Coal-tar pitch, the residue from distilled coal tar, is used to manu- 

 facture coal-tar coatings by cutting hack the pitch with coal-tar solvents 

 and usually adding mineral filler (extender pigmentsl such as- magnesium 

 silicate. Most recent developments of the use of coal tar in coating systems 

 is in the coal-tar epoxy systems. These systems contain epoxy resins, 

 pigments, solvents, curing agents, coal-tar pitch and gelling agents. The 

 broader use of coal-tar pitch with a greater variety of resins is continually 

 under development at present. The development of plastic resins for use in 

 new coating formulations is part of the research and development activities 

 discussed in Section IX, Plastics. 



C2) . Cathodic Protection . Maximizing the efficiency of corrosion 

 control requires a thorough understanding of the environment and its vari- 

 ability to which structural materials are exposed. This is particularly true 

 of metals. However, environmental considerations are also important in con- 

 sidering the durability of all construction materials. 



(a) Environmental Variability . Optimizing cathodic protection 

 systems in the marine environment requires a detailed knowledge of the 

 seasonal variability of dissolved oxygen, temperature, and the saturation 

 rate of the water with respect to carbonates. Although complete protection 

 can and is being achieved on many structures without any prior knowledge of 

 these variables, the design of the most economical system utilizing a com- 

 bination of impressed current, sacrificial anodes, and coatings is not 

 possible without detailed knowledge of the environmental conditions. 



The four variables important to corrosion for which there exists a large 

 enough data base to permit general surface water mapping of the oceans are 

 temperature, salinity, dissolved oxygen, and pH. Other variables may also be 

 of importance but enough data are not yet available to significantly evaluate 

 their impact on a global scale. 



Premature anode material failures as a result of variation in environ- 

 mental conditions are being examined in more detail. Attention is being 

 given to electrochemical reactions and conditions at the anode- environment 

 interface when chloride and sulfate ions are discharged in the anodic process, 

 affecting the anode material. 



(b) Buried and Embedded Steel . Although some ten cathodic 

 protection criteria for buried steel structures have been used throughout the 

 world, a universally acceptable criterion is still not available. Frequently 

 different criteria give conflicting evaluations of the state of protection. 

 This situation has been due primarily to the lack of suitable electrochemical 

 procedures to monitor and evaluate the actual state of protection at the 

 structure-soil interface. A great deal of investigation of this problem, 

 both in the field and laboratory, is being done and reported regularly in the 

 literature. 



Some studies indicate that the current density for cathodic protection of 

 embedded steel in concrete is controlled primarily by the rate of oxygen 

 diffusion through the concrete. Measurements indicate that the resistance to 

 oxygen diffusion may be ten times higher through the interface between 

 cement paste and steel than through the concrete cover. • 



394 



