Aragonite Layers Research Articles

The Biological Crystals in Chamid Bivalve Shells: Diversity in Morphology and Crystal Arrangement Pattern

Chamid bivalves are marine organisms that live in high-energy environments and are cemented to hard substrates. To avoid shell damage, the organisms form thick, densely ornamented shells. Shell material consists of aragonite, and the ornamentation may be either aragonitic or calcitic. The latter can be developed as scaly spines, rows of blades, or comarginal, radial arched lamellae. We investigated biological crystal morphology and mode of assembly of Chama arcana and Chama gryphoides shells. Structural characteristics were obtained from electron backscatter diffraction (EBSD) measurements, complemented with laser confocal and BSE imaging. We found a wide range of crystal morphologies and sizes, ranging from irregularly shaped calcite and/or aragonite prisms to tiny and thin aragonite laths. We observed four different modes of crystal assembly patterns: 1. strongly interlocked dendritic calcite units forming the ornamentation blades; 2. aragonite laths arranged to lamellae forming the outer shell layer, the layer adjacent to the calcite; 3. aragonite laths arranged into blocks comprising inner shell layers or aragonitic ornamentations; and 4. shell portions consisting of aragonite prisms, structured in size and crystal orientation, at muscle attachment sites. These four different types of crystal arrangements were observed for the shells of the investigated chamid species; however, they had slightly different strengths of structuring and slight variations in crystal organisation. Additionally, we observed unique microstructural features in Chama shells: We report ornamentation crystals resembling idiomorphic calcite and novel, twinned entities found at the changeover between the aragonitic layers. We highlight and discuss these differences and anomalies in this contribution.

Structural and Functional Analysis of the Amorphous Calcium Carbonate-Binding Protein Paramyosin in the Shell of the Pearl Oyster, Pinctada fucata.

Amorphous calcium carbonate (ACC) is an important precursor phase for the formation of aragonite crystals in the shells of Pinctada fucata. To identify the ACC-binding protein in the inner aragonite layer of the shell, extracts from the shell were used in the ACC-binding experiments. Semiquantitative analyses using liquid chromatography-mass spectrometry revealed that paramyosin was strongly associated with ACC in the shell. We discovered that paramyosin, a major component of the adductor muscle, was included in the myostracum, which is the microstructure of the shell attached to the adductor muscle. Purified paramyosin accumulates calcium carbonate and induces the prism structure of aragonite crystals, which is related to the morphology of prism aragonite crystals in the myostracum. Nuclear magnetic resonance measurements revealed that the Glu-rich region was bound to ACC. Activity of the Glu-rich region was stronger than that of the Asp-rich region. These results suggest that paramyosin in the adductor muscle is involved in the formation of aragonite prisms in the myostracum.

Evidence of a Scheduled End for Prism Growth in the Shell of Pinctada margaritifera: Closure of the Calcite Biomineralization Area by a Specific Organic Membrane

The shell of the pearl oyster Pinctada margaritifera is made up of two layers: an outer layer of calcite prisms and an inner layer of aragonite tablets. Recent studies have shown that the calcite layer develops in a series of steps. We found that the end of prism growth and the start of aragonite deposition are also complex processes. Contrary to the common belief that prism growth is interrupted by the expansion of the aragonite layer, we found that a specific membrane covers the calcite surface before aragonite deposition starts. The earliest aragonite depositions occur as granular spots located only on the surfaces covered by this organic membrane This membrane appears to be the final stage of the calcite biomineralization cycle. This new understanding of calcite development has implications for shell biomineralization research and the production of pearls.

Investigating the potential of African land snail shells (Gastropoda: Achatininae) for amino acid geochronology

Aragonitic calcium carbonate (CaCO3) terrestrial mollusc shells with complex shell microstructures, such as those in the African subfamily Achatininae, have the potential to be used to build amino acid geochronologies across the African continent. However, as different microstructural shell layers are likely to have different protein compositions, sampling strategies need to be developed to identify the most appropriate shell portion to target. To test possible variability in protein degradation rates between microstructural layers, sampling of a single microstructural shell layer (the ‘nacreous’ layer) was compared to sampling all three aragonitic layers ('3AL') in modern and fossil shells. Reliable isolation of the nacreous layer in all samples proved impractical, and additional complications arose due to mineral diagenesis induced by sampling with a drill. Pleistocene fossils of Lissachatina sp. and modern specimens of Achatina tavaresiana Morelet, 1866 were shown to have an intra-crystalline protein fraction. The 3AL shell portion adhered more closely to closed-system behaviour in heated modern, and fossil, samples. The intra-crystalline protein degradation (IcPD) patterns of Achatininae fossil samples were not consistent with IcPD under forced degradation experiments at high temperatures in the laboratory. However, reliable degradation trends were observed in the 3AL shell portion, demonstrating the potential of fossil achatinids for building relative amino acid geochronologies across Africa.

