AT508512A1 - PROCESS FOR REMOVING CORROSIVE ANIONS - Google Patents

Description

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The invention relates to a method for removing corrosive anions from the porous solid pore solutions, such as e.g. of concrete or brick walls, using zinc to bind the anions.

Corrosion of steel reinforcement is one of the most important causes of increased maintenance and repair costs and subsequently for shortening the useful life of reinforced concrete structures. Corrosion of the steel reinforcement is caused by the penetration of chlorides into the concrete cover and / or by carbonation of the concrete cover. In the remediation of chloride loaded reinforced concrete, there are essentially three methods used in the prior art: (1) mechanical removal of the chloride contaminated concrete beyond the reinforcement; Coating the reinforcement with a corrosion protection, usually epoxy resin; Reprofiling with reprofiling mortar or shotcrete; (2) electrochemical chloride removal by migration of chlorides from the concrete to the concrete surface in a strong electric field generated by a current flow of about 1 to 5 A / m2 between an external anode and the reinforcing steel; (3) cathodic or galvanic corrosion protection of the steel reinforcement by current flow (usually 1 to 5 mA / m2) between an external anode, which is usually mounted on the concrete surface, and the reinforcing steel.

Method (1) involves high material and labor, high waste and heavy traffic due to the arrival and removal of the material and thus high direct and indirect costs. Method (2), e.g. in DE 422 90 72 A1, only during a relatively short period, e.g. from 1 to 3 months, and only chlorides between the external anode and the steel reinforcement are removed. Chlorides, which are located behind or next to the steel reinforcement, can diffuse again to the steel reinforcement after the conclusion of the chloride withdrawal and cause again corrosion. In addition, this method can not be applied to prestressed components. Method (3), e.g. described in AT 410.440 B and AT 413.822, although an effective method, it must be maintained over the entire duration of use -1 - Μ · · · I «Μ ················· ··· ···· • · tf · · ft · ··· «• * * fl * · f | a component are used to effectively prevent chloride corrosion of the steel reinforcement.

All three methods are based on different amounts of chloride removal: In process (1), the chloride is completely removed with the concrete, in process (2) a large portion of the chloride is removed from the concrete by the migration of chloride ions in a strong electric field. however, the portion of the chloride that is outside the electric field remains in the concrete. The chloride removal in process (3) occurs in the microscopic range (about 20 to 100 pm) of the steel reinforcement in the concrete. However, depending on the type of anode, migration is due to capillary chloride transport in the pore structure due to wet / dry cycles, e.g. during rainy and dry periods, superimposed. Therefore, these procedures must be applied over the entire service life of the concrete components.

Another problem with concrete, but especially in brick buildings and masonry is the nitrate content, which in addition u.a. This leads to efflorescence on the surface, which is why in addition to chloride and other corrosive anions concrete removal of nitrate is desirable.

The aim of the invention was therefore to provide a method by which such corrosive anions reliably and permanently from the pore solutions of the respective components or walls, i. generally that of porous solids can be removed.

DISCLOSURE OF THE INVENTION

According to the present invention, this object is achieved by methods for removing corrosive anions from the porous solid pore solutions, e.g. Concrete or brick walls and components achieved, which is characterized in that a zinc-containing aqueous-based coating is applied to the porous solid or the solid is impregnated with the coating, after which the corrosive anions in the pore solution or may be allowed to migrate into the coating and / or caused to migrate to or into the zinc-containing coating ".

• · · · · ··· ·· I I • I · · · · · · · · · ·

In order to enter there with the zinc, a water-insoluble or sparingly soluble compound. Such compounds are understood herein to mean, in particular, both true zinc salts and complex compounds of zinc which are insoluble or sparingly soluble in water.

In this way, according to the present invention, the corrosive anions are precipitated from the pore solution of the porous solid and permanently removed from the latter by this immobilization. Even with recurring dry / wet cycles, as is the case with weathered components and masonry, the rule is that in the zinc compound, e.g. as zinc salt or complex, bound anion not in solution.

