Electrodes made of commercially pure titanium (CP-Ti) and a CoCrMo alloy are immersed at an open circuit in a phosphate buffer saline electrolyte at room temperature for different durations prior to electrochemical analyses. Open circuit potential measurements, electrochemical impedance spectroscopy measurements, and cyclic potentiodynamic polarization (CPP) scans are used to assess the impact of the immersion time on derived property values. Stable passivation layers formed on both materials during immersion. The corrosion potentials determined from the anodic legs of CPP scans become more cathodic, and the corrosion currents decrease to lower values after longer immersion times. Measured currents indicate the layers formed on CP-Ti stabilize during forward anodic scans and persist to the vertex potential, whereas passivation breakdown occurs during anodic scans with CoCrMo with active corrosion at voltages up to the vertex potential. The characteristics of the return cathodic legs of CPP scans represent the surface conditions at the vertex potential: characteristic corrosion property values derived from the test responses represent passive surfaces on CP-Ti and leached surfaces on CoCrMo rather than intrinsic properties of those materials.

Electrochemical tests were conducted to assess the corrosion performance of a developmental multiphase cermet nuclear waste form in different conditions. Corrosion currents were monitored as the surface equilibrated in an acidic brine solution during potentiostatic tests at different imposed potentials. Electrochemical impedance spectroscopy was used to quantify differences in the electrical properties of surfaces stabilised at different potentials. The passivating oxides that will stabilise the surface under disposal conditions are formed and maintained during testing. The measured corrosion currents were two orders of magnitude lower for passivated surfaces than for polished surfaces over a wide range of redox conditions. The electrochemical results were correlated with microscopic analyses of the corroded surfaces and physical models of the surface passivation and degradation processes are proposed. The measured corrosion rates are suitable for the long-term performance modeling of cermet nuclear waste forms.

A set of cast alloys was made by alloying 316L stainless steel powder with 5, 10, and 15 wt-% Mo to study the effects of the Mo content on the microstructures and corrosion behaviours. Sigma phases formed in all alloys and Laves phases co-precipitated in alloys with Mo additions of 10 and 15 wt-%. Alloys with higher Mo concentrations had greater volume fractions of both intermetallic phases, which contained higher Mo but lower Ni levels than the austenite. Potentiodynamic and potentiostatic tests were performed in pH 4 solution containing 10 mM NaCl to provide an aggressive chemical environment. All cast alloys had higher corrosion resistance than 316L SS. Electrochemical impedance spectroscopy showed the Mo content had a significant beneficial effect on the electrical properties of the passivating layers under highly oxidising conditions, but only a minor effect under moderately oxidising redox conditions.

Three alloys were made to represent waste forms for ZIRLO (R) cladding and metallic fuel wastes by alloying ZIRLO (R) and a surrogate metallic fuel waste mixture with various amounts of Inconel 718, 304 stainless steel, trim chromium, and trim copper. The objective was to establish if metallic wastes that contain ZIRLO (R) can be manufactured into durable waste forms for geological repository. Specimens were metallurgically characterized and subjected to electrochemical tests to assess their corrosion resistance. Tests were conducted in acidic and alkaline electrolytes prepared with and without NaCl to simulate ranges of environmental redox and chemical conditions that could occur within a breached waste package in a disposal facility. The microstructures and potentiodynamic scans of these alloys are presented. Surface analyses after the potentiodynamic scans indicate intermetallics enriched in iron, nickel, and copper corroded at relatively low potentials in Cl- solutions and were less durable than the zirconium-chromium phases.

The electrochemical corrosion behaviors of two multiphase alloys representing waste forms made with 316 L stainless steel and different amounts of surrogate metallic fuel wastes were measured and related to the microstructures. Potentiodynamic (PD) scans were performed in an acid brine solution and the corroded surfaces were characterized with scanning electron microscopy (SEM) to compare the electrochemical responses to the corrosion of specific phases. PD scans for the two multiphase alloys, 316 L stainless steel, and pure palladium were compared to understand the complex corrosion behavior of these multiphase alloys also recently classified as multi principle element alloys (MPEAs) and to determine the effects of alloying elements and noble metals present in constituent phases on the corrosion behavior.

