The influence of the ferritic and austenitic phases and of silicon on the Corrosion resistance of alloyed cast steels

The paper deals with the influence of the α/gamma; phase ratio and silicon on the corrosion resistance of low carbon alloyed cast steels in strongly oxidizing media. Electrochemical investigations in 65% HNO₃ + 5g Cr⁶⁺ ions/I showed that none of the four tested cast steels, with γ_R = 100%, γ_R = 75%, γ_R = 50% and γ_R = 0%, achieved a passive state, and that the corrosion process was under cathodic control. Under such conditions the calhodic activation overpotential is considerably greater than the anodic activation overpotential, and, as well as this, the Tafel coefficient for a cathodic reaction has higher values than those which are characteristic for an anodic reaction. The austenitic cast steel is the most resistant to corrosion, since for the corrosion process to take place the greatest activation energy is needed. The results of investigations of corrosion resistance in 12M HNO₃, at 115°C, for 240 hours, confirmed the results of the electrochemical measurements and the predominant role of the austenitic cast steel with γ_R = 100%. By means of AES analysis it was found that the good resistance to corrosion of such a cast steel is based on the building up of a thin mechanically-protecting barrier, which mainly consists of silicon.     

Stainless steel containers for the storage of low and medium level radioactive waste

Corrosion investigations have been performed on the austenitic structural steel AISI 304L, in comparison with the structural steel AISI 316L, in an aerated and a de-aerated solution, which was leached from low and medium level radioactive waste. On the basis of measured potentio-dynamic anodic polarization curves and the results of cyclic polarization tests, it was found that both types of steel, as well as the corresponding welds, had a high pitting potential and a high protective potential, which means that they have a strong tendency to form a compact and corrosion-resistant passive film. The repassivation capability of both types of steel prevents the occurrence of stress corrosion cracking at the level of concentration of chloride ions which corresponds to the described type of waste, whereas absorbed atomic hydrogen does not reduce toughness or cause hydrogen embrittlement. The results of the research work confirmed that it is possible to use AISI 304L structural steel for the construction of containers for the temporary, 30-year storage of low and medium level radioactive waste.     

The susceptibility to sulphide stress cracking of low alloy steels

The susceptibility to sulphide stress cracking (SSC) of low alloy Mn-V and Cr-Mo-Nb steels in standard NACE solution was determined. The critical stress (S_c) and threshold stress (σ_th) of steels were determined in both as-received and heat treated condition. In the as-received state the steel shows a highly susceptibility to SSC dominantly due to its microstucture. By quenching and tempering of steels at high temperature the SSC resistance was increased. It was found that the susceptibility to SSC for Cr-Mo-Nb steel is lower than that of Mn-V steel at a comparable yield strength. It was explained by the presence of different second phases in the ferrite matrix and spheroidized carbide particles resulting from the higher tempering temperature. In addition to Fe₃C, the presence of Cr₃C₂, Mo₂C and NbC in the ferrite matrix of Cr-Mo-Nb steel play an important role in their resistance to SSC. In all cases tested was S_c>σ_th which suggests of the importance of plastic deformation during the test. The microfractographic analysis of specimens before and after SSC testing have been done. Occurrence of cracks in the near outer surface of non-failed tensile specimens during 720 hours test suggested that SSC could be explained as a combination of hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC).     

Investigations into the stress corrosion cracking of stainless steel alloyed with nitrogen

The resistance to stress corrosion of austenitic stainless steel alloyed with nitrogen (ASS N25) was determined in comparison with AISI 316L steel. The research into stress corrosion cracking was performed by using corrosion elongation curves in a 50% CaCl₂ solution, at 100°C, under axial tensile loadings defined as various percentages of Rp_0,2, and with anodic polarization at a current density of 1.0 mA/cm². The increased resistance of the nitrogen‐alloyed steel to stress corrosion is based on the occurrence of NH₄⁺ ions, and the repulsive action of the negatively charged nitrogen atoms towards the Cl⁻ ions on the interface between the passive layer and the corrosion medium. Additionally, the interstitially dissolved nitrogen is able to accelerate the local deformation hardening of the material at the crack tip, for which reason the critical stress needed for stress corrosion to occur, or for further growth of the crack if it has already occurred, is increased.     

Hydrogen embrittlement of Microalloyed structural steels

The effect of Cr, Mo, V, Nb, Mn, their carbides and different microstructures on the corrosion and hydrogen uptake of two fine-grained microalloyed steel grades has been investigated. In practice, fine-grained structural steels appear to be more or less susceptible to hydrogen induced cracking, probably because of various strong interactions between the absorbed hydrogen and the various alloying elements or their carbides, acting as traps. The effect of microstructure was examined by cathodic polarization, by measuring the reduction of area, and by SEM examination of the fracture surface. In addition, hydrogen permeation transients for one microalloyed steel grade and Armco iron were measured, and the resistance of both steel grades to sulphide stress-cracking according to the NACE Standard TM-01-77 was also determined.     

Prediction of the remaining lifetime of stainless steels under conditions of stress corrosion cracking

The prediction of the lifetime of metal structures and equipment under conditions of stress corrosion is very complicated because of the complexity of this process of degradation. Recently a new method, based on the so‐called corrosion elongation curves, has been found, which can be used to predict the time to failure under these conditions. By upgrading of these curves (and thus obtaining Upgraded Corrosion Elongation Curves – UCEC's) it has been possible to obtain a precise definition of the time needed for the initiation of the corrosion crack, and for its stable growth. It is upon this basis that diagrams for the prediction of remaining lifetime (DPRL's) have been developed. DPRL's can also be used to predict the values of various critical parameters which have to be achieved if a stress corrosion crack is to occur.     

The role of social support networks in proxy Internet use from the intergenerational solidarity perspective

Digital inequalities research has documented a set of practices related to people’s Internet use that questions the binary division between Internet users and non-users. In particular, among older adults, a considerably large group of individuals has been identified as not using the Internet by themselves; rather, they ask members of their personal networks to do things online for them—they “use” the Internet by proxy. Since previous research shows that children and grandchildren are important sources of help when it comes to Internet use, the current paper indicates that the notion of intergenerational solidarity is a sound conceptual basis for understanding the relationship between social support networks and proxy Internet use among Internet non-users. Notably, the concept of functional solidarity as a dimension of intergenerational solidarity is advanced, as this relates to the frequency of the intergenerational exchange of resources and services encompassing various types of assistance and support offered between two generations. Empirically, this paper investigates how the two types of social support networks and their characteristics are associated with proxy Internet use. The results from multivariate analyses of survey data from a nation-wide representative sample show that when comparing emotional support and socializing networks, only the latter is associated with proxy Internet use: Internet non-users who have (grand-)children in their socializing support network are more likely to engage in proxy Internet use. The results also indicate that non-users who are younger, more educated, have children, and live in urban areas are more inclined to engage in proxy Internet use, regardless of the type of social support. The findings indicate the importance of empirical investigation related to different aspects of functional solidarity, as the effects on proxy Internet use depend on the type of social support.