CORROSION OF VERY FINE ELECTROLYTIC IRON POWDERS

E. P. Zhelibo UDC 629.762.274.66.047

Electrolyt ic methods of manufacture of powders have always attracted a grea t deal of attention among invest igators because of the ease with which e lec t ro lys is can be controlled and the simplicity of the apparatus employed. By varying electrolyte composit ion and e lec t ro lys is parameters it is possible to produce powders of different physicochemical proper t ies [1-3]. To ensure high quality of electrodeposi ted powders, they are subjected to cer ta in operations aimed at purifying them f rom electrolyte , oxide, and other contaminant inclusions and protecting them against cor ros ion . Much has been writ ten in the l i tera ture about a variety of ways of drying finely divided metals.

To decrease the flammabili ty of metal powders, their sur faces are t reated with various substances p r e - venting contact between the metal and atmospheric oxygen, such as sulfuric acid solutions with inhibitors, water glass , fatty acid salts, sodium arsenide, heavy metal azala tes [4-8], benzine solvent, benzene, and other organic l iq- uids, and also aqueous solutions of surfactants of the fatty and aromat ic se r i e s [9, 10]. Adsorption of these substances slows down the diffusion of oxygen, thereby decreas ing the rate of oxidation of metal part icles . A semipass ive state can, of course , be crea ted by thin oxide films preventing fur ther oxidation of metal. On the other hand, even methods such as those described above cannot always provide adequate protection against co r ros ion to very fine powders.

Electrodeposi t ion of metals on passivated e lect rodes [11, 12] enables very fine nonpyrophoric powders to be produced because a surfactant, in par t icular the widely used oleic acid, is adsorbed on the surfaces of p a r - t icles at the instant of their formation. However, as has been shown, inter alia, by chemical analyses, x - r ay s t ruc tura l investigations, and studies of magnetic propert ies , such powders contain appreciable quantities of oxides and organic impurit ies, which have a marked deleterious effect on the propert ies of parts . For instance, the actual magnetic proper t ies of very fine iron powders attain a level of not more than 10~ of that predicted by theory [13 ]. Clearly, oleic acid adsorbed on the sur faces of fine powder part icles may prevent their spon- taneous combustion, but does not adequately protect them against cor ros ion . To obtain a be t te r understanding of the oxidation p rocesses taking place during the preparation, washing, and drying of iron powders, let us analyze our own experimental resul ts and data published in the l i terature .

The organosol of a metal forming on a passivated electrode is a pastelike mass contaminated with the e lectrolyte (an aqueous chloride or sulfate solution) and organic substances (toluene or oleic acid and its salts) . As washing liquids water , ethyl alcohol, and acetone in various proport ions and o rders are commonly employed. In some cases toluene, benzene, and xylene are used in the last stage of washing for surface passivation. A metal powder washed free f r o m electrolyte and organic substances contains a substantial amount of water. Powder drying is performed by various methods, in vacuum, air, or an inert a tmosphere.

During the format ion Of a powder, because of the large surface energy of finely divided metal, oleic acid reac t s with it and fo rms iron sal ts [14]. As the organic medium general ly contains onty a very smal l quantity of the acid (from 0.2 to 1.0~o), then, according to [15], a chemical react ion resul t ing in the formation of a neutral sal t and water can occur:

Fe + 2RCOGH-+(RCOO)2Fe+2H; I

2H -}- ~- O 2 -+ H~O.

However, such a react ion can proceed to the full extent only when the acid completely covers the surfaces of the powder par t ic les . It has already been established [16] that oleic acid molecules do not arrange them- selves evenly on a solid surface. Apart f r o m this, the presence of a double bond in the center of a molecule along the carbon chain prevents close packing of acid molecules on the surface of an adsorbent [17]. Bearing also in mind the high degree of development of the powder surface (the specific surface varies , depending on

Institute of Colloid Chemistry and the Chemist ry of Water, Academy of Sciences of the Ukrainian SSR. Translated f rom Poroshkovaya Metallurgiya, No. 6(234), pp. 4-9, June, 1982. Original art icle submitted March 16, 1981.

