材料期刊网

The corrosion of Q235 and 5Crl/2Mo steels in synthetic refining media containing naphthenic acid and/or sulphur compounds was studied to evaluate naphthenic acid corrosion (NAC), sulphidic corrosion (SC), and their interaction. Corrosion dependencies on the test duration, temperature, total acid number (TAN) and content of sulphur compound were assessed. Specimens after NAC and SC tests were characterized by SEM/EDX, and XRD. It is found that in liquid phase of media containing only naphthenic acid and at temperature about 230 degrees C, 5Crl/2Mo and Q235 steels have almost the same NAC rate, and above 230 degrees C, 5Crl/2Mo has a higher NAC rate than Q235 has due to the higher NAC activation energy (63.2 kJ mol(-1)) of 5Crl/2Mo than that of Q235 (54.0 U mol(-1)), indicating that increasing temperature accelerates NAC rate of 5Crl/2Mo more than that of Q235. In oil containing only dimethyl disulphide, the growth of SC film follows parabolic kinetics, and the film of Q235 grows faster than that of 5Crl/2Mo while SC rate of Q235 is higher than that of 5Crl/2Mo. In oil containing both naphthenic acid and dimethyl disulphide, 5Crl/2Mo has a lower corrosion rate than Q235 has. On the basis of "naphthenic acid corrosion index" (NACI), the benefits of 5Crl/2Mo over Q235 should ascribe to that the pseudo-passive film for 5Crl/2Mo has better NAC resistance than that for Q235. This is close related to the existing of additional chromium sulphide (Cr5S8) on the pseudo-passive film of 5Crl/2Mo, in contrast with the pyrrhotite (Fe7S8) and troilite (FeS) on the film of Q235. (c) 2005 Elsevier Ltd. All rights reserved.