Definition - What does Subsurface Corrosion mean?
This is characterized by the formation of isolated particles of the products of corrosion beneath the corroding metal surface. This is due to the reaction of some constituents of the alloy with inward diffusion of sulfur, nitrogen and more so, oxygen.
In the tubing medium, corrosion of any given metal must be controlled. The control helps to withstand the aqueous phase of hydrocarbons and dissolved acid gases, which include CO2 and H2S, and other salts like the chloride ions.
There are probes that can be used to detect subsurface corrosion under a conductive material. This is used especially in the inspection of a lap joint on the skins of aluminum airplanes, inspection of the lap joint on aluminum sheets and the detection of corrosion under thick lap joints of aluminum.
Corrosionpedia explains Subsurface Corrosion
There are several factors that cause subsurface corrosion. These include the presence of acid gases as well as bacteria, acid treatment of the wells, erosion problems, under-deposit attack and galvanic corrosion cells that result from the selection of a mismatched metallurgy.
It is easier to safeguard a system because all that is needed is to capture and manage the system parameters. These parameters include the temperature, pressure, compatibility and the composition of the fluid. The mitigation options that can be used include the carbon steel and chemical treatment selection and implementation of coatings and linings.
The corrosion of the external surface of a well casing happens due to the electrochemical cell occurrences along the casing length. Such cells arise from the differences in the oxygen levels and variations of the temperature, as well as different soil strata. The control of subsurface corrosion is vital to avoid the risk of metal loss. Down-hole materials have to meet the resistance to corrosion and the criteria for mechanical resistance. Certain steps have to be followed for the selection of the material, incorporation of the environmental analysis, calculation of the rate of corrosion and the selection of material based on the limits that have been established.
There are benefits that come with the use of nonmetallic coating. This includes the ability to achieve targeted water injection rates to deliver clean water that is free from corrosion byproducts. This eliminates souring and plugging effects.