Search Results for Contaminants

Cracks, with no other damage, were observed in a niobium alloy (Nb-106) part when it was pulled from several months of protective storage for assembly into a rocket nozzle. SEM views showed the cracks to be intergranular, with contaminant particles on a large number of the grain facets. EDX analysis showed they consisted of niobium and fluorine. Plastic replicas, prepared by standard TEM techniques, were analyzed with selected-area electron diffraction, showing a pattern match for niobium tetrafluoride. Auger analyses showed electron spectra containing peaks representing carbon, oxygen, nitrogen, fluorine, and chlorine. Investigation into the processing history of the part showed the tenacious oxide film formed by the affinity of niobium for oxygen - even when heat treated in a vacuum – was removed with a combination of strong acids: nitric, hydrochloric, hydrofluoric, and lactic, resulting in the contaminants found on the surface. Thus, residues of the cleaning acid on the part had caused SCC during storage, with the tensile stresses necessary to generate SCC assumed to have been residual stresses from the heat treatment. Recommendation was made that more stringent cleaning procedures to remove any trace of the cleaning acids be used.

A batch of bimetal foil/epoxy laminates was rejected because of poor peel strength. The laminates were manufactured by sintering a nickel/phosphorus powder layer to a copper foil, cleaning, then chromate conversion coating the nickel-phosphorus surface, and laminating the nickel-phosphorus side of the clad bimetal onto an epoxy film, so that the end product contained nickel-phosphorus sandwiched between copper and epoxy, with a chromate conversion layer on the epoxy side of the nickel-phosphorus. Peel testing showed abnormally low adhesion strength for the bad batch of peel test samples. Comparison with normal-strength samples using XPS indicated an 8.8% Na concentration on the surface of the bad sample; the good example contained less than 1% Na on the surface. After 15 min of argon ion etching, depth profiling showed high concentrations of sodium were still evident, indicating that the sodium was present before the chromate conversion treatment was performed. A review of the manufacturing procedures showed that sodium hydroxide was used as a cleaning agent before the chromate conversion coating. Failure cause was that apparently the sodium hydroxide had not been properly removed during water rinsing. Thus, recommendation was to modify that stage in the processing.

A Ti-6Al-4V alloy pressure vessel failed during a proof-pressure test, fracturing along the center girth weld. The girth joints were welded with the automatic gas tungsten arc process utilizing an auxiliary trailing shield attached to the welding torch to provide inert-gas shielding for the exterior surface of the weld. A segmented backup ring with a gas channel was used inside the vessel to shield the weld root. The pressure vessel failed due to contamination of the fusion zone by oxygen, which resulted when the gas shielding the root face of the weld was diluted by air that leaked into the gas channel. Thermal stresses cracked the embrittled weld, exposing the crack surfaces to oxidation before cooling. One of these cracks caused a stress concentration so severe that failure of the vessel wall during the proof test was inevitable. A sealing system at the split-line region of the segmented backup ring was provided, and a fine-mesh stainless steel screen diffuser was incorporated in the channel section of the backup ring to prevent air from leaking in. A titanium alloy color chart was furnished to permit correlation of weld-zone discoloration with the degree of atmospheric contamination.

One inch diam Type 304 stainless steel piping was designed to carry containment atmosphere samples to an analyzer to monitor hydrogen and oxygen levels during operational and the design basis accident conditions that are postulated to occur in a boiling water reactor. Only one of six lines in the system had thru-wall cracks. Shallow incipient cracks were detected at the lowest elevations of one other line. The balance of the system had no signs of SCC attack. Chlorides and corrosion deposits in varying amounts were found throughout the system. The failure mechanism was transgranular, chloride, stress-corrosion cracking. Replacement decisions were based on the presence of SCC attack or heavy corrosion deposits indicative of extended exposure time to chloride-contaminated water. The existing uncracked pipe, about 75 percent of the piping in the system, was retained despite the presence of low level surface chlorides. Controls were implemented to insure that temperatures are kept below 150 deg F, or, walls of the pipe are moisture-free or the cumulative wetted period will never exceed 30 h.

An outer fan-duct assembly of titanium alloy Ti-5Al-2.5Sn (AMS 4910) for a gas-turbine fan section cracked 75 mm (3 in.) circumferentially through a repair weld in an arc weld in the front flange-duct segment. Examination of the crack with a binocular microscope revealed no evidence of fatigue. A blue etch-anodize inspection showed the presence of an alpha case along the edges of the repair weld. The alpha case, a brittle oxide-enriched layer, forms when welds are inadequately shielded from the atmosphere during deposition. The brittleness of this layer caused transgranular cracks to form and propagate in tension under the thermal stresses created by the repair-weld heat input. The crack resulted from contamination and embrittlement of a repair weld that had received inadequate gas shielding. Thermal stresses cracked the oxide-rich layer that formed. The gas-shielding accessories of the welding torch were overhauled to ensure that leak-in or entrainment of air was eliminated. Also, the purity of the shielding-gas supplies was rechecked to make certain that these had not become contaminated.

An aluminum brass seawater surface condenser failed due to pitting after less than one year of service. Large pits filled with a green deposit were evidenced under the nonuniform black scale present over the entire inside surface of the tube. The black deposit was identified as primarily copper sulfide, with zinc and aluminum sulfides while the green deposit was revealed to be copper chloride. The combination of sulfide and chloride attack on the tubes was concluded to have resulted in the failure. Injection of ferrous sulfate upstream of the condenser which could aid the formation of protective oxide films was recommended.