THE CORROSION-RESISTANT PROPERTIES OF X33CrNiMn23-8 FOR HIGH-THERMAL LOADED ENGINE OUTLET VALVES. Krzysztof Adamaszek a Zbigniew Jurasz a

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1 THE CORROSION-RESISTANT PROPERTIES OF X33CrNiMn23-8 FOR HIGH-THERMAL LOADED ENGINE OUTLET VALVES Krzysztof Adamaszek a Zbigniew Jurasz a a Automotive Research and Development Center Bosmal, Bielsko-Biała, PL, ul. Sarni Stok 93, s: krzysztof.adamaszek@bosmal.com.pl., zbigniew.jurasz@bosmal.com.pl Abstract: Two different experimental methods to the mathematical descriptions of the kinetics of high temperature oxidation of steel (X33CrNiMn23-8) are presented. In the first method microthermogravimetric technique has been used, whereas the second used results of measurements of thickness oxidised after period of time oxidation (discontinuous method). It has been found that the slowest step of the overall reaction rate is the diffusional transport of matter through the scale layer, build of Fe 3 O 4 and Cr 2 O 3 oxides, the latter oxide forming the barrier layer at the metal/scale interface. The activation energy of the reaction determined by first method equals about 200 kj/mol is comparable with that of pure chromium oxidation. The activation energy determined by second method equals about 170 kj/mol. Results also contain morphology of scale with their phase composition. 1. INTRODUCTION Mechanism of high-temperature corrosion of steel applied on outlet valves is unusual complexes especially in high-pressure combustion engines [1, 2]. In this work preliminary results of corrosion-resistance properties of X33CrNiMn23-8 valve steel applied in combustion engine industry. The research was carried out in air. 2. EXPERIMENT AND METHODS Table 1. The chemical composition of oxidised austenitic steel Grade of steel Chemical composition, % C Mn Si Cr Ni Mo W P S X33CrNiMn Kinetics of oxidation was measured by two methods. In the first method weight gain of oxidised specimen was measured continuously by using microthermogravimetric technique, whereas in the second method growth of the thickness of layers was measured discontinuously after period of time oxidation given in table 2. The specimens in first method have had disc shape with 20 mm diameter and thickness c.a. 1 mm. Gain of weight was recorded automatically with accuracy 10-6 g in function of time oxidation at synthetic air flow condition, contains 21% O 2, 79% N 2 through working chamber of apparatus at one followed constant temperature 873, 1073 and 1373K for 100 h. In the second method the series of specimens (5 for each of one of mentioned below temperature) have had cylindrical shape with 30 mm diameter and length l = 40mm and were oxidised in electrical chamber furnace at followed constant temperature 1213, 1253, 1293, 1333 and 1373 K for respectively h, 646 h, 230h, 181 h, 101h. 1

2 3. DISCUSSION OF RESULTS 3.1. Determination of kinetic oxidation by continuos method On the basis of obtained microgravimetric results can be stated, that slowest process, which determined the rate of corrosion process of X33CrNiMn23-8 steel is reagents diffusion. The growth of compacted scale conducted acc. to parabolic law (fig.1): 1.0,. p O2 = 2, Pa 1173 K m/s / mgcm , 1073 K 873 K 0.0, czas / h Fig.1. Kinetics of oxidation of X33CrNiMn23-8 steel at several temperatures in parabolic system [4]. The figure 2 shows that parabolic constants in function of reciprocal temperature has linear dependence, which proved that mechanism of diffusion reaction is changeless. This dependence allow to determined activation energy c.a. 200kJ/mol (fig.2) T / K k p / g 2 cm -4 s p O2 = 2, Pa E a = 200 kj/mol T / K -1 Fig.2. Temperature dependence of oxidation of X33CrNiMn23-8 steel [4] 2

