Corrosion protection

Abstract: Corrosion protection systems which make use of a barrier (2) which is placed between a corrodible substrate (3) and a counter-electrode (2). The barrier can provide more uniform current distribution on the substrate, and/or enable the counter-electrode to be more easily maintained or replaced, and/or reduce the rate at which the current density on an elongate electrode changes with distance from the power source, and/or provide a controlled environment around the electrode to improve its efficiency.

Description: This invention relates to the corrosion protection of pipes, vessels and other corrodible substrates. This application is a divisional application from European Patent Application No. 87306336.6 (publication no 0253671). The parent application relates to the use of distributed anode protection systems, and the present application relates to the use of discrete anode protection systems. It is well known to protect substrates from corrosion by establishing a corrosion-protecting potential difference between the substrate and the counter-electrode. Preferably a DC power source is used to establish the desired potential difference between the substrate as cathode and an anode which is composed of a material which is resistant to corrosion, e.g. platinum, graphite, or a conductive polymer. Reference may be made for example to US Patent Nos. 3,515,654 (Bordalen), 4,502,929 (Stewart et al), 4,473,450 (Nayak et al), 4,319,854 (Marzocchi), 4,255,241 (Kroon), 4,267,029 (Massarsky), 3,868,313 (Gay), 3,798,142 (Evans), 3,391,072 (Pearson), 3,354,063 (Shutt), 3,022,242 (Anderson), 3,053,314 (Brown) and 1,842,541 (Cumberland), UK Patent Nos. 1,394,292 and 2,046,789A, Japanese Patent Nos. 35293/1973 and 48948/1978, and European Patent Publication No. 01479777. The known corrosion systems suffer from serious disadvantages, in particular a failure to obtain sufficiently uniform current distribution on the substrate. This disadvantage can arise from the use of one or more discrete electrodes; or from the use of a distributed electrode, e.g. a platinum wire, whose radial resistance to the substrate is low, so that at high currents the current density on the anode decreases rapidly as the distance from the power source increases; and/or because the substrate is shielded (including those situations in which the substrate has a complex shape which results in one part of the substrate being shielded by another part of the substrate). The flexible elongate anodes disclosed in US Patents Nos. 4,502,929 and 4,473,450, which comprise a low resistance core surrounded by a conductive polymer coating, are very useful in mitigating this disadvantage, but they cannot be used at the high current densities which are required in certain situations, for example the protection of structures which have no protective coating thereon. Another disadvantage is the relatively short life of anodes (including the electrical connections thereto), especially when exposed to environments which are highly corrosive or which contain oily contaminants (and in the case of platinum anodes, when exposed to fresh potable water), and the difficulty and expense of repairing or replacing the anodes when this becomes necessary. GB 936470 describes a cathodic protection system in which a graphite or iron anode is placed in well or conduit having porous walls which is at least partially filled with water to such a level that the water will permanently or periodically seep away through the porous walls into the soil and cause a certain concentration of salt therein. The reference states that a particularly great effect may be obtained when care is taken that a slight excess of pressure prevails at least periodically in the well or conduit compared to the water pressure within the surrounding soil. US 3616354 similarly describes a cathodic protection system in which anodes are positioned in deep bore holes in the ground, in the presence of an aqueous liquid electrolyte. The borehole is lined, and the liner is perforated at the bottom of the bore hole, directly surrounding the position of the anodes, to permit ion flow therethrough. We have now devised a new cathodic protection system in which a plurality of tubes containing ion-permeable sections can transport electrolyte from a vessel remote from the substrate in which the anode is placed, to the vicinity of the substrate. The present invention provides a method of cathodically protecting an electrically conductive substrate from corrosion which method comprises establishing a potential difference between the substrate as cathode and a discrete anode which is located in an anode chamber which contains an electrolyte and which provides a barrier, having at least