US3129095A - High silicon cast iron - Google Patents

Description

April 14, 1964 Filed May 9, 1965 Area of Excess Brihleness W. A. LUCE ETAL HIGH SILICON CAST IRON Area of Poor Corrosion Resistance Poor Corrosion M Resistance Walter A. Luce Glenn W. Jackson I Earl Ryder Mars 6. Fonlana INVENTORS United States Patent Ofiice 3,129,095 Patented Apr. 14, 1964 3,12%,095 EHGH SELECQN CAST IRON Walter A. Luce, Glenn W. Jackson, and Earl Ryder, Dayton, and Mars G. Fontana, Columbus, Ohio, assignors to The Duriron Company, lino, Dayton, {)hio, a corporation of New Yorl:

Filed May 9, 1963, Ser. No. 279,140 11 Claims. (Ql. 7-l26) This invention pertains to high silicon cast iron alloys possessing extraordinary resistance to chemical attack throughout a wide spectrum of corrosive environments; and it pertains especially to alloys which exhibit substantially improved chloride resistance while maintaining good resistance in other media. The novel alloys are particularly useful in areas where relatively high operating temperatures and critical concentrations of the corrosive media are encountered. concomitantly with such corrosion resistance, the novel alloys possess adequate thermal shock resistance, mechanical strength and hardness such that they oifer important practical advantages over previously available alloys in the fabrication of chemical processing equipment and in other fields where corrosion problems are encountered, especially where combined with erosion problems and variable temperature conditions.

It is well recognized that hydrochloric acid and particularly the chloride ion provide some of the most highly corrosive service conditions that a metal or alloy may be called upon to withstand. While chloride ion resistance is not the only consideration, such resistance in alloys of the type under discussion here is a basic factor, necessarily to be considered. The generally good chloride corrosion resistance of prior high silicon iron alloys under moderately severe conditions of concentration and temperature is well-known. Typical alloys of this type which have been in use for some time are those sold under the trade names Duriron and Durichlor by the Duriron Company. Duriron has a nominal composition of about 14.5% silicon, 0.65% manganese, 0.85% carbon, the balance being iron. Durichlor has about the same composition except for the substitution of about 3% molybdenum for a corresponding amount of iron. These alloys are economical to use and provide unexcelled resistance under many conditions to most organic and mineral acids and other corrosives. While the effectiveness of 'Duriron above normal temperatures drops ofi appreciably in hydrochloric acid service, Durichlor remains elfective in practice for most concentrations of this acid up to temperatures of 50 to 60 C. and up to 80% C. where impurities, such as ferric or other oxidizing ions, are absent. However, where highly oxidizing metal ions are present, such as ferric, cupric, or mercuric, even Durichlor corrodes at a faster rate than is generally considered commercially acceptable. At temperatures above 60 C., the choice of practical materials is largely restricted to super-alloys, the latter containing relatively small amounts of iron and being predominant in nickel, chromium and molybdenum. These are expensive and usually highly strategic materials. Chlorimet-Z, for example, which is the trade name for a typical commercially available alloy of this type, contains about 62% nickel, 32% molybdenum, 3% iron and a maximum of about 1% silicon. Such an alloy has excellent resistance to severe corrosives of a reducing nature, e.g. boiling hydrochloric acid in all concentrations. But even this superalloy is attacked quite readily under aerated conditions or where any substantial amount of ferric chloride or other oxidizing agent is present. This specific difiiculty can be corrected by using a different type of alloy, for example Chlorimet-3 which dilfers from Chlorimet-2 principally in that chromium is substituted for about one half of the molybdenum content of the latter alloy. This change, however, substantially reduces the resistance of the alloy to hydrochloric acid as compared with Chlorimet-Z under ordinary, non-oxidizing conditions. Thus although better than Chlorimet-Z in the presence of oxidizing contaminants, the loss in corrosion resistance is only partially compensated by Chlorimet-3.

There are a number of other metals and alloys, including such pure metals as titanium and zirconium, which have good corrosion resistance under severe service, but these are generally subject to disadvantages of excessively high cost, problems of availability, or the further fact that while they are useful in certain limited or specific service, they do not provide over-all effective resistance in corrosive media of both highly oxidizing and highly reducing types. Furthermore resistance to abrasion and erosion of these is limited.

