The corrosion behavior and corrosion inhibition mechanism of carbon steel in concentrated LiBr solution at elevated temperature by LiNO3, Li2MoO4 and LiNO3/Li2MoO4 mixed inhibitors have been investigated. These inhibitors were effective in reducing corrosion, but pitting was likely to occur in the LiBr solution with LiNO3 at some concentrations. With Li2MoO4/LiNO3 initiation of pitting occurred as with LiNO3, but its progression was prevented. The structure and composition of the surface films were analyzed by XRD, AES, SEM-EDX and XPS. A higher OH- concentration in the surface films gave a lower corrosion rate. The surface films formed in the LiBr solutions with Li2MoO4 and LiNO3/Li2MoO4 mixed inhibitors were composed of two layers, consisting of Li-Mo oxide including many OH- ions and oxygen-deficient Fe oxide. On the other hand, with LiNO3 inhibitor, Fe3O4 was formed. The relationships between the amount of corrosion and surface films structures were considered in terms of an interfacial ion-selective diffusion layer.
Hydrogen was charged to plain carbon steels at 653-723 K by an electrochemical method using a molten NaOH electrolyte. The catholic potential was kept at-1.7V vs. Air/O2-(ZrO2) during the charging, and the current density at this potential was equivalent to the hydrogen pressure of 4-8MPa. Methane bubbles are observed at pearlite/ferrite, cementite/ferrite and non-metallic inclusion (MnS)/ferrite interfaces. The number and radius of methane bubbles increase with increasing of charging temperature and time. The charging steel surface was accompanied with decarbulization. The decarbulization depths showed the Arrehenius relation and they were controlled by the diffusion of carbon in steels.
An electrochemical method for monitoring hydrogen content permeated through steel pipe at elevated temperatures (-673 K) was, first time, applied to an actual hydrodesulfurizing plant for light oil. The monitoring equipments include a molten sodium hydroxide electrolyte, a stabilized zirconia reference electrode, Pt-wire counter electrode, a high-purity alumina cell, potentiostat and recorder. The monitoring of ionic current of permeated hydrogen was performed with high reliability during over one month. The measuring currents were fundamentally similar values as ones calculated from the assumption that the process flow was pure hydrogen gas. But, after several days, the permeation currents decreased to 1/2-1/5 of the calculated value. This decrease is closely related to the formation of FeS film at the inner surface of pipe.
Corrosion behavior and pitting potential for Fe in aqueous fluoride solutions were investigated by potentiodynamic and potentiostatic polarization measurements. It was found that the solution pH and total fluoride concentration, [F]total affect the pitting potential which moves to more noble direction with decreasing [F]total and increasing pH values. Furthermore, it was found that the chemical morphology of fluoride in solution affects the corrosion behavior of Fe. Uniform dissolution is observed in HF, active state pits occur in HF+F- and polishing state pits occur in F-.
In studies of erosion-corrosion in “dry” gaseous environments at elevated temperatures, and in wet aqueous environments at room temperatures, there have been recent attempts at mapping such processes. Such maps show the transitions between the erosion-corrosion regimes and thus can be an aid to materials selection in such environments. By plotting the magnitude of the wastage rate on the maps, as a function of the main process parameters, the conditions under which erosion-corrosion damage is minimized are easily identified. The purpose of this paper is to review the approaches to erosion mapping in aqueous and in dry environments. The work carried out on erosion maps for composites and ceramics will also be discussed. The limitations to existing work on the mapping approaches are also be addressed. It is shown how erosion maps can be a potentially powerful tool in minimizing damage due to erosion-corrosion. Further work which needs to be carried out to extend such approaches is discussed. Finally, the use of such maps to construct materials selection maps for both dry and wet erosion-corrosion processes are illustrated.