Corrosion
Resistance of Titanium:
The
corrosion resistance of titanium is the result of a tenacious surface oxide
composed of titanium dioxide that autogenously repairs itself when damaged in
the presence of even very low levels of oxygen or water. This ceramic-like
corrosion resistance of titanium can be relied upon to resist corrosion in
seawater.
Commercially pure titanium is immune to general corrosion in seawater and
brackish water to temperatures as high as 130°C. Low levels of alloying
additions such as palladium in the case of Grades 7, 11, 16 and 17 or nickel or
molybdenum in the case of Grade 12 will extend general corrosion resistance to
temperatures in excess of 260 °C. Commercially pure titanium (Grades 1, 2, and
3) is immune to crevice corrosion in aerated seawater to temperatures of at
least 70°C. In deaerated seawater, commercially pure titanium will resist
crevice corrosion to temperatures as high as 94°C. When higher service
temperatures are required or crevices cannot be engineered out of the process
equipment titanium grades containing alloy addition can be applied to provide
protection from crevice corrosion.
Pitting is the localized attack of the exposed metal surface in the absence of
crevices. Titanium is highly resistant to pitting attack in seawater unless
impressed currents higher than plus-5 volts are applied. Titanium is routinely
used in impressed current systems as the anodic breakdown potential exceeds
that of most common engineering materials.
Hydrogen Damage
Titanium is resistant to hydrogen damage in a wide range of applications
including galvanic couples and impressed current systems. The naturally
occurring oxide film on titanium protects the base metal from hydrogen
absorption which would result in reduced ductility of the metal. Factors
required for hydrogen damage to titanium are: mechanism for generating nascent
hydrogen; metal temperature > 80°C; solution pH <3 or >12
Galvanic corrosion is not normally a concern for titanium due to the noble
nature of the metal. Coupling with dissimilar metals will not result in
corrosion issues as long as the entire system remains passive. If active
corrosion is occurring in the system, then potential for hydrogen damage to
titanium is possible. Factors which influence galvanic corrosion are the
cathode to anode surface area ratio, the solution chemistry and temperature as
indicated in the section on hydrogen damage. Avoiding galvanic corrosion can be
accomplished by coupling with a more compatible metal, electrical insulation of
the connection or designing the system in 100-percent titanium.
Erosion Corrosion
The hard adherent oxide on titanium provides a high level of protection from
erosion corrosion in flowing seawater even when sand particle are entrained in
the process steam. Velocities as high as 30 meter/second are acceptable for
titanium when no sand is present and 5 meters/sec when heavily laden with sand.
Microbiologically influenced corrosion (MIC) has been reported for all
engineering metal and alloys with the exception of predominantly titanium and
high chromium/nickel alloys. MIC can occur over a wide range of temperature to
100°C; however, titanium is not affected by microbial influenced corrosion in
flowing or stagnant seawater service.
Materials commonly selected for seawater heat exchanger and piping systems
include alloys which are predominately copper and/or nickel and titanium. Each
of the materials has benefits and limitations in seawater service. Titanium is
resistant to all forms of corrosion in seawater to temperature exceeding 70°C;
super duplex alloys have a maximum reported service temperature of 40°C, but
are susceptible to pitting of welds at much lower temperatures.
Titanium has twice the strength of copper-nickel alloys and is nominally half
the density. The higher strength means thinner wall sections, the higher
velocity limitations for flowing seawater allows smaller diameter pipe both of
which add to space and weight savings.
Summary
The industrial titanium market has expanded globally both in terms of supply
and application to process plant equipment. The expanded supply base has
brought improved availability, reliable delivery and more economical pricing to
the market; the expanded application base has provided a robust reference list
of successful applications for titanium to a variety of industrial
applications. These success stories are fuelling even more interest in using
titanium products to combat corrosion and extend reliability of equipment in
harsh seawater service.
Case Study:
This business case for titanium was
demonstrated in 2010 with the largest-ever industrial project for titanium,
when the Ras Al Khair desalination plant was constructed utilizing near 6,000
MT of titanium tubing. The application of titanium on this large scale was a
result of the proven 40-year history of titanium in power generation and
thermal desalination service together with the improved delivery for titanium
products and the rising price of copper alloys. The government of Saudi Arabia
commissioned the Ras Al Khair (formerly Ras Al Zwar) multi-stage
flash-evaporation (MSF) desalination plant with 100-percent titanium tubes
based on capital costs being better than the costs with historical material
selection. The life of the MSF plant is expected to be more than 50 years,
which also impacts capital amortization and subsequent operating costs.
