Pitting and crevice corrosion of offshore stainless steel tubing
Stainless steel tubing on oil and gas platforms is regularly employed in process instrumentation and sensing, as well as chemical inhibition, hydraulic lines, impulse lines and utility applications, over a wide range of temperature, flow and pressure conditions.
Gerhard Schiroky, Swagelok Company
Anibal Dam, BP Exploration & Production Inc
Akinyemi Okeremi, Shell International Exploration & Production
Charlie Speed, Consultant
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Unfortunately, all over the globe, including the Gulf of Mexico, the North Sea, the Gulf of Guinea, the China Sea, the Caribbean and so on, corrosion of 316 stainless steel tubing has been observed (see Figure 1). Corrosion is a serious development that can lead to perforations of the tubing wall and the escape, under pressure, of highly flammable chemicals.
The two prevalent forms of localised corrosion are pitting corrosion, which is often readily recognisable, and crevice corrosion, which can be more difficult to observe. There are many factors that contribute to the onset of localised corrosion. The selection of inadequate tubing alloy and suboptimal installation practices can lead to deterioration of tubing surfaces in a matter of months. It has been speculated that today’s minimally alloyed 316 stainless steel tubing with close to 10.0% nickel, 2.0% molybdenum and 16.0% chromium may experience corrosion more readily than the more generously alloyed 316 tubing products that were produced decades ago.
Contamination is another leading cause for surface degradation. Such contamination may be caused by iron particles from welding and grinding operations; surface deposits from handling, drilling and blasting; and from sulphur-rich diesel exhaust. Periodic testing of seawater deluge systems, especially in combination with insufficient freshwater cleansing, may also leave undesirable chloride-laden deposits behind.
Pitting and crevice corrosion
Pitting corrosion of tubing can in most cases be readily recognised. Individual shallow pits and, in later stages, deep and sometimes connected pits can be observed by visual inspection with the unaided eye (see Figure 2). Pitting corrosion starts when the chromium-rich passive oxide film on 316 tubing breaks down in a chloride-rich environment. The higher the chloride concentration and the more elevated the temperature, the higher the likelihood for breakdown of this passive film. Once the passive film has been breached, an electrochemical cell becomes active. As shown in Figure 3a, iron goes into solution in the more anodic bottom of the pit, diffuses toward the top and oxidises to iron oxide, or rust. The concentration of the iron chloride solution in a pit can increase as the pit gets deeper. The consequence is accelerated pitting, perforation of tubing walls and leaks. Pitting can penetrate deep into the tubing walls (see Figure 3b), creating a situation where tubing could fail.
Crevices are very difficult, or even impossible, to avoid in tubing installations. They exist between tubing and tube supports, in tubing clamps, between adjacent tubing runs, and underneath contamination and deposits that may have accumulated on tubing surfaces. Relatively tight crevices pose the greatest danger for crevice corrosion to occur. General corrosion of tubing in a tight crevice causes the oxygen concentration in the fluid that is contained within a crevice to drop. A lower oxygen concentration increases the likelihood for breakdown of the passive surface oxide film. The result is the formation of a shallow pit. Unlike in pitting corrosion described above, the formation of a pit on tubing that is surrounded by a crevice will lead to an increase of the Fe++ concentration in the fluid contained in the gap. Because of the strong interaction of the Fe++ ions with the OH- hydroxyl ions, the pH value drops. Chloride ions will also diffuse into the gap, being attracted by the Fe++ ions. The result of these events is an acidic ferric chloride solution that can lead to accelerated corrosion of tubing within the crevice.
Ideally, tubing should resist all forms of corrosion, including general, localised (pitting and crevice), galvanic, microbiological, chloride-induced stress corrosion cracking and sour gas cracking. The tubing should also have adequate mechanical properties, especially when fluid pressures are high. Resistance to erosion comes into play when fluids contain potentially erosive particles. The environmental impact of the tubing should also be of concern; aquatic life can be harmed by small concentrations of copper ions that can be readily released by copper-zinc alloys.
The resistance of an alloy to localised tubing corrosion can be estimated by calculating from its chemical composition the alloy’s Pitting Resistance Equivalent Number, or PREN. The most frequently used relationship is: PREN = %Cr + 3.3 %Mo + 16 %N. The higher the PREN value of an alloy, the higher its resistance to localised corrosion; ie, the higher its critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT). These critical temperatures can be experimentally determined following common testing procedures such as ASTM G48 and ASTM G150.
The importance of selecting the optimal alloy for an installation is rather convincingly demonstrated in Figure 4. When installed side by side, austenitic 316 stainless steel tubing experienced heavy corrosion, while no signs of corrosion were detected on alloy 2507 superduplex tubing. In a Gulf of Mexico installation of alloy 2507 tubing, only a very small number of cases of external chloride crevice corrosion damage were identified. Perforations leading to the loss of containment of system fluids were not observed. The only instances where crevice corrosion damage occurred involved the use of plastic support strips and neoprene gaskets.
Numerous alloys have been used or have presented themselves as candidates for use in installations that require resistance to seawater corrosion. The most frequently used alloys have been the 300-series austenitic stainless steels, mainly 316 and in some cases 317. Alloys with at least 6% molybdenum, the so-called “6-moly” alloys, have performed well in offshore systems. Typical 6-moly alloys include 254SMO, AL6XN and 25-6Mo. More recently, alloys with slightly more than 6% molybdenum have been introduced: 654SMO, AL6XN Plus, 27-7Mo and 31. The published properties of these alloys suggest that they would perform well in chloride environments. Nickel alloys such as 825, 625 and C-276 are more frequently used for their performance in sour gas applications. Of these alloys, 625 and C-276 have demonstrated excellent resistance to localised corrosion. Ferritic alloys such as Sea-Cure and AL29-4C are resistant to attack by aqueous chloride solutions and are primarily used as heat exchanger tubing. Tungum is a copper-zinc alloy that has been used because of its relative ease of installation. However, it carries disadvantages: lack of hardness indicates susceptibility to erosive wear; low yield strength restricts its use to low pressures or requires high wall thickness; and corrosion liberates copper ions that can be detrimental to sea life.
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