MARITIME: Rope Corrosion Mystery Unravelled
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Wire ropes are complex machines and the safety of crew and vessels depend on the correct selection and proper maintenance of wire ropes.
Even more important is the correct lubrication and coating of marine wire ropes to protect them from corrosion, premature wear and rapid failure.
Wire rope management should take pride of place on all vessels as well as on winches, cranes, derricks and synchrolifts.
Specialised coatings and wire rope lubricators are necessary to ensure the correct protection to prevent corrosion.
However, equally important is the correct selection of a wire rope of the right design and construction for the specific location and application.
Premature corrosion of critically important wire ropes located in a severe marine application became a major headache for engineers at a Cape based firm of consulting engineers. However, a team of Cape Town based corrosion specialists solved the problem.
The solution was developed as a joint initiative between The Centre for Materials Engineering at the University of Cape Town and Chemical investigation Services, an independent technical investigative practice.
The problem appeared when specially developed galvanized steel wire ropes installed in a Cape marine application, subject to very severe in-service conditions, started showing signs of significant visual corrosive activity. The corrosion began to appear within a relatively short period after installation. The design engineers were concerned that the corrosion potentially threatened the wire rope integrity and their safe usage under load.
The supplier had originally provided the wire ropes on the basis that they could function for at least 30-50 years in the chosen marine environment.
Says Rob Knutsen: “The Centre for Materials Engineering and Chemical Investigation Services have done a number of joint test projects and we complement each other very well. We were approached in 2007 to comment on the premature corrosion activity seen on these galvanized wire ropes. After making a proposal to the engineering consortium, we were contracted to carry out accelerated corrosion studies with a view to establishing the underlying failure mechanism. This we successfully did and presented our findings to the consulting engineers in July 2008.”
The wire ropes that were at the centre of this investigation were galvanized with a special zinc alloy known as Galfan. This galvanizing alloy consists of zinc, aluminium and other special alloying elements. Galfan as a galvanizing treatment is supposed to provide far better corrosion resistance than conventional galvanizing.
“When we first went on site in 2007 to view the wire ropes in their marine application it was quite evident that they had not lived up to expectations” says Simon Norton of Chemical Investigation Services.
“Professor Knutsen and I designed an accelerated corrosion test pattern to stress test pieces made up of new wire rope, old corroded wire rope and coated wire ropes. We set out to test in the severe laboratory atmosphere for +1000 hours and then examined the corrosion attack process”, explains Norton.
Wire ropes are complex machines with a range of mechanical, tribological and material properties at play when they are exposed to a harsh environment of sea spray, rain, dust grit, wetness and variable UV radiation from the sun.
Wire rope is wound from many strands of individual wires which are often coated, for example, with a Galfan layer. Wire ropes are used for winches, mine cages hoists, dragline equipment and drydock synchrolifts. Wire ropes thus fulfil a critical function while being exposed to dust, grit, rock fines and corrosive atmospheres.
Explains Knutsen: “After carrying out our first accelerated corrosion test pattern for more than 1000 hours, we cut open all the wire rope test pieces and after mounting and polishing them, examined them using powerful optical microscopes. This allowed us to clarify the mechanism of corrosive attack that had so severely affected the wire ropes. Our partners Chemical Investigation Services reviewed recent literature on aspects of marine corrosion and this led us to understanding the mechanism of corrosion that had occurred in the wires of these ropes. We reported our full findings to the consortium of engineers who were intrigued by the intricacy of the wire rope corrosion process and the complexity of the marine environment in which the ropes had worked.”
Zinc is normally used as a coating on steel surfaces or steel wires so as to act as a sacrificial material when the steel object is exposed to atmospheric corrosive conditions. Zinc galvanizing not only offers a barrier layer but also offers what is termed ‘galvanic’ protection to the underlying steel surface.
“Our next step in solving the problem was to subject new test pieces of wire rope to a carefully designed cyclic accelerated corrosion test pattern. Our objective was to select a suitable coating system which would provide an additional protective barrier on the wire ropes. This time before we started testing we coated the wire rope test specimens with a range of dedicated protective coatings. After running the cyclic accelerated test program for 28 days we could make a clear distinction between good and bad coatings,” Norton says.
Laboratory based accelerated corrosion testing is often no substitute for field testing, where test panels or in this case test pieces of wire rope are placed at a specific site and exposed to the actual conditions of salt spray, drying cycles, wetness, rain washing, sand abrasion and ultra violet radiation from the sun. The combination of these factors can have devastating impact on materials and anti-corrosion coatings.
“While our laboratory based work had clarified many things for us about the nature of the corrosion of the wire ropes and the role that coatings could play in either worsening corrosion or preventing it, it was only when we examined our first set of on-site test pieces that we saw the severe impact of UV radiation, time of wetness and drying on the condition of the wire ropes. We were able to inform our clients about the variability of the marine environment and how critical this was to their design process,” says Knutsen.
Over a period of nearly 15 months the joint venture between the Centre for Materials Engineering at the University of Cape Town and Chemical Investigation Services has successfully solved a serious engineering headache for the consulting engineers who were responsible for designing and managing the wire rope project.
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