Alan Robinson, managing director of Arc Energy Resources, discusses the benefits of weld overlay cladding to protect marine equipment and components from corrosion; and reviews the options available using corrosion resistant alloys.
Paints and coatings are readily available to protect surfaces of a ship’s superstructure from saltwater in the atmosphere, but how can marine engineers ensure the long-term integrity of mechanical components and equipment such as pumps, valves and piping against aggressive corrosion caused by the sea itself, either below the surface or in areas subjected to significant amounts of saltwater spray.
In order to maintain the efficient and reliable operation of marine equipment, ships’ engineers need to know what techniques are available to extend the life of new plant or refurbish worn or corroded components and, just as important, what are their cost benefits compared to the use of expensive base materials or replacing the part?
There are a number of options available to protect marine surfaces but the final choice will normally be based on an evaluation of factors such as the application (above or below the water?), required service life, operational priorities, installation deadlines and, of course, budget restraints.
Where budget is not a constraint, engineers can simply specify components in corrosion or wear resistant alloys known to withstand marine conditions. However, this is rarely the case and other, more cost-effective options are usually sought.
Where shipboard components such as valves, pumps, pipe, flanges or fittings require protection, weld overlay cladding is certainly a versatile option, providing the assurance of a heavy-duty metallurgically-bonded protective layer that will not be undermined in chloride environments.
For use in aggressive underwater applications such as propellers and their shafts, dredger buckets and chains, weld overlay cladding should be considered the default option because it will provide additional protection and extended service life for new equipment; as well as refurbishing worn or older equipment that is already badly corroded or eroded.
When repairing equipment, the affected areas can in many cases be pre-machined and, using automated weld overlay cladding or specialised manual welding, rebuilt with a corrosion resistant alloy (CRA) such as complex nickel aluminium bronze. Typically, the repair will be superior to the original metal!
After first identifying the surfaces (new or old) that need to be protected, engineers can choose from a number of welding processes and a wide range of cladding alloys. The final choice will depend on the size and geometry of the components to be clad and the alloy best suited to combat the environment and the surface degradation.
Of the main welding processes, automated GTAW (TIG) is ideally suited to smaller components and applications where subsequent machining will be required. GMAW (MIG), submerged arc and electroslag welding processes are used where larger areas and thicker deposits are required. Faster deposition rates mean these methods also offer cost savings. A wider selection of consumable materials, which may not be produced in the standard solid wire form, is also available.
Even equipment housed in engine rooms and other enclosed areas could benefit from weld overlay cladding. For example, a conventional and inexpensive stainless steel deposit would be adequate for the flange seal faces of valves and pumps that may suffer corrosion during their normal service.
For the most corrosive applications, the use of a higher grade stainless steel, complex nickel chromium or hardfacing alloys is normally recommended. Whilst these tend to be prohibitively expensive if used in solid form, a 3mm thick layer applied to the affected surface will offer the same operational performance and could lead to significant savings from the extended life expectancy of the equipment.
In marine environments weld overlay cladding offers clear advantages because protection can be applied specifically to the areas under attack, eliminating the need to produce a whole component or piece of plant from an expensive corrosion resistant material.
However, the overwhelming advantage is its versatility. Whatever the shape or size of the equipment or component – from a deck pump to power transmission train – there is a process that can be applied and an alloy to counteract the different levels of corrosive attack.
Weld overlay cladding is a proven and recognised cost saving technology that is already well established in the oil and gas industry offshore and onshore. Now, engineers and designers in marine environments can share the significant benefits, both practical and financial, of increased life expectancy.
About the Author: Alan Robinson, Arc Energy Resources, E-mail: [email protected] Web: www.arcenergy.co.uk