US3086678A - Means for the prevention of disintegration damage to the walls of tankers occasioned in carrying corrosive fluids - Google Patents

Description

April 23, 1963 e. BECKER 3,086,678

I MEANS FOR THE PREVENTION OF DISINTEGRATION DAMAGE TO THE WALLS OF TANKERS OCCASIONED IN CARRYING CORROSIVE FLUIDS Filed Jan. 23. 1957 lnuernor This invention relates to protective means for the prevention of disintegration damage to the walls of tankers engaged in carrying corrosive fluids.

In the structural steel parts of the holds of tankers considerable disintegration damage for example in the form of pitting and surface corrosion occurs after a comparatively short operating time. As the heavily attacked horizontal floor surfaces of the holds are at the same time parts of the outer skin of the ship renewal of the whole bottom of the ship is often necessary after a few years. The high costs involved adversely affect the profit from the transport of raw oil in bulk.

It is known that corrosion damage is caused by electrochemical processes. The electric currents leading to the damage result from differences in potential as a consequence of different composition of the upper surface of the structural part in the presence of active electrolytes such as for example sea Water which is often present in the holds of tankers and in particular in the presence of depolarising material such as oxygen released in the electrolyte.

In recognition of these known causes numerous meas ures for the prevention of corrosion damage in the holds of tankers have already been proposed. They have consisted essentially in the measure of protecting the endangered structural parts with coats of paint or other covering layers which for example can be obtained by the action of protective anodes. Inhibitors of difieernt kinds have also been recommended as a corrosion protection.

None of these processes however is suitable to ensure in an economic manner a certain protection of the container walls from disintegration damage of every kind. Thus, protective coatings of bitumen, tar or asphalt which have the advantage of cheapness and good adherence on the base material, have not proved to be suitable as a protective coating of the tank because of their partial solubility in the raw oil to be transported. Expensive artificial resin paints insoluble in raw oil also could not be used as they do not adhere firmly enough to the base material and thus after a short time become loosened from their bases. As a result not only the protection against corrosion is wholly or partly a failure but also substantial disturbances in the oil loading system occur due to pipes becoming stopped up with paint residue. As regards the alkaline protective layers produced electrolytically by magnesium anodes on the cathodic surfaces during the ballast journey, these are far too weak particularly with the usual tank cleaning procedures by hot water spraying to be able to provide sutficient protection. It has been proved that the inhibitors added to the raw oil cannot give any certain protection from disintegration damage since on the one hand the raw oils in their chemical composition are Very different and therefore it is very difficult in practice to determine the etfective inhibitors under the prevailing conditions and on the other hand the required quantities of inhibitors are often so great that their use for economic and technical reasons is excluded.

In addition to the aforementioned ideas regarding the causes of disintegration damage in the loading contain- 3,03%,fi78 Patented Apr, 23, 1963 ice ers of tankers it has already been shown in literature that the usual mechanical hot water spraying in the cleaning of these containers favours the development of the damage. With the usual tank cleaning process in each minute over 1800 litres of sea water at about C. and a line pressure of at least 12 kg./cm. are sprayed from two rotating nozzles of 10 mm. in diameter each onto the container walls. This has given rise to the idea that thereby the protecting covering layers formed on the container Walls are damaged or removed so that hereby a corrosion attack takes place on the upper surface parts becoming thereby metallically clean. Up till now it has not been possible to suggest any effective measure for the prevention of these damages under the said spraying conditions.

The present invention stems from the cognition that the water spray cleaning of the containers of tankers not only promotes the pitting corrosion to a large extent but in most cases is also its starting cause. Cavitation phenomena appear to play a substantial role.

A confirmation of this knowledge has been given by experiments in which from a nozzle of 5 mm. diameter Water at a temperature of 60 C. and a line pressure of 10 kg./cm. is sprayed onto steel plates, whose upper surface was not treated after hot rolling at a distance of 1 meter. Different steels were subjected to this treatment for several weeks. In this connection all test pieces thus treated showed at preferred locally defined places of the upper surface an average hourly disintegration of about 0.01-0.02 mm. The experiments showed that under the foregoing conditions on the other hand no disintegration damages were caused when test plates with very smooth, for example polished upper surfaces were used.

The invention has as its object the protection of structural container parts in tankers, which are usually made from heavy and medium plates and of course having a comparatively rough upper surface from water impact damage and corrosion.

According to the invention it is proposed to deposit on the structural container parts to be protected covering layers of small thickness (about 3 to 4 mm.) of preferably organic materials having a good adhesive power on the base material and then to deposit, on these covering layers sheets of suitable strength and elasticity (because of bending of the hull of the ship during travel) with an upper surface as smooth as possible, the sheets being deposited and in particular pressed, so that no gap remains between the cover layer and sheet.

Since, as a result of the covering over of the deposited covering layer by sheets it is immaterial in accordance with the invention whether the covering or protection layers have the ability of dissolving in the liquid constituting the load, inexpensive materials such as bitumen, tar or asphalt may be used.

