Robert Bayles

Carburization-induced passivity of 316 L austenitic stainless steel

A low-temperature (450-500°C) gas-phase process for introducing substantial amounts of carbon, without carbide formation, into 316L austenitic stainless steel has been developed. This process, termed low-temperature colossal supersaturation (LTCSS), provides surface carbon concentration as high as 14 atom % and dramatically improves the localized corrosion resistance of 316L austenitic stainless steel in ambient temperature seawater. In particular, the LTCSS-treated steel increases the seawater breakdown potential by more than 600 mV. This result is remarkable, as traditional carburization methods have historically decreased the corrosion resistance of stainless steels.

Comparison of SCC Thresholds and Environmentally Assisted Cracking in 7050-T7451 Aluminum Plate

Aerospace alloys, often aluminums, are frequently exposed to corrosive environments resulting from naval service. These environments may produce significant changes in crack growth characteristics in these materials. An experiment was designed to characterize the effects of environment on crack growth thresholds and fracture characteristics for existing cracks in aluminum 7050-T7451 plate material. This data will be comparatively analyzed against aluminum 7075-T7631, an alloy with known susceptibility to corrosion, in order to determine a relative susceptibility of 7050-T7451, generally considered a superior aluminum alloy in terms of strength and corrosion resistance. The resulting data and subsequent analysis can in turn be used in more accurate determination of aircraft component service life in common corrosive environments experienced by aircraft in naval service.

Interstitial hardening of type 316L stainless steel to improve corrosion resistance and mechanical properties

A low-temperature (450°C to 500°C), gas-phase interstitial hardening (IH) process that introduces substantial amounts of carbon, without carbide formation, into austenitic stainless steels was inve...

A High-Performance Corrosion-Resistant Iron-Based Amorphous Metal - The Effects of Composition, Structure and Environment on Corrosion Resistance

New corrosion-resistant, iron-based amorphous metals have been identified from published data or developed through combinatorial synthesis, and tested to determine their relative thermal phase stability, microstructure, mechanical properties, damage tolerance, and corrosion resistance. Some alloy additions are known to promote glass formation and to lower the critical cooling rate [F. Guo, S. J. Poon, Applied Physics Letters, 83 (13) 2575-2577, 2003]. Other elements are known to enhance the corrosion resistance of conventional stainless steels and nickel-based alloys [A. I. Asphahani, Materials Performance, Vol. 19, No. 12, pp. 33-43, 1980] and have been found to provide similar benefits to iron-based amorphous metals. Many of these materials can be cast as relatively thick ingots, or applied as coatings with advanced thermal spray technology. A wide variety of thermal spray processes have been developed by industry, and can be used to apply these new materials as coatings. Any of these can be used for the deposition of the formulations discussed here, with varying degrees of residual porosity and crystalline structure. Thick protective coatings have now been made that are fully dense and completely amorphous in the as-sprayed condition. An overview of the High-Performance Corrosion Resistant Materials (HPCRM) Project will be given, with particular emphasis on the corrosion resistance of several different types of iron-based amorphous metals in various environments of interest. The salt fog test has been used to compare the performance of various wrought alloys, melt-spun ribbons, arc-melted drop-cast ingots, and thermal-spray coatings for their susceptibility to corrosion in marine environments. Electrochemical tests have also been performed in seawater. Spontaneous breakdown of the passive film and localized corrosion require that the open-circuit corrosion potential exceed the critical potential. The resistance to localized corrosion is seawater has been quantified through measurement of the open-circuit corrosion potential (E{sub corr}), the breakdown potential (E{sub crit}) and the repassivation potential (E{sub rp}). The greater the difference between the open-circuit corrosion potential and the repassivation potential ({Delta}E), the more resistant a material is to modes of localized corrosion such as pitting and crevice corrosion. Cyclic polarization (CP) was used as a means of measuring the critical potential (E{sub crit}) relative to the open-circuit corrosion potential (E{sub corr}). Linear polarization (LP) has been used to determine the corrosion current (i{sub corr}) and the corresponding corrosion rate. Other aspects of the materials will also be discussed, as well as potential applications.

Evaluating Stress-Corrosion Cracking Susceptibility Using a Torsion Test

A torsion test has been devised that provides for plane strain constraint in small specimens during fracture toughness testing. This method has been extended for stress-corrosion cracking and a simple torsion load frame has been built to provide for step loading of the specimens. This paper describes using the torsion technique to measure KISCC for aluminum alloy 7075 having two thermo-mechanical treatments.Copyright

Stress-Corrosion Cracking and Fatigue Crack Growth Behavior of Ti-6Al-4V Plates Consolidated from Low Cost Powders

Titanium is highly desirable for a wide range of applications because of its combination of high strength, low density and outstanding corrosion characteristics. However, the cost of titanium, produced by conventional technology, is high compared to steel and aluminum, which is a result of high extraction and processing costs. New approaches are being investigated maintaining required quality while lowering the cost of finished products. Ti alloy powder, Ti-6Al-4V, manufactured by a low cost hydride-process and consolidated into flat products (sheet, plate), were studied. The results of the study were compared with the properties obtained from plates of Armstrong Titanium consolidated powder. To remove the prior history of consolidation, the plates are beta annealed and the test results are compared with “as received” condition. The mechanism of the fatigue crack growth rate difference, fracture toughness, and stress-corrosion cracking resistance in terms of the respective Ti-6Al-4V microstructure differences will be discussed.

Environmentally Assisted Cracking in Advanced Aerospace Aluminums

Aerospace alloys, often aluminums, are frequently exposed to corrosive environments resulting from naval service. These environments may produce significant changes in crack growth characteristics in these materials. An experiment was designed to characterize the effects of environment on crack growth rate and fracture mechanism for existing cracks in aluminum 7050-T7451 plate material. This data will be comparatively analyzed against aluminum 7075- T7631, an alloy with known susceptibility to corrosion, in order to determine the relative susceptibility of 7050-T7451, generally considered a superior aluminum alloy in terms of strength and corrosion resistance. The resulting data and subsequent analysis can in turn be used in more accurate determination of aircraft component service life in common corrosive environments experienced by aircraft in naval service.

Non-Destructive Evaluation of Thermal Spray Coating Interface Quality By Eddy Current Method

Thermal spray coating is usually applied through directing molten or softened particles at very high velocities onto a substrate. An eddy current non‐destructive inspection technique is presented here for thermal spray coating interface quality characterization. Several high‐velocity‐oxy‐fuel (HVOF) coated steel plates were produced with various surface preparation conditions or spray process parameters. A quad‐frequency eddy current probe was used to manually scan over the coating surface to evaluate the bonding quality. Experimental results show that different surface preparation conditions and varied process parameters can be successfully differentiated by the impedance value observed from the eddy current probe. The measurement is fairly robust and consistent. This non‐contact, nondestructive, easy‐to‐use technique has the potential for evaluating the coating quality immediately after its application so that any defects can be corrected immediately.