Technical Article
Material Selection for Corrosion Resistance
When choosing the best materials for superior corrosion resistance , relying on reputable suppliers to guide component decisions can save time and money in the long run .
By Buddy Damm , Senior Scientist , Swagelok Company
Fluid systems for offshore oil and gas platforms can be extraordinarily complex . The systems often include nearly 50,000 feet of tubing , more than 20,000 components , 10,000 fittings , and as many as 8,000 mechanical connections . Since offshore industrial fluid systems are challenging to service , designers must carefully consider which materials they select for these applications . One of the most important elements to think about is the system ’ s corrosion resistance .
When building a system , components should be made of materials that prevent corrosion damage from affecting the system ’ s overall performance . Though not the only consideration , component materials matter . Suppliers should certify the initial quality of material and manufacturing , as well as have the materials science expertise to support the choices over the system ’ s life span . Such expertise and guidance can be the difference between system success or failure ( Figure 1 ).
Figure 1 : Building an industrial fluid system to withstand corrosion damage starts with choosing the right materials in the design phase .
How to Maintain Quality
Corrosion is virtually inevitable in the harsh conditions of offshore oil and gas
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platforms . There are multiple forms of corrosion , and each poses a unique threat . Corrosion occurs when metal atoms oxidize , causing the metal ’ s surface to lose
POP-A-PLUGS P-A-P
PLUGS material and the thickness of the component wall to shrink . This could ultimately lead to premature mechanical failure .
In addition , the chemical composition of the fluid being transported may have an impact on a component ’ s corrosion resistance . An emphasis on quality at each step must prevail , from bar stock qualification to the component ’ s final inspection .
Corrosion Mechanisms and Mitigation Strategies
Designing for corrosion resistance requires an understanding of the potential types of corrosion , which can be categorized as general corrosion , localized corrosion , or stress-assisted corrosion .
General corrosion occurs when a susceptible alloy is exposed to a sufficiently corrosive environment . For example , low-alloy steels exhibit generalized corrosion when exposed to a humid or moist environment . For more aggressive environments , such as acids , alloys with superior corrosion resistance such as stainless steels or nickel-based alloys are necessary to minimize the impact of general corrosion .
Localized corrosion occurs when the passive oxide layer that normally protects an alloy from corrosion suffers localized attack . Pitting corrosion and crevice corrosion are the most common examples of localized corrosion . As the chemical reactions of pitting and crevice corrosion progress , the local environment becomes more aggressive , accelerating damage . Selecting alloys with higher Chromium ( Cr ), Molybdenum ( Mo ), and Nitrogen ( N ) contents enhances resistance to pitting and crevice corrosion .
Stress-assisted corrosion occurs when a susceptible alloy is placed in a corrosive environment and subjected to static or cyclic tensile loads . Common examples include chloride ion ( Cl- ) stress corrosion cracking ( SCC ) and sulfide ( H 2
S ) stress cracking ( SSC ). reNTals , rePaIrsrs, solUTIoNs .
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Figure 2 : Components made with metals that have higher chromium and nickel content , like 316 stainless steel , resist corrosion more naturally than lesser-quality stainless steels .
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