RESULTS AND DISCUSSIONS 



The results presented and discussed herein also include the corro- 

 sion data for stainless steels exposed on the STUs for the International 

 Nickel Company, Inc. Permission for their incorporation in this report 

 has been granted by Dr. T. P. May, Reference 12. 



Results from the Annapolis Division, Naval Ship Research and De- 

 velopment Center, another participant in the NCEL exposures, are also 

 included, Reference 13. 



Deep ocean corrosion results from the Atlantic Ocean (Reference 14) 

 and surface seawater corrosion data from the Pacific Ocean (Reference 

 15) are included for comparison purposes. 



The corrosion resistance of the stainless steels is by virtue of a 

 very thin stable film on the surface of the alloy which results from 

 the alloying of carbon steels and chromium. Chromium, being a passive 

 metal (corrosion resistant) , imparts its passivity to steel when alloyed 

 with it in amounts of 12 percent or greater. These iron-chromium alloys 

 are very corrosion resistant in oxidizing environments because the pas- 

 sive film is maintained in most environments when a sufficient amount of 

 oxidizing agent or oxygen is present to repair any breaks in the protec- 

 tive film. 



The corrosion resistance of the stainless steels is further en- 

 hanced by the addition of nickel to the iron-chromium alloys. This 

 group of alloys is popularly known as the 18-8 (18 percent chromium - 

 8 percent nickel) stainless steels. 



In general, oxidizing conditions favor passivity (corrosion resis- 

 tance) while reducing conditions destroy it. Chloride ions also destroy 

 passivity. 



The stainless steels usually corrode by pitting in seawater. Pits 

 begin by breakdown of the passive film at weak spots or at inhomogeni- 

 ties. The breakdown is followed by the formation of an electrolytic 

 cell, the anode of which is a minute area of active metal and the 

 cathode of which is a considerable area of passive metal. The large 

 potential difference characteristic of this "passive-active" cell 

 accounts for considerable flow of current with attendant rapid cor- 

 rosion at the small anode (pitting) . 



Pitting is most likely to occur in the presence of chloride ions 

 (for example, seawater) , combined with such depolarizers as oxygen or 

 oxidizing salts. An oxidizing environment is usually necessary for 

 preservation of passivity with accompanying high corrosion resistance, 

 but, unfortunately, it is also a condition for occurence of pitting. 

 The oxidizer can often act as depolarizer for passive-active cells 

 established by breakdown of passivity at a specific point or area. The 

 chloride ion in particular can accomplish this breakdown. 



Aerated seawater (near neutral chloride solution) can pit stainless 

 steels. Pitting is less pronounced in rapidly moving seawater (aerated 

 solution) as compared with partially aerated stagnant seawater. The 



