GB2488586A

GB2488586A - Metal coated with protective composition

Info

Publication number: GB2488586A
Application number: GB1103655.5A
Authority: GB
Inventors: Kjell Hatteland; Trond Johan Hubertz
Original assignee: EMT RES AS
Current assignee: EMT RES AS
Priority date: 2011-03-03
Filing date: 2011-03-03
Publication date: 2012-09-05
Also published as: GB201103655D0

Abstract

A metal surface in contact with flowing fluids, especially the inside of a water pipe 1 used in a hydroelectric power station is coated with a composition 2. Multiple coatings may be used. A tribo-electric charge builds up as the fluid flows along the coated surface. This may repel charged particles suspended in colloidal solutions and hence reduce fouling. This may also provide cathodic corrosion protection. The coating preferably comprises a two component epoxy resin containing polyvinyl chloride (PVC), polyethylene, polypropylene or especially silicone oil. Electrodes may be used to remove undesirable electric charges.

Description

Method for Coated Surface Protection The present invention relates to a method for reduction or prevention of fouling of a coated metal surface in contact with a flowing fluid stream and or for reduction or prevention of corrosion of a coated metal surface in contact with a flowing fluid stream, especially the present invention relates to a method involving coating surfaces with a coating with modified triboclcctric properties.

Prior art

One of the mostly used methods for iron, steel or metal protection is simply to coat the surface. Many types of additives arc added to the coatings or paintings depending on what kind of protection is wanted. Examples of additives are zinc or aluminium in order to protect steel against corrosion, and copper or other bio-toxic compounds in order to avoid fouling. r

Both galvanization of steel structures and adding zinc/aluminium to the coatings are methods to create additional negative charge to the steel compared to the surrounding fluid which is water in most cases. This is an electrochemical process o by which the steel is achieving a cathodic protection while the added metals are acting as offer anodes'. However, the protection effect will be reduced as the anode material is consumed by the process over time.

Pores in the coating will always lead to certain conduction pats between the steel and the surrounding fluid leading to a gradual degradation of the protection performance of the coating. The latest generation of these coatings is typically based on a two-component epoxy or polyurethane, which reduces the extent of micro pores substantially compared to earlier solvent based paintings. This increases the electrical insulation level for the new coatings.

Another reason to apply coating to surfaces is to minimize fouling or any kinds of deposit build ups on the surface. This is important for application where low flow resistance is economically important such as in turbine pipes for hydroelectric power plants and propulsion of ship hulls.

Build up of deposits on surface layers is of complex nature. II involves biology, electrochemical as well as electrostatic and electrokinetic processes. Often deposits on the wall are colloidal particles in the flow that settle on the surface. They can be minerals and/or organic material, dead or alive.

The surface of suspended particles in fluid flow have an electric charge relative to the fluid caused by molecular charge imbalances on the interface between the particle and the fluid. This is the reason why colloidal particles may not grow to lager aggregates of particles along the flow. When particles with the same charges come close to each other they will reject each other. On the other hand, if the charge of the solid wall (e.g. a pipe wall) is of opposite charge of the particle surface, the particles will tend to attach to the wall and form deposits. These forces are associated with the theory of the zeta potential and the double layer formation (EDL). r

o There is another electrical charge phenomenon that occurs in fluid flow ducts an channels called the "Streaming Potential" and which is resuhing from the rubbing o between fluid molecules and wall molecules. This is called the triboelectric effect.

As the flow progresses in the pipe (or duct/compartment) the charge will gradually build up and a resulting electric reverse current will be formed. The magnitudes of these potentials and currents will depend on 1) the current leakage through the coating i.e. the conductivity of the coating material and its cracks and fissures; 2) the conductivity of the fluid (which also is associated with the pH-value of the fluid), 3) the geometrical configuration of the flow system (i.e. the distance to end effects (length/diameter ratio)) and above all 4) the triboelectric potential of the wall surface coating material and 5) the flow velocity in the system. The triboelectric potential is widely varying between different materials.

