LABORATORY STUDIES INVOLVING CORROSION IN COMPLEX, MULTICOMPONENT GASEOUS ENVIRONMENTS AT ELEVATED-TEMPERATURES

The basis for the development of laboratory test programmes to provide data relevant to the selection and design of alloys for use in comple x, multi-component gases, such as those produced during coal gasificat ion, is discussed and some of the advantages and disadvantages are con sidered. The results from recent laboratory studies involving exposure s of alloys in environments of low oxygen and high sulphur partial pre ssures at 300-700 degrees C are reviewed and compared, with emphasis o n non-equilibrium gases that approximate to those of gasifier systems. Conventional Cr2O3-forming alloys develop sulphide scales in this tem perature range, but these do not necessarily lead to unacceptable rate s of metal loss. The corrosion rate is usually determined by the compo sition and integrity of the inner scale layer, and can be very low if the layer consists of an oxide or a mixture of oxides and acceptably l ow if it consists of a refractory metal sulphide or a chromium-rich su lphide, such as FeCr2S4. However, mixed oxides plus sulphides are much less protective, often giving unacceptably high corrosion rates. of o xide-forming elements, particularly silicon or aluminium, to iron (nic kel)chromium-base alloys can facilitate the formation of the oxide lay er, thereby giving improved corrosion resistance.