428 0038-5735/82/2106-0428 $07.50 �9 1982 Plenum Publishing Corporat ion

the process pa rame te r s , f r o m 10 to 70 m2/g [18]) and the fact that part icle growth takes place in an e lect rolyte i.e., under conditions of keen competi t ion f r o m inorganic components (oxygen and chlorine ions), we can a s - sume that the powder surface is not completely covered with oleic acid, and the bare areas can suffer attack. Evidence in support of this hypothesis is provided by the following data. With increasing e lec t ro lyte concen- t ra t ion powder deposits as a rule become c o a r s e r [19]. The resul t s of an e lec t ron microscopica l invest iga- tion show that in such a case iron powder par t ic les attain a large size (a length of the o rde r of 3-5 tzm) [20]. Chemical and x - r a y diffraction analyses have established that with increasing e lec t ro ly te concentrat ion and, hence, par t ic le size, the unoxidized f ract ion of a powder diminishes . Thus, e.g., finely divided i ron produced f rom fer rous chloride solutions at an e lect rolyte concentra t ion of 300 g/1 iter can contain large amounts of oxides in all its specimens , yet this is not the case with powders obtained under the same conditions f rom less concentrated solutions.

To find out whether oxidation takes place during e lec t ro lys i s or subsequent operations, x - r ay diffraction photographs have been taken of f reshly prepared and dried powders. As is shown in [21], the fo rmer was free f r o m oxides, whereas in the la t ter powder substantial amounts of oxides were found. It follows, therefore , that oxidation occurs mainly during powder washing and drying.

Shortening the t ime of p rocess ing of a powder substantially improves the magnetic proper t ies of parts made f r o m it [13 ]. This, too, indicates that the oxidation of a n~tal powder occurs chiefly during the washing and drying s tages . The phenomenon descr ibed using a coarse powder as an example is less pronounced in the case of a fine powder, but this mere ly points to different ra tes of co r ros ion : Cor ros ion is rapid in the fo rmer case and much s lower in the lat ter . In both cases , however, the same mechanism of co r ros ion is, of course , operat ive. Evidently, the var ia t ion of co r ros ion rate as a function of powder part icle size is, as was noted above, a consequence of changes in the degree of packing of surfactant molecules on the surfaces of par t ic les .

As the washing of powders is per formed in air, the unprotected surface of the finely divided iron is ox- idized by a tmospher ic oxygen. Hydrated Fe 3+ ions are brown in color ~2]. When a powder is turned this color by washing in toluene, which readi ly dissolves oxygen [23], this may be indicative of the following m e - chanism of cor ros ion , involving the part icipat ion of oxygen [24, 25],

I Ye + O2 --~ 2FeO + -2- O 2 --~ Fe20~ ,

and the format ion of a t r ivalent iron salt,

Fe203 + 6RCOOH -~ 2 (RCOO)sFe ~- HzO

Much evidence, reviewed in [26], has already been repor ted in the l i terature that metal l ic oxides reac t with fatty acids. In addition, in ~7, 28] the presence was established, using different methods, of surface films composed of t r ivalent i ron oleates on metall ic iron powders produced in a two- layer bath.

The above-descr ibed mechanism of co r ro s ion of finely divided iron would, of course , be predominant if the chemical reac t ion proceeded rapidly, with the part icipat ion of a large amount of the acid. To obtain good magnetic proper t ies , in par t icu lar high remanent induction, in powders, it is necessary to ensure that the metall ic phase content of specimens is as high as possible. For this purpose, powders are washed so as to reduce to a minimum the amount of organic impuri t ies present in them. And, as only a little acid is added to the organic layer , the amount of oleic acid (chemisorbed and chemical ly unbound) in a powder is small . After the washing of a powder some of the liberated active centers on the surface of the finely divided metal can enter into various reac t ions , e.g., with a tmospher ic oxygen.

Since the reac t ion of an organic acid with a metal is much slower than oxygen adsorption, it is reasonable to assume that such an iron powder contains also a large quantity of oxides. Thus, t r iva lent i ron oxides and oleates are presen t in a dry powder.

