Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, sometime we see rust on Stainless steel. It is ‘stain-less’ not ‘stain impossible’. Sometime we see rust on Stainless steel.  It means it can corrode. Why???

It has some common forms of corrosion in Stainless steel:

1. Pitting corrosion: (pitting corrosion of stainless steel in a chloride-containing environment)

• Pitting corrosion is an electrochemical oxidation-reduction (redox) process, which occurs within localized holes (cells) on the surface of metals coated with a passive film.

• The passive layer on stainless steel can be attacked by certain chemical species. The chloride ion Cl- is the most common of these and is found in everyday materials such as salt and bleach. Pitting corrosion is a form of localized corrosion, which produces attacks in the form of spots or pits. Pitting corrosion may occur in stainless steels in neutral or acid solutions containing halides*, primarily chlorides (Cl^–), such as seawater.
• Pitting corrosion attacks most often take place at points where the passive layer might be weakened, for example by slag inclusions, a damaged surface or imperfections in the passive layer. Once the attack has started, the material can be completely penetrated within a short time.
• Corrosion products (Fe(OH)₃) form around the pit resulting in further separation of its electrolyte.
• Pitting corrosion is avoided by making sure that stainless steel does not come into prolonged contact with harmful chemicals or by choosing a grade of steel which is more resistant to attack.
• The pitting corrosion resistance can be assessed using the Pitting Resistance Equivalent Number ( PREN ) calculated from the alloy content:

Pitting resistance equivalent numbers (PREN) are a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels, based on their chemical compositions. The PREN (or PRE) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.        

Pitting resistance equivalent number (PREN) is a predictive measurement of the pitting corrosion resistance of various types of stainless steel.

In general: the higher PREN-value, the more corrosion resistant the steel. Steels with PREN-values above 32 are considered seawater (corrosion) resistant. Duplex steel with PREN values 40 or above is used in hydrogen sulfide environments.

It is standard in the oil and gas extraction industries. It depends on type:

• Ferritic stainless steels require a minimum PREN of 35
• Duplex stainless a minimum PREN of 40
• Super austenitic stainless a minimum PREN of 45

Summay: 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.