Long-term corrosion protection for hulls and water jets
When completed later this year, the two Chantier’s de l’Atlantique-built Corsaire 14000 vessels will each have BAC impressed current cathodic protection systems installed to protect their hull and waterjet propulsion units.
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Corsaire 14000 has four waterjets, two of 2m diameter and two of 1.4m
diameter. The ICCP systems for these two giants are the largest that
BAC has ever supplied and, together with those for two smaller units,
are said to provide the best solution to protect the jet tunnels and
components which subjected to high velocity water flow a
greater-than-normal risk of corrosion.
To control and monitor the BAC installations, reference electrodes fitted adjacent to the anodes and electrically connected into the system, provide a remote control and monitoring facility with an alarm on the bridge.
For the two hull and waterjet protection systems, BAC has used its purpose-designed mixed metal oxide anodes. These comprise a titanium disc with a mixed metal oxide (MMO) coating that activates the titanium electrically, enabling it to function as an anode.
Installation of both anodes and reference electrodes is completed through a welded-in-place assembly with watertight Teflon seals.
Contact: BAC Corrosion Control, Tel: +44(0)1952 290321, Fax:+44(0)1952 290325, Email: bac@group.com
Nonmetallic materials
4.24. What kinds of non-metal things do people use at home and at work in the office? Entitle the text below. Compare metals and non-metals as structural materials.
For example: Both metals and non-metals may be machined.
Two large groups of nonmetallic materials used in structural engineering are polymers and ceramics. Polymers and ceramics are non-metals, however, many plastics may be machined like metals. Ceramics are often employed by engineers when materials which can withstand high temperatures are required.
Almost all biological systems are built of polymers like wood, bone and leather, leaf, veins and cells. In the 20th century man learnt to make polymers of his own. However, most simple man-made polymers show stiffness which is much less than that of metal, wood or bone. That is because wood and bone are composites: they are really made up of stiff fibres or particles, embedded in a matrix of simple polymer.
Man has learnt how to make high-performance composites: glass, carbon, or Kevlar-fibre reinforced plastics (GFRP, CFRP, KFRP). These new materials are stiff, strong and light. They are used in aerospace, shipbuilding, transport, medical and sporting goods.
Composites are stiff and strong. But they are bound by polymers and their properties can change radically with a small change in temperature. Proper design with polymers requires a good understanding of their properties.
Polymers are less stiff, less strong and less tough than most metals. However, by crystallising, or by cross-linking, or by orienting the chains, new polymers are being made which are as stiff as aluminium. They have quickly found their way into production.
Most ceramics show brittleness: if you have dropped a plate on the kitchen floor and seen it disintegrate, you might question whether ceramics have a role as load-bearing materials in engineering. However, ceramic structures have survived longer than any other works of man. The great pyramid of Giza is solid ceramic, so is the Parthenon and the Great Wall of China. Ceramics may not be as tough metals, but for resistance to corrosion, high temperatures, wear and decay are unsurpassed.
Today, cement and concrete replace stone in most large structures. In the past 30 years, a number of high-performance engineering ceramics have been developed. They have a potential to replace metals in many very demanding applications.
No designer can afford to neglect the opportunities now offered by polymers, composites and ceramics. But it is a mistake to imagine that metal components can simply be replaced by components of these newer materials without engineering rethinking.