Sea Technology

MAR 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Material Selection For Seawater Sensors Nickel Superalloys Offer Optimal Corrosion-Resistance By Karmjit Sidhu S eawater attacks metals at varying water depths, resulting in corrosion accelerated by the different levels of oxygen, temperature, pH, salinity, chlorinity, biological activity, electrical conductivity and velocity fow rates present in seawater at various depths. Sensors, including pressure and linear position sensors, are widely used for control and safety functions at various depth levels for applications in offshore platforms, desalination systems, mooring cables, seafoor wellheads, and oil and gas gathering systems. During operation, sensors can be submerged in seawater at varying depths, based on tides, up to 1,000 feet. To operate in these environments, sensors must be constructed from corrosion-resistant materials so that units can provide continuous information under hostile conditions. As demand for sensors to be in contact with seawater and sea fog increases for applications such as loading systems, subsea mooring cables, control valves, chokes, desalination plants and platform stability, specifying sensor materials depends upon the proper selection of alloys suitable for the subsea application and service environment. Whether corrosion comes from varying seawater depth levels, galvanic effects or biological attack, matching the proper materials for service application is the top priority for good sensor performance over a long period of time. Material selection is also often affected by system-reliability requirements, availability, cost and manufacturability. Seawater Characteristics Leading to Corrosion Containing high levels of salts, dissolved oxygen, carbon dioxide and micro-organisms, seawater environments are highly corrosive. Corrosion rates vary by the combination of location, temperature and micro-organism activity. Stagnant or polluted waters are additional triggers that often promote sulfate-reducing bacteria (SRB) that can affect sensor materials��� performance. The major constituents of seawater containing negative ions are chloride, sulfate, bromine and bicarbonate, while positive ions are sodium, magnesium, calcium and potassium. Seawater increases localized corrosion of stainless steels and other active-passive materials as a result of dissolved chlorides and other salts. This type of corrosion occurs in the form of pitting, crevice or intergranular. The high electrical conductivity of seawater promotes macro cell corrosion and increases galvanic corrosion, which accelerates temperature rise as well, further promoting corrosion. The pitting of a 316L pressure-sensor diaphragm 0.030 inches thick with less than one-month service near the Great Barrier Reef in Australia. For instance, pitting occurred in an American Sensor Technologies Inc. (Mount Olive, New Jersey) 316L 0.03-inch-thick pressure-sensor diaphragm, with less than one-month service near the Great Barrier Reef in Australia. This was part of a desalination application where seawater was pressurized to approximately 1,500 pounds per square inch before being converted to drinking water. The unit was replaced with another product from American Sensor Technologies constructed of a nickel superalloy, Inconel 625, that was much more corrosion-resistant than 316L stainless steel in warm, shallow seawater. Corrosion by Micro-Organisms in Seawater Microbially induced corrosion (MIC) is a very serious problem that affects sensor operation based on different service conditions and materials used in sensor construction, especially low-grade austenitic stainless steels. It is a corrosion process involving material degradation that normally occurs on welded joints and leads to weld failure if not checked and treated in time. In oxygen-rich environments, aerobic bacteria are active, while anaerobic bacteria can be found in low-oxygen environments. Bacteria can be described as slime-forming, acid- MARCH 2013 / st 15

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