Objectives of the invention An objective for the present invention is to provide a method for reducing or inhibiting fouling, scaling or deposits on coated surfaces in contact with a fluid flow.

Another objective is to provide a method that at the same time limits the corrosion of coated surfaces.

These and other objectives are obtained by the solutions presented by the present invention.

The present invention provides a method for reduction or prevention of fouling of a coated metal surface in contact with a flowing fluid stream and or for reduction or prevention of corrosion of a coated metal surface in contact with a flowing fluid stream, wherein the method comprises modifying the triboelectric property of the coating with respect to the relevant fluid stream, whereby the electrical streaming charge potential between the fluid and the coated metal is modified in a desired direction.

In one aspect the method for reduction or prevention of fouling according to the present invention comprises adding a triboelectric modifier to the coating, wherein the modifier is selected to provide a reduced charge difference or oppositely charged electrical streaming charge potential compared to the charge of particles in 0 the fluid stream most relevant to causing fouling.

o In another aspect the method for reduction or prevention of corrosion according to the present invention comprises adding a triboelectric modifier to the coating, wherein the modifier is selected to modify the triboelectric property of the coating in a direction which will contribute to an added cathodic protection of the metal surface.

According to one embodiment the triboelectric modifier is added to the bulk coating before the coating is applied to the metal surface. In another embodiment the triboelectric modifier is added to a dedicated layer being in contact with the fluid stream.

The method according to the present invention can be adapted to obtain primarily corrosion protection in one or more areas and primarily prevent fouling in other areas of the surface. In some embodiments both effects can be obtained simultaneously.

The method may further in some embodiments comprise in end areas where undesired changes in the streaming charge potential is experienced compensating these locally by means of exposing an electric DC potential on to short circuiting end elements or close to these or by adding an assisting electrode inserted in the fluid stream close to these end areas.

in one aspect of the present invention the metal surface to be protected is the surface of a water transporting equipment in a hydroelectric power station. In one embodiment of this aspect the coating is a two component epoxy coating and the triboelectric property is modified by adding a modifier selected from the group comprising silicone oil, vinyl, polyethylene, or polypropylene to the coating, preferably silicone oil.

Brief description of the drawings

Figure 1; illustrates how a charged particle 10 is repelled from a surface 2 that has the same charge polarity.

0 Figure 2; illustrates triboelectric build up as streaming potential with reverse current.

0 Figure 3; schematically illustrates a disk rotameter for measurement of triboelectricity in a water pool.

Figure 4; illustrates a hose experiment verifying the build up of a streaming potential inside the hose.

Figure 5; is a photograph illustrating the effect of the present invention.

Principal description of the invention

The present invention provides a method for protection of a coated iron, steel or metal surfaces being in contact with flowing fluids against fouling, scaling and/or corrosion by modifying the triboelectric property of the coating. The triboelectric charge being built up across the coating is contributing to repelling of charged particles suspended in colloidal solutions. The charge will also, when adjusted to adequate polarity, improve the cathodic protection of the coated metal.

As particles in a colloidal solution are charged, these particles will be rejected from a coated surface if the coated surface exhibits a voltage charge with the same sign as the colloidal particles. The present invention is based on manipulating the build up of streaming potential by means of selecting a proper material composition of the S coating with a desired triboelectric potential and thereby avoid build up of deposits on the wall.

There are several mechanisms that contribute to the build up of an electrical potential on a coated surface being in contact with a fluid. Firstly there is the phenomenon of the build up of an electrical double layer (EDL) caused by the charge imbalances that will occur on the surface molecules of the solids and the fluid molecules on the surface. This is prevailing between solids and fluids at rest or in laminar fluid flows such as in capillary flow and micro channels. In larger scale flows where the boundary layer forces are significant the triboelectric phenomenon will take over dominance in determining the surface potential. This r effect, as we understand it, is likely to be rooted in the same condition of molecular charge imbalance on the surface as is the mechanism of the double layer formation.

It is unknown whether these two mechanisms interact in the formation of a o streaming potential in turbulent flow system, or whether the triboelectric effect (the rubbing effect) supersedes the other.

The two effects mentioned above are classified as electrokinetic mechanisms.

However, there could also be cases where electroehemical effects take place and where it would be relevant to take the corrosion potential into account in judging surface potentials and where such potentials may interact with the electrokinetics.

Additional there will also be an exchange of charges between the coating itself and the metal (adherence forces). All these mechanisms will contribute to the build up of charge between the coating and the fluid and between the fluid and the coated metal plane and consequently also to the triboelectric property of the coating.

The present invention does not deal with one or more specific electrokinetic or electrostatic mechanism that could contribute to the surface charges, but relates to the overall modification of the triboelectric property of the coating, with the purpose of improving its resistance to deposits and/or to add corrosion protection the coated metal surface.

The present invention will now be described in further detail with reference to the S enclosed figures.

Figure 1 shows pipe 1 with an inner coating 2. The surface of the coating 2 is negatively charged. This is illustrated with negative charged circles 3 on the surface of the coating 2 forming a double layer (EDL). The size of the charges has been selected purely for illustrative purposes. A negatively charged particle 10 is flowing through the pipe following the path S and is repelled by the coating surface that has gained a charge of same polarity as that of the particle.

Fig.2 shows the build up of a streaming potential 7 and thereby an increased voltage drop across the coating along a pipe 1 with an inner coating 2, the surface of the r coating can be either positive or negatively charged depending on the triboelectric o properties of the coating materials. The arrow 3 indicates the direction of the fluid flow. The voltage is "discharged" by a reverse current 11 through the water column 0 and by imperfections as cracks or micro pores in the coating 13. Conduction current 12 may also "discharge" the potential. Where the water is in contact with non-isolated parts, as both ends 6, 8 of the visualized pipe, the potential is shortened to ground or zeroed.

Several experiments have been conducted showing that different coating types charges the surrounded fluid differently and even with opposite polarity.

One of these experiments is depicted in Figure 3. The setup was based on emerging a rotating coated disk 21, 22 into a pooi 23isolated from ground containing tap water and measuring the electrical potential created between the disk metal 21 and the water. The voltage build up in the water was measured with a specially designed amplifier buffer 30 with a very high input impedance using an electrode 29 mounted in the water. The disks 21 were 10 cm in diameter, coated with a coating 22 and rotated with a speed of in range 1000 rpm by means of a DC motor 24. The motor 24 was isolated from the setup in order to avoid electrical disturbances by an isolated joint 25. A shaft 6 connecting the joint 25 with disk 21 contained isolation27 in the section in contact with the water. The rotating shaft was stabilised by an isolated bearing 28 placed at the bottom of the water pool 23. The electrical charge between water and ground, defined by the metal part of the disk 21 was then measured with a Digital Multimeter (DMM) 31.

In one experiment the disk that was employed was coated with a vinyl-tare based coating and a voltage of in range +100 mY was measured between the water (connected to + terminal of the measuring instrument) and the disk metal after a couple of minutes rotation. A similar measurement on a disk coated with two-component epoxy showed a voltage of almost the same magnitude, but with opposite polarity.

Then the epoxy coating was electrically modified by adding silicone oil which has a r very strong negative triboelectric property similar as vinyl. In this case the voltage o build-up in water was strongly reduced, almost non-existing, which indicated that the added silicon had moved' the streaming potential in positive direction towards o the vinyl-tar.

A second experiment that clearly demonstrated the triboelectric voltage build up along an extended non-conducting surface being in contact with flowing water is depicted in figure 4. In this experiment a plastic hose 42, assumed to be made of PVC, was used. The inner diameter was 12 mm, the length of the hose about 25 m and a tap water flow of 3-5 mIs. The outlet end of the tubing was located in a big water pool 44 totally isolated from ground and a copper electrode 25 was immersed in the pool. The inlet of the tubing was a copper pipe 41, and tap water with a conductivity in range of 5 rn/S was used. A digital multimeter 43 was connected between the inlet pipe 41 and the copper electrode 45 in the outlet water pool 44.

The input resistance of the multimeter was 20 MOhms, which was considerably less that the resistance through the water column of the hose being around 140 MOhms.

After flow onset through the tubing an electromotoric force in range of 1.5 vohs was measured over the water string (taking into account the difference in impedance between the multimeter and the water column). The potential disappeared as soon as the water flow was turned of again.

Strong indications of triboelectric forces have also been seen inside coated steel pipelines at two hydroelectric power plants. This was seen by measuring a small voltage across several joints along the extended pipe. These joints have mostly a certain resistance (they are not totally electrically shorting the two ends) due to corrosion and oxidation in the interface. Although small, in range of 10-30 mY DC, we could measure a voltage with the same polarity over the joints for the two power plants. Both pipes were coated with epoxy coating. At one power plant a stop of the turbines was done and the voltage dropped significantly. We also tried to measure the voltage drop across the joints of another pipe structure where the coating was very old and with a non-epoxy coating. No voltages were seen as on the two other pipes, and it is believed that this was due to all imperfections in the isolation property of the coating, shortening any triboelectric voltages to ground. r

o A test rig was been built at Nedre Leirfoss power plant near Trondheim (Norway) primarily for the purpose of studying friction losses in "larger pipes" compared to o small laboratory setups. The pipe was 10 cm in diameter and totally 50 m length. At several locations there was mounted small pin electrodes being in contact with water and these were connected to a high impedance buffer amplifier similar as for the rotameter experiment depicted in figure 3. All pipe sections were new and coated with a two-component epoxy painting of in range 300 tm in thickness. The test rig could be adjusted giving a max flow of 15-20 litres/sec. Also in this test setup a clear triboelectric voltage build up at all electrodes was recorded. The voltage changes were dependent on the flow rate and were typically in the range of to 50 mV DC.

There are several types of material that can be added to a coating in order to modify the triboelectric property of a coating. The following list shows some common materials sorted in degree of triboelectric strength, ranging from strong positive to strong negative. Note that both silicone oil and vinyl are both strong negative.

Most positive (+) Air, Human Hands, Skin, Asbestos, Rabbit Fur, Glass, Human Hair, Mica, Nylon, Wool, Lead, Cat Fur, Silk, Aluminium, Paper, Cotton, Steel,(almost neutral), Wood, Lucite, Sealing Wax, Amber, Rubber Balloon, Hard Rubber, Mylar, Nickel, Copper, Silver, Brass, Synthetic Rubber, Gold, Platinum, Sulphur, Acetate, Rayon, Polyester, Celluloid, Polystyrene, Orlon, Acrylic, Cellophane Tape, Polyvinylidene chloride (Saran), Polyurethane, Polyethylene, Polypropylene, Polyvinyl chloride (Vinyl), Kel-F (PCTFE), Silicone oil, Teflon, Silicone Rubber Most Negative (-) The above list gives examples of triboelectric modifiers that can be employed in the present invention. However the present invention is generally not limited to these examples as an applicable triboelectric modifier can be selected from any compound that provides the desired modification of the triboelectric effect with respect to the relevant fluid and fouling particles. r

o Vinyl-tar coating has earlier been used in turbine pipes for many years, but is now cf prohibited due to the assumed toxic impact of the tar on the environment. However, o inspections on hydroelectric power stations have shown that some pipes coated with vinyl-tar have been operating for several decades with a minimum of fouling tendencies nor traces of corrosion. This could of course be due to the toxic impact from the tar on live biological organisms as bacteria and algae. However, it is believed that the most dominant exposure of deposits to the pipe walls is happening during spring times were lots of mud and humus (non-living material) components are washed out of the soil (forming a colloidal solution).

Pipelines with epoxy based coatings have also been inspected. Some of these pipes have shoved considerably more fouling build up than pipes treated with vinyl-tar coating although the coating was of much newer date.

