Galvanized Iron and Steel

GALVANIZED IRON AND STEEL. Of all the meth ods which have been adopted to coat the surface of iron and steel to offer resistance to corrosion the applying of a coat of zinc, al though a very old device, seems up to the present to be the most effective. There are four methods of applying zinc to the sur face of iron or steel, these being as follow:— (1) The Hot Dip Process.

(2) Electrolytic or Zinc-Plating.

(3) Sherardizing.

(4) Spraying of Hot Metal.

Of these the Hot Dip Process is the one most widely used, and the one which, when properly carried out, gives the best coating for exposure to the atmosphere.

The Hot Dip Process.

It seems that the idea of using zinc for coating other metals arose as soon as this metal was is simple. All that is necessary is to have a wrought iron or mild steel pot containing molten spelter, the heat required being applied on the vertical sides. The article requiring to be galvan ized is first thoroughly cleaned in hydrochloric acid, dried, and carefully dipped in the molten zinc after the latter has been sprinkled with sal-ammoniac. The surplus spelter is then drained off and the article plunged into water, washing away any spent sal-ammoniac that may remain on the surface.

The manufacture of galvanized mild steel sheets is one of the most important of the metal industries, both in America and Great Britain where about 900,000 tons of sheets are produced annually.

In the carrying out of the process the sheets are first pickled in batches either in hydrochloric or sulphuric acid, and after the scale has been completely removed they are washed in running water. For the actual galvanizing a plant is used of the character shown in fig. 1. The clean wet sheet is passed throng a flux box contain ing sal-ammoniac and so on through the molten metal, leaving the surface of the bath by a pair of horizontal rollers that serve the purpose of regulating the thick ness of zinc on the sheets. The amount of spelter taken up on the surface of a sheet may vary from 14 to 3 4oz. per square foot, depending upon the kind of process used and the condition of same.

In the galvanizing of sheets efforts are usually made to pro duce them with the largest and best form of spangles (fig. 2), as this type of sheet possesses good selling properties. There is a dif ference of opinion as to what causes the variation in the size of spangles, but without entering very deeply into the scientific explanation it may be said that the spangling is controlled by anything which alters the surface tension of the zinc, the follow ing being the governing factors:— 0) The kind of steel used.

(2) The method and quality of the pickling.

(3) Variation in the sheet annealing factors.

(4) The condition of the sheet surface.

(5) The kind of spelter used.

(6) The temperature of the galvanizing bath.

(7) The length of time the sheet is immersed in the zinc.

(8) The method of galvanizing.

Wire for fencing, telegraph lines and other purposes is gal vanized by a continuous process in which the wires, running known in Europe, during the first half of the 18th century. The earliest patent for Hot Dip Galvanizing was taken out by Crawford in England in 1837. Why the term "galvanize" was chosen is difficult to understand, as no particular galvanic action takes place in the process other than that which is common to all forms of chemical reaction. There is, however, a galvanic or electrolytic action brought into operation when the coated iron is exposed to the atmosphere, on account of the zinc being electropositive to iron, this action resulting in the protection of the latter.

In its application to the coating of small articles the process parallel, are passed through an annealing furnace, an acid cleaning cistern, washing and fluxing tanks, and on through the molten zinc of the galvanizing bath, and finally to the winding blocks, as shown in fig. 3.

To regulate the thickness of the coat on the wires it is usual to have a bed of moist sand or other material on the surface of the out-going end of the bath through which the coated wires pass.

The coatings on wire vary very considerably, both in quality and thickness. Two kinds of coatings are shown in fig. 4, the one which is hard and brittle breaking off when the wire is bent, whilst the other is flexible and is tenaciously bound to the surface of the wire. When wire is required to have a thin flexible coat it is usual to pass it through wipers instead of drawing it through a sand bed.

Electrolytic or Zinc-plating Process.

The cold or electro galvanizing process is now commonly used for certain classes of work. Its advantages over hot galvanizing are : (I) Greater economy in the use of zinc, and somewhat healthier conditions.

(2) Complete control over the thick ness of coating required up to a certain maximum.

(3) The depositing of a coat of pure zinc.

(4) Its suitability for articles such as steel springs, etc., which may be affected by the temperature of the molten zinc, if coated by the hot process.

(5) No distortion or buckling in the coating of flat surfaces as with the hot process.

Its disadvantages are .

( I ) The time required is much greater than with the hot process.

(2) It is very difficult to obtain a thick, non-spongy coat.

(3) The coating is usually not as bright as that obtained with hot galvanizing.

(4) Much greater care is required and greater difficulties are encountered to obtain a correct coat than with hot galvanizing.

(5) For articles that have to be made watertight the electro lytic process does not have the same "soldering" effect as the hot process.

To show clearly the difference between the coatings of the hot and cold methods of galvanizing, figs. 5 and 6 should be closely compared.

Many different solutions for the electrolyte are used in con nection with the cold process, but the basis of all of them is zinc sulphate.

Sherardizing.

This consists of heating the material to be coated in metal drums or boxes whilst being surrounded with zinc dust containing metallic zinc. This process is particularly suitable where a thin clear protection coat is required, where the pattern or design on the article will not be interfered with, but it can only be applied to comparatively small articles.

Metallic Spraying.

This is a process which consists of applying a fine spray of molten zinc to the surface of the steel, which has been previously cleaned and warmed. In this process the zinc in the form of a wire or strip is fed at a uniform rate through an oxy-hydrogen or other flame, or through the electric arc, and from these projected on to the surface which is required to be coated. In practice a spe cially prepared metal spray pistol is used for producing the spray.

This method seems to offer some promising results in the way of coating large vessels whilst they are fixed in position. The metal coating, of course, does not alloy with the base metal but simply adheres to it almost the same as a coating of paint or varnish. (See WIRE and WIRE