Corrosion and principles of cathodic protection:
Metals exist in their stable state as oxides or ores. Therefore, the metallic state tends to return to the stable state, that is, the oxide state. The phenomenon of metal returning to the metal oxide state is called corrosion. Corrosion is an electrochemical process in which electrons are exchanged.
The general form of oxidation or corrosion reaction is as follows:
M → M+ + e-
The generated electron moves on the surface of the metal and reaches the cathode areas (areas that need electrons in their reaction). In acidic electrolytes, the cathodic reaction is accompanied by the release of hydrogen gas.
2H+ + 2e- → H2
Corrosion can be briefly defined as the destruction and destruction of metals due to environmental factors. According to the laws of thermodynamics, no factor can completely prevent the corrosion process. For this reason, it is possible to use methods related to postponing or reducing the rate of corrosion, which is known as corrosion engineering. In fact, the role of the corrosion engineer is to reduce the corrosion rate of the structures in contact with the electrolyte.
The damage caused by the corrosion of metal structures in contact with the electrolyte can be divided into two direct and indirect parts, some of the direct damage of corrosion are: destruction or destruction of bridges, explosion of pipelines of oil and gas products. , the destruction of ship bodies and vessels, the destruction of oil platforms, etc., the indirect damage of corrosion can include environmental pollution, loss of life, wastage of products and exorbitant maintenance costs. Based on this, monitoring and checking the corrosion process (corrosion monitoring) is necessary and necessary. In short, corrosion monitoring is: measuring and checking the corrosion process based on a written and recorded plan and predicting the damage caused with the aim of helping to outline the corrosion process and finally predicting the necessary measures to control and prevent the effects Its harmful.
Due to the vastness and abundance of different facilities, processes and equipment in the vicinity of the structures buried in the electrolyte, it is very complicated to detect the corrosion process in them. Obviously, it is necessary to use appropriate tools in different environmental conditions in corrosion analysis and as a result, take effective measures to reduce the corrosion rate.
Corrosion monitoring equipment such as: DCVG, C-SCAN, CIPS, CORROSION COUPON, etc. are used to check and diagnose the efficiency of cathodic protection systems.
Corrosion protection of structures in contact with electrolyte is primarily done by coatings, and cathodic and anodic protection systems are complementary to coatings, as a result, the quality of coatings is of great importance, and in this regard, inspections The relevant should be done. Considering that the coatings may be applied in the relevant factory or at the construction site, their inspection method is also different, the most common inspection method of the coating is Holiday Detector, considering that the said test does not determine the adhesiveness of the coating and It only shows the cracks in the coating, the visual inspection of the coating should also be done.
Cathodic protection can be implemented in two methods: sacrificial and current injection. Cathodic protection in a sacrificial way is actually the formation of a galvanic cell. Cathodic protection by current injection method is such that the structure under protection is connected to the negative pole of the rectifier and the anodes are connected to its positive pole. Unlike sacrificial anodes, current injection anodes do not need to have active or anodic properties compared to steel, and even to increase the life of the cathodic protection system, it is necessary for current injection anodes to have good corrosion resistance, for this reason, active metals are rarely used as anodes. Current injection is used. Therefore, if the current application anodes are directly connected to the pipeline, in fact, the current supply source causes the aforementioned anodes to play an active role and apply current.