Preparation of reactive urchin-like recycled concrete aggregate by wet carbonation: Towards improving the bonding capability of the interfacial transition zone in recycled aggregate concrete

To enhance the bonding capability between recycled concrete aggregate (RCA) and paste, an innovative carbonation method was developed to modify the surface characteristic of RCA by promoting the formation of needle-like aragonite using accelerated carbonation in Mg(NO3)2 solution under an elevated temperature (75 °C). The evolution of surface microstructure, phases and reaction kinetics was investigated using multiple testing methods including scanning electron microscopy, nanoindentation, X-ray diffraction, etc. The results revealed that reactive urchin-like RCA could be prepared within less than an hour after exposing to CO2. The urchin-like wrapping with a thickness of about 100 μm was seen rapidly grown on the surface of RCA, consisting of an outermost layer of aragonite coating, a thin layer of brucite and a silica-rich layer. The mineral wrapping induced by carbonation significantly modified the roughness, topography and geochemistry of RCA's surface entirely, contributing to enhanced bonding strength between RCA and new mortar (33.54%).

Paleoenvironmental study of the Late Preclassic period in the Northern Mesoamerican Frontier

The Northern Mesoamerican Frontier was a complex multicultural region characterized by frequent human settlement changes and shifts in agricultural conditions during the Late Preclassic period (~400 BCE-150 CE). Here, we report a high-resolution paleoenvironmental record from the varved sedimentary sequence of the crater maar La Alberca which spans the Late Preclassic (~400 BCE-150 CE) to part of the Early Classic period (~150 CE-250 CE) corresponding to Late Chupicuaro phase (400 BCE-100 CE) and Mixtlan phase (0–250 CE). Our work aims to study the paleoenvironmental conditions during the rise of agriculture in the Northern Mesoamerican Frontier and provide insights related to landscape alteration by human activity. To reach these aims, a multiproxy investigation was conducted by means of varve counting, high-resolution XRF scans, magnetic susceptibility, pollen data and fecal stanol biomarkers as a proxy for human population change. Our results reveal two varve type. Type 1 is characterized by the alternation of detrital-organic layers and aragonite layers, type 2 by alternating detrital-organic layers with an organic layer formed by diatom frustules and aragonite layers. This study suggest that the increase of erosion by human activity during the Late Chupicuaro phase (400 BCE-100 CE) and the start of the Mixtlan phase (0–250 CE) coincide with a high percentage of Amaranthaceae pollen, a rise of sedimentation rates, increase in nutrient content and the increase of human waste flux interpreted with the (Coprostanol + epi) :((Coprostanol + epi)+cholestanol biomarker. Moreover, a wetter period (~137 BCE-37 CE) interpreted during the Late Chupiacuaro phase and the start of the Mixtlan phase could suggest favorable environmental conditions for the establishment of agriculture.

Characterization of Magnetic Nanoparticles from the Shells of Freshwater Mussel L. fortunei and Marine Mussel P. perna

Magnetite (Fe3O4) nanoparticles were extracted from the shells of freshwater Limnoperna fortunei (Dunker 1857) and marine Perna perna (Linnaeus 1758) mussels, followed by full physical and chemical characterization using ICP-OES, UV–Vis, EDX, Raman, and XRD spectroscopy, VSM magnetometry, and SEM and TEM techniques. Considering their spatial distribution, the ferrimagnetic particles in the shells had low concentration and presented superparamagnetic behavior characteristics of materials of nanometric size. Transmission electron microscopy (TEM, especially HRTEM) indicated round magnetic particles around 100 nm in size, which were found to be aggregates of nanoparticles about 5 nm in size. The TEM data indicated no iron oxide particles at the periostracum layer. Nevertheless, roughly round iron (hydr)oxide nanoparticle aggregates were found in the nacre, namely, the aragonite layer. As the aragonite layer is responsible for more than 97% of the shell of L. fortunei and considering the estimated size of the magnetic nanoparticles, we infer that these particles may be distributed homogeneously throughout the shell.

