Typical forms of corrosion within oil and gas extraction
Materials used in oil and gas extraction are affected by several different types of corrosion, often caused by seawater and spray. Common types of corrosion are pitting and crevice corrosion. Stress corrosion cracking (SCC) can affect standard austenitic grades if the material temperature is above 60°C (140°F). These types of corrosion are all localized attacks; general corrosion need not be considered for stainless steels in seawater.
High temperatures, high chloride contents and low pH values increase the risk of localized attacks in any chloride-containing environment. Of these, temperature is usually the most influential factor.
However, there is a fourth important consideration: the electro-chemical corrosion potential of the environment. In seawater, this potential is affected by biological activities on the steel surface. Since seawater is, in a sense, a living corrosive environment, it is sometimes difficult to define exactly what the service conditions will be. At normal seawater temperatures, a biofilm will form on the steel surface and result in a corrosion potential of +300 to +500 mV/SCE.
At temperatures above ~40°C (100°F) the biological activity will cease and the corrosion potential will drop. The use of continuous chlorination, to stop marine growth, may increase the corrosion potential to values as high as +600 to +800 mV/SCE. This, however, can be avoided through the use of intermittent rather than continuous chlorination.
Benefits over Cu and CuNi-based alloys
Stainless steels are very resistant to erosion corrosion compared with Cu and CuNi-based alloys, which are very sensitive to this form of attack. Water in harbors, around offshore platforms, and near chemical plants is often contaminated with, for example, ammonia (NH 3) and sulphides (S 2-). These compounds, even in very small quantities, cause localized attacks on Cu-based alloys, while stainless steels are not affected at the impurity levels involved.
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