Thus there has long been a need in industry for less expensive alloys which will stand up under a wide variety of the more severely corrosive as well as erosive environments to which industrial equipment is necessarily exposed, or tor alloys which will permit operation under conditions not practical heretofore but nevertheless desirable to employ. One of the objects of the present invention accordingly is to provide an economical alloy having improved corrosion resistance under severe service conditions. While effective resistance to severe chloride conditions, such as prolonged exposure to strong hydrochloric acid, is particularly important, it is a further object of the invention to provide alloys which at the same time have superior corrosion resistance in many other corrosive media frequently encountered in industry. Along with these objectives, the invention further provides alloys which are easier to found and from which sound castings of good strength can be obtained. A further object is to provide alloys having excellent surface characteristics for highly erosive and abrasive application.

The new alloys are high silicon cast irons modified by the inclusion of certain elements, more particularly chromium, most of which are held in solid solution at least in the preferred embodiments. As used herein, the term high silicon cast iron designates iron-silicon alloys in which the silicon content is above the level, generally about 6% to 8% silicon, where a definite break or transition occurs in the matrix as a result of ordering of the silicon and iron atoms, tending to produce brittleness in the alloy, and carbon for the most part is present in the form of free graphite randomly dispersed in the essentially continuous silico-ferrite matrix. In all cases, chromium in prescribed amount is included in the alloy, as this provides the key to the startling improvement aiforded by the invention. Other additions optionally included for specific applications are molybdenum, and sometimes boron and columbium. The alloys may contain without appreciable adverse effect the usual manganese and trace impurities of sulfur, phosphorus and other elements generally occurring in conventional high silicon irons. Elements such as aluminum, cobalt, antimony, tin, manganese, silver, lead, titanium totalling in some cases as much as about in the alloys, have little or no adverse effect upon the corrosion resistance in a majority of corrosive environments. The same is true, but to a markedly lesser extent, of additions of nickel or copper. However, in all cases the corrosion rates in the more severe corrosive environments are increased appreciably by the inclusion of more than small or residual amounts of nickel or copper.

The unique combination of remarkably improved corrosion resistance and adequate mechanical properties which characterize the alloys of this invention is achieved by the discovery that chromium, which ordinarily decreases corrosion resistance in reducing type media, has just the opposite effect in high silicon cast iron alloys of the type discussed above, provided the chromium is present within critical limits. Prior high silicon cast iron founders have deliberately avoided the inclusion of chromium in significant amount, with the result that if it was present in prior alloys, residual amounts only, i.e. less than 1% at most, were involved. There is good reason for this, since, as mentioned above, previous work in this field teaches that corrodibility, particularly in hydrochloric acid and similar reducing media, increases with chromium content, and the exclusion of this element has accordingly been the rule for corrosion service applications.

The addition of chromium alone to high silicon cast iron, in the amounts hereinafter specified, shows superior results for example as a material for impressed current anodes in the cathodic protection of steel structures or equipment. This is true in salt water as well as high temperature fresh water service, both of which have presented extremely troublesome metal corrosion problems in the past. Heretofore, standard high silicon cast irons such as Duriron and Durichlor mentioned above have proved to be among the most practical types of alloys available for this protective anode service, but these alloys are limited to conditions where the temperature does not exceed around 60 C. in the case of Durichlor or 50 C. in the case of Duriron. The use of the new chromiumcontaining high silicon cast iron alloys is not so limited, as will appear more fully hereinafter.

Those alloys within the scope of the invention which exhibit still greater resistance to chemical corrosion under more severe conditions include, in addition to chromium, supplemental amounts of molybdenum. The addition of molybdenum produces a very substantial increase in resistance, particularly in hydrochloric acid, but must be carefully controlled to avoid adverse effect on the mechanical properties of the resulting alloys. The further addition of boron when molybdenum is present is also sometimes desirable to facilitate casting of the metal in the foundry and to reduce any tendency of the alloy to spall.