Evolving Eocene–Miocene seasonality decoded from aragonitic laminae deposited within the Qaidam Basin, North Tibetan Plateau

A thorough understanding of sub-decadal paleoclimate variability typically depends on ultra-precise geochronological constraints that are rarely obtained. While the seasonally resolved nature of varved successions enlarges the opportunity, their identification beyond Quaternary ages is difficult; thus is frequently met with skepticism. The Cenozoic strata in the Qaidam Basin, for instance, feature expanded rhythmite successions, whereas the periodicity of these laminae remains to be explored. Targeting Eocene- (VE; ∼42.0 Ma), Oligocene- (VO; ∼26.9 Ma) and Miocene-aged (VM; ∼19.5 Ma) intervals, herein we report layer-based petrographic observations coupled with mineralogical and phase-specific geochemical analyses of millimeter-scale rhythmic alterations between yellowish- and grayish-colored aragonitic layers. Noting a constant δ13Ccarb offset (∼ 2.8‰) between yellowish- and grayish-colored laminae, these isotope data imply seasonally paced alternations between microbial induction and inorganic carbonate precipitation. In addition, covariations between inorganic oxygen isotope (δ18Ocarb) and elemental ratios (e.g., Mn/Fecarb, Sr/Basilic and Chemical Index of Alteration, CIAsilic) reveal evolving seasonality, capturing Eocene–Oligocene aridification followed by an early Miocene reversal. Therefore, the varved fill of the Qaidam Basin serves as an annually resolved, and as yet under-explored, archive of evolving Cenozoic paleoseasonality in the aftermath of the Indo–Asia collision.

LATE JURASSIC INTERIOR-JEWELRY QUALITY AMMONITES FROM THE ULYANOVSK REGION: THE MINERAL COMPOSITION AND GEMOLOGICAL CHARACTERISTICS

Decorative articles made of ammonites are currently in high demand. The mineral composition and gemological characteristics of interior-jewelry ammonites from the Upper Jurassic of the Ulyanovsk Region have first been studied in detail. The ammonites are characterized by a pearl layer showing a bright varycolored iridescence. A complex of ammonite investigations comprised determinations of their microhardness, density, and luminescence; their optical-petrographic analysis; determinations of the mineral and chemical composition; and the electron-probe and electron-microscopic investigations. The ammonites consist mainly of apatite (22-88 wt. % ) and calcite (6-64 wt. %). Quartz, gypsum, pyrite, zeolites, ilmenite, rutile, goethite, zircon, and organic matter are present. Impurity elements recorded in elevated concentrations are (in wt. % ) Sr, 0.237; Ba, 0.011; and Pb, 0.023; and those recorded in low concentrations (less than 0.01 wt. %) are Ni, Cu, Zn, Rb, U, Th, Y, and As. The outer walls and septa of an ammonite shell have completely lost their original aragonite composition and consist of apatite and calcite with inclusions of pyrite and organic matter. The pearl layer iridescence is related to lamellar layers of apatite, that inherited the structure of lamellar layers of aragonite. The iridescence color is determined by the size and order of the lamellars and does not depend on their mineral composition. Ammonites of the Ulyanovsk Region are of commercial interest as interior-decorative specimens. Internal fragments of the shells can be used in jewelry. Ammonites are collected in steep ravine walls, which requires no capital investment and does not violate the environment.

Holocene paleoclimate variability in the eastern Mediterranean, inferred from the multi-proxy record of Lake Vouliagmeni, Greece

This study presents a Holocene multi-proxy record from Lake Vouliagmeni, eastern Gulf of Corinth, Greece. The lake is subjected to intense climatic and tectonic forces causing stratigraphic variations with laminated sediments frequently interrupted by homogenous sections and event deposits. Lamination couplets consist of aragonite layers alternating with detrital and organic residues formed during periods of seawater intrusion and stratification of the lake water. The discontinuous occurrence of laminated deposits excludes a varve based chronology from being established but highlights the susceptibility of the lake to record environmental and climatic changes. Our synthesis model for regional climatic reconstruction and local environmental changes derives from δ 18 O and δ 13 C data from laminated and homogenous sediments studied separately and depending on the dominant carbonate mineral . This is further strengthened by high-resolution geochemical proxies, diatom and sedimentological data. Regional climatic signals from key sites and possible links to the Lake Vouliagmeni record are explored in response to atmospheric circulation patterns. Phases of overall humid conditions are recorded by increased inflow of siliciclastic material to the lake and negative δ 18 O bulk values. In contrast, periods of marine intrusion and enhanced evaporation are recorded by aragonite precipitation, positive δ 18 O Ar values and laminations. The laminations formed in the lake seem to occur during periods of sea water intrusion into the lake, which led to pycnocline stabilization and stratified lake waters. • δ 18 O bulk and δ 13 C bulk reflect variations in precipitation and detrital influx. • Periods of stratified lake waters and pycnocline stabilization are displayed with aragonite laminations. • Humid phases are recorded from 10 to 7.8, 5.2 to 4.4 and 1.5 to 0.2 ka cal BP with lower δ 18 O bulk . • Increased summer temperatures occur from 7.4 to 6.8 and 5.4 to 4.5 ka cal BP and are reflected with increased carbonate accumulation.

