Silica Scale Formation Research Articles

Comparative investigation of structure, mechanical properties, and oxidation resistance of Mo-Si-B and Mo-Al-Si-B coatings

Mo-Si-B and Mo-Al-Si-B coatings were deposited by DC magnetron sputtering of MoSiB and MoAlSiB composite targets fabricated by the self-propagating high-temperature synthesis method. The structure, element and phase composition of coatings were studied by means of scanning and transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and glow discharge optical emission spectroscopy. To evaluate their oxidation resistance, the coatings were annealed in air in the temperature range of 1200–1700°C during different time slots between 10min and 5h. The obtained results demonstrated that the Mo-Si-B coatings possess higher hardness, improved oxidation resistance and better thermal stability compared with their Mo-Al-Si-B counterparts. The 7-μm thick Mo-Si-B coatings were shown to successfully withstand oxidation during short-time exposure for 10min at a temperature as high as 1700°C due to the formation of protective silica scale. The oxidation of Mo-Al-Si-B coatings was accompanied by the diffusion of aluminum to the coating surfaces and the formation of a single Al2O3 layer at 1200–1300°C and a double Al2O3-SiO2 layer at 1500°C which were less protective against oxidation. The surface oxidation processes were also accompanied by phase transformations inside the oxygen-free part of both Mo-Si-B and Mo-Al-Si-B coatings with the formation of MoB and Mo5Si3 phases.

Relating Silica Scaling in Reverse Osmosis to Membrane Surface Properties

We investigated the relationship between membrane surface properties and silica scaling in reverse osmosis (RO). The effects of membrane hydrophilicity, free energy for heterogeneous nucleation, and surface charge on silica scaling were examined by comparing thin-film composite polyamide membranes grafted with a variety of polymers. Results show that the rate of silica scaling was independent of both membrane hydrophilicity and free energy for heterogeneous nucleation. In contrast, membrane surface charge demonstrated a strong correlation with the extent of silica scaling (R2 > 0.95, p < 0.001). Positively charged membranes significantly facilitated silica scaling, whereas a more negative membrane surface charge led to reduced scaling. This observation suggests that deposition of negatively charged silica species on the membrane surface plays a critical role in silica scale formation. Our findings provide fundamental insights into the mechanisms governing silica scaling in reverse osmosis and highlight the potential of membrane surface modification as a strategy to reduce silica scaling.

Influence of Sodium Hydroxide on the Austenitic 330Cb Alloy Oxidation at High Temperature

Sodium hydroxide is found on metallic conveyor belts as residue of the degreasing process of metallic screws put inside heat treatment furnaces at 900 °C. The 330Cb (Fe-34Ni-23Cr-1Nb-1.55Si) alloy is used to build these conveyor belts. With time, the amount of sodium hydroxide increases on the 330Cb steel surface. The aim of this work is to show the influence of the NaOH amount on the 330Cb oxidation during 48 h, at 900 °C. Results show that, on this austenitic steel, a protective silica scale formation is promoted by low oxygen containing gaseous environments and the high alloy silicon content. With low NaOH deposits the oxide layer is adherent and a SiO 2 cristobalite subscale is formed. It is also demonstrated that sodium combines with niobium to form NaNbO 3. When the NaOH amount exceeds 0.265 mg.cm-2 on the alloy surface, severe oxidation is observed leading to a fast growing FeCr 2 O 4 oxide scale.

Real-time direct detection of silica scaling on RO membranes

Silica scaling of RO membranes was evaluated via real-time direct surface imaging demonstrating a capability for detecting the onset of silica scale formation and its evolution. Silica scaling was detected significantly earlier than by traditional flux decline measurements. The observed rate of silica particle nucleation followed classical nucleation theory while the growth of individual silica particles, at the early stages of silica scaling, was governed by diffusional growth. SEM and optical images of the membrane surface suggest that silica scaling occurs through the formation of both primary silica particles and their agglomerates (~1–30µm), as well as a gel-like silica film embedded with silica particles both of which contribute to permeate flux decline. At low silica saturation index at the membrane surface (SIm≤1.93) silica gel film formation resulted in a smoother and less porous film than at higher silica saturation (SIm≥2.72). At the higher silica saturation levels (SIm=2.72–3.50), silica scaling resulted in larger observed particles as well as rapid permeate flux decline. The silica scale layer thickness was in the range of ~0.1–3.5µm, with surface roughness being higher by a factor of 2.6–8.3 relative to the native membrane. Results of the present study suggest that there is merit in exploring the application of the present approach for early detection and monitoring of silica scaling in RO plants in support of strategies for silica scale mitigation.

