Pitting of Type 304 Stainless Steel with Hydrogen Evolution as a Cathodic Reaction at Elevated Temperatures

Abstract One of the important problems in determining the usefulness of stainless steels as cladding materials in high-temperature gas-cooled reactors is the effect of chemical reactions between clad and coolant on the cladding strength and ductility. The mechanism by which the coolant (CO2 in this investigation) affects the strength properties of Type 304 stainless steel are being investigated in the range 1300 to 1700 F (704–927 C). Creep- and stress-rupture results obtained on sheet materials in wet and dry CO2 and argon are compared. The effect of annealing in CO2 on the tensile strength and ductility is also reported. The question of whether the strengthening observed in CO2 was due to oxidation or carburization was investigated. Experiments on the effect of various partial pressures of oxygen in argon showed that the creep rate was minimum at approximately 10 ppm. The creep rate in CO2 at equivalent stress and temperature was one-third the minimum rate observed in oxygen. Chemical analyses, metallography, and experiments with isotopic carbon showed that carburization occurred in pure flowing CO2 in the temperature range studied. From this evidence it was concluded that the strengthening observed in CO2 was primarily due to carburization. The creep- and tensile-fracture strains were adversely affected by exposure to CO2 with the magnitude of the effect dependent on the time and temperature of exposure.

Download Full-text

Mechanism of Chloride Stress Corrosion Cracking of Austenitic Stainless Steels

CORROSION ◽

10.5006/0010-9312-25.11.462 ◽

1969 ◽

Vol 25 (11) ◽

pp. 462-472 ◽

Cited By ~ 57

Author(s):

P. R. RHODES

Keyword(s):

Stainless Steel ◽

Crack Propagation ◽

Stress Corrosion ◽

Hydrogen Evolution ◽

Stress Corrosion Cracking ◽

Crack Tip ◽

Corrosion Cracking ◽

304 Stainless Steel ◽

Type 304 ◽

Type 304 Stainless Steel

Abstract Electrochemical studies were made in aqueous LiCl, MgCl2, and MgBr2 solutions and in ZnCl2/KCl molten salt to clarify the corrosion reactions related to stress corrosion cracking (SCC) of austenitic stainless steel and to better define environmental variables critical to the occurrence of chloride SCC. Type 304 stainless steel electrodes were employed, with complementary SCC tests made with U-bend Type 304 stainless steel specimens. Several conclusions critical to an understanding of the mechanism of chloride SCC resulted from these investigations: (1) SCC was observed in concentrated MgBr2 solutions, (2) H2O must be present in the electrolyte, as SCC did not occur in dry molten ZnCl2/KCl, and (3) H2 evolution from corroding specimens may be facilitated by anodic polarization. Present studies do not support a model equating crack propagation with stress assisted anodic dissolution. Rather, evidence is presented that hydrogen evolution at the crack tip occurs and is a critical precursor to crack initiation and propagation. A model of SCC requiring hydrogen evolution at the crack tip is proposed, with emphasis being placed on the effect of anodic reactions within the crack in maintaining high acidity near the crack tip. Recent publications suggest that the role of evolved hydrogen in SCC may be related to formation of hydrogen induced martensitic platelets along paths of crack propagation.

Download Full-text