Investigating controls of shell growth features in a foundation bivalve species: seasonal trends and decadal changes in the California mussel

Abstract. Marine bivalve mollusk shells can offer valuable insights into past oceanographic variability and seasonality. Given its ecological and archeological significance, Mytilus californianus (California mussel) presents the opportunity to examine seasonal and decadal changes recorded in its shell over centuries to millennia. While dark–light growth bands in M. californianus shells could be advantageous for reconstructing past environments, uncertainties remain regarding shell structure, environmental controls of dark–light-band formation, and the amount of time represented by a dark–light pair. By analyzing a suite of M. californianus shells collected in 2002, 2003, 2019, and 2020 from Bodega Bay, California, we describe the mineralogical composition; establish relationships among the growth band pattern, micro-environment, and collection season; and compare shell structure and growth band expression between the archival (2002–2003) and modern (2019–2020) shells. We identified three mineralogical layers in M. californianus: an outer prismatic calcite layer, a middle aragonite layer, and a secondary inner prismatic calcite layer, which makes M. californianus the only Mytilus species to precipitate a secondary calcite layer. Within the inner calcite layer, light bands are strongly correlated with winter collection months and could be used to reconstruct periods with moderate, stable temperatures and minimal upwelling. Additionally, modern shells have significantly thinner inner calcite layers and more poorly expressed growth bands than the archival shells, although we also show that growth band contrast is strongly influenced by the micro-environment. Mytilus californianus from northern California is calcifying differently, and apparently more slowly, than it was 20 years ago.

Investigation of nacre nanostructure by analyzing its structural color pattern

Produced by light interacting with structures at micrometer or nanometer levels, structural color can be utilized to investigate details of material structures. In this article, we studied the pattern of interference color generated from repetitive aragonite–conchiolin double layers on colorless nacreous pearls. Based on qualitative wave analysis and quantitative electromagnetic computation, we theoretically concluded such patterns are mainly determined by aragonite layer thickness. We also demonstrated how to predict the aragonite layer thickness and estimate the conchiolin refractive index variation on a Tahitian pearl with near-colorless body color and strong iridescence. We believe this approach offers a new perspective to study nanostructures in biology and mineralogy.

Effects of extra feeding combined with ocean acidification and increased temperature on the carbon isotope values (δ13C) in the mussel shell

Ocean acidification (OA) and global warming present future challenges for shell producing organisms such as mussels through reduction in the carbonate available to produce shells in these and other valuable aquaculture species. Molluscs control their shell growth through biomineralisation, but the response of the mechanisms behind biomineralisation to OA conditions are relatively unknown. It is unclear how much carbon is taken into the shell from the environment compared to the uptake through the food source. Shell production is energetically costly to molluscs and metabolic processes and energetic partitioning may affect their ability to perform the underlying mechanisms of biomineralisation under OA. It is possible that additional food consumption might alleviate some impacts caused by acidification. We assessed the ability of extra feeding to alter the impacts of OA and increased temperatures on adult Mytilus edulis. Carbon isotopes (δ13C) were used to examine the change in biomineralisation pathway in mussels. OA did not alter the δ13C directly in separate analyses of the shell calcite and aragonite layers, mantle tissue and extrapallial fluid. However, ambient treatments with increased temperatures altered the mussel biomineralisation pathway in the shell calcite using CO32− instead of HCO3− as the main source of carbon. The proportion of metabolic carbon uptake into the mussel shell calcite layer increased under OA, with additive effects when exposed to increased temperatures and extra feeding. The proportion of metabolic carbon uptake is higher (7%–11%) in the shell aragonite layer compared to calcite, under ambient treatments. OA initially reduced the metabolic carbon uptake into the shell aragonite, but after a period of 4-months with extra feeding, the mussels were able to adjust their metabolic carbon uptake to a level experienced under ambient treatments. This indicates that an abundance of food resources may enable changes in mussel biomineralisation pathways to compensate for any decrease in seawater inorganic carbon associated with OA. The impact of OA on phytoplankton varies from species to species, changing the structure of the community which could provide sufficient food resources to maintain metabolic carbon uptake for mussel shell growth. This study of δ13C isotopic values has identified changes in biomineralisation pathway relating to the mussel metabolic carbon uptake from their food source, with varying results for the aragonite and calcite shell polymorphs. The implications of these findings suggest that some bivalve species with different shell composites may cope better under OA than others, demanding further study into species-specific biomineralisation pathways.

