100

FIGURE 7.1 Corrosion pitting potential of alloy 31 vs. other alloys in artificial seawater.

is inadequate in certain applications and is being replaced by the standard and advanced 6Mo alloys.

7.4.7.5 Alloy33

Alloy 33, a recent innovation, is a chromium-based austenitic wrought superstain-less steel (33 Cr, 32 Fe, 31 Ni, 1.6 Mo, 0.6 Cu, 0.4 N). This alloy has excellent resistance to both acidic and alkaline corrosive media, mixed HNO3/HF acids, hot sulfuric acid, localized corrosion, and stress corrosion cracking. Due to its high nitrogen content, this alloy has excellent mechanical properties. Its high pitting resistance equivalent (PRE) makes it a very cost-effective alloy in comparison to some stainless steels and some lower alloys.

In comparison to other high chromium alloys such as alloy G-30, 690, and 28, alloy 33 shows excellent corrosion resistance behavior. This alloy on a cost—performance basis has the potential of being an excellent alternative to many alloys currently in use such as 825, 904L, 20, 28, 6Mo alloys, G-3, G-30, and in some cases even alloy 625. The alloy has been successfully used in high-temperature concentrated sulfuric acid and is undergoing tests in a wide variety of diverse industries and environments. Details on the development and properties of this alloy are presented in the open literature [21, 22].

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