EP0559879A1 - Use and selection of coating and surface materials to control surface fouling and corrosion using zeta potential measurement - Google Patents

Abstract

The invention describes a method of selecting potential coating materials for high temperature applications. This process involves measuring the zeta potentials of a fouling substance and a potential coating or surface. The figure represents the experimental device used for the measurement of the zeta potential. The pump (1) injects electrolyte into the loop (2). Heating tapes (3) are wrapped around the tubing and are regulated by temperature controllers (4). A YSZ electrode (5) and a reference electrode (6) with external pressure regulation measure the pH. The system pressure is measured by a pressure gauge (7) and maintained by a relief valve (8). The solution is cooled with a cooling jacket (9). The pulverulent substance (11) to be measured is packed in a rulon column (10) and held by platinum screens and teflon filters (13). The electrode contacts are made by means of rulon tips (14). A differential pressure transducer (15) is provided.

Description

USE AND SELECTION OF COATING AND SURFACE

MATERIALS TO CONTROL SURFACE FOULING

AND CORROSION USING ZETA POTENTIAL MEASUREMENT

Field of the Invention

This invention relates to the use and selection of coating and surface material to prevent fouling deposit formation on surfaces, using zeta potential measurement to select appropriate materials.

Background of the Invention

Electrokinetic phenomena involve either the movement of charged particles through a continuous medium or the movement of a continuous medium over a charged surface. The four principal electrokinetic phenomena are electrophoresis, electroosmosis, streaming potential and sedimentation potential. These phenomena are related to one another through the zeta potential, , of the electrical double layer which exists in the neighborhood of the charged surface.

The distribution of electrolyte ions in the neighborhood of a negatively charged surface and the variation of potential, ψ, with distance from the surface are well known in the art (see, for example, Zeta Potential in Colloid Science - Principles and Applications; R.J.Hunter; Academic Press, N.Y. (1988) ) . Two different layers of ions appear to be associated with the charged surface. The layer of ions immediately adjacent to the surface is called the inner Hel holtz (IH) layer; the second layer of ions is the outer Helmholtz (OH) layer.

Ions of the IH layer are held to the charged surface by a combination of electrostatic attraction, specific adsorption forces and chemical bonds. The thickness, δ, of this* layer is assumed <- be equal to the ionic radius of the specifically adsorbed ionic species.

The second layer of ions is the OH layer. The boundary between the two layers is the limiting inner Helmholtz plane. The ions outside the OH layer are acted upon only by electrostatic forces andthermalmotions ofthe liquid environment (Brownian motion) , and they form a diffuse atmosphere of opposite charge to the net charge at the OH plane. The net charge density of the ion atmosphere of the diffused layer decreases exponentially with distance from the limiting OH plane.

Thediffused layer forms one half of an electrical double layer, and the charged surface plus the inner and outer Helmholtz layers form the other half. The effective distance of separation 1/k between the two halves of the double layer is determined by the concentration of electrolyte (ionic strength) . For an electrolyte of univalent ions inwater at 25°C (77°F) , the relationship for 1/k from the Debye-Huckel theory is described by an equation.

where EQ is the permittivity of free space D is the dielectric constant R is the gas constant T is the absolute temperature F is the Faraday constant I is the ionic strength, defined as:

where, C_; is the concentration, and

Z; is the valency of the ionic species.

Variation of potential, , with the distance x from the charged surface decreases linearly with increasing distance x in the region of the inner and outer Helmholtz layers. In the region of the diffused layer, φ decreases exponentially with increasing distance x.

In electrokinetic phenomena, a displacement occurs at some plane (plane of shear) between the charged surface and its atmosphere of ions. The position of the slipping plane is known to be located in the OH layer. The potential of the plane of shear is the f-potential. From the theories of Gouy and Chapman, for spherical particles the following equation holds:

where

* - «!* '

q_e is the charge on the particle. Here 1/k is the effective thickness of the double layer, D the dielectric constant of the liquid, and a the particle's radius at the plane of shear. For flat surfaces, a fifth equation holds, where e is the charge per unit area of surface:

C= 4πe (5) Dk

Thus, the equations 2 and 3 above show that the ξ- potential is determined by the net charge at the plane of shear and 1/k, the effective thickness of the ion atmosphere. In turn, the ^-potential controls the rate of transport between the charge surface and the adjacent liquid. The relationship between rate of transport v_E and the f-potential which is valid for all four electrokinetic phenomena is given by a sixth equation, where v_E is the velocity of the liquid at a large distance from the charged surface, E is the field strength (V/cm) , and η is the viscosity of the liquid.

