HOT DIP GALVANIZING OF STEEL WINDOWS
This article is based on information supplied by Mr. E. F. Pellone, Research Manager of the Crittall Manufacturing Co. Ltd. We are indebted to Messrs. Industrial Finishing (British)
RIOR to 1914 the metal window was a comparatively expensive article seldom used except for expensive buildings; however, during the 1914-1918 war the British light structural engineering industry received considerable impetus from the necessity of adopting a standar- dised production technique. This, and the development of fast welding methods, made possible a good quality window at competitive prices. As a result, by 1939 the metal window industry had become of appreciable importance in the national
economy.
As the industry grew in size it became evident to the leading manufacturers_that if the metal window was permanently to take the place of its wooden predecessor considerable research must be undertaken into the best ways in which it could be permanently protected against corrosion. In this country, such conditions are onerous. Apart from mechanical damage and abrasion in transit or on site, a window frame may be exposed to marine condi- tions, industrial atmosphere and periods of excessive humidity. After considering various methods of improving the resis- tance of the steel to corrosion it became obvious that the most effective method was Hot Dip Galvanizing. This rust-proofing method the Crittall Company have adopted. Modernised by Research
Although Hot Dip Galvanising is a com- paratively old process, it has been com pletely modernised by exhaustive research, with particular reference to steel windows.
Among its advantages are:-
(1) The window is completely covered
with zinc.
(2) The zinc actually forms a hard alloy
on to the steel,
Fig. 1.
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(3) The coating is non-porous and non-
absorbent.
(4) The coating is both ductile and robust so that heavy hammer blows will not detach it, whilst rough usage in transit and on site will do it no harm if the hot dip galvanizing is properly carried out.
(5) It need not, and indeed should not, be painted before despatch and can be left long periods without painting. (6) The coating deposited is in the neighbourhood of 2.0 oz. per sq. ft. or 0.003 in. in thickness.
The older disadvantage of distortion of the welded structures has been overcome to some extent by using manufacturing methods which minimise residual stresses in the structure. It has, however, been completely eliminated by careful attention to entering and withdrawal speeds at the galvanizing pot itself and rigid temperature control.
The hot dip galvanized window is then despatched to site unpainted. If the win- dows are etched (dipped in a solution of weak phosphoric acid and zinc phosphate) before depatch from the factory they may be painted at any time. If etching is not carried out at the factory, either a mordant should be applied on site or the windows should be allowed to weather naturally for a minimum period of two months before painting.
Life of Coating.
From a study of corrosion samples all over the world it is evident that in normal marine or rural atmospheres a hot dip galvanised coating will last many years in the unpainted condition. So far as fre quency of repainting is concerned, this is
determined by the life of the paint film itself and not as with steel, upon the speed with which rust can form. With a reason- able paint film a period of many years can be expected before repainting.
In industrial atmospheres hot dip gal vanized windows have been thoroughly tried out and proved to be, in our opinion, the most satisfactory and economic finish.
Such windows have been used satis- factorily in gas works, tanneries, foundries, plating shops, sugar beet factories, swim- ming baths, laboratories and so on.
The essential operations of this process as applied to windows may be summarised as follows:
(1) Pickle to remove all scale and to present a clean surface upon which galvanizing will take.
(2)
Washing to remove all iron salts and acid.
(3)
Fluxing. i.e, dipping in a 30 per cent. solution of No. 20 flux.
(4) Drying.
(5) Galvanizing.
(6) Etching or phosphating. Pretreatment
Where throughputs are small, such as 20 tons per week or less and small capital expenditure is required, then hydrochloric acid is the normal pickling agent and a 15 per cent. solution (cold) is used, i.e. one part of commercially concentrated acid to one part of water. With a freshly made up tank the pickling time of hot rolled mild steel is 10-15 minutes. As the iron content builds up in the tank however, the pickling time slows to 30-40 minutes in spite of topping up with fresh acid, then this solution has to be dumped.
The disadvantages of hydrochloric acid
Fig. 2.
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