DIAGNOSIS

Corrosions

Reinforced concrete

Buried structures

Corrosion

Corrosion is the destruction of metals occurring during chemical or electrochemical reactions when they are in contact with an electrolyte.

The equilibrium of metal in the electrolyte is the following :     Fe <=> Fe++ + 2 e-

It corresponds to a certain potential difference between the metal and the electrolyte in its contact. Every destruction of this equilibrium, caused by the departure of electrons from the metal, leads to the corrosion phenomenon.

Practically, these corrosion phenomena can be due to :

Contact between the metal and its environment, which is particularly the case of metal structures contacting any heterogeneous electrolyte, such as unclean concrete, some liquids or ground;

Galvanic couples due to the association of different metals;

The influence of stray currents in the vicinity of other current sources, the most frequent example being an electrified railroad.

Some figures : Illustration of the importance of corrosion :

A 1 Ampere current leaving the metal corresponds to the dissolution of 10kg Iron per year.

Corrosion mitigation costs 2 to 4% of the GNP of industrialized countries every year.

30% water is lost due to leaks caused by corrosion.

To tackle these corrosion phenomena, SEM has a full team of specialists in Belgium and Grand-Duchy of Luxemburg.

The first step of our intervention consists in making a diagnosis, for reinforced concrete structures, for buried structures, for tanks as for other metallic structures.

Reinforced concrete

Steel rods in a sound reinforced concrete are spontaneously protected against corrosion. This is due to the high alcalinity  of the environment, allowing the formation of a protective passive film on the rods. However, this passive film may disappear when the concrete cover is altered (infiltration of water containing chloride ions, for example), and may let the corrosion spread. Corrosion reduces the cross-section of rods and increases their volume by a factor of 3 to 7, generating spalling and cracks, which on their turn allow the infiltration of other aggressive elements, accelerating the degradation process.

Thus corrosion directly depends on the concrete humidity, ambient temperature, chlorides diffusion and presence of oxygen. These factors are linked to the cover thickness, but even with a sufficient cover, chlorides may penetrate into the concrete and generate corrosion.

Some effects of this corrosion can be easily visually detected: rusted rods causing concrete burst, falling debris, the decrease of the useful rods cross-sections, ... However, these are only the visible degradations. To determine the real extent of damages, SEM uses a patented technique of corrosion diagnosis, specially adapted to reinforced concrete, which allows :

Knowing the degradation progress ;

Better defining and planning the nature and volume of necessary repair works ;

Eliminating the risks of seeing the degradations reappear, if only a part of damages are treated.

The basic principle of the diagnosis consists in measuring a potential difference between the network of rods and a reference electrode laid on the concrete facing. Collected data are then processed by computer software, to obtain a color scan of the corrosion status of the concrete.

Following this diagnosis, we are able to propose you remediation solutions, either by cathodic protection or by specific coating of rods.

Buried structures

The causes of corrosion of buried structures are numerous: metal heterogeneity (welds, steel bolting used on cast iron flanges, ...), piping passing through soils of different natures, presence of stray currents, chemical corrosion due to the presence of severely corrosive products, ...

The risks associated to this type of corrosion are important :

Risks of undetected product leaks, corresponding to an economical loss for the operator, and possible risks of explosion and pollution ;

Embrittlement of metallic elements of the structure ;

Increase of costs of monitoring and maintenance, replacement and operations.

Our corrosion diagnosis system allows the corrosion qualification and quantification :

Qualify each zone of the concerned metallic structure (zones of current exit, or the so-called "anodic zones"),

"Proportionally" quantify each corrosion situation to locate the high risk sectors.

To do this, we continuously measure one electrical tension, and one electrical resistance :

The surface electrical tension: the tension between the piping and the ground surface, measured with a reference electrode;

The surface tension is periodically switches on and off, each cycle during a few milliseconds: this allows determining the extent of the damages on the piping ;

The transfer impedance: measurement of the electrical resistance between the piping and the ground surface.

Once the diagnosis is complete, results can be used to protect the locations at risks, particularly with a cathodic protection.

S.E.M.

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