The term “corrosion resistant alloys” (CRA's) covers a wide range of materials from ferritic stainless steels to nickel base or titanium alloys. Nickel, chromium and molybdenum contents increase steadily as we progress from ferritic, duplex, super-duplex, austenitic, 6% or 7% molybdenum super-austenitic stainless steels to nickel-chromium-molybdenum-tungsten alloys. As this progression is made the corrosion resistance, and cost, increase dramatically. A definitive ranking of the general and localized corrosion resistance of these materials in a variety of environments is provided, with special emphasis on effects of welding and choice of filler metals.
The term “corrosion resistant alloys” (CRA's) covers a wide range of materials from ferritic stainless steels to nickel base or titanium alloys. Nickel, chromium and molybdenum contents increase steadily as we progress from ferritic, duplex, super-duplex, austenitic, 6% or 7% molybdenum super-austenitic stainless steels to nickel-chromium-molybdenum-tungsten alloys. As this progression is made the corrosion resistance, and cost, increase dramatically. A definitive ranking of the general and localized corrosion resistance of these materials in a variety of environments is provided, with special emphasis on effects of welding and surface condition. The alloys considered are INCONEL® alloy C-276 (N10276), INCOLOY® alloy 27-7MO (S31277), INCOLOY® alloy 25-6HN (N08367), INCOLOY® alloy 865 (S35115), Austenitic Stainless Steel 316 (S31600), Duplex Stainless Steel 2205 (S32205) and Duplex Stainless Steel 2507 (S32750). The alloy C-276 represents the standard top of the line “work horse” ®INCONEL and INCOLOY are registered trademarks of Special Metals Corporation Group of Companies nickel base alloy used for a wide range of acid and chloride environments in the Chemical Process and a variety of other industries. Alloy 25-6HN represents the standard 6% molybdenum - nitrogen super-austenitic alloy. Alloy 27-7MO is an advanced 7% molybdenum alloy with higher nickel, molybdenum and nitrogen levels that provide corrosion resistance approaching that of alloy C-276 in many applications. Alloy 2205 is a standard duplex stainless steel with improved resistance to localized corrosion and stress corrosion cracking (SCC) compared to some austenitic stainless steels like SS 316. Alloy 2507 is a “super duplex” stainless steel with improved corrosion resistance compared to the standard duplex stainless steels. Though there is a wealth of data available for each individual class of alloy, it is difficult to find a good comparison of the corrosion resistance of all alloy types. Published papers such as References 1 through 6 provide information on austenitic and duplex alloy pitting or general corrosion resistance but do not make comparisons to higher alloyed materials. Also, there is very little published data describing the corrosion resistance of welded specimens, though in Reference 1 Stenvall does address pitting corrosion of welded specimens. The intent of this study is to provide a comprehensive comparison of all alloy types and to determine the effects of welding on corrosion resistance. To this end, the corrosion resistance of the alloys described above has been compared in a variety of acid. With the exception of SS 316, all were tested in the as-welded condition using standard filler metals.
