The effects of grain boundary segregation of Phosphorus (P) on the intergranular corrosion (IGC) and stress corrosion cracking(SCC) of mild steel in hot 55% $Ca(NO_3)_2$ solution have been investigated. The kinetics of IGC and the SCC mechanism of mild steel were also studied. Moreover, a research of the effect of plastic deformation on the hydrogen permeation through 3.3 Ni-1.6 Cr steel was carried out to elucidate the effect of dislocation transport of hydrogen. The effect of grain boundary segregation of P on the hydrogen permenation was investigated by using the results of above research.
In chapter III, the kinetics of IGC Of mild steel and the effects of grain boundary segregation of P on that in hot 55% $Ca(NO_3)_2$ solution have been investigated. From the result of current-time curves, it is concluded that the IGC Occurs by the process of grain boundary dissolution followed by repassivation and quasi-uniform corrosion. IGC current increases until the IGC fissure grows to some critical depth. By the repassivation of pre-attacked grain boundaries, the current decreases until all the grain boundaries within the specimen are attacked. The current after repassivation does not exbit a periodic damping but is rather nearly constant because of the quasi-uniform corrosion of repassivated grain surfaces.
The grain boundary segregation of P makes the IGC attack more easily by interupting the formation of passivating films. The effects are well explained by the kinetic model of IGC suggested. In addition, as a proper IGC susceptibility parameter, the average current density of grain boundary dissolution (stage II) and repassivation (stage III) normalized by grain boundary area is proposed in the light of IGC kinetics.
In chapter IV, the effects of grain boundary segregation of P on the SCC and on the SCC mechanism of mild steel in hot 55% $Ca(NO_3)_2$ solution have been investigated by using the slow strain rate and constant load methods. The SCC susceptibility was examined as a function of applied potential, strain rate and test temperature.
The extent of P segregation effect on SCC is greatly affected by the kinetics of dissolution of passivating film developed on the specimen surface depending upon the SCC test condition. That is, the effect of P segregation at grain boundaries is revealed markedly in the presence of stable passive films on the specimen surfaces. This result is discussed in terms of the stability of passivating films as a function of applied potential, strain rate, test temperature and loading method. The IGC susceptibility seems to be necessary for the occurrence of SCC susceptibility in case of the P-doped mild steel. From the experimental SCC results and SEM fractography, the SCC mechanism of mild steel proved to be strain-induced anodic dissolution irrespective of grain boundary segregation of P.
In chapter V, the effect of plastic deformation on the hydrogen permeation through 3.3 Ni-1.6 Cr Steel was investigated by using gas phase charging and electrochemical detection methods at 29 C. At the strain rate of $1.0×10^{-6}$/sec, the stress-strain curves both with and without hydrogen permeation showed the dynamic strain aging. The corresponding permeation measurements allow to distinguish the contribution of dislocations generated by plastic deformation to hydrogen transport from that to hydrogen trapping. A quantitative analysis gives us an example that the release rate of the dislocations moving out from the surface is calculated as $8.4×10^{-12}/㎠$/sec in average and 1-2 hydrogen atoms trapped to a dislocation is transported by a dislocation at a constant strain rate of $1.0×10^{-6}$/sec.
In chapter VI, the effect of grain boundary segregation of P on the hydrogen permeation through 3.3 Ni-1.6 Cr steel has been investigated by employing the gas phase charging and electrochemical detecting permeation method with and without plastic deformation at 30℃.
The experiment of hydrogen permeation with plastic deformation (the straining permeation experiment) showed the P segregation effect on the hydrogen trapping clearly. The trap binding energy and the trap density of P to hydrogen were found to be about-51 kJ/mol and $9.0×10^{-7}$ mol/ ㎤$, respectively, by the hydrogen permeation experiment without plastic deformation (the static permeation experiment). These results indicate that the P segregated at grain boundaries may act as trap sites of hydrogen.
The trap binding energy and the trap density of Ni-Cr steel (without P segregation) were found to be about-38.5 kJ/mol and $6.0×10^{-4}$ mol/ ㎤$, respectively. These results are attributed to the effect of shallow trap sites (e.g., alloying element, grain boundary, inter-phase boundary, etc) in the Ni-Cr steel.
