Critical Research
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Can corrosion-resistant nickel alloys be used as liners of supercritical water
oxidation reactors?
Nickel magazine, March 18, 2003 -- Members of an international consortium
looking into supercritical water oxidation (SCWO) as a means of destroying organic chemicals
are cautiously optimistic about research at the H. H. Uhlig Corrosion Lab at the Massachusetts
Institute of Technology (MIT).
Preliminary results indicate that SCWO reactors may not have to be lined with
platinum, but instead could possibly be lined with less expensive nickel alloys.
In the SCWO process, hazardous or toxic organic contaminants in aqueous wastes are rapidly oxidized at
temperatures of about 600ºC, pressures of 24 to 30 megapascals, a pH range of 2 to 12, and oxygen
concentration levels ranging from parts-per-million up to percentage levels. Under these conditions, water
acts like a dense gas, becoming very soluble to organic substances and gases like oxygen and nitrogen,
creating a highly corrosive environment. Many organic compounds are completely oxidized in single-phase
reactions to carbon dioxide, water and various acids.
The consortium, which includes of some of the world's largest producers of nickel alloys (including
Special Metals, Haynes, ThyssenKrupp VDM and Mitsubishi) and the Nickel Development Insitute, met recently at
the annual conference of NACE International, which was held in San Diego, California, U.S.A. in March.
The consortium was formed in October 2000 to explore several questions, one of
which is: Can operators control the rate of corrosion of the reactor shell, and thus
control the liner's susceptibility to failure, by controlling the feedstock parameters to a SCWO
reactor?
Although no actual feedstocks will be tested, the researchers want to understand what parameters cause high
rates of corrosion in these alloys. This will allow operators to select the most cost-effective material
based on the type of feedstock.
Using sensors developed at Penn State University, researchers at MIT are testing various
nickel alloys at many different temperatures, oxidizing potentials and pH conditions.
Although limited data have been generated so far, they indicate that nickel alloys may
perform satisfactorily at relatively high oxidizing and pH conditions -- conditions which are
ideal for the destruction of dangerous chemicals inside these reactors.
In the first tests, samples of N06022 wire showed significant "de-alloying" of nickel at a pH less than 2 and no
corrosion at a higher pH of 4.5.
"Initial experiments look promising," says Ronald Latanision, Professor of Materials Science and Engineering
at MIT. "Nickel-chromium alloys have a larger operating 'window' than other alloys, therefore show the most
potential for use in this service."
"Platinum is not the answer for this technology," Latanision says. "It's too expensive."
SCWO will be discussed at a session at the next NACE annual conference, to be held in New Orleans, Louisiana,
U.S.A. from March 28 to April 1, 2004.
 Ronald M. Latanision |
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