Microstructure and oxidation behaviour of Fe–Cr–silicide coating on a niobium alloy

The cyclic oxidation performance of a Fe–Cr-modified silicide coating on Nb-alloy C-103 was evaluated in air at temperatures between 1100 and 1500°C and the microstructural changes examined. The coating was oxidation resistant and the duration of protection increased from 60 minutes at 1100°C to 255 minutes at 1500°C. The formation of the surface glassy silica layer and its ability to heal the cracks in the coating enhanced the oxidation performance at high temperatures. The outer NbSi2 layer served as a reservoir of Si and sustained the formation of the protective silica scale. Spallation of the silica scale and the concomitant depletion of the NbSi2 layer towards the regeneration of the silica scale limit the oxidation life of the coating.

Oxidation protective glass–ceramic coating for higher manganese silicide thermoelectrics

In this work, a silica-based glass–ceramic coating has been deposited on higher manganese silicide (HMS) thermoelectrics in order to provide oxidation protection at high temperature (up to 600 °C). The coating showed good densification behaviour and an excellent thermomechanical and chemical compatibility with the HMS substrate, suppressing the silica scale formation on the HMS at high temperatures. The thermoelectric properties of the glass–ceramic-coated HMS showed excellent stability after oxidation at 600 °C in air for 500 h.

Silica scale formation and effect of sodium and aluminium ions -29Si NMR study

Silica scale formation on reverse osmosis (RO) membrane surface is a significant problem for operation of high recovery RO desalination plant.

Isothermal and cyclic oxidation behaviour of hot-pressed MSi2 compounds (with M = V, Ti, Cr)

The oxidation resistance of MSi2 compounds with M = V, Ti, Cr was investigated from 450 to 950 °C in air under isothermal and cyclic conditions. Vanadium, chromium and titanium disilicide were not subjected to the pest phenomenon at 650 °C over 800 1-h cycles. The results demonstrated very low weight gains regardless of the testing conditions. Oxidation tests were also performed over long duration (1000 h) to identify the oxidation products. The MSi2 compounds were all subjected to the simultaneous oxidation of M and Si despite the formation of a protective silica scale. Increasing the duration of oxidation enhanced the protective properties of the silica scale. Therefore, short-term measurements (by thermogravimetry) did not allow an extrapolation of the MSi2 lifetime. The formation of molten V2O5 induced a higher oxidation rate of VSi2 and delayed the establishment of the protective silica scale compared with CrSi2 and TiSi2.

Isothermal oxidation behavior and growth kinetics of silicide coatings formed on Nb–1Zr–0.1C alloy

Isothermal oxidation behavior of the single layer NbSi2 and the double layered (NbSi2+Nb5Si3) silicide coated Nb–1Zr–0.1C (wt.%) alloys has been studied between 800 and 1300°C in static laboratory air. The single layer coated alloy shows a lower weight gain as compared to the double layer coatings at all temperatures. The formation of amorphous silica scale embedded with Nb2O5 oxide particles is observed beyond 1100°C, and a superior oxidation resistance is detected between 1100 and 1300°C. A kinetic equation relating the thickness of the coating layer with the various process parameters for pack siliconizing of Nb-alloy has been established.

The interplay between cationic polyethyleneimine and anionic polyelectrolytes for the control of silica scale formation in process waters

Stabilization of mono- and disilicic acids is accomplished by using a cationic polymeric chemical additive, polyethyleneimine (PEI), in supersaturated silica solutions (the starting solution contained 500 ppm/8.3 mM sodium orthosilicate, Na2SiO3·5H2O, expressed as SiO2) at pH = 7. The PEI polymeric system contains an excess of amine groups (~25% primary amines, ~50% secondary amines and ~25% tertiary amines) that become protonated at circumneutral pH, hence charging the molecules positively. PEI was found to be efficient as a silica scale inhibitor only at the low dosage of 10 ppm. Its inhibitory activity was found to drop as its concentration increased. It was hypothesized that PEI is entrapped into the colloidal silica matrix and deactivated. Thus, one way to maintain its activity was to create blends of PEI and anionic polymers. Anionic polyelectrolytes that were tested included polyacrylic acid (PAA), phosphonated polyacrylic acid (PAA(PO3H2)2), carboxymethylinulin (CMI), poly(acrylamide-co-acrylic acid) (PAM-coAA), phosphonomethylated chitosan (PCH) are also studied for their silica scale inhibition efficiency. It is observed that the silica inhibitory activity of PEI was improved upon its combination with anionic polyelectrolytes.

