The statistical median corrosion rate data for the steels after 

 400 days of exposure were treated by linear regression analysis to 

 determine whether a mathematical expression could be obtained for 

 calculating corrosion rates from oxygen concentration, temperature, 

 and oxygen and temperature combined. The surface data were obtained 

 from Figure 5 and the depth data from Figure 9o 



A linear expression, MPY = 0.5176'02 + 1=127, was obtained 

 for the effect of oxygen. 



MPY = mils penetration per year 



O2 = concentration of oxygen in milliliters per liter of 

 sea water. 



Corrosion rates calculated using this expression agreed very well with 

 those calculated from weight loss determinations after 400 days of 

 exposure as shown in Figure 1. The corrosion rates of the steels 

 increased linearly with oxygen concentration. 



However, this expression is not applicable to other exposure 

 time periods; for example, after 200 or 300 days of exposure. Curves 

 of experimental corrosion rates for 200 and 300 days of exposure are 

 not straight lines as shown in Figure 1. For these time periods, 

 the corrosion rates of steels do not increase linearly with oxygen 

 concentration; they more closely approach a hyperbolic relationship. 



Uncorroded steels corrode at high rates when first immersed in 

 sea water or any oxygenated electrolyte because of the free access 

 of the dissolved oxygen to the surface of the steel. As the time of 

 exposure increases and the film of corrosion products forms, the 

 corrosion rate decreases because the access of oxygen to the un- 

 corroded surface is impeded by the corrosion product film. When 

 the film of corrosion products becomes of such a thickness and per- 

 meability that oxygen diffuses to the surface at a constant rate, 

 the corrosion of the steel becomes constant with time and is known 

 as being under diffusion control. This explains the non-linear 

 increase in corrosion rates of steels with increase in oxygen con- 

 centration after only 200 or 300 days of exposure; i.e., they were 

 not completely under diffusion control. 



Corrosion rates calculated from exponential expressions for 

 temperature and temperature and oxygen combined did not agree with 

 experimental corrosion rates. 



66 



