Pertti Aaltonen

Metallurgical effects on the corrosion resistance of a low temperature sensitized welded AISI type 304 stainless steel

Abstract The low temperature sensitization (LTS) of a welded AISI Type 304 stainless steel was studied. Both the microstructural changes and the degree of sensitization (DOS) were examined in samples aged for times up to 1500 days at 350–500°C. The DOS was observed to increase at temperatures below the normal sensitization range and the increase could be attributed to the growth of chromium-rich grain boundary carbides. The (T, r) dependence for LTS was determined on the basis of intergranular corrosion (I.G.C.) tests. These results indicated a departure from a linear Arrhenius temperature dependence, suggesting a change in the growth mechanism of M23C6 grain boundary carbides at low temperatures. The possible reasons for this change are discussed in terms of the observed microstructures.

Evaluation of the EPR technique for measuring sensitization in type 304 stainless steel

Abstract The electrochemical potentiodynamic reactivation (EPR) technique has been used to measure the degree of sensitization in type 304 (UNS S30400) stainless steel with 0.052% C. In the first p...

Study of Snoek-type relaxation in hydrogenated Inconel 600

Abstract Anelastic behaviour was studied in hydrogenated Inconel 600 alloy. Two complex peaks were detected in the temperature range 80–500 K. The low-temperature peak (100–270 K) was recognized as a hydrogen-induced Snoek-like relaxation peak, while the high-temperature peak may be caused by some transient processes. The low-temperature peak is attributed to the transition of hydrogen atoms between the energetically equivalent octahedral sites, which defines the fine structure of the peak. Non-monotonic dependence of the peak height on the charging current density indicates the possible formation of hydride phases.

Electrochemical methods for testing the intergranular corrosion susceptibility of stainless steels

Abstract In addition to the standardized intergranular corrosion testing methods to detect sensitization in austenitic stainless steels, electrochemical test methods, i.e. the potentiokinetic reactivation test (EPR) and the anodic polarization and constant potential etching methods in perchloric acid solution containing sodium chloride, have been developed. The results of these electrochemical measurements are compared with those obtained from copper sulfate-sulfuric acid tests.

Internal Friction Study of Environmental Effects on Metals and Alloys

Abstract The internal friction method was applied for studying structural changes occurring in bulk metal due to its interaction with the environment. It was shown that the active dissolution of metal or its oxidation is accompanied by the generation of non-equilibrium vacancies. Diffusive transfer of vacancies into the bulk causes considerable rearrangements of the defect structure of the crystal and, therefore, changes in its anelastic response. Environmental effects have been studied in pure copper subjected to oxidation in a 0.3 N solution of NaNO 2 . Such an interaction resulted in the formation of Hasiguti peaks. A similar effect was found in admiralty brass after oxidation in hot tap water, resulting in selective dissolution of zinc. Low-temperature internal friction peaks were also found in the range 150–200 K in Al–5 Mg industrial alloy subjected to active dissolution in 0.05 N KOH solution. The results obtained are discussed in terms of the applicability of the internal friction method for predicting the susceptibility of metals and alloys to environmentally assisted cracking.

Low-frequency internal friction of pure copper after anodic polarisation in sodium nitrite solution

Abstract Low-frequency internal friction (IF) of pure copper after anodic polarisation (AP) in passivating sodium nitrite solution was studied. In the temperature range of 100–300 K a set of peaks was detected and they were recognised as the Hasiguti peaks. Excessive vacancy generation at the Cu\Cu 2 O interface was the reason for these peaks. It was assumed that excessive vacancies diffuse into the bulk metal and osmotic stress in copper caused by them results in dislocation multiplication and rearrangement of dislocation structure.

Cyclic crack growth evaluation of a 20MnMoNi55 piping steel in high-oxygen reactor water

Abstract Samples of a low alloy steel piping material taken from the full scale corrosion fatigue test loop of the Heissdampfreaktor (HDR) plant have been tested at 240°C in high oxygen reactor water. The small-scale specimens (CT25) were exposed to a similar loading spectrum to that which has been used in the full-scale corrosion fatigue tests at the HDR-plant. During the autoclave tests cyclic crack growth rates were determined. Fracture surface investigations were performed not only for the laboratory test specimens but also for the fracture surface of a sample taken from the HDR test loop piping after the full scale test. In this paper the autoclave testing results and fracture surface observations are presented and compared to those obtained in the HDR piping tests.

Stress corrosion and thermal fatigue — Experiences and countermeasures in austenitic SS pipings of finnish bwr-plants

Abstract A summary of the existence of pipe cracking in Finnish BWR plants is presented covering both thermal fatigue and IGSCC cases. Countermeasures against cracking are evaluated and the measures applied are summarized. Also the results of a research program to monitor ageing of the weld heat affected zones in a pipeline section of a shut-down cooling system are summarized.

Characterization of Austenitic Stainless Steel Pipe Welds After Prolonged Power Plant Exposure

Abstract An austenitic stainless steel (AISI 304) pipe weldment, which had been in operation about 45,000 h at 230°C in the Ringhals 1 BWR plant, was characterized using electrochemical sensitization testing methods (electrochemical potentiokinetic reactivation [EPR] and double-loop EPR), as well as transmission electron microscopy. The weld heat-affected zone (HAZ) showed a sensitized zone between two and five mm from the fusion line in which grain boundary carbides formed almost a continuous chain, and Cr-depletion was evident.

Overview of the nuclear power plant structural integrity research in Finland

Abstract An overview of the Finnish programmes concerning safety related research in pressure boundary components of nuclear power plants is presented. The focus is on recent results in the areas of fracture mechanics, large scale component tests, irradiation damage, environmental influence on materials as well as nondestructive testing (NDT).