnson, iehairer Heights, Uhio, as-

signor to The Duriron ilompany, line, Dayton, Ohio, a corperation of ew York No Drawing.

Application August 24, 1950,

Serial No. 181,312

(oi. ll.7--65) 1 21 Claims.

This invention relates to surface-modifying metal articles by action of an impregnating or alloying metal; and it has to do more particular hr with a process of surface-modifying metal articles which is essentially characterized by the inclusion of a step or stage wherein the surface of the metal article or "Work to be altered, or such portion of it as is to be impregnated or cased, is exposed, while maintained at a reaction temperature within a range sufficiently below its melting point to prevent its undergoing either liquation or substantial deformation, to the action of the impregnating or alloying metal in available form contained in a layer of sintered (i. e. incipiently or partially fused) but suitably refractory material, resembling in its general characteristics a ceramic or slag-like composi tion which, at the stated temperature, has a pasty consistency and adheres to the surface of said article, thereby shielding it from the atmosphere of the heating furnace while maintaining the source of the impregnating or alloying metal in intimate contact with said work.

Surface modification of metal articles for various purposes, and particularly such treatment of ferrous metal articles, in order to render them more resistant to rusting and other types of corrosion, as well as to enhance their ability to withstand mechanical wear, constitutes the subject matter of a very extensive prior patent and journal literature. Some of the methods proposed. have found more or less successful practical ap plication in industry. Generally speaking, however, commercial success has been far from outstanding where the surface-modifying procedure has involved impregnating or surface-alloying an iron or steel article with such a metal as silicon, for example, to provide the article with a corro-- sion and wear resistant case. All the methods of achieving this that have been proposed. heretofore, insofar as they are capable of producing a reasonably satisfactory case, are cumbersome, more or less complicated, and generally not commercially practicable. Some involve heating the articles in special mixtures of granular solid materials. Others require the use of fused salt baths, or of special atmospheres, air tight containers or reaction chambers. Long heating cycles at very high temperatures are the rule, with close control of operating conditions at all stages usually desirable but difficult to attain. All these factors have greatly restricted application of such processes, even where their rela tively extremely high cost-might not in itself be prohibitive.

It is an object of the present invention to provide a greatly simplified and relatively inexpensive procedure whereby ferrous and other metal articles may be materially improved as regards resistance to corrosion especially, and commonly to wear also, through impregnation or surfacealloyin with a metal or a plurality of metals which, singly or together, are capable of conferring upon the base metal (1. e. the metal of which the given article is composed.) the improved property or properties sought.

A further object of the invention is to extend materially the practical application of metal impregnation and surface-alloying for the protection of ferrous and other metal articles.

With the foregoing general objects in view, as well as others which will become evident hereinafter, the nature of the invention will now be more fully set forth and explained by describing in detail certain specific embodiments of the invention that are regarded as particularly advantageous and will serve to make clear the underlying principles involved. These embodiments should be understood, however, to be only illustrative of many specifically different forms which the novel process may take within the scope of the invention pointed out in the accompanying claims.

In general outline, the process of the invention may be described as one in which the metal article whose surface is to be altered by the impregnating metal is provided initially with a substantially dry, adherent coating or enclosing layer of a mixture comprising finely divided refractory slag-forming materials and a temporary binder to render the mixture coherent, said mixture including the impregnating metal either as such or in other available form, and one or more c e s adapted. to facilitate sintering (i. e.

pregnating metal upon the underlying or basis metal surface; the coated article is then heated to a temperature that is high enough to sinter or partially fuse the adherent mixture into a rather viscous agglomerate which clings to the surface of said article and substantially seals the same, said temperature being insufficiently high, however, to render the mixture thinly fiuid or endanger the integrity of said article; the heating is continued within this temperature range until surface modification of the metal article by the impregnating or alloying metal has been accomplished to the extent desired; the resultant product is then cooled to solidify the adherent sintered slag-like coating, which is desirably more or less glassy or porcelain-like in character; and such of the solidified coating as may still adhere to the cooled article is usually removed.

together with the impregnating metal or source compound thereof, preparatory to furnacing the thus-coated article, is one which although simple enough from the standpoint of manipulative procedure, must be carried out with proper care and attention to certain operational details necessary to observe in order to obtain optimum results. This is because the properties of the protective case formed by the action of the impregnating metal in the ensuing furnacing or firing operation depend in large part upon the characteristics, both physical and compositional, of that initial coating layer which, for convenience, is herein referred to as the preliminary (i. e. green or unfired) ceramic blanket or envelope.