The corrosive anions are, in particular, chloride, nitrate or mixtures thereof which, especially in the case of masonry or walls and components of reinforced concrete, have harmful effects, e.g. on pipes and pipes running in them, but especially on the steel reinforcement of reinforced concrete. The invention therefore relates in particular to the corrosion protection of reinforced concrete.

The zinc-containing coating is preferably electrically conductive so as not to interfere with any additional corrosion protection that may be present and, if appropriate, to be itself replaceable as part of galvanic electrodes.

The zinc in the zinc-containing aqueous-based coating is preferably selected from zinc salts, metallic zinc in powder form, metallic zinc moldings and mixtures thereof. As only zinc present in ionic form in the pore solution can form a compound with the anions to be removed, it is obvious to the person skilled in the art that in the case of metallic zinc as the zinc component of the zinc-containing coating, the zinc must be ionized in situ in order to obtain the zinc To meet the purposes of the present invention. This also applies to those cases in which a mixture of zinc salt and metallic zinc is contained in the zinc-containing coating and the metallic zinc is also mixed with anions. 3 # # ♦ · · · · · · • • • • · · · Should react, ie should not serve exclusively as an anode material, as will be described in more detail later.

Zinc salts present in the zinc containing coating are preferably selected from zinc oxide, zinc hydroxide and mixtures thereof, since these salts, optionally after hydration, form sparingly soluble salts or complexes with chloride and nitrate in water, e.g. Oxychlorides, hydroxochlorides, chlorozincates, and their nitrate analogues (also called oxide chlorides by recent nomenclature), as demonstrated by EDX and ESEM screens.

Because of ease of manufacture, zinc-containing coatings containing a zinc salt and / or finely divided zinc powder are preferred over those having zinc embedded components for many applications. Nevertheless, in addition to or as an alternative to zinc salts and powders, one or more metallic zinc moldings are often embedded in an aqueous based coating to form the zinc-containing aqueous based coating, preferably using zinc foil and zinc mesh moldings. Such zinc-containing coatings with embedded moldings can serve, for example, as galvanic anodes or external current anodes and enable control of the local field strength of the electric field by the shape of the respective component.

Accordingly, a preferred embodiment of the method according to the invention is characterized in that the corrosive anions are caused by applying an electric field to mi-grate to or in the coating. In this way, a faster and more complete removal of the anions from the porous solid is effected, as if the transport takes place solely due to natural diffusion processes.

For this purpose, the electric field is preferably applied between at least one anode, which is applied to the coating or embedded in the coating, and at least one provided within or on the opposite side of the porous solid cathode, wherein in the case of steel reinforced concrete ins -4 - ···· I ·· ··· ····················· ···························································································································· In this way, the corrosive anions are quickly and deliberately pulled away from the solid, in particular from the steel reinforcement to be protected against corrosion, by the electric field and immobilized as insoluble or sparingly soluble zinc salts.

The anode used is preferably an electrically conductive coating or galvanic anode applied to the porous solid, which is particularly preferably at least partially formed by the zinc-containing coating. In particular, use is made of a galvanic anode which consists of a material which is transformed into an electrically conductive binder, e.g. a mortar, embedded zinc molding, e.g. a zinc grid, i. which is entirely formed by the zinc-containing coating as defined herein. In this case, in addition to the zinc component (serving as a sacrificial anode), the binder may contain further zinc in the form of zinc salts or zinc powder in order to further enhance the action of the anode.

As an anode, but also as an additional anode in addition to the above galvanic zinc sacrificial anode, in certain embodiments of the invention, an external current anode can be used, which is electrically connected, for example, with the steel reinforcement of reinforced concrete and to reinforce the electric field and thus the migration rate of the anions through the pore solution.

Particularly preferred embodiments of the method according to the invention, in which the surface of the porous solid is protected against ingress of water from the outside, thereby preventing that enter with entering from the outside water additional amounts of corrosive anions in the pore solution of the porous solid, as for example in buildings by splashing or in buildings surrounded by water, such as Bridge piers, that would be the case.

Preferably, the surface of the porous solid is thereby formed by applying a waterproof coating to the zinc-containing coating, embedding a waterproof film in the zinc-containing coating, or a combination thereof -5- # ···························································. # · · · · # ♦ ··· t Water entry protected, for which, for example curable resin compositions are used as paints or plastic films.

