June iS, 1908] 



NA TURE 



159 



how TEMPERATURE GALVANISING. 



nPHE coating of iron with zinc in order to protect it 

 "^ from atniosplieric corrosion is an industry which dates 

 bade to 1S46, and is employed upon a very large scale. 

 The original process, and still the chief one in vogue, was 

 to dip the cleaned iron surface into a bath of molten zinc. 

 The zinc forms an alloy upon the surface of the iron, and 

 as zinc is very little acted upon by the atmosphere, whereas 

 iron rapidlv rusts and corrodes, a protective coating is 



thus 

 only 



with 

 keep 



Fig. 



obtained. Furthermore, in the case of tanks, buckets, &c., 

 it is not necessary that the joints should be absolutely 

 water-tight, because when dipped into the molten zinc 

 any slight leaks are filled in with this metal. 



The great advantage of employing zinc as a coating 

 for iron is that it is electropositive to this metal, and 

 thus the tendency is for the zinc to go into solution^ and 

 become oxidised,' instead of the iron with which it is in 

 contact being acted upon. The zinc, however, takes on 

 a protective coating of oxide or oxycarbonate, and 

 further corrosion is prevented, or at any rate will 

 take place very slowly. 



There are, however, disadvantages in connection 

 hot galvanising. In the first place it is expensive to 

 large quantities of zinc in the molten condition. Secondly, 

 there is considerable loss of zinc through oxidation. 

 Thirdly, the zinc tends to alloy with the iron and to form 

 a difficultly fusible alloy. The higher the temperature of 

 the bath the greater the tendency for 

 alloy formation, but, on the other 

 hand, the thinner the coating obtained 

 upon the metal to be galvanised, and 

 thus in this direction a saving in cost. 

 The alloy of iron and zinc produced 

 is extremely hard, and is sometimes 

 used for making bearings, but as it is 

 difficult to separate the iron from the 

 zinc it is a distinct disadvantage to the 

 galvaniser. Finally, the hot zinc is apt 

 to destroy the temper of the iron, and 

 sometimes to distort the shape of the. 

 article to be galvanised. 



Consequently, it is necessary to keep 

 the bath at as low a temperature as 

 possible, the excess of zinc being re- 

 moved by passing the sheets through 

 rollers (Fig. i).' 



Cold galvanising or electrolytic 

 galvanising has therefore been sug- 

 gested, and is actually employed for 

 specific purposes, but there are a good 

 many reasons which have prevented it 

 coming into general use. In the first 

 place, the articles to be galvanised 

 require a more thorough cleansing than 

 is the case with hot galvanising, the 

 least trace of grease being fatal to a 



good deposit. If there happen to be Haws or the joints 

 are not quite tight, these are not filled in by the wet 



1 The diagrams accompanying this article have been reproduced from the 

 Transactions of the Glasgow Technical College Scientific Society from a 

 paper read by Mr. S. Cowper-Coles. 



galvanising process. There are also difficulties in connec- 

 tion with the use of zinc anodes owing to disintegration. 

 .As a rule, therefore, lead or iron anodes are employed, 



and the strength of 

 the electrolyte is 

 maintained constant 

 by circulating it 

 through a filter bed 

 consisting of coke 

 and powdered zinc or 

 coke and zinc oxide 

 (Fig. 2). The dis- 

 advantage that flaws 

 are not coated by 

 electrolytic galvan- 

 ising is in certain 

 cases made use of. 



For example, steam 

 tubes employed for 

 marine boilers may 

 have slight flaws 

 which are practically 

 impossible to locate 

 by mere inspection, 

 but which, if the 



. tubes were actually 



used in boiler con- 

 struction, might lead 

 to very serious acci- 

 dents. If such tubes are electro-galvanised, the flaws 

 are made apparent owing to the inequality of the 

 galvanising, probably partially produced by local action. 



Fic. 2. 



Therefore electro-galvanising has been, and is, employed 

 for coating steel tubes used for the construction of boilers 

 for the navy, not to act as a protective coating, but in 

 order to expose flaws or inequalities. 



Electro-deposited zinc is of a uniform grey colour, and 



NO. 2016, VOL. 78] 