This preliminary blanket or envelope is formed by suitably applying to the metal article, as by dipping, brushing or spraying, a fluid, slurrylike composition generally similar in its characteristics to a wet grog or slip, and therefore commonly so termed herein. Its exact composition may vary Widely within the scope of the invention, depending upon the nature of the metal or alloy of which the article is composed, and the nature of the impregnating metal, factors which will necessarily determine the most suitable furnacing or firing temperature to be employed in any given instance, and hence what should be the composition of the slagor glass-forming mixture in order for it to have the desired viscous consistency at such furnacing temperature. Essentially, however, the grog or slip comprises a suspension, in a reasonably volatile liquid vehicle that may in some cases be aqueous and, in

others, more desirably non-aqueous, of the various finely divided solid mineral materials including the impregnating metal source, i. e. either the metal as such or in the form of a compound from which it becomes available in the firing operation, which are to be present in the preliminary ceramic blanket or envelope aforesaid; plus a binder which holds said materials firmly coherent in the. blanket or envelope during and after evaporation of the liquid vehicle therefrom prior to firing. Said binder may desirably be a substance that is soluble in said vehicle.

As already indicated, the composition of the grog should be such as to provide a more or less vitreous slag that will serve as a carrier medium for maintaining the available impregnating metal in very intimate shielded contact with the surface of the metal article during the firing or furnacing operation; and to this end the grog composition should always include ingredients adapted to produce such a slag-like carrier. The slag-forming mixture should have its mineral components so proportioned that its sintering or incipient-fusion temperature or temperature range reasonably approximates the firing or furnacing temperature that is to be employed in said firing operation. This is readily done by applying compositional principles well known in the ceramic art. As a general rule it is highly desirable for achievement of optimum results, although not invariably indispensable, that the ingredients of the grog employed in practicing the invention shall include a halogen-containing, more particularly a fluorine-containing, fiuxing agent such, for example, as a halide of an alkali metal or alkaline earth metal, or a complex halide of more than one such metal.

At the present time, the invention finds its most important practical application in the impregnation, with silicon, of ferrous metal articles, more especially steel and fabricated steel prodthat of the underlying metallic case.

nets of all kinds, to form thereon'ajcorrosion resistant layer or case. This particular aspect or phase of the invention will accordingly be more especially referred to and emphasized in the following detailed description notwithstanding the iact that, in general, the invention is applicable to the impregnation or surface-alloying of any metal article or product with any other metal capable of penetrating thereinto at a temperature below the melting point of the basis metal.

Let it be assumed by way of example that the process is to be applied to a plate, tube or bar of mild steel. The procedure in this case may desirably consist of the following steps:

(a) Sand-blasting or otherwise cleaning the steel articles to be treated.

(b) Coating the articles by dipping them in a slip or slurry-like grog of suitable fluidity, consisting of ferrosilicon, sodium fluoride and silica, together wtih a liquid binder.

(c) Drying the coated articles, prior to firing, to drive off the volatile portion of the liquid binder. Where the grog coating is relatively very thick, the drying operation may desirably be continued at increasingly high temperatures into what amounts to a pre-firing operation (e. g. at 1200 F.), since this tends to preserve the integrity of the grog coating and its good adherence to the work throughout the remainder of the process.

((1) Firing the dry (and, if necessary, prefired) grog-coated articles in a suitable furnace chamber at a temperature within the range of 1800 to 2000 F., say approximately 1950 F. in a typical instance, to effect impregnation of the surface of the articles with silicon; the grog coating or envelope being converted into a viscous slag which serves effectively to protect the underlying metal surface and thus obviate the necessity of employing any special furnace atmosphere. A good way to support the articles in the firing furnace is to place them upon a layer of silica flour, for example.