In particularly preferred embodiments, a galvanic zinc sacrificial anode, i. a zinc component, e.g. a zinc grid embedded in an electrically conductive binder matrix, e.g. a corresponding mortar used to remove the corrosive anions. When using such a galvanic anode, for example, after two years of operation showed that about 30% of the originally present in the concrete cover chloride (ds 3% of the cement weight) had enriched near the anode in the coating where the chlorine was present in poorly soluble form , After five years of operation, 60% of the zinc was immobilized in the coating. £

If the surface of the solid electrolyte in which the zinc anode is embedded was impregnated with epoxy resin, so that the surface became water impermeable, and the panels were subsequently exposed to the weather, i. Dry / wet, sun / rain cycles) it was found that after two years of operation 70% of the chloride and after 5 years 90% of the chloride had been removed from the concrete cover. The chloride content in the concrete cover was thus < 0.4%, based on the cement weight, and thus was below the limit above which corrosion of the steel is possible.

If a coating whose matrix has a pH of at least 9, preferably > 11, admixed with zinc sulfate before the concrete is applied, the immobilization of the chloride and thus the speed of the chloride removal are significantly improved.

With a minimum life of the galvanic zinc anode of 10 years, a renewal of the zinc anode due to the permanent chloride removal from the concrete cover by the inventive method is not required. -6- • I ····························································································· I »· # · · · · · ·« ·· «·» «

A further improvement in the rate of chloride removal occurs when the zinc anode is applied as a zinc foil to the concrete, and in particular to a coating on the concrete described above, and fixed.

Cathodic corrosion protection by means of external current anodes, e.g. Titanium anode anodes (eg as described in EN 12.696, June 2000 edition, item C.1.3., Entitled "Activated Titanium") or by conductive coating (eg described in AT 410.440B) also cause migration of chloride ions away from the rebar and towards the anode. However, especially in devices with titanium anode anodes, the migration is superimposed by the capillary transport of chloride ions towards reinforcement during rainy periods. Titanium anode anodes are usually embedded in 5 cm shotcrete applied to the surface of the reinforced concrete component to be protected. There, highly enriched chloride fronts with a chloride content of up to 8% of the cement weight can be transported capillary to the reinforcement at the anode. Switching off the cathodic corrosion protection under these circumstances would lead to a very high risk of corrosion.

In anodes for cathodic corrosion protection consisting of conductive paints applied to the concrete surface of the reinforced concrete structural element to be protected, e.g. described in AT 410,440 B, migration is the predominant transport mechanism leading to significant chloride removal from the concrete cover and high chloride accumulation near the anode. However, the disadvantage here is the development of chlorine gas at the anode at current densities of > 2 mA / m2. This problem is solved by the method according to the invention in its entirety.

Namely, when the anode is applied to an electroconductive coating containing finely divided zinc powder, e.g. about 1.5 to 2.5 kg / m2 concrete surface, so " corroded " The zinc during the operation of the cathodic corrosion protection and forms anodic reaction products to which the chloride ions migrating to the anode are bound. As a consequence, even at current densities of> 2 mA / m 2, no more chlorine gas evolution is observed at the anode. Mixing zinc salt 7 · # # · · ····ΦΦΦ · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ······ιιιιιιιιιιιιι·························φφφφφ φ φ φ φ φ φ φ φ · φ φ · · φ φ φ ·· φ φ φ zen, such as Zinc sulfate or zinc hydroxide, or zinc oxide or hydrates thereof, increases the effectiveness of the chloride bond. In principle, the admixture of the abovementioned zinc compounds at sufficient concentration would be sufficient to sufficiently bind chloride ions, but these compounds have a strong retarding effect on the setting behavior and the hardening of hydraulic binders and thus can be added to the known binder mixtures from which electrically conductive coatings are produced. only to a small extent, usually in amounts of < 5% by weight, preferably < 2 wt.%, To be mixed.

