This section is from the book "Notes On Construction In Mild Steel", by Henry Fidler. Also available from Amazon: Notes On Construction In Mild Steel.
The durability of foundation bolts, rag bolts, and the like depends largely upon this, and there can be no doubt that the condition of much iron and steel work in such situations is greatly a matter of speculation.
In all probability in respect of the great mass of iron and steel structures now existing, a future generation of engineers and architects will not lack employment in their renovation or repair.
The durability of iron and steel work in certain marine structures is frequently very powerfully influenced not only by corrosion, but also by the erosive action of sand or shingle. The cast-iron piles of promenade piers, where exposed to the action of shingle set in motion by tidal or wave action, are sometimes found to be seriously affected thereby, the scouring nature of the shingle or sand producing effects even more serious than oxidation. The same may be said of outfall sewer pipes and the like. Probably the best protection in these cases is a substantial coating of Portland cement mortar or concrete. Roman cement is also used for the same purpose.
The protection of steel surfaces commences, or at all events in good work ought to commence, at a very early period in the construction.
Plates and bars are, as a rule, delivered in the girder or roof builder's yard in the condition in which they come from the rolling mills, that is to say, with no protective medium on their surfaces, unless the thin greyish.black scale, called mill-scale, which is formed during the processes of rolling, and is loosely attached to their surfaces, can be so called.
This scale, which occurs in thin flakes, is but slenderly attached, and can be more or less easily removed. As a consequence, paint applied to such a surface is not unlikely to come off with the scale, leaving bare places unprotected and exposed to oxidation, although in work where the process of "pickling," now to be described, is not used, the paint and the scale probably get mixed up to a certain extent together, unless the surfaces are carefully scraped before painting, which should be the case when pickling is not employed, or if the process of cleaning by means of the sand blast is not adopted.
Where the class of work requires the complete removal of this scale the process of "pickling" is frequently adopted, and it is possibly the absence of this precaution which has, in some cases, led to complaints as to the premature oxidation of mild steel.
The process consists in the immersion of the steel plate or bar in a bath of diluted acid, commonly specified to consist of one part of hydrochloric acid to nineteen parts of water. This liquid is contained in timber tanks of substantial construction, and of dimensions suitable to the largest plate or longest bar to be dealt with. Plates should not be laid flat upon one another when "pickled," but retained in a vertical position, with sufficient space between each. For this purpose tanks for plates are frequently fitted with racks at the ends, into which the plates are dropped, and thus held upright, the handling of the plates being also facilitated.
Bars are laid horizontally and loosely together. The action of the dilute acid upon the surface of the plates or bars is accompanied by the ebullition of a considerable amount of gas, which rises to the surface of the liquid in numerous bubbles, large and small, and this ebullition is a measure of the proper activity of the bath.
After an immersion which may average from seven to fourteen hours, according to the freshness or activity of the bath, and governed to some extent by the time of year, the bath being somewhat more active in summer time, the plates or bars are hoisted out of the bath, and thoroughly washed with pure water by means of a hose. They should then be stacked until they are required for the further shop processes of marking off, punching or drilling, shearing, etc.
The student will observe, in passing, the effect produced upon wrought-iron chains which have been immersed in the acid for a considerable time, in laying bare and bringing to view the fibrous nature of this material.
The surfaces of the plates or bars, when properly pickled, after removal from the bath, will be found clean and smooth to the touch, and free from all appearance of scale, but will soon take on the appearance of premature oxidation, with an evenly distributed thin light yellow rust when exposed to the weather and not further protected.
In order to prevent this, and for the protection of the material during the period, often of considerable length, occupied in the various shop processes of conversion into riveted work, and during which time the plates or bars are often exposed in the contractor's yard to all the vicissitudes of weather, it is frequently specified that the whole of the steelwork shall be covered with a coating of boiled linseed oil, which is brushed on, and if of proper quality will be found in about twenty-four hours to have dried to a tough film, forming an effective preliminary and temporary protection until such time as the first coat of paint can be applied. Manufacturers will occasionally object to this process on the grounds of the slippery nature of the coating making the handling of the plates, etc., somewhat difficult; but it may be argued that the advantages counterbalance the objection.
The following analyses of boiled linseed oil, made as samples of the oil actually used in extensive contracts and used for protecting steelwork in the manner above described, will be found of interest as showing what chemical composition should be looked for, and the variations which will occur in ordinary practice.
Specific gravity at 60° Fahr. ...
Unsaponifiable matter ...
1.390 per cent.
Ash • • • • • • • • . . . • • •
Specific gravity at 60° Fahr. ...
Unsaponifiable matter ...
1.20 per cent.
Ash ... ••• ••• • • •