Corrosion The electron microscope provides information on intergranular corrosion of austenitic stainless steels byMarnC.Fontana
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suxqitibility of austenitic 1 8 4 .stainless st ' . Intergranular corrosim is of great interest to the chemical industry and the field of corrosion because these stainless steels are widely used. This subject wa8 iiisciissed several coluiiin. An crtremely interesting and pertinent .problein covering a new approach was reportetl i n a paper titled, "Carbide Precipitation in Type 304 Stainless n Electron Microscope Study," by E. M. Mahla and elsen of the Eiigineering Research Laboratory, E. I. du Pont de Semours & Coinpan.y, lnc., Wilmington, I)rl. This paper vas presented on October 24 before a technical session of tlie American Society for Metals (1950 preprint So. 16) during the Sational Metals Congress in Chicago. Fermission t,o review this paper in this column was received f r m the Anieriean Society fur Metals. The invcstigation was carried out on 18-8s (Type 304) stainless steel x i t h a variet.y of heat treatments including several serisitisiilg conditions. Sensitization consists of heating in the approxiinate temperature range 900"to 1400"F. to precipitate carbides and indiice intergranular corrosion. The carbides mere isolated by dissolving the spechnens in a solution of bromine aiid anhydroiis methanol. This leaves a midue containing carbides and other insoluble constitiients. These residues w r e prepared on a "Fonnvar screen" for examination by tlie electron microscop at high magnifications and also for electron diffraction. The actual distribution of the carbides in the metal cannot be shown, of course, and the total ainount or voiuine of carbides was not determined. The usual procedure involving the light microscope for examining precipitated carbides shows the pattern illirstrated in Fignrc 1 . TIE dark spots at the grain boiindaries are cornmoirly called precipitated carhiiies. Actually they are holes HI..
or etcliirig pits and do not, sliwv the
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tore of the carbide precipitate. All other figures are electron pliot~oniicnigraphs. Figure 2 shows the residue of a quench-aiiiiealed (1950' to 2000" F.water quench) lX4S specimen. The composition and structure of this residue is iinknoivn, and it is not the chromium carbide iiieiititicd iii other residues. This specimen is heat treated for best cixmsion resistance and showed a corrosion rate of 0 mils 1 x 7 year in t,hc boiling %,I nitric acid test.. Figure 3 stiows the carbide structure of 18-8s senhitized by heating at 1200O 12. for 1 hour. The nitric acid corrosion rate \vas 110 m i l s per year. Electroii diffraction idcntifietl this inaterial as chromium carbide with the formula (XX,. The dendritic or leaflike pattern of tlrr carbide is evident. Figures 4 and 5 slim\- tlie carhide structure after heating at 1200' F. for 13 inimrtes and 2000 hours, respectively. Sotc the geoiiietrir Ionns of tlie carbide after the long-time heat treatment rvhich niiniiiiizcs sweeptihility to intergraiiiilar attack. This is a fiiic structure slroa-n a t 30,oClO diameters. Figure 6 also shows a geonietric patt.ern for a specinieo lieiited for 18 hours at 1300"17. As a result of this study the authors conclude that t l i e carbide particles form and undergo transforniatimi a t sensitizing temperatures as fullom: precipitation of ilendrites growth of dendrites --t fragmentation of dendrites -i change tu inore stable geometric forms. Another interesting point brought up in this paper is that, cold work after sensitization rcstores cormsim resistance. (ContinuedOIL p u p 7 4 .'l i ---)
Figure 1 ( L e f t ) . Photomicrograph of Polished and Etrhed 18-8s S e n s i t i d for 1 Ifow a t 12WP F. Ordinary Light Mirroscmpy,750 Diameters. Figure 2 (Center). Residue from ineealod 18-8s (13,lm Diameters). Figure 3. ( R i g h f ) Carbide 18-85 Heated for 1 llow at 12W" F. (13,W Dianletcrs)
Corrosion This may be diie t
