Two cases are presented on improvement in corrosion resistance by alloying design for equipments related to energy systems. Metal dusting, which is often encountered in CO containing gas environments, is a prominent corrosion problem for the equipment of GTL (gas to liquid) process.The resistance to the metal dusting is remarkably increased by Cu addition to a Ni-base alloy in addition to the high Cr content for the formation of the protective oxide film which acts as a barrier against the CO gas, because the added Cu, which is prone to segregate on the alloy surface, prevents dissociation of the adsorbed CO molecule and therefore the C entry at the defect sites of the oxide film. Highly pressurized hydrogen is one of the most effective media for storage and transport of hydrogen for fuel cells. However, the pressurized hydrogen causes various materials to suffer hydrogen environment embrittlement (HEE). The susceptibility to the HEE of 300 series of austenitic stainless steels largely depends on the degree of deformation-induced martensitic transformation at the crack tip. Therefore, the high resitance to the HEE is obtained by the alloying design which minimizes its transformation in the austenitic stainless steels.
DACROTIZED® film is one of the anti-corrosion films which are practically applied to automobile small components. In this paper, the anti-corrosion performances of two kinds of DACROTIZED® films in 3 wt% NaCl solution for 90 days were quantitatively evaluated using various electrochemical methods. The mild steel coated with the film which is composed of zinc flake, aluminum flake and chrome binder was used as specimen. The cathode protection duration of this film was found to be about 30 days through the result of corrosion potential monitoring. An EIS study showed almost capacitive response at low frequency range after 75 days immersion. The red rust which was the corrosion products of steel was observed simultaneously. On the other hand, the sacrificial zinc anodes in the film which containing 3% boron compound as additives was kept working over 90 days. The absolute value of impedance of the film containing boron compounds was twice as large as that of the film without boron. The polarization curve transition with immersion time revealed that zinc corrosion products would gradually accumulate in the film and form stabilized layer after about 14 days. It was suggested that boron compounds not only suppress the excessive dissolution of zinc but also improve barrier properties of the film.
High temperature oxidation behavior of an intermetallic compound of NbAl3 was investigated at 1073 and 1173 K, in oxygen containing a trace amount of NaCl vapor. Particular attention has been given to the effect of NaCl vapor concentration on the depression of the pesting oxidation. In oxygen without NaCl vapor, mass gain of NbAl3 was quite large and the pesting oxidation occurred. On the other hand, in oxygen with several tens of volume ppm NaCl vapor, mass gain was reduced within the experimental time region, down to negligible value in oxygen with 123 ppm NaCl vapor. Moreover, the pesting phenomenon disappeared when the mass gain was lower than 0.05 kg m−2. X-ray diffraction and SEM observation revealed that specimens without the pesting were covered with a relatively dense Al2O3 film, whereas the specimens that occurred pesting, formed the scale consisting of mainly AlNbO4. From the thermodynamic discussion, it was considered that NaCl vapor produces a small amount of chlorine gas, inducing chlorination of Al selectively under lower potential of oxygen at the alloy/scale interface, subsequently, produced AlCl3 is oxidized to form a protective Al2O3 film. This Al2O3 formation mechanism was proposed as the pesting depression model. Consequently, from all the result, an ability to prevent pesting oxidation by adding a trace amount of NaCl vapor to oxygen became apparent.
In order to evaluate the effects of various surface conditions and the heating rate during aging on corrosion of carbon steel which is used as a regenerator vessel material in the absorption refrigeration cycle, electrochemical measurements of carbon steel in a concentrated lithium bromide solution were carried out. Aging means a passivation process for vessel material by heating it in a concentrated lithium bromide solution containing a passivator as an inhibitor at the beginning of refrigerator operation. As a result, it was found that when polished steel as a defect of mill scale on carbon steel was corroded to some extent or when a metal/metal crevice was formed on polished steel surface, macrocells possibly continued to occur between the polished steel and the carbon steel covered with mill scale during or after the aging. On the other hand, it was found that when the heating rate during the aging was low, the macrocell current was large and that there was higher corrosion risk than at high heating rates.