With a gaseous corrosion medium a material reacts to the chemical corrosion without any involvement Electrolytes to a Metal oxide. This corrosion process already takes place at room temperature, but is accelerated by high temperatures. A is created on the metal surface Oxide layer, the z. B. must be removed before soldering a copper pipe. When steel pipes are hot-bent, tarnishing occurs and a layer of scale forms, which can be attributed to chemical corrosion processes.
Also on the surface of PVC pipes creates a Oxide layerwhich must also be removed by peeling off or by applying solvents before these pipes are glued.
On Aluminum and copper surfaces Oxidation creates a desired, dense and solid protective layer that must not be destroyed by external influences, because otherwise the material will be corroded and destroyed.
Biochemical corrosion (biocorrosion)
The biogenic sulfuric acid corrosion are in Sewage systems known for decades. Because of the sharp decline in water consumption and the associated waste water runoff, this issue is becoming more and more explosive. But also in Heater-, solar, Cool- and Drinking water systems is the Bio-corrosion beautiful always been a problem, only it was neglected or ignored and was accepted as "God given" or normal. Here one also speaks of "Biofouling".
In the Pipe material decision in Sewage systems knowledge and experience of flow and transport processes and the interactions between runoff and pipe material are important. The only thing that helps here is regular maintenance of the ducts and the removal of deposits biological and biochemical processes initiate by high pressure cleaning.
Growth of microorganisms
The biogenic sulfuric acid corrosion takes hold cementitious materials and those with Cement linings at. This is a necessary prerequisite for biogenic sulfuric acid corrosion Sulphide evolution. Have a decisive influence organic pollution of sewage. The Sulphide evolution by the wastewater temperature, the discharge conditions and by deposits in the sewer network.
bacteria are also able to use in Heating, Drinking water-, solar and Cooling systems to exist because in every water there is life. Everything biological in a plant is called a bacterium, even if it can be fungi or algae.
Bacteria need a source of energy. This can be heat or (which is preferred by them) a certain chemical reaction. They nest in bumps or pores to form colonies there. Above all, they prefer oily surfaces, plastics and hard coatings. Bacteria require a certain constancy of living conditions, especially at the beginning of settlement.
Sulphate-reducing bacteria Sodium sulphite, which was previously often used to bind oxygen in heating and cooling water, combines with oxygen to form sodium sulphate. Since a certain amount of oxygen is to be expected constantly, the oxygen binding agent has to be added continuously. That has three consequences
|• The electric conductivity of the heating water increases, which favors the flow of the corrosion current|
|• The Sulfate concentration increases, which can lead to the formation of gypsum crystals if there is hardness in the water|
|• The water may tip over under certain circumstances. In this context, this means the formation of hydrogen sulfide. The water acidifies, smells and is poisonous|
The polluters are sulfate-reducing bacteria, the sulfate to sulfide convert. This creates oxygen, which leads to the oxidation of metals. Temperature and pressure play almost no role for the bacteria. For this reason, sulfite is now only used as an oxygen binder when it is under constant observation.
A Bacterial colonization (Stratification) is able to ye chemical environment largely self to determine. Even if the PH value of the water z. B. 9 is can be on the surface of the metal or plastic one PH value of 4,5 (acetic acid) are available. The In these gelatinous layers arise biological processesthat can be so diverse that they are practical Not to dominate are. Treatment with toxic chemicals and other biocides is also unsuccessful in most cases because they only work on the surface. The bacteria form in the underlying bacteria Resistances and can absorb and process the products created by the means.
|The Legionella and E. coli bacteria (Coli bacteria) are only relevant in drinking water installations.|
The Colibacterium (Escherichia coli - E. coli) has acid-forming properties and occurs in human and animal intestines and has an important function in the immune system. There is also harmful coli bacteria which produce toxins and can cause infections in the presence of immunodeficiency. Such intestinal infections are under the name EHEC colitis known.
React with electrochemical corrosion two different metals or metal compounds with a electrically conductive liquid (electrolyte). This type of corrosion occurs in drinking water installations, among other things. when processing copper and galvanized pipes that are integrated in a circuit (circulation line) or that are in direct contact. But the alternately arranged different metals can also be bad in heating, solar and cooling systems, since in these systems it is always a question of cycles.
Such an arrangement creates a galvanic element. A closed electrical circuit is created here. The atoms of the less noble metal (e.g. zinc) give off electrons and go into solution as positively charged ions. The less noble metal is decomposed in the process. The resulting excess electrons flows through the conductor to the more noble metal (e.g. copper).
The potential difference between the different metals is called tables or diagrams electrochemical series shown. The metals are classified according to their electrical potential (difference in charge compared to hydrogen). Hydrogen has the value “zero”. The more negative the potential of a metal compared to hydrogen, the less noble it is and the more positive its potential, the more noble it is.