EP0029494A1 - Vessel protected against internal corrosion - Google Patents

Abstract

1. Container, protected from internal corrosion with the aid od anodes, supplied by parasitic current, characterized in that between a first line a, which corresponds to the perpendicular from the anode (A) to the container wall (B), and a second line b, measured between the anode (A) and any point (P) on the container that is situated at equally large straight-line distances from two adjacent anodes (A), the relationship : 1.15a=<b=<1.5a. holds good.

Description

The invention relates to a container protected against internal corrosion with the aid of anodes supplied with external current.

A large number of facilities in industry, commerce and the home are exposed to corrosion by aqueous electrolytes. The ongoing removal of the damage caused by this is often associated with considerable costs. The task of corrosion protection technology is to prevent such damage from the outset or to limit it to a technically and economically justifiable minimum dimension.

The electrochemical corrosion of metals and the protective measures used to prevent them take place at the interface between the solid phase of a metal and the liquid phase of an electrolyte. If a current flows through both a metallic and an electrolytic conductor, then free electrons are created or consumed at the metal / electrolyte phase boundary. Since the emission of electrons means oxidation, the absorption means reduction, the passage of a current across the phase boundary is always associated with a chemical reaction. This process is known as electrolysis if the reactions are caused by an external voltage. If they run voluntarily on spatially separated electrodes (due to an internal EMF), one speaks of a galvanic element. If they occur on a metal surface at the same time, it is corrosion.

The metals tend to go into solution. Positively charged ions diffuse out of the atomic lattice of the metals, while the negatively charged electrons remain in the lattice structure. The metal charges negatively. This leads to a potential difference compared to the electrolyte. The metal dissolves on the less noble metal surfaces (anodes); the attacking electrolyte takes up electrons on the nobler metal surfaces (cathodes).

The anodic and cathodic partial reactions can take place in the same zone of the metal surface. In practical corrosion events, however, it is more common for anodic and cathodic areas to be distributed in a geometrically separated manner on the metal surface. Local or corrosion elements with corresponding local anodes are formed which have a different potential than the electrolyte. This leads to material removal on the base electrode.

With passive corrosion protection, the surface to be protected is separated from the attacking electrolyte by coating, with active corrosion protection, one intervenes directly in the corrosion processes. All kinds of coatings are used as passive corrosion protection: paint and plastic coatings with layer thicknesses of a few to up to a few millimeters, metallic coatings, glass enamel, cement mortar and the like electrochemical way through anodic or cathodic protection, with the latter to be distinguished between protection methods with galvanic (sacrificial) anodes and those with external current.

For the internal protection of containers, cathodic protection with electrodes supplied with external current and insulated in the container has proven itself for a number of years. The electrodes are connected to the positive pole of a direct current source, the container to the negative pole. The electrons used in the cathodic partial reactions are supplemented by the external power supply, whereby the anodic partial reactions, ie the metal dissolution of the container walls and internals, are reduced. Corrosion is practically completely suppressed when the current flowing from the external current source via the (anodically connected) electrode built into the container into the container walls or internals corresponds in size to at least the cathodic partial current. This current strength is therefore referred to as the minimum operating current strength of the cathodic corrosion protection I _k . In the practice of cathodic corrosion protection, less the absolute value of the minimum operating current I _k , rather the so-called minimum operating current density of the cathodization i _{k, is used} as the operating variable, which _measures the quotient of I _k3 measured in mA and the container surface F including its internals, measured in m ² , represents.

The quantity i _k depends on a whole series of variables, for example on the concentration of dissolved oxygen, the concentration of any oxidizing agents that may be present, the electrolytic conductivity, the temperature, the pH value, the flow rate and the like. It accordingly indicates considerable spectrum, but also has a specific value for a specific case. This can be determined in the laboratory and determined in practice by examining the opened container at certain time intervals with any accuracy.

Instructions for the distribution of the anode material within the container can already be found in German patent specification 2 144 514. Information about the average distances between the anodes and the tank wall as a function of the electrolytic conductivities of the water is already included there.

