When pursuing suspected smugglers through the waters off northern Australia or rushing humanitarian aid to a cyclone-ravaged Pacific island, the last thing the Commanding Officer of a naval ship needs to worry about is whether the hull will leak, or a critical system will fail, due to corrosion.
Australia’s maritime industry operates in a wide variety of open water and coastal environments ranging from hot, humid tropical, to windy, freezing sub-Antarctic. According to international standards, most of these are classified as having very high to extreme corrosion severity, containing high levels of salt-laden aerosols.
The Royal Australian Navy (RAN) has all its water-borne assets and most support infrastructure exposed to these environments. For the RAN, corrosion has consequences in addition to the economic ones faced by other organisations.
The RAN consists of approximately 50 warships including frigates, destroyers, amphibious landing ships, submarines and patrol boats. It also operates minehunters, resupply vessels and hydrographic survey ships. In addition to its vessels, the RAN’s rotary wing aircraft are integral to its operations. Generally, warships can tolerate higher levels of corrosion causing structural damage than aircraft.
All of the RAN’s vessels, equipment and structures must be protected to minimise the impact of corrosion. Traditionally, naval maintenance was carried out on fixed-time schedules such as the rolling hull survey of RAN frigates. Such programs do not allow for the impact of the actual operating environment encountered by individual vessels and its impact on operational availability.
However, when an asset is managed effectively, the impact of corrosion can be minimised. According to a Department of Defence spokesperson, the RAN has implemented a “whole of life” management plan for its assets as recommended by government reviews carried out in 2008 and 2011.
“The plans factor the overall costs for maintenance into the acquisition process,” the spokesperson said. “A culture of accountability has also been instituted which considers the risk-versus-cost benefits of deferring preventative maintenance.”
There are a number of ways to estimate the financial cost of corrosion of RAN assets, including scaling the costs of a foreign navy operating similar warships. According to the RAN spokesperson, the economic impact of corrosion and its degradation of naval assets and infrastructure is estimated to be between $135m and $650m annually, depending on the metric used to calculate the cost of maintenance and remedial repairs.
The RAN is now planning condition-based maintenance, which optimises maintenance costs by only intervening when a monitored system’s condition indicates a problem. Corrosion Prognostic Health Management (CPHM) uses these principles to predict current and future corrosion conditions based on platform usage in an operational environment. CPHM uses a combination of sensors and models to predict and plan future maintenance activities and operations.
“Moving from a time-based to a condition-based maintenance system through use of environmental and corrosion sensors, allied with corrosion prognostics and modelling, should allow targeted maintenance and more efficient use of limited resources,” the RAN spokesperson added. “It will also allow improved scheduling of maintenance, resulting in optimum usage of platforms and equipment.”
Ship staff on a warship are directed to report changes in appearance or obvious signs of corrosion. If staff have the relevant training, they may apply corrective maintenance if it is safe and timely to do so. However, maintenance at sea or on operations is usually only carried out if it is high priority. The majority of maintenance is carried out while the ship is alongside or in dry-dock in accordance with the technical maintenance plan.
The RAN’s mission includes maintaining Australia’s sea lines of communication and defending Australia’s sovereign interests. Additionally, it undertakes humanitarian missions around Australia and the Asia Pacific region. From a long-term perspective, corrosion shortens the “life of type” of vessels, which can create a capability gap as it may be several years before replacement vessels come into service and may have a large impact on the ongoing maintenance budget as the older vessels are kept for a longer time than planned.
It is important to manage corrosion in the defence industry using preventative measures over both short and long timescales. Unidentified or untreated corrosion issues can lead to unplanned corrective maintenance that can affect availability, readiness, safety and capability of RAN vessels and rotary wing aircraft for operations, all of which result in significant costs.
Protective coatings are critical to mitigating corrosion as they form the barrier between the metal and the aggressive marine environment. The RAN spokesperson stated that a paint specification document that details the coatings systems to be applied to every part of the ship, from underwater hulls, superstructure and decks to all internal areas, is prepared for each vessel prior to construction.
