Corrosion

Corrosion means the gradual deterioration of a material, caused by the progressive decay of its intrinsic properties, by chemical-physical interaction with the surrounding environment. For metals and steel, corrosion involves the progressive loss of the mechanical, functional and structural characteristics typical of the metallic state. Corrosion affects and conditions all human activities.

Planning of protection and maintenance

The task of stopping the advance of degradation already rests with the designer who, together with the choice of suitable materials for the construction of the work, should always take care of their protection.  Prevention is the only advisable and effective defensive strategy from a technical and economic point of view. During the selection phase of the protection with appropriate materials, techniques and design measures, it is also essential to plan the maintenance, planning the time of the first intervention and the frequency of the following ones. In fact, each anticorrosive system is characterized by its own duration, which depends on the intrinsic characteristics of the protection, on the design features and on the environmental conditions.

If scheduled maintenance interventions are lacking, corrosion may lead to the need for particularly onerous restoration procedures, to the point of determining, in extreme cases, the need for a total refurbishment of the product. The lack or insufficiency of maintenance is not always due to lack of funds, but can simply be caused by neglect and unpredictability. And this occurrence is by no means unlikely. On the other hand, there are not rare cases in which there are conditions of impracticability of repairs due to intrinsic, logistical and technical difficulties, which limit the effectiveness of the interventions. An example is the case of corroded reinforced concrete, for which structural recovery is often very complex and expensive, and which can, at times, even become impossible.  When conditions of particular application difficulties occur, the total cost of maintenance can even exceed the initial cost of construction.

In light of these simple considerations, the opportunity to choose anticorrosive methods capable of ensuring lasting protection becomes evident. The ideal choice consists of a system capable of protecting the product for its entire useful life, which minimizes or cancels the need for maintenance after its application. Therefore, the key to tackling the problem related to corrosion consists in choosing a protection that does not require particular attention from the owners or managers of the work, capable of providing total prevention, which does not involve any degradation of the structural resistances and is not characterized by particular maintenance needs.

The consequences of corrosion

• The economic damage:   despite the fact that there has been an increase in sensitivity towards the problem of corrosion protection for some time, however, the order of magnitude of the figures regarding costs remains very high. The corrosion phenomenon involves direct damage estimated at tens of billions of euros per year (40,000 billion of the old lire according to a research from a few years ago) in Italy alone.
• The social damage:  the part that can be monetized, considered in the calculation, is only a fraction of the total: the direct damages must be added to the damages for the reduction of the efficiency and usability of the works.  For production plants, even simple maintenance interventions may require long downtime with consequent considerable economic damage. The social cost originating from the interruption or even the limitation of the service of civil installations, such as public and private buildings, theaters, schools, sports and recreational facilities, architectural works of cultural interest is evident. The consequences on the artistic heritage are simply incalculable, when corrosion literally causes the dissolution of valuable works, as happened to the torch of the Statue of Liberty. The original work was replaced in the 1980s  The effects of corrosion on safety can be dramatic or even tragic: corrosion implies a reduction in the mechanical strength of the steel, with an evident effect on the safety and reliability of the structures built. Particularly interested in the phenomenon is the steel used as a load-bearing structure for constructions and buildings. Cases of major structural failures with tragic consequences are quite rare, but unfortunately not unlikely. Serious pitfalls can also be present in less heavy installations such as, for example, furniture, lighting poles, fences, etc.   etc. in which the corrosion damage can progress in a much less conspicuous and controllable way, to the point of causing its collapse.

All this makes it necessary for the designer or anyone who has decision-making power on the future of the work to have a cultural background of knowledge of the causes and mechanisms by which corrosive processes evolve. Only in this way is it possible to distinguish the most suitable of the technological solutions available to combat corrosion and reduce its risk.

The causes and the mechanism of corrosion

Steel, an alloy of iron and carbon, is obtained from the iron and steel extraction of the metal from oxidized species such as natural minerals or rusty scrap. This process requires energy, which is supplied to the system in the form of heat.

Steel is, therefore, in a state in which free energy is greater than the materials from which it is obtained. This condition generates thermodynamic instability: the metal spontaneously tends to return to the oxidized form of origin. Corrosion corresponds to the dissipation of the energy accumulated by the steel during the iron and steel process and occurs with spontaneous reactions with irreversible damage. The phenomenon, in addition to being favored by thermodynamics, has a commonly fast kinetics in normal environments of use, due to favorable temperature conditions and the presence of activating substances such as acids and pollutants.

Corrosion progresses with electrochemical mechanisms

From a conceptual point of view, the corrosion process coincides with the functioning of a galvanic element in short circuit, in which areas destined for oxidation and areas where reduction takes place differ. For the reaction to take place, the simultaneous presence of an oxidizing agent (oxygen in the air) and a conductive solution formed by electrolytes (salts) and pollutants (acids or alkalis, inorganic or organic compounds) is required. These conditions are normally encountered, given the common presence of water vapor condensation (in addition to brackish aerosols near the sea), in which the soluble parts of dust and chemical species produced by pollution dissolve.

