Joseph R. Kish

Physical Characterization of Cathodically-Activated Corrosion Filaments on Magnesium Alloy AZ31B

In this study, the filiform-like corrosion behavior of Mg alloy AZ31B was characterized using analytical microanalysis. Recent electrochemical measurements have shown that the corrosion filaments on AZ31B support enhanced cathodic reaction kinetics, but there has been little strong physical evidence published to explain this behavior. Hence, the specific aim of the investigation was to contribute to the understanding of the physical origin of the “cathodic activation” of the corrosion filaments. Highlights of this investigation include the presence of through-thickness cracks within the corrosion filaments, Al-Mn particles (identified as Al11Mn4 with energy-dispersive x-ray spectroscopy (EDS) quantification) in the preexisting films and filament corrosion products, and a significant enrichment of Zn at the corrosion filament/metal interface after aging of the filament. Diffraction patterns of the corrosion filaments indicated they were composed of nanocrystalline magnesium oxide (MgO), which was suggested...

Influence of edge effects on local corrosion rate of magnesium alloy/mild steel galvanic couple.

The effect of the insulator-mixed-material edge on the galvanic corrosion rate of magnesium alloy (AE44)-mild steel (MS) couple is experimentally studied using scanning vibrating electrode technique (SVET), profilometry, and classical electrochemistry. The local and average corrosion rates estimated from the experimental depth of anodic attack profile of AE44-MS couple are validated by 2D and 3D corrosion numerical models. Our study demonstrates experimentally and theoretically that the presence of the insulator edge increases the local current density, which enhances the corrosion rate. The extent of the local corrosion rate enhancement and its effect on the overall corrosion rate of the mixed material is discussed and depends on the mixed materials geometry and the edge type.

Corrosion Control of Type 316L Stainless Steel in Sulfamic Acid Cleaning Solutions

Abstract An investigation was conducted to better understand the corrosion behavior of Type 316L (UNS S31603) stainless steel in sulfamic acid (NH2HSO3) cleaning solutions and the relative merits o...

Corrosion of Bed Nozzle Alloys in a Wood-Waste Fluidized Bed Power Boiler

Abstract A field test was conducted to identify a more corrosion-resistant material than Type 310H (UNS S31009) stainless steel, from which to fabricate bed nozzles for a fluidized bed power boiler that burns salt-laden wood-waste (hogged) fuel. Test nozzles fabricated from Type 310H stainless steel, Alloys 556 (UNS R30556), 59 (UNS N06059), HR160 (UNS N12160), and 625 (UNS N06625) were installed, subsequently removed, and examined as a function of time. Of the various alloys tested, Alloys 625 and HR160 had the best performance in terms of the amount of thickness loss, and the extent of internal damage exhibited. However, those alloys were still susceptible to corrosion by the fireside (fluidized bed) environment and, therefore, will require replacement over time. Despite the observed corrosion, those two alloys significantly extended the nozzle life beyond that of Type 310H stainless steel. Nozzle design was found to have a strong influence on the apparent corrosion resistance of Alloy 625 and, therefor...

Technical Note: Corrosion Resistance of Alloy 33 (Fe-33Cr-32Ni) in Supercritical Water

This study examines the corrosion resistance of wrought Alloy 33 (Fe-33Cr-32Ni) exposed in a static autoclave containing 25 MPa supercritical water at 625°C for 500 h. Gravimetric measurements in combination with electron microscopy analyses showed that the formation of a thin, dense external single-layer Cr2O3-based scale provides Alloy 33 with a promising corrosion resistance. However, the microstructure was not stable, as Cr-rich α-ferrite precipitates formed in the subsurface during the short exposure. The suitability of this alloy as a candidate fuel cladding for the Generation IV supercritical water-cooled nuclear reactors concept is discussed in light of the findings presented.

On the Oxidation Resistance of Alloy 800HT Exposed in Supercritical Water (SCW)

The purpose of this study was to examine the structure and composition of the oxide scales formed on Alloy 800HT in a supercritical environment after a relatively short exposure time (circa 500 h) to provide a more complete physical description of the metallurgical factors that influence the protectiveness of the scale formed. This was achieved using transmission electron microscopy and associated techniques on site-specific cross-section samples prepared by focused ion beam milling. No physical evidence in support of oxide scale exfoliation was found. Rather, the formation of the discontinuous magnetite (Fe3O4) outer nodular layer is believed to be controlled to a large extent by the underlying microstructure and the presence of ϵ-martensite plates in particular. The formation of a thin compact corundum-type ([Cr,Fe]2O3) base scale is considered to be sufficiently encouraging to justify an effort to reduce or eliminate the small volume fraction of ϵ-martensite from the starting microstructure of Alloy 80...

Prediction And Measurement of Corrosion Rates of Stainless Steel In Concentrated Sulfuric Acid

When exposed to concentrated sulfuric acid, stainless steel exhibits unique electrochemical behavior. This behavior can be observed as an oscillation in open-circuit potential between the active and passive states. The transient nature of the corrosion behavior under these conditions results in a distinct challenge for measuring and predicting corrosion rates. Using a series of commercial alloys with various nickel contents, this paper outlines the utilization of electrochemical experimentation to refine the prediction of corrosion rates. The paper also discusses some of the difficulties associated with many traditional electrochemical techniques such as potentiodynamic scans when used for characterizing systems that undergo oscillations in open-circuit potential.

Corrosion of High-Alloy Superheater Tubes in a Coastal Biomass Power Boiler

Abstract Metallurgical examinations were conducted on a set of damaged high-alloy (Type 310H [UNS S31009] stainless steel and Alloy 625 [UNS N66250] weld overlay) superheater tubes removed from a c...

Characterization of Localized Filament Corrosion Products at the Anodic Head on a Model Mg-Zn-Zr Alloy Surface

Localized filament corrosion products at the anodic head on a model Mg-1%Zn-0.4%Zr alloy surface were characterized by electron microscopy techniques of site-specific lamella prepared by focused ion beam milling. It is revealed that the anodic head propagates underneath a largely intact thin and dense MgO surface film and comprises dense aggregates of nano-crystalline MgO within a nano-porous Mg(OH)2 network. The findings contribute new supportive direct imaging insight into the source of the enhanced H2 evolution that accompanies anodic dissolution of Mg and its alloys.

Effect of surface preparation on the corrosion resistance of friction stir linear lap welded AZ31B-H24

ABSTRACT The effect of surface preparations on the atmospheric (continuous NaCl salt fog) corrosion resistance of bare (uncoated) linear friction stir lap welded magnesium (Mg) alloy AZ31B-H24 joints was investigated. The surface preparations evaluated included as-received (as-welded for the weld zones), mechanically abraded and acid-cleaned. Electrochemical potentiodynamic polarisation, X-ray photoelectron spectroscopy and Auger electron spectroscopy measurements were made to elucidate the link between the surface preparation and the relative corrosion resistance observed across the microstructural zones of the joints. Heavy metal surface contamination of the as-received material, rather than the air-formed oxide or differences in alloy grain size resulting from variations in thermo-mechanical processing history, was found to be the controlling factor affecting the corrosion mode observed and, in turn, the relative corrosion resistance across the joint. Removal of the heavy metal contamination changed the corrosion mode from pit-like to filament-like, which in turn homogenised the corrosion resistance across the joint in the continuous salt fog exposure.