The corrosion rates of alloys that will be present in spent fuel waste packages are being measured to represent the generation rates of H2 in a breached package under a range of Eh-pH conditions. The H2 concentration is a crucial parameter in repository performance analyses because the presence of even small amounts of H2 in a breached waste container can significantly decrease the oxidative-dissolution rate of directly-disposed spent UO2 fuel. Potentiostatic tests are being conducted in pH 4, 7, and 10 buffer solutions spiked with NaCl at several potentials that span the ranges of redox and chemical conditions that could occur in a breached waste package. The corrosion currents are monitored as the alloys corrode and changes in the electrical properties of the metal surfaces are measured using electrochemical impedance spectroscopy. Results of tests with 316L stainless steel, Zircaloy-4, and 4320 low alloy carbon steel conducted at several voltages in pH 10 solutions with added NaCl are presented as examples. The dependencies of alloy corrosion rates on these variables and the sensitivity of the fuel dissolution rate are discussed.

This paper introduces a new acoustic emission (AE) characteristic to determine the deformation stage of metals under quasi-static loading. The AE characteristic is extracted from the derivative of the cumulative AE energy with respect to time. The reproducibility of the developed feature is demonstrated on a set of A572 Grade 50 steel coupons loaded up to different levels of plastic deformation, and then evaluated using the AE data recorded from aluminum 1100. To address the prior load-dependence of AE data, nonlinear ultrasonics is studied to quantify the presence of plastic deformation as a way to determine the initiation strain during quasi-static load testing typically used for evaluating metallic structures with the AE method. The sensitivity of nonlinear ultrasonics is compared with the linear ultrasonics showing the higher sensitivity of third harmonic nonlinearity coefficient to plastic deformation. The microstructural changes affecting ultrasonics are identified with metallographic characterizations.

Long-term corrosion rates of waste forms must be predicted for the range of environmental conditions that could occur during geologic disposal. Several standard electrochemical methods and microscopy were applied to a multiphase alloy/oxide composite material to study the effects of multiple phases on test responses. Electrochemical impedance spectroscopy showed application of those methods altered the electrical properties of the stabilized surfaces and the corrosion rates. Those methods provide qualitative indications of the corrosion behavior, but alternative methods that do not disturb the surface properties are required to reliably quantify the corrosion rates of multiphase waste forms for long-term disposal assessments.

The corrosion resistance of alloys in specific environments can be improved by tailored additions of passivating trim metals. A combination of metallurgical and electrochemical methods were used to evaluate and optimize the corrosion resistance of several multiphase steel-based alloy waste forms under conditions pertinent to long-term geological disposal. Test results are used to optimize the amounts of trim metals added to enhance the corrosion resistance of radionuclide-bearing phases. The effects of additives partitioning into different phases and galvanic interactions between those phases on the passive corrosion rates were measured under controlled chemical conditions spanning ranges that could occur in a disposal facility. Tests quantifying the benefits of adding trim Mo, Cr, and Ni to HT-9 steel to mitigate pitting, promote passivation, and produce a durable waste form are discussed.

Weld evaluation processes are usually conducted in the post-weld stage. In this way, defects are found after the weld is completed, often resulting in disposal of expensive material or lengthy repair processes. Simultaneously, weld quality inspections tend to be performed manually by a human, even for an automated weld. Therefore, a proper real-time weld quality monitoring method associated with a decision-making strategy is needed to increase the productivity and automaticity in weld. In this study, acoustic emission (AE) as a real-time monitoring method is introduced for gas metal arc weld. The AE system is designed to cover a wide range of frequencies from 5 to 400 kHz. Additionally, the welding parameters (weld current, voltage, gas flow rate, and heat input) are recorded concurrently with AE. Different types of weld defects are artificially created to generate different signals. For the automated decision-making system, machine learning algorithms are used. Several features extracted from the AE and welding parameters feed into a machine learning algorithm. A new AE feature as the rate of AE energy accumulation extracted from time driven AE feature is defined. For decision-making, supervised learning models are trained and evaluated using testing data. General classification methods—such as Logistic Regression—predict each data-point separately. In this study, Adversarial Sequence Tagging method is applied to predict the presence of four weld states as good, excessive penetration, burn-through, porosity and porosity-excessive penetration. We explore the prediction task as a sequence tagging problem where the label of a data-point depends on its corresponding features as well as neighboring labels. When all the AE features as well as heat input are used in the feature set, the sequence tagging and logistic regression algorithms achieve a prediction accuracy of 91.18% and 82.35%, respectively, as compared to metallographic analysis.

