What Causes Externally Influenced Corrosion and How Can It Be Mitigated?
In order to combat corrosion in offshore oil and gas environments, it is important to understand the challenges associated with the environment. That’s why we’ve created a three-part Ask Swagelok series that explains the types of offshore corrosion—including internally and externally influenced corrosion—and ways to mitigate it.
In this second video, Buddy Damm, senior scientist of metallurgy, explains what externally influenced corrosion is, the types of it that exist, and how an operator can prevent or mitigate it.
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BETH NIESER:
Hello and welcome to Ask Swagelok. I'm Beth Nieser and I'm here with Buddy Damm, who's a senior scientist of metallurgy here at Swagelok. Today, Buddy, you've been telling us a little bit about corrosion in offshore oil and gas applications, and you also mentioned internally influenced and externally influenced corrosion. Can you explain some of the different types of externally influenced corrosion and some of the mitigations for it?
BUDDY DAMM:
Yeah. One common type of externally influenced corrosion is uniform corrosion. As the name implies, you're uniformly corroding something, consuming the material. Mitigation for this could be using a thicker component so that during the corrosion process over the life of the component, it still remains functional. You can also paint the component or choose a more corrosion-resistant alloy.
Another type of common corrosion offshore is galvanic corrosion. This happens when you take two dissimilar metals and connect them, like a bolt holding a plate together. If one material is more susceptible to corrosion than the other, coupling them accelerates corrosion in the more susceptible material. To mitigate this, make good choices in material selection and avoid overly strong galvanic coupling.
Another type of corrosion is locally influenced corrosion, which includes pitting corrosion and crevice corrosion. This occurs due to a passive oxide film that forms on most metals, providing protection against corrosive environments. Higher chrome content in stainless steel enhances this oxide layer, improving resistance to pitting and crevice corrosion.
Chloride stress corrosion cracking is another concern. If a component is under stress and exposed to chloride-rich environments or dissolved salts in seawater, the combination can attack the material and cause cracking. Mitigation includes increasing the nickel content in the material.
For both localized corrosion and chloride stress corrosion cracking, Swagelok specifies materials with enhanced chemistries. For example, the ASTM standard for chrome is 16–18 wt%, but Swagelok specifies a minimum of 17%. For nickel, ASTM is 10–14%, and Swagelok specifies a minimum of 12%. Swagelok also offers alloys with higher chrome, molybdenum, nitrogen, and nickel to enhance corrosion resistance.
Other important offshore concerns include fretting corrosion, which happens when two sliding components, like tubes, rub against each other. Properly designing tubing runs and supporting tubes at regular intervals prevents fretting. Fatigue is also a concern; cyclically loaded components accumulate fatigue, and in corrosive environments, this accelerates due to corrosion-assisted fatigue. Supporting tubes reduces vibration and cyclic loading, mitigating fatigue.
BETH NIESER:
It sounds like one of the key takeaways is that adding certain elements to stainless alloys can help mitigate corrosion. Can you summarize?
BUDDY DAMM:
Yes. Increasing chrome, molybdenum, and nitrogen content enhances the passive oxide layer, helping avoid pitting and crevice corrosion. Increasing nickel content improves resistance to chloride stress corrosion cracking. For extreme cases, nickel-based alloys are effectively immune, and duplex stainless steels, with low nickel and higher chrome, provide excellent resistance through a ferrite-austenite duplex microstructure.
BETH NIESER:
Okay, great. Well, thank you for that information, Buddy. And thank you all for joining us for Ask Swagelok.