The format ion of i ron oxides and oleates lowers the ra te of cor ros ion . However, these f i lms do not com- pletely passivate metals and consequently do not actually prevent co r ros ion ; a dry powder continues to oxidize during s torage , but its ra te of co r ros ion steadily falls. Under these conditions the physicochemical proper t ies of the powder change. While in a f reshly produced powder crysta l l ine oxides a re virtually nonexistent, and only amorphous oxides are found which are not detected by x - r a y diffraction, with time they begin to appear in increasing quantit ies. This phenomenon has been c lear ly revealed in a M~Jssbauer effect study of very fine tin powders [29]. As the diffusional permeabil i ty of crys ta l l ine oxides is much lower than that of amorphous

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ones, with t ime the ra te of co r ro s ion of a powder falls. On the bas is of exper imenta l data obtained it is pos - sible to visualize the s t ruc tu re of an e lec t ro ly t ic powder par t ic le as consist ing of a metal l ic nucleus sur rounded by l aye r s of oxides in the amorphous and crys ta l l ine s tates , o leates , and oleic acid.

Evidence in support of the contention that such a powder contains a substantial quantity of a nonmagnetic const i tuent is provided by resu l t s of chemical analyses for metal l ic i ron (whose amount in spec imens of s tand- ard powders does not exceed 50"~) and data yielded by magnetic measurements .

Thus, use of e lec t ro ly t ic powders produced on passivated e lec t rodes for the manufacture of high-quality par ts whose physieochemical p roper t i e s r emain stable with t ime presen ts diff icult ies, which, however, need not prove insurmountable. To achieve this goal, it will be necessa ry , apar t f r o m varying e l ec t ro lys i s condi- tions, to: conduct the washing of powders in iner t envi ronments ; introduce powders, immediate ly a f te r they have been washed f ree f r o m all t r aces of e lec t ro ly te and organic contaminants , into a varn ish compound when it is requi red to apply varn ish coatings or into a binder in the manufacture o f , e.g., s in te red pa r t s ; modify the par t ic les of fine metal powders with h igh-molecula r -weight organic compounds in the course of e lec t rodepos i t ion and then encapsulate them by chemical means (according to [30, 31], chemical encapsulat ion of sol id-phase par t i c les is the best method of improving the proper t ies of par t s made f r o m them); and p e r f o r m the heat t r e a t - ment of powders in reducing a tmospheres and then encapsula te the metal par t ic les in inorganic compounds. Thus, e.g., by mere ly reducing fine Fe and F e - C o alloy powders in a hydrogen a tmosphere it is possible to improve the p roper t i es of permanent magnets made f r o m such powders 2-2.5 t imes [32 ].

Investigation of the above-mentioned topics will undoubtedly help produce high-quality par ts f r o m m i c r o - powders e lec t rodepos i ted on e lec t rodes passivated by organic compounds.

L I T E R A T U R E C I T E D

1. F. Eisenkolb, Advances in Powder Metallurgy [Russian translat ion], Metal iurgiya, Moscow (1969). 2. W.D. Jones, Fundamental Pr inc ip les of Powder Metallurgy, Edward Arnold, London (1960). 3. O .K. Kudra and E. B. Gitman, Elec t ro ly t ic Product ion of Metal Powders [in Russian], Izv. Akad. Nauk

UkrSSR, Kiev (1952). 4. B . L . Zlobinskii , V. G. Ioffe, and V. B. Zlobinskii , Flammabil i ty and Toxici ty of Metals and Alloys

[in Russian], Metallurgiy a, Moscow (1972). 5. A . V . Pomosov, E. E. Krymakova, V. P. Gargala, et al., ' Method of protect ion of metal powders

agalnst oxidation," Inventor ' s Cer t i f ic ate No. 298691 ; Byull . Izobret . No. 11 (1979). 6. A . P . Brynza and N. A. Kormshchikova, "P ro tec t ion of i ron powder by heavy-meta l azelates ," Poroshk.

Metall . , N~o. 8, 13-17 (1966). 7. L . P . Kuz 'min and V. L. Kiseleva, "E lec t ro ly t i c production and proper t i es of i ron powder," Zh. l>rikl.

