Superaustenitic stainless steels have shown superior crevice corrosion resistance in comparison to other stainless steel alloys. They are optimized for use in critical environments in a number of applications. One of these alloys, UNS N08367 has been considered for evaluation. The resistance to crevice corrosion of UNS N08367 welds made by laser, gas tungsten arc (with and without flux) and friction stir welding were compared and evaluated. The study was performed in flowing, ambient temperature natural seawater. The resistance to crevice corrosion was evaluated on frequency of initiation and depth of propagation.
The United States Navy is evaluating non-magnetic stainless steels as possible candidate materials in ship hulls. Sedriks and Dudt1 have documented an excellent review on the use of superaustenitic stainless steels for naval combatant hulls. According to this review, the reason for focus on superaustenitic stainless steels is their superior crevice corrosion resistance in comparison to other stainless steel alloys. These alloys that have high levels of chromium and nickel, typically contain an average of 6 percent molybdenum and may also be alloyed with nitrogen. They are optimized for use in critical environments in a number of investigations. One of these alloys, UNS N08367 has been considered for evaluation. Although there is experience in the ship building community in using lower-alloyed materials for building submarine and mine counter measures vessels, there is only a limited knowledge and experience in the technology base for considering higher alloyed materials such as UNS N08367 for this purpose. For any ship system, it is critical to consider the cost of corrosion protection such as sacrificial coatings, impressed currents, etc. The superaustenitics are inherently the best materials in terms of corrosion resistance. However, the welds within itself and with other dissimilar metals are of primary interest, particularly when the information on weld performance under actual seawater exposure is limited. Any shipbuilding program will involve the use of dissimilar welds. In particular, galvanic corrosion issues due to dissimilar welds of UNS N08367 with commonly used weld deposit materials in natural seawater were evaluated and the results described in a recent report2. The shipbuilding also involves welding of UNS N08367 alloy to itself for designing trusses. For this purpose, several types of welding technologies such as conventional welding (control), gas tungsten arc welding (GTAW) with and without a flux, laser and friction stir welding have been considered. The objective of this task is to evaluate the crevice corrosion performance of UNS N08367 welds made with different welding technologies as cited above by exposing them to natural seawater and recommend the best welding technology for ship building application. The Naval Research Laboratory was tasked to assess the crevice corrosion resistance in natural seawater of UNS N08367 welds which were prepared by different welding technologies. The welded 4-inch x 6-inch x ¼-inch thick UNS N08367 specimens, each prepared with a crevice former as specified in ASTM G 78-01, were exposed to refreshed, unfiltered, ambient temperature natural seawater for 6 months and one year.