Safety-of-flight is paramount for the RAN’s air assets – this demands very strict inspection and maintenance programs to mitigate risk of failure. Failure of a key component in flight could result in accidents leading to loss of the helicopter and loss of life.
Areas of naval vessels subject to continuous water immersion, such as the underwater hull, bilges and tanks, are considered critical as these areas are more prone to coating failures and corrosion. Corrosion and loss of metal in tankage is a major factor in determining the life of commercial vessels. Freeboard areas, which are subject to continuous water spray while underway, and ship decks are also considered critical.
Research into the development of specialised protective coatings for military applications is supported by the Australasian Corrosion Association (ACA). The organisation works with private companies, not-for-profit bodies and academia to research all aspects of corrosion prevention and mitigations. The ACA provides an extensive knowledge base that supports best practice in corrosion management, thus ensuring all impacts of corrosion are responsibly managed, the environment is protected, public safety enhanced and economies improved.
Several ACA members are currently engaged in a range of Defence-related projects. Professor Maria Forsyth, Chair in Electromaterials & Corrosion Sciences at Deakin University, and Senior Research Fellows Dr Katerina Lepkova and Dr Laura Machuca Suarez at Curtin University’s Western Australian School of Mines: Minerals, Energy & Chemical Engineering are investigating anti-corrosives such as lanthanide-based ones to mitigate microbially induced corrosion and special anti-corrosive pigments.
A related anti-corrosive pigment project is being undertaken by researchers Dr Sam Yang and Dr Tony Hughes at the CSIRO’s Materials Science & Engineering division, specifically looking at ways to transport these pigments to defect sites.
Associate Professor Scott Wade from Swinburne University’s Faculty of Science, Engineering & Technology is researching microbiologically influenced corrosion of piping materials and mitigation options, and high-velocity oxygen-fuelled (HVOF) coatings for marine hydraulic applications.
For many decades, the RAN employed solvent-based gloss alkyds as the topside coating for vessels, but these were not very durable, often failing in as few as six months. In the 1990s, a polyurethane coating was introduced but this has since been replaced by a Low Solar Absorbing (LSA) Polysiloxane coating, which has a colour-stable pigment that provides improved visible camouflage in the waters around northern Australia. The polysiloxane also has improved thermal protection reducing the cost of cooling ships, where trials of patrol boats in norther Australian waters showed that the surface temperature of the polysiloxane coating could be as much as 15 degrees Celsius cooler than traditional coatings.
The development of rapid-cure, ultra-high solids, two-pack epoxy amine coatings technology offers potential advantages for the RAN, especially when dealing with complex internal surface geometries found inside sea water ballast tanks, which feature baffles with cut-outs between bays and numerous longitudinal and transverse stiffeners to provide requisite rigidity and strength. Historically, corrosion on these edges would appear after a period in service, largely as a result of the poor edge retention of conventional coatings that were used.
The latest epoxy amine coatings are applied using specialised plural component high-pressure spray equipment where the two components are mixed at, or close to, the tip of the spray gun and generally require each component to be pre-heated to reduce flow viscosity. The coatings can be spray-applied at high film build without sagging, have improved edge retention and produce very low emissions of volatile organic compounds. These coatings cure very quickly and can be walked on within a few hours, delivering a swift return to service.
The specification of high-performance coatings for RAN ships must be supported by rigorous quality assurance inspections. It is imperative that coatings are applied as per specification or manufacturer’s recommendations, which is best achieved through the use of independent inspectors who are required to witness the condition of the substrate and undertake measurements, such as dry film thickness at key ‘hold points’, during the surface preparation and painting processes to ensure that the environmental conditions are suitable for painting. These include weather conditions, substrate temperatures, and dew point.
Other corrosion management technologies applicable to naval vessels and infrastructure include research into anti-corrosive coatings, for superstructure, underwater hull, tanks and bilges as well as cathodic protection of hulls, bilges, ballast tanks, seawater piping systems.