Surface oxidation of nickel base alloys and stainless steel under pure oxygen atmosphere: Application to oxygen safety

Abstract

Irradiation by a high power laser beam has been used since several years as an innovative approach for evaluating the reactivity of metallic materials with a pure oxygen atmosphere. Indeed, materials selection for oxygen service is a key factor of mitigating the ignition of equipment also called as oxygen fire. Pulled by the need having always more efficient industrial processes, the oxygen usages are evolving toward steadily higher pressures or temperatures. In such operating conditions, the evaluation of the metallic materials resistance to the oxygen fire ignition and their property to propagate based on the oxidation kinetic is the only way to avoid the oversizing of equipment. In this context, the use of laser in evaluating the reactivity of metallic materials is a well-adapted method enabling the accurate control of the energy deposition transmitted to the tested material during oxidation as well as giving access to the energy threshold required to trigger the oxygen fire ignition. This paper investigates the oxidation kinetics on nickel-based alloys (Inconels 600 and 625) and on stainless steel 310, using a laser or a furnace preheating on small metallic parts. The scope of the study is the oxidation step taking place before the heating of the sample up to its ignition temperature. Process parameters are oxygen pressure (1 and 40 barg), oxygen content (air versus pure oxygen), duration, and temperature. A numerical model is used to couple a sample surface condition (machined, sandblasted, polished) with laser parameters (power, duration) needed to reach a predetermined preheating temperature of the sample. In order to determine the threshold heat input for materials' ignition by a laser in the oxygen atmosphere, it is necessary to know the absorption coefficient of the laser at the sample surface. This parameter is measured for various oxide layer thicknesses.