Symbiotic worms in the inner aragonitic layer of Leptodesma (Bivalvia) from the Přídolí (Upper Silurian) of Saaremaa Island, Estonia

Multiple calcareous worm-like boring casts occur on the inner surface of the calcitic external layer of the bivalve Leptodesma sp. from the Kaugatuma Formation (lower Přidoli, Upper Silurian) of Saaremaa Island, Estonia. Most of the boring casts are oriented with their apertures towards the posterior margin of Leptodesma sp. The worm-like casts were originally borings in the aragonitic layer of the valve. The borings were filled with sediment and the aragonitic layer of the bivalve later dissolved. The orientation of the borings indicates that they were likely made during the life of Leptodesma. The boring worms likely used the feeding currents of the bivalve to capture suspended nutrients, so they may have been kleptoparasites. Based on the morphology of borings in Leptodesma, they likely housed some type of filter-feeding polychaete annelid.

Biomineralization of the Cambrian chancelloriids

Abstract As extinct animals that flourished during the Cambrian explosion, chancelloriids have a unique body plan lacking guts but with a flexible integument and a suite of star-shaped, hollow sclerites. Due to this body plan, along with the paucity of knowledge on sclerite biomineralization, the phylogenetic position of chancelloriids within the Metazoa is still controversial. Integration of analyses of diverse fossils from Cambrian stage 2 to the Wuliuan Stage of China and Australia indicates that chancelloriid sclerites possess an encasement-like organic layer and a fibrous aragonitic layer. The organic layer is inferred to be a specialized trait derived from the epidermal integument of the animal body. The sclerites were likely biomineralized by using the outer organic layer as a template to absorb cations and precipitate crystal nuclei, reflecting a strategy adopted by a range of eumetazoans with a developed epidermis. Therefore, the hypothesis that chancelloriids represent an epitheliozoan-grade animal and an early explorer of template-based biomineralization is supported.

A comparative study on the mechanical and structural design of nacre in gastropod and bivalve molluscs

The lesson learnt from nature is presented through the examination of shells from four mollusc species (two gastropods and two bivalves). Individual aragonite tablets together with the organic network have been studied in terms of tablet dimensions, tablet stacking sequence, texture and the weight fraction of the organic present in these bio-composite shells. Mechanical properties viz. elastic modulus and hardness at the macro (bulk), micro (tablet) and nano (aragonite nanograins) levels have been determined using the experimental technique of nanoindentation together with the theoretical formulations offered by the Mori-Tanaka method and other such mechanical models for composites. XRD studies confirm the strong c-axis crystallographic orientation - aligned parallel to the direction of growth and crystallization of nacreous tablets, and fractography performed on the shell specimens suggests a difference in the stacking of individual aragonite layers for the gastropod and bivalve molluscan class, respectively. DSC and TG-DTA data helped in characterizing the organic material present in the shell specimens and in calculating the organic weight and volume fractions across species. Further to this, the DSC data revealed that the nature of the organic in the brick and mortar assembly of nacre, was similar in nature across the two molluscan classes reported in this study. EBSD studies performed on our system revealed a difference in crystallographic texture for the two different tablet stacking (columnar and sheet nacre). It has been shown experimentally that the shells of gastropods (columnar nacre) are orthotropic in nature, while the bivalve mollusc shells (sheet nacre) are anisotropic in nature.

