Despite the inherent corrosion resistance of stainless steel pipes, stainless steel pipes installed in marine environments experience different types of corrosion during their expected life.This corrosion can lead to fugitive emissions, product loss and potential risks.Offshore platform owners and operators can reduce the risk of corrosion by specifying stronger pipe materials that provide better corrosion resistance.Afterwards, they must remain vigilant when inspecting chemical injection, hydraulic and impulse lines, and process instrumentation and sensing equipment to ensure corrosion does not threaten the integrity of installed piping and compromise safety.
Localized corrosion can be found on many platforms, vessels, ships, and piping in offshore installations.This corrosion can be in the form of pitting or crevice corrosion, either of which can erode the pipe wall and cause fluid release.
The risk of corrosion is greater when the operating temperature of the application increases.Heat can accelerate the destruction of the protective outer passive oxide film of the tube, thereby promoting the formation of pitting corrosion.
Unfortunately, localized pitting and crevice corrosion can be difficult to detect, making these types of corrosion more difficult to identify, predict and design for.Given these risks, platform owners, operators and designees should exercise caution when selecting the best piping material for their application.Material selection is their first line of defense against corrosion, so getting it right is important.Fortunately, they can choose using a very simple but very effective measure of localized corrosion resistance, the Pitting Resistance Equivalent Number (PREN).The higher the PREN value of a metal, the higher its resistance to localized corrosion.
This article will review how to identify pitting and crevice corrosion and how to optimize tubing material selection for offshore oil and gas applications based on the material’s PREN value.
Localized corrosion occurs in small areas compared to general corrosion, which is more uniform on the metal surface.Pitting and crevice corrosion begin to form on 316 stainless steel pipes when the metal’s outer chromium-rich passive oxide film ruptures due to exposure to corrosive fluids, including salt water.Chloride-rich offshore and onshore marine environments, as well as high temperatures and even contamination of the tubing surface, increase the potential for degradation of this passivation film.
pitting.Pitting corrosion occurs when the passivation film on a length of pipe is destroyed, forming small cavities or pits on the surface of the pipe.Such pits are likely to grow as electrochemical reactions take place, causing the iron in the metal to dissolve into the solution at the bottom of the pit.The dissolved iron will then diffuse towards the top of the pit and oxidize to form iron oxide or rust.As the pit deepens, electrochemical reactions accelerate, corrosion intensifies, and can lead to perforation of the pipe wall and lead to leaks.
Tubing is more susceptible to pitting corrosion when its outer surface is contaminated (Figure 1).For example, contamination from welding and grinding operations can damage the passivating oxide layer of the pipe, thereby forming and accelerating pitting corrosion.The same goes for simply dealing with contamination from pipes.Additionally, as the brine droplets evaporate, wet salt crystals that form on the pipes do the same to protect the oxide layer and can lead to pitting corrosion.To prevent these types of contamination, keep your pipes clean by regularly flushing them with fresh water.
Figure 1 – 316/316L stainless steel pipe contaminated with acid, brine and other deposits is highly susceptible to pitting corrosion.
crevice corrosion.In most cases, pitting can be easily identified by the operator.However, crevice corrosion is not easy to detect and poses a greater risk to operators and personnel.It usually occurs on pipes that have tight spaces between the surrounding materials, such as pipes held in place with clips or pipes that are tightly installed side by side.When brine seeps into the crevice, a chemically aggressive acidified ferric chloride (FeCl3) solution forms in the area over time and causes accelerated crevice corrosion (Figure 2).Because crevices themselves increase the risk of corrosion, crevice corrosion can occur at temperatures much lower than pitting corrosion.
Figure 2 – Crevice corrosion may develop between the pipe and the pipe support (top) and when the pipe is installed close to other surfaces (bottom) due to the formation of a chemically aggressive acidified ferric chloride solution in the crevice.
