I/EC
Corrosion
Metals Resist Hot Sulfuric Acid by John L. Campbell, Stainless Foundry & Engineering, Inc.
Modifications
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• ROFESSOR S. W. PARR of the Uni versity of Illinois announced to the American Institute of Metals in 1915 the development of a platinum sub stitute for making scientific instru ments. He primarily wanted a calorimeter bomb which would resist corrosive attack of acids resulting from the combustion of coal. His personal interest in fuel technology resulted in the development of an alloy known as lllium G. As many as 50,000 coal determinations have been run in one bomb without ob jectionable attack of the surface by the hot sulfuric and nitric acids re sulting from the combustion. O n e of the outstanding applica tions of lllium G is in the manufac ture of cellulose products such as rayon and cellophane by the viscose process. Here the setting or spin ning bath contains 10 to 1 5 % sul furic acid, substantial amounts of zinc, sodium, and ammonium sul fates, and hydrogen sulfide. The unusual resistance of lllium G to this combination of chemicals definitely established its use. In the chamber process for manu facturing sulfuric acid, lllium G jour nal bearing assemblies in pumps are run with no other lubricant than the chamber acid. Temperature Requirements
Years ago, pumps, valves, acid regulators, and other process com ponents in contact with flowing acids
seldom had to withstand tempera tures above 185° F. Today, many contact acid plants operate at 230° and some at 300° F. A relatively minor increase in temperature greatly accelerates acid attack. Most standard metals and alloys will not stand up under these elevated tem peratures. Alloy Modifications
T o answer industry's demand for alloys capable of withstanding hot process acid, particularly sulfuric, Stainless Foundry & Engineering, Inc., Milwaukee, Wis., developed lllium 98. As a modification of lllium G, lllium 98 has found a wide range of applications in contact acid plants where high temperatures would hnve caused failure of other alloys. Several industrial p u m p manufacturers use lllium 98 impellers for acid pumps. Impeller life in these pumps has doubled when compared with similar impellers made of lllium G. lllium 98 is much superior to Stainless 20 alloy and lllium G in the difficult 60 to 9 8 % range of sulfuric to 80° C. However, the alloy is attacked by sulfuric above 80° C. in the 60 to 9 0 % range. We usually consider a metal questionable if it has a corrosion rate above 0.02 inch per year. lllium 98 also exhibits ex cellent resistance to nitric, phos phoric, acetic, and other nonhalogen acids.
IHKC
PER CENT SULFURIC ACID I T 8 0 · C.
Left and center graphs show comparative corrosion data.
Curve on right indicates resistance of lllium Β VOL. 53, N O . 11
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NOVEMBER 1961
125 A
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CORROSION Properties
Available as a castable alloy, IIliurn 98 has the following nominal composition: 5 5 % nickel, 2 8 % chro mium, 8.5% molybdenum, 5.5% copper, 1.25% manganese, 1.0% iron, and 0.05% carbon. Copper is essential for good sulfuric acid re sistance; nickel keeps all the copper in solution. The roles of chromium, molybdenum, and manganese in re sistant alloys are well known. Typical mechanical properties are: 54,000 p.s.i. tensile strength; 18% elongation in 2 inches; 2 2 % reduc tion in area ; and 1 52 to 167 Brinell hardness. Illium 98 is readily ma chinable using techniques standard for stainless steels (carbide tooling and rigid, adequately powered ma chines). It is weldable by arc, in ert-arc, or acetylene. Welding rod is available. The cost of Illium 98 will be IV2 to 2 times Stainless 20, or about 10% more than Illium G. Satisfactory applications have been made in pumps, valves, and fittings for sulfuric service, in contact with moist SO2 gas or waste sulfite liquors, and in alkylation or sulfonation equipment. Like other corrosion re sistant alloys, Illium 98 is not univer sally resistant. It should not be used in contact with chlorine, bromine, and iodine or their compounds. Other Alloy Modifications
Another modification of Illium G came about in a search for bearing al loys for maximum wear and galling resistance in high concentrations of hot sulfuric acid. This new alloy is Illium B. Illium Β makes available to industry a new series of wear- and corrosion-resistant casting alloys which are characterized by useful properties not found in ordinary austenitic materials. Machinable cast ings in four different hardness ranges are offered the design engineer to combat problems where corrosion resistance is of primary importance, but where resistance to erosion, wear, and galling of similar structured metals must also be considered. Illium Β is a precipitation-harden ing alloy. In its four hardness ranges the alloy possesses a unique metallur gical structure in which a controlled amount of a hard phase is precipi tated in an austenitic base material. The amount of hard phase precipi tated and the hardness of the matrix
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INDUSTRIAL AND ENGINEERING CHEMISTRY
are carefully controlled to give se lective hardness ranges. An alloy with the hard materials dispersed in a hard matrix, such as Illium B-3 or B-4, works well in cutting operations where both wear and corrosion are encountered. An alloy with a dis persion of hard materials in a rela tively soft matrix, such as Illium B-l and B-2, answers many bearing problems. Grades B-l and B-2 are machin able using standard stainless steel techniques; with grades B-3 and B-4 the alloy should be solution treated, machined, and then hard ened. All grades of Illium Β are weldable by heliarc and oxyacetylene methods. As a castable alloy, Illium Β has the following nominal composition (variation in chemistry is necessary to obtain the hardness ranges de sired): nickel, 47.00-52.00%; chro mium 2 8 % ; molybdenum, 8.5%; copper, 5.5%; silicon, 2.5 to 6.5%; iron, 2.00 to 3.50%; manganese, 1.25%; boron, 0.05-0.55%; and carbon, 0.05%. Successful Applications
Illium Β is being successfullyemployed for rayon staple fiber cutters, pump impellers, chemical pump bearings and seals, and other special process equipment where corrosion and abrasion are factors. Illium Β is not recommended in strongly oxidizing environments. It is useful, however, in both oxidizing and reducing environments. Most outstanding is its ability to resist sul furic acid containing small concentra tions of nitric acid. Field tes ling of new alloys supple ments data collected in the labora tory. With careful study of data returned from field testing, we can develop alloys to meet specific corro sion problems. Quite often, modifi cations of older existing alloys such as Illium G produce new alloys such as Illium 98 and Illium B.
CB
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