Effect of ultrasonication on polymerization of silicic acid in geothermal water

To prevent silica scale formation on solid surfaces in geothermal water, the changes in silica concentration by ultrasonication were examined. The polymerization rate of monosilicic acid was enhanced by ultrasonication at 500 kHz and was maximum at pH = 8.5. This is because hydroxyl radicals accelerate the reaction between monosilicic and polysilicic acids. Large polymer particles were formed by ultrasonication at 500 kHz. The formation ratio of large particles increased with ultrasonic power and was maximum at pH = 6.0. These results indicate the possibility of the reduction in monosilicic concentration before silica scale formation.

Study of the Formation and Influence Factors of Silica Scale in ASP Flooding

ASP Flooding has a better displacement efficiency, but there is still a serious problem of wells scaling. Researched on formation mechanism and influence factors of silica scale in ASP flooding with alkali, the formation of silica scale is mainly due to the alkali dissolution reaction of reservoir rock to produce the soluble silicon ions and form silica scale in formation conditions. Laboratory experiments simulated formation mineralization aqueous solution, analyzed the morphology of silica scale by scanning electron microscopy, and studied the role played by calcium, magnesium, aluminum ion, polyacrylamide, surfactant. The experimental results show that: calcium, magnesium, aluminum ions, polyacrylamide, and surfactant can promote the formation of silica scale, and the morphology characteristics of silica scale are different.

PH modification for silica control

ABSTRACT Lowering solution pH slows the polymerization of silica and formation of silica scale. In batch systems, lowering the pH of approximately 200 ppm silica solutions prevents scale formation for over 300 h. Silica scale forms most quickly near pH 8. Solutions with pH 3.6–3.7 can maintain silica levels of 1,000–3,000 ppm for roughly 90 h. Bench-scale membrane testing showed that silica scale formation lag times of approximately 72 h were achievable after lowering the pH to 4.5–4.7, which might allow flushing of silica-laden solutions through, for example, flow reversal, before scale formation occurs during water treatment.

Formation of Silica Scales in the Water Utilization Process and its Prevention Methods

A huge amount of industrial water is necessary for continuous development of industries in the world. Various mineral scales forming at facilities on the ground within the water utilization process due to dissolved salts in the natural waters. Silica scale is a significantly serious problem to prevent the effectively operation of industries due to its hard structure and low solubility. This paper only focus on formation of silica scales, summarized its formation mechanism, influence factors and prevention methods.

Influence of various gaseous environments on SiO2 formation on the 330Cb (Fe-35Ni-18Cr-1Nb-2.15Si) alloy at 900 °C

The influence of various gaseous environments on the 330Cb (Fe-35Ni-18Cr-1Nb-2.15Si) oxidation during 144h, at 900°C, has been studied. In air, a non-adherent Mn1.5Cr1.5O4 and Cr2O3 containing scale is formed and no continuous silica scale is observed at the alloy/oxide interface. In nitrogen, the chromia scale is adherent and a continuous cristobalite subscale is detected at the alloy/oxide interface. In Ar-5vol.% H2, the oxide scale is mainly composed of chromia and manganese chromite. A thick cristobalite scale is also present at the alloy/oxide interface. On this austenitic alloy the protective silica scale formation is promoted by low oxygen containing gaseous environments and the high silicon content.

Alkaline/Surfactant/Polymer Flood: From the Laboratory to the Field

Summary After two decades of relative calm, chemical enhanced-oil-recovery (EOR) technologies are currently revitalized globally. Techniques such as alkaline/surfactant/polymer (ASP) flooding, originally developed by Shell, have the potential to recover significant fractions of remaining oil at a CO2 footprint that is low compared with, for example, thermal EOR, and they do not depend on a valuable miscible agent such as hydrocarbon gas. On the other hand, chemical EOR technologies typically require large quantities of chemical products such as surfactants and polymers, which must be transported to, and handled safely in, the field. Despite rising industry interest in chemical EOR, until today only polymer flooding has been applied on a significant scale, whereas applications of surfactant/polymer or alkaline ASP flooding were limited to multiwell pilots or to small field scale. Next to the oil-price fluctuations of the past two decades, technical reasons that discouraged the application of chemical EOR are excessive formation of carbonate or silica scale and formation of strong emulsions in the production facilities. Having identified significant target-oil volumes for ASP flooding, Petroleum Development Oman (PDO), supported by Shell Technology Oman, carried out a sequence of single-well pilots in three fields, sandstone and carbonate, to assess the flooding potential of tailor-made chemical formulations under real subsurface conditions, and to quantify the benefits of full-field ASP developments. This paper discusses the extensive design process that was followed. Starting from a description of the optimization of chemical phase behavior in test-tube and coreflood experiments, we elaborate how the key chemical and flow properties of an ASP flood are captured to calibrate a comprehensive reservoir-simulation model. Using this model, we evaluate PDO's single-well pilots and demonstrate how these results are used to design a pattern- flood pilot.

