Incoloy Alloy 800: steam corrosion experiments under heat transfer and isothermal steam corrosion testing of various superheat materials in a high pressure loop. Final report

In order to investigate superheated steam corrosion of superheated materials at high heat fluxes, a high-pressure steam loop was constructed and operated over a period of 2 years. During this time thin-walled Incoloy 800 pipe material in sealed bonding to insulated resistance heaters was subjected to steam corrosion at temperatures of 4500 to 7500C and heat fluxes of 200 watts/cm2. Under the selected conditions of exposure and exposure time no corrosion-intensifying effect was produced by the special situation of a thermally highly stressed enveloping pipe in a stepped-up, fast steam flow. Results of a number of investigations are describable by parabolic time laws and may be extrapolated to longer use times. However, allowances must be made for the unreliability of growth stresses in the metal/oxide system resulting from increasing oxide layer thickness and cyclic stresses. Cold forming operations on the surface increased the corrosion resistance of the enveloping pipes considerably. The superheated steam corrosion of the isothermally exposed sheet metal samples exhibited scarcely any corrosion-intensifying effect of dynamic high-pressure steam exposure. Coarse-grained sheet metal samples of austenitic CrNi steels oxidized substantially more than fine-grained samples while cold-worked surfaces or structures displayed a remarkable long-term stability. Excessive hydrazine additions to the feed water damaged all worked CrNi steel samples and all versions of the nickel-base alloys as a result of nickel reduction accompanied by cancellation of the protective character of the otherwise very stable Ni--Cr spinel layer. (auth)