^s 4πη

The conditions for validity of this sixth equation are that the double layer thickness (1/k) must be small compared to the radius of curvature of the surface; the substance of the surface must be nonconducting; and the surface conductance of the interfacemust be negligible.

The equations which relate f-potential to the streaming potential may be obtained from Equation 6 by use of Poiseuille's law for laminar flow through a capillary. For electrophoresis and sedimentation potential, v_E is the velocity of the particles. E is the applied field strength for electrophoresis, whereas it is the gradient of potential developed by the sedimentation of charged particles in the sedimentation effect.

The effect of electrolyte concentration on the f- potential is also well known. Characteristically, an increase in electrolyte concentration produces a decrease in f-potential, and ions of high charge of opposite sign to that of the surface can completely reverse the sign of the f-potential. The explanation for these two effects is also well known: an increase in electrolyte concentration reduces f-potential by reducing 1/k, as indicated by Equations 1-3 and 6, given above. Reversal of charge by ion adsorption occurs in the double layer and this gives rise to a ^-potential of opposite sign to the original value.

Knowledge of high temperature {"--potentials and the point of zero charge (pzc) , as well as other physicochemical properties, is important to control particle deposition, particle removal, coating adhesion, microbial deposition, the fabrication of advanced ceramics and other technological applications where surface charge of particles plays a role.

For example, oxides are typically produced as corrosion products in the feedwater systems of commercial nuclear and fossil fuel power plants. Corrosion products deposited on surfaces involved in critical measurements can effect the reliability of those measurements. For example, such corrosion products deposited on critical surfaces of the Venturis that are used to measure mass flow rates in power plants will result in higher energy utilization and costs. Thus, it would be highly beneficial if coatings or surfaces could be discovered which have physicochemical properties whichwould in theoryrepel foulingmaterials of concern for a particular application, for example, the same sign of surface charge at the operating pHvalue as the charge of the oxide corrosion products; those substances likely will repel the fouling or corrosion products via electrostatic repulsion and thus prevent fouling or corrosion of the surface of interest.

There is no evidence in the literature of measurement systems capable of measuring {"-potentials at elevated temperatures. Although the use of coatings to prevent fouling of feedwater Venturis in power plants has been studied, specific coating and surface candidates with the desired surface charge have not been identified.

Thus, it is an object of this invention to provide coatings and surfaces resistantto fouling and repulsive to foulingmaterials, particularly athigh temperature.

It is a further object of this invention to provide coatings and surfaces resistant to fouling and repulsive to fouling materials selected by the use of zeta- potential measurement at high temperature.

It is a further object of this invention to provide a method of selecting coatings and surfaces resistant to fouling and repulsive to fouling materials selected by the use of zeta-potential measurement at high temperature.

DESCRIPTION OF THE DRAWINGS

FIGURE 1 shows theexperimental arrangementused for zeta potential measurement. In the presently preferred embodiment, automated computer control of the measuring process is accomplished using components shown in the FIGURE and software developed to monitor and control the system.

FIGURE 2 is an enlarged and expanded view (central figure) of the rulon column section of the loop. As explained in the text, the substance or surface to be measured is held in place by platinum screens and, if necessary, tsflon filters as the electrolyte flows through and contacts the substance or surface.

FIGURE 3 shows the results of zeta potential measurement vs. pH at 235°C for Fe₃0₄.

FIGURE 4 shows the results of zeta potential measurement vs. pH at 235°C for the PdO.

FIGURE 5 shows results of zeta potential measurement vs. pH at 235°C for the W0₃.

FIGURE 6 shows results of zeta potential measurement vs. pH at 235°C for the Fe₂0₃.

FIGURE 7 shows the results of zeta potential measurement vs. pH at 235°C for Fe₃0₄ in solutions aerated for different periods.

FIGURE 8 shows results of zeta potential measurement vs. pH at 235°C for an actual foulingmaterial collected from the feedwater duct of a power plant.

FIGURE 9 shows results of zeta potential measurement vs. pH at 235°C for Ti0₂.

FIGURE 10 shows results of zeta potential measurement vs. pH at 235°C for Ta₂0₅. FIGURE 11 shows results of zeta potential measurement vs. pH at 235°C for NbjOj.

