US3190749A - Alloy article having a porous outer surface and process of making same - Google Patents

Description

United States Patent 3,190,749 ALLDY ARTICLE HAVING A PORGUS QUEER SURFACE AND PROCESS Gi MAKING SAME Richard A. Fleming, North Tonawanda, N31, assignor to E. I. du Pont de Nemours and Company, Wilmington, Dei., a corporation of Delaware No Drawing. Filed July 23, 1963, Ser. No. 296,900 Claims. (Cl. 75-122) This application is a continuation-in-part of my application Serial No. 58,918, filed September 28, 1960, and now abandoned.

This invention relates to nickel, platinum, .and palladium alloy articles having porous surface structures and to methods for preparing said article's.

In accordance with the present invention, articles of nickel, platinum, or palladium alloys, especially ferrousnickel alloys and ferrous-nickel-chromium stainless steel alloys, can be prepared that have porous and sponge-like structures useful as catalysts or as lubricant accommodat-ors. Surface porosity is best accomplished by selectively removing or leaching out with a liquid metal or salt melt one component from a multiple component solid alloy. By way of example, in the formation of porous surfaces the preferential dissolution of copper in Cu-Ni alloys can be accomplished with liquid Ag at elevated temperatures as reported by Harrison and Wagner (Acta. Met. 7, 722 [1959]). A related method can be cited in the formation of Raney nickel from Ni-Al alloys in which aluminum is dissolved with the aid of concentrated NaOH.

The present invention relates to the formation of porous surfaces on alloy articles containing at least one metal selected from the subclass of Group VIII of the Periodic Table consisting of nickel, platinum, and palladium by the selective removal of a portion of said metal using a melt containing a IIA metal such as calcium, strontium, barium,

and magnesium. It has been found in accordance with the invention that the degree of porosity, including the number of interstices per unit area, depth of interstice penetration and the final composition of the alloy treated can be controlled so that very unique articles may be formed.

The mechanism of thepresent invention is related to liquid-t-o-solid diffusion in which the relative solubilities of Cr, Ni, Co, Al,-Si, and Mn in liquid HA metals and solid ferrous articles are taken advantage of in the preparation of diffusion alloy coatings. Detailsof this concept are disclosed in copending application Serial No. 139,369, filed September 20,1961, now abandoned, which is a continuation-in-part of applications Serial No. 44,015, filed July 20, 1960, abandoned, and Serial No. 835,171, filed Augustll, 1959, also abandoned. Alloy coatings form because of the tendency'of the alloying metal to move from the liquid phase to the solid phase. Control is primarily achieved by varying the concentration of a particular diffusing element in the melt.

It has been found in accordance with the present invention that if a metal article is contacted in a certain temperature range with .a melt of a IIA metal, preferably calcium, wherein the metal is an alloy containing at least 8% by weight of a metal component which is soluble in the melt and a second metal component which is relatively insoluble in the melt, unique and -useful articles having controlled surface porosity are formed. Metals which may form the soluble metal component of the alloy article treated are the art recognized subclass of Group VIII of the Periodic Table consisting of nickel, platinum, and palladium. Metals which may form the second or insoluble component of the alloy article treated are titanium,

zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, or cobalt. Nickel is preferred as the metal constitutin the soluble component and chromium and/ or iron are preferred as the metals constituting the insoluble component of the alloy article treated. I

A pure HA metal melt promotes the most porosity; increasing the content of the soluble metal in the melt or decreasing the amount of HA melt by incorporating diluent materialsin the melt reduces the dissolving tendency of the melt and the depth of porosity as well. As expected, when treated under the same conditions, those articles having the highest original content of soluble metal component develop the most surface porosity. Alloy articles containing from about 880% by weight of the soluble metal component and from 92-20% by weight of the insoluble metal component are found to be suitable for treatment in accordance with the invention to prepare articles having a porous surface. Alloys having less than 8% of the soluble metal do not seem to develop the porosity characteristic of articles of the invention and alloys having more than 80% of the soluble metal generally develop too extensive a degree of porosity to retain suitable strength for most end uses of the resulting article. More preferably, the proportion of soluble metal in the starting alloy article is in a range of from about 18-40% by weight with the insoluble metal component being in the range of from about 82-60% by weight.

It is found that to form the articles of the invention,

the contacting method described should be carried out in a temperature range of from 1050 C. to 1300 C. At temperatures below 1050 C. little porosity develops and the time for any porosity to be obtained becomes excessive. The upper temperature selected for the contacting method of the invention will be dependent in large part on the composition of the alloy article treated and the boiling point of the HA metal containing melt.