If at least one zinc compound is admixed in sufficient amounts to the electrically conductive coating, so that sufficient chloride can be bound so that no significant or at least a greatly reduced chloride evolution occurs.

This embodiment of the invention is advantageous in particular because, after about 5 to 10 years, up to 90% of the chloride has been removed from the concrete cover and corrosion protection by means of cathodic corrosion protection and therefore any possible renewal of the anode are no longer necessary.

The invention will be described in more detail below by way of non-limiting examples.

EXAMPLES

example 1

A test piece was made of steel reinforced concrete (50 x 50 x 10 cm), with a cement content of 320 kg / m3, aggregate 0 to 16 mm, w / c = 0.47, 0.2 wt .-% liquefier , Chloride content 3.0% by weight of the cement weight, steel reinforcement 0 16.0 mm, lattice spacing e = 100 mm, concrete coverage 3 cm. -8- ························································································································································································································· · ··· · · · · «

The test piece was coated with an electrically conductive mortar with a layer thickness of about 2 mm. On this mortar, a zinc grid (2.1 kg / m2, mesh size 3 cm, strand thickness 1.2 x 2 mm) was mechanically fixed and embedded in the same mortar with a layer thickness of 4 mm. After curing of the mortar, the zinc mesh was connected to the steel reinforcement and at 80% r. F. and stored at room temperature 20 ° C. at intervals of 6 months, the chloride profile in the composite system coating / concrete was measured on removed drilling dust. The results are shown in Table 1 below.

Table 1

Time (months) Chloride content [wt% / cement weight] in the coating and concrete coverage (in cm from the surface of the specimen) Coating 0-1 cm 1-2 cm 2-3 cm 3-4 cm 0 0 3.0 3.0 3.0 3.0 6 1.9 2.5 2.4 2.0 2.3 12 3.5 1.2 2.1 1.9 2.0 18 4.3 0.8 1 7 1.5 1.6 24 5.1 0.6 1.5 1.3 1.4 30 5.8 0.5 1.3 1.1 1.3 36 6.1 0.5 1.1 1 , 0 1.2

The current flow between anode and cathode averaged 2 mA, i. 8 mA / m2 concrete surface.

The example shows that the chloride is migrated into the coating and fixed there. -9- 99 99 9 · 9 9 9 9 9 9 9 9 9 9 9 9 9 99 9 • ♦ ······································· 9 9

Example 2

It was the same experimental design (test plate, coating, zinc anode) as selected in Example 1. In contrast to Example 1, the test plate provided with the zinc-containing coating was artificially irrigated (10 mm water in 2 hours) for 2 hours once a week and then dried with infrared light for 2 hours.

The results are shown in Table 2 below.

Table 2

Time (months) Chloride content [wt% / cement weight] in the coating and concrete coverage (in cm from the surface of the specimen) Coating 0-1 cm 1-2 cm 2-3 cm 3-4 cm 0 0 3.0 3.0 3.0 3.0 6 1.8 2.7 2.6 2.4 2.5 12 3.2 1.8 2.2 2.0 2.2 18 4.3 1.5 1 9 1.7 1.8 24 4.9 1.3 1.7 1.6 1.8 30 5.4 1.0 1.5 1.4 1.6 36 5.6 0.9 1.4 1 , 3 1.4

Due to weathering, the average current flow was 4 mA, i. 16 mA / m2 concrete surface.

The above results show that by artificial weathering, i. the generation of wet / dry cycles, and the resulting capillary water transport in the concrete cover, slows down the chloride migration to the anode and "blurs" it. has been. However, the results also show that most of the chloride, when it enters the coating, is fixed there. -10- «· v * · v« t · t ··· ··· t · «· · · · · · · · I | t t · t · ··· ·····

I t I t # ···· I ·· · · ··········

Example 3

The same test arrangement (test plate, coating, zinc anode) as in Example 2 was chosen. In contrast to Example 2, the surface of the zinc-containing coating was coated with an epoxy resin (two-component system, hardening after 12 hours) and impregnated so that liquid water could no longer penetrate.

The results are shown in Table 3 below.

