Effect of Cr and Si contents in the ferritic stainless steels on the occurrence of reddish brown oxide scale in the kerosene-combustion atmospheres has been studied in the O2+H2O+Ar mixed gases and the combustion portions of kerosene heaters. Furthermore, their effects have been studied in detail by the observation and analysis of the produced oxide scales. In the results, the oxidation behaviour of Cr steels in the O2+H2O+Ar mixtures showed good coincidence with those in the kerosene heaters. Namely, Cr and Si show good effect to prevent the formation of the reddish brown oxide scale. It was found that this good effect can be attained by increasing Cr and Si in the steel following a condition Cr(%)+4Si(%)≥27. When a reddish brown oxide scale occurs, (Cr, Fe)3O4 as well as (Cr, Fe)2O3 was produced in the initially formed oxide scale. In order to prevent the formation of the reddish-brown oxide scale, it was found that the initially formed oxide scale must be M2O3 type which can be attained by the highly Cr-enriched scale (Cr/Fe ratio≥3). Furthermore, Si may enrich Cr in the scale and prevent the oxidation of Fe.
The oxidation behavior of Cr- and Si- bearing ferritic stainless steels has been studied in the O2+H2O+Ar mixed atmospheres at 600°C, in an attempt to differentiate the effect of O2 and H2O on the occurrence and growth of nodular scale and to clarify the effect of Cr and Si contents on the oxidation resistance. In the results, the reddish brown scale grew on the SUS 430 steel in above 10% H2O containing atmosphere regardless of O2 concentration. On 11%Cr-2%Si steel, the reddish brown scale developed in the above 15% H2O and under 10% O2 mixed Ar atmospheres. On the other hand, 18%Cr-2.5%Si steel showed good resistance to reddish brown scale in all conditions examined. It has been observed that O2 promotes the formation of Cr-enriched M2O3 type scale suppressing the nodular scale formation. However, once nodular scale is formed, O2 accelerates its growth. While, it has been found that H2O promotes the formation of M3O4 type scale and causes nodular scale.
The effect of fluoride ion on crevice corrosion for FPM/matal of type 304 stainless steel was studied in terms of critical potential, Vcrev, in neutral NaCl/NaF mixed solutions at 303K. It was found that fluoride ion has inhibitive effect to the chloride ion-induced crevice corrosion. Linear relationships were found to exist between the logarithm of the minimum F- concentration required to inhibit crevice corrosion and the logarithm of Cl- concentration at constant potentials. For a given F- concentration, the minimum Cl- concentration required to induce crevice corrosion is given by the following equation: log[NaCl]=αlog[NaF]+β [molarity] This equation suggests that Cl- and F- competitively adsorbed on the metal surface, which can be described by Freundlich adsorption isotherm.
In this study TiN films were formed under various conditions onto SUS 304 stainless steel substrates by a hollow cathode discharge (HCD) type ion plating process and by an activated reactive evaporation (ARE) type ion plating process, and amount of defects on their films was measured by electrochemical measurement and by optical microscope. And then both results were compared. In electrochemical measurement their samples were polarized in 0.5M-H2SO4+0.05M-KSCN solution and the pinhole ratios (Ri/2) were calculated by the rate of critical passivation current density of a coated and non-coated specimens. In case of ARE process, it is found that Ri/2 decreases by increasing film thickness and bias voltage. In case of HCD process, Ri/2 decreases with the increase of film thickness, but it dose not change by the increase of bias voltage. The pinhole ratio (Rr) was obtained by measuring the size of each pinhole and counting the numbers of pinholes. Ri/2 coincides with Rr comparatively in case of HCD, but this is not the case for ARE. This is because that the pit which do not penetrate through the film to the substrates or the unusual products formed on ARE specimens are also considered defects in the film.
This review describes how to prevent stainless steels as architectural member from aesthetic degradation caused by corrosion. It was mentioned that proper maintenance and appropriate selection of the materials are important, in order to let the architecture have long enough service time. Nature of atmospheric corrosion on stainless steels are discussed, using rating number method as well as recently developed spot density vs. area ratio plot technique. Results of exposure tests gave guideline for alloy selection relating role of alloying elements and environmental corrosiveness. Rusting process was well characterized by the spot density vs. area ratio chart, which may lead to a diagnostic measure for selecting conditions of cyclic corrosion tests.