Geoscience Reference
In-Depth Information
reactions as well as to structural changes due to partial annealing of the alloy dur-
ing conditions of prolonged high-temperature service. This is further complicated
by the geometry of the vessel, because the effective stress is more intense in areas
of high curvature. In addition, it is noted that the creep-rupture strengths decrease
significantly with increasing temperature, thereby lowering the maximum permissi-
ble pressure of an experiment at higher temperature.
Alloys of either stainless steel or titanium have a tendency to form an imperme-
able oxide surface, which prevents continued oxidation. Thus, the corrosion resis-
tance of an alloy depends upon the permeability, reactivity, and solubility of this
oxide layer in the corrosive fluid. Obviously, a layer so permeable that solutions
can penetrate and contact the unoxidized metal can react with the underlying metal.
The corrosion can be crevice corrosion, stress corrosion cracking, and intergranular
attack. The crevice corrosion can be prevented through proper agitation of the ves-
sel and polishing of the interior surface.
Intergranular corrosion can be prevented through the use of low-carbon stainless
steel, or by alloying with small amounts of metals forming very stable carbides.
Stress corrosion can be retarded through proper annealing of the alloy used for mak-
ing autoclaves, and through the use of molybdenum-containing austenitic steels
[4]
.
The hydrothermal experimenter should pay special attention to the systems con-
taining hydrogen under hydrothermal conditions. Hydrogen at high temperature
and/or high pressures can have a disastrous effect on alloys used in the making of
autoclaves. It reduces the strength of the autoclaves through any one of the follow-
ing processes: hydrogen embrittlement, irreversible hydrogen damage, or metal-
hydride formation. These problems could be overcome through careful selection of
alloys containing small additives such as Ti, Mo, V, heating in an H
2
-free atmo-
sphere, and using alloys with low thermodynamic activity.
The autoclave selection is usually done by considering the above-discussed aspects
accordingly, for the type of material or compound under investigation, the medium
in which the reaction is taking place, and the experimental pressure
temperature
conditions. For example, the first author of this handbook experienced a dangerous
situation while using hastelloy high-pressure autoclave during the growth of
-Al
2
O
3
fine crystals using glycol-based solvent. Generally, hastelloy contains a high amount
of nickel, which can act as a catalyst and triggers an exothermic reaction in such an
organic system. Although this experiment was carried out at 300
C in a high-pressure
hastelloy autoclave with a pressure
α
temperature range of 4 kbar and 500
C, the auto-
clave leaked at 300
C. However, researchers have successfully synthesized
-Al
2
O
3
at 300
C and lower pressure using much cheaper SS316 autoclaves. Some crystals
can be grown readily within the autoclave without any lining, liners, or cans. For
example, the growth of quartz can be carried out in low-carbon steel autoclaves. The
low-carbon steel is corrosion resistant in systems containing silica and NaOH, because
the relatively insoluble NaFe-silicate forms and protectively coats the ground vessel.
In contrast, the growth of berlinite crystals requires a Teflon lining or beakers because
phosphorus is highly corrosive; it can even corrode platinum if used for a long time.
Therefore, the corrosion resistance of any metal under hydrothermal condition is very
important. For example, turbine engineers have long known that boiler water with
α