Table 3

Time (months) Chloride content [wt% / cement weight] in the coating and concrete coverage (in cm from the surface of the specimen) Coating 0-1 cm 1-2 cm 2-3 cm 3-4 cm 0 0 3.0 3.0 3.0 3.0 6 2.2 2.2 2.4 2.2 2.5 12 3.7 1.1 2.0 1.7 1.9 18 4.5 0.7 1.5 1 , 4 1.5 24 5.2 0.5 1.3 1.2 1.4 30 5.9 0.4 1.2 1.1 1.3 36 6.5 0.4 1.1 1.0 1.1

The current flow between anode and cathode averaged 1.8 mA, i. 7.2 mA / m2 concrete surface.

The results show that with complete suppression of capillary water transport in the concrete and coating, chloride was extracted much faster and more efficiently than from the weathered concrete specimen - even more efficiently than in the " vapor diffusion open " Test specimen from example 1. -11- ····· * »··· ······ * ··· • ······································································ ·· ♦ · · · · ····· «···· ··· ··

Example 4

The same test arrangement (test plate, coating, zinc anode) as in Example 3 was chosen. In contrast to example 3, a mixture of 1% zinc sulfate, 1% zinc hydroxide and 1.5% zinc oxide, all values based on the weight of the fresh mortar, was admixed to the electrically conductive mortar.

The results are shown in Table 4 below.

Table 4

Time (months) Chloride content [wt% / cement weight] in the coating and concrete coverage (in cm from the surface of the specimen) Coating 0-1 cm 1-2 cm 2-3 cm 3-4 cm 0 0 3.0 3.0 3.0 3.0 6 2.3 2.1 2.2 1.9 2.1 12 4.3 1.0 1.7 1.5 1.7 18 5.1 0.6 1, 4 1.2 1.3 24 5.9 0.5 0.9 0.7 0.8 30 6.6 0.5 0.7 0.6 0.7 36 7.2 0.5 0.6 0 , 5 0.7

The results show that the addition of zinc salts (zinc hydroxide, zinc sulfate) and zinc oxide significantly increases the efficiency of the chlorine removal compared to the zinc grid alone.

After 3 years, the chloride content in the concrete cover averaged only 0.6%. At this chloride content, there is only a slight risk of corrosion. It may be extrapolated that after a further 2 to 3 years, the chloride would have been withdrawn so far that for the reinforcing steel no corrosion risk would exist anymore. -12- m 9 mww 9 9 9 9 9 9 9 9 9 99 9 9 9 9 9 9 9 9 9 999 99 9 9 9 9 9 9 9 9 · 9 9 999 · 9 9 9 9 9 9 9 9 9 9 9

Example 5

A test plate was produced as described in Example 1. The test panel was coated with an electroconductive mortar to which 2 kg of zinc powder having a grain size of 0.02 to 0.6 mm was mixed in a layer thickness of 3 mm. After the coating had cured, a graphite-containing electroconductive coating 0.3 mm thick was applied to the surface of the coating. An electrical lead made of Cu / Nb / Pt wire was embedded in the electrically conductive coating. After the coating had cured, a voltage of 2.5 volts was applied between the electrically conductive coating acting as the anode and the reinforcing steel. After about 2 weeks, the current flowing between the anode and cathode stabilized at about 15 mA / m2 concrete surface.

The results are shown in Table 5 below.

Table 5

Time (months) Chloride content [wt% / cement weight] in the coating and concrete coverage (in cm from the surface of the specimen) Coating 0-1 cm 1-2 cm 2-3 cm 3-4 cm 0 0 3.0 3.0 3.0 3.0 6 2.7 2.0 2.1 1.7 1.9 12 4.5 1.3 1.6 1.4 1.5 18 5.3 0.6 1.2 1 , 1 1.2 24 6.0 0.5 1.0 0.8 0.9 30 6.8 0.4 0.7 0.5 0.6 36 7.5 0.4 0.5 0.4 0, 4

The results show that the chloride was efficiently bound in the zinc-containing coating. Unlike the experiment in which the anode was applied to the concrete without coating, no evolution of chlorine gas was noted. -13- ·· • · • ♦ · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·

After three years, the chloride content in the concrete cover was only 0.4% on average. At this chloride content is only a very low risk of corrosion.