one ion-permeable section therein, between the anode and the substrate, the electrolyte being fed into the chamber, and being driven by pressure from the chamber through said at least one ion-permeable section of the barrier, characterised in that (i) the anode chamber comprises: (a) a separate vessel in which the discrete anode is placed, which vessel is positioned remote from the substrate to be protected, and (b) a plurality of tubes in communication with the vessel, which tubes constitute the barrier and contain ion permeable sections, and (ii) the pressure is provided by a hydrostatic pump which, in use, pumps the electrolyte from the remote vessel in which the anode is placed, via the barrier tubes, to the vicinity of the substrate. Advantageously, the invention may provide an improved current distribution on the substrate, enable the anode to be more easily maintained or replaced, and provide a controlled environment around the anode to improve its efficiency, e.g. by reducing contamination or by making it possible to surround the anode with an electrolyte which is different from the electrolyte which contacts the substrate. The barrier preferably comprises a plurality of ion-permeable sections. Preferably ion-permeable sections include simple apertures, for example a hole in the wall of a tube, or an opening at the end of a tube. Ion-permeable sections which are composed of an ion-permeable material, e.g. a glass frit, can also be used, especially when it is desired to have the anode contacted by an electrolyte which is different from that which contacts the substrate. The size and/or the spacing of the ion-permeable sections can be uniform or non-uniform, depending upon the desired current distribution on the substrate. The ion-permeable sections are preferably of fixed dimensions. The distance between adjacent ion-permeable sections is preferably less than 10 times, particularly less than 4 times, the distance between the ion-permeable sections and the substrate. An important factor in determining the size of the apertures can be the need to ensure that anodic reaction products, e.g. gaseous chlorine, do not block the apertures. Unless the conditions of operation are such that anodic reaction products remain dissolved in the electrolyte or can be easily vented, care must be taken to prevent harmful build-up of such reaction products between the anode and the barrier. In some case positive benefit can be derived from such reaction products, e.g. to lessen fouling of marine structures. To assist in the dispersion of such reaction products, the system is operated in such a way that hydrostatic pressure drives the electrolyte through the ion-permeable section(s) towards the substrate. Such hydrostatic pressure, which is provided by a pump, can have the alternative an/or additional advantages of (1) reducing the danger that the ion-permeable sections will be blocked by contaminants present in the electrolyte between the barrier and the substrate, for example oily contaminants in the water layer at the bottom of an oil storage tank, and/or (2) making it possible, when it is desired to surround the anode with and electrolyte which is different from the electrolyte which contacts the substrate (e.g. when protecting a potable water tank with a platinum anode), to prevent substantial contamination of the anode electrolyte by the substrate electrolyte with minimal contamination of the substrate electrolyte by the anode electrolyte. The barrier must not be electronically connected to the substrate or the anode, and is preferably composed of (including coated by) an electrically insulating material, e.g. a plastic. The tubular barrier may be of round or other cross section. In one embodiment, a plurality of tubes which are joined together to form a branched structure may be used. In such a branched structure, the branch tubes are preferably of smaller cross than the main tube, for example so that the total cross-sectional area of the branch tubes is no greater than the cross-sectional area of the main tube. The tube or tubes can be heated by an internal or external heater to reduce the viscosity of the electrolyte therein (including to prevent it from freezing) and/or to reduce its resistivity. The tube or tubes can be arranged as a continuous loop, so that electrolyte circulates through them, or can simply terminate in an open end (i.e.. an ion-permeable section) or a closed end. According to the invention, the discrete anode is placed in a vessel remote from the substrate, and electrolyte is pumped from the anode vessel to the vicinity of the substrate via the tubes which constitute the barrier and which contain (including terminate in) ion-permeable sections. In such a system, it is