During the past two decades, the 6-Mo superaustenitic alloy UNS N08367 has been tested and used in marine environments around the world. The results of planned tests and actual service experience are enumerated. Both successes and failures are examined. These provide guidance on environmental limits for the alloy?s successful use.
Seawater handling systems are used in industries such as power and desalination plants, shipping, offshore gas and oil production, etc. Seawater environments are very corrosive since they contain a high concentration of chloride salts (mainly sodium chloride), dissolved oxygen, carbon dioxide and are biologically active. In stagnant or polluted conditions, additional corrosive species such as ammonia or sulfide ions, and reducing conditions can be created. Such environments are particularly detrimental to the performance of the traditional copper-based alloys that historically have been the mainstay of seawater service. Oxidizing biocides such as chlorine or ozone frequently are used for control of biofouling. The oxidizing, chloride-rich environment of seawater promotes localized corrosion processes such as pitting, crevice corrosion and chloride stress-corrosion cracking (SCC) in stainless materials.
Therefore, only materials that possess extremely high corrosion resistance can be used in such environments. The 6-Mo super austenitic stainless steels such as UNS N08367 (AL-6XN®) are commonly used in marine environments due to their high resistance to pitting, crevice corrosion and chloride SCC. Alloys such as S31254, S32654, S34565, S44660, S44735 and N10276 are also used. The UNS N08367 alloy has been tested and used in marine environments around the world during the last two decades. This paper briefly summarizes those results and provides the guidance on environmental limits for successful use of N08367 alloy.
A typical composition of UNS N08367 alloy is shown in Table 1. Compositions of the conventional austenitic stainless steels Type 304 (UNS S30400) and Type 316 (UNS S31600), are given in Table 1 for comparison. The high levels of molybdenum and nitrogen present in UNS N08367 alloy to confer corrosion resistance also provide better mechanical properties than exhibited by conventional austenitic stainless steels. Typical mechanical properties of UNS N08367 and Types 304 and 316 are summarized in Table 2.