EP0285593B1 - Method for increasing the corrosion resistance of a quenched and self-tempered reinforcement bar made of steel - Google Patents

Abstract

Process in which, during the quenching and self-tempering, a layer protecting against corrosion is formed at the surface of the bar. This layer can be produced by phosphatising with manganese, by oxidation with blowing-in an oxidising agent or by the application of a paint or of a resin. The phosphatising can be preceded by pickling. The protective layer can then be coated with a film of drying oil.

Description

The present invention relates to a method for improving the corrosion resistance of a hardened and self-tempering steel rebar.

The steel bars used for the reinforcement of concrete are subjected to corrosion both during their storage, their transport and their implementation on construction sites as during their lifetime in concrete.

They are first exposed to corrosion exerted by various aggressive agents contained in the atmosphere. This action is particularly severe in a marine atmosphere, due to the attack of steel by chlorides; this can result in excessive rusting, with detachment and loss of material, which can lead to unacceptable reduction in section or undesirable damage to the anchoring ribs.

When embedded in concrete, the reinforcements are normally protected against corrosion thanks to the alkaline character (pH about 12.5) given to concrete by the lime it contains. However, there are frequently significant problems of corrosion of reinforcements in concrete, mainly under the action of external aggressive agents which diffuse through it. These problems are encountered in particular when the concrete is of poor quality, but also when the thickness of the concrete facing is low, for construction reasons, or when the concrete is cracked as a result of tensile stresses in service. These diffusion phenomena lead to carbonation of the concrete by absorption of the CO₂ of the atmosphere, resulting in a decrease in the pH of the concrete; this results in a fall in the protective power of the concrete and the restoration of the vulnerability of the reinforcements to aggressive agents such as chlorides which also diffuse through the concrete. These phenomena can lead to flaking of the concrete and the exposure of the reinforcements; this results in an acceleration of corrosion which in turn causes a deterioration of the aesthetic appearance and can even jeopardize the safety of the construction .

Two main techniques are also known which are intended to combat the aforementioned phenomena and thus to ensure the protection of the reinforcements.

A first type of technique consists in applying a protective film to the concrete intended to prevent the penetration and diffusion of atmospheric agents in the concrete. This film, applied to the finished part or construction, however, does not provide any protection for the reinforcement during its storage, transport or use. In addition, its protective action may diminish or even disappear over time under the effect of atmospheric erosion. Finally, this action is compromised in the event of degradation or damage to the protective film.

The second type of technique consists in depositing, on the reinforcement, a layer of protection against corrosion, for example by dip galvanizing or by coating with epoxy resin.

These two techniques are however very expensive. In addition, they can only be applied after the manufacture of the reinforcement; they therefore require additional operations which lengthen the manufacturing times and ultimately have an unfavorable effect on the price of the reinforcements.

In this regard, reference may be made in particular to the document CHEMICAL ABSTRACTS, vol. 90, no.16, April 1979, page 230, abstract no 125340a, Colombus, Ohio, US; V. S. VANIN et al .: "Nonisothermal phosphating and oxidizing of steel", & ZASHCH. MET. 1979, 15 (1), 110-12, as well as in the document CHEMICAL ABSTRACTS, vol. 104, no.2, January 1986, page 259, abstract no.9793j, Colombus, Ohio, US: "Corrosion inhibition of prestressed steel wires and rods"; & JP-A-60 110 381 (SUMITOMO ELECTRIC INDUSTRIES LTD), 15-06-1985.

The first document briefly describes the formation of a layer of phosphate, respectively of oxide, on a piece of steel during the quenching by addition of manganese phosphate, respectively of KMnO₄, to the quenching bath. The second document discloses a process for forming a resin layer on a preheated steel rebar.

The object of the present invention is to provide a method for protecting concrete reinforcements against corrosion, which does not have the aforementioned drawbacks and which, in particular, can be implemented during the manufacturing process of these reinforcements.

