Other Marine Applications
Jet Pump Dredger
Since the metal spraying process was pioneered in the U.K. by Metallisation Ltd. of Dudley, West Midlands, one of its principal applications has been as an anti-corrosive protective treatment for structural and other steelwork. The Zinc Spraying of a Jet Pump Dredger provides an interesting example of the very widespread use of metal spraying. The dredger extracts sand by high-pressure suction jet, and has an output of 250 tons of sand per hour.
The vital statistics of the dredger are length 41m (135ft), width 6.7m (22ft), and height from the water line to the top of the hoist 4.55m (15ft). Fabricated in mild steel the dredger will spend its entire working life afloat on the quarry lake, so that the problem of corrosion is quite considerable.
Using Metallisation Flame spray equipment, the contractor Thomas Gilks & Son, UK has grit blasted the steelwork and sprayed with zinc. Below the water line a deposit of 100mm (0.004”) zinc has been applied followed by two coats of bitumastic paint; above the water line the dredger hull and deck structure have been grit blasted and zinc sprayed to a thickness of 100mm (0.004”).
In addition to the normal corrosive attack of water, the method of operation creates corrosive and abrasive conditions above the water line so that the best available anti-corrosive treatment is essential. It is anticipated, that the metal sprayed zinc will protect the dredger throughout its entire operational life without further treatment.
Sea Water Mains
A series of 1.8m (6ft) diameter pipes fabricated in mild steel to form a massive Sea Water Main at a Chemical Plant in the UK. As part of the chemical process, seawater is pumped through the Main, which is obviously subject to very considerable corrosive attack from the seawater. To afford the best possible anti-corrosive protection therefore, these massive pipes were grit blasted and then metal sprayed with 100mm (0.004”) zinc using Flame Spray equipment. The metal sprayed surface was then followed by a zinc chromate primer and a finishing coat.
Armature Shaft – Falmouth Dry Dock
The armature shaft on the No 2 main pump motor head, became so badly worn that it was deemed un-serviceable. A deposit of S20/S2 chrome steel was sprayed to a depth of 2.5mm (0.100”) using the Arc Spray system. No disconnection of the armature windings was necessary, a simple cardboard mask preventing the ingress of metal dust during spraying, and after re-machining the component was back in service in less than 24 hours.
Marine Piston
The illustrated piston from a massive marine diesel engine provides an interesting example of more than one characteristic of arc spraying.
The piston, which is 508mm (20”) in depth and approximately 305mm (12”) in diameter, came from a marine engine that had been in use for a considerable number of years, it was not possible to obtain a replacement.
The outer surface was so badly worn that an overall deposit of 1.56mm (1/16”) on the radius was necessary, with heavier build-ups in localised areas where the wear was excessive. Before spraying, the piston was machined and the recessed areas were masked with a No Bond masking compound to prevent adhesion of the sprayed deposit in these areas. A bond coating of aluminium bronze was deposited without any prior grit-blasting, followed by a final deposit of S20/S2 steel. After spraying, the piston was machined and ground to finished size. A point, which is well brought out by the photograph, is the sharp definition, which was achieved on the edges of grooves, holes and recessed surfaces.
Submarine Hull
The above image shows a submarine hull subjected to the thermal spray process in the 1930's. In this instance the hull was sprayed using Zinc with the MK16 Flame Spray Pistol (superseded model). Examples like this give an insight into the history of the Metal Spray process and how long it has been in use.
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