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
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