Characteristics and Crystal Structure of Calcareous Deposit Films Formed by Electrodeposition Process in Artificial and Natural Seawater

In this study, we tried to form the calcareous deposit films by the electrodeposition process. The uniform and compact calcareous deposit films were formed by electrodeposition process and their crystal structure and characteristics were analyzed and evaluated using various surface analytical techniques. The mechanism of formation for the calcareous deposit films could be confirmed and the role of magnesium was verified by experiments in artificial and natural seawater solutions. The highest amount of the calcareous deposit film was obtained at 5 A/m2 while current densities between 1–3 A/m2 facilitated the formation of the most uniform and dense layers. In addition, the adhesion characteristics were found to be the best at 3 A/m2. The excellent characteristics of the calcareous deposit films were obtained when the dense film of brucite-Mg(OH)2 and metastable aragonite-CaCO3 was formed in the appropriate ratio.

Carbon Steel Corrosion and Cathodic Protection Data in Deep North Atlantic Ocean

Many parameters may influence the corrosion and the cathodic protection current demand in natural seawater. These are potential, temperature, dissolved oxygen content, biofilm and fouling activity, hydrostatic pressure, and calcareous deposit formation. In this study, the influence of the exposure depth on the corrosion, cathodic protection current demand, and nature of the calcareous deposit formed on carbon steel was investigated at 1,020 m and 2,020 m depth. For this purpose, a set of coupons, cathodic protection, and environmental sensors were exposed in Azores in the Atlantic Ocean for 11 months. The higher corrosion rate and current density observed at 1,020 m can be explained by the higher temperature and oxygen diffusion. The cathodic current demand decrease with time can be attributed to the calcareous deposit formation. The current densities after 11 months are in agreement with the literature with 143 mA/m2 and 124 mA/m2 at 1,020 m and 2,020 m depth. Calcareous deposits formed, characterized by Raman spectroscopy, x-ray diffraction, and scanning electron microscopy/electron dispersive x-ray spectroscopy, highlight (i) the favored formation of calcite and hydrocalcite at the expense of aragonite in deep and cold water, (ii) the presence of a thin deposit after 11 months, (iii) the decrease of the Ca/Mg ratio with immersion depth, (iv) the presence of CaMgCO3 compounds, and (v) a higher decrease of the current demand with time in deep water, suggesting the formation of a more protective deposit. The capacity for aluminum-gallium and aluminum-indium galvanic anode were in agreement with the literature for long-term exposures.

Hydrogen absorption and hydride formation in pure titanium T40 (grade 2) and TA6V ELI (grade 23) under cathodic polarization in artificial seawater

Different kinetics of hydrogen absorption in T40 (grade 2) and TA6V ELI (grade 23) under cathodic polarization in artificial seawater have been highlighted. These polarizations were made by applying potentials from -0.8 to -1.8V/SCE in artificial seawater and NaCl solution. Four stages were identified and related in term of hydrogen ingress, hydrides formation and calcareous deposit growth. The formation of γ and δ-hydrides have been observed, localized and characterized using several techniques. On T40, hydrides form as a layer that increases the surface roughness and clusters form in the bulk after first moments of hydrogen absorption. Whereas in TA6V ELI, hydrogen is absorbed by β-phase leading to a volume expansion of this phase. Then after reaching the hydrogen solubility limit of β-phase, hydrides form on interfaces α/β or α/α and in α grains. For long durations, the hydrogen ingress is limited by the subsurface hydrides and the stabilized calcareous deposit. These different steps are time depend on processes which need to be tacked into account to improve knowledge of hydrogen embrittlement in titanium alloys.