Oxidation behavior of Mo-Si-B-(X) alloys: Macro- and microalloying (X= Cr, Zr, La2O3)

High melting Mo-Si-B based alloys exhibit good mechanical properties and oxidation resistance at very high temperatures. The alloy composition Mo-9Si-8B (at.%) shows excellent oxidation behavior between 900°C–1,300°C as a consequence of protective silica scale formation. Below 900°C, a protective oxide layer does not form as a consequence of simultaneous and competitive Mo- and Si-oxide formation. Macro- and microalloying strategies as well as pre-treatment prior to oxidation in air were examined in order to determine their impact on the oxidation performance. Alloying with Cr is a suitable means for protective Cr-oxide scale formation at intermediate temperatures; adding small amounts of reactive elements was found to have a strong impact on the silica scale forming ability. Furthermore, the oxidation behavior can be controlled and enhanced by selective oxidation in oxygen-deficient atmospheres prior to exposure to air.

High-Temperature Oxidation Performance of Mo-Si-B Alloys: Current Results, Developments and Opportunities

Ni-base superalloys are approaching the melting point as their fundamental limitation. For high-temperature components one possibility aiming at a further increase of efficiency, e.g. of jet turbines, is the use of refractory metals. Mo as base material is suitable for operating temperatures far beyond 1200°C. As a consequence of the formation of volatile Mo-oxides, it exhibits no intrinsic oxidation resistance when exceeding 700°C. Mo-Si-B alloys have melting points around 2000°C and retain good mechanical properties and oxidation resistance at very high temperatures. In air, the three-phase Mo-Si-B alloy dealt with in this paper shows excellent oxidation behaviour between 900°C-1300°C as a consequence of the formation of a protective silica scale. Below 900°C, alloys of this class suffer from catastrophic oxidation, leading to an evaporation of Mo-oxide and giving rise to a linear rate law of the weight loss. A protective oxide layer is not formed as a consequence of simultaneous and competitive Mo- and Si-oxide formation. Several approaches are possible to improve the oxidation performance of Mo-Si-B alloys, especially in this moderate temperature range. These include classical alloying, e.g. with Cr aiming for protective Cr-oxide scales, addition of small amounts of reactive elements for microstructure-refinement as well as selective oxidation of silica in oxygen-deficient atmospheres prior to operation in air. The results presented show promising opportunities and indicate that an oxidation protection from room temperature up to 1300°C requires a combination of the suggested approaches.

The inhibition abilities of multifunctional polyelectrolytes for silica scale formation in cooling water systems: Role of the nonionic functional group

The abilities of multifunctional polyelectrolytes to enhance aluminum hydroxide dispersion and inhibit silica scale formation were examined in a pilot cooling water system. The following multifunctional polyelectrolytes were studied: a terpolymer of acrylic acid (AA), 2-acrylamide-2-methyl propane sulfonic acid (SA) and N-vinylpyrrolidone (NVP) (P(AA/SA/NVP)), acrylic acid homopolymer (P(AA)) and a copolymer of AA and SA (P(AA/SA)). The order of inhibition ability was P(AA/SA/NVP)>P(AA/SA)>P(AA), and was consistent with that of the dispersing ability for aluminum hydroxide. Other terpolymers incorporating different nonionic monomers were also examined and factors affecting their inhibition abilities were investigated, based on interaction energies calculated by density functional theory. Based on the correlation between scale inhibition abilities and interaction energies, we elucidated that the effective nonionic monomer of terpolymer for silica scale inhibition had low affinity for aluminum hydroxide and high affinity for H2O and Si(OH)3O−. The affinities of nonionic monomer for aluminum hydroxide and H2O suggested that there was proper conformation of polyelectrolyte adsorbed for effectively dispersing aluminum hydroxide. Also, high affinity of nonionic monomer for Si(OH)3O− suggested that interacting Si(OH)3O− is an important role of inhibition of silica scale formation.

Formation of a silicato complex of zinc in aqueous solution and its accelerating effect on the formation of silica scales in cooling water systems

This study elucidates the effect of zinc (Zn), which is an anticorrosive water additive, on the formation of silica scales from cooling water. In these experiments, the silica scales were analyzed by EPMA, and the results indicate that Zn is sorbed into the silica scales during formation. Measurements of the solubility of Zn(OH)(2) at various concentrations of silicic acid demonstrate that Zn is present as a silicato complex of Zn (SCZ) in cooling water. From adsorption experiments of the SCZ on silica and alumina, which are major components of the silica scales, it can be concluded that the SCZ accelerates the formation of silica scales from cooling water.