important that the resistance of the electrolyte in the tubes should not be too high. Therefore, the resistivity of the electrolyte is preferably less than 50 ohm.cm, particularly less than 20 ohm.cm, so that the tubes can be of a convenient size. In this embodiment, the main tube or tubes conveying electrolyte from the anode chamber to the vicinity of the substrate may for example have an equivalent inner diameter (i.e. of cross-section equal to a circle of that diameter) of 1 to 12 inches (2.5 to 30.5cm), and the branch tubes may for example have an equivalent diameter of 0.5 to 3 inches (1.25 to 7.5cm). Any appropriate DC power source can be used in the present invention. The voltage of the power source is preferably less than 100 volts, particularly less than 50 volts, with the system being designed with this preference in mind. When there is a net transfer of electrolyte through the ion-permeable section(s) of the barrier, electrolyte must be supplied to the anode, and this can be done by recycling electrolyte from the vicinity of the substrate and/or by supplying fresh electrolyte. When build-up of electrolyte in the vicinity of the substrate must be avoided, e.g. in the bottom of an oil storage tank, means must be provided for removing excess electrolyte; the excess electrolyte can be recycled to the anode, if desired or if necessary after filtering or otherwise treating it to remove harmful contaminants. Preferred uses for the present invention include the protection of city water tanks, ballast tanks in ships, oil rigs, cooling tanks for power stations, water tanks for secondary recovery in oil wells, soil storage tanks, heat exchangers, condensers, heater treaters, and buried pipes, in particular pipes buried below the permafrost line, for example oil pipes in frozen tundra. Referring now to the drawing, Figure 1 shows a DC power source 1 which is connected to an anode 2 and a corrodible substrate 3 which is a cathode. Anode 3 and substrate 3 are separated by a barrier 4 which comprises ion-permeable sections 45, and are connected by electrolyte 5 through sections 45. A positive hydrostatic pressure is maintained from the interior of the barrier 4 across the ion-permeable sections 45 by means of pump 6. Figure 1 is a diagrammatic side view in which the substrate 3 is an oil storage tank in which the electrolyte 5 is a highly corrosive aqueous mixture covered by oil 8. The anode is a discrete anode which lies within an anode chamber 21. The barrier 4 comprises, in addition to the part of the anode chamber which lies between the anode and the tank 3, a tube 41 which lies between the anode and the tank 3, a tube 41 which leads from anode chamber 21 to the centre of tank 3 and branch-tubes 42 which communicate with tube 41, which are of relatively small diameter, and which contain perforations 45. Means not shown removes excess electrolyte from the tank 5. Pump 6 maintains a positive pressure across the perforations 45 and thus reduces the danger that they will become blocked by oily contaminants in the water layer. Figure 2 shows in diagrammatic side view a system for protecting a pipe 3 which is buried in the earth or immersed in the sea or other electrolyte. The anode 2 lies within an anode chamber 21 and is surrounded by electrolyte 30. Barrier 4 comprises a tube 41 which extends downwards from the anode chamber 21 and branch tubes 42 which communicate with and extend horizontally from the tube 41 under the pipe 3, and which comprise nozzles 45 covered by protective caps. Tube 41 contains a heater 9 which may be used to prevent the electrolyte from freezing or reduce its viscosity, for example when the tube 41 passes through a layer of earth which is frozen or liable to freezing, and/or to decrease its resistivity. A positive hydrostatic pressure is maintained across the nozzles 45, and the electrolyte lost in consequence is replaced from electrolyte storage tank 23. Figure 3 shows a tube with perforations therein through which ion-containing electrolyte can emerge; the perforations shown are uniformly spaced and of uniform size, but they could be of different sizes and separations in order to provide desired current distribution. Figure 4 shows a tube composed of an ion-conducting membrane through which ions can pass, but non-ionic material cannot. Figure 5 shows a perforated tube which is covered by an ion-conducting membrane. Figure 6 shows a part of a perforated tube in which each perforation is covered by an ion-conducting membrane. Figure 7 shows an open-ended tube through the open end of which ion-containing electrolyte can emerge. Figure 8 shows an open-ended tube whose open end is covered by a porous plug. Figure 9 shows a tube having a plurality of branch nozzles mounted