Investigation of the corrosion cracking of steels 12Kh18N10T, 10Kh17N13M3T, and of the imported steel EN58-J under production conditions of primary aliphatic alcohols

Abstract

For the separation of the light fractions in the production of primary aliphatic alcohols (PAA) by direct hydrogenation of methyl ethers, vertical condensers are used whose tube part is made in the form of a bundle of tubes up to 6 m long of corrosion-resistant steel. The operating conditions in the tube space are:temperature at the inlet up to 250~ temperature at the outlet 40~ pressure 32 MPa; the medium is hydrogen (70%), primary alcohols, methanol; in the intertube space: temperature 30~ pressure not more than 0.5 MPa; medium circulating water.Since in the installations for PAA the condenser pipes made of imported steel EN58-J (12.7 • 2 mm in size), which are similar in chemical composition to the Russian steel 08KhI7NI5MZT (GOST 9941-81), had to be replaced, we carried out investigations with the object of finding an analog of steel EN58-J.We chose the Russian steels 12KhI8NIOT and 10KhI7NI3M2T which we investigated in comparison with steel EN58-J.During the investigation we inspected visually the surface of the tube bundle (of steels 12KhI8NIOT and 10KhI7NI3M2T) and of the tubes (steel EN58-J), we studied the microstructure of the metal before and after operation with the aid of optical microscopes Versamet and Neophot 21. The chemical composition and hydrogen content of the steel after operation were analyzed on instruments DFS-36, CS-044, UTNAS-II.The mechanical tensile properties of the material of the tubes were determined on specimens cut out from different parts of the tube bundle, in accordance with GOST 16006-80.The microhardness of the specimens was measured on an instrument Mikromet II.Chemical analysis of the steels (Table i) showed that their composition corresponded to the reference data [i, 2]. Steel EN58-J was investigated after 13 years of operation of the condensers, specimens of steels 12KhlSNIOT and 10KhI7NI3M2T after 9 months of operation.On the inner surface of tubes that had become unserviceable we did not find any traces of corrosion; this is in agreement with the conclusions of [3]. According to the analysis, the outer surface of the tubes at a distance of ~i m from the upper tube sheet was covered with deposits of water hardener (CaCO 3, FeC03, CaSO4).In the circulating water we found chlorine ions (~50 mg/liter) and dissolved oxygen (~10 mg/liter).On the outer surface, after the deposits had been removed, we found isolated pits and fine transverse cracks 5-10 mm long. A complete picture of the corrosion damage to the tubes was obtained in mechanicala tensile tests of tube sections: plastic deformation of the metal was accompanied by opening of the cracks whose surface, in distinction to cracks arising in tension of the material, was dark (oxidized).In Table 2 we compare the results of mechanical tests of specimens of tubes of different steels after operation, cut out of defective tube sections near the upper tube sheet (~0.5 m) as well as of lower tube sections 0.5 m or more from the tube sheet.It follows from the data of Table 2 that the strength and ductile properties of specimen...