158 PRESERVATION OF METAI^S USED IN MARINE CONSTRUCTION. 



(3) The steel around the sea chests of the bottom blow discharges, if properly 

 painted, does not show more corrosion than does that around the other 

 discharges, and the writer has never seen a ship docked that had any zinc 

 in the bottom blow discharges, they evidently having deteriorated and been 

 blown out soon after the previous undocking. (4) It has been noticed on 

 many occasions that the only steel attacked by corrosion in the vicinity 

 of the zinc plates was that of the steel screws holding the zincs in place. 

 Therefore the writer fails to see the necessity of the zinc protectors, especially 

 as he has just seen the bottom of a ship on which, after having been in the 

 water over six months, there were no signs of corrosion at any point except 

 on the zinc, when the paint film was unbroken. 



■ The use of zincs and other metals electropositive to steel in galva- 

 nizing, electroplating, or other similar covering processes, as a preservative 

 for it, when continuously immersed in water, is a dangerous one and is not 

 to be commended for the following reasons: (i) The metal covering must 

 be electropositive to the steel or metal to be protected, therefore it is more 

 soluble in water and will dissolve more quickly. (2) When the steel is once 

 exposed to the action of the water it corrodes rapidly over the exposed 

 surface and, if not stopped by some similar covering, it will pit through before 

 other surfaces are uncovered. (3) For hull and ship fittings generally a 

 surface once exposed cannot be again covered. (4) The stable oxides of all 

 metals are lower in the potential series than the pure metals, therefore the 

 oxide of the coating may be lower in potential than the metal to be protected 

 and actually does harm instead of good where the steel is exposed. 



For the protection of the hull construction it therefore remains only: (i) 

 to provide the most homogeneous metals possible both in regard to chemical 

 composition and physical structure. (2) Paint the completed structure with 

 a complete film of the best anti-corrosive paint that can be obtained, after 

 having cleaned the surfaces of all foreign matter and rust and dried them 

 thoroughly. After this film of paint is dried give the surfaces a second coat 

 of the same paint ; after this coat is dry paint with an anti-f ouling paint for 

 all outboard, under-water structures and with the best moisture-excluding 

 paint for all other surfaces. (3) Dock frequently and remove any rust that 

 may be found, and repaint as may be necessary. (4) On inboard structures 

 remove all paint that shows active corrosion under it and, when thoroughly 

 cleaned and dry, repaint as before. 



The subject of proper painting is a very important one and one that 

 must be thoroughly understood to obtain good results. There are three 

 necessary points that must be observed and understood as follows :— How to 

 paint, when to paint, and what kind of paint to use for the different con- 

 ditions the paint has to^^stand. 



