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Vol. 230 No.4
PRODUCTION TECHNOLOGY
Preventing pitting and crevice corrosion of offshore stainless steel tubing
Tubing corrosion can lead to perforations and escape of highly flammable chemicals.
Gerhard Schiroky, Swagelok Company; Anibal Dam, BP Exploration & Production Inc.; Akinyemi Okeremi, Shell International Exploration & Production; and Charlie Speed, Consultant
Stainless steel tubing on oil and gas platforms is used 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. Unfortunately, all over the globe, including the Gulf of Mexico (GOM), the North Sea, the China Sea and so on, corrosion of 316 stainless steel tubing has been observed.
The two prevalent forms of localized corrosion are pitting corrosion, which is often readily recognizable, and crevice corrosion, which can be more difficult to observe. The selection of inadequate tubing alloy and suboptimal installation practices can lead to deterioration of tubing surfaces in a matter of months. 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 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 sulfur-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
In most cases, pitting corrosion of tubing can be readily recognized. Individual shallow pits, and in later stages, deep and sometimes connected pits, can be observed by visual inspection with the unaided eye, Fig. 1. Pitting corrosion starts when the chromium-rich passive oxide film on 316 tubing breaks down in a chloride-rich environment. Higher chloride concentrations and elevated temperatures increase the likelihood for breakdown of this passive film. Once the passive film has been breached, an electrochemical cell becomes active. Iron goes into solution in the more anodic bottom of the pit, diffuses toward the top and oxidizes to iron oxide, or rust. The iron chloride solution concentration in a pit can increase as the pit gets deeper. The consequence is accelerated pitting,...