Yuzo Daigo

Effect of Chromium Ion from Autoclave Material on Corrosion Behavior of Nickel-Based Alloys in Supercritical Water

Abstract Seven kinds of corrosion tests were carried out on thirteen kinds of nickel-based alloys in subcritical and supercritical water containing 0.01 mol/kg sulfuric acid (H2SO4) and 800 ppm oxygen (O2) using a Ni-44Cr-1Mo autoclave. The obtained corrosion rates were significantly lower than the previous experiments performed under the same conditions using a titanium autoclave. A compact oxide scale was found on the specimens tested in the Ni-44Cr-1Mo autoclave. The chromium-rich oxide layer would originate from the autoclave material and provide a protective function against corrosion. The corrosion retardation phenomenon has been explained by dependencies of chromia (Cr2O3) solubility on the phase state of water and temperature.

Corrosion Mitigation in Supercritical Water with Chromium Ion

Abstract It was reported that the chromium ion originated from the autoclave material (Ni-44Cr-1Mo) deposited on the specimen surface as chromium oxyhydroxide (CrOOH), and the protective chromium oxide layer mitigated corrosion of the alloys tested in supercritical water containing sulfuric acid (H2SO4). Based on the report, three kinds of tests were conducted at 400°C and 30 MPa in 0.01 mol/kg H2SO4 aqueous solution containing Cr3+ or Cr6+. Corrosion resistance of the specimens became 1.5 ∼ 7.5 times better than specimens without chromium ions, because chromium ions from the aqueous solution was incorporated into the protective chromium oxide on the specimen. It was revealed that reduction of Cr6+ to Cr2O3 (Cr3+) occurred on the specimen surface and in the autoclave. The chemical equations of chromium oxide or chromium oxyhydroxide scale formed on the specimen surface were evaluated.

Compatibility of Nickel-Based Alloys with Supercritical Water Applications: Aging Effects on Corrosion Resistance and Mechanical Properties

Abstract Corrosion-resistant Ni-based alloys are thought to be strong candidates as a structural material for chemical processes using supercritical water (SCW). In recent years, Cr was considered as the most important alloying element to improve corrosion resistance in SCW. However, high Cr content can cause a phase transformation in Ni-based alloys because of poor phase stability at elevated temperatures for SCW processes from 400°C up to 650°C. The change in the microstructure of the alloy can lead to degradation in corrosion resistance and mechanical properties. In this study, thermal aging effects on corrosion resistance and mechanical properties of various Ni-based alloys were investigated through a corrosion test in SCW containing 0.01 mol/kg sulfuric acid (H2SO4) at 400°C/30 MPa and using Charpy impact tests. A reduction in impact toughness was observed for all the alloys tested, and the reduction in corrosion resistance was more significant for alloys with higher Cr content. The importance of sus...

Corrosion Rate and Oxide Scale Characteristics of Austenitic Alloys in Supercritical Water

Corrosion properties were investigated on stainless steels and Ni base alloys in supercritical water containing 0.01mol/kg-H2SO4 at 400°C/30MPa as a function of oxygen concentration ranging from 3ppb to 800ppm. Alloys with high Fe content showed an interesting contrast in corrosion property in sulfuric acid-containing supercritical water between at the high oxygen condition (800ppm) and at the lower oxygen conditions (3ppb and 8ppm). At 8ppm of oxygen concentration or lower, corrosion rate was a unique function of Cr content of the alloys including both Ni base alloys and stainless steels. However, corrosion resistance of the iron-based alloys (316 stainless steels) was remarkably improved when oxygen concentration was increased up to 800ppm. Corrosion rate of alloy C-276, which contains 5.7% of iron, was also somewhat reduced under the high oxygen condition, while corrosion rate of the other “iron-less” Ni base alloys was accelerated as oxygen level was increased. Characteristics of oxide scales, in terms of chemical composition and compound structure, have been examined in connection with the corrosion properties of the alloys.