Dunarea de Jos University of Galati, 47 Domneasca Street, Galati, ROMANIA,

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1 SURFACE TREATMENT OF IRON- BASED SINTERED COMPACTS FOR CORROSION PROTECTION POTECASU Florentina, RADU Tamara, POTECASU Octavian, MARIN Mihaela, CIOCAN Anisoara Dunarea de Jos University of Galati, 47 Domneasca Street, Galati, ROMANIA, Abstract The purpose of this paper is to describe the corrosion behavior evaluation of the steam treated sintered iron parts compared with sintered parts, obtained in similar conditions, but without steam treatment. Steam treatment is a surface treatment developed to improve the corrosion resistance, the hardness and the wear resistance of the sintered steels. In this research, the sintered iron alloys parts were steam treated at 550 C for 60 minutes. After microstructural studies, electrochemical tests were carried out to study the corrosion behavior of the sintered and steam treated sintered specimens. The elecrochemical test results showed that the steam treated sintered specimens exhibit better performance on corrosion testing in comparison with sintered specimens, without steam treatment. Keywords: powder metallurgy, surface treatments, corrosion, electrochemical tests 1. INTRODUCTION Powder metallurgy (P/M) parts are often used in structural parts because of the low production cost in large scale series. This economic benefit is granted by a near net shape and low raw material loss. The characteristics of sintered steels are closeness to those obtained by other classic tehniques [1]. An important problem in sintered steels is their high porosity structure which results in lower values in some mechanical properties. To reduce the influence of open and interconected porosity in iron PM specimens, different secondary operations are apllied such as metal infiltration, plastic or resin impregnation and steam-treatment [2]. Steam treatment is one of the surface treatments applied to sintered components, and is often used for the following benefits: an increased in density [3] and corrosion resistance [4, 5], a complete closure of the interconnected and surface porosity [6], enhanced hardness and wear resistance [7-10], an economic poresealing process as comparing with the copper or plastic impregnation [9, 11]. At first, this treatment was used as an economic way to seal the interconnected pores, characteristic of sintered iron, making the component impervious to liquid and gases. This treatment is claimed to improve the superficial properties of sintered materials [13]. The water vapor in the steam will begin to react with the iron in the part and will form magnetite (Fe 3 O 4 ). 4 H 2 O (gas) + 3 Fe Fe 3 O H 2 (gas) (1) The magnetite (Fe 3 O 4 ) or black iron oxide is an iron oxide that is blue to black in color. When a ferrous component is exposed to steam before reaching 320 C, the water vapor will oxidize the iron to form hematite (Fe 2 O 3 ) on the surface of the part. 6 H 2 O (gas) + 2 Fe 2 Fe(OH) H 2 (gas) (2) 2 Fe(OH) 3 Fe 2 O H 2 O (gas) (3) The Fe 2 O 3 shows up as pink color of the part surface. Steam treating components at a temperature above 650 C is inadequate to quality because the oxide that will form in the reaction of the steam with the iron is wustite (FeO). The FeO is not a stable iron oxide below 650 C. The FeO will revert to Fe 3 O 4 below 650 C [12]. 4 FeO Fe 3 O 4 + Fe (4)

2 2.EXPERIMENTAL PROCEDURE 2.1. Materials The specimens prepared from atomized iron powder and from pre-alloyed iron based powders were analyzed in this paper. The chemical composition of the powder samples, pure iron and iron-based prealloyed powder with Cu, Ni and Mo is presented in Table 1. Tab. 1. Chemical composition of analyzed powders Powder type Cu Mo Ni C P <0.01 P <0.01 P <0.01 In Fig. 1 are presented the SEM images of analzed powders. The powder P 1 is a sponge powder comparing with powders P 2 and P 3 wich are more uniform. (a) (b) Fig.1. SEM images of the water- atomized iron powder: a) P 1, b) P 2, c) P 3 The samples were compressed in a universal mechanical testing machine to a pressure of 600 MPa, the dimensions of disc specimens are 8 6 mm. Uniaxial pressing in the mold is used effectively for mass production of simple components. The green samples were sintering in a laboratory furnace, within a controlled atmosphere. The sintering temperature was approximately C and the sintering time was 60 min. with a heating rate of C/min. All the samples were kept in the furnace for slow cooling to room temperature. Before the sintering temperature is reached, the parts were maintained during 30 min at 500 C to burn lubricant, respectivelly zinc stearate.

