LITERATURE CITED i.

2.

G. N. Dubinin and Ya. D. Kogan (eds.), Progressive Methods of Chemicothermal Treatment of Metals [in Russian], Mashinostroenie, Moscow (1979). A. S. Dukhanin, I. B. Shenderov, A. I. Zemzerov, et al., "The effectiveness of threaded pairs of cut-off valves of sulfocyanided steels," Khim. Neft. Mashinostr., No. 3, 32-33

(1981). 3.

4. 5.

Z. Bemer, V. Shreter, Yu. M. Lakhtin, et al., "The principles of classification, control, and use of nitriding processes," Metalloved. Term. Obrab. Met., No. i0, 19-25 (1979). L. M. Kleiner et al., "Steel," Inventor's Certificate No. 583197 (USSR). Byull. Izobret., No. 45 (1977). E. V. Ryabchenko, "The use of the glow discharge for diffusion impregnation of metals," Tr. Mosk. Aviats. Inst., No. 228, 65-80 (1971).

CORROSION RESISTANCE OF POWDER NICKEL ALUMINIDES IN ALKALINE AND ACID MEDIA A. P. Presnov, R. A. Valieva, G. E. Kuznetsova, N. V. Gladyuk, V. S. Yarovenko, and N. S. Trusova

UDC 620.193.4 669.245

Within the wide range of intermetallic compounds, of great interest for equipment and machinery construction are nickel aluminides NiAI and Ni3AI, having high heat resistance, hardness at elevated temperatures, and resistance to thermal impacts [i, 2]. However, the practical use of these materials obtained by traditional technology (casting) is restricted because of their increased brittleness [2]. In one of the scientific and industrial corporations, powder alloys corresponding to the properties of nickel aluminides NiAI and Ni3AI have been developed, and their production has been organized. The range of use of these intermetallic compounds can be significantly expanded when they are used as a plasma coating. The hardnesses of Ni3AI and NiAI are 2.94 and 3.23 GPa respectively, and their rupture strengths of joints with steel St3 are 0.4-0.5 and 0.37 MPa respectively. Coatings based on NiAI and Ni3AI are now being used successfully for restoration and strengthening of worn surfaces of seating sites, friction pairs, etc. These coatings are also used effectively for prevention of the oxidation of parts and subunits operating at temperatures up to 1300~ and as a sublayer for subsequent spraying of various steels, alloys, and heat-shielding, electrically insulating, and other coatings based on powder-metallurgical and ceramic materials, During the use of equipment, many of its parts experience simultaneously not only wear, but also the effect of corrosive media, which is the reason for fracture of the coating; therefore, information on the corrosion resistance of powder alloys is very important and could significantly expand their practical use. For this purpose, we investigated the corrosion resistance of alloys NiAI and Ni~AI in alkaline and acid media. Investigations ofthe corrosion resistance of alloys NiAI and Ni3AI were carried out on compact (sintered) specimens. For their preparation, the alloys were pressed in the form of 32-mm-diameter and 5.5-mm-high tablets with a specific pressure of 1-1.5 tons/cm 2. After sintering of the tablets, the specimens shrank, and their height was 5 mm, and their diameter was 28 mm. The porosities of the specimens of alloys Ni3AI and NiAI determined hydrostatically were 7-10 and 4-7%, respectively. The corrosion resistance of the alloys was determined gravimetrically on the basis of quantitative and qualitative evaluation of the rate of corrosion of each alloy. In addition, the porosity of compact specimens of the alloys was taken into account in calculations of the corrosion rate, increasing the surface area of the specimen with due regard for the porosity. To verify the possibility of use of the alloys as anticorrosion coatings in parallel with compact specimens, specimens with a powder-alloy layer sprayed onto carbon steel were Translated from Khimicheskoe i Neftyanoe Mashinostroenie,

0009-2355/84/0102-0039508,50

No. i, pp. 25-26, January, 1984.

