peller operates in the open sea water of the Boston Harbor from a barge which serves 

 as a supporting platform. The propeller rig is fitted with suitable brush arrangements 

 to permit cathodic protection to be applied to individual blades. A silver-silver chloride 

 reference electrode is built into the plastic hub cap. 



The purpose of these Boston experiments is to separate the electro-chemical 

 aspects of cavitation damage from the purely mechanical effect. It is assumed that the 

 total damage in a cavitating environment is the combination of corrosion and impinge- 

 ment attack where the total damage is greater than the sum of each individual compo- 

 nent of the damage. It is felt that by arresting the corrosion damage by means of 

 cathodic protection, the severity of mechanical damage can be reduced. So far due 

 to operating deficiencies in the drive mechanism, no data are available. Progress is 

 going forward at present and some information should be reported within the ensuing 

 year. 



Another phenomenon which occurs on a metal surface as a result of a cathodic 

 reaction is the occlusion of hydrogen. The hydrogen atoms adhere to the surface of 

 the metal until removed by oxygen depolarization, mechanical washing or overvoltage 

 in which the atomic hydrogen combines to the molecular state and bubbles off as a 

 gas. One theory worthy of investigation is whether the vapor bubble produced by 

 cavitation boiling can combine with the hydrogen bubble when impinging on a cathodic 

 surface to produce a cushioning effect because of its partial compressibility. ONR is 

 sponsoring some exploratory research on the cavitation damage under cathodic pro- 

 tection conditions at the Hydrodynamic Laboratory of the California Institute of 

 Technology. 



One other point worthy of mention in a discussion of this nature. Corrosion 

 reactions are comparatively slow and therefore accelerated cavitation testing techniques 

 such as the ultrasonic device used by Professor A. T. Ellis may prevent corrosion 

 reactions from having any influence on the total damage produced. Perhaps exposing 

 specimens under cavitation pulses followed by short intervals of quiescense will reveal 

 the more significant data regarding the role of corrosion in a cavitating environment. 



P. Eisenberg 



Mr. Preiser has referred to some suggestions that I have made concerning the 

 nature of cathodic protection against cavitation damage. I should like here to elab- 

 orate somewhat these ideas and thus give the background for the experiments referred 

 to as being ONR sponsored, i.e., work by Dr. A. T. Ellis at the California Institute 

 of Technology. 



As the basis for his experiments on cathodic protection, Petracchi {La Metal- 

 lurgia Italiana, Vol. 41, No. 1, Jan.-Feb., 1949) postulated that cavitation damage is 

 largely attributable to anodic corrosion associated with currents set up between adjacent 

 crystals of the material, such currents resulting from deformation caused by the 

 mechanical stresses produced by collapsing cavities. Actually, similar ideas had been 

 suggested prior to Petracchi's paper — thermal stressing being assumed responsible for 

 such currents. Petracchi applied cathodic protection to a number of specimens in sea 

 water and obtained marked reductions in damage even with very small current densities. 

 On the other hand, experiments can be cited in which very large currents were needed 

 to show any protection. Furthermore, there is the question of what happens in non- 

 conducting materials which are nevertheless susceptible to cavitation damage. Thus, 

 it is necessary to determine the mechanism of cathodic protection in detail — the 

 practical importance is evident. 



The idea of protection produced by a blanket of hydrogen evolved at the 

 cathode (in much the same way as large quantities of air introduced into cavitating 

 turbines) suggests itself immediately. However, for such protection it is likely that 

 extremely large current densities are required to produce sufficient quantities of gas 

 to cushion the cavity pressures. In looking for another mechanism in which the cur- 

 rent densities would be more commensurate with those of Petracchi's experiments, one 



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