Effect of Molybdate on the Stability of Pit Growth in Duplex Stainless Steel 2205

In the present study, the criteria of pit stabilization and salt formation have been investigated for DSS 2205 using the lead-in-pencil electrode in order to explore the contribution of molybdate in stable pit growth. Results show that the Epit shifts to positive values by increasing molybdate concentration. It is found that the increase in Epit could not be referred to the changes in DCcrit and DCsat, since these values are slightly reduced by adding Molybdate to the solution. In contrast, the active dissolution rate of the metal assessed by idiss,max is hindered in the presence of molybdate ions. Therefore, it is proposed that the mechanism for the inhibitory influence of molybdate can be attributed to an increment in the intersection point of idiss,max/idiff,crit (which reflects Ecrit), and idiss,max/ilim (which reflects Esat). Therefore, increasing the Ecrit and Esat at the pit growth process is responsible for the observed raising of the Epit in presence of Molybdate.

Void Hunting

Karst environments are characterized by voids, i.e. sinkholes and conduits of varying size that arise from the active dissolution of carbonate rock by acidic groundwater. These voids, whether air-, water-, or soil-filled, can be difficult to image using near-surface geophysical methods due to the limited investigation depths of most active-source methods. In addition, due to the significant effort it takes to collect active-source data, investigators are often unable to monitor spatio-temporal variations in the subsurface. The ability to detect, image, and monitor subsurface voids improves the understanding of processes that create and transform voids, a vitally important insight across a variety of scientifc disciplines and engineering applications, including hydrogeology, geotechnical engineering, planetary science and even issues of national security. Using a 54-element nodal array (1C and 3C sensors), I image the subsurface of the USF GeoPark with ambient noise surface wave tomography. I also use complementary active-source geophysical datasets (e.g. 2D ERT) collected at the GeoPark to constrain and/or validate the tomography results. I address two research questions with this study: (1) How do ambient seismic methods complement active-source near-surface methods? (2) Can ambient noise tomography resolve voids in the karst environment? In this thesis, I discuss my answers to these questions and present the current state of surface wave methods in the karst environment, including the feasibility for utilizing ambient noise methods to monitor spatio-temporal changes in sinkhole and conduit formation. In addition to the ability to use seismic methods for temporal monitoring, ambient noise provides lower frequencies than what are achievable with active-source seismic methods. Using frequencies from 5-28 Hz, ambient noise tomography is able to image deeper into the subsurface (up to 100 m at 5 Hz) than previous active-source seismic studies at the GeoPark field site. This study yields a more robust and simple method to image voids in covered karst environments and a long-term installation of ambient seismic nodes enables future investigations of spatio-temporal variations in void structures.

Electrolyte Based Modeling of Corrosion Processes in Sulfuric Acid Mixtures – Part 1: Non-Oxidizing Conditions

The corrosion behavior of stainless steels and Ni-base alloys in non-oxidizing sulfuric acid mixtures at concentrations below approximately 30 moles/Kg H2O is modeled. The redox potential in sulfuric acid across a broad concentration range, from 0 to 80 mole percent (0 to 95.6 weight percent), is determined by the proton reduction reaction. Thus, in the absence of other oxidizing species, sulfuric acid behaves as a non-oxidizing (reducing) acid. The calculated corrosion rate, using an electrochemical model up to about 30 moles/Kg H2O (about 75 weight percent) is in agreement with experimental values. The predicted polarization curves of anodic and cathodic processes showed that the alloys in these environments are in active dissolution regime, consistent with experimental data. The model predictions of corrosion rates in H2SO4+HCl, H2SO4+HF, and H2SO4+HCl+HF mixtures are in agreement with weight-loss corrosion data. The corrosion rate of alloys in the non-oxidizing sulfuric acid mixtures correlated to an equivalent alloy composition given by (Ni0.7-Cr0.1+Mo+0.5W). The effect of alloying elements under these conditions may be related to their beneficial effect on active dissolution and proton reduction reaction rates.

Sulfur bacteria promote dissolution of authigenic carbonates at marine methane seeps

Carbonate rocks at marine methane seeps are commonly colonized by sulfur-oxidizing bacteria that co-occur with etch pits that suggest active dissolution. We show that sulfur-oxidizing bacteria are abundant on the surface of an exemplar seep carbonate collected from Del Mar East Methane Seep Field, USA. We then used bioreactors containing aragonite mineral coupons that simulate certain seep conditions to investigate plausible in situ rates of carbonate dissolution associated with sulfur-oxidizing bacteria. Bioreactors inoculated with a sulfur-oxidizing bacterial strain, Celeribacter baekdonensis LH4, growing on aragonite coupons induced dissolution rates in sulfidic, heterotrophic, and abiotic conditions of 1773.97 (±324.35), 152.81 (±123.27), and 272.99 (±249.96) μmol CaCO3 • cm−2 • yr−1, respectively. Steep gradients in pH were also measured within carbonate-attached biofilms using pH-sensitive fluorophores. Together, these results show that the production of acidic microenvironments in biofilms of sulfur-oxidizing bacteria are capable of dissolving carbonate rocks, even under well-buffered marine conditions. Our results support the hypothesis that authigenic carbonate rock dissolution driven by lithotrophic sulfur-oxidation constitutes a previously unknown carbon flux from the rock reservoir to the ocean and atmosphere.

