What is Pitting Corrosion?
Pitting corrosion is a corrosion that occurs in materials having a protective films. It can be seen as an attack with localized holes on the metal’s surface. Pitting corrosion attack can penetrate the materials very rapidly, while some parts of the metal surface remain free from corrosion.
Pitting is vigorous when the solution on the metal surface contains chloride, hypochlorite or bromide ions. Other harmful solutions are those that contain fluorides and iodides, while sulfides and water are known to enhance the pitting process.
How Pitting Corrosion takes place?
When metal is exposed, its available electrons are given up, and thus tiny pits begin to form on the metal surface. This then grows to become a rapid attack that results in massive damage of the metal. The oxidizing cat ion of iron, copper and mercury, among others, enables the formation of pitting even when there is no supply of oxygen in the metal surface. Stainless steel, chromium, passive iron, cobalt, aluminum, copper and associated alloys are all prone to pitting corrosion.
What does Pitting Resistance Equivalent Number (PREN) mean?
The pitting resistance equivalent number (PREN) is a measure of the relative pitting corrosion resistance of stainless steel in a chloride-containing environment. The pitting resistance of stainless steel is primarily determined by its composition. The elements which have a significant impact are chromium, molybdenum, and nitrogen. The formula, which quantifies their respective contribution, is:
PREN = 1 × %Cr + 3.3 × %Mo + 16 × %N
Higher PREN values indicate greater corrosion resistance.
The factor for nitrogen varies according to the source. Some variants of the equation also include tungsten (W).
PREN values are useful for ranking stainless steel grades according to their pitting-corrosion resistance. However, this comparison is only meaningful within the same stainless steel family (austenitic, ferritic, duplex). PREN cannot be interpreted as an absolute value.
Pitting Resistance Equivalent Number (PREN)
The pitting resistance equivalent number (PREN) is a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels based on the chemical compositions of an alloy. To improve the pitting corrosion resistance of stainless steel, such alloying elements like molybdenum (Mo) and/or nitrogen (N) are added. To help in the selection of an appropriate alloy for an application, an equation for the pitting resistance equivalent number has been developed. The higher the PREN value is, the more corrosion resistant the steel is.
Steels with PREN values above 32 are considered resistant to seawater corrosion. Duplex steel with PREN values 40 or above is used in hydrogen sulfide environments. It is standard in the oil and gas extraction industries. An exception is stainless steels with molybdenum content equal to or above 1.5%; these may have a PREN value 30 or more. In these cases the PREN value takes into account tungsten (W) in the alloy and is defined with the formula:
PREN = 1 × %Cr + 3.3 ( %Mo + 0.5 × %W ) + 16 × %N
Selection of the appropriate stainless steel grade depends primarily on the corrosiveness of the application environment and the application’s mechanical property requirements. When more than one alloy meets these requirements, other factors that must be considered include:
- Physical properties
PREN can change depending on the individual elements in the steel or alloy steel composition.
Studies of corrosion failures of stainless steel have indicated that pitting and crevice corrosion types are major problems, and together account for perhaps 25% of all corrosion failures. One reason why pitting corrosion is so serious is that once a pit is initiated there is a strong tendency for it to continue to grow, even though the majority of the surrounding steel is still untouched.
Here is a chart. Remember that this is for comparison sake only and not guaranteed for any specific application: