GB2466499A - Method of providing corrosion protection and removing biofilms - Google Patents

Abstract

The present invention relates to a method for the removal of bio-films and the provision of corrosion protection for electrically conducting objects 1 coated with a dielectric layer 6 in contact with a fluid phase 2. The method comprises: applying a negative DC-potential across the dielectric layer such that the electric conducting object becomes positively charged and the fluid phase becomes negatively charged for a first time period t1. A positive DC-potential is then applied across the dielectric layer such that the electric conducting object becomes negatively charged and the fluid phase becomes positively charged for a second time period t2. Finally, these first and second steps are successively repeated in a cycle, such that the first time period t1 is shorter than the second time period t2. The electrode 3 may comprise copper or aluminum. The dielectric layer 6 may comprise an epoxy with aluminium micro-flakes.

Description

Combined biofilm and corrosion protection This invention relates to a method for combined removal of bio-films and corrosion protection for coated metals.

Background

Protection of coated metal surfaces is normally related to avoiding deposits in forms of bio-film and protection against corrosion. No method is known today to combine these regimes for protection of surfaces in an efficient way and simultaneous over a substantial metal areas. As a consequence there are still lots of problems with both bio-fllm formation and corrosion in application related to steel based turbine pipes for hydroelectric power plants and inside closed containers of steel using water inside as e.g. ballast tanks for ships. Salt water heat exchangers also exhibit the same problems.

Bio-films consist of immobilised cells in a substratum of microbial origin. In flowing aquatic environments, a bio-film consists of a sticky and absorptive polysaccharide matrix encompassing micro-organisms which often forms a layer on the surfaces of pipelines, ship hulls etc. Bio-films create a range of problems for industry, by enhancing corrosion phenomena, by enhancing the flow resistance in process equipment, by forming flocculants which may clog process equipment, and sometimes by being toxic.

One example where bio-films may constitute a serious problem is hydroelectric power plants which convert the potential height energy in a mass of water to electric energy by sending the water from a relatively high altitude water magazine through a turbine pipeline down to a low attitude water turbine connected to a power generator. The flow friction in the turbine pipeline will thus constitute a direct loss of the energy yield since it reduces the conversion of the water's potential height energy to kinetic energy in the turbine. This friction loss of the water height energy is often denoted as head loss.

In order to minimise the head loss, the internal wall of the turbine pipelines is usually coated with an anti-corrosion layer. Nevertheless, it is frequently observed that the head loss in turbine pipelines tends to increase with time, due to build-up of corrosion and bio-film on the inner wall of the turbine pipes. Corrosion in forms of rust will increase both friction and turbulence near the inner pipe wall which again will increase the head loss. Bio-film formation will decrease the effective inner diameter of the piping. Since the head loss is inverse proportional to the diameter in power of 5, it is obvious that even a modest bio-film thickness will affect the overall power plant efficiency.

Thus the head loss due to corrosion and bio-film growth tends to increase over time, and may over years reach magnitudes making it necessary to sand blow and recoat the inner walls of the turbine pipelines. For example, Vrenga Power Plant in Norway bad a head loss of 33 m when it was new in 1959 (of a total water height of 352 m). In 1984 the head loss had increased to 52 m. This increase in head loss reduced the available potential energy of the water by 5.4 %, which obviously S should be avoided.

Prior art

Bio-films may be removed by mechanical or abrasive means; that is by use of scraping, surface grinding etc. Mechanical means may be applied during operation of the process equipment that is to be treated, i.e. a scraping mechanism may often be used when running the process. However, mechanical means have a drawback in that there is a practical limit of their physical dimensions such that they become cumbersome and time-consuming when applied for large systems such as turbine pipelines etc. Another problem with mechanical means is that they necessarily have a physical dimension which will occupy space in the equipment that is to be treated, and thus constitute a flow restriction in them selves.

The problem of space occupation may be solved by spraying fluid borne abrasives onto the surface, such as i.e. sand blowing. This process is considerably faster than mechanical grinding or scraping, but has the drawback that it is necessary to empty the equipment that is to be treated for its working fluid before being able to perform an effective blowing. Further, sand blowing may easily damage the coating such that it becomes necessary to recoat the metallic surfaces which are being treated.

This requires a relatively long and thus expensive shut down period of the process equipment in question.

It is known to use chemical treatments, that is use some kind of chemical (biocide) which is poisonous to the microbes forming the bio-fiim. The biocide may be applied directly into the bulk water in the system. An example of such technology is disclosed in US 7 407 590 which relates to a method for removing bio-films in aquatic media by adding one or more chlorinated hydantoins to the aquatic medium.

One obvious problem with adding the biocide directly to the aquatic media is that for large applications with huge volume flows, the method necessarily becomes costly due to need for large amounts of the biocide to obtain an effective concentration. There are also environmental concerns associated with use of large quantities of biocides.

This problem might be alleviated by applying the biocide as a coating and thus generate an antimicrobial action at the attachment surface of the application. An example of such technology is disclosed in US Patent Application 2008/0063693, which relates to an antimicrobial coating for coating a substrate surface, particularly medical devices that are likely to become contaminated or have become contaminated with micro-organisms as a result of bacterial adhesion and proliferation and methods for preventing bio-film formation by inhibiting microbial growth and proliferation on the surface of medical devices. However, a chemical agent is being consumed during action, such that an antimicrobial coating needs to be renewed at regular intervals. Thus this solution is encumbered with the same problem as for the sand blowing, it becomes necessary to shut down and empty the process equipment for renewal of the coating.

Corrosion of metals may be avoided by use of cathodic protection, which is partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or an impressed current. An example of this technology is given in US 4 510 030 which discloses a method for cathodic protection of an aluminium article against electrochemical corrosive attack by an aqueous medium. The method comprises observing the cathodic potential of the article relative to a reference electrode in contact with the aqueous medium, applying a sufficiently electro-negative potential onto the article to repress the cathodic potential to within the alkali corrosion range when the observed potential approaches the potential when pitting corrosion is initiated, and then shut off the applied potential before the article undergoes sufficient alkali corrosion. The observed potential will then begin to move towards the pitting potential. The process is therefore repeatedly being applied, resulting in a zigzag pattern of the observed potential.

US 4 437 957 discloses a combined cathodic and anodic protection where a structure immersed in an aqueous medium is protected by applying at least two protection units comprising an electrode an alternating potential at least in two regions of the structure. Each protection unit comprises an electrode immersed in the electrolyte and means for measuring and regulating the potential between the structure and each of the electrodes independently of each other. The applied potential may be a rectified alternating potential and may be adjusted to give a potential such that the structure becomes the cathode (cathodic protection) or anode (anodic protection).

WO 2004/094319 discloses a method and apparatus for improvement of flow rates and reduction of fouling in process equipment by constantly applying a positive DC-potential at the walls of the process equipment that exactly opposes the naturally occurring potential due to interaction between the walls and the flowing fluid. The effect is claimed to be caused by a reorientation of the dipolar water molecules near the surface, which again will improve the flow characteristic near the surface. Thus a better degree of washing' of the surface is achieved.

In an earlier application by the same inventor, WO 1999/0 19260, a similar method of constantly applying a positive DC-potential at the walls of the pipes and ducts in order to reduce the flow restriction is disclosed for use in pipes/ducts with water flowing inside, such as for example turbine pipelines for hydro-electric power plants. There were performed experiments with this method at Vrenga Power Plant in the period 1999 to 2003. It was shown that by applying a modest negative voltage of in range 1.1 Volts to the water, the head loss was reduced with 14 % over 4 years. The specific reason(s) behind this reduction of head loss was, however, not fully understood.

Objective of the invention The main objective of the invention is to provide a combined method for removal of bio-film and prevention of corrosion on coated metal surfaces.

Description of the invention

The invention utilises an observation that removal of a bio-film by imposing a positive DC-charge to the coated metal structure relative to the water is faster than the natural build-up of the film. This allows a combination of cathodic corrosion protection by applying a negative DC-potential such that the metallic pipe object to be protected becomes a cathode relative to the fluid phase, interrupted by relatively short periods of reversed potential for removing of formed bio-film.

Thus in a first aspect, the present invention relates to a method for combined removal of bio-films and corrosion protection for electrically conducting objects coated with a dielectric layer in contact with a fluid phase, wherein the method comprises the following steps: 1) applying a negative DC-potential across the dielectric layer such that the electric conducting object becomes positively charged and the fluid phase becomes negatively charged for a first time period ti, 2) applying a positive DC-potential across the dielectric layer such that the electric conducting object becomes negatively charged and the fluid phase becomes positively charged for a second time period t2, and 3) successively repeating step 1) and 2) in a cycle, and wherein -the first time period ti is shorter than the second time period t2.

By the term "electric conducting material" as used herein, we mean any material with volumetric resistivitiy from 102 Ohms*m or lower. One example of suited materials is metals, which have volumetric resistivities in the range of 1-15 nanoOhms*m. The invention may also apply non-metallic materials as long as they exhibit volumetric resistivities from 102 Ohms*m or lower. Another example of an electric conducting material' will be a structure consisting of a metal mesh outside an pipe of non-conducting material, where the grids alternatively has been electrically enforced with conducting paints, rubber etc. By the term "dielectric layer" as used herein, we mean a layer of any material with a high resistance to the flow of direct current, and which is capable of being polarized by an electrical field. Suited materials may be, but are not limited to, plastics, epoxy films, paint, etc. The dielectric layer should have a sufficient dielectric strength to form an effective barrier substantially preventing electric currents across the layer. This will ensure that the imposed DC-potential across the dielectric layer will be established over long distances from the electrodes, and thus make the method according to the invention an effective combined anti-corrosion protection and bio-film removing protection for large surfaces.

The invention will function with any thickness of the dielectric layer as long as the dielectric layer forms an effective barrier for currents across the layer. Thus there is no limitation to the thickness, but in practice the thickness of the dielectric layer will be in the range from an order of magnitude I.tm to an order of magnitude I mm, but may also be outside this range. Thus the dielectric strength should be in the order of magnitude 1 MV/rn or higher.

The term "fluid phase" as used herein, we mean any fluid, liquid or gaseous, which may carry electric charges and which may pose a corrosion problem if obtaining contact with the electric conducting material. The electric conductivity of the fluid phase should be at least as high as the electric conductivity of drinking water, that is at least 0.0005 S/rn or higher.

By the term "negative DC-potential" as used herein, we mean an electric potential across the dielectric layer such that the electric conducting object becomes positively charged (anode) and the fluid phase becomes negatively charged (cathode). In this case the current will try to flow from the electric conducting body to the fluid phase. The negative DC-potential is applied to remove bio-film formed on the dielectric layer, and the time period, ti, in which a negative potential is applied will be denoted bio-film removal period in the following description.

By the term "positive DC-potential" as used herein, we mean an electric potential across the dielectric layer such that the electric conducting object becomes negatively charged (cathode) and the fluid phase becomes positively charged (anode). The electric current will seek to flow from the anode to the cathode, which in this case means that current will try to flow from the fluid phase to the electric conducting body. The applied potential should normally be such that the resulting potential is within the corrosion potential of the electric conducting object, but may in shorter periods be outside the corrosion potential to deliberately enrich the fluid with certain metal ions from the electrode. This may i.e. be copper or aluminium ions, to kill microorganism etc. The positive DC-potential is applied to obtain cathodic protection of the electric conducting material The applied potential should normally be such that the resulting potential is within the corrosion potential of the electric conducting object, but may in shorter periods be set higher than the corrosion potential also in order to increase the voltage distribution range and thus to compensate for voltage drops caused by leakage currents of the coating. The positive voltage applied will thus be more restricted to avoiding unwanted electrolyses of the metal behind the coating or direct electrolysis of the metal where the coating has been damaged. The method of cathodic protection is well known to a skilled person and need no further description, and the time period, t2, in which a positive potential is applied will be denoted cathodic protection period in the

following description.

The DC-potentials will be continuously applied at constant voltage in the range from � 0. 1 V to � 2.5 V during operation, but may in short periods be increased outside these regions for shorter periods. In theory, there is no limitation to the strength of the applied potential in these short periods. The electric field arising from the applied DC-potential will be weakened by the electric resistance of the fluid phase and the electric conducting body, and by eventual imperfections in the dielectric layer causing creep currents across the dielectric layer. Thus it may be necessary to employ more than one set of electrodes placed in a distance from each order to "replenish" the DC-potential when applied on large structures.

Examples of suited electrodes in contact with the fluid includes, but are not limited to, electrodes made of one of stainless steel, copper, aluminium, or zinc, or any combinations of these. Multiple electrodes are applied simultaneously and/or where the electrodes are made of different metal types. There may be employed different electrodes which are set to different voltages, or to equal voltages. The applied switching times may be the same or different for the different electrodes.

As mentioned, the bio-film removal is faster than the build-up of the bio-film, thus an effective bio-film removal may be obtained by employing shorter periods with negative applied potential (bio-film removal) than the periods with positive applied potential (cathodic protection). This is advantageous in that the long term corrosion is often the major problem, such that the cathodic protection period should be longer than the bio-fifm removal period; that is ti should be shorter than t2.

Hiowever, there is no restrictions other that ti <t2. Any conceivable combination of time periods as long as ti <t2 may be applied. The length of time periods may be from an order of minutes to an order of days. It is also envisioned employing periods with lengths measured in months or following seasonal variations which can impact on both bio-film growth rate and corrosion exposure levels. Usually t2 will be considerably longer than ti, by a factor of at least two or more.

