ABSTRACT:

It is generally known that the production route influences the sensitivity of a material to metal dusting. In this study the different production states of UNS N06025 are compared as there are: plate deeply ground and 600 grit laboratory ground, strip as-delivered and 600 grit laboratory ground, and longitudinally welded tubes as-delivered and 600 grit laboratory ground. All samples were exposed for 7750 hours at 600°C in a 37% CO, 7% CO2, 46% H2, 9% H2O atmosphere, Ptot = 20 bar. The differences between the samples originating from different production routes, in terms of mass loss, pit density, pit size and pit depth, are discussed in terms of the effect of surface treatment (ground, brushed, annealed, pickled), grain size, chemical composition and sample thickness.

INTRODUCTION

Metal dusting is a high temperature corrosion damage for alloys used in hydrogen, ammonia, methanol and gas-to-liquids production plants.1 - 7 It has been known for more than 50 years and is observed when iron, nickel or cobalt alloys are exposed to carbon producing atmospheres (i.e. mixtures of CO, hydrogen, water and CO2) typically between 500 and 750°C. CO from the gas atmosphere reacts at the metallic surface to form atomic carbon, which diffuses into the substrate. The metallic substrate supersaturates in carbon and therefore decomposes into a mixture of graphite, oxidic, carbidic, and metallic particles (“metal dust”).8 - 24 This leads to the formation of pits, and possibly loss of containment.4 - 7 Metal dusting corrosion and its prevention have been studied extensively. Additionally there are influences of surface treatments on metal dusting corrosion such as pickling, grinding, machining and grain size 9 - 12, 33, 34 and results for spray and diffusion coatings or alternative coating solutions not based on oxides are also known.

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