Super austenitic Stainless Steels

Stainless steel 316L material used for piping and equipment shows considerable corrosion resistance because of the beneficial effect of molybdenum on the surface properties. It is also observed that the surface treatment (pre-reduced, polished, passivated and chemically treated surfaces) of stainless steel equipment and piping reduces the corrosion process in seawater applications. The corrosion resistance of stainless steel in seawater applications can also be enhanced by bulk alloying the stainless steel with nitrogen, chromium, molybdenum and nickel by converting the stainless steel into super austenitic stainless steel. From leaching studies it is also observed that the release of iron, chromium and nickel from the super austenitic stainless steel to seawater is considerably... 

Alloys containing molybdenum can potentially experience catastrophic oxidation. The super-austenitic stainless steels such as Alloy AL-6XN, a 21Cr-25Ni-6.5Mo-N alloy (UNS N08367), are an example. A heavy molybdenum oxide scale forms, usually as a result of an improper heat treatment or a severe thermal excursion. Removal of such scales prior to service (or return to service), usually by pickling, prevents this problem. 

Super austenitic stainless steels with relatively high nickel content (approx. 20 Cr. 29 - 34 Ni), sometimes referred to as "alloy 20" are more costly than standard austenitic steels but provide excellent resistance to acids and some acid chlorides. 

Super austenitic stainless steels, such as AL-6XN and 254SMO alloys, have high resistance to localized corrosion. Chloride pitting and crevice corrosion in these alloys is very low due to the presence of high molybdenum content (>6%) and nitrogen additions. Steels with higher nickel content exhibit better resistance to stress corrosion cracking than austenitic stainless steels. 

Also, relatively new are the 6% molybdenum super-austenitic stainless steels, which possess the strength of duplex stainless steels and corrosion resistances about midway between the duplex stainless steels and the high-nickel-based alloys. Welding super-austenitic stainless is not a problem with molybdenum-enriched filler metals. Examples of high-molybdenum alloys are AL-6XN (Allegheny Ludlum Corp.) and 254 SMO (Avesta Sheffield, Inc.). Sorell (1994) and Kane (1993) present concise reviews of the various alloy groups. 

Super austenitic, high nickel, stainless steels, containing between 29 to 30 per cent nickel and 20 per cent chromium, have a good resistance to acids and acid chlorides. They are more expensive than the lower alloy content, 300 series, of austenitic stainless steels. 

Duplex, and super-duplex stainless steels, contain high percentages of chromium. They are called duplex because their structure is a mixture of the austenitic and ferritic phases. They have a better corrosion resistance than the austenitic stainless steels and are less susceptible to stress corrosion cracking. The chromium content of duplex stainless steels is around 20 per cent, and around 25 per cent in the super-duplex grades. The super-duplex steels where developed for use in aggressive off-shore environments. 

The fuel rod cladding material of the Super LWR is being developed and tested. For fuel rod design of the Super LWR in the cmicept development phase, typical austenitic stainless steels or Ni-alloys are applied as described in Chap. 2. The principle of the safety criteria for fuel rod integrity is shown in Table 6.6. Since heat transfer deterioration is a much milder phenomenon than boiling transition, the minimum deterioration heat flux ratio was eliminated from the transient criterion related to fuel rod heat-up [7]. The types of abnormalities are separated into loss of cooling and overpower . 

Chlorine dioxide is similar to chlorine with respect to its corrosivity (36). Suitable materials for bleach washers using chlorine and chlorine dioxide are super austenitic 6-7% molybdenum stainless steels such as 25-45 Mo or 25-6 Mo. 

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