Chromized coatings obtained by spontaneously propagating high-temperature synthesis

Abstract

The discovel 3, in 196 7 The application of SkIS processes for obtaining chromized coatings is based on the use of exothermic mixtures. The SHS consists in very intense exothermic interaction of chemical elements in condensed phase suitable for spontaneous propagation in the form of a combustion wave [I]. Enhancing the strength of material by SHS was carried out in the regime of combustion as well as in the regime of thermal self-ignition.The experiments were carried out with mild steel and steels with different carbon content ( Table 1). The microstructure of the hardened layers was investigated under an optical microscope Neophot-21. Phase analysis was carried out on an x-ray diffractometer Dron-2 and an x-ray microanalyzer Jeol Superprobe-733. Microhardness was measured on an instrument PMT-3 with a load of 1 N on the indentor.Thermoclaemical treatment (TCT) of the steels was carried out in an open-type reactor with an inert gas, viz., argon, blown through. The saturating medium was a mixture of powdered aluminum, aluminum oxide, trioxydichrome, metallic iodine. Saturation in the regime of combustion was initiated with the aid of an exothermic pellet or an electric coil. The combustion wave moved at a speed of 10 .3 m/sec. In distinction to the known chromizing methods [2] coating by SHS proceeds under nonisothermal conditions where chemical transport reactions play a considerable role. Such reactions occur on account of gas carriers in powder mixtures when there is a temperature gradient. Chemical transport in a combustion wave is possible because the combustion process entails a successive change of the thermal regimes. That means that at each point of the mixture the temperature changes from the initial value to the combustion temperature.On the basis of an analysis of the reactions accompanying the combustion of SHS mixtures, and also of the results of experiments and of metallographic investigations, we obtained a picture of the formation of a chromized layer in the regime of combustion. This process can be divided into three stages.Stage 1 is the heating of the object after the combustion front has passed through the charge; at that a coating does not form. On the object an unreacted layer forms, it is about 1 mm thick which is equal to the width of the heated zone. Iodine vapors diffuse from the charge into this layer. The iodine vapors react with the chromium reduced from the trioxydichrome, forming chromium iodine Crl 2.Stage 2 comprises the decomposition of CrI and deposition of chromium on the surface. The subsequent rise of the temperature leads to the formation of a modest diffusion zone. At that instant concentration of active chromium atoms on the surface is high. The high chenaical potential due to the concentration gradient dc/dx causes the formation of a diffusion layer which consists of the phase (Cr, Fe)7C 3, Calculations show that a layer with thickness of the order of 10 p,m forms.

Corrosion resistance of chromized 316L stainless steel for PEMFC bipolar plates

“…The void opened to the free surface was noteworthy because it might affect the corrosion behavior. It was reported that the voids were attributed to the differences of diffusion rates between the elements and is known as a Kirkendall effect [18].…”

“…2 shows that the specimen exhibits three distinct layers composed of a thin carbide layers, a Cr-rich layer, and substrate layer. The outer thin carbide layer is known as Cr 23 C 6 due to the low carbon content of 316L stainless steel [18].…”

“…Considering the above-mentioned requirements, the pack cementation technique appears to be an attractive surface treatment to improve corrosion resistance without increasing electrical resistance. The pack cementation technique has been employed extensively to improve corrosion resistance or increasing the surface hardness and the experimental methods and the diffusion mechanism during cementation process is well established [18][19][20][21][22][23].…”

Improved corrosion resistance and interfacial contact resistance of 316L stainless-steel for proton exchange membrane fuel cell bipolar plates by chromizing surface treatment

“…(For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.) [8,16]. The chrome carbide layer is removed when the bare specimen is cemented by process II.…”

“…It is reported, however, that the Cr-enriched surface decreases the electrical conductivity and causes a voltage drop at the interfacial contact [15]. Void formation has also been reported after the pack cementation of Cr for an extended period due to the Kirkendall effect [8,16], which is detrimental to the mechanical strength of the thin steel plates [17]. In this study, the effects of Cr cementation processes under different conditions are investigated, in order to determine the optimum conditions for producing a Cr-enriched layer on stainless steel with improved corrosion resistance without sacrificing ICR.…”