Critical Research

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 Professor of Materials Science and Engineering Professor of Nuclear Engineering Director of The H.H. Uhlig Corrosion Laboratory MIT, Room 8-202 Cambridge, MA 02139-4307 U.S.A. Tel: 1-617-253-4697 Fax: 1-617-253-8745 E-mail: lats@mit.edu

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