Khim., 22___, No. 3, 311-318 (1949). 8. A . P . Brynza , E. S. Rynskaya, M. P. Smovdorchenko, et al., "Atmospher ic co r ro s io n of hydrophobized

copper powders in the p resence of sulfur dioxide," Zh. t>rikl. Khim., 37__, No. 6, 1376-1380 (1964). 9. N .A. Shurmovskaya and R. Kh. Burshtein, ' Iron e lec t rode of a galvanic cell ," Zh. I>rikl. Khim., 3_.00, No.

8, 1176-1184 (1957). 10. 9" Kubaschewski and B. E. Hopkins, Oxidation of Metals and Alloys, But terworth , London (1953)o 11. E . M . Natanson, Colloidal Metals [in Russian], Izv. Akad. Nauk UkrSSR, Kiev (1959).

/

12. E . M . Natanson, E. P. Zhelibo, and E. A. Serpuchenko, "Method of prepara t ion of organosols of metal , alloys, and metal l ic po lymers , " Inventor 's Cer t i f ica te No. 271810; Byull. Izobret . , No. 10 (1970).

13. G . K . Chernov, A. I. Polishchuk, V. K. Shikin, e t al., "Ef fec t of technological fac tors upon the p roper t i es of permanent magnets made f r o m very fine i ron powder," Poroshk. Metall. , No. 10, 87-92 (1973).

14. A . D . Korenev and M. A. Lunina, "Adsorpt ion of fatty acids and soaps on metal powders," Zh. Fiz. Khim., 43_, No. 6, 1605-1608 (1969).

15. T . A . Miskinova and L. G. Gindin, "Cor ros ion of lead in d i e l ec t r i c s , ' Zashch. Met., 1, No. 2, 194-198 (1965).

16. S . N . Tolstaya, "Phys icochemica l pr inciples of adsorpt ion activation of minera l f i l l e r s and pigments in polymer sys tems ," Author ' s Abs t r ac t of Doctora l Disser ta t ion , Moscow (1969).

17. S . G . Daniel, " T he adsorpt ion on metal sur faces of long chain polar compounds f r o m hydrocarbon so lu- tions," Trans . Faraday Soc., 348, No. 47, 1345-1359 (1951).

18. Yu. I. Khimchenko, E. P. Zhelibo, and T. V. Chubar ' , "Ef fec t of e l ec t ro lys i s conditions on the specif ic sur face of ve ry fine iron and cobalt powders," in: Phys icoehemica l Mechanics and Lyophi l ism of D i s - pe r se Systems [in Russian], Vol. 6 (1974), pp. 103-107.

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19. E . P . Zhelibo, T. V. Chubar ' , T. N. A melichkina, et al., "E f f ec t of e l ec t ro lys i s conditions on the p r o p - e r t i e s of fine cobal t powders," Poroshk. Metall . , No. 10, 1-7 (1974).

20. E . P . Zhelibo, K. A. Aryuptna, and E. M. Natanson, " F o r m a t i o n of fine i ron powder on the cathode," Poroshk . Metall . , No. 2, 14-19 (1973).

21. E . P . Zhelibo, "E lec t ro ly t i c deposi t ion of ve ry fine i ron and cobal t powders f r o m concent ra ted solutions," Author ' s A b s t r a c t of Candidate ' s Disse r ta t ion , Kiev (1972).

22. Oka~ Arno~t, Qual i ta t ive Anal i t i sche Chemie, Leipzig (1960). 23. Handbook of Solubility [in Russian], Izd. Akad. Nauk SSSR, Leningrad (1961). 24. E . V . Cherevkova , " C o r r o s i o n of iron in benzene solutions of bu tyr ic acid," Zashch. Met., 5, No. 2, 236-

238 (1969). 25. E . V . Cherevkova , " C o r r o s i o n of i ron in the b e n z e n e - b u t y r i c - a c i d - w a t e r sy s t em," Zashch . Met., 9,

No. 3, 323-325 (1973). /

26. E . M . Natanson and Z. R. Ul 'be rg , Colloidal Metals and Metall ic P o l y m e r s [in Russian], Naukova Dumka, Kiev (1971).

