Initiation of stress corrosion cracking of sensitized type 304 stainless steel

The corrosion potential of stressed sensitized Type 304 stainless steels were studied in oxygen-containing solutions to determine the relation between potential fluctuations and the initiation of stress-corrosion cracking. An in situ scanning vibrating electrode technique was shown to detect currents coming from growing stress corrosion cracks in dilute thiosulfate solutions at temperatures below 90/sup 0/C. The onset of the cracking was clearly indicted by rapid decreases in the potential. It was observed that small cracks, which had grown for a limited period, were not susceptible to reinitiation of the cracking process. The onset of cracking was clearly observable by marked decreases in the potential which continued to decrease if cracking remained active and increased when cracking stopped. At and above 200/sup 0/C there were no potential fluctuation indicative of the onset of stress-corrosion cracking. Small potential differences between platinum and stainless steel were observed demonstrating the presence of a single electrochemically reversible reaction determined the potential of all metals within the stainless steel autoclave, and thus masking the onset of cracking. The presence of only an iron-containing deposit on the platinum suggested that a sufficiently reversible redox couple was set up between deposited iron oxide and a soluble iron species in solution which dominated the potential of the stainless steel and the platinum

Ambient temperature stress-corrosion cracking of sensitized stainless steels. [PWR]

Stress-corrosion cracking of sensitized Type 304 steel in low temperature borated water has been observed. The probable role of low levels of chloride ions or sulfur-containing ions is described, including the relationship of the phenomenon to polythionic acid cracking. The mechanism of the sulfur-induced cracking and its usefulness as a test for sensitization are outlined

Determination of surface oxide compositions on Alloy 600 using Rutherford backscattering

The surface composition of oxides formed on Alloy 600 under conditions similar to those in the primary side of PWR heat exchangers has been studied as a function of potential using Rutherford backscattering and proton inelastic scattering. Electropolished samples of Alloy 600 were exposed at several potentials to a solution of 0.18M H/sub 3/BO/sub 3/(2000 ppM B) with 0.28M LiOH (1.4 ppM Li) at 300/sup 0/C for 450 hours. The potentials relative to an internal hydrogen electrode ranged from -.09 to 750 mV. RBS analysis showed little or no oxide formation on samples exposed at 0 mV. Above 0 mV oxide layers formed whose thicknesses increased with potential. In addition the RBS showed a significantly enhanced concentration of aluminum and silicon in oxide. Both the oxygen and the sum of the aluminum and silicon content appeared to maintain a fixed surface concentration independent of the oxide thickness. Boron and lithium concentration were analyzed with proton inelastic scattering. No lithium was found in any sample. The boron concentration was found to follow the thickness of the oxide

XANES Studies of Chromate Replacements in Oxide Films on Aluminum

The chemistry of conversion coatings on aluminum containing chromate and non-toxic chromate replacements has been investigated using XANES. Chromate conversion coatings contain 20% 6-valent chromium which is gradually lost on immersion in a corrosive environment. The most promising alternative coatings are those based on phosphotungstate. The chemistry of these and coatings containing Mo, V, and Mn are discussed

Fuel cell applied research: electrocatalysis and materials. Quarterly report, July 1-September 30, 1978

A study was undertaken to investigate the electrocatalysis of the formic acid and methanol oxidation reactions at underpotentially deposited metal surfaces at 25/sup 0/C. The test electrode was a smooth polycrystalline platinum disc, on which a metal Bi, Cd, Pb or Tl was underpotentially deposited (UPD) by potential cycling in 1 N HClO/sub 4/. The organic reactant (HCOOH or CH/sub 3/OH) was added to this electrolyte so that its concentration was 0.26 M. Cyclic voltammograms were recorded on the Pt (control) and on the UPD test electrodes at a sweep rate of 50 mV/sec in the potential range of 0 to 1.45 V/RHE. Results are reported. Also, overpotentials in solid electrolyte fuel cells were studied. Alternating and direct current techniques were used to determine the impedance characteristics at the platinum-yttria stabilized zirconia (YSZ) interface in the regions of potentials where the oxygen evolution and reduction reactions take place. The measurements were conducted as a function of time to elucidate aging effects on the kinetics of these reactions. Experiments were conducted at 1000/sup 0/C in the potential range of -600 to +600 mV vs an air reference electrode. Results are presented and discussed. (WHK

Fuel cell applied research: electrocatalysis and materials. Quarterly report, January 1-March 31, 1979

In situ regeneration of surface areas of sintered carbon supported platinum electrodes in 85% H/sub 3/PO/sub 4/ at 150/sup 0/C was studied. The kinetics of oxygen reduction on platinum in trifluoromethane sulfonic acid was investigated. Overpotentials in solid oxide electrolyte fuel cells were studied. Results are presented and discussed. Also, a survey of the status of studies of phosphoric acid and molten carbonate fuel cells is described. (WHK

Molecular characterisation of a candidate gut sucrase in the pea aphid, Acyrthosiphon pisum

The hydrolysis of sucrose, the principal dietary source of carbon for aphids, is catalysed by a gut α-glucosidase/transglucosidase activity. An α-glucosidase, referred to as APS1, was identified in both a gut-specific cDNA library and a sucrase-enriched membrane preparation from guts of the pea aphid Acyrthosiphon pisum by a combination of genomic and proteomic techniques. APS1 contains a predicted signal peptide, and has a predicted molecular mass of 68 kDa (unprocessed) or 66.4 kDa (mature protein). It has amino acid sequence similarity to α-glucosidases (EC 3.2.1.20) of glycoside hydrolase family 13 in other insects. The predicted APS1 protein contains two domains: an N-terminal catalytic domain, and a C-terminal hydrophobic domain. In situ localisation and RT-PCR studies revealed that APS1 mRNA was expressed in the gut distal to the stomach, the same localisation as sucrase activity. When expressed heterologously in Xenopus embryos, APS1 was membrane-bound and had sucrase activity. It is concluded that APS1 is a dominant, and possibly sole, protein mediating sucrase activity in the aphid gut