The object of the present invention is to use the smallest number of anodes in containers which are cathodically protected by electrodes supplied with external current and with optimum current distribution.

The solution to this problem according to the invention consists in that between a first distance a, which corresponds to the perpendicular from the anode to the container wall, and a second distance b, measured between the anode and each container point, which has linear spacings of equal size to two adjacent anodes, the relationship b = (l.15 ... l, 5). a exists.

With the anode arrangement or configuration according to the invention, an optimal protective current distribution is ensured with a minimal number of anodes, both in containers with a circular or oval cross section and in containers with a rectangular cross section. This optimal current distribution is particularly important in systems with externally powered inert anodes, since inert anodes, especially platinum-coated titanium, niobium and tantalum anodes, may only be subjected to certain, maximum DC voltages. The maximum operating voltage for platinum-coated titanium anodes is 12 volts. Additional voltages lead to the rapid destruction of the electrode material due to the breakthrough potential being exceeded (cf. Journal "Galvanotechnik", 59 (1968), No. 8, pp.659-666, esp. P.660).

For containers filled with aqueous electrolytes whose electrolytic conductivity is less than 150 pS / cm, the distance of the anodes from the named container point may be up to 50% greater than the vertical distance of the anodes from the container wall. For containers with liquids whose electrolytic conductivity is greater than or equal to 150 pS / cm, extensive practical and theoretical studies have shown that this distance should be between 15 and 30%, preferably 20%, larger than the vertical distance of the anode from the container wall .

In principle, however, vertical anode distances from the container wall of 700 mm for conductivities greater than or equal to 150 µS / cm and 400 mm for smaller conductivities should not be exceeded.

In the case of containers with a total height of 8 m and more and corresponding anode lengths, the voltage drop within the anodes, in particular in the case of platinized titanium anodes, leads to an uneven current distribution. Since limits are set for an increase in the operating voltage for the reasons stated above, according to a development of the subject matter of the invention, multiple current leads to the anodes become in the longitudinal direction of the anode intended. For this purpose, appropriately designed current feedthroughs are provided in the upper and lower carcasses. At the same time, means are provided on the container wall for holding and / or bracing the anode rods or wires.

To avoid excessive current suction at the current feedthroughs, the anodes are provided with a high-resistance layer in a range of 200 - 300 mm, measured from the current feedthrough. If platinum-plated titanium anodes are used, these areas are not platinized.

In the past, containers that contain not only the aqueous electrolyte but also other substances or materials that are more conductive than the electrolyte, in particular filter containers with gravel layers of different grain sizes and one or more activated carbon layers, have been limited to only carrying out passive corrosion protection measures or, if they do operating conditions allow corrosion inhibitors to be added to the aqueous electrolyte. The containers were either made of a corrosion-resistant material, e.g. stainless steel, concrete, or the inside of the containers were provided with protective plastic coatings. The procedures have, however proven to be less economical and partially ineffective: high investment costs for stainless steel containers; Periodic emptying of the containers and renewal of the protective plastic coatings, which have defects due to water vapor diffusion and which in turn lead to pitting in the container walls. Due to the anode configuration or arrangement according to the invention in connection with anodes which have longitudinal sections of different electrical surface conductivity, the electrical surface resistance of a longitudinal section being higher, the greater the electrical conductivity of the container content assigned to this anode longitudinal section, for the first time it is also possible to use containers of the specified type Protect cathodically using the external current method.

While containers with an inner diameter of less than or equal to 1000 mm make do with a single central anode which extends essentially over the entire container height or length, larger containers have a plurality of anodes which are distributed in the container and are electrically and optionally mechanically connected to one another to provide, which also extend substantially over the entire container height or length.

