Despite the inherent corrosion resistance of stainless steel pipes, stainless steel pipes installed in marine environments are subject to various types of corrosion during their expected service life. This corrosion can lead to fugitive emissions, product losses and potential risks. Offshore platform owners and operators can reduce the risk of corrosion by specifying stronger pipe materials from the outset for better corrosion resistance. Thereafter, they must remain vigilant when inspecting chemical injection lines, hydraulic and impulse lines, and process instrumentation and instrumentation to ensure that corrosion does not threaten the integrity of the installed piping or compromise safety.
Localized corrosion can be found on many platforms, ships, ships and offshore pipelines. This corrosion can be in the form of pitting or crevice corrosion, either of which can erode the pipe wall and cause liquid to be released.
The risk of corrosion increases as the operating temperature of the application increases. Heat can accelerate the degradation of the tube’s protective outer passive oxide film, thereby promoting pitting.
Unfortunately, localized pitting and crevice corrosion are difficult to detect, making it difficult to identify, predict, and design these types of corrosion. Given these risks, platform owners, operators and designees must exercise caution in selecting the best pipeline material for their application. Material selection is their first line of defense against corrosion, so getting it right is very important. Fortunately, they can use 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 look at 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 over the metal surface. Pitting and crevice corrosion begin to form on 316 stainless steel tubing when the outer chromium-rich passive oxide film of the metal is ruptured by exposure to corrosive liquids, including salt water. Marine environments rich in chlorides, as well as high temperatures and even contamination of the tubing surface, increase the likelihood of degradation of this passivation film.
pitting Pitting corrosion occurs when the passivation film on a section of pipe breaks down, forming small cavities or pits on the surface of the pipe. Such pits are likely to grow as electrochemical reactions proceed, as a result of which the iron in the metal is dissolved in solution at the bottom of the pit. The dissolved iron will then diffuse to the top of the pit and oxidize to form iron oxide or rust. As the pit deepens, electrochemical reactions accelerate, corrosion increases, which can lead to perforation of the pipe wall and lead to leaks.
Tubes are more susceptible to pitting if their outer surface is contaminated (Figure 1). For example, contaminants from welding and grinding operations can damage the passivation oxide layer of the pipe, thereby forming and accelerating pitting. The same goes for simply dealing with pollution from pipes. In addition, as the salt droplets evaporate, the wet salt crystals that form on the pipes protect the oxide layer and can lead to pitting. To prevent these types of contamination, keep your pipes clean by flushing them regularly with fresh water.
Figure 1. 316/316L stainless steel pipe contaminated with acid, saline, and other deposits is highly susceptible to pitting.
crevice corrosion. In most cases, pitting can be easily detected by the operator. However, crevice corrosion is not easy to detect and poses a greater risk to operators and personnel. This usually occurs on pipes that have narrow gaps between surrounding materials, such as pipes held in place with clamps or pipes that are tightly packed next to each other. When the brine seeps into the gap, over time, a chemically aggressive acidified ferric chloride solution (FeCl3) is formed in this area, which causes accelerated corrosion of the gap (Fig. 2). Since crevice corrosion by its nature increases the risk of corrosion, crevice corrosion can occur at much lower temperatures than pitting.
Figure 2 – Crevice corrosion can develop between the pipe and pipe support (top) and when the pipe is installed close to other surfaces (bottom) due to the formation of a chemically aggressive acidified solution of ferric chloride in the gap.
Crevice corrosion usually simulates pitting first in the gap formed between the pipe section and the pipe support collar. However, due to the increase in the concentration of Fe++ in the fluid inside the fracture, the initial funnel becomes larger and larger until it covers the entire fracture. Ultimately, crevice corrosion can lead to perforation of the pipe.
Dense cracks represent the greatest risk of corrosion. Therefore, pipe clamps that encircle a large portion of the pipe’s circumference tend to be more risky than open clamps, which minimize the contact surface between pipe and clamp. Service technicians can help reduce the chance of crevice corrosion damage or failure by regularly opening fixtures and inspecting pipe surfaces for corrosion.
Pitting and crevice corrosion can be prevented by choosing the right metal alloy for the specific application. Specifiers must exercise due diligence in selecting the optimum piping material to minimize the risk of corrosion, depending on the operating environment, process conditions, and other variables.
To help specifiers optimize their choice of materials, they can compare the PREN values of metals to determine their resistance to localized corrosion. PREN can be calculated from the alloy’s chemistry, including its chromium (Cr), molybdenum (Mo), and nitrogen (N) content, as follows:
PREN increases with the content of corrosion-resistant elements of chromium, molybdenum and nitrogen in the alloy. The PREN ratio is based on the critical pitting temperature (CPT) – the lowest temperature at which pitting occurs – for various stainless steels depending on the chemical composition. Essentially, PREN is proportional to CPT. Therefore, higher PREN values indicate higher pitting resistance. A small increase in PREN is equivalent to only a small increase in CPT compared to the alloy, while a large increase in PREN indicates a significant improvement in performance over a much higher CPT.
Table 1 compares PREN values for various alloys commonly used in the offshore oil and gas industry. It shows how specification can greatly improve corrosion resistance by selecting a higher quality pipe alloy. PREN increases slightly from 316 SS to 317 SS. Super Austenitic 6 Mo SS or Super Duplex 2507 SS are ideal for significant performance gains.
Higher nickel (Ni) concentrations in stainless steel also increase corrosion resistance. However, the nickel content of stainless steel is not part of the PREN equation. In any case, it is often advantageous to choose stainless steels with a higher nickel content, as this element helps to repassivate surfaces that show signs of localized corrosion. Nickel stabilizes austenite and prevents martensite formation when bending or cold drawing 1/8 rigid pipe. Martensite is an undesirable crystalline phase in metals that reduces the resistance of stainless steel to localized corrosion as well as chloride-induced stress cracking. The higher nickel content of at least 12% in 316/316L steel is also desirable for high pressure hydrogen gas applications. The minimum nickel concentration required for ASTM 316/316L stainless steel is 10%.
Localized corrosion can occur anywhere in a pipeline used in a marine environment. However, pitting 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 installation equipment. Using PREN as a basis, the specifier can select the best pipe grade to minimize the risk of any kind of localized corrosion.
However, keep in mind that there are other variables that can affect the risk of corrosion. For example, temperature affects the resistance of stainless steel to pitting. For hot maritime climates, super austenitic 6 molybdenum steel or super duplex 2507 stainless steel pipes should be seriously considered because these materials have excellent resistance to localized corrosion and chloride cracking. For cooler climates, a 316/316L pipe may be sufficient, especially if there is a history of successful use.
Offshore platform owners and operators can also take steps to minimize the risk of corrosion after tubing has been installed. They should keep the pipes clean and regularly flushed with fresh water to reduce the risk of pitting. They should also have maintenance technicians open the clamps during routine inspections to check for crevice corrosion.
By following the steps above, platform owners and operators can reduce the risk of pipe corrosion and related leaks in the marine environment, improve safety and efficiency, and reduce the chance of product loss or fugitive emissions.
Brad Bollinger is the Oil and Gas Marketing Manager for Swagelok. He can be contacted at bradley.bollinger@swagelok.com.
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