The novel alloys, by virtue of their high proportion of iron and silicon are notably economical to use in comparison with the super-alloys or pure metals heretofore required for comparably severe service conditions. Moreover the new alloys are readily handled by those familiar with high silicon cast iron founding and present no new problems in casting practices.

While corrosion resistance is a major consideration, it is obvious that the alloys must also possess adequate mechanical properties for practical utility, and frequently the dictates of maximum corrosion resistance are in direct conflict with those of useful mechanical properties. The alloys of this invention are unique in respect to com bining excellent chemical resistance over a broad spectrum of corrosive conditions with practical mechanical strength and thermal shock resistance. These mechanical properties are dependent in large measure on the presence of free graphite in the alloy matrix, and it is an essential '4 characteristic of the invention alloys that they are true cast irons, as contrasted to steels. The carbon level and the nature of the carbon content in the invention alloys is accordingly important.

Like other high silicon ferrous alloys, the cast irons of the invention are characterized by rather extreme hardness, to the extent that they are not readily machinable in most instances. The novel alloys, however, are quite fluid in their molten state and thus readily permit casting of intricate parts with very small cross sections, as well as the more uniform, heavier shapes. The castings may be ground to provide a finished surface and any tendency to spall is largely eliminated, where this is a problem, by the inclusion of boron. Also the alloys are easily pulverized and classified according to mesh size for application to a base metal, using powdered metal spray techniques, to form an adherent, hard, dense, corrosion resistant cladding or facing material. Those alloys within the invention having maximum corrosion resistance and including molybdenum have a Rockwell C. hardness rating of up to around 58 and provide excellent bearing surfaces. The straight iron-silicon-chromium alloys, without molybdenum, show hardness readings of around 45 Rockwell C. Minimum tensile strength of at least 12,000 p.s.i. is obtained in all of these compositions, which value is considered to be a permissible low limit, and generally higher tensile strengths obtain in the new alloys, particularly in the preferred compositions. It is however very diflicult to make suitable tensile tests without premature failure in a brittle alloy system like the high silicon cast irons because of misalignment in tensile test apparatus or the influence of slight imperfections in the test specimen. A transverse load value for a given test specimen is thus considered to be a more suitable method in evaluating these relatively brittle alloys. As there is no ASTM or other standard governing the procedure for a suitable transverse load test, the following has been established by the inventors to provide a suitable basis for comparison. A test specimen is cast in the form of a one inch square bar, thirteen inches long. This is placed on twelve inch centers in a testing machine, loaded at the center until breakage occurs and the load recorded in pounds. A rough relationship exists here between transverse load and tensile strength, such that the tensile strength of a specimen can be approximated by multiplying the transverse load value by fifteen.

The corrosive media in which the new alloys are useful include not only the reducing type corrosives, such as hydrochloric acid and chloride ion environments, but oxidizing type corrosives as well. This includes more particularly nitric and sulfuric acids, as well as phosphoric, acetic and chromic acids, and the respective anions of these agents. The alloys are highly resistant to the common caustic reagents also, and show definite improvement in this respect over previous high silicon iron alloys. The alloys are useful not only under moderately corrosive conditions of temperature and concentration in these various agents, but under conditions which have previously made the use of known iron-silicon alloys uneconomical.

In general, the compositional analyses of the preferred alloys of the invention fall within the following component ranges:

Many of the advantages of the preferred alloys can be obtained in useful degree within the following broader ranges of the respective components which, however, are found to be critical to the practical attainment of the unique combination of chemical and mechanical properties of the invention alloys:

It will be understood from What has already been stated and from the discussion to follow that adjustment of individual component percentages within the indicated limits in both the over-all and preferred ranges just mentioned will be necessary to obtain maximum benefits for specific applications or uses of the alloys. However, it should be borne in mind that in all cases it is important to provide a cast iron, i.e. one in which free graphite is dispersed throughout the matrix, since this structure is found essential in the alloys here disclosed to obtaining the desired mechanical properties.

In order to provide a basis for the better understanding of this correlation or adjustment of component percentages, reference will be made to the accompanying drawing illustrating graphically, in a plot of silicon percent vs. chromium percent, the limits to be observed in accordance with the teaching of the invention.