As indicated above, the present invention relates to steel concrete reinforcements subjected to the quenching and self-tempering process which was recently developed by the same applicant BE A 889 575. We can briefly recall the principle of this controlled cooling process which essentially comprises three stages. The first step consists in subjecting a steel bar, as soon as it leaves the last stand of the hot rolling mill and therefore from its end of rolling temperature, that is to say in the austenitic state, to abrupt and short-term surface cooling such that it causes in the bar the formation of a peripheral layer consisting of martensite or bainite, that is to say a quenching structure. This first cooling is carried out with cold water. At the end of this first step, the surface of the bar is at a temperature below the point Ms of the steel used, while the core of the bar is not reached by the sudden cooling. The bar is then, during a second step, subjected to cooling in the air during which the temperatures are equalized in its section: the surface temperature increases under the effect of the heat coming from the heart, this which causes self-tempering of the surface layer of martensite or bainite; simultaneously, the core cools slowly and its initial austenitic structure is transformed into ferrite and carbides. As a general rule, surface and core temperatures are approximately equal to between 400 degrees centigrade and 700 degrees centigrade. From the equalization of the temperatures, the bar cools, throughout its section, to room temperature, without undergoing any further structural transformation. This latter cooling constitutes the third stage of this known process.

The present invention makes it possible to take advantage of the particular conditions which this process presents to achieve the aim aimed at, namely the improvement of the corrosion resistance of this type of concrete reinforcement.

The invention also relates to a rebar having improved protection against corrosion obtained by applying this method.

According to the present invention, a method for improving the corrosion resistance of a steel rebar subjected to a continuous heat treatment comprising a first step which consists in surface hardening of the bar from the end temperature rolling, a second step which consists of cooling in air with temperature equalization in the section of the bar and self-tempering of the hardened surface layer, and a third step which consists of a final cooling of the bar in the air from the equalization temperature to ambient temperature, is characterized in that during at least one of said steps, a protective layer is formed on the surface of the bar made up of a chosen material in a group comprising phosphates, oxides, paints and synthetic resins.

In this regard, it has been found advantageous to use a phosphating agent containing manganese phosphate for the phosphating of the surface of the bar.

According to this variant, said phosphating treatment is applied during the first step of the process, by adding the phosphating agent to the cooling water.

Also according to this variant, said phosphating treatment is applied during the second step of the process, by projecting onto the surface of the bar a liquid or gaseous fluid containing said phosphating agent.

Still according to this variant, said phosphating treatment is applied during the third step of the process by immersing the bar in a solution containing said phosphating agent.

In the context of this variant, and particularly when the phosphating treatment is applied during the third step of the process, it has proved advantageous to combine the phosphating treatment with a stripping operation of the bar. The purpose of such a pickling operation, possibly followed by rinsing, is to eliminate the surface oxides which may have formed during the preceding cooling stages and to promote the formation of the protective phosphate layer on the surface of the bar. Depending on the nature of the substances used, it can also be combined into a single operation stripping and phosphating the bar.

Although the above-mentioned phosphating treatment is preferably applied continuously to a moving bar, it would not, however, depart from the scope of the present invention to carry out this treatment discontinuously during the third step of the process mentioned above. In such a discontinuous implementation, the treatment could for example relate to a plurality of bars, cut to an appropriate length and immersed in a solution of phosphating and possible pickling.

According to another variant of the invention, a stable oxide layer is formed on the bar, by exposing said bar to an oxidizing gas stream such as air or oxygen.

According to this variant, this operation is advantageously carried out in the process zones where the surface of the bar has a high temperature, that is to say before the start of the first step and / or between the second and the third step, in the sense defined above.

According to yet another variant of the invention, a layer of paint resistant to high temperatures is deposited on the surface of the bar.