thereon. Figure 10 shows the arrangement of the tube in Example 1, as described below. The invention is illustrated in the following Example. EXAMPLE 1 In this Example, procedures (A) and (B) are comparative Examples, and procedure (C) is an example of the invention. (A) An 18 x 24 inch (45.7 x 61.0 cm) stainless steel mesh screen was placed on the bottom of a tank. One end of each of six flexible plastic tubes 0.375 inch (0.95 cm) inner diameter x 6 foot (182.9 cm) long was positioned about 1 inch (2.5 cm) from the screen; the ends or the tubes were placed in a rectangular pattern centered over the screen as illustrated in Figure 12, with x being 4.5 inch (11.5 cm) and y being 4 inch (10.2 cm). The other end of each tube was placed in a second tank adjacent the first. The tubes and both tanks were filled with 3% NaCl solution having a resistivity of about 20 ohm.cm. A saturated calomel electrode (SCE) was placed in the first tank in a number of different positions so that the potential of different parts of the screen could be measured. The corrosion potential of the screen was measured to be 0.220V, and was uniform across the screen surface. (B) The apparatus described in (A) was modified by placing a single graphite anode 1 inch (2.5 cm) above in the center of the screen. The anode and the screen were connected to a DC power source of sufficient voltage to maintain a total current of 0.05A. The absolute potential of the screen (i.e. the potential measured by the SCE minus the corrosion potential) was found to be at a maximum of 0.560V. The absolute potential decreased in a radial pattern away from the anode, reaching 0.499V at the edge of the screen, a total difference of 0.061V. (c) The apparatus described in (A) was modified by placing a single graphite anode in the second tank. The anode and the screen were connected to a DC power source, and with the tubes acting as salt bridges between the tanks, sufficient voltage (about 45 VDC) was applied to maintain a total current of 0.05VA. The absolute potential of the screen was found to be at a maximum of 0.550-0.563V directly below each of the tube openings and at a minimum of 0.540V at the edges of the screen, i.e. a difference of at most 0.023V as compared to a difference of at most 0.061V in (B) above.

Claims: A method of cathodically protecting an electrically conductive substrate from corrosion which method comprises establishing a potential difference between the substrate as cathode and a discrete anode which is located in an anode chamber which contains an electrolyte and which provides a barrier, having at least one ion-permeable section therein, between the anode and the substrate, the electrolyte being fed into the chamber, and being driven by pressure from the chamber through said at least one ion-permeable section of the barrier, characterised in that (i) the anode chamber comprises: (a) a separate vessel in which the discrete anode is placed, which vessel is positioned remote from the substrate to be protected, and (b) a plurality of tubes in communication with the vessel, which tubes constitute the barrier and contain ion permeable sections, and (ii) the pressure is provided by a hydrostatic pump which, in use, pumps the electrolyte from the remote vessel in which the anode is placed, via the barrier tubes, to the vicinity of the substrate. A method according to Claim 1, wherein each tube has aperatures in its walls. A method according to Claim 1 or 2, wherein the barrier comprises a first tube extending from the anode vessel and further tubes, which communicate with the said first tube, and which also have aperatures therein. A method according to any preceding claim, wherein the substrate is buried in soil, the electrolyte is driven into the soil, and the anode is located in an anode chamber which is accessible from above ground, so that the anode can be easily maintained or replaced. A method according to Claim 4, wherein the substrate is buried below a permafrost line in the soil and the ion-permeable section is below the permafrost line. A method according to any preceding claim, wherein the substrate is contacted by a mass of liquid electrolyte, for example the sea, and the anode is located in an anode chamber which is accessible separately from the mass of electrolyte so that the anode can be easily maintained or replaced. A method according to any preceding claim, also comprising removing excess electrolyte from the vincinity of the substrate, and optionally recycling it to the anode chamber.

Inventors: HIGHE ALBERT, MASIA MICHAEL, REED JAMES PATRICK, HIGHE, ALBERT, MASIA, MICHAEL, REED, JAMES PATRICK
Applicants: RAYCHEM CORP, RAYCHEM CORPORATION
Application year: 1987
Main classification: C23F13
IPC codes: C23F13