Crevice corrosion usually simulates pitting corrosion first in the crevice formed between a length of pipe and the pipe support clip.However, due to the increasing Fe++ concentration in the fluid within the fracture, the initial crater becomes larger and larger until it covers the entire fracture.Ultimately, crevice corrosion can perforate the pipe.
Tight cracks are the greatest risk of corrosion.Therefore, pipe clamps that wrap around most of the circumference of the pipe tend to present a greater risk than open clamps, which minimize the contact surface between the pipe and the clamp.Maintenance technicians can help reduce the likelihood of crevice corrosion causing damage or failure by regularly opening the clamps and inspecting the surface of the pipe for corrosion.
Pitting and crevice corrosion can be best prevented by choosing the right metal alloy for the application.Specifiers should exercise due diligence to select the optimum piping material to minimize the risk of corrosion based on the operating environment, process conditions and other variables.
To help specifiers optimize material selection, they can compare metals’ PREN values ​​to determine their resistance to localized corrosion.PREN can be calculated from the chemical composition of the alloy, including its chromium (Cr), molybdenum (Mo), and nitrogen (N) content, as follows:
PREN increases with the content of the corrosion-resistant elements chromium, molybdenum and nitrogen in the alloy.The PREN relationship is based on the critical pitting temperature (CPT) – the lowest temperature at which pitting corrosion is observed – for various stainless steels in relation to chemical composition.Essentially, PREN is proportional to CPT.Therefore, higher PREN values ​​indicate higher pitting resistance.A small increase in PREN is only equivalent to a small increase in CPT compared to the alloy, whereas a large increase in PREN indicates a more significant performance improvement for significantly higher CPT.
Table 1 compares the PREN values ​​of various alloys commonly used in offshore oil and gas applications.It shows how the specification can significantly improve corrosion resistance by selecting a higher grade pipe alloy.PREN increases only slightly when transitioning from 316 to 317 stainless steel.For a significant performance increase, 6 Mo super austenitic stainless steel or 2507 super duplex stainless steel is ideally used.
Higher concentrations of nickel (Ni) in stainless steel also enhance corrosion resistance.However, the nickel content of stainless steel is not part of the PREN equation.In any case, it is often beneficial to specify stainless steels with higher nickel concentrations, as this element helps to re-passivate surfaces that show signs of localized corrosion.Nickel stabilizes austenite and prevents martensite formation when bending or cold drawing 1/8 hard pipe.Martensite is an undesired crystalline phase in metals that reduces stainless steel’s resistance to localized corrosion as well as chloride-induced stress cracking.A higher nickel content of at least 12% in 316/316L is also desirable for applications involving high pressure gaseous hydrogen.The minimum nickel concentration required for 316/316L stainless steel in the ASTM standard specification is 10%.
Localized corrosion can occur anywhere on pipes used in marine environments.However, pitting corrosion is more likely to occur in areas that are already contaminated, while crevice corrosion is more likely to occur in areas with narrow gaps between the pipe and the mounting hardware.Using PREN as a basis, the specifier can select the best pipe alloy to minimize the risk of any kind of localized corrosion.
However, keep in mind that there are other variables that can affect corrosion risk.For example, temperature affects the pitting resistance of stainless steel.For hot marine climates, 6 molybdenum super austenitic or 2507 super duplex stainless steel pipe should be seriously considered because these materials have excellent resistance to localized corrosion and chloride stress cracking.For cooler climates, 316/316L pipe may be sufficient, especially if a history of successful use has been established.
Offshore platform owners and operators can also take steps to minimise the risk of corrosion after the tubing is installed.They should keep the pipes clean and flush with fresh water regularly to reduce the risk of pitting corrosion.They should also have maintenance technicians open tubing clamps during routine inspections to look for the presence of crevice corrosion.
Following the steps outlined above, platform owners and operators can reduce the risk of tubing corrosion and related leaks in marine environments, improving safety and efficiency, while reducing the chance of product loss or the release of fugitive emissions.
Brad Bollinger is the Oil and Gas Marketing Manager for Swagelok Company.He can be reached at bradley.bollinger@swagelok.com.
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