3 3.2. Determination of kinetic oxidation by discontinuous method Kinetics of oxidation of X33CrNiMn23-8 steel was also determined by measurement of thickness scale after period of time oxidation (discontinuous method). The specimens have cylindrical shape with diameter 30 mm and length 40mm, and were oxidised in electrical chamber furnace at temperatures for times given in table 2. In the same table are obtained results of thickness of scales. The measurements of thickness of scale was carried out by means of micrometer with accuracy 0.01 mm. Table 2. Increase of thickness of scale l oxidation t of steel X33CrNiMn23-8 in function of time Temperature, K thickness of scale l, µm time of oxidation t, µmin Rate of oxidation was determined by using followed formula on parabolic constants k p = l / t [µm/min -1/2 ]: (1) 1213 K 1950 k p = = µm/min -1/2, K 2400 k p = = µm/min -1/2, K 3060 k p = = µm/min -1/2, 1333 K 3900 k p = = µm/min -1/2, 1373 K 5490 k p = = µm/min -1/ The logarithms of obtained values (0.8432, ; ; and µm/min -1/2 ) in function of reciprocal temperature has linear dependence (fig.3). 3

4 2.00 lgk'p [um/ min -1/2] E a =170 kj/mol /T [1/K] Fig.3. Logarithms of parabolic constant k p [µm/min -1/2 ] in function of reciprocal temperature. Steel X33CrNiMn23-8 On the basis of linear regression analysis of obtained logarithms of parabolic constant k p values in function of temperature was calculated parameter B = , which allow to estimate activation energy from followed equation: E a = B 1000 = kj/mol (2) 3.3. Morphological study The obtained values of parabolic constants allow to concluded that good protected properties of scale follow from compacted oxide layer of Cr 2 O 3 arisen on surfaces of tested steel [5, 6]. This conclusion confirm results of phase analysis, which reveals that scale has two-phases i.e. internal Cr 2 O 3 adherents to steel base, and external Fe 3 O 4. The observation of oxides by means scanning microscope reveals that scales are without cracks, micro-porosity and its crystals are bigger with increasing of temperature (fig.4). 4

5 Fig.4. Crystals of oxides of X33CrNiMn23-8 steel oxidised at T=1173K, p(o 2 ) = 2, Pa. 4. SUMMARY AND CONCLUSION In this work we measured the kinetic of high-temperature oxidation X33CrNiMn23-8 austenitic steel by two methods. Annealing was done in the range from 1073 to 1373K in air. For each method we prepared series of specimens with different shapes i.e. of disc and cylinder. In the first method gain of weight in function of time oxidation was measured, whereas in the second growth of thickness of layer. Although applied specimens and measured technique completely differs the obtained values of energy activation are comparable (difference c.a. 15%). On the basis of literature data for pure chromium [6] we concluded the obtained value of energy activation in the first method (i.e. 200 kj/mol) better describes kinetics oxidation of tested steel. The compactness of scale consisted of oxides Fe 2 O 3 and Cr 2 O 3 influences on rate oxidation because diffusion reaction process was limited, so corrosion course slower, than for constructional carbon steel. BIBLIOGRAPHY 1. BERNHARD M., DOBRZYŃSKI S., LOTH E.,: Silniki spalinowe, WKiŁ, Warszawa 1988, 52 p. 2. MERKISZ J.,: Wpływ motoryzacji na skażenie środowiska naturalnego, Wyd. Politechniki Poznańskiej, nr 1715, Poznań 1993, 22 p. 3. MROWEC S, Z.GRZESIK, J.: Phys. Chem. Solids, 2004, 65, pages JURASZ Z. and etc.: The heat-proof properties of X33CrNiMn23-8 steel for valves in car engines, Proceedings of the 11-th All-Polish Corrosion Symposium, Ochrona przed korozją, 2005, no.11s/a, pages KOFSTAD P.: High Temperature Corrosion, Elsevier Applied Science Publishers Ltd., London and New York, 1988, 390 p. 6. KRISZTAL M.A.: Diffusion Processes in Iron-based Alloys (in Russian), GJNTI, Moskva, 1963, p