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Counter-attack on Corrosion
From a undated Esso-publication. ( Thanks to Jullian Lelliott )
New methods of coating steel promise longer protection against corrosion for Esso's large fleet of tankers.
In one important new development, large sections of a new tanker now under construction are being brought
to the assembly yard already coated, atmospheric conditions being properly controlled while the coating was
applied
Applying Rust-ban 190 to fabricated bulkhead sections in controlled atmospheric conditions.
NO wise shipowner would spoil his ship for a ha'porth of tar, and in the Esso group of companies worldwide much
thought and expense have been directed in recent years at the age-old problem of corrosion. With vessels carrying
passengers and dry cargoes, the problem occurs mainly on the exterior surfaces, particularly those that are expos-
ed to both air and sea. With tankers, however, a special difficulty arises because the many individual tanks, into
which the 'hold' is divided, have to make contact with sea water from time to time.
There are several reasons for this: the tanks may have to be cleaned after discharging a cargo of crude oil; or some of
them may have to be filled with sea water as ballast when the ship is returning empty. Whatever the cause, the effect
is the same;
the metal surface of the tanks loses the film of oil which normally lines and protects it and so is exposed to attack.
Tanks carrying light oil products, such as petrol and kerosene, are particularly exposed because these liquids eva-
porate quickly and so give less protection than the heavier oils.
Until recently there was no really promising solution to this problem. Now, however, research has made possible a
number of new protective coatings which can be effective in these conditions. One type-an Esso product called
Rust-ban,
which is used mainly on Esso tankers - is a zinc silicate coating, which gives excellent protection when applied
satisfactorily. The zinc silicate coating process is in three main stages. First, all impurities must be removed from
the metal surface, which is then blasted with grit or shot until it is composed entirely of white metal. Then, a thin
zinc silicate coating (Rust-ban 190) is sprayed on immediately, before the white metal has time to tarnish. Finally,
an acid curing agent (Rust-ban 195) is applied to the coating, so making a firm chemical bond with the metal.
When the process is complete, the coating is hard and resists abrasion very well.
All this sounds straightforward enough. But the coating needs to be applied in a warm and dry atmosphere, and
this presents a complication in the shipyards of Britain and North-Western Europe, where the atmosphere is
damp for much of the year. At present, however, Esso has on order, from the Netherlands Dock and Shipbuilding
Company in Amsterdam, a 47,000 ton tanker (the first of two) each section of which is being prepared and coated
in a special air-conditioned building. The sections are then taken as sealed units to the shipyard for assembly,
and only the welded joints and damaged surfaces have to be coated in normal atmospheric conditions. This new
development should make it possible for zinc silicate coatings to be used successfully in a wide variety of weather
conditions.
Though a coating of this kind is not cheap, the likely saving on structural replacements far outweighs the cost. A
modern ocean-going tanker, built at a cost of1 1/2 -5 million, can be expected to last up to 20 years; but uncoated
tanks become badly corroded much earlier than that, especially in a vessel carrying light oil products. When tanks
do become corroded, a third of the internal steelwork may need renewal at a cost of up to 350,000 Pound, on a
large tanker, although both amount and cost vary greatly from one ship to another.
It is estimated that zinc silicate coatings can prolong the life of a tanker by ten years, virtually to the end of the
vessel's normal life. The Company can then decide, in the light of economic factors and new developments in
design, whether it is better to scrap the whole ship or to replace the corroded parts. When savings of this order
are reckoned in terms of an Esso Petroleum Company owned tanker fleet which totals 54 vessels in service
amounting to 706,172 deadweight tons, together with ten vessels under construction amounting to 576,155
deadweight tons, they are very impressive indeed.
The huge prefabricated sections of a 47,000 deadweight ton tanker, after receiving their protective
coating, are assembled on the slipway
