Technical Article fuse . As crevice corrosion progresses , the solution within the crevice becomes more corrosive . In such a scenario , the entire surface within the crevice can corrode at an ever-increasing rate .
Typically , crevice corrosion is discovered when technicians remove clamps from existing tubing runs . It usually occurs at lower temperatures than pitting corrosion because it takes less energy for corrosion to occur beneath geometric crevices like those created by the tube clamp .
Keeping Pitting and Crevice Corrosion at Bay
Education is key to keeping corrosion damage to a minimum . Arming system designers and technicians with appropriate materials science knowledge will advance those goals , as will having strict corrosion-prevention protocols in place .
As systems are being designed , choosing the right materials for tubing and tube supports is crucial . The first line of defense is to consider the Pitting Resistance Equivalent Number , or PREN , which is an algebraic function of the Cr , Molybdenum ( Mo ), and Nitrogen ( N ) content of stainless steel . A higher Cr content results in a more Cr-rich passive oxide , and Mo and N enhance the robustness of the passive oxide film . When pitting or crevice corrosion is observed , replacing components made from an alloy with a higher PREN will enhance pitting or crevice corrosion resistance .
PREN = % Cr + 3.3 ×% Mo + 16 ×% N
The ASTM G48 standard for laboratory metrics of critical pitting temperature ( CPT ) and critical crevice temperature ( CCT ) is an invaluable tool for comparing materials for use in corrosive environments . CPT testing evaluates the temperature at which pitting begins on a material in a specific corrosive solution . Similarly , CCT testing evaluates the temperature at which crevice corrosion begins when a predefined crevice is placed on a metal sample in a corrosive solution . Because the local environment in a crevice becomes more aggressive than in a pit , the CCT is always lower than the CPT .
The higher the temperatures for CPT and CCT , the better those metals are for use in corrosive environments like offshore oil and gas platforms . For example , 304L has the lowest CPT value of the materials shown in Figure 5 , while 6Mo and 2507 are the two highest for CPT and CCT temperatures . This indicates that 6Mo and 2507 will be more resistant to pitting and crevice corrosion than 304L and 316L in chloride-bearing solutions . Keep in mind that CPT and CCT values are comparative and not predictive so , even armed with this understanding , further investigation should occur before choosing components for specific applications .
316L austenitic stainless steel ( UNS S31603 ) tubing and fittings are effective in resisting pitting and crevice corrosion in many service environments . If the oil and
Figure 5 . The ASTM G48 standard describes testing to assess the temperature at which pitting or crevice corrosion is first observed in a 10 % ferric chloride solution . Austenitic and duplex stainless steels with higher Cr , Mo , and N contents exhibit greater resistance to pitting and crevice corrosion .
gas platform is in a warmer climate and is prone to saltwater pooling and drying , pitting or crevice corrosion of 316L stainless tubing is more readily observed . However , due to the beneficial addition of Mo , 316L typically performs better than 304L ( UNS S30403 ) stainless steel in these corrosive environments . Furthermore , 316L with a minimum of 17 weight percent Cr can help to enhance localized corrosion resistance . The ASTM standard minimum is 16 weight percent .
For situations in which 316L cannot sustain its corrosion resistance over the tubing ’ s expected life cycle , tubing made from super austenitic ( e . g ., 6Mo or 6HN , UNS N08367 ) or super duplex ( e . g ., 2507 , UNS S32750 ) stainless steels may be a better choice , as these alloys have higher Cr and Mo contents , and the addition of N . Additionally , the higher yield and tensile strength of super austenitic and super duplex stainless steels make it easier to build systems that must be rated to a higher maximum allowable working pressure ( MAWP ). Working with a reliable tubing and fitting supplier who can guide teams through appropriate component choices will keep facilities from having to engage in expensive retrofits .
While choosing the right initial materials can alleviate some corrosion concerns , facilities must also implement precise protocols to prevent corrosion and keep crevices to a minimum . One way is to avoid putting tubes directly against walls or each other . If crevice corrosion occurs in 316L stainless steel
Figure 6 . Training programs covering materials science work best when they focus on local and regional regulations or other geographic-specific factors to ensure relevance for participants . tubing , teams should consider replacing it with 6Mo tubing , which is more corrosion resistant . If properly engineered from the beginning , 316L fittings can be used despite the differences in metals .
Finding a supplier who can offer training in understanding corrosion ( Figure 6 ) — from what it looks like to where it occurs and why — is essential to building corrosion-resistant systems from the
Pitting vs . Crevice Corrosion 101
Pitting Corrosion
• Occurs anywhere along stainless steel surfaces as localized pits
• Appears as reddish-brown oxide deposits and pits along the metal surfaces
• Increases under high temperatures
• Occurs in areas of high chlorine concentration , including deposits from evaporated saltwater
Crevice Corrosion
• Happens in crevices between tubing , tube supports , clamps , adjacent tubing runs , and underneath coatings , dirt , and deposits
• Can only be observed by removing tubing clamps
• Can occur at lower temperatures
ground up . Ensuring designers and technicians who manage the system have this basic knowledge can prevent material failures and keep systems operating at peak performance for longer . Proactive measures can prevent expensive repairs and minimize downtime . Keeping corrosion from spreading will improve profitability , keep employees safer , and improve overall system performance .
Images © 2024 Swagelok Company
About the Author
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Buddy Damm is Senior Scientist for Swagelok .
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