9 1984 Plenum Publishing Corporation

39

subjected to corrosion tests. Specimens with an applied alloy layer were obtained by plasma spraying on a UPU-3D unit. Cylinders of steel St3 with dimensions of 20 x20 mm were used as the matrix for the sprayed layer. For convenience of determination of the nature of corrosion fracture in carrying out investigations, the lateral surface of the sprayed specimens was ground (to a fifth or sixth precision grade). The thickness of the sprayed layer of the powder alloy after grinding was 0.3 mm. The porosity of the layer was determined metallographically, and on all the specimens it was equal to 7-8%. Just as on the compact specimens, the porosity of the sprayed layer was taken into account in the calculation of the corrosion rate. For carrying out corrosion tests, the prepared specimens were held in the corrosive medium for a specific time, and they were weighed before and after the holding with a precision of up to 0.0002 g. The preparation of the specimens for the tests and their treatment after the tests were carried out in accordance with the generally accepted procedure [3]. The rate of dissolution of the alloys was evaluated quantitatively according to the depth of corrosion penetration calculated in accordance with GOST state standard [4] with assignment of the tested alloy to the corresponding stability group according to a 10-grade scale. Qualitative evaluation of thecorrosion fracture of the alloys (the nature of the corrosion) was carried out visually. For the tests, the corrosion media were 5, i0, and 15% potassium hydroxide solutions and 0.5, i, 1.5, 3, 5, i0, and 20% hydrochloric acid solutions. The temperatures of the corrosion media were 18-25, 50, 70, and 90~ The solutions for the tests were prepared from analytically pure grade potassium hydroxide and technical grade hydrochloric acid with distilled water. The corrosion medium with the specimens was placed in glass flasks (for hydrochloric acid media) or steel hermetically sealable vessels (for alkaline media) equipped with reflux condensers and thermometers. The specimens were suspended on a Ftoroplast (Teflon) ribbon, and, to prevent their contact with each other, they were separated by Ftoroplast gaskets. To create a temperature higher than room temperature in the vessels, the vessels with the specimens and the corrosion medium were placed in thermostats with the heating liquid (glycerin or silicone oil), where with a contact thermometer and an electronic relay the temperature was kept constant with a precision of up to •176 The results of the corrosion tests showed that in solutions of potassium hydroxide with a temperature of up to 50~ inclusively the NiAI and Ni3AI alloys were stable in the entire range of concentrations of the alkali both in the liquid phase and in the vapor--gas phase and the rate of their corrosion did not exceed 0.03-0.04 mm/yr. Both alloys were unstable when the temperature of the potassium hydroxide solutions was increased to70~ and higher. In the case of the Ni~AI alloy, with increasing temperature, the corrosion rate increased linearly, indicating uniform corrosion of the alloy. This was also confirmed by visual inspection of the tested specimens. The NiAI alloy had the highest corrosion rate at 70~ A further increase of the temperature of the alkali solution did not increase the corrosion rate of the alloy. The alloy was some what passivated, remaining unstable. As shown by the investigations, in the case of the specimens with the coating, after the tests in potassium hydroxide solutions, because of high uniform porosity of the sprayed layer, there was corrosion fracture of the carbon-steel substrate. For the specimens with a sprayed layer of the Ni3AI alloy, a temperature of 50~ was also limiting. At this temperature, the specimens were stable in all the investigated alkali concentrations, and the corrosion rate was up to 0.i mm/yr. The NiAI and Ni3AI alloys were less stable in hydrochloric acid solutions than in alkaline solutidns. In addition, the NiAI alloy was somewhat more stable than the Ni3AI alloy. It has sufficient corrosion resistance (up to 0.i mm/yr) in hydrochloric acid solutions with concentrations up to 1.5% at room temperature. The Ni3AI alloy was unstable in the liquid phase of the hydrochloric acid solutions in all the investigated concentrations. In the vapor-gas phase, this alloy could be used at room temperature in an acid with a concentration not greater than 10%, and the rate of corrosion of the alloy was 0.029 mm/yr. In the case of the specimens, intensive dissolution of the matrix occurred in the liquid phase of the hydrochloric acid solutions at all the test temperatures. In addition, the corrosion products of the carbon steel were unable to penetrate through the pores in the alloy layer to the specimen surface, and, accumulating under the alloy layer, they promoted its peel40