Welding defects such as lack of penetration, undercutting, crater crack, burn-through and porosity can occur during manufacturing. Assessing weld quality using nondestructive evaluation methods is important for the quality assurance of welded parts. In this paper, the measurement of weld penetration, which is directly related to weld integrity, is investigated by means of ultrasonics. Both linear and nonlinear ultrasonic methods are studied to assess their sensitivities to weld penetration. Welded plates with different penetration depths controlled by changing weld heat input are manufactured using gas metal arc welding (GMAW). Microscopic properties are assessed after the ultrasonic measurements are completed. Numerical models are built using the weld profile obtained from macrographs to explain the relationship between linear ultrasonic and weld penetration. A quantitative correlation between weld morphology (shape, width and depth) and the energy of linear ultrasonic signal is determined, where the increase of weld bead penetration exceeding the plate thickness results in decrease of the energy of the ultrasonic signal. Minimum detectable weld morphology using linear ultrasonics is defined depending on the selected frequency. Microhardness measurement is conducted to explain the sensitivity of nonlinear ultrasonics to both weld penetration and heterogeneity in weld. The numerical and experimental results show that the weld geometry influences the ultrasonic measurement other than the materials’ properties.

The morphology of welding pool (fusion zone and heat affected zone) includes a variety of microstructural constitutions (the formation of new boundaries, pores, and textures) that can affect the integrity of welded joint. Currently, microscopic observations are the common method to characterize the welding pool, which is a destructive post-welding procedure, and requires a complex preparation process. In this paper, linear and non-linear ultrasonic methods are investigated to measure the weld morphology and the presence of porosity nondestructively in welds manufactured with gas tungsten arc welding (GTAW). The weld morphology and the presence of porosity are quantified using microscopic investigations. The measurements show that the ultrasonic energy obtained by linear ultrasonics and the acoustic nonlinearity parameter obtained by nonlinear ultrasonics have good correlation with the weld morphology. The relationship between the ultrasonic energy and the weld morphology is validated further with numerical models. The linear UT at 1 MHz is not sensitive to the presence of porosity as the resolution is limited by ultrasonic wavelength. However, nonlinear ultrasonics has better sensitivity to the inclusions due to sub-wavelength resolution. (C) 2019 Elsevier Ltd. All rights reserved.

In this study, linear and nonlinear ultrasonic (UT) methods are utilized to evaluate the microstructures of A572 low carbon steel samples that were intercritically heat-treated at three temperatures and quenched to room temperature. The microstructures were also qualitatively and quantitatively assessed using standard metallography. ImageJ software was used to measure the amount of the phases. Correlations between ferrite content and linear velocity and the acoustoelastic coefficient are established. The experiments show that the UT linear and nonlinear velocities are affected by varying the amounts of ferrite and martensite. The acoustoelastic coefficient is found to be more sensitive to small changes in ferrite volume fraction.

The degradation rate of spent oxide fuel in a disposal facility will depend on the surface potential, which may be strongly affected by the concentration of dissolved H2 within a breached waste container. The H2 generation rate due to anoxic metal corrosion is a key variable in the fuel degradation model being developed for use in performance analyses of waste disposal facilities. We are using electrochemical methods to measure the corrosion rates of low alloy steel, stainless steel, and Zircaloy-4 to represent H2 generation rates under solution redox (Eh) and pH conditions spanning the ranges that could occur in disposal systems. Potentiostatic tests are conducted to measure corrosion rates at fixed Eh and pH conditions and Electrochemical impedance spectroscopy is performed periodically during those tests to quantify changes in the surface electrical properties as the material attains a constant corrosion rate. The corroded surfaces are examined with scanning electron microscopy after the test to relate the electrochemical measurements to physical changes. Results of tests with 4320 steel in acidic and alkaline brine solutions at an imposed potential of 0.25 VSCE are presented as an example of the experimental method and how the fuel corrosion rate varies with the H2 generation rate.

Welding is a complex manufacturing process, which involves significant physical and chemical changes in materials. Meanwhile, unexpected defects such as lack of penetration, undercutting, crater crack, burn-through and porosity may occur during manufacturing process. Therefore, the assessment of weld quality by means of nondestructive testing is needed. In this paper, the weld morphology is investigated by means of linear and nonlinear ultrasonics. The relationship between weld morphology and ultrasonic features (signal energy ratio extracted from linear ultrasonics and acoustic nonlinearity coefficient extracted from nonlinear ultrasonics) are built. Different penetration morphologies (bead, shallow penetration, excessive penetration, and burn-through) show different ultrasonic signatures. The microscopic observation is used to build the numerical models that validate the experimental measurements. The experimental and numerical results show that linear ultrasonics is influenced by the penetration morphology (shape, width and depth). The acoustic nonlinearity coefficient is influenced not only by the weld morphology but also heterogeneities (e.g. porosity) in the medium. The experimental and numerical results are used to build the regression models such that the ultrasonic features can be exploited to assess the weld morphology in situ without any destructive testing.

