Excerpt from: The Preservation of Iron and Steel Structures by F. Cosby-Jones, The Mechanical Engineer January 30, 1914
This is the method of protection against corrosion that has the most extensive use, owing to the fact that paint is easy of application, and as a product is cheap; further, it has the advantage of being readily renewable to structures, where all other methods are impossible. Paints applied to iron and steel are engineering materials, and, as such, deserve more study and consideration by engineers; the ” factor of safety” of iron and steel takes the effect of corrosion greatly into consideration; therefore if more care be expended upon the surface the factor might be lowered somewhat in certain cases, provided that sufficient care is given to surface preservation. Paint is not a destroyer of rust, nor will it last for ever, and will only protect iron or steel so long as it remains an adhesive and impervious coating. All paint undergoes alteration, as it absorbs oxygen from the atmosphere. The pigment used may accelerate this absorption. With a good paint on application, the oxygen absorbed is 10 per cent. to 15 per cent. of the weight of t.he oil used in the constitution of that film
The properties of a good paint are, that it shall have no ill effects upon the metal itself, adhere firmly, be easy of application, be hard, tough, and elastic, adapting itself to changes of temperature, and form a smooth and even surface impervious to air, water, and gases. Paint should not remain soft underneath, and harden on the surface, and should not be softened by subsequent coats of paint, nor should it flake off, powder or blister; but gradually wear away as its life comes to an end. The duration of the life of paint varies considerably. On one occasion I had an engine room bulkhead chipped and scraped for the purpose of repainting. The ship was built of iron, and was 23 years old at the time. The coats of paint applied during service were readily removed, but the paint applied by her builders on the engine room side of the bulkhead could not be readily removed as it was so tough and adhesive, seeming to be ingrained to the metal. The iron in the engine room side was in a splendid state of preservation, except where it had been rusted through from the alleyways and tops of the bunkers: certain repairs were being executed at the time for this reason. The paint in the bunker side of the bulkhead was red lead 1/8 in. thick. This could be chipped off in large flakes, leaving the metal quite bare.
The value of paint depends upon the nature, quality and preparation of the oils used; the character and proper selection of the pigment, and the care exercised in grinding and mixing; all of which means expert knowledge, constant attention, good machinery, and considerable expense; hence cheap paints are not to be relied upon. A great deal depends upon the manner in which painting is done, and the preparation of the work before commencement. It should be perfectly clean, free from grease, oil, or moisture, and all scale and rust should be removed. The most efficient method of cleaning iron or steel is by pickling. but unless all trace of acid be removed after pickling it might be a dangerous means of inciting corrosion; it is, however, not applicable to large structures. Sandblasting with dry, and not too sharp sand is more efficient means of cleaning metal in structural work that. is already erected. Failing the last method, all scale should be removed with scaling hammers, that are not so sharp as to leave marks upon the solid metal, then steel scrapers and wire brushes used to remove all traces of rust, until the metal shows clean. All oil or grease should be removed by turpentine or benzine. Much painting is done upon metal when in a moist condition, and under such circumstances it is not to be wondered at that good results are not obtained. Wet and damp days should be avoided; the best results will be obtained when the metal is warmer than the surrounding atmosphere, thus avoiding the liability of condensation of moisture in the atmosphere upon the metal.
Good stiff brushes should be used and the paint well brushed into the metal. A coat of paint should be well brushed out, and should be as thin as possible with due regard given to covering power. One coat of paint should be thoroughly set, not skinned over, before another coat is applied over it, otherwise the paint will soon begin to crack. The usual long handled brushes used in painting the hulls of ships are not at all good from a painting point of view, quite apart from the fact that they are the means of wasting much paint on the bottom of the dry dock; the reason being that the work is too far from the operator’s eye for him to see if the surface is properly covered, and the paint well worked into the surface.
All old paintwork that is to be repainted should be thoroughly washed down with clean fresh water, for the purpose of removing any matter that might be incorporated with the new coat of paint, and act in a harmful manner upon it; moreover, any loose paint should be removed. Within certain limits the longer a paint takes to set the greater its durability. A good paint, under normal conditions, will take eight to 12 hours to dry. Paints containing highly volatile ingredients should be scrupulously avoided in finishing coats, because the film will become porous on drying.
Too great importance cannot be attached to the fineness to which a pigment is ground; also to its thorough amalgamation with the medium with which it is mixed, in order to form a homogeneous mass, so that every particle may be impregnated with the medium; if this is not the case, the particles will absorb moisture and transfer it to the body on which the paint is applied. The medium in which a pigment is ground and mixed is of primary importance, as it is upon it the life of a paint mainly depends. The pigment is to give body to the paint, and it is advisable that it should be such, that of itself it should have no deleterious effect upon the material, or to the medium with which it is mixed. Another important feature of a pigment is to protect the medium with which it is mixed from the action of the sun’s rays where it is exposed to their influence, a paint in this respect is nearly always more durable than the medium of itself would be. Tar is not satisfactory either of itself or as a medium for the protection of iron or steel. It cracks badly under the action of the sun, and contains ingredients which hasten rusting. If these ingredients are removed, a material may be obtained which is fairly satisfactory for coating iron or steel not exposed to sunshine and air; such as for water pipes, &c., underground.