The area represented by the quadrilateral abc-d in the drawing delineates the limits of chromium and silicon in the compositions of the novel :alloys. The area thus bounded constitutes the boundary of a phase diagram which is divided by a line b'c' into two regions. This line is defined by the formula:

Percent Si0.7(percent Cr) 7.4=0

and represents the compositional level of chromium and silicon at which the graphitizing tendency of silicon abruptly stops and carbides can readily form. Carbon in the alloys above the solid solution limit of about 0.1 to 0.2% (depending on the alloying elements present) will exist as free graphite in the region to the left of line b'c' and thus of cource produce a cast iron. Substitution of the chromium and silicon values in the above expression will thus give a positive value of X under this condition. This line remains constant irrespective of the total carbon present above the solid solution limit. In the preferred alloys a total combined carbon in solid solution and as free graphite of about 1.0% is normal, however, since the upper carbon limit is influenced by the actual silicon content as well as the presence of certain other elements, the upper limit may reach about 1.5% before rejection of the carbon begins to occur during solidification. In the region to the right of line b'-c' carbides are readily formed, as the carbide forming tendency of chromium overcomes the graphitizing tendency of silicon. In order to produce a cast iron in this particular region, sufficient carbon must first be added to satisfy the carbide-forming tendency and then additional carbon is necessary to provide the free graphite.

Thus line b-c' delineates two regions on a phase diagram where to the left of the line only a homogeneous silico-chromium-ferrite matrix and graphite are in equilibrium while to the right, the matrix material, chromium carbides and graphite (provided the carbon level is adequate) are in equilibrium. When considering the region .to the right of line bc', therefore, it is apparent that the ability for graphite to be formed depends on the amount of silicon, chromium and carbon present. At carbon contents below about 1% it is impossible for graphite to form under any condition in this region, but it can be made to occur by raising the carbon level to some predetermined point depending on the silicon and chromium levels. It has been established by the inventors that where the total amount of carbon exceeds the value determined by the formula:

C=1.660.063 (percent Cr) 0.089(percent Si) 6. then free graphite will exist and a cast iron structure obtained.

The foregoing considerations also apply generally where molybdenum is present up to about 1%, but above that level molybdenum becomes additive with the chromium in respect to the amount of carbon which is tied up, and for such alloys the sum of chromium and molybdenum contents must be taken into consideration.

For purposes of reference, all of the alloy compositions referred to herein are tabulated in Table I and the alloys themselves are accordingly referred to only by their letter or number designation thereafter in subsequent tables.

TABLE I Per- Per- Per- Per- Per- Per- Per- Per- Alloy cent cent cent cent Alloy cent cent cent cent Si Cr G M0 Si Cr 0 M0 The presence or absence of free graphite in various al loys at different silicon and chromium levels is illustrated in Table 11.

TABLE II Graphite Determination at Various Levels of Silicon, Chromium and Carbon Alloy: Remarks F Free graphite. T Do.

Y Do. S No graphite. BB Free graphite. GG Do.

Z Do. AA Some free graphite. DD Do.

EE D0.

FF Do. V Do. CC No visible free graphite. W No graphite. X Do.

G Do. I Do.

M n Do. L Do. HH Some graphite. NN Free graphite.

Free graphite exists in alloysat the high silicon 17%) level, nominal'1% carbon, for chromium levels up to about 12% to 13%, due to the high graphitizing effect of the silicon. Compare alloys F, Z, EE and X. As the silicon level is decreased, so must the chromium content also be decreased, and at minimum silicon (12%), with more than 6% to 7% chromium free graphite disappears. Compare alloys Y, V and W. At the preferred level of silicon (14% to 15%), nominal carbon of 1%, graphite is prevent up to about 10% to 11% chromium (see alloy DD), but virtually no free graphite is found over 11% chromium (see alloy CC). However, as total carbon is increased, graphite again appears at higher chromium levels. Thus at 13.25% silicon, 11.35% chromium, nominal 2% carbon, the graphite structure is obtained, as indicated for alloy HH. Even at low silicon (11.83%), high chromium (15.11%), free graphite is found in the matrix where the total carbon is nominally 3% (see alloy NN).