ing from the matrix. Therefore, when a sprayed layer of NiAI or Ni3AI alloys is used under service conditions, it is necessary that the matrix of the part or subunit on which the alloys are applied be corrosion-resistant. LITERATURE CITED I. 2.

V . S . Sinel'nikova, V. A. Podergin, and V. N. Rechkin, Aluminides [in Russian], Naukova Dumka, Kiev (1965), pp. 50-51. R . K . Evans, "New aerospace materials debut of Farnborough 78," Met. Mater., No. 12, 32-34

(1978). 3. 4.

Guideline Technical Data RTM 26-01-21--68. Methods for Corrosion Tests of Metallic Materials [in Russian], NIIkhimmash, Moscow (1968). State Standard GOST 13819--68. Corrosion of Metals [in Russian], Izd. Standartov, Moscow (1972).

FEATURES OF THE STRUCTURE AND PROPERTIES OF COATINGS APPLIED BY THE METHOD OF MICROARC OXIDATION V. N. Malyshev, G. A. Markov, V. A. Fedorov, A. A. Petrosyants, and O. P. Terleeva

UDC 621.794.61:621.357.8 669. 715

The method of microarc oxidation makes it possible to obtain on a part bacically new coatings strongly adhering to the base and characterized by high mechanical, heat-resistant , and wear-resistant properties [i, 2]. One of the important advantages of it is the capability of applying the coatings on the inner surfaces of parts. The structure of coatings applied by the microarc oxidation method to DI6T aluminum alloy was investigated with the use of scanning electron microscopy and chemical and x-ray diffraction analyses. It was found that the coating is inhomogeneous. Its structure and composition change across the thickness and the microinhomogeneity observed in analyzing the microstructure of a transverse microsample and confirmed by the data of coating microhardness measurements is characterized by the formation of a honeycomb type structure. Consequently the coating may be considered as a composition material with uniformly distributed hard islands surrounded by softer veins. X-ray diffraction investigations on a DRON-2.0 diffractometer established that the base of the coating is solid phase solutions of the components of the aluminum alloy, primarily the oxides ~-A1203 and y-A1203 (up to 60%). On the x-ray diffraction pattern obtained in CoKe-radiation from the coating separated from the base in addition to the lines corresponding to a- and y-AI203 lines were noted the identification of which indicates the presence of complex oxide compounds of elements in the composition of the metal and the electrolyte and chemically combined both with the aluminum oxides and with each other. The data of chemical analysis of the coating also confirms the presence of the oxides K202, Na2Oa, Si02, and Fe20~. The porosity of the investigated coatings was determined by petrographic analysis with the use of a system of texture analysis of the images (TAS Leitz). The porosity was calculated on the basis of the first stereometric identity [3] as the ratio of the area occuring in voids to the total area of the investigated portion of the sample. It was found that the porosity of the coating depends upon the conditions of microarc oxidation and varies:from 0.2 to 14%. By selecting the proper conditions of microarc oxidation and the electrolyte it is possible to obtain practically porosity-free coatings. The data of microhardness measurements of the coatings on a PMT-3 tester confirmed their structural inhomogeneity (Fig. i). The highest coating microhardness occurs at a distance of 30-60 Dm from the interface. Closer to the surface layer the microhardness drops somewhat Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. i, pp. 26-27, January, 1984.

0009-2355/84/0102-0041508.50

9 1984 Plenum Publishing Corporation

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