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PROCESSES: SUPERFICIAL PROTECTION OF STEELS AGAINST OXIDATION AND CORROSION
1.Oxidation.
The layer of scale forms continuously during the process of hot lamination. The layer of rust (secondary) is eliminated just before the sheet enters the rolling mill. Nevertheless, the time that goes by between the pickling stage and the first roller (approx. 5 seconds) is enough to give way to the formation of another layer of rust, the tertiary, which is unavoidable. Its properties, depending on the chemical composition of the steel and the conditions of the process, have a great impact on the final superficial quality of the steel. The customers requirements regarding the properties of the layer of scale are especially strict with the aim of guaranteing a suitable answer of the material in subsequent steps of the process such as pickling, stamping or direct coatings. Studies carried out by qualified staff from CTM on the growth and mechanical behaviour of the scale layer show that its properties can be controlled up to the point that its behaviour does not have to be the expected one, that is to say, fragile and abrasive (detrimental for the determination of aspects such as the lamination rollers) but in suitable conditions, it could present characteristics such as ductility and great adhesiveness and evenness to steel; this would bring about many advantages i.e. reduction of the pickling costs, increase of the productivity, etc.
This monitoring of the propeties takes on a higher relevance when it comes to the oxidation of special alloys such as steels with a high Si content. This kind of steel has the inevitable drawback of forming at high- T (1170ºC) of Fe2SiO4 (fayalite). This drawback is related with the so-called catastrophic oxidation of steels (Si increases the kinetics of oxidation above 1170ºC and delays the ones below). These studies are being carried out in collaboration with the Universidad de Gante (Belgium), the Universidad Autónoma de Nuevo Léon (Mexico) and the University of Freiberg (Germany).
schematic representation of the hot lamination process and the forming of scale.
2. Galvanizing process.
Given its versatility and low cost, steel is the material most used worldwide. Therefore, it constitutes a determinant factor in the development of industry. Its main handicap is that in the presence of aggressive atmospheres, a high level of corrosion may appear. It is for this reason that steel has to be conveniently protected with a proper coating. These coatings have to present excellent forming properties and a good adhesion to the steel substrate. The processes of transformation such as galvanizing make it possible to offer global, agile and versatile solutions which are adapted to the global and changing environment in which we find ourselves.
The galvanizing coat presents a series of advantages that we cannot find in other kinds of coatings. Low cost against usefull life, low level of corrosion, coating metallurgically joined to the steel, easy to check, great resistance to mechanical damages. The coating of steels by immersion in zinc is a very effective technique that has been used for more than 150 years. Zinc on its own has a very good corrosion resistance through the formation of protective superficial layers against oxides and carbonates. The zinc layer that forms on top of the steel protects it in two different ways: barrier protection and galvanic protection. In the first case a physical barrier forms itself between the steel of the substrate and the environment. Galvanic protection makes it possible for steels products to remain for decades without corrosion. This explains why zinc acts as a sacrificial anode in the presence of humidity and prevents steels from oxidating.
The zinc coating processes are varied according to their practical application. By way of illustration we can mention the continuos hot dip galvanizing or batch (galvanizing of pieces once they are formed) and thermogalvanizing. The galvanizing process is affected by different variables which can have a considerable influence on the appearance and surface quality of the final product. CTM accumulates an extensive experience in the study and characterisation of the processes of galvanization as well as in the proper identification and microstructural characterisation of the coatings optimising this way to the maximum the surface quality requirements of the customer. The most recent works have been carried out with the University of Gante (Belgium) and companies such as GONVARRI.
Examples of corrosion and oxidation in steels pieces.
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