DIAGNOSIS OF DAMAGE OF METAL STRUCTURES UNDER THE INFLUENCE OF CORROSION IN THE REGION OF DJIJEL
Mohammed-Amin BOUMEHRAZ
Civil engineering department, Faculty of sciences and technology.
University of Biskra.
amine18gc@yahoo.com
Mekki MELLAS
Faculty of sciences and technology.
University of Biskra.
Abstract: The region of Djijel is characterized by a Mediterranean climate, its winters are cold and rainy unlike its autumns. It is also famous for its dry summers with a temperature ranging between 20 °C and 35 °C in summer, and average relative humidity of the air is high and can reach about 70%. Several physical and chemical damages were observed on the metal structures implanted in the Djijel region, these changes have directly affected their sustainability, such as: corrosion of steel profiles or reinforced concrete frames, cracks or/and coating bursting under the effect of alkali-aggregate reactions or corrosion of steel. In this study, we show some pictures from damage caused by corrosion of metal structures, then we explain the main causes of corrosion of steel profiles or degradation of reinforced concrete, and finally we propose some solutions or recommendations to avoid or reduce this structural damage.
Key-words: Durability, corrosion, metal structures, humidity.
1. INTRODUCTION
Djijel is characterized by a Mediterranean climate, it is characterized by cold winters and little rain unlike the fall, including precipitation that reaches about 1200 mm / km.
Summers are dry, with temperatures ranging between 20 ° C and 35 ° C in summer. The region gets wet between December and January, the rate of rainfall between November and January can reach 57% of the rainfall of the year. The duration of sunshine is about 3,000 hours per year, and the average relative humidity is higher, which may reach about 70% [1]. The catchment areas of Djijel, with the exception of the coastal zone, are characterized by a dense river network, and low coverage and low permeability geologic, which promotes surface flow of rainwater, we can mention the main ouedsof the region, and which are from east to west: OuedZhor, ElkebirWadi, Wadi Nile Djendjen, Mencha, Kissir, Bouchayed, Taza and Guelil [2].
These characteristics of the study area negatively affect the sustainability of metal constructions and / or mixed constructions, which are carried out to see the underground structures, bridges, factories, ... .etc.
Several physicochemical damages were observed on the metal structures or composed structures serving in Djijel.
Generally, these structures cannot withstand aggressive chemicals from the external environment, these changes have directly affected their sustainability, such as: corrosion of steel profiles or frames of reinforced concrete, cracks or / and bursting coating of concrete under the effect of alkali- aggregate reaction and corrosion of steel, they can destroy the adhesion between aggregates and cement pulp. [3]
2. CORROSION MECHANISM OF STEEL
Generally, the corrosion of a metal, means the conversion of the metal (iron element) in various compounds (oxides and hydroxides) in a chemical action (atmospheric oxygen action in the presence of water, oxygen, salinity pH, carbon dioxide), physical (current velocity, temperature, pressure, solid particles in water
(sand,...) or biological (plant or animal biological fouling, bacteria ....) [4]. Steel corrosion with rust has two phases (see Figure (01)):
The first phase is the incubation phase, in this phase the aggressive agents such as carbon dioxide (CO2) or chlorides (Cl-) present in the surrounding environment, they are stayed on the profile surface steel, and / or they have penetrated into the concrete to the reinforcement.
finally, the corrosion propagation phase which begins when these aggressive bodies are present in fairly high concentrations in steels, this phase corresponds to the growth of mildew, and the dissolution of metal, which may then cause the bursting of the cover concrete for composite structures. [5]
Figure 01: Corrosion mechanism of steel metal structures.
2.1. Mécanisme de la corrosion (phénomène électrochimiques)
La corrosion des aciers, selon le phénomène électrochimique implique l’existence d’une anode, d’une
cathode et d’un milieu électrolytique pour constituer une pile [6], la réaction anodique ou la réaction de dissolution du métal suive l’équation (01) :
Fe → Fe 2+ + 2 e- (01)
In the alkaline medium, the ions released by the anodic reaction consumed at the cathode by reduction of oxygen according to equation (02), the ferrous ions then react with the ions (OH-) to form ferrous hydroxide slightly soluble according to the equation (03): [7]
H2O + ½ O2 + 2 e- → 2 OH- (02) Fe 2+ + 2 OH- → Fe (OH) 2 (03)
The reactions proceed in the presence of oxygen to Fe(OH)3 and Fe2O3, and slow oxidation leads to Fe2O3.H2O hydrated oxides and magnetite Fe3O4. Rust is then formed a mixture of these three constituents [8]. The most favorable relative humidity so that the corrosion occurs is 70% to 80%, and even more than 80%, the oxygen diffusion is significantly reduced but the rust steels increases, generally, no rust formation when the concrete element is totally immersed in the aerated water and the influence of temperature being significant at above 10 ° C [9]. Photos (01) and (02) show some examples of corrosion of metal structures under the effect of high humidity in the area east of Jijel.
Photo 01: Corrosion of metal walkway Milia, Jijel.
