GB1578984A

GB1578984A - Corrosion-protection of metal reinforcements for concrete

Info

Publication number: GB1578984A
Application number: GB2701777A
Authority: GB
Original assignee: WR Grace and Co
Current assignee: WR Grace and Co
Priority date: 1976-06-28
Filing date: 1977-06-28
Publication date: 1980-11-12
Also published as: JPS533935A; DE2729579A1; IT1085087B; FR2356740B1; SE7707477L; FR2356740A1

Description

(54) CORROSION-PROTECTION OF METAL REINFORCEMENTS FOR CONCRETE (71) We, W. R. GRACE & Co., a Corporation organized and existing under the laws of the State of Connecticut, United States of America, of Grace Plaza, 1114 Avenue of The Americas, New York, New York 10036, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention is directed to the protection against corrosion of metals used as reinforcement in cementitious bodies such as concrete.

Hydraulic cements are binders which set by the action of water, and the most common are alite cements of which the predominant example is portland cement. A set mixture of hydraulic cement binder and sand is a mortar, and a set mixture of hydraulic cement binder sand and aggregate is concrete. When set, all these may be called "cementitious bodies". Metal is often present in cementitious bodies, especially concrete, as reinforcement. The present invention plovides a method of precoating metal to protect it from corrosion that may occur when the metal is in situ in a cementitious body. The invention provides corrosion-treated metal articles, which may be in use or intended for use as concrete reinforcement.

Hydraulic cements (including alite cements) encounter various corrosion environments. In some, the environment is an inherent part of the cement, e.g. as by use of calcium chloride accelerator, or the use of chloride containing materials. Other environments may be extraneous, e.g. use of calcium chloride and/or salt in snow and ice removal, exposure to salt spray or brines, and the like Such environments tend to attack and corrode metal pieces within or in contact with the cement. The present invention provides coatings for the metal that we believe inhibit such corrosion.

According to the present invention, a corrosion inhibitor present as coating on the surface of metal articles suitable for use as reinforcement for concrete, consists of or includes calcium nitrite. This can be a corrosionXinhibiting coating on structures of metal including iron, steel, aluminum, steel-aluminum alloy grids, reinforcing rods, girders, etc., which in use are in contact with corrosion environments in alite cements.

The use of nitrite salts, preferably sodium nitrite, to reduce the corrosion of tanks of ships used for transporting liquid cargoes, has been proposed in Patent Specification 686,184; we have found that for our intended use, namely for the corrosion protection of metal to be used as reinforcement in a cementitious body, calcium nitrite is far superior to sodium nitrite-calcium- nitrite exhibits less efflorescence, and prevents corrosion as measured both by nonexpansion of concrete and by weight loss in the presence of sodium chloride.

Example I There are several ways to cost the metal piece with the inhibitor. The simplest way is simply to make up an -aqueous solution of the inhibitor, say for example, 10 to 40 weight percent, dip the work piece into this solution, pull it out, and let it air dryor oven dry. This will give an effective coating of the inhibitor on the metal piece; The metal piece can then be placed as astructural member in concrete, mortar, or the like. Spraying the solution with a hose should also be effective; also, wiping or brushing the inhibitor onto the metal.

There are various other systems of coated ing the metal piece with the inhibitor. In one system, the inhibitor is admixed into a slurry of hydraulic cement, in an amount of about 1 to 2 weight percent, based on the weight of solids in the cement. The metal piece is then dipped into the slurry,; then withdrawn, then permitted to cure, and after curing is then available for use in alite cement matrix.

Numerous other systems of coating the metal piece with calcium nitrite will be evident to those skilled in the art.

The coated metal articles of this invention can be used in association with many standard systems of alite cement corrosion control, including powder epoxy coatings; polymer-impregnated concrete; waterproof membranes; dense concrete, low watercement ratio and superplasticizers; integral addition of wax beads; expansive cements; fly ash; latex modified mortar topping; inorganic polymer addition; and polymer concrete. (Examples of suitable latices for latex modified mortar topping are acrylic, polyvinyl acetate, polyvinyl alcohol, vinylidene chloride, styrenebutadiene, and copolymers of these.) Any of the above mentioned systems should work quite well in concert with the coated articles of this invention. However, special mention should be made of certain of these.The first case is methods of treating concrete that would reduce the water permeability. In these cases a harsh mix with a low water/cement ratio would tend to make the calcium nitrite coating more effective. Hcwever, because of the placing difficulties with a harsh mix an admixture could be used to reduce the water and maintain the same plasticity of the mix. In these cases a plasticizer could be used such as copolymers of naphthalene formaldehyde resins or the more normally used water reducing agents such as 80% calcium lignosulfonate and 20% triethylamine added at an addition rate of 0 1 to 0 4 percent s/s cement.

