Pellumb Jakupi

Crevice Corrosion of Ni-Cr-Mo Alloys

Ni-Cr-Mo alloys were developed for their exceptional corrosion resistance in a variety of extreme corrosive environments. An alloy from this series, Alloy-22, has been selected as the reference material for the fabrication of nuclear waste containers in the proposed Yucca Mountain repository located in Nevada (US). A possible localized corrosion process under the anticipated conditions at this location is crevice corrosion. therefore, it is necessary to assess how this process may, or may not, propagate if the use of this alloy is to be justified. Consequently, the primary objective is the development of a crevice corrosion damage function that can be used to assess the evolution of material penetration rates. They have been using various electrochemical methods such as potentiostatic, galvanostatic and galvanic coupling techniques. Corrosion damage patterns have been investigated using surface analysis techniques such as scanning electron microscopy (SEM) and optical microscopy. All crevice corrosion experiments were performed at 120 C in 5M NaCl solution. Initiating crevice corrosion on these alloys has proven to be difficult; therefore, they have forced it to occur under either potentiostatic or galvanostatic conditions.

The Influence of Microstructure on the Corrosion of Magnesium Alloy ZEK100

The corrosion behavior of Mg alloy ZEK100 was investigated in water and in chloride-containing environments. The alloy is composed of an α-Mg matrix and a wide-spread dispersion of a Mg-Zn-Nd phase...

Microstructural Feature Analysis of X65 Steel Exposed to Ripple Load Testing Under Near Neutral pH Conditions

Newly designed miniature Compact Tension (CT) specimens, designed according to standard ASTM dimension ratios, and machined out of previously in-service X65 pipeline steel were exposed to super-imposed cyclic loading at high mean stresses in NS4 solution to determine the behaviour of X65 steel to ripple loading under near neutral pH conditions. Electron Back-Scatter Diffraction (EBSD) was used to study the microstructural grain geometry to determine if it influences stress-corrosion cracking (SCC) initiation and propagation. Prior to ripple load testing, finely polished X65 surfaces were subjected to EBSD measurements to characterize the microstructure’s geometry; i.e., grain and grain boundary orientations and texture. On the same locations where EBSD maps were recorded, a grid of cross-shaped resist markings — approximately 1–5 μm in size — were deposited every 15 μm across the analyzed surfaces. Following microscopic analyses the specimens were pre-cracked and re-examined to determine whether the crack initiation procedure preconditions the residual strain (quantified by grain misorientations) around an induced crack. Then, ripple load testing at stress levels characterized by load ratios (R) greater than 0.9 was performed, while simultaneously monitoring the open-circuit potential (OCP) at room temperature. The originally characterized surface was again re-examined to determine if the crack tip propagated preferably along a specific crystallographic grain orientation by comparing the shifts in each cross-shaped grid. Results from this investigation will help determine if there is a link between microstructural grain geometries and transgranular stress corrosion cracking.© 2014 ASME

The Influence of Temperature on the Passive Film Properties of ASTM Grade-7 Titanium

The influence of temperature on the passivity of highly corrosion-resistant alloys is often critical to their industrial performance. We have been studying the effects of temperature (up to ~150°C) on Ti alloys, such as ASTM Grade-7 (Ti-7), in neutral saline solutions, using open circuit potential measurements and electrochemical impedance spectroscopy (EIS). As the temperature increased, up to ~80°C, the oxide film resistance increased and the capacitance decreased, consistent with thickening of, and defect annealing within, the passive film. At higher temperatures, the passive film displayed decreasing resistance and increasing capacitance, and a second time constant developed at low frequencies, indicating the introduction of pores or cracks into the oxide. Thus, this thin oxide film ruptures under certain conditions, leading to a temporary loss of corrosion protection. Such oxide breakdown and repair yields potential transients (“electrochemical noise”), which contain information on the electrochemical behaviour of the material.