Asphaltum is a natural bituminous material from which bituminous compositions and paints should be made, which will give good results used in enclosed spaces, where extremes of temperature do not exist, and the material is not exposed to sunshine and atmospheric influences.
Of paint mediums, linseed oil is the most widely known and extensively used. It belongs to the class known as drying oils, that are able to dry without the addition of driers, though this is seldom allowed to occur naturally, driers being added. There is a great deal of difference between the drying properties of various samples, and the endurance of a film of dried oil. This oil is now brought from many parts of the world and there is an ever increasing demand for its supply, which renders it more difficult to procure stored and matured oil. A good quality cold pressed oil may contain 5 per cent. of water, a hot pressed 10 per cent., and oil may be made to take up 20 per cent. of water by mechanical means. The complete elimination of all water and aluminous matter from oil is an essential feature in the preparation of paint suitable for ironwork. China wood oil has been used by the Chinese for centuries on their junks with splendid results in endurance; it is now used by many makers of ready-mixed paints, in conjunction with other oils, it causes paint to dry hard, and work well. It. cannot be employed by painters who mix their own paints, as the treatment is not simply mixing, but a complicated process. Soya bean oil is derived from a particular class of bean extensively grown in Manchuria; it is sometimes used in the manufacture of fiat paints, or may be compounded with other oils for certain other classes of paint. Menhaden oil is obtained from the menhaden fish, which is found on the North Pacific Coast of America; properly treated it is a useful oil in the preparation of heat-resisting paints.
Corn oil may be used in the preparation of paste paints in conjunction with linseed oil, it enables the paint to remain soft for a considerable period. A varnish made from china wood oil, or linseed, or both, in conjunction with hard gum resin, constitutes an ideal medium for ships’ paints. The introduction of gum resins is by no means a new idea, having been used at as early a date as 1600 A.D. to form a protective coating to armour. The gum resins used are natural exudations from trees, and are endowed with various properties according to the source from which they are derived. They come from many countries, East Africa, Brazil, New Zealand, &c. Having been buried in the grounds for thousands of years, when dug up they are in a fossilised condition, and constitute some of the most permanent organic materials. The manufacture of varnish is an art which is the outcome of years of experience. Respecting pigments, there are but few that cannot be mentioned as being of themselves bad, in connection with some particular purpose; but combinations, in certain proportions, can, as a rule be made suitable to meet most cases. In order to illustrate the nature of some paints found
to be best adapted for certain specified conditions, two different examples of ingredients contained are given as follows: (1) Zinc chromate, red lead, willow charcoal, asbastine, china clay, boiled linseed oil, raw linseed oil, china wood oil, varnish, turpentine, driers. (2) Zinc chromate, white lead, zinc oxide, willow charcoal, silica, boiled linseed oil, china wood oil, turpentine, driers.
Satisfactory paints can be compounded particularly applicable to each of tile following conditions: (1) Funnel paints to withstand heat and retain colour in sea air. (2) Radiator paints in all colours to stand steam heat without blistering or chipping. (3) White enamels for all interior and exterior first-class work, either fiat or glossy, and capable of withstanding sea air for lengthy periods. (4) Cork insulating paint upon which cork dust is applied, then varnished or painted. This prevents condensation by the retention of heat.. (5) Refrigerator paints that will withstand hoar frost, &c. (6) Antifouling paints containing materials which arrest the growth of seaweed, molluscs, &c. (7) Fillers and rivet compositions for levelling up surfaces, which may be applied by knife or brush.
The constitution of all paints should be in accordance with the nature of the work that they have to perform, and should be compounded to give the best results obtainable under those conditions; therefore it behoves the engineer to give close study to these matters, and superintend the mixing of the best materials he can obtain for the purpose, or to put himself into the hands of a reputable firm of manufacturers, advising that firm of the exact purpose for which the material is to be used, when he should obtain an even better result if only due to the more intimate mixing and grinding of the material.
In conclusion, all structures should be composed of the same metal as far as other considerations will allow; surfaces should be even and free from depressions, cracks, and crevices wherein water can lodge; pockets that allow of the collection of water should be avoided; all members should have free air circulation about them; provision should be made, where necessary, for the drainage of water; joints, rivet heads bolts and nuts should be given special attention to ensure exclusion of water, and all parts that are not easily visible should have careful inspection. When painting has to be done it is advisable that each coat should be of a different shade in order to facilitate inspection and to check the number of coats used. Above all, before a protective coating is applied the metal must be clean and dry, then a good material used and the work must be properly carried out.Home
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