According to a development of the subject matter of the invention, the anode longitudinal sections of different electrical surface conductivity are formed by section-wise coating otherwise good-conducting anodes with resistance material and / or insulating material. Another possibility of creating differently electrically conductive anode sections is to provide the anode base material with sections with electrically well-conductive coatings or layers. In the case of titanium anodes in particular, this leads to extremely favorable technical and economic results if, in accordance with a preferred embodiment of the invention, the anode base material titanium is coated in sections with a 2.5 to 10 μm thick platinum layer. The anode sections spared from the platinum plating are coated with a thin oxide film with a comparatively high surface resistance immediately after the anodes are put into operation. In the case of filter containers with activated carbon layers, the conductivity of which far exceeds that of the remaining container contents, a current leakage from the passive sections of the anodes is thus reliably prevented.

It has also proven to be particularly advantageous to use these in the case of filter containers with activated carbon layers Layers associated with the anode longitudinal sections to be made 100 to 200 mm longer than the layer thickness of the activated carbon, the anode longitudinal sections projecting beyond the activated carbon layer on both sides. This bilateral overlap of the anode longitudinal sections ensures perfect operation even during and after the necessary backwashing of the filters, in which the filter material is whirled up to a greater or lesser extent. In particular when using platinum-coated titanium anodes with platinum-free anode longitudinal sections covered with titanium oxide, no damage to the platinum-coated and non-platinum-coated anodes was found during or after backwashing. It has also been shown that such anodes can be rammed into containers already filled with filter layers (gravel / activated carbon) without being damaged. Here, the ends of the anodes can be sharpened to a pile.

The invention is explained in more detail below with reference to the drawing, in which exemplary embodiments of the subject matter of the invention are shown in simplified form.

• Fig.l einen Horizontalschnitt durch einen Behälter mit rechteckförmigem Querschnitt,
• Fig.2 einen Horizontalschnitt durch einen Behälter mit kreisrundem Querschnitt,
• Fig.3 einen Längsschnitt durch einen stehenden Behälter
• Fig.4 .eine beispielsweise Ausführungsform einer Halte-und/oder Verspanneinrichtung für Drahtanoden
• Fig.5 eine beispielsweise Ausführungsform einer Haltevorrichtung für Stabanoden
• Fig.6 einen Längsschnitt durch einen Tragbolzen einer Stromdurchführung
• Fig.7 einen Längsschnitt durch einen kathodisch geschützten Filterbehälter
• Fig.8 einen Längsschnitt durch eine platinierte Titananode.

In the drawing shows

• 1 shows a horizontal section through a container with a rectangular cross section,
• 2 shows a horizontal section through a container with a circular cross section,
• 3 shows a longitudinal section through a standing container
• 4 shows an example of an embodiment of a holding and / or bracing device for wire anodes
• 5 shows an exemplary embodiment of a holding device for rod anodes
• 6 shows a longitudinal section through a support bolt of a current feedthrough
• 7 shows a longitudinal section through a cathodically protected filter container
• 8 shows a longitudinal section through a platinized titanium anode.

In Fig.l, a plurality of rod-shaped and wire-shaped platinum-coated titanium anodes A, uniformly distributed over the circumference, are installed insulated from the container walls in a container B with a rectangular cross section. All anodes are connected to a direct current source (not shown). The vertical distance the anode A from the container wall is labeled a. P denotes tank points which have the same spacing b from the two immediately adjacent anodes A. To achieve an optimal current distribution for the cathodic protection of the container walls with a minimum number of anodes, the following relationship applies between the two sections a and b:

These two inequalities result in anode distances d between 1.14 a and 2.24 a.

In the case of containers with a circular cross section, as shown schematically in FIG. 2 and where the same parts are provided with the same reference symbols, the same relationship applies between the distance a of the anodes from the container wall and the distance b of the container points P from the adjacent anodes A. . Here, too, the corresponding anode distances d can be determined, which can be done most simply by drawing / geometrically.