DETAILED DESCRIPTION OF THE INVENTION

The experimental arrangement for high temperature f-potential measurements is shown in Fig. 1 and in Fig. 2. An electrolyte is pu_.pcd into the loop by a commercial HPLC pump 1. The loop 2 is made of 1/4" and 1/8" stainless steel tubing. Heating tapes 3 wrapped around the stainless steel tubing are controlled by commercial temperature controllers 4 that sense the temperature at both ends of the rulon column. High temperature pH is measured with a yttria-stabilized zirconia (YSZ) pH electrode 5 and an external pressure balanced reference electrode (EPBRE) 6. The pressure of the system is measured by a pressure gauge 7 and pressure is maintained by adjustable pressure release valve 8. The solution is cooled to room temperature by passing it through a cooling jacket 9 before it is discharged from the system.

The substances to be studied are packed into a rulon column 10 which is tightly fitted inside the stainless steeltubing 2. Rulon provides the electrical isolation of the substance powder 11 from the stainless steel tubing wall. Powder is held in the column by Pt screens 12 that also serve as electrodes formeasuring streaming potentials. For very fine powders, teflon filters 13 are used in addition to Pt screens. Contacts to the electrodes are made through rulon ferrules 14. The column region 10 of the loop is thermally well insulated to avoid heat losses and maintain the column at the required temperature. Rulon columns 10 are made to different lengths such that they provide easily measurable pressure drops across the column. In a newer design, column lengths can be changed simply by changing the thickness of the center rulon ring 10. The powder 11 is packed in the center opening of the ring and the three rings 10 are held tightly by pipe fittings that also provide the high pressure seal. Nominal thicknesses of rings are about 3/32" for outer electrode holder rings and about 3/16" for center ring. Ring O.D. and I.D. are about 3/8" and 3/16", respectively. The outer stainless steel casing 2 supports the rulon column 10 to take the high pressure. The rulon column 10 can be easilymodified to accommodate solid surfaces to study surface charge of coatings. Thus, both powders and solid surfaces can be used to determine the {"-potential of the material of interest. When solid surfaces are used, their distance of separation should be small (10-100 mils) so that adequate pressure drops along the length of the separation can be attained by varying the electrolyte flow.

Design of the column and the electrode contacts are key parts of the high temperature {"-potential measuring system. Experimental difficulties at high temperature andpressurehave preventedmeasurements of {"-potentials above about 95°C. The novel design of powder packed column and electrode contacts make itpossible tomeasure {"-potentials of materials up to and beyond the supercritical temperature of water.

This invention discloses coatings or surfaces to protect measurement surfaces from fouling. The invention makes use of novel techniques to measure both the pH of aqueous solutions and {"-potentials of substances, coating and surfaces as well as fouling and corrosion products, at high temperatures to discover candidate coatings and surfaces that can be used to prevent fouling. In a preferred embodiment, this invention uses measurement of {"-potentials at elevated temperatures, in the range of about 235°C, to identify candidate coating and surface materials, although measurement at lower and higher temperatures, and under a wide range of pressures and pHs, is also possible using the method of this invention.

Deposition of the candidate anti-fouling coatings and anti-fouling surfaces identified herein will be useful when applied to measuring surfaces.

In one embodiment, a coating, which could be applied to a measuring surface, is selected by virtue of its having physicochemical properties which will act to repel particular fouling or corrosion products which are present in a fluid in contactwiththemeasuring surface. The selected coating could then be applied, alone or in a formulation as, for example, by sputtering deposition, electroplating, electrolessplating, ion implantation, and other coating methods known to those skilled in the art, to the measuring surface. Application of the coating to the measuring surface would serve to protect the measuring surface from deposition of all or some of foulingmaterialor corrosionproductspresent in a fluid to be contacted with the measuring surface.

In another embodiment, a surface, which could serve as a measuring surface, is selected by virtue of its having physicochemical properties which will act to repel particular fouling or corroding materials which are present in a fluid in contactwiththemeasuring surface. The selected surface material could be formed as part of or as an addition to the measuring surface, to form a device or the like. Selection of this surfacematerial as the measuring surface would serve to protect the measuring surface from deposition of all or some of foulingmaterial or corrodingmaterial present in a fluid to be contacted with the measuring surface.

In a preferred embodiment, the feedwater Venturis used to measure mass flow rates in power plants, or, for example, orifice plate surfaces or flow tap surfaces or other surfaces susceptible to fouling or corrosion, particularly at high operating temperatures, may be protected.

