Phillip D. Hailey

Corrosion Resistances of Iron-Based Amorphous Metals with Yttrium and Tungsten Additions in Hot Calcium Chloride Brine & Natural Seawater: Fe48Mo14Cr15Y2C15B6 and W-Containing Variants

Yttrium-containing SAM1651 (Fe{sub 48.0}Cr{sub 15.0}Mo{sub 14.0}B{sub 6.0}C{sub 15.0}Y{sub 2.0}), has a critical cooling rate (CCR) of approximately 80 Kelvin per second, while SAM2X5 (Fe{sub 49.7}Cr{sub 17.7}Mn{sub 1.9}Mo{sub 7.4}W{sub 1.6}B{sub 15.2}C{sub 3.8}Si{sub 2.4}) with no yttrium has a higher critical cooling rate of approximately 600 Kelvin per second. SAM1651s low CCR enables it to be rendered as a completely amorphous material in practical materials processes. Chromium (Cr), molybdenum (Mo) and tungsten (W) provide corrosion resistance; boron (B) enables glass formation; and rare earths such as yttrium (Y) lower critical cooling rate (CCR). The passive film stability of these Fe-based amorphous metal formulations have been found to be superior to that of conventional stainless steels, and comparable to that of Ni-based alloys, based on electrochemical measurements of the passive film breakdown potential and general corrosion rates.

Corrosion Resistance of Amorphous Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 Coating: A New Criticality Control Material

An iron-based amorphous metal with good corrosion resistance and a high absorption cross section for thermal neutrons has been developed and is reported here. This amorphous alloy has the approximate formula Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 and is known as SAM2X5. Chromium, molybdenum, and tungsten were added to provide corrosion resistance, while boron was added to promote glass formation and the absorption of thermal neutrons. Since this amorphous metal has a higher boron content than conventional borated stainless steels, it provides the nuclear engineer with design advantages for criticality control structures with enhanced safety. While melt-spun ribbons with limited practical applications were initially produced, large quantities (several tons) of gas-atomized powder have now been produced on an industrial scale, and applied as thermal-spray coatings on prototypical half-scale spent-nuclear-fuel containers and neutron-absorbing baskets. These prototypes and other SAM2X5 samples have undergone a variety of corrosion testing, including both salt-fog and long-term immersion testing. Modes and rates of corrosion have been determined in various relevant environments and are reported here. While these coatings have less corrosion resistance than melt-spun ribbons and optimized coatings produced in the laboratory, substantial corrosion resistance has been achieved.

Crevice Repassivation Potential of Alloy 22 in High-Nitrate Dust Deliquescence Type Environments

The nitrate ion (NO{sub 3}{sup -}) is an inhibitor for crevice corrosion of Alloy 22 (N06022) in chloride (Cl{sup -}) aqueous solutions. Naturally formed electrolytes may contain both chloride and nitrate ions. The higher the ratio R = [NO{sub 3}{sup -}]/[Cl{sup -}] in the solution the stronger the inhibition of crevice corrosion. Atmospheric desert dust contains both chloride and nitrate salts, generally based on sodium (Na{sup +}) and potassium (K{sup +}). Some of these salts may deliquescence at relatively low humidity at temperatures on the order of 150 C and higher. The resulting deliquescent brines are highly concentrated and especially rich in nitrate. Electrochemical tests have been performed to explore the anodic behavior of Alloy 22 in high chloride high nitrate electrolytes at temperatures as high as 150 C at ambient atmospheres. Naturally formed brines at temperatures higher than 120 C do not induce crevice corrosion in Alloy 22 because they contain high levels of nitrate. The inhibitive effect of nitrate on crevice corrosion is still active for temperatures higher than 100 C.

Enhanced Corrosion Resistance of Iron-Based Amorphous Alloys

Iron-based amorphous alloys possess enhanced hardness and are highly resistant to corrosion, which make them desirable for wear applications in corrosive environments. It was of interest to examine the behavior of amorphous alloys during anodic polarization in concentrated salt solutions and in the salt-fog testing. Results from the testing of one amorphous material (SAM2X5) both in ribbon form and as an applied coating are reported here. Cyclic polarization tests were performed on SAM2X5 ribbon as well as on other nuclear engineering materials. SAM2X5 showed the highest resistance to localized corrosion in 5 M CaCl{sub 2} solution at 105 C. Salt fog tests of 316L SS and Alloy 22 coupons coated with amorphous SAM2X5 powder showed resistance to rusting. Partial devitrification may be responsible for isolated pinpoint rust spots in some coatings.

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.

Comparative Study on the Corrosion Resistance of Fe-Based Amorphous Metal, Borated Stainless Steel and Ni-Cr-Mo-Gd Alloy

Iron-based amorphous alloy Fe{sub 49.7}Cr{sub 17.7}Mn{sub 1.9}Mo{sub 7.4}W{sub 1.6}B{sub 15.2}C{sub 3.8}Si{sub 2.4} was compared to borated stainless steel and Ni-Cr-Mo-Gd alloy on their corrosion resistance in various high-concentration chloride solutions. The melt-spun ribbon of this iron-based amorphous alloy have demonstrated a better corrosion resistance than the bulk borated stainless steel and the bulk Ni-Cr-Mo-Gd alloy, in high-concentration chloride brines at temperatures 90 deg. C or higher. (authors)

Thermogravimetric Thin Aqueous Film Corrosion Studies of Alloy 22; Calcium Chloride Solutions at 150°C and Atmospheric Pressure

The extent of reaction of alloy-22 with limited amounts of aqueous calcium chloride (CaCl{sub 2}) was investigated. Alloy-22 is a highly corrosion-resistant nickel-chromium-molybdenum-tungsten alloy. Specimens were polished to a mirror finish prior to aerosol salt deposition. An aqueous film was formed by deliquescence of deposited CaCl{sub 2} at 150 C and 22.5% relative humidity (RH). The reactant gas was a continuous flow of purified humidified laboratory air. The reaction progress as a function of time was continuously measured in-situ by a micro-balance. An initial weight gain due to deliquescence of the CaCl{sub 2} was observed. A steady weight loss was observed over the next 72 hours, after which no further weight change was observed. During this weight loss, white precipitates formed and the specimens surface became visibly dry. The precipitate crystals were identified as Ca(OH){sub 2} by post-test Raman spectroscopy; however, energy dispersive X-ray spectroscopy indicated that there was a significant amount of chlorine contained in them.

Long-Term Immersion Testing of Alloy 22 and Titanium Grade 7 Double U-Bend Specimens

Double U-bend specimens of Alloy 22 (N06022) and Titanium Grade 7 (R52400) were exposed to a naturally aerated concentrated Basic Saturated Water (BSW) electrolyte at 105 C for over six years. Different type of discoloration of the Ti Gr 7 and Alloy 22 specimens was observed. General Corrosion was minimal and not distinguishable under a scanning electron microscope. None of the tested specimens suffered environmentally assisted cracking (EAC) or localized corrosion under the tested conditions. The specimens retained their residual stress after the long environmental exposure.