Other factors also enter into consideration in determining the scope of the alloy compositions to obtain the unique properties of the invention alloys, and further illustrate the criticality of the compositional limits. As shown in Table III, low transverse load values become a determining factor at the higher silicon and chromium levels, notwithstanding additional carbon. This brittleness begins to be significant at about 15% silicon, and above 17% there is insufficient mechanical strength and thermal shock resistance in the alloys to make them practical. In the case of chromium, increased brittleness appears at around and becomes determinative at chromium levels. (See Table III.)

TABLE III Mechanical Properties Practical considerations in industry dictate that even for anode service, which is one of the least demanding from the standpoint of mechanical properties, the alloy material from which the anodes are cast should provide a minimum transverse load value of around 800 pounds and deflection of 0.025". Short of this, excessive breakage of the anodes occurs simply from handling during storage and shipment. Thus, whereas a preferred composition within the scope of the invention, such as alloy T in Table III, shows a transverse strength of 1291 p.s.i. and deflection of 0.036", an alloy differing from this only in that it contains about 3% more silicon and thus falls just outside the indicated limits of the invention, has less than half the strength and deflection of the preferred alloy. (See alloy F.) Quite apart from corrosion resistance, therefore, alloys such as alloy F are impractical.

At the lower limits of the given composition ranges, poor corrosion resistance in the alloys becomes chiefly determining. For silicon, the minimum practical level is around 12% and for chromium it is about 2%. Preferably silicon is at least 14% and chromium at least 3%. The results of corrosion tests on various alloys, both within and outside of the scope of the invention for comparison purposes are given in Table IV. The abbreviations used in this and other tables hereinafter relating to corrosion rates have the following meanings:

40% BN--40% boiling nitric acid 10% BN-l0% boiling nitric acid 30% BS30% boiling sulfuric acid 30% S 80-30% sulfuric acid at 80 C.

% BH-20% boiling hydrochloric acid 20% H 3820% hydrochloric acid at 38 C.

20% H 38*-20% hydrochloric acid at 38 C. also containing from 0.5 to 1.0% FeCl 20% H 80-20% hydrochloric acid at 80 C.

The corrosion rates given in each instance, unless otherwise indicated, are the mean rates for five 48-hour test periods determined in accordance with A.S.T.M. procedures set out in Specification No. A279-44T of the Society.

than the above conditions.

8 TABLE 1v Corrosion Rates, Mils Penetration per Year Alloy 40% 10% 30% 30% 20% 20% 20% EN EN BS S H 38 H 38 H 80 180 14 2, 470 2,129 1, 520 60 9 2, 900 207 1 203 15 8 60 25 20 1 430 13 4 l 26 1 240 l, 603 l, 60L 547 441 488 12 18 1o 4 43 Nil 414 Nil 35 15 4 20 N11 237 10 34 75 117 n D urichlor ll 23 27 125 1 Dissolved.

Increasing the chromium content at the low silicon level does not correct poor corrosion. See corrosion rates for alloy L, Table IV. At the preferred silicon level (1415%) but high chromium level, both the mechanical properties as well as corrosion resistance are adversely affected. (See alloy M, Tables III and IV.) And at higher silicon levels (17%-19% high chromium content (20%), the mechanical properties are completely deteriorated. (See alloy N, Table III.)

The foregoing limits of silicon and chromium also apply where molybdenum is present in the alloys. Molybdenum is known to improve the resistance of high silicon iron alloys in chloride ion environments, and this is true of the chromium containing high silicon iron alloys of the invention. Highest corrosion resistance commensurate with adequate mechanical strength is obtained at from 2.5% to 3.5% molybdenum, in an alloy having from 14% to 15% silicon, 3% to 10% chromium. Although resistance to nitric acid decreases with more than 3.5% molybdenum, alloys containing as much as 5% molybdenum are still practical for specific uses. But where bet ter mechanical properties are desirable or essential, a molybdenum content of from about 0.5% to 1.0% gives best over-all results.

In alloys containing molybdenum it is sometimes desirable to add a small amount of boron as a grain refiner to alleviate a tendency of the metal to spell when ground to provide a finished surface. Boron tends to reduce the eifectiveness of chromium and, when present, the chromium level should be maintained near the high limit. The upper limit of boron should not exceed about 1%, as corrosion resistance is depreciated to an uneconomic level.

