Compositions – Preservative agents – Anti-corrosion
Reexamination Certificate
1999-05-11
2002-06-11
Yoon, Tae H. (Department: 1714)
Compositions
Preservative agents
Anti-corrosion
C252S389100, C252S390000, C106S014050, C106S014120, C106S014130, C106S014140, C106S014150, C106S014160, C106S014170, C106S014180, C106S014410, C106S014420, C106S014430
Reexamination Certificate
active
06402990
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to the inhibition of corrosion of reinforcing steel embedded in a hardened concrete structure. In particular, this invention provides compositions and a method for the reduction of the corrosion rate of already corroding steel reinforcements embedded in a hardened concrete structure as well as for the protective corrosion inhibition of uncorroded steel reinforcements embedded in a hardened concrete structure exposed to aggressive environments.
BACKGROUND OF THE INVENTION
Durability limitations of steel reinforced concrete are well documented. Corrosive environments (e.g. the presence of chlorides), carbonation of concrete structures, poor workmanship and other factors can quickly cause corrosion of the reinforcing steel. Chloride ions in concrete can originate from the ingress of de-icing salts, seawater or air-borne salts, diffusing to the reinforcement through the pore network of concrete, as well as from contaminated aggregates or from contaminated mixing water (cast in chlorides).
Carbon dioxide as well as other acidic corrodents present in the air react with the free alkali contained in concrete. Over a period of time the pH value of the outermost concrete layer decreases, resulting in a reduction of the natural protection of the steel embedded in reinforced concrete.
Normally, reinforcing steel embedded in concrete is protected because the concrete cover acts as a barrier and the high pH value of the pore fluid assures a passive state. Both the presence of chloride ions at concentrations above a given threshold level and carbonation can put reinforcing steel into an active state and result in corrosion rates that markedly decrease the expected service lives of reinforced concrete structures. Thousands of bridges and other structures made of reinforced concrete need to be repaired worldwide as a consequence of corrosion of the steel reinforcement.
The present invention relates to corrosion-inhibiting compositions for the rehabilitative repassivation of corroded reinforcing steel embedded in hardened concrete as well as for the precautionary, protective corrosion inhibition of uncorroded reinforcing steel embedded in hardened concrete structures exposed to corrosive environments. Said corrosion-inhibiting compositions contain corrosion inhibitors with high buffer capacities as well as water repellent agents, and cause a reduction of the corrosion rate of corroding reinforcing steel embedded in a hardened concrete structure as well as delay the onset of corrosion of uncorroded reinforcing steel embedded in a hardened concrete structure respectively.
Corrosion inhibitors are compounds or compositions that, when added in small concentrations to a corrosive environment, decrease the corrosion rate. The use of corrosion inhibitors is wide-spread and well established. For example protection systems based on corrosion inhibitors are used to limit the corrosion of process equipment such as cooling systems, pipelines, or central heating systems. Various types of corrosion inhibitors designed for different applications are available in the marketplace. Corrosion inhibitors may be classified into the following groups: Anodic oxidizing passivators (e.g. chromates, nitrites), anodic non-oxidizing passivators (e.g. molybdates), cathodic corrosion inhibitors (e.g. oxygen scavengers such as sulfites or cathodic poisons such as arsenates), film-forming inhibitors of the adsorption type (e.g. amines, alkanolamines). The film-forming corrosion inhibitors, which are typically organic compounds, represent the largest group of corrosion inhibitors and may exhibit anodic, cathodic or mixed behaviour.
THE PRIOR ART
Several methods have been suggested to protect reinforcing steel embedded in concrete against corrosion:
World Pat. Appl. No. 96/22951 (Berke et al.) discloses a corrosion-inhibiting admixture for hydraulic cement compositions. Said admixture is based on an anodic corrosion inhibitor such as calcium nitrite and a cathodic corrosion inhibitor such as calcium stearate or other organic acids of high molecular weights (C
7
to C
21
) including salts and esters thereof.
U.S. Pat. No. 5,262,089 (Bobrowski et al.) discloses an admixture for inhibiting the chloride-induced corrosion of steel reinforcements in concrete. Said admixture is based on an emulsion containing an ester of an unsaturated fatty acid, a surfactant and a saturated fatty acid ester as well as a saturated fatty acid, an amphoteric compound, a glycol and a soap; the admixture is added to the concrete prior to placement.
PCT Patent application Ser. No. 96/27695 (Curatolo et al.) describes a method of controlling effectively chloride-induced corrosion of steel reinforcement members in concrete by the addition of a corrosion-inhibiting admixture to freshly prepared concrete. Said admixture comprises i) one or more waterproofing agents; ii) one or more mono-, di-, or trialkanolamines and iii) optionally, one or more additional corrosion-inhibiting agents, with the proviso that if the waterproofing agent is capable of forming a salt with ii) and/or iii), then at least a portion of i) and ii) and/or iii) are present in said admixture in their salt form in combination with each other.
Swiss Pat. No. 686 368 (Buerge at al.) is related to corrosion-inhibiting admixtures for cementitious building materials which are containing metal parts, e.g. metallic reinforcements. Said admixtures are based on reaction products of amino compounds with one or more inorganic and/or organic acids. The admixtures may contain excessive amounts of the amino compound and/or un-reacted amino- and acid compounds.
According to the four patents above, corrosion inhibitors of the admixture type are added to concrete prior to placement, thus prior to hardening. None of these prior art methods, however, discloses a rehabilitative and/or protective application of corrosion-inhibiting compositions to the surface of a hardened, reinforced concrete structure. U.S. Pat. No. 5,435,846 (Tatematsu et al.) discloses a cement additive for inhibiting concrete deterioration. Said cement additive contains a powder mixture of inorganic ion exchangers as well as optionally nitrite ions. It inhibits the alkali-aggregate reaction and the corrosion of reinforcing steel, thereby protecting concrete from deterioration. Also disclosed is a method of inhibiting or repairing concrete deterioration, comprising charging or applying a cement composition to a concrete crack or to a concrete body whose surface has peeled or been damaged.
However, said method of coating and/or patching damaged concrete structures with a cementitious material is labor-intensive and costly, as well as (regarding patching applications) limited to restorations of not fair-faced concrete structures.
Canadian Pat. No. 1,258,473 (Martin et al.) describes a method of inhibiting corrosion of iron or steel reinforcements in concrete which comprises incorporating in a slurry of the concrete an effective quantity of a corrosion-inhibiting composition having as its major ingredient a water-soluble hydroxyalkylamine having a molecular weight in the range of about 48 to about 500. An additional method relates to placing a concrete slurry containing said hydroxyalkylamine in corrosion-inhibiting proximity to said reinforcement members embedded in cured concrete structures.
This method relates to corrosion-inhibiting compositions of the concrete admixture type. Said corrosion-inhibiting compositions are mixed with concrete before placing the concrete to either cast or to repair reinforced concrete structures. According to the patent it is not intended to use a solution of said corrosion-inhibiting compositions to impregnate the surface of hardened reinforced concrete. Furthermore, many low to medium molecular weight types of hydroxyalkylamines are featuring disadvantages such as strong ammonia-like odours, high alkalinity and corrosiveness.
U.S. Pat. No. 4,609,573 (Omata et al.) describes a process for inhibiting corrosion of steel built in an inorganic material by ap
Bürge Theodor A.
Mäder Urs
Marazzani Beat
Burns Doane Swecker & Mathis L.L.P.
Sika AG, Vorm, Kaspar Winkler & Co.
Yoon Tae H.
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