For liquids with electrolytic conductivities less than 150 pS / cm, the range b = 1.15a ... 1.5a defined above can be fully used. Assign the liquids in the tank electroly table conductivities greater than or equal to 150 pS / cm, the distance b should be between 15 and 30%, preferably 20% longer than the anode distance a. In general - and this applies to all types of containers and conductivities - the distance between the anode and the container wall should not exceed 700 mm.

In the standing container shown in FIG. 3, wire-shaped, platinized titanium anodes A with an anode configuration according to FIG. 2 are installed. Additional anodes A 'at the upper and lower end of the container serve to cathodically protect the upper and lower cells C ₁ and C ₂ . With container heights greater than 8 m and the corresponding anode lengths, the voltage drop in the longitudinal direction of the anode can assume such large values with conventional anode construction (core made of titanium, 2.5 to 10 pm thick platinum coating) that in the container part facing away from the anode feed side the required anode potential for a complete cathodic Protection is no longer sufficient. With such containers, the protective current is supplied to the anodes at a plurality of spatially separated locations. This is shown schematically in Figure 3. Current feedthroughs D are provided in the upper and lower carcasses of the container, all of which are connected in parallel and with the positive pole of the direct current source G are connected. The negative pole of the direct current source G is connected to the container B. The anodes A are subdivided in the central area of the container and held or clamped there by a combined holding and bracing device V. As can be seen from FIGS. 3 and 4, a radially inward-pointing carrying iron T is fastened to the container wall B at the level of the separation point. A sleeve M is attached to the free end of the carrying iron T. A plastic plug St is inserted into the sleeve M. This has a continuous threaded bore running in the longitudinal direction of the anode. If wire-shaped anodes are used, as shown in FIG. 4, a titanium clamping bolt Sp provided with a recessed thread is screwed into this threaded bore, which is provided with a horizontal-axis bore at its free end. The length of the external thread is less than half the length of the sleeve or plug. With anodes screwed in on both sides, there is therefore no electrical connection between the anode parts. During the assembly of the anodes, the anode end that was bent at right angles is inserted into the hole mentioned and secured with a grub screw Ms.

One or more anode spacing elements V 'are furthermore arranged between the current feedthroughs D and the holding and tensioning devices V described above arranges. These consist of carrier iron T '(FIG. 3) also attached to the container walls, the free ends of which are provided with an insulating sleeve I, through the bore of which extends in the longitudinal direction of the anode, the wire anodes A are drawn.

In the case of rod anodes, the holding device arranged in the center of the container differs from the one described above essentially in the design of the titanium clamping bolt. As can be seen from Fig. 5, the clamping bolt Sp ^{l has} an external thread at the socket-side end, while the other end is provided with a blind hole. Anodes with external threads are screwed into this threaded hole and secured with a radially acting grub screw Ms. Even when installing rod anodes additional distancing elements may be used for the anodes between the Stromdurchführun ^g s D and holding devices, which consist essentially of a supporting iron and insulating sleeve for receiving the anode.

The current feedthroughs D are constructed in a similar manner to that described and shown in FIG. 2 of the German utility model 1998 364. The support bolt designated there with the reference number 2 is however replaced by a titanium support bolt according to the present Fig. 6. The bolt Bt is provided with a collar Bb in the middle section. The left part of the bolt pointing towards the inside of the container has an external thread onto which an internally threaded porcelain cap Bk is screwed. The porcelain cap has an axially running bore, the diameter of which is slightly larger than the outside diameter of the titanium clamping bolts Sp and Sp 'according to FIGS. 4 and 5, respectively to which the wire or rod anodes are attached. Rod anodes can be screwed directly into the blind hole. With the help of the other bolt part, which is partially provided with an external thread, the current lead-through is attached to the container wall or the insulation, as described in the aforementioned utility model. In a departure from this known current feedthrough, however, the cable connection is made with the aid of a threaded blind hole at the outward-pointing bolt end, which is additionally provided with a flattened area Bf for holding back.

The holding or tensioning devices for wire and Rod anodes and the associated current feedthroughs are simple in construction and optimally do justice to the conditions when installing the cathodic protection system at the manufacturing or installation site.

