Marine Systems

The corrosion of metals and alloys in natural and chlorinated seawater has long presented challenges for those responsible for materials selection and has been much studied. A number of alloys have been successfully used in seawater services. Copper-nickel alloys and the 300-series austenitic stainless steels are normally considered minimum starting points for materials selection in seawater, with duplex stainless steels, nickel-based alloys, and titanium specified for the more severe services. The choice of an appropriate material for seawater service is a difficult decision that has to be made by a designer prior to specification of the system.

Seawater piping systems and their associated components (pumps, valves, etc.) on naval surface ships suffer from relatively high failure rates in service. As a result, they are a continuing maintenance burden. The costs associated with seawater valves alone have been identified as a significant driver in the overall maintenance budgets of naval surface ships. Such costs have driven ship designers to investigate the benefits associated with alternate materials such as commercially pure titanium to mitigate the corrosion and erosion problems associated with the current copper-nickel piping. However, titanium is galvanically incompatible with most current valve materials. Therefore, new materials that are both compatible with titanium piping and cost effective must be identified for surface ship applications.

Galvanic corrosion is the most frequent cause of unexpected corrosion failures in seawater. It has caused failures of ship fittings and deckhouse structures, fasteners, hull plating, propellers, shafts, valves, condensers, and piping. In sea atmospheres, galvanic corrosion causes failures of roofing, gutters, and car trim. The reason that galvanic corrosion causes so many failures is that it can occur any time that two different metals are in electrical contact in seawater. Since most structures and devices are made of more than one kind of metal, this diversity of materials is common and frequently overlooked in corrosion prevention activities.

Crevice corrosion is another major consideration in component design and alloy selection for seawater service and will often dictate materials selection when crevices cannot be avoided. The nickel-based alloys have been used extensively for demanding applications, where excellent overall corrosion resistance and long term equipment reliability are needed. However, even the highly alloyed nickel-based alloys, those with high molybdenum contents, are susceptible to crevice corrosion, depending on the specific seawater environment. Materials selection to combat crevice corrosion in seawater is especially complicated due to wide variations in seawater properties from location to location, difficulty in comparing various alloy ranking results, and difficulty in extrapolating test results to an anticipated service life in seawater applications.