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Cast duplex stainless steel, alloy 2205, a mix of ferrite and austenite, was heat-treated at 845 • C which produced additional phases, e.g., sigma and chi. The new phases depleted the surrounding grains of passivity-promoting elements such as Cr, causing alloy sensitization and a corresponding decrease in pitting resistance. A low temperature gas-phase interstitial hardening (IH) surface modification process was used to surface alloy sensitized samples. Anodic polarization curves showed that after IH treatment, the pitting resistance, determined by the pitting potential, of sensitized samples was restored to values of the as-cast material. Duplex stainless steels (DSS), a mix of ferrite and austenite, are finding applications in the oil and gas, chemical, food and beverage, power, transportation, and paper industries 1-5,10,11 because of their good corrosion resistance and mechanical properties. However, when DSS are exposed to temperatures between 650• C and 950• C, additional phases, e.g., sigma and chi, form. The formation of these phases results in a depletion of passivity-promoting elements, e.g., Cr, in the surrounding grains, resulting in sensitization and decreased corrosion resistance.1-11 The Cr concentration in the depleted regions has been reported to be in the range of 12.5 to 19.5 at%. 5,8,9 In recent years, low temperature (generally lower than 500• C, but alloy dependent) surface modification processes using gas or plasma interstitial hardening (IH) have been developed for introducing substantial amounts of carbon and/or nitrogen (generally between 10 to 15 at%) into stainless steels without the formation of detrimental carbides or nitrides. [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] As implied by the name, carbon and nitrogen reside in interstitial lattice sites. The surface alloyed region, the case, ranges in thickness between 5 and 30 μm depending on treatment time and temperature. To form a case, strong carbide/nitride forming elements, such as Cr, are essential (for example, References 13,16,18,22). Interstitial hardening of a number of stainless steels, including austenitic, martensitic, and duplex, has been shown to improve the hardness and wear, fatigue, galling, and corrosion resistance.12-26 IH treatments have been shown to be effective in improving the pitting resistance in stainless steels with Cr concentrations as low as 13 at%. 19,21 In previous work, it was shown that the inherently good pitting resistance of DSS alloy 2205 was retained while increasing the crevice corrosion and fatigue resistance. 25 The inherent pitting resistance (high value of Epit) of alloy 2205 is a result of high concentrations of passivity-promoting alloying elements. As discussed above, when a stainless steel becomes sensitized, the alloying elements (i.e., Cr) that provide passivity, in addition to being strong carbide formers, are depleted in the regions adjacent to where the new phases form. This communication discusses the application of IH to restore the pitting corrosion resistance ...

A high density, low porosity Al-Co-Ce metallic coating has been successfully applied to an AA-2024-T351 substrate using a new PTS process in thicknesses ranging from 100-600 µm. The three proposed modes of corrosion protection brought about by coating (e.g., a localized corrosion barrier, a sacrificial anode to supply sacrificial cathode protection of any exposed AA 2024-T351, and active inhibitor release and protection of AA 2024-T351) have been verified to function and the results are promising. Although XRD has identified nanocrystalline peaks in the as-sprayed deposit, the best PTS applied coating samples have approached the barrier properties of fully amorphous melt spun ribbons of similar composition with regard to resistance to pitting. However, high surface area PTS coatings produced superior capabilities for active inhibitor release. IntroductionAluminum alloys are widely used in aerospace applications, where their high strength-toweight ratios make them ideal structural metals. They also have good atmospheric corrosion resistance as a result of the aluminum oxide films that form on the alloy surfaces. Unfortunately, the protective properties of these films may break down locally and allow extensive corrosion to propagate through the bulk material. A number of methods are used to achieve better corrosion protection of these structural aluminum alloys, including mechanical cladding, chromate conversion coatings, and the application of corrosion inhibiting paints which typically contain slightly soluble chromate pigments. Mechanical aluminum cladding, for instance, is widely used and provides good corrosion protection as a sacrificial anode by suppressing local pitting, however, lacks an active inhibitor and its corrosion resistance could be lost during surface repair. An additional factor to consider is that cladding is not field-replaceable. Field-replaceability is a benefit for many military and commercial applications where component removal is non-trivial and equipment downtime is very costly. Corrosion-inhibiting paints may provide soughtafter self-healing capability; however, these typically require the use of chromate-or lead-based pigments and, due to the toxicity of both lead and chromate, are being phased out from use based on environmental concerns. It is extremely desirable to develop a metal coating for high-strength aerospace alloys, such as AA 2024-T351, that can protect in three important ways: (i) serve as local corrosion barrier with respect to both uniform corrosion and local corrosion along with tunable corrosion barrier properties, (ii) act as a sacrificial anode (i.e., have an opencircuit potential (OCP) that is ca. 20-300 mV below that of AA 2024-T351), and (iii) 59 60 supply soluble ions that function as inhibitors to mitigate corrosion of high performance aluminum structures, especially at locations where the coating was scratched or removed. These three functions are not mutually exclusive. However, optimization of each in a spray applied coating may not be achieved in the same w...

A high density, low porosity Al-Co-Ce metallic coating has been successfully applied to a AA-2024-T351 substrate using a pulsed thermal spray (PTS) process in thicknesses ranging from 100-500 micrometers. The three proposed modes of corrosion protection brought about by the coating (e.g., a localized corrosion barrier, a sacrificial anode to supply cathodic protection of any exposed AA 2024-T351, and active inhibitor release and protection of AA 2024-T351) have been verified to function simultaneously. Although XRD has identified nanocrystalline peaks in the as-sprayed deposit, the best PTS applied coating samples have approached the localized corrosion properties of fully amorphous melt spun ribbons of similar composition. However, high surface area PTS coatings produced superior capabilities for active inhibitor release.