Use with Water Reducing Agents A water-reducing agent is a material added to cement during the manufacture of the concrete to improve the placeability or workability of the mix which allows normal hardening of the concrete to take place, which eventually produces 28-day strengths that are at least 10% stronger than the mix not containing the admixture. Generally a reducing agent causes 5% or greater reduction of water in the mix. Water-reducing agents are described in ASTM C-494.

A preferred formulation using the cal cium nitrite coated articles of this invention involves a water-reducing agent. There are at least two reasons for this. In the first place an ordinary mix making a water reducing agent may tend to form large random bubbles against reinforcing rods during placement of the concrete or cement.

Our experimental work shows that the surface of the reinforcing rod exposed to such large random bubbles shows aggravated corrosion, as compared to surfaces of the rod not in contact with such bubbles. In the second place the use of such water reducing agents results in the formation of a more dense cement or concrete, and in consequence reduces ingress of corrosive salts from the environment.

A preferred water reducing agent is 80% calcium lignosulfonate and 20% calcium chloride added at an addition rate of 01 to 0 4 per cent. Other water reducing agents include additives such as triethylamine formate or polysaccharides.

Use with Retarders As is known in the cement art, sodium gluconate, calcium lignosulfonate, saccharide type materials and the like, have a known characteristic of improving the placeability and strength as well as retarding the set of the concrete mix. Hence simply by the use of any of the standard retarders, the corrosion control properties of sodium or calcium nitrite as used herein should be enhanced.

Use wilh Air Entraining Agents Air entraining agents are added to concrete during, e.g. bridge construction, to improve durability. They work by providing small expansion chambers within the concrete mix such that when water freezes within the concrete, concrete does not spall and crack. We believe that when an air entraining agent is used the mean free path for the introduction of chloride into the concrete is kept to a maximum, reducing the chloride concentration at the surface of the coated metal articles of this invention, thereby making less inhibitor necessary.

As an air entraining agent Vinsol Resin sold commercially by Hercules Powder Co.

is recommended. This material is of the family of wood resins. Also suitable are saponified tall oil resins and sodium lauryl sulfonate (this being an anionic surfactant) and Triton X-100 (this being a nonyl phenol ethylene oxide condensate, a non-ionic surfactant).

Use with Accelerators The calcium nitrile-coated articles of this invention can be used with alite cement containing accelerators. Such accelerators are well known to those skilled in the cement and the concrete art and are listed in standard texts of this subject.

Use with Pozzolans Pozzolans whether natural or artificial (e.g. fly ash, blast furnace slag, and the like) are useful with the coated metal pieces of this invention in that these added materials fill up voids in the concrete and/or cement mix, thereby reducing the amount of inhibitor required for equivalent corrosion control.

WHAT WE CLAIM IS: - 1. A metal article suitable for use as reinforcement in a cementitious body treated to protect it against corrosion, by a surface coating containing calcium nitrite as corrosion inhibitor.

2. Article according to claim 1 in which the surface coating is a composition which contains also hydraulic cement.

3. Article according to claim 2 in which the calcium nitrite inhibitor is 1-2% by weight of the cement.

4. Article according to claim 1 in which the coating consists substantially only of calcium nitrite.

5. Article according to claim 1, 2, 3 or 4 in which the metal is iron, steel, zinc, aluminum, or aluminum-steel alloy.

6. A cementitious body containing metal elements which have been precoated with calcium nitrite.

7. A body according to claim 6 in which the elements have been precoated with a composition containing a hydraulic cement and calcium nitrite.

8. Method of treating a metal piece suitable for use as reinforcement in a cementitious body to protect the piece against corrosion, which comprises applying there to a coating of calcium nitrite.

9. Method according to claim 8 in which the piece is coated with a composition which is an aqueous solution containing 1040% of calcium nitrite.

10. Method according to claim 8, in which the piece is coated with a composition comprising a hydraulic cement and the calcium nitrite.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. like) are useful with the coated metal pieces of this invention in that these added materials fill up voids in the concrete and/or cement mix, thereby reducing the amount of inhibitor required for equivalent corrosion control. WHAT WE CLAIM IS: -

1. A metal article suitable for use as reinforcement in a cementitious body treated to protect it against corrosion, by a surface coating containing calcium nitrite as corrosion inhibitor.