A d , 1922
THE JOURNAL O F INDUSTRIAL AND ENGINEERING CHEMISTRY
323
Failure of Cast and High Silicon Iron in Fuming Sulfuric Acid' By T. F. Banigan HERCULES POWDER COMPANY, HERCULES, CALIFORNIA
Experiments carried out by the writer on metallic silicon and silicon-iron alloys indicate that the failure of cast iron and malleable castings in fuming sulfuric acid is due to oxidation of the particles of silicon in the casting to particles or crystals of silicon dioxide, which, being larger in volume than the original silicon particles, place the casting in a condition of internal strain. Cast iron, with as low silicon content as ordinary steel, should be as satisfactory as the latter for use with fuming sulfuric acid, and any reduction in silicon content should improve it for this use.
T is frequently desirable in the manufacture and use
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of fuming sulfuric acid to employ cast-iron equipment, but castings of this material are likely to crack suddenly after being used for considerable periods of time with either hot or cold sulfuric acid containing free so3 Cracking also occurs, to a less degree, with anhydrous mixtures of nitric and sulfuric acids and is considerably more severe with hot than with cold acids. Numerous cases have come to the attention of the miter where heavy cast-iron pipes used with fuming sulfuric acid, in places where they were exposed to no unusual strains, have cracked longitudinally with loud reports, though the pipes on later examination showed practically no evidence of corrosion by the acid. Similarly, cast-iron valves and pumps in such service crack from no apparent cause before the metal shows sufficient wear to account for such failure. Malleable castings used in attempts to overcome this difficulty have failed likewise. Breaks in such pieces of equipment while in operation involve such serious danger to human life, loss of material, and stoppage of production that some manufacturers eliminate the danger by substituting steel, which is more expensive but free from this defect. Lunge2 quotes Knietsch, in explanation of this peculiarity of cast iron as follows:
Whereas cast-iron vessels are very suitable for the manufacture of hydrated sulfuric acid, this is not the case for fuming acids, for these, although they corrode the metal but slightly, cause it to crack, sometimes quite suddenly with a loud report. Evidently the fuming acid penetrates into the pores of the metal, where it is reduced to SO2 and HnS, with formation of COz from the carbon of the cast iron-all of them gases with somewhat low critical temperatures which produce high tensions in the interior of the metal.
EXPERIMENTAL EXPT. 1-Test bars of a well-known high-silicon iron alloy, which is extremely resistant to nonfuming sulfuric acid, were immersed in 15 per cent oleum, or 103.37 per cent Hi304 a t room temperature for 19 days. The bars were then washed with water and found to be severely corroded and covered with gelatinous silica. Loss in weight amounted to 4.73 per cent. The sludge produced in this test was found ' to contain a considerable amount of free carbon. EXPT. 2-A weighed quantity of amorphous silicon was introduced into 103.37 per cent HzS04 and kept a t about 92' 0. for 8 days. The mixture was then drowned in distilled water and filtered through a weighed Gooch crucible which was then heated at less than dull red heat and Received November 17, 1821. *"Sulfuric Acid and Alkali," 4th edition, Vol. 1, Part 1 , 322.
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weighed. The increase in weight was calculated to SiOp. Filtration was difficult on account of the gelatinous silica present. Increase in weight showed that 24.64 per cent of the total silicon had been oxidized to SiOz. EXPT. 3-A weighed quantity of amorphous silicon was treated as in Expt. 2 except that it was allowed to stand a t room temperature for 21 days. Increase in weight corresponded to 40.20 per cent of total silicon oxidized to SiOz. EXPT. 4-In order to determine with certainty that the increase in weight of silicon in the preceding experiments was due to action of the free 803, a weighed amount of amorphous silicon was kept for 8 days a t 100" C. in 96 per cent HzS04. No increase in weight resulted. EXPT. 5-A sample of silicon carbide was introduced into 103.37 per cent Hi304 and heated at 100" C. for 7 days. No change in weight resulted.
CONCLUSIOKS FROM EXPERIMENTS 1-Silicon either in free condition or alloyed with iron is readily oxidized by sulfur trioxide. 2-The failure of high-silicon iron alloys in fuming sulfuric acid is due to rapid oxidation of silicon to the more voluminous Si02which tears down the surface of the casting, thereby continually exposing fresh surface to the action of the SO3. 3-The failure of cast iron and malleable iron in fuming sulfuric acid is apparently due to SO3 penetrating into the pores of the casting and oxidizing the silicon t o SiOg, which, on account of its large volume, produces a condition of internal strain. Action of SO1 on silicon carbide in the casting is not responsible for failure. Carbon in the casting appears to be little, if a t all, affected under these conditions, and in the usual amounts probably has no injurious effect.
Retirement of E. G. Rippel Mr. E. G. Rippel, who organized the Buffalo Foundry and Machine ComDanv about twenty-one years ago, has recently retired from active duty as sales manager of the corporation. Mr. Rippel has always believed in a progressive development policy, to which our chemical industry is indebted for many i m p o r t a n t pieces of plant apparatus. He has been an ardent supporter of the National E x p o s i t i o n of Chemical Industries and is to be credited with considerable contributions toward its success. After a much-needed rest, plansfor the future may be announced by Mr. Rippel. E. G. RIPPEL