According to this variant, this layer of paint is advantageously deposited by spraying, this deposit preferably being carried out between the first and the second stage of the process, that is to say in the zone thereof where the surface of the bar is at a low temperature. The paint is then quickly annealed and dried at high temperature during the surface reheating of the bar which occurs during the second stage of the process.

According to yet another variant of the invention, a thin film of plastic resin is deposited on the bar, for example of the type soluble in water and used in other applications as additives to concrete to make it impermeable to moisture.

According to this variant, this resin film is advantageously deposited by projection or immersion during the third step in the sense defined above.

According to an additional characteristic of the process of the invention, an oil film is deposited on said protective layer consisting in particular of phosphate, oxide, paint or resin. For this purpose, a desiccant oil is preferably used.

Such an oil film completely closes the pores and any defects in the protective layer. In addition, a desiccant oil remains fluid and oily for a period which depends on its composition; it thus very effectively protects said layer against any external agent and it thus clearly reinforces the protection provided by the phosphate, oxide, paint or resin layer. When the corresponding duration has elapsed, the oil dries and therefore does not alter the behavior, in particular, the adhesion of the bar in concrete.

By way of example, an operating sequence corresponding to a pickling treatment followed by a manganese phosphating is described below, applied to a bar during the third step of the quenching and self-tempering process.

During the final cooling, the bar was immersed for about 10 min in a pickling solution consisting of 0.5 N HCl supplemented with 2.5 g / l of hexamethylenetetramine, which was at room temperature. After rinsing with running water, the bar was immersed in an aqueous solution containing 10% by weight of a phosphating agent, the temperature of the solution was between 90 ° C and 95 ° C, the ratio of total acidity to free acidity was 7: 1 and the immersion time was approximately 10 min. After rinsing with running water followed by drying with hot air, the bar phosphated was coated with an oil film by soaking in an oil bath at room temperature.

The bar thus treated showed no trace of oxidation during prolonged storage and exposure to an industrial atmosphere.

In all the cases envisaged above, the method of the invention is implemented during the quenching and self-tempering process. It does not require significant additional equipment and it provides both temporary protection against atmospheric corrosion and permanent protection against corrosion in concrete. Finally, it would not be outside the scope of the present invention to apply simultaneously or successively several of the aforementioned variants, during the quenching and self-tempering process of the bar.

Claims (10)

1. Process for improving the corrosion resistance of a steel reinforcing bar subjected to a continuous heat treatment comprising a first stage which consists of surface quenching of the bar from the temperature at the end of rolling, a second stage which consists of cooling in air with equalisation of the temperatures in the section of the bar and self-tempering of the quenched surface layer, and a third stage which consists of final cooling of the bar in air from the equalising temperature to room temperature, characterised in that during at least one of the said stages a protective layer is formed at the surface of the bar, consisting of a material chosen from a group comprising phosphates, oxides, paints and synthetic resins.

2. Process according to Claim 1, characterised in that a phosphating agent containing manganese phosphate is employed for phosphating the surface of the bar.

3. Process according to either of Claims 1 and 2, characterised in that a pickling operation is performed before or during the said phosphating treatment of the bar.

4. Process according to Claim 1, characterised in that a stable oxide layer is formed on the bar by exposing the said bar to an oxidizing gas stream.

5. Process according to Claim 1, characterised in that a layer of paint which is resistant to elevated temperatures is deposited on the surface of the bar.

6. Process according to Claim 5, characterised in that the said layer of paint is deposited by spraying.

7. Process according to either of Claims 5 and 6, characterised in that the said layer of paint is deposited on the surface of the bar immediately after the quenching stage of the said quenching and self-tempering process.

8. Process according to Claim 1, characterised in that a film of resin in liquid state is deposited on the surface of the bar.

9. Process according to any one of Claims 1 to 8, characterised in that a film of oil is deposited on the said protective layer.

10. Quenched and self-tempered steel reinforcing bar provided with a protective layer against corrosion, made by a process in accordance to any one of the preceding claims.