Zinc phases

Dezincification Dezincification is corrosion of a brass alloy containing zinc in which the principal product of corrosion is metallic copper. This may occur as plugs rilling pits (plug type) or as continuous layers surrounding an unattacked core of brass (general type). The mechanism may involve overall corrosion of the alloy followed by redeposition of the copper from the corrosion products or selective corrosion of zinc or a high-zinc phase to leave copper residue. This form of corrosion is commonly encountered in brasses that contain more than 15 percent zinc and can be either eliminated or reduced by the addition ox small amounts of arsenic, antimony, or ph osphorus to the alloy. 

Edwards and Schrader—IR investigations support common formate intermediate in water-gas shift and methanol synthesis over Cu/ZnO. Edwards and Schrader,234 using careful reduction procedures (95%N2/5%H2), were able to obtain direct evidence by infrared spectroscopy of the formation of active OH groups on Cu/ZnO, that formed surface formates on the surface of the zinc phase (1576, 1381, 1366, 2970, and 2878 cm-1, respectively for OCO asymmetric, OCO symmetric, and C-H stretching bands) upon exposure to CO. In the presence of CO and H20, the formate intensity initially increased, followed by the production of C02, indicative of water-gas shift. A carbonyl band was also observed at 2093 cm-1. The authors... 

Magnesium (Mg), 15 320-381. See also Aluminum— magnesium phase diagram Aluminum-magnesium- zinc phase diagram MgB2 entries activated, 12 835 analytical methods for, 15 348 atmospheric exposures of, 15 369 beer as dietary source of, 3 588 behavior on contact with chemicals, 15 372t... 

Aluminum—magnesium—zinc phase diagram Magnesium—zinc phase... 

To find out the phase composition of the intermetallic compound layers formed, X-ray patterns were taken immediately from the polished surfaces of the Ni-Zn and Co-Zn cross-sections. Annealing and subsequent cooling the specimens of the type shown in Fig. 3.12b in most cases resulted in their rupture along the interface between the zinc phase and the intermetallic layers, with the latter remaining strongly adherent to nickel or cobalt plates. Therefore, preparation of the cross-sections for X-ray analysis presented no difficulties. These could readily made by successive grinding and polishing the plate surface until the Ni or Co phase was reached. In total, four layer sections parallel to the initial interface were analysed for each cross-section. Simultaneously, layer composition on each section of the interaction zone was determined by electron probe microanalysis. 

Similar X-ray data were obtained with Co-Zn reaction couples. The intermetallic layer adjacent to the zinc phase was identified as the y2 phase. According to P.J. Brown,280 the monoclinic unit cell of this intermetallic compound, containing 26 zinc atoms and 2 cobalt atoms (space group C2/m), has the parameters =1.3307 nm, =0.7535 nm, c=0.4992 nm, and P=126.8°. The intermetallic layer adjacent to the cobalt phase was mainly the Yi phase with a distorted y-brass structure.142... 

Both systems are suitable to check whether or not there is a directly proportional relationship between the width of the homogeneity range of a compound and the growth rate of its layer, predicted by the diffusional theory.5 It is clear that in view of the presence of the liquid zinc phase during preparation of Ni-Zn and Co-Zn reaction couples, all the inter-metallic phases had equal and favourable conditions to form their nuclei at the interface between nickel or cobalt and zinc, which could then readily grow during subsequent isothermal annealing. 

In the couples of the type shown in Fig. 3.12b the cohesion between dissimilar metals after prolonged isothermal annealing was so poor that the nickel or cobalt plate could be taken out without any effort by hand from the zinc matrix. Therefore, more or less acceptable kinetic data were only obtained with the couples of the type shown in Fig. 3.12c, in which the nickel or cobalt plate is seen to be surrounded by the zinc phase. However,... 

In the great majority of cases, a line of the markers located in the zinc phase displaces a few micrometres aside from a line located in the other phases, indicative of the crack formation at the interface with zinc. To understand the further course of the reaction-diffusion process after the rupture of any reaction couple, it is necessary first to analyse the growth kinetics of the same compound layer in different reaction couples of a multiphase binary system. This will be done in the next chapter. 

The binary oxygen-zinc phase diagram is depicted in Fig. 1.5 [58]. Above 200°C only the binary compound ZnO is stable. At low temperatures also zinc peroxide (ZnO2) is reported, which can be prepared by chemical synthesis [59]. The melting point of ZnO is 1975°C. The sublimation of ZnO occurs congruently by decomposition to the gaseous elements according to ... 

In this reaction three phases are involved the solid zinc phase, the aqueous solution, and the gaseous phase formed by the evolved hydrogen. 

Cramer, E.M., F.H. Ellinger and C.C. Land, 1960, Plutonium-Zinc Phase Diagram, in Extractive and Physical Metallurgy of Plutonium and its Alloys, ed. W.D. Wilkinson (Interscience Publishers, New York) pp. 169-180. 

The distribution ratios of the minor elements. As, Cd and Pb, were calculated by dividing the average content of these elements in the condensed zinc phase by that in the matte phase. 

Silver is much more soluble in zinc than in lead. In the Parkes process, zinc is added to a lead melt A silver-rich zinc phase forms on the surface and is drawn off When zinc is removed by distillation, raw silver remains. Very pure silver is obtained by a subsequent electrolytic process. 

One of the pyrometallurgical methods used is the Imperial Smelting Technique. A roasted zinc concentrate is charged together with coke into a blast furnace. At 1000°C zinc is reduced and its vapor passes from the top of the furnace into a condenser. Here the zinc vapor is cooled by molten lead and the two metals form a molten alloy, which is allowed to cool to 44f)°C. At this temperature the metal system has separated into a lead phase and a zinc phase. The lead is circulated for continued cooling purposes. Crude zinc produced by this process contains about 2% Pb, 0.3% Cd and 0.05% Fe. It is refined by distillation in two columns [33.5]. In the first one, zinc is purified from lead and iron. The separation is based on the fact that the boiling point of zinc is 907°C, while lead boils at 1749°C and iron at 2861°C. In the second column, zinc is separated from cadmium (boiHng point 767°C). Zinc metal with 99.9% purity is obtained. 

Dezincification refers to the selective leaching of the zinc phase in alloys such as brasses that contain more than 15 percent Zn. The gross appearance and size of a part that has suffered dezincification is often unchanged except for the formation of a copper hue. The part, however, will have become weak and embrittled, and therefore subject to failure without warning. To the trained observer, dezincification is readily recognized under the microscope, and even with the unaided eye, because the red copper color is easily distinguished from the yellow of brass. 

Figure 9.21 A region of the copper-zinc phase diagram that has been enlarged to show eutectoid and peritectic invariant points, labeled E (560°C, 74 wt% Zn) and P (598°C, 78.6 wt% Zn), respectively.

The most common copper alloys are the brasses, for whieh zine, as a snbstitntional impurity, is the predominant alloying element. As may be observed for the eopper-zinc phase diagram (Figure 9.19), the a phase is stable for eoneentrations up to approximately 35 wt% Zn. This phase has an FCC crystal structure, and a-brasses are relatively soft, ductile, and easily cold worked. Brass alloys having a higher zinc content contain both a and p phases at room temperature. The P phase has an ordered BCC crystal structure and is harder and stronger than the a phase consequently, a + p alloys are generally hot worked. 

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