(6) Cooling the fired articles to ordinary tem perature, and removing, as by hammering or tumbling, any of the vitrified slag coating which may not have spalled off during the later stages of the cooling. It is indeed desirable to so proportion the ingredients of the grog composition as to encourage this spalling off of the slag coating or envelope in the later stages of the cooling down. This is merely a matter of adjusting the composition of the vitreous material so that its coeflicient of expansion differs sufiiciently from spalling off of the fired ceramic blanket may also be encouraged by water quenching the work from. a red heat.

The solid components of the fiuid grog should all be in powdered or finely divided form. In preparing the grog, the solid ingredients are commonly dry-mixed as powders and then suspended in the liquid binder composition by adequate stirring or other mode of agitation.

Ferrosilicon is ordinarily used as the source of silicon because of its ready availability and relatively low cost. It should be used in the form of a powder which will all pass through a 100- mesh sieve, with approximately passing through ZOO-mesh. The silicon content should be not less than 20% by weight and may run as high as It is possible to use even silicon metal in powdered form, but the cost is much greater and it appears to offer no advantage over ferrosilicon for the purposes of this invention. As a general rule, use of a ferrosilicon containing from 39% to 75% silicon gives the most satisfactory results in practice. It is sometimes found desirable to use in the same grog ferrosilicons of different silicon contents in order to obtain satisfactory suspension of the particles of solid matter. A ferrosilicon of high silicon content is, of course, of lower density than one of lower silicon content. Furthermore, by using ferrosilicons of differing silicon contents in suitably proportioned blends or mixtures, it is possible to obtain slag blankets that spall off automatically or, at any rate, are rather removable, upon cooling the fired article.

It is possible, of course, to employ available silicon in forms other than those mentioned here inabove. Thus, silicon carbide could in some cir cumstances serve as the silicon source; the considerably higher firing temperature which its use requires is a factor sufficient in itself, ordinarily, to render the use of ferrosilicon, for e:-:- ample, decidedly more advantageous.

As regards the fluoride component of the grog or slip, sodium fluoride, in the form of a dense white powder of such fineness that it will all pass ZGil-mesh, has been found usually to give optimum results. Aside from the fact that it acts as a flux, the exact function of the fluoride is not fully understood. However, it is believed possible that, at the temperature at which silicon impregnation of the underlying steel is effected in the furnacing operation, the fluoride may serve as a source of fluorine available to promote, perhaps catalytically, the penetration of silicon into the underlying steel surface. From this standpoint, the

fluoride may be regarded as a .ixing catalyst; but, regardless of theory, it obviously has a most favorable effect upon the impregnating or alloying action or reaction, expediting it materially. Other fluorine-bearing compounds can also be used for this purpose instead of or in conjunction with sodium fluoride. Among such compounds are, for example, barium fluoride, calcium fluoride, sodium fluosilicate (or silicofluoride) also the corresponding potassium salts. Chlorides oi the alkali metals and alkaline earth metals, as well as of other metals, may also sometimes be usefully included as fluxing agents, but in general are not as effective as fluorides in promoting the penetration of the impregnating or alloying metal into the work being treated.

The silica may be in the form of silica flour. However it is often advantageous to employ, instead, silica aerogel because of its lower apparent density and consequent greater eifectiveness as a suspension agent for the fluoride and ferrosilicon. A silica aerogel available from Monsanto Chemical Company under the trade name Santocei is satisfactory. The air volume within the silica particle is about 94% 'of this particular form of silica aerogel, which therefore has great powers of suspension. Besides acting as a suspending agent, this or any other form of silica employed in the grog supplies at least a part of the acid component required for fluxing the basic constituents of the grog.

