Recent studies on the cellular synthesis of various glucosides have taught us that in contrast to the catabolism of glucose the anabolic processes require the mediation of uridine diphosphoglucose (UDPG) T or related nucleotides. 1 2 The biosynthesis of the anabolic mediator, UDPG, proceeds via UTP and G-1-P by means of a specific enzyme UDPG pyrophosphorylase.3' 4 We shall use the term UDPG synthetase instead of UDPG pyrophosphorylase in order to be more explicit about the problem with which we are dealing in this paper, i.e., cellular synthesis of UDPG and of polyglucoside compounds. In the course of our studies on mutants of E. coli K-12, we have encountered a mutant, Gal 23,5a, which by the token of microbiological assays on EMB galactose agar would be classified as a "galactose-negative" mutant. It was found to have only very low capacity to incorporate galactose into insoluble carbohydrate compounds. When cells of Gal 23 were grown on ammonia mineral medium with glycerol as a carbon source, addition of galactose to the medium induced a cessation of growth comparable to that seen in transferaseless mutants.6 Surprisingly enough, enzymatic assays of the enzymes of galactose metabolism' revealed that this galactose-negative and galactose-sensitive strain contained all three enzymes, i.e., K, T, and E. All these observations focused our attention toward a genetic block in the pathway of synthesis of UDPG, especially the enzyme UDPG synthetase. This hypothesis has been fully substantiated as will appear from the present study. Meantime Fukasawa et al.5 described two mutants of E. coli K-12 which they found to be defective in UDPG synthetase. They also reported that these mutants contain no detectable amount of hexose in their cell walls, and that their major cellwall components are heptoses and hexosamines. These observations will be discussed in conjunction with our own findings.

The steam oxidation resistance of 80%Ni-20%Cr metallic coatings has been evaluated at four different steam temperatures in the range of 600-750°C. Substrate used for the study was 9Cr-1Mo type steel. The thermal spray coatings were carried out using two different processes, viz., atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF) spray. Thickness of the coatings was about 40 and 60 mm respectively. The results show that the thick and dense HVOF coating showed a better steam oxidation resistance than the thin porous APS coatings. At prolonged aging (1 000 h) the HVOF coating showed the best protection till the temperature range of 650°C. Beyond this temperature the presence of Fe 2 O 3 was noticed at the coating surface. The reason for the protectiveness and failure at higher temperatures (above 650°C) are discussed in detail.KEY WORDS: steam oxidation; Cr-Mo steel; Ni-Cr coating; high velocity oxy fuel spray and atmospheric plasma spray.The effectiveness of a protective oxide scale depends on several factors. Ideally, it must be free of stoichiometric imperfection, porosity, crack, stress and be resistant to exfoliation and stable in service. Chromium oxide is one of such oxides that can exhibit such properties. To form a complete chromium oxide outer layer in Fe-Cr system, it requires around 12 % Cr in the air and oxygen environments. 1)However, in the steam environment, the amount of Cr required is around 25 %, to form a complete protective chromium oxide layer.7) Adding large amount Cr into the material will deteriorate the mechanical properties of the material. Thus, a way to incorporate a large amount of alloying elements to form a protective oxide is applying them as a coating either by diffusion or by deposition retaining the bulk mechanical properties of the component. Chromizing is widely used for steels to improve their steam oxidation resistance. 8,9) However, there are limitations to apply this technique for the large component and onsite modification. Thermal spray is an alternative technique in which deposition can be made at thicker level and flexibility to deal with onsite modifications.A few earlier attempts were made on the thermal spray coating for fossil power plants though the thermal spray process is extensively used for gas turbine applications. 10,11) A work from Aguero et al.,12) who attempted thermal spray coating FeAl, FeCrAl and NiAl powders on 9Cr-1Mo type steel to improve the steam oxidation resistance for USC boiler applications. In that FeCrAl and NiAl showed the promising results. Also, the thermal spray was attempted for the super heater/re-heater components where the material severely suffers on fireside corrosion. The powder used for the study is Fe-Si intermetallics with the undercoat of NiCr alloy. 13) In the present study, Ni-Cr based alloy powders have been coated on modified 9Cr-1Mo steel and the effect of coatings on steam oxidation were evaluated by X-ray diffraction (XRD), scanning electron microscope (SEM), energy despersive spectroscopy (EDS) and electr...

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The present work is focused on the steam oxidation resistance of 50%Ni-50%Cr metallic coatings produced using atmospheric plasma spray (APS) and high velocity oxy fuel (HVOF) spray processes on 9Cr-1Mo type steel substrate. Thickness of the coatings obtained in the HVOF and APS processes were about 60 and 40 mm respectively. The steam oxidation resistance of the coatings was evaluated at four different temperatures viz., 600, 650, 700 and 750°C. The results showed that the thick and denser HVOF coating yielded a better steam oxidation resistance than the thin and porous APS coatings. At prolonged aging (1 000 h), the HVOF coating showed the best protection in all tested steam temperatures. APS coating performed satisfactorily well till the 100 h of test duration. But, it started the scale initiation at the coating interface and incorporation of scales occurred at 1 000 h of steam oxidation test. The reason for the protectiveness by HVOF coating and failure of APS coatings at prolonged aging are discussed in detail.

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