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ABOUT CORROSION

INTRODUCTION

Pipelines, tanks and other structures are protectedpreventively from corrosion by isolation. However, even the smallest damage of the isolation leads to corrosion. The effect of corrosion is evident in the mass loss of metal resulting in huge economic and ecological damages.

CorrosionCorrosion

LOWER LIFESPAN

The most frequent situation in practise is that the pipelines are endangered by synchronous interaction of negative influences. These could be aggressive substances in the soil, stray currents due to electrified transist, alternating current from the high voltage transmission lines, contact with metal having a different contact potentials etc.Each of these leads to the showing sings of corrosion, drill failures and also lower lifespan of the pipelines. The expected lifespan of a pipeline network is about 50 years, however, due to intensive corrosion, that period of time could be reduced to only a couple of years. Through application of cathodic protection and drainage systems, it is possible to stop corrosion completely and prolong the lifespan of the pipelines for several decades.

ELECTROCHEMICAL NATURE OF CORROSION

Corrosion

Being in contact with the soil which contains some proportion of humidity, mineral substances and oxygen, a metal construction undergoes an electrochemical reaction, starting to decompose according to the following reaction scheme:
(One of possible reactions)
The equation presents the conversion of iron into iron hydroxide, after being in contact with water and oxygen, resulting in damage of the metal construction.

STRAY CURRENT CORROSION (DC INTERFERENCE)

Corrosion

Stray current occurs due to electrified transit, such as a DC powered trains. As a consequence, a part of the current which flows back in the rails streams, as stray current, into the soil and pipelines. Due to the polarity of the railway feeding-in (positive pole at overhead electrical line and negative pole at track), stray current leaves mainly close to transformer substations and causes there a loss of material that leads soon to pitting corrosion at a speed of up to 10mm/per year. The intensity of corrosion depends on many factors: on isolation of tracks, on isolation of the pipeline network, on specific ground resistance, on electric load etc.

 

 

AC INDUCED CORROSION (AC INTERFERENCE)

Corrosion

Alternating current creates a magnetic field under which an induction of voltage on the pipelines occurs, resulting in enormous corrosion damages at the pipelines at the spots of reduced isolation resistance, at spots where the current leaves the pipe on its way to the ground. At that spot, the current makes a perforation if Jac is greater than 30 A/m2 (EN12954). The magnitude of the induced AC voltage depends on many factors: geometry and the interacting position of high-voltage power cables and pipelines, electric loads of power cables, load of different phases, isolation resistance at a pipeline, specific ground resistance. In general, the greater the electric load is, the longer the route in relative parallel is and the better the pipe isolation is, it is more certain that momentous voltage will be induced on a pipe.

THE CORROSION OF REINFORCING STEEL IN CONCRETE STRUCTURES

Corrosion

Concrete is a porous material, which absorbs contaminates from the surrounding environment. The corrosion of reinforcing steel in steel-reinforced concrete and pre-stressed concrete structures is a consequent of presence of Oxygen, water and Chlorides in the form of salt. The growth in volume of corrosion products leads to tensile stress on concrete and cracking of covering concrete layer.