The grog composition can be rendered especially eifective if it contains, in addition to the ingredients already mentioned, a soda lime or .borosilicate glass in powdered form all passing through ZGO-mesh. This is of somewhat lower melting point than the vitreous slag or glassy material that would be formed Without this addition. Moreover, this addition provides a con venient means of controlling and regulating the fluidity of the vitreous blanket produced in the firing operation so that it shall be of the desired degree of viscosity. This viscosity should be such that the blanket or envelope does not flow materially at the maximum firing temperature employed, but yet is soft enough to permit escape of occasional gas bubbles outwardly there-through, with prompt rescaling of the vent openings to restore substantial gas tightness of the envelope.

The liquid binder employed may vary widely in character, and the viscosity of the grog, which is of great importance in obtaining proper application of the grog to the work initially, is of course controlled by the amount of liquid binder that is mixed with the powdered solid components. The liquid binder is a solution or dispersion of an agglutinant binding material proper in a solvent or liquid vehicle. While the active binder component may be either organic or inorganic, the best results are ordinarily obtained by using an organic material, such as a cellulose ester, for example, carried by an appropriate non-aqueous solvent vehicle. For example, a satisfactory liquid binder composition for present purposes may be obtained by diluting one part by volume of an ethyl-cellulose clear lacquer (e. g. Du Pont #1234) with three parts of a fast drying lacquer thinner (e. g. Du Pont #3651). In general, the liquid binder employed should be such that, when a grog employing it is applied to the work and the Work is charged directly into a hot furnace after a preliminary heating-up to drive oif the volatile solvent, the coating even when relatively thick will not flake or blister. In actual practice, of course, the work, after being coated with the grog, is heated moderately to drive oif the solvent and dry the grog-coated work before firing.

Although the use of water or a partly aqueous medium as the liquid portion of the liquid binder composition is frequently quite impracticable be cause of the very slow rate at which the grogcoated work would have to be dried in order to avoid causing the coating to flake or blister, and because of the difficulty of eliminating substantially all traces of the liquid vehicle in the drying operation, nevertheless water may be employed to advantage where such slow rate of drying is not seriously objectionable, because of the sub stantial saving in cost of liquid vehicle thereby realized. In such case, it is feasible to use sodium silicate or a water-soluble gum, for example, as the active binder component.

The desired case thickness is obtainable by heating the work long enough at the temperature most practical under the circumstances. The higher the temperature and the longer the time of heating, the thicker is the case. For cases of high silicon iron the temperature should not exceed 2100 F. At about this temperature, the sintered coating or envelope, which up to this point remains properly viscous or plastic, starts to become too fluid and, at 2200 FL, will run oif the work. Thin cases of silicon iron will form as low as 1550 F. Generally speaking, operation of the present process at much below about l'iili) F. is not commercially practicable. The most desirable practical operating range seems to be from about 1750 to 1959 or 2000" F., with 1950 F. representing typical good practice.

Thin cases of high silicon iron (1--l l% sill con) will form in three minutes at l850 Operating on -inch steel plate at a furnace temper'ature of 1950 F., high silicon iron cases of varying thicknesses depending upon the length of firing time are obtainable as follows:

At heat: Case thickness minutes 0.007 inch. minutes 0.017 inch. minutes 0.031 inch. 90 minutes 0.047 inch.

For steel parts that require machining after the protective case has been formed, it is usually necessary to hold the furnace temperature for two to three hours in order to form a case that will provide sufficient thickness for machining.

The heat used to produce the reaction that forms the protective case can be derived from gas, coal or oil. Radiant heat from electric furnaces or induction heating can also be used. No special furnace atmosphere is required.

In order to further illustrate the principles of the invention as applied to siliconizing metal articles or products, more particularly those composed of steel, several specific examples will now be given. In the various formulae, the proportions are given in parts by volume unless other wise indicated.