In FIG. 7, an embodiment of a filter container that is cathodically protected with the aid of external current supply is shown in simplified form. For reasons of clarity, only a single, centrally arranged platinum-plated titanium anode A is provided in the filter container, which is quite sufficient for filter containers with an inner diameter of up to 1000 mm. For containers with larger diameters - in practice these can be 3 m and more - a central anode is not sufficient. Anode configurations according to Fig.l or Fig.2 are then to be provided in such filter containers.

Filter gravel layers S ₁ , S _{2 of} different grain sizes are located on a nozzle base R in filter container B. An activated carbon layer S _{3 is} arranged above the layer S ₂ . The liquid to be filtered is fed in at the nozzle W ₁ in the upper granule C ₁ and discharged at the nozzle W ₂ in the lower granule C ₂ . The centrally arranged anode A is fastened in an insulated manner from the container walls by means of brackets H. Your power is supplied on upper end, which is symbolized by the arrow I _k drawn there. As can be seen in FIG. 8, the anode A consists of a titanium core K which is provided with a platinum coating P of 2.5 to 10 μm in length in sections E, G. In a longitudinal section F, namely where the anode penetrates the activated carbon layer S ₃ in the installed state, the platinum plating is left out. In the installed state and practically immediately after the cathodic protection has been put into operation, a titanium oxide layer Q which is stable against chemical and physical attacks is formed in the longitudinal section F, the surface resistance of which practically completely prevents the current from escaping in this area. With usual layer thicknesses of the activated carbon of approx. 200 mm, it is advantageous to make the length of the region of the anode which is left free from the platinum plating 100 to 200 mm longer than the thickness of the activated carbon layer.

The anode A is e.g. pointed at the lower end. In this way, it can be rammed into filter containers already loaded with filter material without any special effort.

As already explained above, anode configurations are available for filter containers with inner diameters of over 1000 mm tion with several anodes distributed in the container. The dimensioning rules shown at the beginning with reference to FIGS. 1 and 2 apply in an analogous manner to these embodiments. Furthermore, separate power supply lines, as have been explained in connection with FIG. 3, are required for filter containers with total heights of more than 8 m.

In the case of filter containers with several layers of activated carbon, the above-described platinings must of course be omitted at the corresponding locations on the anodes.

Claims (26)

