Iron goes into solution in the more anodic bottom of a pit, diffuses toward the top, and oxidizes to iron oxide (rust). A lower oxygen concentration increases the likelihood for breakdown of the passive surface oxide film. General corrosion of tubing in a tight crevice causes the oxygen concentration in the fluid that is contained within a crevice to drop. Relatively tight crevices pose the greatest danger. They exist between tubing and tube supports, in tubing clamps, between adjacent tubing runs, and underneath contamination and deposits that may accumulate on tubing surfaces. Pitting can penetrate deep into the tubing walls, creating a situation where tubing could fail.Ĭrevices are difficult, or even impossible, to avoid in tubing installations. The consequences are accelerated pitting, perforation of tubing walls and leaks. The concentration of the iron chloride solution in a pit can increase as the pit deepens. Iron goes into solution in the more anodic bottom of the pit, diffuses toward the top, and oxidizes to iron oxide. Once the passive film is breached, an electrochemical cell becomes active. (Below) Pitting often can be seen with the unaided eye. (Above) Corrosion of 316 stainless steel tubing. The higher the chloride concentration and the more elevated the temperature, the more likely the breakdown of this passive film. Pitting corrosion starts when the chromium-rich passive oxide film on 316 tubing breaks down in a chloride-rich environment. Individual shallow pits, and in later stages, deep and sometimes connected pits can be seen with the unaided eye. Pitting corrosion of tubing usually is readily recognized. Periodic testing of seawater deluge systems, especially in combination with insufficient freshwater cleansing, may leave undesirable chloride-laden deposits behind. 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. It is speculated that today's minimally alloyed 316 stainless steel tubing, with about 10% nickel, 2% molybdenum, and 16% chromium, may more readily corrode than the more generously alloyed 316 tubing products produced decades ago.Ĭontamination is another leading cause for surface degradation. Inadequate tubing alloy and suboptimal installation practices can lead to deterioration of tubing surfaces in a matter of months. Many factors contribute to the onset of localized corrosion. The two prevalent forms of localized corrosion are pitting, often readily recognizable, and crevice, which can be more difficult to see. Corrosion is a serious development that can lead to perforations of the tubing wall and the escape, under pressure, of highly flammable chemicals. Corrosion of 316 stainless steel tubing has been observed in offshore applications around the world. Oil and gas platforms regularly use stainless steel tubing in process instrumentation and sensing, as well as in chemical inhibition, hydraulic lines, impulse lines, and utility applications, over a wide range of temperatures, flows, and pressures. Shell International Exploration & Production Safe construction demands proper materials
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