J. R. Scully

Corrosion-resistant metallic coatings

We describe recent computational and experimental studies on the corrosion properties of metallic coatings that can be tailored (tuned) to deliver up to three corrosion-inhibiting functions to an underlying substrate. Attributes are tuned by a selection of alloy compositions and nanostructures, ideally in alloy systems that offer flexibility of choice to optimize the corrosion-resisting properties. An amorphous Al-based coating is tuned for corrosion protection by on-demand release of ionic inhibitors to protect defects in the coating, by formation of an optimized barrier to local corrosion in Cl− containing environments, as well as by sacrificial cathodic prevention. Further progress in this field could lead to the design of the next generation of adaptive or tunable coatings that inhibit corrosion of underlying substrates.

Metallurgical factors influencing the corrosion of aluminum, Al-Cu, and Al-Si alloy thin films in dilute hydrofluoric solution

The corrosion behavior of sputter-deposited Al, Al-Cu, and Al-Si alloy thin films in dilute hydrofluoric (HF) acid solution was investigated. These materials maintain a thin aluminum oxide film in dilute HF solutions and, consequently, are susceptible to localized corrosion. Pit densities increase for the alloys with Cu and, to a lesser extent, Si additions. Open circuit potentials (OCP) are more positive for such alloys relative to the OCP of pure Al. Metastable pits in Al-Cu alloys are formed in Cu-depleted zones at grain boundaries which are galvanically coupled to adjacent θ-Al2Cu precipitates. Metastable pits in Al-Si alloys are formed in the Al matrix which is galvanically coupled to adjacent elemental Si nodules. θ-Al2Cu has different electrochemical characteristics than Al, even though both maintain a thin Al oxide in dilute HF solutions. θ-Al2Cu has a more positive OCP than pure Al and facilitates cathodic reactions at enhanced rates relative to pure Al. Hence, its presence raises the potential of the adjacent pure Al grain boundary to potentials which increase the probability of metastable pitting. Evidence is also presented which suggests that metastable pit growth may be cathode limited. A new hypothesis describing one mechanism by which θ-Al2Cu supports cathodic electron transfer re-actions is discussed.

The influence of palladium on the hydrogen-assisted cracking resistance of PH 13-8 Mo stainless steel

We compare the hydrogen-assisted cracking resistance of wrought PH 13-8 Mo stainless steel alloyed with 0.4 to 1.0 wt pct palladium to the conventional alloy when aged to yield strengths of 1170 to 1250 MPa. Intergranular hydrogen cracking is suppressed with Pd in both static load and constant extension rate tests conducted with electrochemical hydrogen charging. These results are analyzed to elucidate the role of Pd in suppressing intergranular cracking. Palladium is found both in substitutional solid solution in the martensitic phase and also in the form of randomly distributed PdAl precipitates in all Pd-modified alloys. Interfacial segregation of Pd to grain boundaries and lath boundaries is not observed at any levels above a detection limit of approximately 0.5 monolayers. Hydrogen permeation analyses indicate that hydrogen ingress is not inhibited by Pd but that apparent diffusion coefficients are lowered relative to the conventional alloy. Lower diffusion coefficients are consistent with the creation of a strong but reversible hydrogen trap, identified as the uniformly distributed PdAl phase. We hypothesize that PdAl trap sites force a redistribution of trapped hydrogen, which lowers the amount of interfacially segregated hydrogen at prior austenite grain boundaries for the electrochemical conditions applied. These assertions are supported by a simplistic trapping model for PH 13-8 Mo which shows that both the hydrogen trap binding energy and the trap density for the PdAl trapping site are greater than the hydrogen trap binding energy and density for prior austenite grain boundaries.

Predicting the Effect of Applied Potential on Crack Tip Hydrogen Concentration in Low-Alloy Martensitic Steels

Abstract The dependency of hydrogen environment embrittlement on applied potential of an ultrahigh-strength steel was explained. This is accomplished through (1) definition of crack chemistry and p...

Long-Term Effects of Cathodic Protection on Prestressed Concrete Structures: Hydrogen Embrittlement of Prestressing Steel

Abstract The issue of safe cathodic protection (CP) limits for prestressing steel in concrete was addressed in regard to concerns over hydrogen embrittlement (HE). The local environment at the stee...

