The main influencing factors of corrosion of OCTG pipes
PH value: Hydrogen ion is a cathode depolarizer. Generally, the lower the PH value is, the more serious the metal corrosion becomes. When the pH value is greater than 6.6, the corrosion rate begins to decrease.
Cl- or total salinity: Cl- is the main factor that destroys the passivation film, and is an accelerator that accelerates the generation of pitting corrosion pits. In many environments, for example, CO2 corrosion, Cl- is the main factor that destroys the CO2 corrosion film and accelerates the corrosion pit. The increase in salinity improves the conductivity of the liquid, and the increase in harmful ions accelerates corrosion; it is easy to produce precipitation and corrosion under scale.
Temperatures: Generally, the corrosion rate increases with increasing temperatures. The main reason is that the diffusion speed of particles increases and the reaction speeds up.
Flow rates: This is an extremely important and complex factor. It includes physical, mechanical and electrochemical content, and mainly has the following three forms of damage, namely cavitation, abrasion and corrosion.
Dissolved oxygen: The concentration of dissolved oxygen is the main influencing factor of corrosion, and the deoxygenation of drilling mud and sewage reinjection is very important.
Others: Factors such as moisture, protective film, impurities, oxygen, bacteria, etc.
Corrosion mechanism of OCTG pipes
The corrosion mechanism dominated by electrochemical corrosion: The ferrite of the steel is used as the anode, and the cathode is the carbonite with higher potential, forming a corrosion couple. The anode gradually dissolves, that is, electrochemical weightless corrosion.
Corrosion characteristics: There are partial corrosion and high perforation rates, showing pitting and table top pit corrosion for
OCTG pipes. Stress corrosion cracking is formed. Hydrogen sulfuric acid is easily ionized, and the generated H+ causes hydrogen corrosion of low strength steel and hydrogen embrittlement of high strength steel.
The roles of water in the medium
① Water is an indispensable medium for electrochemical reactions.
② Corrosion with CO2 is the corrosive effect that occurs by dissolving with CO2 in water to form acidic substances (carbonic acid and hydrogen sulfuric acid).
③ Acidic conditions exacerbate ionization and electrochemical corrosion.
Sulfate reducing bacteria (SRB) corrosion characteristics and mechanism: The
OCTG casing has been in a dead water anoxic state for a long time, and the sulfate reducing bacteria (SRB) multiply and restore the sulfate to H2S, causing severe corrosion. The manifestations are hydrogen corrosion of low strength steel and hydrogen embrittlement of high strength steel.
Stress corrosion: Because of the heavy weight of the drilling tool itself and its rotation, especially for directional well drilling, the friction force between the drilling tool and stratum or OCTG casing is very great, which means that there is a great stress. The interaction in the corrosive medium causes corrosion cracking.
Mechanical wear and man-made scratches: Partial wear or scratches under stress can become the origin of corrosion failure in particular.
Basic measures of corrosion resistance of OCTG pipes
1. Follow the standard design and material selection, and choose different materials and steel grades for different service environments. High alloy pipes like stainless steel pipes are also used in corrosive environments (the cost is 4 times more than that of carbon steel), but stainless steel pipes also have the problem of pitting corrosion of Cl-.
2. Adopt appropriate and effective protective coatings such as anti-corrosion coatings and metal plating protection technology.
3. Adopt other auxiliary methods such as adding corrosion inhibitor and closed water injection system to reduce the dissolved amount of O2, CO2 and H2S and using sulfate reducing bacteria fungicide.
4. Use cathodic protection; establish a corrosion monitoring system; grasp the dynamics of corrosion and prevent problems before they occur.