Articles prepared in accordance with the present invention are characterized by having a porous outer surface containing interstices of comparatively uniform dimensions along the grain boundaries extending inwardly in .a direction normal to the surface. The interstices are substan tially circular in cross section with the total cross sectionalarea of the individual interstices being essentially cons-taut along their inward length so that in their longer dimen sion they are U-shaped rather than V-shaped in configura tion. While the degree of porosity #(the number of interstices per unitarea) and the depth of penetration of the interstices are controlled by the relative concentrations of soluble metal in the solid article being treated and the melt, the individual hole diameters appear to be scarcely affected by these factors. The interstices have diameters of approximately /6 mil plus or minus a factor of 2 3. It is found that the soluble metal is preferentially depleted in the porous outer surface of the article. Obviously, where the treatment of the invention is practiced on bulky articles, the difference in composition of the porous surface article of the invention is only slightly different from that of the original article. However, in cases where the treatment of the invention is practiced on very thin articles, such as alloy foils, the composition of the porous surface article of the invention may be appreciably different'frorn that of the original article, although it has been found that invariably an appreciable content, usuallynot lower than 1% by weight of the soluble metal, remains present in the treated article.

If desired, chromium and other diffusing elements such as disclosed in copending application Serial No. 139,369 can be simultaneously diffused into the article being treated from which the soluble component is being removed. This provides a convenient means to maintain the content of the insoluble component of the article, such as chromium, constant at the surface or to significantly enrich it in content at the surface or introduce addi tional alloying elements at the surface of the article.

A better understanding of the invention will be gained from the following description which more clearly illustrates the articles of the invention and the preferred modes of preparing the same. In this description, all percentages are -by weight unless otherwise noted.

A series of identical panels of Incoloy 2" x /2" x A having alloy compositions of 21.6% chromium, 32.3% nickel, 43.0% iron, and 3.1% trace metals, were each treated for about an hour in a bath of molten calcium (550 grams) at about 1100 C. The first panel was treated for an hour after which 20 grams of nickel were added to the bath before the second panel was treated. Between each treatment time 20 grams of nickel were added to the bath. The bath was maintained at about 1100 C. and the panels were fastened to a rotating stirrer (about 100 rpm.) which was immersed in the bath. In the above manner 6 consecutive panels were each treated for one hour and thereafter their surface compositions were analyzed by X-ray fluorescence to determine the average composition of the top 0.10.2 mil of the panel.

It should be noted that about the same small amount of chromium was removed from each sample during treatment and the amount of nickel removed is greatly dependent on the nickel-calcium ratio of the bath.

The surfaces were examined visually and panels 1 through 5 all had interstices at their surfaces; panel No. 6 was virtually unchanged. The majority of these interstices were associated with grain boundaries and grain boundary intersections. The number of interstices per unit area decreased in the order l 2 3 4 5 6.

The panels were then cross sectioned and examined to determine the depth of the affected surface layer.

Depth (mils) of the From the above, it can be seen that the depth of the affected zone is critically dependent on the bath composition.

The experiments described above show that the amount of porosity produced by treatment of a given Fe-Cr-Ni solid alloy in a Ca-Ni liquid bath is dependent on the nickel content of the bath. A series of alloys of differing nickel content treated together in the same bath of calcium (or alternatively in Ca-Ni) yielded different degrees and depths of surface porosity much as in the aforementioned experiment. This time the bath composition Was fixed and the composition of the solid was varied. Those solid alloys with highest nickel contents developed porosity to the greatest depth and degree. Nevertheless, it was found in this testing that porous articles of the invention could be readily formed by treating type 304 stainless (18-20% Cr, 8l1% Ni, balance essentially Fe) and type 316 stainless (16-18% Cr, -14% Ni, balance essentially Fe) in a melt of calcium.

A reduced chromium content on the sample surface tends to reduce corrosion resistance. Consequently, an experiment was carried out to show that the chromium 4 content can be increased (or alternatively maintained constant) while the nickel content is reduced with attendant development of porosity. The first-mentioned experiment was repeated with but one modification. The initial bath contained powdered chromium (40 gm.) in addition to calcium (500 gm.). The sequence of treatments of stirred Incoloy samples with intervening nickel additions (but no further addition of chromium) was carried out as before. Surface analyses are reported below:

Bath Composition Average Surface Comp0sition of Solid Panel Gms. Ca Grns. Cr Gms. Ni Percent Percent Percent Cr Ni Fe 500 40 0 71 0. 4 Balance 500 40 20 56 6 D0. 500 40 40 48 10 D0. 500 10 00 40 17 Do. 500 40 8O 25 Do. 500 100 24 39 D0.

Surface examination again indicated that holes of about /6 mil diameter were present on all samples except No. 12; the holes were primarily associated with grain boundaries. The number of holes per unit area decreased in the order: 7 8 9 10 11 12. Since chromium is sparingly soluble in melts of IIA metals, only a small amount of chromium need be incorporated in the melt to favor liquid-to-solid diffusion.

Cross section examinations established the following depths of the affected areas:

Depth (mils) of the porous Panel: surface layer These'porous-surfaced articles are extremely resistant to corrosion.

The above examples are illustrative of treating nickel 7 alloys in which iron is the insoluble metal component.

Nickelalloys composed of nickel and any one or combination of the other metals mentioned hereinbefore as suitable as an insoluble component can also be treated to yield articles of the invention having the desired porous surface. For example, nichrome alloys containing approximately -80% by weight nickel, balance chromium, can be treated to yield highly porous structures through the selective removal of nickel. The amount of nickel selectivelyremoved can be controlled by varying the amount of nickel in the melt. Thus, articles having a porous surface predominantly comprised of chromium with a small residual amount of nickel can be formed by treating chromium-rich nichrome articles with a melt having a controlled, low nickel concentration.