In a preferred embodiment, the physicochemical property to bemeasured is the {"-potential. Thus, the {"-potential of the coating or surface material is compared to the {"-potential of the fouling material or corroding material, at conditions of temperature and pH that correspond to conditions under which the measuring surface will operate.

Such coatings and surfaces will, for example, reduce power losses currently encountered due to venturi fouling and corrosion, and allow more accurate measurements via the measuring surfaces of interest which are critical components in those measurements.

In a preferred embodiment, the high temperature {"-potential of oxides as a function of pH has been measured to determine the oxide's pzc's and the sign of the surface charges. {"-potential measurement is not limitedto oxides, however, andthe {"-potentials of other fouling and corrosion products, as well as those of a variety of candidate coating and surface materials, are within the scope of this invention.

In a preferred embodiment, {"-potentials of various oxides at high temperature have been measured using the above- described apparatus. For example, the variation of {"-potential with pH for Fe₃0₄ at 235°C is shown in FIG. 3 as an illustration. In the presently preferred embodiment, {"-potentials are calculated from slopes of the graphs of streaming potential (ΔE) vs. differential pressure (ΔP) .

In a presently preferred embodiment, automated computer control of the {"-potential measuring process is used. Thepresentlypreferredcomputer interface usedwith the loop, to control the fluid flow rate and to acquire data, is presented for illustration purposes in Figure 1.

To better describe the use of this apparatus, the following example is provided, which is not intended to in any way limit the scope of this invention.

Example

In this experiment, metal oxides having the same sign of surface charge as fouling or corroding material oxides, such as Fe₂0₃ and Fe₃0₄, typically found in the feedwater systems of commercial nuclear and fossil fuel power plants, are identified.

If the fouling or corroding material oxides and the candidate metal oxide have the same sign of surface charge at a given pH value, then the candidate metal oxide likely will prevent fouling and/or corrosion of the venturi surface, used to measure mass flow rate of the feedwater entering the system.

Measurement of the high temperature {"-potential of oxides as a function of pH to determine their pzc's and the sign of the surface charge was carried out. PdO and W0₃ (Aldrich Chemicals) powders were separately placed in the rulon column, held in place by platinum screens (and additionally teflon filters) . Although a variety of electrolytes (such as KC1, KN0₃ or any inert electrolyte, at concentrations in the range from about 10"³M to about lO^M) could be used, 10^_3M KN0₃ was circulated through the measuring loop, contacting and passing through the PdO and W0₃ powders in the rulon column.

The pressure in the loop was held at approximately 600 psi; the temperature of the electrolyte in the column was held at 235°C for the course of the measurements. pH of the electrolyte solution was also measured and correlated with the temperature measurements. {"-potential is calculated from the slope of the graph of streaming potential (ΔE) vs. differential pressure (ΔP). E _ E₀Dr²R _{^ (7)}

ΔP 4 η 1 *

R = Resistance of the powder packed column r = radius of the column 1 •= length of the column.

The entire system of varying the flow rate of the HPLC pump and the acquisition of the ΔE and ΔP data was under computer control.

Using the measuring apparatus described herein, both PdO and W0₃ show negative {"-potentials at 235°C over the pH range 2 to 9 and 2 to 6.5, respectively, (Figs. 4 and 5) . Thus, it is clear that at the operating pH's of boiling water reactors (BWRs; pH5.6) and pressurized water reactors (PWRs; pH7.2), both PdO and W0₃ remain negatively charged.

Fouling and corroding particulates commonly found in venturi ducts of power plants are Fe₂0₃ and Fe₃0₄. Thus, characteristics of these compounds were compared to characteristics of PdO and W0₃. At these pH's, Fe₂0₃ is negatively charged (Fig. 6) at 235°C and hence, in principle, should not deposit on PdO and W0₃ surfaces due to electrostatic repulsion.

Fe₃0₄, on the other hand, has a small positive charge at 235°C under BWR conditions, indicating some degree of interaction between Fe₃0₄ and both PdO and W0₃. Under PWR conditions, Fe₃0₄ has either a small negative or a small <_;Iιarge depending on the oxygen content present in the oxide (Fig. 7). Thus, Fe₃0₄ might be slightly attracted or repelled from PdO andW0₃ surfaces. The exact interaction can only be estimated once tests areperformedwith actualpower plant foulingmaterials.

