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.
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:
