403 



The corrosion rate of uncoated plain carbon steel in aerated quiescent 

 seawater is normally as high as 0.25 to 0.^40 mm/year (10 to 16 thousandths of 

 an inch per year [MPY] ) for the first few months of immersion-^' '-'. 

 Gradually, as the fouling and corrosion product layer builds up, the above 

 rate decreases^ to 0.03 to 0.13 mm/year (1 to 5 MPY). In addition, this 

 rate depends directly on the concentration of dissolved oxygen present in the 

 seawater^ and on the pH of the seawater^. Within the range of pH 4 to 10 

 the corrosion rate of steel is independent of pH and depends only on how 

 rapidly oxygen can be supplied to the steel surface. As the pH becomes more 

 basic than 10, however, steel becomes passive in seawater and the corrosion 

 rate drops rapidly to a negligible value^. 



The well known corrosion behavior of steel in seawater described above 

 may partially account for the good condition of the recovered nuclear waste 

 container. Although dissolved oxygen measurements were not made at the drum 

 recovery site, it is generally found that the dissolved oxygen in the deep 

 ocean decreases rapidly with depth from 7 ppm at the surface to a minimum of 

 about 0.5 ppm at 750 meters, then rising again typically to 2 or 3 ppm at 

 great depth. Occasionally the dissolved oxygen at great depth may rise again 

 to a value as high as that at the surface or even higher. 



Given the good condition of the recovered drum, it can be speculated that 

 the dissolved oxygen in both the water and the sediments at the drum recovery 

 site was low (perhaps 1 to 2 ppm). This, coupled with the low temperature to 

 be expected at great depth, would account for the relatively low corrosion 

 rates observed. In addition, the concrete inside the drum was saturated with 