As sheets, preferably fine plates of about 1 mm. thickness are suitable, the upper surface of which, turned towards the water spray or the inside of the container, is provided with a high gloss by means of annealing lacquers or enamel or also by artificial resin layers. Unprotected fine plates may be used having a smooth upper surface. Soft annealed cold rolled plates or hot rolled plates are suitable which after rolling have a smooth and firmly adhering rolling skin so that its smooth sufrace finish corresponds approximately to that of the cold rolled plates. The surface of these plates should have a roughness depth of between 1 and 10 microns as taught by DIN 4763, published in Deutsches Normen, September 1954. Unprotected fine plates are cheaper in prime costs than plates protected by lacquer, enamel or the like but have a shorter life as in time a roughneing of their upper surface takes place by eletcrochemical influences which (a) The copper content of which is below say 0.12% and in which the sum of the contents of tin, arsenic and nickel is below say 0.10%,

(b) The copper content of which is below say 0.12% and in which the sum of the contents of tin, arsenic and nickel is over say 0.15%,

(c) The copper content of which is over say 0.30% and in which the sum of the contents of tin, arsenic and nickel is below, say 0.10%,

(d) The copper content is over say 0.30% and in which the sum of the contents of tin, arsenic and nickel is over say 0.15%.

Non-metallic materials of sufiicient elasticity and stren th with suitable smooth surface finish such as for example, artificial resins and the like may also be used as protective sheets.

The protective sheets are preferably bent down or chamfered at the sides about 2 mm. deep for better anchoring and for lateral fixing of the intermediate sheets and pressed into the covering layer. The sheets may also or in addition he fixed mechanically by suitable metal or non-metal holding means, for example by means of clips or clamps. In any case the covering layers are completely covered by the protective sheets.

The carrying out of the measures proposed according to the invention may for example be efiected in such a manner that suitably treated fine sheet metal bands are placed between longitudinal bearers and over the covering layer located on the floor of the tank or that larger and smaller sheet metal plates similar to tiles are placed close against one another on the covering layer. Since the covering layers are practically completely covered by the sheets, as already mentioned above, there is no solubility of the covering layer in the material to be transported. I

By"virtue of"the proposed measures the plates of the ship are not affected by the cleaning jet of the sea water. The walls of the container are protected in an economic manner from water impact damage as well as from corrosion. The protective sheets also withstand the heaviest treatment by hot Water spraying at high pressures and any bending of the body of the ship. Of course after a long time in particular when it is a question of sheet metal sheets without upper surface protection disintegration damage can occur. The damaged plates or plate parts may however be replaced in a short time and at small cost by new ones. Even if after a long time when there is insufficient control pitted sheets are left on the structural parts of the container there is always under the sheets the covering layer which prevents for a time the exposure of the plates of the bottom of the ship. Even when finally the plates of the bottom of the ship are exposed at small locally defined places no serious damage can occur here as on the small surfaces only the occurrence of small amounts of current is possible and particularly large cathodic araes, which as is known are the requirement for dangerous pitting damage, cannot be formed.

Proposals are already known to protect the walls of containers from corrosion in that in the containers continuous sheet metal skins of corrosion resistant material or special chemical composition are inserted. This measure however is very expensive and cannot be practically and economically carried out in the containers of tankers. The protective arrangement according to the invention on the other hand does not require continuous sheet metal skins nor expensive alloy sheet metal material. It is only a question that the upper surface of the sheets is sufficiently smooth and that by means of the sheets both the covering layer lying underneath as well as the structural parts of the container are protected from the action of the water impact and from chemical reaction with the loaded material.

The drawing illustrates two embodiments of the invention.

FIG. 1 shows a part of the bottom of the ship 1 with the longitudinal bearers 2. A bituminous covering layer 3 is deposited on the bottom of the ship 1 between the longitudinal bearers 2. A thin plate 4 which is chamfered or beat down at the longitudinal bearers lies flat on this covering layer as a covering sheet.

FIG. 2 again shows the bottom of the ship 1v and a longitudinal bearer 2, the covering layer 3 and the thin plate 4. The difierence between the illustration 1 and that of illustration 2 consists in the fact that there is not a single large covering sheet lying between the longitudinal bearers but a number of small chamfered tilelike sheets placed against one another.

What I claim is:

Protective means for the prevention of disintegration, particularly pitting, in the holds of tankers, comprising a thin protective coating of bituminous adhesive applied to the inner surface of the hold to be covered, and an exposed inner, self-protective lining of thin steel plates applied to said coating in intimate contact therewith and completely covering said coating, said plates having a chemical composition insuring high resistance against pitting and having a smooth polished inner surface, the roughness depth of which is between 1 and 10 microns, said steel plates being applied to said coating by bending down the sides of said plates and pressing the bent-down portions into said coating.

\ References Cited in the file of this patent UNITED STATES PATENTS Re. 19,537 Pierce Apr. 16, 1935 Re. 22,246 Baird Jan. 12, 1943 1,268,987 McMullen June 11, 1918 1,327,917 Kellar Jan. 13, 1920 1,748,575 Maskrey Feb. 25, 1930 1,790,643 McGary et al. Feb. 3, 1931 1,932,726 Egloff et al. Oct. 31, 1933 1,952,705 Eglofi et al Mar. 27, 1934 2,186,185 Walker Jan. 9, 1940 2,246,955 Scheuch June 24, 1941 2,343,225 Pray et al. Feb. 29, 1944 FOREIGN PATENTS 481,320 Germany Aug. 19, 1929

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