The corrosion rs.te.3 of the steels exposed at nominal depths of 

 5,500 and 2,350 feet, in the Pacific Ocean also decreased with time 

 of exposure and were consistently lower than the surface corrosion 

 rates. These Lower corrosion rates are attributed to the combined 

 effects of the differences between the variables at the surface and 

 at the two depths; temperature, pressure and oxygen, concentration. 



Also, the corrosion rates at a depth of 2,350 feet were lower 

 than those at a depth of 5,500 feet. In this case, the lower corro- 

 sion rates at a deptli of 2,350 feet are attributed to the combined 

 effects of the differences between the variables at tne two depths; 

 temperature, pressure aivJ oxygen concentration, Table 1. 



Because of the interdependence of one variable on another, the 

 above differences in the corrosion rates cannot be attributed chiefly 

 to any one variable. For example, the solubility of oxygen in sea 

 water is increased as the pressure is increased at constant temperature 

 but at constant pressure the solubility of oxygen decreases as the 

 temperature increases. 



In their discussion of the effect of temperature, the inter- 

 dependence of the effect of temperature and other rate factors on 

 corrosion is discu;>sed by LaQue and Copson state: " "In general, the 

 effect of temperature on the. corrosion rate depends on its influence 

 on the factors controlling the corrosion reaction,. Temperature may 

 affect the corrosion rate through its effect on oxygen solubility and 

 availability. As the temperature rise? the oxygen solubility in 

 an aqueous solution decreases. Opposed to this is the fact that the 

 diffusion rate of oxygen increases with temperature. The corrosion 

 rate of steel in aqueous s?LijLinns with free access of air reaches 

 a maximum at about 175*^F. On the other hand, in a closed system 

 where the pressure was allowed to increase, the corrosion, rate in- 

 creased linearly at ab(.)ut 3 percent per degree which suggests control 

 by the diffusion rate of oxygen to the steel. Temperature may affect 

 corrosion through its effect on pH. The disscjciat ion of water in- 

 creases with temperature with the result that the pH decreases with 

 temperature (beco.mes more acid). Temperature may also affect corro- 

 sion rate through its effect on films. It may increase the solubility 

 of corrosion products in some cases in other cases cause the pre- 

 cipitation of protective film.- and in stitl other situations change 

 the characteristics of corro-i.jn products to render them more imper- 

 vious to oxygen diffusion." According to H. H. Uhlig:-^^ ''When 

 corrosion is controlled by diffusion of oxygen, the corrosion rate, 

 at a given oxygen concentration, approximately doubles for every 30'^'C 

 rise in temperature." However, T.aQue^^ has pointed out that in flowing 



