X-Message-Number: 4962 Date: Wed, 11 Oct 1995 00:02:43 -0700 (PDT) From: Doug Skrecky <> Subject: more heavy metal madness TIME MACHINE CONSTRUCTION PART II (Spring 1994 Canadian Cryonics News) By Doug Skrecky Titanium is the only affordible metal with which one could build a secure time capsule which could reliably survive the millenia. Titanium comes in various grades which offer various levels of corrosion resistance. Grades 7 & 11 alloys which incorporate 0.15% platinum offer a performance in oxidizing acids which can only be beaten by precious metals themselves. Grade 16 with 0.05% platinum offers virtually the same performance at a significantly reduced cost. Grade 12 with 0.3% molybdinum and 0.8% nickel is the next step down, while grade 2 or unalloyed titanium is the least resistant. *1 However ALL titanium alloys including grade 2 have proven to be invulnerable to ALL ambient temperature ground waters including seawater. The most severe threats these fluids pose to metals is due to the presence of chloride and microbiologically induced corrosion (MIC). Grade 2 titanium is immune to both at ambient temperatures. *1 *2 The price of grade 2 titanium is also LESS than that of resistant nickel alloys. Thus the only materials which could offer a substantial degree of corrosion resistance at a price significantly lower than titanium are the stainless steels. Unlike titanium alloys, stainless steels exhibit a very wide range of corrosion resistances because of their large differences in composition. However all derive their resistance from a microscopic surface layer of a brittle ceramic composed of chromium oxide, which forms on steels alloyed with at least 12% chromium. Better grades of stainless have chromium contents ranging up to 30% as well other alloying additions which help to stabilize the chromium oxide layer. Additions which are generally helpful in this regard include molybdenum, nitrogen, tungsten, silicon and vanadium. *3 The later three additives see little use since tungsten is relatively ineffective in raising corrosion resistance, silicon makes the alloy brittle while vanadium is simply too expensive. Up to 6% molybdenum and 0.4% nitrogen are commonly used to help stabilize the chromium oxide layer, though new experimental alloys with higher nitrogen contents are under development. Copper has been found to possess a synergism with nitrogen, possibly because it tends to inhibit the formation of chromium nitride inclusions. *6 *7 Recent research has also found that phosphorus has a dramatic effect in increasing corrosion resistance, though no commercial alloys as yet contain deliberate phosphorus additions. *4 *5 Some minor constituents of stainless alloys such as sulfur also have been found to be detrimental since pitting tends to initiate at manganese sulfide inclusions. *6 Calcium treatment of the melt has been found to eliminate these inclusions and significantly improve both corrosion resistance as well as low temperature ductility. *8 *9 The least expensive stainless steels are the 12% chromium steels such as 3CR12. The most common stainless steel (used for cutlery) is type 304 which possesses 18% chromium, 8% nickel and a price tag double than of 3CR12. With the further addition of 2.5% molybdenum we have type 316, which is twice as expensive as type 304. Increasing alloying additions to 6% molybdenum, 0.2% nitrogen and 18% nickel we have 254SMO, which is (you guessed it) twice as expensive as type 316. With 254SMO we are starting to get close to the price of grade 2 titanium so if we are to consider such an alloy as a replacement for titanium it had better be good. It isn't. Although 254SMO has a high resistance to chlorides it has experienced a few failures due to MIC. *10 The reason for this appears to be related to the harmful effect that additions of over 2.5% molybdinum have on resistance of stainless steels in some highly oxidizing acids. This is further supported by the fact that pickling stainless steel welds to increase their surface chromium content and render them resistent to highly oxidizing acids also renders them invulnerable to MIC. *11 Although the details of the corrosion mechanisms involved in MIC are still not yet fully understood we can be reasonly certain that an increased chromium content coupled with a more modest molybdinum content would likely offer an improved resistance to MIC. (Nitrogen is neutral with respect to highly oxidizing acids.) An example of such an alloy is SAF 2507 with 25% chromium, 3.8% molybdinum, 7% nickel and 0.27% nitrogen. In addition to (we believe) not being succeptible to MIC, this high strength alloy is also less expensive than 254SMO because less of this alloy is required for structural applications to obtain the same yield strength. Such high strength stainless steels currently offer the only reasonably convincing lower cost alternative to titanium. To further cut costs the nickel content of many of these alloys has been reduced so that low temperature ductility suffers. However a similar but cheaper alloy SAF 2205 has been selected for use in pipelines in the arctic and since SAF 2507 itself has a similar impact toughness this would seem not to be a serious defect. *12 In any case high strength stainless alloys with an increased nickel content such as Remanit 4565S are starting to become available on the market as well. *13 *1 "Understanding and Preventing Crevice Corrosion of Titanium Alloys" 57-62 October 1992 Materials Performance *2 "A Case for Titanium's Resistance to Microbiologically Influenced Corrosion" 58-61 January 1991 Materials Performance *3 "Effects of Alloy Composition and Microstructure on the Passivity of Stainless Steels" 376-389 Vol.42 No.7 1986 Corrosion *4 "The Corrosion Behaviour of Cr-P Alloys in Hydrofluoric Acid" 599-613 Vol.34 No.4 1993 Corrosion Science *5 "Corrosion Behaviour of Boron and Phosphorus-Implanted Fe-40Cr Alloy in Reducing Acid Solution" 127-132 Vol.64 1993 Applied Surface Science *6 "Crevice Corrosion Resistance of Commercial and High-Purity Experimental Stainless Steels in Marine Environments 574-581 Vol.46 No.7 1990 Corrosion *7 "Corrosion Study of Industrially Sintered Copper Alloyed 316L Austenitic Stainless Steel" 46-50 Vol. No.1 1991 British Corrosion Journal *8 "Electochemical Tests to Assess Resistance to Crevice Corrosion in Sea Water of Some Duplex Stainless Steels" 45-47 Vol.21 No.1 1986 British Corrosion Journal *9 "The Effect of Calcium Additions to Aluminium Containing Flux on the Mechanical Properties of Structural Steel" 93-96 Vol.64 No.1 1993 Steel Research *10 "Ranking Alloys for Susceptibility to MIC - A Preliminary Report on High-Mo Alloys" 55-57 January 1991 Materials Performance *11 "Microbiologically Influenced Corrosion of Austenitic Stainless Steel Weldments" 52-54 January 1991 Materials Performance *12 "Overview: Super Duplex Stainless Steels" 685-700 Vol.8 1992 Materials Science and Technology *13 "Highly Alloyed Stainless Steels to Cope With Corrosion" 117-123 December 1990 Tappai Journal Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=4962