1. Containers protected against internal corrosion by means of externally powered anodes, characterized in that between a first distance a, which corresponds to the vertical from the anode (A) to the container wall (B), and a second distance b, measured between the anode (A) and each container point (P), which has the same linear spacing to two neighboring anodes (A), the relationship b = (1.15 ... 1.5) .a exists. 2. Container according to claim 1, characterized in that it is filled with liquids whose electrolytic conductivity is less than 150 pS / cm. 3. Container according to claim 1, characterized in that between the first distance a and the second distance b, the relationship b = (1.15 ... 1.3) .a, preferably b = 1.2.a, exists. 4. Container according to claim 1 and 3, characterized in that it is filled with liquids whose electrolytic conductivity is equal to or greater than 150 µS / cm. 5. Container according to claims 1 to 4, characterized in that the first distance a with conductivities greater than or equal to 150 pS / cm is a maximum of 700 mm, otherwise 400 mm. 6. Container according to claim 1, characterized in that the current supply to the anodes (A) with the aid of in the container wall (B) or the pummeling (C ₁ , C ₂ ) built-in current feedthroughs (D), wherein the anodes (A ) are covered with a high-resistance layer in a range from 200 to 300 mm, measured from the current feedthrough (D). 7. A container according to claim 6, characterized in that when using platinized titanium anodes in said area, the anodes (A) are free of platinization. 8. Container according to one or more of the preceding claims 1 to 7, characterized in that a current feedthrough (D) is provided at least at one container end. 9. A container according to claim 8, characterized in that the current lead-through is secured in an insulated manner in the container wall or container body, preferably made of titanium supporting bolts (Bt), on the end of which facing the inside of the container for wire anodes, a clamping bolt (Sp) also preferably made of titanium is fastened, to which in turn the anodes (A) are attached, while rod anodes are fastened directly in the supporting bolts . 10. Container according to one or more of the preceding claims 1 to 9, characterized in that the anodes (A) are additionally held in the container (B) by holding and / or tensioning devices (V) 'and / or anode spacing elements (V') . 11. A container according to claim 10, characterized in that the holding and / or bracing device comprises a support arm (T) fastened to the container wall, at the free end of which a preferably made of titanium clamping bolt (Sp) is fastened insulated, to which in turn the Anode (A) is attached. 12. A container according to claim 11, characterized in that said support arm end carries a sleeve (M), a plug (St) in the bore of the sleeve Insulating material is used and the plug is provided with a bore running in the longitudinal direction of the anode for receiving the clamping bolt (Sp). 13. Container according to one of claims 9 to 12, characterized in that means (Ms) for securing the anode (A) are provided at the anode-side end of the clamping bolt (Sp). 14. Container according to claim 10, characterized in that the anode spacing elements (V ') comprise a support arm (T') fastened to the container wall, the free end of which carries an insulating sleeve (I), through the bore of which extends in the longitudinal direction of the anode, the anode (A) is carried out. 15. Container according to one or more of the preceding claims 8 to 14, characterized in that in the longitudinal direction of the anodes (A) a plurality of current feedthroughs (D) are provided for containers whose anode lengths exceed 7500 mm. 16. A container according to claim 15, characterized in that the current feedthroughs are arranged in the container body (C ₁ , C ₂ ), the anodes on the Holding and / or tensioning devices (V) are electrically isolated from one another and the protective current (I _k ) is fed separately to the partial anodes formed in this way. 17. A container according to claim 16, characterized in that the partial anodes are electrically connected in parallel with respect to the direct current source (G). 18. Container according to one or more of the preceding claims 1 to 17, characterized in that in the case of containers with different conductivity layers of the container contents, the anodes (A) have longitudinal sections (E, F, G) of different surface conductivity, the electrical surface resistance of a longitudinal section being all the higher is, the greater the electrical conductivity of the container contents assigned to this container section (S ₁ , S ₂ , S ₃ ). 19. A container according to claim 18, characterized in that in the case of containers with an inner diameter of less than or equal to 1000 mm, a central anode (A) extending essentially over the entire container height or container length is provided (FIG. 7). 20. A container according to claim 18, characterized in that a plurality of anodes (A) distributed in the container are provided for containers with an inner diameter greater than 1000 mm, which extend essentially over the entire container height or container length. 21. Container according to one of claims 18 to 20, characterized in that the anode longitudinal sections (E, F, G) of different electrical surface conductivity are formed by section-wise coating of otherwise highly conductive anodes (A) with resistance material and / or insulating material. 22. Container according to one of claims 18 to 20, characterized in that the anode longitudinal sections (E, F, G) of different electrical surface conductivity are formed by section-wise chemical and / or electrochemical treatment of the respective anode sections. 23. A container according to claim 22, characterized in that the anodes (A) consist essentially of titanium, niobium or tantalum and in sections with a 2.5 to 10 µm thick platinum layer (Pt) are provided. 24. Container according to one of the preceding claims 18 to 23, characterized in that in the case of filter containers which contain an activated carbon layer (S ₃ ), the anode longitudinal section (F) associated with this layer has a layer (Q) of high electrical surface resistance. 25. A container according to claim 24, characterized in that the anode longitudinal section (F) associated with the activated carbon layer (S ₃ ) is 100 to 200 mm longer than the layer thickness of the activated carbon and projects beyond it on both sides. 26. A container according to one or more of the preceding claims 18 to 25, characterized in that when using rod anodes, these are constructed in a pile-like manner and are rammed into the filter layers (S ₁ , S, ₂ , S ₃ ) from the loading side of the container.

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