It is therefore reasonably to believe that the difference between these two coating types, with respect to withstanding fouling build-up, most likely is related to also the different triboelectric properties of the two coating types.

Figure 5 shows three test pieces that was immersed in a creak containing water flow during spring/summer for 3 months. The water flow direction was from left to right.

The water temperature was 10 to 20 °C. One was coated with vinyl-tar the lower one (black one), another with unmodified epoxy in the middle and the third one on the top with epoxy modified with a silicone oil. The picture shows a reasonably clear indication that the fouling on the modified epoxy painting has been reduced compared to the unmodified epoxy painting. It is difficuh to see the degree of deposits on the vinyl-tar coated test piece due to the black colour. The degree of fouling was, however, visually judged to be something in between the modified epoxy and the epoxy coated test pieces.

By modifying the coating's triboelectric properties the antifouling properties of the coating has, in this experiment, been measurable ahered. r

It should also be mentioned that these triboelectric properties also will have positive impact on the corrosion properties of the coating if the streaming potential contributes to giving the pipe metal an added cathodic protection. It could therefore o be desirable to have coatings with different triboelectric properties at different locations of a long and complex assembly, which aims at locally solving the most dominant problem: corrosion or fouling/deposits.

A resent inspection was carried out of the steel lined pressure shaft at Uvdal 1 hydroelectric power plant. The internal coating was of the vinyl-tare sort and more than 20 years old. The inspection showed a perfect surface without corrosion nor fouling all the way except at the upper and lower end of the shaft where considerable corrosion and some fouling was observed. This is believed to be caused by a break down of the protecting streaming potential due to electrical short circuits to ground through the pipe rupture safety valve at the top end of the shaft and through the uncoated turbine at the lower end of the shaft.

Such puncture of the protecting potential at the ends can be compensated locally by means of exposing an electric DC potential on to the short circuiting units or close to these or to add an assisting electrode inserted in the water close to these units. r

Claims

CLAIMS1. Method for reduction or prevention of fouling of a coated metal surface in contact with a flowing fluid stream and or for reduction or prevention of corrosion of a coated metal surface in contact with a flowing fluid stream, wherein the method comprises modifying the triboelectric property of the coating with respect to the relevant fluid stream, whereby the electrical streaming charge potential between the fluid and the coated metal is modified in a desired direction.
2. Method for reduction or prevention of fouling according to claim 1, wherein the method comprises adding a triboelectric modifier to the coating, wherein the modifier is selected to provide a reduced charge difference or oppositely charged electrical streaming charge potential compared to the charge of 1-particles in the fluid stream most relevant to causing fouling. r
3. Method for reduction or prevention of corrosion according to claim 1, o wherein the method comprises adding a triboelectric modifier to the coating, wherein the modifier is selected to modify the triboelectric property of the O coating in a direction which will contribute to an added cathodic protection of the metal surface.
4. Method according to claim 1, 2 or 3, wherein the triboelectric modifier is added to the bulk coating before the coating is applied to the metal surface.
5. Method according to claim 1, 2 or 3, wherein the triboelectric modifier is added to a dedicated layer being in contact with the fluid stream.
6. Method according to any one of the claims 1-5, wherein triboelectric property is modified to provide primarily corrosion protection in one or more areas and primarily prevent fouling in other areas of the surface.
7. Method according to any one of the proceeding claims, wherein the method further comprises in end areas where undesired changes in the streaming charge potential is experienced compensating these locally by means of exposing an electric DC potential on to short circuiting end elements or close to these or by adding an assisting electrode inserted in the fluid stream close to these end areas.
8. Method according to any one of the proceeding claims, wherein the metal surface is a water transporting equipment in a hydroelectric power station.
9. Method according to claim 8, wherein the coating is a two component epoxy coating.
10. Method according to claim 8 or 9, wherein the triboelectric property is modified by adding a modifier selected from the group comprising silicone oil, vinyl, polyethylene, or polypropylene to the coating, preferably silicone oil. r