Estudo da difusibilidade e solubilidade do hidrogênio no aço inoxidável bifásico martensítico-ferrítico

Abstract

The development of the oil and gas industry demands materials capable of performing in sour environments with low pH, presence of chlorides, pressure of H2S and protonic hydrogen. They must be resistant to corrosion and embrittlement by hydrogen. The ferritic-martensitic stainless steel was developed from the supermartensitic steel, with addition of chromium (14 Cr), resulting in islands of delta ferrite arranged in the martensitic matrix, which provides greater resistance to corrosion. This work investigates the heterogeneity of the biphasic microstructure. It presents difficulty to be evaluated in fine samples, considering that these may be less representative. For this reason, the present work aims to study the "alloy architecture" - considering the % of the area and the morphology of the delta ferrite in the martensitic matrix - to correlate it with the diffusivity and the solubility of the hydrogen in steel. To achieve these objectives, a microstructural analysis was performed by SEM and OM; tests of Thermal Desorption Spectroscopy and hydrogen electrochemical permeation were also conducted. The electrochemical tests were performed on fine samples of about 0.5 mm at room temperature. The methodology consisted in obtaining the % of the ferrite area in the microstructure of the samples, from the micrographs with the aid of Image J. The TDS indicate the trap sites present in the sample, which could interfere in the diffusion of the hydrogen. The electrochemical permeation tests in samples with different amounts of ferrite revealed the influence of this phase on the acceleration of the diffusion of hydrogen in the biphasic microstructure. Based on the comparison of the theoretical values with the values obtained in the tests, it was discussed how the amount and distribution of the ferrite affect the results obtained for the steel parameters.