Impressed-Current Cathodic Protection of Steel-Reinforced Concrete Pilings: Protection Criteria and the Threshold for Hydrogen Embrittlement

Abstract Safe cathodic protection (CP) limits for prestressing steel in concrete and the adequacy of CP using established criteria were evaluated in regard to hydrogen embrittlement (HE). Impressed-current CP was applied to laboratory scale pilings at current densities from 0.1 μA/cm2 to 3.0 μA/cm2 via a skirt anode located at the waterline. Adequate CP was achieved at positions 25 cm (9.8 in.) above to 50 cm (19.7 in.) below the waterline, according to the 100-mV depolarization criterion, at an apparent applied current density of 0.33 μA/cm2. However, the −780 mVSCE criterion was not met for currents as high as 1.33 μA/cm2 for these positions. Hydrogen production, absorption, and permeation in steel first was observed via embedded hydrogen sensors 50 cm and 25 cm above the water line at an applied current density of 0.33 μA/cm2. Observation of hydrogen production verified concerns that the local oxygen concentration might be depleted readily at modest CP levels and that local pH levels may be below 12.5....

Controlling Hydrogen Embrittlement in Precharged Ultrahigh-Strength Steels

Abstract An Fe-13Co-11Ni-3Cr-1Mo-0.2C steel alloy, processed for ultrahigh-strength and fracture toughness, exhibits three distinct hydrogen trap states in a complex precipitation-hardened martensitic microstructure and is susceptible to severe hydrogen embrittlement (HE) at threshold stress intensity levels as low as 20 MPa√m. The causes of HE susceptibility include very high crack-tip tensile stresses and a reservoir of diffusible hydrogen that is trapped reversibly with a binding energy, Eb, of 11.5±0.5 kJ/mol at (Fe,Cr,Mo)2C precipitates. This reversibly trapped hydrogen repartitions to interstitial sites proximate to the highly stressed crack tip and, subsequently, may retrap at martensitic lath interfaces to produce substantial local hydrogen concentrations and transgranular embrittlement. These results are pertinent to the control of HE in this modern ultrahigh-strength steel with a cadmium-plated coating and codeposited hydrogen (H). Thermal desorption spectroscopy demonstrates that 190°C baking r...

The Effect of Microstructural Variation on the Hydrogen Environment-Assisted Cracking of Monel K-500

The influence of microstructural variation on hydrogen environment-assisted cracking (HEAC) of Monel K-500 was evaluated using five nominally peak-aged lots of material tested under slow-rising stress intensity loading while immersed in NaCl solution under cathodic polarizations. Minimal variation in HEAC resistance among material lots was observed for an applied potential of −950 mVSCE (Eapp, vs saturated calomel), whereas lot-to-lot variability in the fracture morphology demonstrates a significant difference in the HEAC resistance at the less negative potential of −850 mVSCE, suggesting that relatively severe H environments produce sufficient crack-tip H to minimize the impact of metallurgical differences. Sensitivity analyses accomplished by varying the inputs used in decohesion-based, micromechanical models imply significant variations in HEAC resistance are possible for realistic changes in grain boundary toughness, hydrogen uptake behavior, and yield strength. Grain size, impurity segregation (including the effects of gettering elements), grain boundary character/connectivity, and crack path tortuosity are also considered in the context of HEAC susceptibility. Yield strength, global hydrogen content, as well as impurity segregation to grain boundaries, especially boron and sulfur, are speculatively considered to be the dominant contributions in determining HEAC resistance. Modifications that would incorporate the effects of grain boundary segregation are proposed for the KTH model; detailed validation of such changes require high-fidelity and quantitative inputs for the degree of grain boundary segregation. Regardless, fracture mechanics-based HEAC results, detailed microstructural characterization, and micromechanical modeling were successfully coupled to gain insights into the influences governing the microstructure-dependent HEAC susceptibility of Monel K-500.

Tunable multifunctional corrosion-resistant metallic coatings containing rare earth elements

Abstract: A variety of approaches enable deposition of metallic coatings containing rare earth (RE) elements for corrosion protection. Al-TM-RE amorphous and amorphous nanocrystalline alloys are one type of multifunctional material for protection of Al alloys. Ce and Al-Ce coatings have also been deposited. Results for high density, low porosity multifunctional Al-Co-Ce metallic coatings applied to an AA-2024-T351 substrate using a pulsed thermal spray (PTS) are discussed in detail. Three proposed modes of corrosion protection are provided by the coating (i.e. a localized corrosion barrier, a sacrificial anode to supply cathodic protection of any exposed AA 2024-T351, and active inhibitor release) to protect AA 2024-T351. Chemical protection is afforded by either Ce(OH) 2 2+ or Ce 3+ release.