In a neutral environment, the cathodic half-reaction of electron consumption is given by the reduction of dissolved oxygen in the condensation that forms on the surface of the metal. H2O+½ O2+2e-®2(OH-) cathodic reduction half-reaction. The metal dissolution half-reaction can be schematized simply with Me®Me +++ 2e-anodic dissolution half-reaction. In corrosive processes, different areas of the same metal surface can act as anodes – with oxidation of the metal to M ++ ions and simultaneous release of electrons – or as cathodes – sites of the reduction reaction, in which the aggressive environment acquires electrons.

Let’s observe what happens when a drop of water falls on the steel: in the presence of pollutants commonly present in the atmosphere (such as SO2 and NOx), reactions are triggered that lead to the deposition of rust in the center of the drop, while corrosion is less evident at the edges. . The oxygen present in the air diffuses inside the drop, meeting in solution the metal surface which, in the outermost area, acts as a cathode, provides electrons for the reduction reaction and remains immune from corrosion. At the same time, in the center of the drop, where oxygen has a much lower concentration, an anodic zone is created where the metal releases iron Fe ++ ions in solution and ensures the supply of electrons to the process. In summary, the two anodic and cathodic semi-reactions involved in the corrosion of steel can be summarized in the overall reaction: H2O + ½ O2 + Fe®Fe (OH) 2 By the action of other oxygen, ferrous hydroxide Fe (OH) 2 oxidizes to ferric Fe (OH) 3 which precipitates around the anodic zone. Finally, the atmospheric CO2, dissolved in the drop, reacts with a part of these hydroxides forming the respective carbonates. The result of this complex phenomenon is rust – a mixture of hydroxide, oxide and iron carbonate.

In reality, the typical generalized corrosion of steel is described by the model of the mixed electrode – a metal surface in which the anodic dissolution of the metal and the simultaneous reaction of cathodic reduction of oxygen takes place on uniformly dispersed micro-areas. The phenomena described occur on a microscopic scale and are so closely interpenetrated as to allow a uniform deposition of corrosion products. It is the typical appearance of a rusty piece of iron. The layer of rust that forms on the surface of the iron cannot protect the underlying metal against aggressive external agents because it is porous and not very compact. Therefore, in the absence of appropriate protective measures, the corrosion once started can continue until the complete anodic dissolution of the iron. Corrosive processes are therefore favored by the presence of agents that improve surface conductivity or contribute to the exposure of activated metal surfaces.

This is why the presence of humid air and salts (present in dust or other atmospheric suspensions) is obviously harmful and the destructive action of acids is evident. The differentiated aeration conditions, in which the oxygen concentration is different from point to point, also favor corrosion, since the oxygen concentration gradients differentiate between cathodic and anodic areas. Examples of this are the very strong corrosion that is found in the outcrop points of partially buried structures. Also consider the acceleration of degradation that is evident if two metal surfaces overlap. In this case, in fact, there is a difference in oxygen concentration between the outside (part exposed to the air) and the inside (underground part or cavity, respectively).

Galvanic corrosion

Interesting is the case of galvanic corrosion caused by coupling with direct contact between two different metals: metal materials do not all show the same tendency to oxidize. This depends on the greater or lesser propensity they have to release electrons, or to dissolve them in solution as ions.

Electrochemical potentials:   to have a clearer idea of the phenomena involved in this type of phenomena, we must necessarily refer to the standard reduction potentials, which are indicated in the series shown in tab. 1.1. They are the result of complex formalizations of the electrochemical properties.  

For a very simple reading of the table, we can state that metals that have a lower potential are characterized by a greater drive to the transfer of electrons, or oxidation. This, in qualitative terms, implies that, if two different metals are brought into contact in the presence of conductive condensate, the one that is characterized by the lowest standard reduction potential corrodes, that is, it releases electrons, faster. This is also expressed by saying that metals with a higher potential are more noble, they remain reduced in contact or do not corrode. It is clear that the contact between two metals determines a situation that is all the more disadvantageous for the less noble the more the metals are distant in the series, that is, the greater the difference between the potentials.

The galvanic coupling effect can reserve nasty surprises since it can critically accelerate the corrosion of the less noble metal parts in contact between two metals. Particularly harmful are also the current dispersions – the so-called stray currents – which can determine anode potentials capable of causing the acceleration of corrosion. Bad electrical insulation of underground structures in industrial or urban sites, incorrect grounding installations in buildings, the use of certain electromechanical equipment and contacts of various kinds, can determine the anode actions of direct electric currents in the ground, the conduction of which is favored by the presence of humidity and salts present in the soil.