The method according to the first aspect utilizes a set of important mechanisms for achieving a combined removal of bio-fjlm and a protection of a coated metal surface over a substantial area: i) Electrical charges can be passed over a wide area or long distances when the dielectric layer is a good isolator, such as i.e. two-component epoxy or polyurethane paintings which have an excellent electrical isolation, ii) Algae and other types of bacteria tend to have a negative charge relative to the surrounding water or fluid, iii) Bacteria and other organisms survive and make new cultures only within a small and preferably constant p11-range, iv) protection of steel can partly be done by forming hematite on the steel surface (Fe203 passivation oxide on the surface), and could plug micro-pores or micro-cracks in a dielectric layer when the layer is exposed to a specific voltage drop between the fluid phase and the electric conducting body, v) steel can electrically be corrosion protected by simply connecting the steel to a negative voltage compared to the water voltage potential.

The removal of bio-film is basically achieved by charging the fluid phase negative compared the dielectric layer coated electric conducting body. By this a capacitor is formed, with the fluid phase as one pole, and the electric conducting body as the IS second positive pole separated by the dielectric layer. The way of regarding the structure of i.e. a coated metal plate in contact with a conducting fluid as a capacitor is most likely a new way of thinking' at least when it comes to the painting industry. A strong indicator for this is that two major painting companies did not have any specifications for the specific capacitance value (pF/cm2) of their coating products. Thus the present inventor had to measure this by himself, after having received a painting sample.

As bacteria mostly are negative charged compared to the surrounding water, they will have a tendency to be electrically forced away from the negative charged coating surface. In addition, the bacteria will see a huge step in pH level near the surface, which will inhibit or at least strongly reduce their ability to created new cultures. Also, the reversing of the polarity steadily alters the p11-level along the coated surface and prevents certain bacteria from been adapted' to a certain pH level.

List of figures Figure 1 shows a schematic drawing of an embodiment of the invention applied on a coated pipeline.

Figure 2 shows a diagram over observed head loss at a hydroelectric power plant with and without applied negative voltage to the turbine pipeline.

Figure 3 shows a photograph of a section of the inner coated wall of the turbine pipeline at a hydroelectric power plant after treatment with a negative voltage Embodiments of the invention The invention will be described in further detail by way of examples of embodiments of the invention. These examples should not be considered as a limitation of the general idea of employing an imposed DC-potential on an electric conducting body coated with a dielectric layer in contact with a fluid phase, such that the imposed DC-potential results in a cyclic alternating pattern between periods of cathodic protection of the electric conducting body and periods of bio-film removal from the dielectric layer.

First example embodiment The first example embodiment of the invention is applied on a turbine pipeline of a hydro-electric power plant, as shown in Figure 1.

The figure shows a pipe I in which water 2 is flowing in the direction of the arrow.

A modified manlid 3 mounted on the pipe is electrically isolated from the pipe I by non-conducting materials 5 and being electrically connected to a metal plate 4. The metal plate 4 is made of a corrosion resistant and electric conducting metal, such as high chrome steel etc., and has no dielectric layer such that it obtains electric contact with the water 2 flowing in the pipe 1. The pipe I is a conventional turbine pipe made of steel, and is having a dielectric layer 6 in the form of an anti-corrosion coating made of an epoxy based painting enriched with aluminium micro-leaflets (by the manufacturer). The thickness of the coating is 100-300 micrometer. Thus the pipe I is electrically insulated from the flowing water 2. The water is untreated fresh water.

A DC electric potential generator 7 is connected with one polarity to the pipe I and with the other polarity to the isolated manlid 3. The DC electric generator 7 is regulated by a controller (not shown) which ensures that the imposed DC-potential is constant and is regulated such that during the bio-film removal, ti, the water 2 becomes negatively charged while the pipe I becomes positively charged relatively to each other. And in the cathodic protection period, t2, the controller reverses the polarity of the output from the DC electric potential generator 7 such that the water 2 becomes positively charged relative to the pipe 1. In this embodiment, the imposed DC-potential during bio-film removal should be approximately 1 volts.

For old pipes the bio-film period, ti, should initially be set to some few months to two years depending on the thickness of the bio-film that has been build up during several years.

Special attention must be taken to the aluminium micro-leaflets which has been added to the coating for this special embodiment of the invention. Aluminium leaflets was added to the epoxy coating primarily to made a diffusion proof layer (working as roof tiles) to protect the steel. As the coating wears away over time, the aluminium leaflets will be exposed to the water. Due to a finite isolation value (or resistor value) of the coating, the aluminium will then act as an offer anode, although the currents are very small though the coating. It is thus mandatory that the negative voltage on the water is terminated when the bio-film has been removed and that the voltage polarity is switched to a positive value which both will protect the steel behind the coating as well as the aluminium leaflets from further corrosion (initiate cathode protection).