In nonlinear ultrasonic techniques, the material nonlinearity is observable in the ultrasonic signal through the generation of higher harmonics (HH). The HH generation, however, can be triggered by many sources. Any variation in the micro-, meso-, and macroscopic scales of the structure may collectively lead to HH generation. This paper presents a finite element approach with mesoscale heterogeneities explicitly modeled for the nonlinear wave propagation. The aim of this paper is to understand HH generation due to the non-mesoscale variation and non-uniform deformations introduced by the uniaxial tensile test. The study is divided into two parts: first, the effect of non-uniform plastic deformation resulted by geometrical variation of structures on HH is studied. The effect of non-uniformity due to mesoscale variations on HH is then analyzed. The numerical studies and predictions are crossly validated with nonlinear ultrasonic experiments and microscale imaging, including X-ray diffraction scanning. Numerical and experimental studies both indicate that non-uniform variations in different length scales affect the generation of both second- and third-harmonics and that both second- and third-harmonics acoustic nonlinearity parameters grow with the increase of plastic strain level. However, the third-harmonics acoustic nonlinearity coefficient is more sensitive when micro-, meso- and macrostructural variations exist.

Nearly all manufactured products in the metal industry involve welding. The detection and correction of defects during welding improve the product reliability and quality, and prevent unexpected failures. Nonintrusive process control is critical for avoiding these defects. This paper investigates the detection of burn-through damage using noncontact, air-coupled ultrasonics, which can be adapted to the immediate and in-situ inspection of welded samples. The burn-through leads to a larger volume of degraded weld zone, providing a resistance path for the wave to travel which results in lower velocity, energy ratio, and amplitude. Wave energy dispersion occurs due to the increase of weld burn-through resulting in higher wave attenuation. Weld sample micrographs are used to validate the ultrasonic results.

The automated weld quality assurance can improve efficiency and productivity. This paper presents the development of real-time weld quality assurance approach for gas tungsten arc welding (GTAW) using acoustic emission (AE) and air-coupled ultrasonic testing (UT). The major weld defect of interest in this paper is burn through, that is, melting through the base metal during welding that creates a hole/gap. The in situ monitoring system evaluates the changes in weld size leading to burn through by changing the weld heat input. Different categories of burn through are defined that include melting of the back of the plate without any molten metal exiting to formation of a hole in the plate. It is demonstrated that complete air-coupled UT cannot be used simultaneously with welding due to the influence of the magnetic field that develops in the weld torch during welding, which weakens the ultrasonic signal. Consequently, a rolling UT transmitter is combined with air-coupled UT receiver to increase the signal/noise value. Wave dispersion is detected due to the different levels of burn through. While UT method provides quantitative information about the weld state, any localized surface discontinuity causes sudden surges in the AE energy indicating non-uniform welding qualitatively. It is concluded that passive and active nondestructive evaluation methods should be combined to monitor weld quality real time for qualitative and quantitative assessment.

Welding is a key manufacturing process for many industries and may introduce defects into the welded parts causing significant negative impacts, potentially ruining high-cost pieces. Therefore, a real-time process monitoring method is important to implement for avoiding producing a low-quality weld. Due to high surface temperature and possible contamination of surface by contact transducers, the welding process should be monitored via non-contact transducers. In this paper, airborne acoustic emission (AE) transducers tuned at 60 kHz and non-contact ultrasonic testing (UT) transducers tuned at 500 kHz are implemented for real time weld monitoring. AE is a passive nondestructive evaluation method that listens for the process noise, and provides information about the uniformity of manufacturing process. UT provides more quantitative information about weld defects. One of the most common weld defects as burn-through is investigated. The influences of weld defects on AE signatures (time-driven data) and UT signals (received signal energy, change in peak frequency) are presented. The level of burn-through damage is defined by using single method or combine AE/UT methods.

Alloys were made by alloying 5, 10, 15, 17.5 and 20wt-% Mo with Type 316L stainless steel. Sigma phases containing 21-29wt-% Mo formed along the austenite grain boundaries with the addition of 5wt-% Mo and increased with additions up to 15wt-% Mo, but they decreased with further additions. Laves phases containing 33-40wt-% Mo co-precipitated at additions of 10wt-% Mo which increased with further Mo increases. The corrosion resistance, assessed by potentiodynamic polarisation in a 10mM NaCl solution adjusted to pH 4, increased relative to Type 316L for alloys made with 5 and 10wt-% added Mo, but decreased with further additions due to preferential corrosion of the Laves phase. The alloy made with 10wt-% added Mo had the highest corrosion resistance due primarily to the high Mo content of the austenite.