Hard to Soft: Biogenic Absorbent Sponge-like Material from Waste Mussel Shells

Summary Mussel shells are a source of renewable CaCO3 and can be obtained from aquaculture waste streams. Valorization of waste shells relieves strain on landfills and aligns with the goals of a circular economy. Here, we present the preparation of an absorbent CaCO3 material, soft calcite (SC), prepared from waste blue mussel shells under mild conditions. Calcite is the only polymorph present in SC, despite the presence of aragonite in the shell. SC has a nest-like morphology, unlike the highly ordered calcite in the shell. SC is formed by the reassembly of calcite prisms and may be held together by the organic matrix “glue” from the original shell as evidenced by NMR spectroscopy. This inorganic sponge adsorbs dyes from aqueous solution and absorbs crude oil from seawater with good recyclability. Our results demonstrate how a natural structure can be modified in a sustainable fashion to yield a higher-value material.

Molecular Characterization of a Novel Shell Matrix Protein With PDZ Domain From Mytilus coruscus.

Mollusk shells are products of biomineralization and possess excellent mechanical properties, and shell matrix proteins (SMPs) have important functions in shell formation. A novel SMP with a PDZ domain (PDZ-domain-containing-protein-1, PDCP-1) was identified from the shell matrices of Mytilus coruscus. In this study, the gene expression, function, and location of PDCP-1 were analyzed. PDCP-1 was characterized as an ∼70 kDa protein with a PDZ (postsynaptic density/discs large/zonula occludes) domain and a ZM (ZASP-like motif) domain. The PDCP-1 gene has a high expression level and specific location in the foot, mantle and adductor muscle. Recombinantly expressed PDCP-1 (rPDCP-1) altered the morphology of calcite crystals, the polymorph of calcite crystals, binding with both calcite and aragonite crystals, and inhibition of the crystallization rate of calcite crystals. In addition, anti-rPDCP-1 antibody was prepared, and immunohistochemistry and immunofluorescence analyses revealed the specific location of PDCP-1 in the mantle, the adductor muscle, and the aragonite (nacre and myostracum) layer of the shell, suggesting multiple functions of PDCP-1 in biomineralization, muscle-shell attachment, and muscle attraction. Furthermore, pull-down analysis revealed 19 protein partners of PDCP-1 from the shell matrices, which accordingly provided a possible interaction network of PDCP-1 in the shell. These results expand the understanding of the functions of PDZ-domain-containing proteins (PDCPs) in biomineralization and the supramolecular chemistry that contributes to shell formation.

Effect of Number of Nuclei and Nucleus Position on Shell Growth and Mabé Pearl Coating in Pteria penguin Cultured in Coastal Waters of Southeast Sulawesi, Indonesia

The aim of this study was to determine the effect of additional nucleus insertion and its corresponding position on the shell growth of Pteria penguin as well as the thickness of the resulting mabe pearls. The treatment applied was the insertions of nucleus: A (one into the left shell), B (two into the left shell), and C (one into each sides of the shell). After 4 mo of culture, the growth and structure of the layers in the pearl were observed. The development of pearl layers was observed using scanning electron microscope. Results show that the number of nuclei implanted in the shell has no significant impact on shell growth or thickness as well as the number of aragonite layers of mabe pearls. Growth data show changes in oyster shell height, length, and thickness of 11.55–14.78, 18.48–22.05, and 2.59–6.14 mm, respectively, for all treatments. The average thickness of nacre was 0.29–0.33 and 0.19–0.22 µm for the base and top of resulting mabe pearls, respectively. In general, the addition of two nuclei in a single pearl oyster can be used to increase mabe pearl production in the coastal areas of Palabusa Bay, Southeast Sulawesi, Indonesia, which is suitable for the growth of pearl oysters.

Evolution of Microstructure and Corrosion Behavior of Plasma-Sprayed Ca2SiO4 Coatings with Exposure Periods in Simulated Seawater

Ca2SiO4 coatings with and without metallic bonding layer (NiCr) have been prepared by atmospheric plasma spraying. Microstructure and corrosion behavior of the coatings with various exposure periods in simulated seawater were investigated by means of scanning electron microscopy, energy dispersive spectrometry, potentiodynamic polarization and electrochemical impedance spectroscopy. The results revealed that the after immersion, a layer of urchin-like aragonite (CaCO3) was formed on the surface and a dense silica-rich layer was found between CaCO3 and Ca2SiO4 layer. Besides, self-sealing behavior was observed in the Ca2SiO4 layers. Compared with the immersed coatings, as-sprayed coatings exhibited the highest corrosion current densities, the lowest polarization resistance and charge transfer resistance. The presence of the metallic bonding layer slightly decreased the corrosion resistance of Ca2SiO4 coatings. The plasma-sprayed Ca2SiO4 coatings have a potential use in marine corrosive environments.