Example I In this instance the grog or slip has the following composition:

1 part silica flour 2 parts sodium fluoride (dense white grade)- through ZGO-mesh 6 parts 50% ferrosiliconthrough 150-mesh 6 parts 90% ferrosilicon-through 150-mesh These ingredients are mixed dry, and the dry mixture worked into a dipping slurry by the use of a liquid binder composition in the form of an ethyl-cellulose clear lacquer. The work is coated by dipping it in this slurry or grog while keeping it constantly stirred to prevent settling. After thoroughly drying the coat by gently heating the work to evaporate off the volatile portion of the liquid binder composition, firing the work at a temperature of 1950 R, usually for as little as 15 minutes, gives an excellent case approximately 0.015 inch thick.

By substituting silica aerogel (e. g. Santocel) partly or wholly, for silica fiour as a suspending agent, the tendency of the other solid components of the grog to settle out is greatly lessened. Also it becomes possible to apply a heavier coating of grog to the work without encountering so much tendency of the grog to slip from the work being treated. Where such heavier coating of grog is applied, it is generally well to slow down the drying time of the liquid binder composition somewhat by adding thereto a suitable proportion of a slower drying thinner or solvent, or using less thinner, in the manner well known in the lacquer and paint arts. Where the binder composition dries too fast there is a tendency on the part of the green ceramic blanket or envelope to crack and blister, resulting in an imperfect case upon firing. It is very important that the dried grog coating be as uniform and smooth as possible, and that it adhere firmly to the surface of the underlying work at all points.

Example II In this procedure, which is especially suitable where the production of a relatively thick (e. g. 0.05 inch or more) high silicon iron case on the work is desired, the grog should be applied to the work in several successive coats, each being dried before application of the next coat.

8 In this instance, the first coat is a thin priming coat of grog having the following formula as regards its solid components, which are finely powdered as before:

1 part silica aerogel 2 parts sodium fluoride 12 parts ferrosilicon This priming grog provides thorough and immediate coverage of the work. The early thin case thus gives protection from oxidation during prolonged firing periods.

For the second and subsequent coats, the grog formula as regards solid components may be as follows:

6 parts silica aerogel 3 parts sodium fluoride 3 parts AFSL (air floated soda lime) powdered glass 12 parts 50% ferrosilicon The purpose of including the air floated soda lime glass in this grog formula is to favor early interaction of the grog ingredients in the firing. Its

ction is to cause early fluxing of the acid and basic elements of the grog.

Example lll When iron oxide in powdered form (all through ZOO-mesh) is added to any of the formulae given in the foregoing examples, in proportion up to 50 of the combined volumes of the ferrosilicon and sodium fluoride, such addition promotes an exothermic reaction as the silicon case forms at elevated temperatures, and this may increase to some extent the efiectiveness of the action of the iiuxing catalyst, thereby permitting the use of lower furnace temperatures.

Example IV By adding certain other metals in available form to the grog or slip, such metals may also be caused to penetrate the underlying metal surface along with silicon, thus producing cases of modified characteristics. Thus, in any or" the foregoing examples less brittle high silicon iron cases can be formed by adding copper to the indicated siliconizing formula in volume proportion of about 1 part of fine copper powder (through 300-mesh) to 12 parts of ferrosilicon. This, when fired in the usual way, forms a low copper silicon alloy that is somewhat more ductile but has lower corrosion resistance. An even better result is obtainable by adding the copper in the form oi red copper oxide.

Additions of manganese (e. g. as ferromanganose) to the ferrosilicon tend to make a harder case and also to give the case surface a silvery brightness. Also, adding manganese causes somewhat greater fluidity both of the case and of the ceramic envelope in the firing operation, thus tending toward formation of a more eutectic alloy.

Adding powdered nickel to the ferrosilicon in proportion of from 1 to 2 parts of nickel to 12 parts of ferrosilicon has a pronounced effect on the case, forming what appears to be a finer grain structure which, under prolonged oxidation tests, gives better protection to the steel base than do silicon cases containing no nickel. Here again, the nickel apparently forms a eutectic alloy which enables the protective case to form at lower temperatures.

Ordinary furnace atmospheres during the firing would have definitely adverse effect upon the characteristics of the silicon-iron case formed, as by causing formation of a loose superficial case, for example, it may often be desirable to further guard against this possibility, without resorting to prepared. atmosphere protection. This can be done by finally dipping the work, to which a grog coating has already been applied as in any of the foregoing examples, in a special grog or slip having the following volume formula as regards its solid components:

1 part iron oxide (F6203) 2 parts borosilicate powdered glass This gives an especially effective sealing effect during the firing.