Remarkable improvement in corrosion resistance under impressed current conditions, i.e. in anode service, is obtained by the addition of chromium to high silicon iron alloys which are frequently used in this type of service. As shown by Table V, the improvement in consumption rates and freedom from selective attack occurs only at chromium levels in excess of 2%. The data below represents results obtained in synthetic sea water at 93 C. which is extremely severe in its eifect on all impressed current anodes of previously known alloys.

TABLE V Anode Test Data-Synthetic Sea Water93 C.

at 2 Amps/Sq. Ft.

Analysis, Percent Consumption Alloy Rate, /lbs./ Type of Attack amp. yr. Cr M0 0 1.0 Nil 0. 95 14. 82 Badly pitted. 2. 0 Nil 0. 95 5. 75 Do.

3. 35 Nil 1.00 0. 94 Uniform.

6.10 Nil 0. 02 1. 00 Do.

1.0 3.0 0.95 12.90 Badly pitted. 2.0 3.0 0.95 0.95 Slight pitting. 3. 10 2. 72 0. 96 1. 34 Uniform.

The importance of chromium in these anode applications is further illustrated in Table VI by results obtained in synthetic sea water at 79 C. which is much less severe For purposes of comparison,

data for anodes of Duriron and Durichlor are also given.

TABLE VI Anode Test DataSynthetic Sea Water79 C. at 2 Amps/Sq. Ft.

Analysis, Percent Consump- Alloy 1gion Rate,

s.am r. Si Cr Mo I ply Duriron 14. 50 Nil Nil 0.90 33 1. 40 Nil 0.92 11.0 2.0 Nil 0.88 4.5 2. 30 Nil 0. 79 1.07

Nil 3.0 0. 90 1.0 2.88 0.98 26.4 1.38 2. 84 0.97 4.2 2.00 2.86 1.0 3.3

1 Tests of these alloys were discontinued before completion because of eitlcessrvle rates of selective attack which rendered the anodes substantia y use ess.

The criticality of the minimum 2% chromium level is well illustrated by the foregoing. Selective attack is so excessive as to cause substantially complete destruction of the anodes where no chromium is present, and that type of attack is largely eliminated only when the 2% level is reached. It will be appreciated that where selective attack occurs, this can render an anode useless, due to breaking off of lengths or large chunks, even though the normal consumption rate may not be excessive. A maximum consumption of around two pounds per ampereyear represents an economic limit, and generally a rate of less than one pound per ampere-year is desired.

For special applications, as where the alloy is exposed to some of the most destructive types of corrosive media, for example boiling hydrochloric acid, inclusion of columbium along with molybdenum is useful. For the most part, additions of elements other than chromium and molybdenum to high silicon cast iron, provided these are not over %,have little effect upon the corrosion resistance in the more usual corrosive media. But as corrosive conditions become more severe, chemical attack on the alloys increases with the addition of such elements as, and in the approximate amounts of 3% nickel, 5% cobalt, 5% copper, 3% antimony, 2% titanium, 8% manganese and 3% aluminum.

As is evident from the foregoing the invention alloys are sharply distinguished from the more conventional stainless or stain resistant alloys, as many are often referred to in the art. The character of corrosion resistance possessed by the new alloys here disclosed is of an entirely different order from that needed or contemplated for the so-called stain resistant low-silicon ferrous alloys. In the context of corrosion resistance here contemplated, a corrosion rate of 20 mils penetration per year is acceptable for use in most chemical equipment fabrication although in some instances involving very severe conditions a 50-60 mil per year rate will suffice. But in some exceptionally critical areas a 5 mil per year rate maximum may be necessary. As a group, the alloys of this invention are unique in meeting the requirements in respect to corrosion resistance and at the same time maintaining adequate strength, thermal shock resistance and other mechanical properties to make them practically useful.

The alloys of the invention may be heat treated in the usual manner for high silicon cast irons, and do not require any change in established foundry procedure or require any special care.

This application is a continuation-in-part of our prior copending application Serial No. 836,885, filed August 31, 1959, now abandoned.

