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.
General remarks - The destructive effects of oxidation - Desirability of an exhaustive inquiry into the best methods of protection and the relative efficiencies of various coatings - Effects produced by locomotive gases and other agencies - Influence of these considerations upon certain types of construction - Inaccessible positions - Durability of foundation bolts, and the like, of marine structures - Attrition of shingle - Mill scale and its removal - The process of pickling mild steel - Proportions of acid bath - Tanks - Period of immersion - Subsequent processes - Coating of boiled oil - Analyses of boiled linseed oil - First coat of paint - Oxide of iron paint - Analyses of oxide of iron paint - Analyses of lead colour or grey paints.
The destructive effects of oxidation upon the surfaces of metallic structures must necessarily engage the most careful consideration of the designer, and as our experience of the life.history of such structures increases and enlarges with the lapse of time, it becomes increasingly evident that the neglect of such precautions as are in our power must have a serious effect in shortening the period of time within which a metallic structure, such as a steel girder or roof principal, can maintain its working efficiency.
The powerful influences of oxidation in structural iron or steel work are everywhere in evidence, while the relative protecting efficiency of the various preservative coatings used can only be determined by the lapse of time, and a careful comparison of the conditions under which they are used ; and notwithstanding the number of coatings which are in the market, an exhaustive inquiry into their relative efficiency and the best possible means of protecting steelwork is still required.
The rapidity with which oxidation can be set up in any structure varies so much with circumstances that it is impossible to give any general rule or estimate by which to calculate its probable effects.
As an example, we have no difficulty in observing the very destructive effects produced by the gases issuing from a locomotive funnel. Such structures as the lower member of the wind screen at the end of a railway station, placed immediately over the position usually occupied by the engine when standing, and consisting, it may be, of angles and lattice bars of light scantling, have been wasted to destruction, while the webs of plate girders in similar positions have had large holes eaten right through in a few years.
The webs of plate girders in promenade pier construction at the point of junction of the girder with the pier decking have also been known to be eaten through into holes, and the plane of junction of the metallic structure of highway road bridges with the surface of the road or footways appears also peculiarly liable to the destructive attacks of oxidation or electrolysis, combined with the chemical effect of the impurities of roadway drainage.
It is, unfortunately, frequently the case that the parts most affected are precisely those least able to be reached by the painter's brush, and least liable to be detected, and this consideration has some force with respect to various details of construction which are, from every other point of view, desirable and efficient. For example, it is frequently found convenient to frame the upper and compressive member of a roof truss with two angles in place of one tee, or similarly the bracing members, or even the tie bars, when they are flats or eye bar links. Such construction is attractive in several ways. Connecting plates are easily inserted, rivets are in double shear, and the whole detail works out neatly. But it is to be feared that when the time comes round (often too long delayed) for repainting, those surfaces which are too close together to be conveniently reached by the paint brush will be neglected, unless special precautions be taken to ensure that they receive their fair share of the protective coating.
A similar example on a larger scale may be found in the tension diagonals of large lattice girders, frequently formed of two separate flat bars placed close together, or nearly so, but not so close as to prevent the entrance of moisture, which gradually trickling down, and perhaps also led by capillary attraction, sets up a powerful oxidising action between the surfaces, to such an extent as in course of time to swell the bars by an excessive accumulation of rust out of their true line, and, while reducing their cross-section, also adds to the stresses in action in the bar.
An instance of this is to be found in a well-known metropolitan bridge, where it has been found necessary to tack the bars together by rivets or bolts, and otherwise to render the surfaces watertight.
It would appear desirable in such cases so to tack the separate bars together at the edges as to render the interior surfaces as far as possible impervious to moisture, while the effective sectional area in tension need not be reduced to any greater extent than would ordinarily occur in the riveted connection at the ends of the bar.
The interior surfaces of riveted box girders and of certain sections of riveted columns are obvious examples of construction where the paint brush can never reach, unless the closed cell be of sufficient dimensions to admit of the working inside of man or boy. This can be realized in the design of large box girders, say, of 60 feet and upwards, and in such instances covered manholes are sometimes provided for access.
It is so far satisfactory to find that in small box girders, inaccessible inside, where the ends are practically hermetically sealed and the entrance of moisture or impure air prevented, experience appears to show that few signs of deterioration are to be found after the lapse of some years.
The imbedding of ironwork in Portland cement or concrete also tends to preservation, although much depends upon the amount of close contact actually obtained and the adhesion between the surfaces.