3 After cooling to room temperature the samples were steam-treated. The steam treatment was carried out in a furnace with steam atmosphere at 550 C for 60 min. Steam-treated specimens were air-cooled to room temperature. on the surface of samples was formed a blue layer of iron oxide, typical for this process. By steam treatment is ensured a closure of interconnected and surface porosity. In order to evaluate the effect of surface treatment on the sintered specimens, steam treated sintered steel parts were subjected to corrosion resistance tests and electrochemical characterization. The corrosion behavior of the sintered and steam treated sintered specimens were investigated by potentiodymanic polarization technique. After 40 min exposure at open circuit potential, measurements were carried out in a conventional three electrode electrochemical cell, consisting of the specimen, the saturated calomel electrode (SCE) as the reference and the platinum plate as the counter electrode. The tests were carried out in a 3.5% NaCl electrolyte solution. The polarization curves were obtain using a Voltalab PGP 201 Potentiostat at a scan rate of 2mV/s. The test area (0.5 cm 2 ) was obtained by embedding the specimens and leaving free plane surface to assess the surface quality of sintered and steam treated sintered specimens. 3. Results and discussion 3.1. Microstructural studies Fig. 2 shows the porosity values of steam treated and sintered (non-treated) specimens. From this figure, it is clear that all three types of steam treated specimens demonstrate considerable decrease in porosity content in comparison with the non-treated specimens. Fig. 2. Interconnected porosity of sintered and steam treated specimens Microstructural studies were carried out after polishing specimen faces using optical (Olympus BX 50). Photomicrographs were obtained at a magnification of 400X. Optical micrographs representative of typical microstructures are reported in Figs. 3a c and Figs. 4a - c. Fig. 3a, b and c shows the microstructures of sintered specimens and it clearly shows open pores.

4 (a) (b) Fig. 3. Optical photomicrographs of sintered specimens: a) P 1, b) P 2, c) P 3. In the steam treated specimens porosity was seen inside all specimens (Fig. 4a - c), but performed oxide layer on the surface of the specimens was able to close the opening of interconnected porosities. However, oxide layer was seen in some areas, as shown in Fig. 4b. Thin layer of oxide influences the corrosion protection of the substrate [2, 12, 13]. (a) (b) Fig. 4. Optical photomicrographs of steam treated specimens: a) P 1, b) P 2, c) P 3.

5 3.1. Corrosion test results The anti-corrosion performances of the sintered and steam treated sintered specimens in 3.5% NaCI solutions are studied, and compared using potentiodynamic polarization technique. In each series of specimens, the least corrosion resistant result was subjected to series P 1, which has the highest porosity. These results are in agreement with other works which show the negative effect of porosity on corrosion resistance of porous specimens [1 8, 10 12]. From Fig. 5, it can be seen that the corrosion potential of 60 min steam treated samples is higher than the sample non-treated one. This is attributed to the steam treated layer and the closed interconnected pores.in the steam treated specimens, the superior protection is afforded by the thin magnetite (Fe 3 O 4 ) under layer which has contributed to corrosion protection. a b Fig. 5. Tafel plots for specimens: a) sintered and b) steam treated samples at 550 C. Different parameters like i corr, cathodic ( c) and anodic ( a) Tafel constants derived after the extrapolation of the Tafel plots are summarized in Table 2. Table 2. Different electrochemical parameters obtained by Tafel extrapolation method Sample a c i corr, ma/cm 2 corrosion rate (mm /year) P 1 sintered P 2 sintered P 3 sintered P 1 steam treated P 2 steam treated P 3 steam treated

6 4. CONCLUSIONS The results confirmed that the steam treatment applyed to sintered specimens can decrease the surface porosity through the formation of a homogeneous layer mainly composed of iron oxide. The corrosion resistance investigations showed that a steam treatment of 60 min can be improved significantly and deserves further extensive study. The prealloyed samples with Cu, Ni and Mo present an improve the corrosion resistance of sintered steels compared with unalloyed samples. REFERENCES [1] Akimenko V.B., Gulyaev I.A., Sekachev M.A., Kalashnikova O. Y., Air-atomized iron powders Past, Present, Future, Metallurgia, 2007, 51, no , 624. [2] Beiss P., Steam treatment of sintered parts. Powder Metall 1991;34: [3] Mair G., Relationship of density to hardness of steam treated sintered iron, Powder Metallurgy Int, 1993, 25, no 3, p. [4] Saunders S.R.J., Monteiro M., Rizzo F., The oxidation behaviour of metals and alloys at high temperatures in atmospheres containing water vapour: A review, Progress in Materials Science, 2008, 53, p. [5] Volenık K., Volrabova H., Neid J., Seberıni M., Structure of oxidation products of sintered steel in superheated steam, Powder Metall., 2004, 21, p. [6] Franklin P., Davies B.L., The effects of steam oxidation on porosity in sintered iron, Powder Metallurgy, 2001, 20, p. [7] Razavizadeh K., Davies B. L., The effects of steam treatment on the wear resistance of sintered iron and Fe-Cu alloys, Wear, 1981, 69, p. [8] Da Silva W.M., Binder R., De Mello J.D.B., Abrasive wear of steam-treated sintered iron, Wear, 2005, 258, 1-4, [9] De Mello J.D.B., Hutchings I.M., Effect of processing parameters on the surface durability of steam-oxidized sintered iron, Wear, 2001, 250, p. [10] Molinari A., Straffelini G., Surface durability of steam treated sintered iron alloys, Wear, 1995, p. [11] Stephen L,. Feldbauer, Steam Treating; Enhancing the Surface Properties of Metal Components. [12] Straffelini G., Trabucco D., Molinari A., Oxidative wear of heat-treated steels, Wear, 2001, 50, 1-12, p. [13] Razavizadeh K., Davies B.L., Combined Effects of Steam Treatment and Age Hardening on Mechanical Properties of Sintered Fe-Cu Alloys, Powder Metallurgy, 1982, 25, p.