Page 757 - Corrosion Engineering Principles and Practice

708 C h a p t e r 1 5 H i g h - Te m p e r a t u r e C o r r o s i o n 709

Chemical reactions between these deposits and the protective
surface oxide can lead to destruction of the oxide and rapid corrosive
attack. In gas turbines, oxidized sulfur contaminants in fuel and
chlorides from ingested air (marine atmospheres) tend to react to
form salt deposits. The presence of sodium sulfate, potassium
sulfate, and calcium sulfate together with magnesium chloride have
been reported in such deposits for compressor-stage components
[14]. Sodium sulfate is usually regarded as the dominant component
of the salt deposits.
Testing has indicated that in commercial nickel- and cobalt-based
alloys, chromium additions play an important role in limiting this
type of damage. Alloys with less than 15 percent of chromium as
alloying addition are considered highly vulnerable to attack.
Refinery heaters and boilers that are fired with low-grade fuels may
be vulnerable to such corrosion damage, especially if vanadium, sulfur,
and sodium contaminants are present at high levels. Vanadium
pentoxide and sodium sulfate deposits assume an important role in this
type of environment. The melting point of one of these mixed compound
deposits (Na SO -V O ) can be as low as 630°C, at which point
5
4
2
2
catastrophic corrosion can set in. In these severe operating conditions
the use of special high-chromium alloys is required. A 50Ni-50Cr alloy
has been recommended over the use of 25Cr-12Ni and 25Cr-20Ni alloys
for hangers, tube sheets, and other supports. Ash and salt deposit
corrosion is also a problem area in fireside corrosion of waste incinerators,
in calcining operations, and in flue gas streams.
15.4.8 Corrosion by Molten Salts
Corrosion damage from molten salts can occur in a wide variety of
materials and by different mechanisms. It has been pointed out that
although many studies have been performed, quantitative data for
materials selection and performance prediction are rarely available [15].
Molten salt corrosion is usually applicable to materials retaining the
molten salt, as used in heat treating, solar and nuclear energy systems,
batteries, fuel cells, and extractive metallurgical processes. Some factors
that can make molten salts extremely corrosive include the following:
• By acting as fluxes, molten salts destabilize protective oxide
layers (on a microscopic scale, this effect contributes toward
fuel ash corrosion described above).
• High temperatures are typically involved.
• Molten salts are generally good solvents, preventing the
precipitation of protective surface deposits.
• Direct chemical reaction between the containment material
and the salt.
• The presence of noble metal ions in the molten salt, more
noble than the containment material itself.