Prevention of marine encrustation on bronze propellers

Abstract

From the time that they are immersed into a marine environment, bronze propellers are prone to attack by marine organisms, such as barnacles, coral and algae, which attach themselves to the bronze metallic surface, creating lumps on the propeller, which adversely affect its balance and cause impedance and vibration of the propeller and its boat in the water. Ant-fouling paints are either too toxic for the marine environment of lack smoothness on the surface. These problems have been overcome by a combination of known steps, namely, polishing the propeller to prepare it for electroplating, cleansing to remove all traces of dirt and grease, electroplating with copper to a depth of at least 0.005″ or 0.15 mm, followed by spraying with a standard solution (5%) of sodium hypochlorite and sodium chloride in a suitable container to form a firmly adhering conversion coating of basic cupric chloride and then sealing for at least twenty-four hours. The preferred procedure for polishing is 60# grit size at 3500 sfm for roughing, followed by 180# grit size at 5500 sfm for finishing using grease as a polishing aid. Electroplating with copper to the minimum depth provides a smoothing effect ranging from 70 to 90 per cent. The container in which the hypochlorite spraying is carried out is preferably kept sealed until just before fitting and launching. A life expectancy of five (5) years can be anticipated with minimal maintenance every time the vessel is slipped.

The claims defining the invention are as follow:-  
     
         1 . A process for the prevention of marine encrustation on bronze surfaces, in particular those of propellers, to provide protection from marine activity for an extended period of time, characterised by the following steps:- 
 (a) the cast propeller is first polished to the propeller production standard;    (b) the whole polished propeller is then cleansed, to remove all traces of dirt and grease;    (c) the cleansed propeller is then electroplated with copper to a depth of at least 0.005″ or 0.15 mm;    (d) the electroplated propelle is then placed in a suitable container and sprayed with a standard solution of sodium hypochlorite;    (e) the container is then sealed for at least twenty-four hours.    
     
     
         2 . A process according to  claim 1 , wherein the polishing procedure is 60# grit size at 3500 sfm for roughing, followed by 180# grit size at 5500 sfm for finishing using grease as a polishing aid.  
     
     
         3 . A process according to  claim 1 , wherein the surface cleansing consists of one or more of alkaline cleaning by dipping or electrolytic means, vapour degreasing and solvent cleaning.  
     
     
         4 . A process according to  claim 1 , wherein the copper electroplating baths are alkaline.  
     
     
         5 . A process according to  claim 4 , wherein the copper electroplating bath is an alkaline cyanide bath.  
     
     
         6 . A process according to  claim 4 , wherein the copper electroplating bath is an alkaline pyrophosphate bath.  
     
     
         7 . A process according to  claim 1 , wherein the copper electroplating baths are acid.  
     
     
         8 . A process according to  claim 7 , wherein the copper electroplating bath is an acid sulphate bath.  
     
     
         9 . A process according to  claim 7 , wherein the copper electroplating bath is an acid fluoborate bath.  
     
     
         10 . A process according to  claim 1 , wherein the container is kept sealed until just prior to launching.