In the various formulae given in the foregoing examples, the proportions of the various ingredients may vary considerably without greatly affecting the characteristics of the case obtained, provided care be taken to observe the requirement that, in the furnacing operation, the resultant slag that is formed shall not become completely molten or substantially fluid at the firing temperatures employed. However, the formulae given represent good practice, and reasonable approximation to them is therefore recommended for best results.

Where the invention is to be applied to chromizing a metal article, the procedure is similar to that hereinabove described for siliconizing, but with available chromium replacing available silicon in the grog or slip applied to the work, and

the firing temperature being in general somewhat higher. To provide available chromium, ferroohrome in powder form (all through lOO-mesh) may be used, taking the place of ferrosiiicon in the siliconsizing formulae given hereinabove. The carbon content of the ferrochrome should most desirably not exceed about 0.10. High carbon ferroehrome will not form a satisfactory chrome-iron case.

Good chrome-iron cases that can be bent without rupturing the case can be produced by using low carbon ferrochrome powder (through 100- mesh) in a grog or slip of the following formula:

2 parts silica aerogel 2 parts sodium silicofluoride 6 parts ferrochrome mixed to the consistency of paint with a suitable liquid binder composition such as a thinned ethyl cellulose lacquer.

The invention can also be applied to formation of a bright metallic copper case over a steel base. To this end, metallic copper or, still better, red copper oxide, both in powder form (all through loll-mesh), may replace ferrosilicon in the foregoing siliconizing grog formulae.

A further application of the invention is in forming a protective silicon case on solid molybdenum and/or titanium metal which, when unprotected, is very easily oxidized at elevated temperatures.

The adherence of porcelain enamel to a steel base may be substantially increased byflrst providing the steel base with a case of silicon, chrome or oth r suitable metal that will alloy with such base in the manner herein disclosed, thus lessen-- ing the well-known tendency of porcelain to chip or flake from its base.

In the best mode of practicing the invention, the green but dry grog or ceramic coating applied to the work preparatory to firing usually has a thickness on the order of not much more than approximately four times that of the metallic case that subsequently forms underneath it. Where the coatings are relatively thick, say on the order of one-fourth inch or more, there is greater danger than with thinner coatings that the metal-to-coating contact may be disturbed in the subsequent manipulation of the coated work. Thus, in order to minimize cracking and separation of such thick grog coatings from the underlying metal due to thermal shock, it is advisable not merely to dry the work at temperatures moderately above the vaporizing point of the liquid vehicle employed in the fluid grog or slip applied, but also to pro-fire the coated work at a temperature on the order of 1200 F. or thereabouts, before introducing it into the firing or reaction furnace, wherein the temperature is commonly maintained at from 1800 to 2000 F.

The novel mixtures of finely divided solid materials herein disclosed as useful in practicing the novel process above described, whether said mixtures or compositions be in dry pulverulent form or worked up into a wet slurry or slip with a suitably volatile liquid vehicle, are to be understood as constituting a part of the invention. When in dry form, prior to being worked into a slurry or slip for application to the work to be treated, such a mixture may or may not include, already incorporated therein, a solid binding or cementing agent which is soluble in or miscible: with the subsequently added liquid vehicle in such manner as to exert the desired binding action in the dried but green coating of raw mix with which the work is provided prior to firing, and also to aid in promoting good adherence of said coating to the worl; prior to the sintering stage. Thus, where a non-aqueous liquid vehicle is to be employed, the dry mix may have ethyl cellulose, a varnish gum or resin, a solid bitumen, or the like, or any two or more of these, incorporated therewith in powder form to provide the binder component; said liquid vehicle being so selected or composed as to have the proper solvent or disper-sing action thereupon when eventually employed. If, on the other hand, an aqueous liquid vehicle is to be employed in preparing the wet grog or slip, the binder component thus optionally pre-incorporated in the dry mix may be any one or more of many substances available in powder or other suitably comminuted form which, upon solution in or mixture with water, exerts effective agglutinating action upon the other mix components. Examples of such substances are sodium silicate, starch, a starch derivative or conversion product such as dextrin, or animal lue.

