Corrosion of Steels and Nickel Alloys in Superheated Steam

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Abstract
The corrosion behavior of the 18‐8 stainless steels and some nickel alloys in superheated steam was found to be dependent on the surface preparation. Those treatments which left a severely cold worked surface resulted in reduced corrosion attack in subsequent exposure to steam as compared with annealed metal surfaces. The corrosion rate of electropolished annealed type 304 stainless steel in oxygenated (30 ppm) steam rose rapidly as the temperature increased from 540° to 650°C. At 600° and 650° the measured rate decreased with increasing time during 80 day static tests. The corrosion appeared more linear with time in dynamic tests at 650°C (∼7 mg/dm2‐day). In these tests there appeared to be no effect of velocity (30–91 m/sec) or oxygen and hydrogen content on the corrosion rate. Flaking and loss of the outer corrosion coating was severe in the dynamic tests. The corrosion rate of type 406 was too small to be precisely measured in all of the tests, static and dynamic, in which it was simultaneously exposed with type 304. A coating equivalent to about metal penetration was formed on the initial exposure of 406. No flaking of the outer coating was evident in 80 day tests. The corrosion attack of the 400 series steels in static oxygenated steam went down with increasing chromium content, particularly above about 13 w/o Cr. Both type 430 (16 w/o Cr) and 446 (25 w/o Cr) exhibited greater short term corrosion resistance than type 304 to 650 °C, 42 kg/cm2 steam. The nickel based alloys, particularly Inconel 625, were also more corrosion resistant than type 304 in short static and dynamic tests in 650 °C, 42 kg/cm2, superheated steam.

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