The oxidation characteristics of Ti-14Al-21Nb (mass%) were studied in a temperature range 1000K to 1300K in O2, N2-21 vol%O2 and Ar-21 vol%O2 under atmospheric pressure. The oxidation rates in N2-O2 are lower than those in O2. The oxidation rates in Ar-O2 are almost equal to those in N2-O2 below 1200K. However, at 1300K the oxidation rate in Ar-O2 is nearly equal to that in O2. The oxidation curves are almost parabolic, except for those in O2 and Ar-O2 at 1300K, for which temporal acceleration was observed. The scale consists of three layers: outermost layer of sintered rutile grains, intermediate alumina rich layer, and innermost layer consisting of a mixture of rutile, alumina and niobium oxide grains. In N2-O2, a nitride layer (TiN), an Nb-rich layer (Nb2Al) and an Al-rich layer (TiAl) are additionally formed in this order beneath the three layered scale. These differences in the oxidation characteristics are attributable to the folowing observations; 1. A more continuous alumina rich layer is formed because of the reduced oxygen pressure below 1200K. 2. A continuous TiN layer is formed near the scale/substrate interface. 3. By the selective nitridation of Ti near the scale/substrate interface, Al is enriched beneath TiN, and TiAl is formed.
Transition mode of hydrogen sulfide cracking from hydrogen induced cracking (HIC) to sulfide stress corrosion cracking (SSCC) types has been investigated as a function of the ratio (σ/σ0.2) of applied stress to proof stress. The tensile specimens of SS 400 carbon steel have been used for sulfide stress corrosion (SSC) testing in accordance with the NACE TM 0177 standard in 5 mass% NaCl solution containing saturated hydrogen sulfide for 720h with/without various applied stress ratio. After the tensile test of the SSC specimens, the observation of the number of cracks, crack angles, crack arrangement and hydrogen content has been carried out against the various values of σ/σ0.2. The SSCC susceptibility increases with increasing in applied stress ratio from 0.1 and agrees with the stress ratio dependence of crack numbers and hydrogen contents. Mode transition from the HIC to SSCC types takes place beyond 0.35 in σ/σ0.2. As the applied stress ratio goes up to over this critical value, the crack angles over 45 degree appear in addition to lower angles, and crack arrangement ratio of interval between two cracks to the crack length exceeds 0.12 which value brings about the change from coalescence to no-coalescence of adjacent cracks. It has become apparent that the evaluation of the mode transition from the HIC to SSCC types in hydrogen sulfide cracking of carbon steel is possible from the observation of applied stress ratio, crack angles and crack arrangement ratio.
Corrosion performance of type 304 ULC stainless steel in plutonium nitrate solution was experimentally studied. Corrosion rate of 304 ULC stainless steel was accelerated by co-existent plutonium in nitric acid solution. It is caused by trans-passive dissolution based on the shift of corrosion potential to the noble position. It is thought that potential shift is caused by increase of the cathodic current on the 304 ULC surface due to the reduction of Pu(VI) to Pu(IV). It is suggested that the Pu(IV) which is reduced on 304 ULC surface would be re-oxidated to Pu(VI) by HNO3 at bulk in the solution. Redox potentials of Pu(VI)/Pu(IV) and HNO3/HNO2 which are introduced by calculation from thermodynamic data support the possibility that reaction at high temperature.
Transient behaviors of corrosion current of Type 316 stainless steels during a slow strain rate test (SSRT) in high-purity oxygenated water at 288°C were investigated and their correlation with stress corrosion cracking (SCC) was discussed. Corrosion current at corrosion potential between a specimen under slow-rate straining and a counter electrode which was made of the identical alloy to the specimen was continuously monitored using a zero resistance ammeter. Spike-like current pulses were observed after yielding. The current pulses had short rise-time, typically a few seconds, and slow decay behavior, 100-1000 seconds; This characteristic was considered to correspond to the slip dissolution processes. Although the current pulses were observed both on the solution treated material and on the sensitized one, characteristics of the current pulses reflected well the difference in SCC susceptibility of the samples. Current pulses from the sensitized material had higher peak and slower decay rate than those from the solution treated one. Capability of the current noise monitoring for detecting SCC initiation in austenitic stainless steels in high-temperature high-pressure water was demonstrated and it was suggested that the current noise can provide real-time evaluation of SCC behavior.
Chemical adsorption and the corrosion inhibition effect of inhibitors on metals and oxidized metals are closely related to the hard and soft acids and bases principle. A stable adsorption bond forms between the metal surface acting as a soft acid and an inhibitor classified as a soft base, resulting in a high inhibition effect on metallic corrosion. An inhibitor of a hard base is adsorbed on the oxidized metal surface acting as a hard acid without difficulty.