What is claimed is:

1. The process of surface-modifying a metal article by the action of an impregnating or alloying metal, which comprises providing the article initially with a substantially dry, adherent coating comprising an unsintercd mixture of finely divided slag-forming solid mineral materials associated with an agglutinant binder that renders the mixture coherent, said mixture including a source of the impregnating metal, silica and a metal halide, so proportioned as to facilitate sintering of the mixture at suitably elevated temperature nondestructive to said article and promote action of the impregnating metal upon the underlying or basis metal surface; then heating the coated article to a temperature not lower than about i550 F. that is high enough to sinter or partially fuse the coating mixture to viscous consistency and thereby substantially seal the underlying surface of said article, but is insufficiently high to render the coating mixture thinly fluid or to endanger the integrity of said article; continuing the heat ing within such temperature range until the sired impregnation has been effected; then cooling the article and removing therefrom sin tered coating mixture adhering thereto.

2. The process set forth in claim 1, wherein the coating with which the metal article is initially provided is formed by applying to its surface a fluid grog comprising said mixture and a liquid vehicle in which said binder is dispersed and which is readily volatilizable, and said liquid vehicle is evaporated off to dry the grog coating prior to the sintering operation.

3. The process set forth in claim 1, wherein at least a part of the metal halide component is a fluoride.

4. The process set forth in claim 3, wherein said binder is an organic substance, and said liquid vehicle is non-aqueous.

5. The process set forth in claim 2, wherein said grog coating is built up on the work to a desired final thickness by applying the fluid grog in a plurality of successive steps and drying the coated work after each application.

6. The process set forth in claim 2, wherein said grog coating is built up on the work to a desired final thickness by applying the fiuid grog in a plurality of successive steps and drying the coated work after each application, the grog appl ed in the first step being thinner than that subsequently applied.

7. The process set forth in claim 2, wherein said grog coating is built up on the work to a desired final thickness by applying the fiuid grog in a plurality of successive steps and drying the coated work after each application, the ap plied in the first step being thinner than that subsequently applied, and the composition of the subsequently applied grog being such as to favor fiuxing interaction of the grog ingredients at lower firing temperature than that of the grog first applied.

8. The process of surface-modifying a metal article by impregnating it with silicon to form a corrosion-resistant case thereon which comprises coating such metal article with a fluid grog or slip comprising a mixture of finely divided solid mineral materials including a source of silicon, silica and a metal halide fiuxing agent, so proportioned as to render the mixture sinterable at a temperature non-destructive to said metal article, associated with, an agglutinant binder and a readily volatilizable liquid vehicle in which said binder is dispersed; evaporating off said liquid vehicle to leave a substantially dry, coherent coating adhering to the article; and firing the article at a temperature not lower than about 1550 F. that is sufiiciently high to sinter the coating mixture to viscous consistency, but not high enough to render it thinly fluid or to deform the article; finally cooling the article and removing any sintered material adhering thereto.

9. The process set forth in claim 8, wherein the metal of the article is steel, and said grog comprises essentially a mixture of ferrosilicon, silica and sodium fluoride, associated with said binder and a sufficient quantity of said liquid vehicle to give the mixture a slurry-like consistency.

10. The process set forth in claim 9, wherein the silica content of the grog is at least partly in the form of an aerogel.

11. The process set forth in claim 10, wherein the ferrosilicon employed in the fluid grog comprises different grades as regards silicon content, but the overall ferrosilicon component analyzes, by weight, not less than 20 per cent silicon.

12. The process set forth in claim 9, wherein the grog mixture further contains powdered iron oxide in proportion up to 50 per cent by volume of the ferrosilicon and sodium fluoride contents taken together.