Photo 02: Corrosion and partial destruction of metal walkway in el-Ancer, Jijel.
2.2. Attack by chloride ions
The steels of the corrosion rate is higher in the presence of chlorine ions, the latter reach the superficial layers of steel profiles and / or concrete reinforcement in sufficient quantities, these steels will then be depassivated and corrosion will start in oxygen then spread. In the case of very low concentrations of chlorine ions, barely exceeds 0.01%, alter the morphology of the passive layer to give the compound Fe(OH)2 and (OH-) ions are consumed by the equations (04) and (05) [8]. Photo (03) has the corrosion profiles beams a metal factory cork Ouled Salah - Jijel under the high humidity effect was caused by evaporation of the water used for production.
Fe + 3Cl- → Fe Cl3- + 2 e-(04) Fe Cl3- + 2 OH- → Fe (OH) 2 + 3Cl-(05)
Photo 03: Corrosion of profiles beams plant cork Ouled Salah- Jijel.
The electrons released by the oxidation reaction move through the metal to the cathodic sites. From these reactions, the process leads to a decrease in pH and a recycling of the chloride ions [9]. Hydrolysis of the latter result in the acidification of matter and the insoluble ferrous hydroxide formation whose evolution gives rust, thereby forming the swelling of the concrete, and it causes disintegration of the coating, or by bursting cracking [3]. Photo (04) has the coating of reinforced concrete bursting under the effect of corrosion of steel of a composite bridge in the city of Jijel.
Photo 04: Concrete frames of corrosion and the coating bursting concrete of a composite bridge in the city of Jijel.
3. CAUSES OF CORROSION
In this section we include the main causes of corrosion of steel profiles and / or reinforcement of the concrete used for the manufacture of metallic and composite structures.
3.1. Causes due to climate impacts
The wet corrosion occurs in wet, that is to say when the relative humidity exceeds 60%, these environments are neutral, acidic or alkaline. We note that the relative humidity of the Djijel region reached more than 70% in the winter months, generally the steel corrosion is accelerated in the relative humidity is between 70% - 80%, because of the presence of adequate oxygen percentage. [8]
Temperature plays an important role, said that the corrosion rate doubles substantially between 20 °C and 40 °C, to a moisture content greater than 80% [8], especially the Jijel region's average temperature between 20 °C and 35 °C during the summer months.
3.2. Causes due to the implementation
Lack of implementation of reinforcement, or errors in the provision of steels.
Incorrect assembly of steel profiles of different shapes, are: for beams, columns, slabs ... .etc.
The mixing too short, and / or a fast pouring concrete negatively affects the compactness of the concrete, and then the mechanical compressive strength.
The absence of formwork spacers may cause coating failure.
A too short or too powerful vibration can cause inhomogeneities, and also a too strong vibration above major diameters of bars placed too near the surface induces early cracking. [10]
The higher temperature and moist cure short accelerate the rate of corrosion of reinforcement of reinforced concrete. [8]
3.3. Causes due to maintenance
Poor protection of steel structures because of wrong use of paints, especially the assembly areas or nodes.
Lack of maintenance after commissioning for bridges and steel bridges, made in areas of high humidity or the presence of areas of wadis.
The brushing the concrete foundations of metal structures submerged in the waters of Oued by a single layer of bituminous paint, allowing the penetration of chloride ions and reinforcement corrosion of reinforced concrete.
4. PROPOSED PRECAUTIONS
The protection against corrosion of a metal frame can be obtained either by an anticorrosion coating, or by weathering steel or stainless steel use to increase the lifespan and avoid the risk of corrosion. Generally, protective coatings applied by paintings are composed of a base layer, single or multiple, which serves protection against corrosion (rust paint) and liaison with the topcoat, these coatings are characterized by a seal , hue, brightness, texture and resistance to external influences.
And as protective coatings applied by dipping (hot galvanizing), this method allows to treat hard to reach places or areas of joint arrangements by bolting. [11]
areas Repair corroded metal structures by the use of antifouling paints, generally, before application of different paints, clean corroded parts. This cleaning is done in several steps and with the use of a pressure washer (max 500 to 1000 bar), which removes all the concretions and some of the corrosion. Then you have to sand the surface, which turns the iron to "white" and
give roughness to the surface. When this step is done, it must immediately make the paint to prevent the metal oxide and thus that painting no longer has any use. [4]
Provide maintenance every year, especially for metal structures made to areas of high relative humidity.
Making a concrete good mechanical resistance to compression or by ensuring: a good mixing of concrete, good vibration so good compactness and low porosity, and also the moist cure,...etc.
5. CONCLUSION
We conclude from this study that the metal constructions made in the Djijel region are short-lived, so poor durability, due to high humidity or reached over 70%. Generally, reinforced concrete structures (bridges, buildings, art works,....etc.) Were performed in the Djijel region, is increased durability compared to metal structures or / and mixed, so the realization of structures reinforced concrete is better choice due to a long service life.
6. REFERENCES
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