EN
Among various cooling sources for marine vessels and facilities in coastal areas, seawater is one of the sustainable sources and possesses low cost, but it is one of the harshest corrosive media for heat exchanger materials. There is a lot going on to degrade this environment including: high levels of chlorides and dissolved oxygen, biologically active in high temperatures with suspended solids and temperature fluctuations and suspended solids. Having knowledge of material compatibility is not something very academic to the ship operator and marine engineer but is essential as it can help in reliable ship operation, minimize downtime and also help in cost control of the ship's lifecycle.
Understanding the Corrosive Environment
Heat exchangers of seawater-based cooling systems can pose various combined risks. General corrosion thins metal surfaces slowly, while the pitting corrosion forms small holes in the metal and can result in a failure of the tube at a critical location. Galvanic corrosion takes place when two different metals are connected in the conductive seawater electrolyte, causing an increased attack on the less noble. Crevice corrosion occurs when there is lack of water flow in the crevice under gaskets and tube-to-tube sheet connections. Microbiology induced corrosion (MIC) is phenomena caused by microorganisms, bacteria which breed inside the metals and drastically increase the corrosion processes. The choice among these options would demand due consideration of the appropriate material, with an eye not just to elemental composition, but also the condition of the material service-ie, to its environment of exposure to seawater, temperature, current, and chlorination-regime.
Common Alloy Families and Their Performance
Copper alloys such as cupronickel (90/10 and 70/30) and aluminium brass, have long been common materials for seawater heat exchanger tubes. They have good corrosion resistance, excellent thermal conductivity and are said to be biofouling resistant. However, they suffer from erosion-corrosion at high flow speeds, and accelerated attack when polluted with sulfides. Stainless steels, in duplex, high-strength austenitic and super-austenitic configurations provide excellent tensile strength and pitting resistance properties. High-strength steels, regardless of strength form, are subject to crevice corrosion at the temperatures shown above, and chlorination must be controlled to limit its occurrence.
Nickel based alloys have very high corrosion resistance over a wide range of temperatures but are found to be among the more expensive alloys and this may make them too expensive for use in the more modest in terms of value applications. However, the aluminum alloys are very light in weight and inexpensive, but they have poor seawater resistance and generally need to be used with special cladding to prevent perforating the tube.
The Case for Titanium
The titanium's corrosion resistance and strength make it the best material for seawater-cooled heat exchangers. Near impervious against attack of chlorides, pitting and crevice corrosion is ensured by its passive oxide film even in warm, chlorinated sea water. Titanium tubes hold good resistance to damage by erosion, MIC (microbial induced corrosion) and fatigue and can be used at a high velocity. In most cases the long-term service life of a titanium vessel or facility (typically of over 30 years without need for replacement) is the most cost effective option when including both initial capital and lifetime maintenance expenses as compared to the cost of copper alloys. When it comes to particularly challenging jobs where the integrity of operation is essential, including cooling of offshore platform systems or to the LNG tank top circulation, no material would seem better than titanium.
Mitigation Strategies Beyond Material Selection
When deciding on a particular alloy other factors like usage and design methods are also equally important. Tubes lifespan could be enhanced by use of cathodic protection either with sacrificial anode or by use of impressed current, which will prevent galvanic attack or pitting on the tube material. Coatings provide a positive protection in the form of a physical barrier either using rubber linings to epoxy in solvent-free system, thus delaying corrosion rate on carbon steel elements. Scale/corrosion inhibitors can be effective in preventing under deposit corrosion by having controlled doses to provide protection against pitting as the deposits would not cause attack on the material. Routine monitoring on factors like corrosion potential, water chemistry, tube wall thickness helps in early detection of degradation thereby preventive maintenance can be taken care before catastrophic failure happens.
Conclusion
Material compatibility for use in seawater-cooled heat exchangers calls for a thoughtful approach that takes into account the corrosive nature of seawater, the properties of the materials selected and the cost of installation and maintenance. As part of SME Group, we have over 20 years of expertise in marine engineering and heat exchanger services to assist our clients in making these complex decisions. With the help of our workshop of more than 5000 m² in Nantong and our ISO certification of quality systems our team of over 100 engineers and technicians are qualified to offer full-scale solutions from alloy selection counseling to theoretically planning the retrofit to supplying spare parts and conducting repair and maintenance. From changing tubes on a titanium clad-steel condenser to making higher performance changes, we are here to help. Reach out now to our technical team, ask for a quotation for any service or discover more about our services in terms of case studies and product documentation.