13. As a new composition of matter, useful in surface-modifying metal articles by the action of an impregnating or alloying metal, a mixture comprising a source of such impregnating metal, together with silica and. a metal halide fluxing component, all intimately commixed in finely divided condition and so proportioned that the mixture will sinter or partially fuse into a viscous slag-like mass at a predetermined temperature within the range of 1500 to 2100 F.

14. The new composition of matter defined in claim 13, wherein the metal halide component includes a fluoride.

15. The new composition of matter defined in claim 14, wherein silicon is the impregnating metal.

16. The new composition of matter defined in claim 13, wherein at least part of the silica content is in the form of an aerogel.

17. The new composition of matter defined in claim 13, wherein silicon is the impregnating metal and at least part of the silica content is in the form of an aerogel.

1s. The new composition defined in claim 13, said composition being in substantially dry, noncohering condition, and further including a finely divided solid agglutinant binder component dispersible in a given liquid vehicle with which the mixture is to be used to produce a slurry-like composition.

19. The new composition defined in claim 13, wherein at least part of the silica content is in the form of an aerogel, said composition being in substantially dry, non-cohering condition and further including a finely divided solid agglutinant binder component dispersible in a given liquid vehicle with which the mixture is to be used to produce a slurry-like composition.

20. The new composition defined in claim 13, mixed with a liquid vehicle in proportion to produce a mass of slurry-like consistency, said mass including an agglutinant binder dispersed in said liquid and effective, upon evaporation of said vehicle, to render the residual mass coherent.

21. The new composition defined in claim 13, wherein at least part of the silica content is in the form of an aerogel, said composition being mixed with a liquid vehicle in proportion to produce a mass of slurry-like consistency, said mass including an agglutinant binder dispersed in said liquid and effective, upon evaporation of said vehicle, to render the residual mass coherent.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 20,719 Ihrig May 10, 1938 928,398 Patterson July 20, 1909 1,735,000 Dely Nov. 12, 1929 1,784,570 Becket Dec. 9, 1930 1,853,370 Marshall Apr. 12, 1932 2,109,485 Ihrig Mar. 1, 1938

Claims (1)

1. THE PROCESS OF SURFACE-MODIFYING A METAL ARTICLE BY THE ACTION OF AN IMPREGNATING OR ALLOYING METAL, WHICH COMPRISES PROVIDING THE ARTICLE INITIALLY WITH A SUBSTANTIALLY DRY, ADHERENT COATING COMPRISING AN UNSINTERED MIXTURE OF FINELY DIVIDED SLAG-FORMING SOLID MINERAL MATERIALS ASSOCIATED WITH AN AGGLUTINANT BINDER THAT RENDERS THE MIXTURE COHERENT, SAID MIXTURE INCLUDING A SOURCE OF THE IMPREGNATING METAL, SILICA AND A METAL HALIDE, SO PROPORTIONED AS TO FACILITATE SINTERING OF THE MIXTURE AT SUITABLY ELEVATED TEMPERATURE NONDESTRUCTIVE TO SAID ARTICLE AND PROMOTE ACTION OF THE IMPREGNATING METAL UPON THE UNDERLYING OR BASIS METAL SURFACE; THEN HEATING THE COATED ARTICLE TO A TEMPERATURE NOT LOWER THAN ABOUT 1550* F. THAT IS HIGH ENOUGH TO SINTER OR PARTIALLY FUSE THE COATING MIXTURE TO VISCOUS CONSISTENCY AND THEREBY SUBSTANTIALLY SEAL THE UNDERLYING SURFACE OF SAID ARTICLE, BUT IS INSUFFICIENTLY HIGH TO RENDER THE COATING MIXTURE THINLY FLUID OR TO ENDANGER THE INTEGRITY OF SAID ARTICLE; CONTINUING THE HEATING WITHIN SUCH TEMPERATURE RANGE UNTIL THE DESIRED IMPREGNATION HAS BEEN EFFECTED; THEN COOLING THE ARTICLE AND REMOVING THEREFROM ANY SINTERED COATING MIXTURE ADHERING THERETO.