27. L. Yu. Ivanova, A. K. Aver inov, B. M. Machul ' sk i i , e t al., "Compos i t ion of a magnet ic iron powder p r o - duced e lec t ro ly t ica l ly , " Abs t r ac t s of P a p e r s to an All -Union Sc i en t i f i c -Techn i ca l Conference on Future Deve lopments in Magnetic Recording Technology and Magnetic C a r r i e r Manufacture [in Russian], Shostka (1980), pp. 59-60.

28. L. Yu. Ivanova, B. M. Machul ' sk i i , and A. I. Alekseenko, "Role of oleic acid in the product ion of metal powder by the e lec t ro ly t i c p roces s , " Abs t r ac t s of P a p e r s to an All-Union Se ien tLf ie -Technica l C o n f e r - ence on Future Deve lopments in Magnetic Recording Technology and Magnetic C a r r i e r Manufacture [in Russian], Shostka (1980), pp. 61-62.

29. I. Ya. Dekhtyar , E. P. Zhelibo, V. G. Kushnir , et al., ' M~ssbauer e f fec t in superconduct ing t in o r g a n o - sols ," in: P r o b l e m s in Atomic Science and Engineer ing: Fundamental and Applied Superconductivi ty [in Russian], Vol. 1 (1977), pp. 10-14.

30. T . A . Makovskaya and V. A. Stolyarova, "Microencapsu la t ion and i ts applicat ion in paint and va rn i sh technology," in: Paint and Varn ish Mater ia l s and The i r Appl icat ions [in Russian], Vol. 1 (1980), pp. 11-13.

31. V . D . Solodovnik, Microencapsu la t ion ~n Russian], Khimiya, Moscow (1980). 32. I . N . F ran t sev ich , T. M. Shvets, S. A. Mikhalyuk, et al., " P e r m a n e n t magnets f r o m very fine i ron and

i r o n - c o b a l t alloy powders," Vestn. Akad. Nauk Ukr. SSR, No. 7, 58-63 (1 973).

PROPERTIES OF R6M5 STEEL* POWDERS OF VARIOUS

PARTICLE SIZES AND THE STRUCTURE OF THE STEEL

IN THE SINTERED CONDITION

S. S. K i p a r i s o v , V. I . T r e t ' y a k o v , V. S. P a n o v , M. M. S m i r n o v a , a n d Y u . F . K o t s ~

UDC 621.762

Today s in te red R6M5 h igh-speed s t ee l tools a r e being increas ingly used in industry. A number of v a r i - ants of the P/M p roces s by which these tools can be produced have been examined in the l i t e ra tu re [1-4], and the fact that a single technique has not ye t been genera l ly adopted sugges t s that fu r the r r e s e a r c h in this f ield

is n e c e s s a r y .

The p re sen t work was under taken with the a im of de termining op t imum pres s ing and s in ter ing conditions for powders of var ious par t ic le s i zes . Compress ib i l i t y and s in te rab i l i ty expe r imen t s were c a r r i e d out on R6M5 s tee l powders of par t ic le s izes 100, 54, and 22 ~m. Star t ing powders we re obtained by comminut ing R6M5 high- speed s t ee l waste (swarf) in a ba l l mill . The mill ing of the waste to the r equ i red par t ic le s i zes and sieving of r e su l t an t powders we re p e r f o r m e d by methods employed in the h a r d - m e t a l industry [5-7]. The yie ld of good- quality powders of all par t ic le s i z e s w e r e m o r e than 90%.

*A 0.85% C - 6 ~ W-5~/0 Mo-4T0 C r - 2 % V (nominal composit ion) h igh-speed s t e e l - T r a n s l a t o r . tThe authors wish to thank A. N. Nikolaev of the Gork i i Polytechnic Insti tute for helpful d i scuss ion in the e x - pe r imen ta l par t of the work. SA 0.75% C - 1 8 % W-0 .7% M o - 4 % C r - 1 . 2 % V h igh-speed s t ee l - T r a n s l a t o r .

Moscow Inst i tute of Steel and A l l ~ s . T r a n s l a t e d f r o m Poroshkovaya Meta l lurg iya , No. 6(234), pp. 9-15, June, 1982. Original a r t i c le submit ted Apri l 22, 1981.

0038-5735/82/2106-0431507.50 �9 1982 Plenum Publishing Corpora t ion 431