Alloy depletion

Hydrochloric acid test 1 10% HCl 24 h in boiling solution 1. Appearance of sample after bending around mandril 2. Mass loss per unit area (a) Redox pxjtential = -I-0-32 (b) Corrosion potential = -0-2 0-1 1. Alloy-depleted area 2. Not for 0-phase... 

Defects that must be appraised continuously include white spot" and "freckle. While spot is an area of alloy depleted in the lower melting-point alloying additions. Freckle is a mid-radial channel segregation resulting from a deep mcli pool and a steeply sloping liquidus profile. This segregation develops in the liquid before solidification. 

Assuming this mechanism to occur it is obvious that the stability and the time for dissolution of the continuous Ti-rich nitride layer will depend on various parameters such as alloy composition, growth rate and growth mechanism of the scale, as well as on the phases present in the alloy depletion layer at a given time. 

Hydrochloric acid 1 10% HCI 24 h in boiling solution 1. Appearance of (o) Redox potential 1. Alloy-depleted area o... 

Due to the high chromium contents, duplex alloys are sensitive to 885 (475°C) embrittlement. This generally limits their usage to 600°F (SIS C) maximum for pressure vessels. Due to the presence of nickel, chromium, and molybdenum they are also susceptible to the formation of affect mechanical properties and corrosion resistance due to alloy depletion. The temperature range of 1100°F (593 C)-1600°F (882°C) and most rapidly at about 1450°F (788°C). The deleterious effects of phase formation are not obvious at the elevated temperature but can become a factor at room temperature. The formation of a phase in these alloys is sufficiently rapid to have an effect on properties due to slow cooling (air) after anneal. A measurable effect as a result of exposure in this temperature range due to welding has been demonstrated. 

Figure 8. Comparative AES depth profiles of microscopic areas of a 909< Ni. 10% Cr alloy exposed to low-pressure oxidation at 600°C. On grain VI. ihe Ni/Cr ratio in the oxide is similar to that in the alloy. By contrast, on grain VII. the oxide is strongly enriched, and the underlying alloy depleted, in Cr. The two grain faces have different crystallographic orientations.

Both the intermetaUic phases and the carbides are rich in molybdenum, tungsten, and chromium and therefore create adjacent areas of alloy depletion that can be selectively attacked. Carbide precipitation can be retarded considerably by lowering carbon and silicon this is the principle behind HasteUoy Alloy C-276. 

Schematic of alloy depletion by the corrosion process (in this example by oxidation, i.e., chromium-rich surface oxide scale formation). The dark areas are those of high (original) Cr content of the alloy. By Cr consumption due to scale growth also chromium carbides in the metal subsurface zone become dissolved (cross section of the surface area).

Chemical analysis of the metal can serve various purposes. For the determination of the metal-alloy composition, a variety of techniques has been used. In the past, wet-chemical analysis was often employed, but the significant size of the sample needed was a primary drawback. Nondestmctive, energy-dispersive x-ray fluorescence spectrometry is often used when no high precision is needed. However, this technique only allows a surface analysis, and significant surface phenomena such as preferential enrichments and depletions, which often occur in objects having a burial history, can cause serious errors. For more precise quantitative analyses samples have to be removed from below the surface to be analyzed by means of atomic absorption (82), spectrographic techniques (78,83), etc. 

The recovery of vanadium from these slags is of commercial interest because of the depletion of easily accessible ores and the comparatively low concentrations (ranging from less than 100 ppm to 500 ppm) of vanadium in natural deposits (147,148). In the LILCO appHcations the total ash contained up to 36% 20 (147). Vanadium is of value in the manufacture of high strength steels and specialized titanium alloys used in the aerospace industry (148,149). Magnesium vanadates allow the recovery of vanadium as a significant by-product of fuel use by electric utiUties (see Recycling, nonferrous LffiTALS). 

In tire steady state, where these two rates are equal, the depletion of the surface, and hence the lowering of the surface concentration, and therefore the free evaporation rate of manganese below tire initial value for the alloy, which is given above, is... 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

A useftil applicadon of time-dependent PL is the assessment of the quality of thin III-V semiconductor alloy layers and interfaces, such as those used in the fabri-cadon of diode lasers. For example, at room temperature, a diode laser made with high-quality materials may show a slow decay of the acdve region PL over several ns, whereas in low-quality materials nonradiative centers (e.g., oxygen) at die cladding interface can rapidly deplete the free-carder population, resulting in much shorter decay times. Measurements of lifetime are significandy less dependent on external condidons than is the PL intensity. 

The use of equipment close to the temperature at wliich the material was diffusion treated will result in continuing diffusion of chromium, aluminum etc., into the substrate, thus depleting chromium with consequent loss in oxidation and corrosion resistance. For aluminum, this effect is noticeable above 700°C in steels, and above 900°C in nickel alloys. For chromium, the effect is pronounced above 850°C for steels and above 950°C for nickel alloys. 

Localised corrosion can occur if the passivating element of an alloy is locally depleted from the matrix. This may occur in the sensitized regions of stainless steels for example, as outlined above. Other alloy components too. 

In overlay bearings operating above about 140°C, the tin or indium in the overlay diffuses towards, and alloys with, the underlying copper, depleting the overlay and reducing its resistance to corrosion. This depletion by diffusion can be combatted by the use of a diffusion barrier or dam , e.g. a nickel-rich layer between the bearing alloy and, the overlay . 

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