Titanium Continues to Demonstrate Its Value in Seawater Service, Especially Desalination
The technical case for titanium’s application to seawater service was well established in the early 1970’s and performance of titanium over the last 40 years has validated the technical case. Widespread use of titanium for process plant application has grown significantly over the last 40 years in spite of comparative cost with competing alloys. This challenge began to reverse in the early 2000s as global expansion of titanium production accelerated and the cost for competing alloys continued to rise.
Even considering the recent collapse of base metal prices, titanium still has an economic advantage over materials comprised predominately of copper and/or nickel. While the price gap with these more common materials of construction has narrowed titanium still brings an economic benefit to projects.
Given the expanding global production of titanium in recent years, the application base for titanium was able to grow, which allowed the stocking of service centers worldwide to support existing and developing applications. According to information presented at the 2015 TITANIUM Conference and Exhibition, global titanium sponge production, the total output estimated for China, Russia, Japan and the United States, registered 150,000 metric tons (MT) in 2014. Concurrent with this improved market position for titanium, the global nickel and copper industries faced increasing demand, rising energy prices and most importantly declining ore grades. Producing commodities from low-grade resources requires higher energy input and capital intensive plants for the processing of large tonnage and low-grade, run-of-mine ore.
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.
There are two basic technology categories for desalination systems: membrane/reverse osmosis, and thermal distillation (evaporation). Thermal distillation is broken out into three sub categories: MSF; multiple effect evaporation (MED), and vapor compression. Titanium finds most of its applications in the thermal distillation category, use for tubing, valves and water-storage vessels. By contrast, titanium has only limited use in reverse osmosis systems, primarily in pump heads.
With titanium often the economic choice for seawater application the importance of the technical case cannot be overstated and should be revisited. Many application engineers have ignored the potential for titanium based on the misconception that titanium is hard to find and if found was too expensive to consider. For that reason the technical case for titanium in seawater service is presented here in this article.
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. The 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.
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.
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.
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 fueling even more interest in using titanium products to combat corrosion and extend reliability of equipment in harsh seawater service.
(Editor’s note: Rob Henson is the chair of the International Titanium Association (ITA)’s Industrial Sub Group and the business development manager at VSMPO Tirus US)