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Several oxide dispersion strengthened (ODS) alloys were tested for cyclic, high velocity, oxidation, and hot corrosion resistance. These results were compared to the resistance of an advanced, NiCrAl coated superalloy. An ODS FeCrAl were identified as having sufficient oxidation and hot corrosion resistance to allow potential use in an aircraft gas turbine without coating.
Marine atmosphere and laboratory stress corrosion test results on smooth and precracked specimens from 7075, 7475, 7050, and 7049 alloy plates (1.25 and 3.0-in. thick) are presented. It is shown that for a given strength level, alloys 7050-T7X and 7049-T7X have superior short-transverse stress corrosion resistance (SCR) to 7X75-T7X. At typical strength levels above the minimum, for example, SCR of these alloys is considerably better than that of 7075-T76, and approaches that of 7075-T73. Alloy 7475 maintains an advantage in the area of fracture toughness, however, because it can be thermally processed to give particularly clean microstructures. Results from precracked specimens are in good qualitative agreement with those obtained from smooth specimens. Although both specimen types are capable of distinguishing between -T6, -T76 and -T73 tempers in relatively short time periods the precracked specimen provides more information about crack growth rates.
It is pointed out that any proper design of interference fit fastener, interference fit bushings, or stress coining processes should consider both the stress-corrosion susceptibility and fatigue-life improvement together. Investigations leading to such a methodology are discussed. A service failure analysis of actual aircraft parts is considered along with the stress-corrosion susceptibility of cold-working interference fit bushings. The optimum design of the amount of interference is considered, giving attention to stress formulas and aspects of design methodology.
Candidate materials were tested for their ability to limit corrosion under conditions that approximate those found in typical solar-energy system. In addition to presentation of data, report also includes discussion of different forms of corrosion and recommendations for future work.
Combinations of chemical compounds that reportedly reduce the corrosion of aluminum in fresh water were evaluated. These included combinations of borates, nitrates, nitrites, phosphates, silicates, and mercaptobenzothiazole. Eight of fifty inhibitor combinations evaluated gave excellent corrosion protection and compared favorably with sodium chromate, which has generally been considered standard for many years.
Inhibitors which appeared promising in previous tests and additional inhibitors including several proprietary products were evaluated. Evaluation of the inhibitors was based on corrosion protection afforded an aluminum-mild steel-copper-stainless steel assembly in a hot corrosive water. Of the inhibitors tested two were found to be effective and show promise for protecting multimetallic solar heating systems.
The effectiveness of several potential fuel additives in reducing the effects of sodium sulfate-induced hot corrosion was evaluated in a cyclic Mach 0.3 burner rig. The potential inhibitors examined were salts of Al, Si, Cr, Fe, Zn, Mg, Ca, and Ba. The alloys tested were IN-100, U-700, IN-738, IN-792, Mar M-509, and 304 stainless steel. Each alloy was exposed for 100 cycles of 1 hour each at 900 C in combustion gases doped with the corrodant and inhibitor salts and the extent of attack was determined by measuring maximum metal thickness loss. The most effective and consistent inhibitor additive was Ba (NO3)2 which reduced the hot corrosion attack to nearly that of simple oxidation.
Samples of a cobalt-base alloy, Mar M-509, were subjected to hot corrosion in a Mach-0.3 burner rig. The corrodent was NaCl added as an aqueous solution to the combustion products of a sulfur-containing Jet-A fuel. The metal temperature was fixed at 900 C. The extent of hot corrosion increased by a factor of three as the fuel-to-air mass ratio was increased from 0.033 to 0.050. Because the depositing salt was always Na2SO4, the increased attack appeared to be related to the gas composition.
Cast specimens of nickel-base superalloys 713C and Mar-M200 with nominal additions of 0, 0.5, and 1 wt% Si were evaluated for oxidation and corrosion resistance, tensile and stress-rupture properties, microstructure, and phase relations. Results are compared with those of an earlier study of the effects of Si in B-1900. Si had similar effects on all three superalloys. It improves oxidation resistance but the improvement in 713C and Mar-M200 was considerably less than in B-1900. Hot-corrosion resistance is also improved somewhat. Si is, however, detrimental to mechanical properties, in particular, rupture strength and tensile ductility. Si has two obvious microstructural effects. It increases the amount of gamma-prime precipitated in eutectic nodules and promotes a Mo(Ni,Si)2 Laves phase in the alloys containing Mo. These microstructural effects do not appear responsible for the degradation of mechanical properties, however.
Studies have been performed to examine the effect of NaCl vapor on the Na2SO4-induced hot corrosion of the alumina former NiAl. In the incubation period associated with such hot corrosion, NaCl(g) has been shown to be effective in removing aluminum from below the protective alumina scale and redepositing it as Al2O3 whiskers on the surface of the Na2SO4-coated sample. Similar effects seen in simple oxidation are associated with isothermal rupturing of the protective alumina scale.