If it is assumed that the actual power plant fouling deposits resemble theventuri deposits used as exemplary in this example (i.e., a mixture of Fe^ and Fe₃0₄ isolated from an actual power plant venturi deposit) , then the surface charge of the fouling deposits is either close to zero or negative under both BWR and PWR operating conditions (Fig. 8) at 235°C and hence should not deposit on PdO or W0₃ surfaces.

Similar results, with reference to Fe₂0₃ and Fe₃0₄, were obtained with Ti0₂ Ta₂0₅, and Nb .-. These results are depicted in Figures 9-11.

Thus, the {"-potential approach can be used to identify a variety of potential coating materials for preventing the fouling and corrosion of venturi surfaces. These results indicate that PdO and W0₃ show unique behavior compared with many other oxides investigated. PdO, W0₃, Ti0₂ Ta₂0₅, and Nb^ are potentially useful as coating materials for venturi surfaces that would prevent fouling and corrosion.

Claims

What is Claimed is:

I. A coating or surface to prevent fouling and corrosion of a measuring surface in contact with fouling or corroding material in a fluid.

2. A coating or surface of Claim 1, wherein said surface is a device surface.

3. A coating or surface of Claim 2, wherein said device is used to measure mass flow rate of a fluid.

4. A coating or surface of Claim 1, wherein said measuring surface is a venturi surface.

5. A coating or surface of Claim 1, wherein said measuring surface is an orifice plate surface.

6. A coating or surface of Claim 1, wherein said measuring surface is a flow tap surface.

7. A coating or surface of Claim 1, wherein said coating or surface is a member of the group consisting of metals, alloys, oxides and ceramics.

8. A coating or surface of Claim 1, wherein said coating or surface is W0₃.

9. A coating or surface of Claim 1, wherein said coating or surface is PdO.

10. A coating or surface of Claim 1, wherein said coating or surface is Ti0₂.

II. A coating or surface of Claim 1, wherein said coating or surface is Ta₂0₅.

12. A coating or surface of Claim 1, wherein said coating or surface is Nb₂0₅.

13. A coating or surface of Claim 1, wherein said coating or surface is selected to have a physicochemical property the effect of which is to repel fouling or corrodingmaterial present in said fluid contacting said measuring surface.

14. A coating or surface of Claim 13, wherein said physicochemical property is the zeta potential.

15. A method to select a candidate coating or surface to be used as a coating or surface at which comprises a. measuring a physicochemical property of a surface fouling or corroding material; b. measuring the physicochemical property of a candidate coating or surface, such measurement taken within the range of operating temperatures and pH's to which said candidate coating or surfacewill be exposed; c. comparing said physicochemical property of said fouling or corroding material with said {"-potential of said candidate coating or surface; and, d. selecting as an appropriate candidate said candidate coating or surface with a physicochemical property the sign of which is the same sign as that of said fouling material.

16. Amethod of Claim 15, wherein said physicochemical property is the {"-potential.

17. Amethod of Claim 15, wherein said candidate coating or surface is selected from the group consisting of metals, alloys, oxides and ceramics.

18. Amethod of Claim 17, wherein said candidate coating or surface is PdO.

19. A method of Claim 17, wherein said candidate coating or surface is W0₃.

20. A method of Claim 17, wherein said coating or surface is Ti0₂.

21. A method of Claim 17, wherein said coating or surface is Ta₂0₅.

22. A method of Claim 17, wherein said coating or surface is Nb->0₅.

23. A method of Claim 15, wherein said measurements are under computer control.

24. A coating or surface selected by the method of Claim 15.

25. A method to select a candidate coating or surface to be used as a coating or surface which comprises a. measuring the {"-potential of a surface fouling or corrosion material; b. measuring the {"-potential of a candidate coating or surface, such measurement taken within the range of operating temperatures and pH's to which said candidate coating or surface will be exposed; c. comparing said {"-potential of said fouling material with said {"-potential of said candidate coating or surface; and, d. selecting as an appropriate candidate said candidate coating or surface with a {"-potential the sign of which is the same sign as that of said fouling material.

26. A method of Claim 25, wherein said candidate coating or surface is selected from the group consisting of metals, alloys, oxides and ceramics.

27. Amethod of Claim 26, wherein said candidate coating or surface is PdO.

28. Amethod of Claim 26, wherein said candidate coating or surface is W0₃.

29. A method of Claim 26, wherein said coating or surface is Ti0₂.

30. A method of Claim 26, wherein said coating or surface is Ta₂0₅.

31. A method of Claim 26, wherein said coating or surface is Nb-jOj.

32. A coating or surface selected by the method of Claim 25.