What is claimed is:

1. A corrosion resistant high silicon cast iron containing from 12% to 17% silicon, 2% to 15% chromium and more than 0.1% to about 3.0% carbon, the balance being substantially all iron; said cast iron within the range of 10 alloy compositions represented by the area bbcc' in the accompanying drawing having a carbon content always in excess of the value defined by the expression:

C=1.660.063 (percent Cr) 0.089 (percent Si) to provide free graphite randomly dispersed in the ferrous matrix.

2. A corrosion resistant high silicon cast iron containing from 12% to 17% silicon, 2% to 15% chromium, more than 0.1% to about 3.0% carbon, up to 5% molybdenum, the balance being substantially all iron; said carbon being present in amount within said range suflicient to provide free graphite randomly dispersed in the solidified ferrous matrix.

3. A corrosion resistant high silicon cast iron containing from about 14% to 15% silicon, from about 3% to 10% chromium and carbon from about 0.2% to 1.5%, the balance being substantially all iron; said cast iron within the range of alloy compositions represented by the area b-bcc' in the accompanying drawing having a carbon content which is always greater than the value defined by the expression:

C: 1.660.063 (percent Cr) -0.089 (percent Si) to provide free graphite randomly dispersed in the ferrous matrix.

4. A corrosion resistant high silicon cast iron as defined in claim 3, which also includes up to 1% molybdenum.

5. A corrosion resistant high silicon cast iron containing from 14% to 15% silicon, from 3% to 10% chromium (and from 0.8% to 1.5% carbon, the balance being substantially all iron, said carbon being present in part in said alloy as free graphite randomly dispersed in the ferrous matrix.

6. A corrosion resistant high silicon cast iron containing from 14% to 15 silicon, from 3% to 10% chromium, up to 3.5% molybdenum and from 0.8% to 1.5% carbon, the balance being substantially all iron, said carbon being present in amount within said range sufiicient to provide free graphite randomly dispersed in the ferrous matrix.

7. A corrosion resistant high silicon cast iron containing about 14.5% silicon, 4.5% chromium, 0.5% molybdenum, 1% carbon, the balance being substantially all iron.

8. An impressed current anode formed of high silicon cast iron alloy having a compositional analysis of 12% to 17% silicon, 2% to 15% chromium and 0.1% to 3% carbon, the balance being substantially all iron; said alloy within the range of compositions represented by the area b'b-c--c' in the accompanying drawing having a carbon content which is always greater than the value defined by the expression:

C=1.66-0.063 (percent Cr) -0.089 (percent Si) to provide free graphite randomly dispersed in the ferrous matrix, said alloy having a transverse load capacity of at least 800 pounds.

9. An impressed current anode as defined in claim 8, wherein the composition of said alloy is from 14% to 15% silicon, 3% to 10% chromium, 0.2% to 1.5% carbon, the balance being substantially all iron.

10. An impressed current anode as defined in claim 8, wherein said alloy also includes up to 1% molybdenum.

11. An impressed current anode formed of high silicon cast iron alloy having a compositional analysis of about 14.5% silicon, 4.5% chromium, 0.5% molybdenum and 1.0% carbon, the balance being substantially all iron, said alloy having a transverse load capacity of at least 800 psi References Cited in the file of this patent Tetsu to Hogane (in Japanese), 1957, vol. 47, June, pages 652 to 657.

The Journal of the Iron and Steel Institute (British), No. 2, 1956, vol. 183, page 338.

Claims (1)

1. A CORROSION RESISTANT HIGH SILICON CAST IRON CONTAINING FROM 12% TO 17% SILICON, 2% TO 15% CHROMIUM AND MORE THAN 0.1% TO ABOUT 3.0% CARBON, THE BALANCE BEING SUBSTANTIALLY ALL IRON; SAID CAST IRON WITHIN THE RANGE OF ALLOY COMPOSITIONS REPRESENTED BY THE AREA B''-B-C-C'' IN THE ACCOMPANYING DRAWING HAVING A CARBON CONTENT ALWAYS IN EXCESS OF THE VALUE DEFINED BY THE EXPRESSION:

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