Second example embodimeni The second example embodiment of the invention is an alternative application on a turbine pipeline of a hydro-electric power plant.

This embodiment employs two electrodes of which one is of stainless steel (working electrode) and the other of copper (offer electrode) which is located upstream compared to the first.

The concept follows the configuration shown in Fig. I with an additional isolated manlid mounted upstream compared to the first one and with some few meters distance. Both electrodes are located at the inlet part of the pipe. For a period of time the water is connected to negative voltage of approximately I volt relative to the pipe by means of the working electrode, in order to remove bio-film as previously depicted Thus the pipe is given a positive voltage relative to the water.

However, the copper electrode, located upstream, is also connected to a positive voltage, relative to the water. This voltage can be equal or even more positive compared to the relative voltage applied on the pipe. Thus an enrichment of the water with copper ions will happen and some of these copper ions will follow the water flow through the pipe. Because the pipe itself is connected to a positive voltage compared to the water inside, the copper ions will tend to stay in the water and ideally in the middle of the water column. However, since the coating is not ideal and the water column is connected to ground at the power plant station, the positive voltage gradient of the pipe will gradually decline to zero or could even go a little negative if the natural corrosion potential of the pipe steel dominates at other end. Thus a gradual deposit of the copper ions will occur along the pipe. These copper ions are toxic to algae or bacteria. When the process has worked for a predetermined time or time interval, the relative potential of the pipe and also the copper electrode is switch to a negative voltage compared to the working electrode being in contact with the water. By this, the enrichment of copper ions is stopped, the copper ions deposited along the pipe wall are even stronger electrically attached to the coated surface and the cathode protection of the steel is initiated.

Verificatioii of the invention The bio-film removal part of this invention has been verified at two hydro-electric power plants in Norway. One of these is the Vrenga power plant as earlier mentioned.

For this installation two electrodes were used, one upstream for the pipe-rupture valve and another downstream for the pipe-rupture valve. The coating was more than 20 years old, and of an early epoxy type enriched with aluminium leaflets according to available information. Pipe-rupture valves have metal pacts electrically in contact with the water and will thus puncture the voltage charge of voltage distribution along the coated pipe. Both electrodes were set to in range 1.1 volts negative (-1.IV) compared to the pipe (or ground) for some years.

An important observation which has direct impact to the inventive method in this application, is that the mechanisms of removing bio-fllm worked faster compared to the natural process of building up the blo-flim. This is shown in Fig. 2, which depicts the head loss of the hydroelectric turbine pipe over time as measured at Vrenga Power plant in Norway. Head loss reduction for this power plant has after resent investigations primarily been connected to bio-film thickness removal. The figure shows both the natural grown in head loss, and the reduction of head loss after a positive voltage had been established between the metal pipe and the water inside over some few of years. The natural development in head loss for this power plant and the initial reduction of head loss by applying the negative DC-voltage to the water, as such, have been known to the public for some years. However, the detailed explanation and mechanisms behind the major head loss reduction achieved at this power plant, has been unknown to people outside the project.

Latest inside inspection of the turbine pipe has discovered that almost all bio-film bad been removed over the last years. In Fig.3, the inside surface of the pipe after removal of the bio-film is shown. Normally the inside should have been covered by a black, slippery and several mm thick bio-film. Instead a clear coating surface with mostly its natural colour is seen. Even the formation structures from the welding process of the pipe can be clearly seen on the picture. The electrode for this installation was made of stainless steel.

The last six months the electrode voltages has been switched off and no increase in head loss or accelerated build-up of bio-film has been observed, For this last period the natural corrosions potential of approximately 0.7 volts between steel and water is most likely dominating the voltage drop across the coating. This voltage represent a small positive charge of the water compared to the metal pipe, but is of course not enough to achieve the desired cathodic protection of the pipe.

At another Norwegian power plant a head loss reduction of in range of 10 0% was achieved after 13 months of applying a negative voltage to the water. This power plant had only one modified manlid working as the electrode, and this electrode was mounted downstream for the pipe rupture valve. The voltage imposed to the water S was -3 Volts, which was considerably lower than for the Vrenga power plant. The turbine pipe had been coated with a two component epoxy based coating 11 years earlier.

Cathodic protection of a metal surface by use of an imposed DC-potential is a conventional and well known technique. There is thus no need for presenting results verifying that the cathodic protection period of the inventive method functions as stated.