Seawater corrosion factors oxygen

Seawater is a unique environment. A recent worldwide test program was recently completed by ASTM Task Group Gl.09.02.03 to evaluate the corrosivity of seawater at a number of sites [/5]. Though these results indicated the uniqueness of natural seawater, corrosivity was site-specific and influenced by numerous factors. Seawater can vary widely in terms of chemical composition, dissolved oxygen content, temperature, salinity, pH, carbonate levels, flow, degree of fouling, biological activity, and pollution [16. ... 

As a general rule, the corrosion reaction rate in seawater increases as the temperature is increased. This applies only when the effect of temperature alone is a factor. Other variables such as oxygen concentration, diffusion rates, salinity, calcareous deposit formation, and biological activity vary as a function of temperature and must also be considered as to how it affects the overall corrosion rate of a material, component, or system. 

While seawater is a ubiquitous environment and quite similar in terms of chloride content and pH, the corrosivity is site-specific, and likely to be influenced by a myriad of other factors such as temperature, dissolved oxygen concentration, flow, degree of fouling, bacterial activity, and pollution. All of these factors often are interrelated. The test program followed the guidelines provided in ASTM G 52 (Standturd Practice for Conducting Surface Seawater Exposure Tests on Metals and Alloys). 

Base metals and alloys used in medical and dental devices are corrosion-resistant due to the presence of an oxide film on the surface that is protective [49]. These materials are not corrosion-resistant initially as is evident from their positions in the electromotive force series. The galvanic series, a listing of electrode potentials measured in seawater, indicates the changes in the noble and active tendencies of these materials in practical use for this given environment. Passivity is dependent on environmental factors such as solution pH, temperature, ions, oxygen, etc. Some ways of minimizing corrosion of these materials follow. Others are given in the discussion of the types of corrosion that can occur. 

These corrosion types occur in seawater on metals that are exposed in the active state such as unalloyed or low-alloyed steels. The main factor in the rate of corrosion of these non-passivatable materials access to the surface by solute oxygen. In oxygen-free seawater, the reaction rate is practically zero. 

Evaluation of this seawater must consider its movement as well as its oxygen content, since both factors are of equal importance when it comes to transporting oxygen to the steel surface. This is why the corrosion rates in the tidal zone (TZ) are greater by a factor of 1.5 than in the immersion zone (IZ) [15]. 

The mainly similar behaviour of unalloyed and low-alloyed structural steels concerning the corrosion rate in seawater is finally due to the fact that the rusting process is controlled by oxygen access, which is not a material-dependent factor. On the other hand, the kinetics of the partial reaction iron dissolution, which determines the rest potential, is a material-dependent factor. The electrically conductive contact of materials or material parts with different rest potentials results in the formation of elements that may cause increased local corrosion depending on the potential level and area ratio. In practice, such elements occur at welding seams. 

Brackish waters containing less than 1% NaCl have been handled successfully in steel pipes. Seawater imder quiescent conditions can also be stored in steel vessels. The pitting tendency in such cases should be controlled by removing dissolved oxygen. Steel pipes can have a life expectancy of 2 to 5 years in mine waters, depending on their composition. The main factors controlling the corrosion of steel in natural waters follow ... 

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