Example 6

A brick (25 × 12 × 6.5 cm) of baked clay was repeatedly soaked with a solution of 3% by weight of sodium chloride and 2% by weight of sodium nitrate, so that the brick had a content of 0.5 after drying % Chloride and 0.3% nitrate, based on the weight of the brick.

The brick surface was coated with the electrically conductive mortar of Example 1, which contained 3% by weight of zinc hydroxide, 3% by weight of zinc oxide and 1% by weight of zinc sulfate in a layer thickness of 6 mm. After the mortar had set, the mortar was rendered hydrophobic with a commercial polysiloxane-based water repellent. The brick was placed on a permanently damp cloth (cotton, 1 mm). Subsequently, the brick was exposed to an artificial wind generated by a fan with a wind speed of about 10 km / h.

After about 1 year, about 95% of the chloride and about 80% of the nitrate had been removed from the brick by the process according to the invention and bound in the coating.

Thus, it is clearly demonstrated by the above examples that the process of the present invention enables easy, rapid and permanent removal of corrosive anions from the porous solid state pore solutions such as concrete and brick walls. -14-

Claims (17)

• · · · · ··········································································································································································································································· Process for removing corrosive anions from the porous solid state pore solutions, eg Concrete or brick walls and components, characterized in that a zinc-containing coating is applied to the porous solid on an aqueous basis or the solid is impregnated with the coating, after which the corrosive anions are allowed to migrate in the pore solution to or in the coating and / or to migrate to the zinc-containing coating to form a water-insoluble or sparingly soluble compound there with the zinc. 2. The method according to claim 1, characterized in that the corrosive anions are cloride and / or nitrate. 3. The method according to claim 1 or 2, characterized in that the porous solid body is a component or a wall made of reinforced concrete. 4. The method according to any one of claims 1 to 3, characterized in that the zinc-containing coating is electrically conductive. 5. The method according to any one of the preceding claims, characterized in that the zinc is selected in the zinc-containing aqueous-based coating of zinc salts, metallic zinc in powder form, moldings of metallic zinc and mixtures thereof. 6. The method according to claim 5, characterized in that the zinc salts are selected from zinc oxide, zinc hydroxide and mixtures thereof. A method according to claim 5 or 6, characterized in that one or more metallic zinc moldings are embedded in an aqueous-based coating to form the zinc-containing aqueous-based coating. -16- «« v mw vm · • ···························································································································································································································· 9 9 ····· 999 ·· 99 · 8. The method according to any one of claims 5 to 7, characterized in that the molded parts of zinc foils and zinc lattices are selected. 9. The method according to any one of the preceding claims, characterized in that the corrosive anions are brought by applying an electric field to migrate to or in the coating. 10. The method according to claim 9, characterized in that the electric field between at least one anode, which is applied to the coating or embedded in the coating, and at least one provided within or on the opposite side of the porous solid cathode is applied. 11. The method according to claim 10, characterized in that the porous solid body consists of reinforced concrete, the steel reinforcement is used as the cathode. 12. The method according to claim 10 or 11, characterized in that an anode applied to the porous solid galvanic anode is used as the anode. 13. The method according to claim 12, characterized in that the galvanic anode is at least partially formed by the zinc-containing coating. 14. The method according to any one of claims 10 to 13, characterized in that an external current anode is used as the anode or additional anode. 15. The method according to any one of the preceding claims, characterized in that the surface of the porous solid is protected against ingress of water from the outside. -17- «« 9 9 9 99 99 9 9 9 9 9 99 9 9 99 9 9 9 9 9 999 99 9 9 9 99 9 99 99 9 999: 9999 99999 9 16. The method according to claim 15, characterized in that the surface of the porous solid is protected by applying a waterproof coating on the zinc-containing coating and / or embedding a waterproof film in the zinc-containing coating against water ingress. 17. Use of a galvanic zinc anode for removing corrosive anions such as chloride and nitrate from the pore solutions of porous solids. Vienna, on ^^ Feb, 2010 -18-