Seawater minimizes pitting corrosion of aluminum-alloy test samples. Of three corrosion-inhibiting methods evaluated using (a) chromate inhibitors in saltwater, (b) surface treating sample via anodizing or alodine treatment, and (c) synthetic seawater, synthetic seawater was most effective test medium, since it is more uniform than fresh seawater.
The type and cause of corrosion in spot welded joints were determined by X-ray and chemical analysis. Fatigue and static tests showed the degree of degradation of mechanical properties. The corrosion inhibiting effectiveness of WD-40 compound and required renewal period by exposing typical joint specimens were examined.
Sodium chloride is believed to be the primary source of turbine engine contamination that contributes to hot corrosion. The behavior of NaCl-containing aerosols ingested with turbine intake air is very complex; some of the NaCl may vaporize during combustion while some may remain as particulates. The NaCl can lead to Na2SO4 formation by several possible routes or it can contribute to corrosion directly. Hydrogen or oxygen atom reaction with NaCl(c) was shown to result in the release of Na(g). Gaseous NaCl in flames can be partially converted to gaseous Na2SO4 by homogeneous reactions. The remaining gaseous NaCl and other Na-containing molecules can act as sodium carriers for condensate deposition of Na2SO4 on cool surfaces. A frozen boundary layer theory was developed to predict the rates of deposition. The condensed phase NaCl can be converted directly to condensed Na2SO4 by reaction with sulfur oxides and O2. Reaction of gaseous NaCl with Cr2O3 results in the vapor phase transport of chromium by the formation of complex Cr-containing gaseous molecules. Similar gaseous complexes are formed with molybdenum. The presence of gaseous NaCl was shown to affect the oxidation kinetics of Ni-Cr alloys. It also causes changes in the surface morphology of Al2O3 scales formed on Al-containing alloys.
An experimental program was undertaken to identify effective substitutes for part of the Cr in 304 stainless steel as a method of conserving the strategic element Cr. Although special emphasis was placed on tensile properties, oxidation and corrosion resistance were also examined. Results indicate that over the temperature range of -196 C to 540 C the yield stress of experimental austenitic alloys with only 12 percent Cr compare favorably with the 18 percent Cr in 304 stainless steel. Oxidation resistance and in most cases corrosion resistance for the experimental alloys were comparable to the commercial alloy. Effective substitutes for Cr included Al, Mo, Si, Ti, and V, while Ni and Mn contents were increased to maintain an austenitic structure.
The corrosion of sodium sulfate coated cobalt alloys was measured and the results compared to the cyclic oxidation of alloys with the same composition, and to the hot corrosion of compositionally equivalent nickel-base alloys. Cobalt alloys with sufficient aluminum content to form aluminum containing scales corrode less than their nickel-base counterparts. The cobalt alloys with lower aluminum levels form CoO scales and corrode more than their nickel-base counterparts which form NiO scales.
Sodium sulfate induced hot corrosion of B-1900 and NASA-TRW VIA at 900 C was studied with special emphasis on the chemical reactions occurring during and immediately after the induction period. Thermogravimetric tests were run for set periods of time after which the samples were washed with water and water soluable metal salts and/or residual sulfates were analyzed chemically. Element distributions within the oxide layer were obtained from electron microprobe X-ray micrographs. A third set of samples were subjected to surface analysis by X-ray photoelectron spectroscopy. Evolution of SO2 was monitored throughout many of the hot corrosion tests. Results are interpreted in terms of acid-base fluxing mechanisms.
The effect of SO3 pressure in the gas phase on the Na2SO4 induced hot corrosion of Co-Cr, Ni-Cr, and Co-Cr-Al alloys was studied in the temperature range 700 to 750 C. The degradation of the Co-Cr and Ni-Cr alloys was found to be associated with the formation of liquid mixed sulfates (CoSO4-Na2SO4 or NiSO4-Na2SO4) which provided a selective dissolution of the Co or Ni and a subsequent sulfidation oxidation mode of attack which prevented the maintenance of a protective Cr2O3 film. A clear mechanism was not developed for the degradation of Co-Cr-Al alloys. A pitting corrosion morphology was induced by a number of different mechanisms.
Sodium chloride is one of the primary contaminants that enter gas turbine engines and contribute, either directly or indirectly, to the hot corrosion degradation of hot-gas-path components. The paper surveys the results of laboratory experiments along with thermodynamic and mass transport calculations, intended for elucidating the behavior of sodium chloride in combustion environments. It is shown that besides being a source of sodium for the formation of corrosive liquid Na2SO4, the NaCl itself contributes in other indirect ways to the material degradation associated with the high-temperature environmental attack. In addition, the experimental results lend credence to the conceptual scheme presented schematically (behavior of NaCl in a turbine engine combustion gas environment) and resolve conflicting aspects of relevant NaCl misconceptions.