High-temperature corrosion of steels in acid solutions. Part 3. Inhibitory protection of steel by nitrogen-containing heterocyclic organic compounds and inorganic oxidants. Review

The literature and patent data on the protection of steels in high-temperature acid solutions by nitrogen-containing heterocyclic organic compounds and inhibitors of oxidative type were summarized. Composite inhibitors based on substituted triazoles can provide effective protection of low-carbon and chromium-nickel steels in these environments (200 °С). The mechanism of the protective action of these inhibitors is based on the ability of substituted triazoles to form on the steel surface in acid solutions the protective layers chemically bonded to the metal surface. Often these protective layers are basically composed of a polymer complex compound containing triazole molecules and Fe cations (in the case of nickel-chromium steels — Ni and Cr). It is noted that oxidizing additives can be used to protect steels which are in high-temperature acidic media at a passive state. Using the example of corrosion of chromium-nickel steel in H3PO4 solutions (130 °C), it is shown that additives of oxidants (Cu(II) and NO3–) stabilize the passive state of the metal in this medium, preventing the possibility of its active dissolution. Based on analysis of the literature data, a conclusion was made about the prospects of using compositions based on nitrogen-containing heterocyclic compounds for protecting steels in high-temperature acidic environments. These compositions are superior in important technological parameters to mixed inhibitors based on unsaturated organic compounds.

Electrochemical Evaluation of Chrysin Flavonoid as Corrosion Inhibitor for Mild Steel in Sulphuric Acid Solution

Recently research is focused on natural organic compounds as metallic corrosion inhibitors demonstrating good corrosion protection and efficiencies. Steel corrosion behavior in acid media was evaluated in the presence of a pure natural flavonoid metabolite named Chrysin present in different plants. The evaluation of corrosion protection was studied using polarization curves, electrochemical impedance spectroscopy (EIS) and electrochemical current density under potentiostatic conditions. Polarization curves present active dissolution and at high overpotentials two passivation regions were found. Slight corrosion protection was obtained from EIS measurements and potentiostatic curves at three different anodic potentials: -370, +216 and +600 mV, revealed a more stable passive film in the presence of Chrysin at both passive regions. High corrosion protection was obtained on the film formed at +600 mV during the first 4 hours of immersion.

Enzymatic degradation of liquid droplets of DNA is modulated near the phase boundary

Biomolecules can undergo liquid–liquid phase separation (LLPS), forming dense droplets that are increasingly understood to be important for cellular function. Analogous systems are studied as early-life compartmentalization mechanisms, for applications as protocells, or as drug-delivery vehicles. In many of these situations, interactions between the droplet and enzymatic solutes are important to achieve certain functions. To explore this, we carried out experiments in which a model LLPS system, formed from DNA “nanostar” particles, interacted with a DNA-cleaving restriction enzyme, SmaI, whose activity degraded the droplets, causing them to shrink with time. By controlling adhesion of the DNA droplet to a glass surface, we were able to carry out time-resolved imaging of this “active dissolution” process. We found that the scaling properties of droplet shrinking were sensitive to the proximity to the dissolution (“boiling”) temperature of the dense liquid: For systems far from the boiling point, enzymes acted only on the droplet surface, while systems poised near the boiling point permitted enzyme penetration. This was corroborated by the observation of enzyme-induced vacuole-formation (“bubbling”) events, which can only occur through enzyme internalization, and which occurred only in systems poised near the boiling point. Overall, our results demonstrate a mechanism through which the phase stability of a liquid affects its enzymatic degradation through modulation of enzyme transport properties.

Effects of the α/γ-Phase Ratio on the Corrosion Behavior of Cast Duplex Stainless Steel

The effects of the α/γ-phase ratio on pitting corrosion initiation and growth in cast duplex stainless steel were studied, including the preferential dissolution of the two phases inside the pits, using pitting potential measurement and potentiostatic polarization measurement with a high concentration of chloride ions and a low pH. The initiation of pitting was not dependent on the α-phase ratio. The γ phase preferentially dissolves when a high potential in the active dissolution region is applied, and the α phase preferentially dissolves when a low potential is applied. In addition, with an increase in α-phase ratio, the potential range where the α phase preferentially dissolves enlarged toward the higher potential side. The growth rate of stable pitting increased with the α-phase ratio. Dissolution of the α phase increased with an increase in the α-phase ratio. This phenomenon is presumably caused by the decreased amount of Cr in the α phase, resulting from the increased α-phase ratio, as well as by Cr depletion around Cr nitrides.

Effects of Magnetic Field on Uneven Dissolution of Iron in Sulfuric Acid Solution with Chlorides

The effects of magnetic field on anodic dissolution and passivation of iron in a sulfuric acid solution with chlorides are investigated by electrochemical measurements and surface observations. In the anodic potentiodynamic polarization curve, the potential for the drastic current drop is not significantly affected by the potential sweep rate under 0 T, which moves in the negative direction with increasing potential sweep rate under 0.4 T magnetic field that is parallel to the working electrode surface. The uneven surface produced during the potentiodynamic polarization hinders the transition from active dissolution to passivation. The area fraction of the locally accelerated dissolution increases with prolonging polarization time at high potentials where the surface film precipitation-dissolution process is the rate-determining step for metal dissolution. Pretreatment under potentiostatic polarization at 0.4 T magnetic field produces an uneven surface that would result in unrecoverable electrochemical states after switching from 0.4 T to 0 T, depending on the applied potential. The positive-feedback mechanism for the magnetic field effect and the surface morphological effect is proposed. The results demonstrate the direct magnetohydrodynamic effect and its resultant uneven surface on the anodic behavior of iron.