Since the stainless steel articles are believed to have particular practical importance it is contemplated that the insoluble metals titanium, zirconium, vanadium, tantalum, niobium, molybdenum, tungsten, manganese, and cobalt will usually be included as additional alloying elements with iron and chromium to make up the second or insoluble metal component of the alloy article to be treated. For instance, an alloy containing about 20% nickel, 20% cobalt, 21% chromium, 29% iron, 3% molybdenum, 2.5% tungsten, and 4.6% other metals and nonmetals can be treated in a melt of calcium to yield an article of the invention. Obviously, moving machine parts having corrosion-resistant stainless steel porous sur faces have obvious utility, particularly in the preparation of piston rings, heads, bearings, and the like.

Although the above examples relate to nickel alloys,

it is to be appreciated that platinum or palladium alloys wherein these metals are alloyed in like manner With an insoluble metal component may be acted on in the same manner to produce similar articles. Moreover, the above examples are illustrative only of the treatment of bulk substrates. However, the principles of the present invention can be utilized to treat thin substrates. For instance, alloy foils when treated in the same manner contain perforations which in some cases can be made in such number for the treated article to resemble a sieve.

While other modifications of this invention'which may be employed within the scope of the invention have not been described, the invention is intended to include all such as may be comprised within the following claims.

I claim:

1. An article comprising an alloy consisting essentially .of from up to 80% by weight of at least one metal selected from the subclass of Group VIII of the Periodic Table consisting of nickel, platinum, and palladium with the remainder being essentially at least one additional alloying metal selected from the group consisting of titanium, zirconcium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, and cobalt; said article being characterized in having a porous outer surface containing interstices along the grain boundaries extending inwardly in a direction normal to the surface, said interstices being substantially circular in cross section with the total cross sectional area of the individual interstices being essentially constant along their inward length.

2. An article comprising an alloy consisting essentially of from up to 80% by Weight nickel and the remainder being essentially at least one additional alloying metal selected from the group consisting of titanium, zirconium, vanadium, niobium, tantalum, chromium, molybenum, tungsten, manganese, iron, and cobalt; said article being characterized in having a porous outer surface containing interstices along the grain boundaries extending inwardly in a direction normal to the surface, said interstices being substantially circular in cross section with the total cross sectional area of the individual interstices being essentially constant along their inward length.

3. An article comprising an alloy consisting essentially of from up to by weight nickel and the remainder being essentially iron and chromium; said article being characterized in having a porous outer surface containing interstices along the grain boundaries extending inwardly in a direction normal to the surface, said interstices being substantially circular in cross section with the total cross sectional area of the individual interstices being essentially constant along their inward length.

4. The article of claim 2 in which iron is the alloying metal with nickel.

5. The article of claim 2 in which chromium is the alloying metal with nickel.

6. The method of forming a porous outer surface on an alloy consisting essentially of from 8 to 80% by weight of at least one metal selected from the subclass of Group VIII of the Periodic Table consisting of nickel, platinum, and palladium and from 92 to 20% of at least one alloying metal selected from the group consisting of titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, and cobalt, comprising contacting said alloy with a melt of 21 HA metal at a temperature of from 1050-1300 C.

7. The method of forming a porous outer surface on an alloy article consisting essentially of from 8 to 80% by weight nickel and from 92 to 20% by weight of at least one alloying metal selected from the group consisting of titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungstein, manganese, iron, and cobalt, comprising contacting said alloy with a melt of a IIA metal at a temperature of from 1050-1300 C.

8. The method of claim 7 in which iron and chromium are the alloying metals with nickel.

9. The method of claim 7 in which iron is the alloying metal with nickel.

10. The method of claim 7 in which chromium is the alloying metal with nickel.

References Cited by the Examiner UNITED STATES PATENTS 2,740,730 4/56 Banus 117l14 DAVID L RECK, Primary Examiner.

Claims (1)

1. 2. AN ARTICLE COMPRISING AN ALLOY CONSISTING ESSENTIALLY OF FROM UP TO 80% BY WEIGHT NICKEL AND THE REMAINDER BEING ESSENTIALLY AT LEAST ONE ADDITIONAL ALLOYING METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, VANADIUM, NIOBIUM, TANTALUM, CHROMIUM, MOLYBENUM, TUNGSTEN, MANGANESE, IRON, AND COBALT; SAID ARTICLE BEING CHARACTERIZED IN HAVING A POROUS OUTER SURFACE CONTAINING INTERSTICES ALONG THE GRAIN BOUNDARIES EXTENDING INWARDLY IN A DIRECTION NORMAL TO THE SURFACE, SAID INTERSTICES BEING SUBSTANTIALLY CIRCULAR IN CROSS SECTION WITH THE TOTAL CROSS SECTIONAL AREA OF THE INDIVIDUAL INTERSTICES BEING ESSENTIALLY CONSTANT ALONG THEIR INWARD LENGTH.