Specifications define a couple hundred stainless steel grades, differing from one another mainly in chemical composition. Composition is a fundamental characteristic of stainless steel because it determines the alloy’s corrosion resistance, microstructural phase balance, mechanical properties, and physical properties. The table below lists the most important stainless steel grades and the most important molybdenum-containing grades. The most widely used grades are austenitic Types 304 and 316 and ferritic Types 409 and 430.
Table 1: Chemican composition and Pitting Resistance Equivalent Number (PREN) of common ferritic, austenitic and duplex stainless steels with and without molybdenum.
The Pitting Resistance Equivalent Number (PREN) is a measure of the relative pitting corrosion resistance of stainless steel in a chloride-containing environment. Higher PREN values indicate greater corrosion resistance. The formula for PREN is:
PREN = %Cr + 3.3*%Mo + 16*%N
This formula suggests that molybdenum is 3.3 times more effective than chromium at improving pitting resistance, which is true within limits. Chromium must always be present in stainless steel to provide basic corrosion resistance. Molybdenum cannot provide this basic resistance, but it significantly enhances a stainless steel’s corrosion resistance, as the formula shows.
The table shows that ferritic, austenitic and duplex stainless steels with different levels of pitting resistance are available, and the figure below shows that it is possible to select several different grades with similar pitting resistance. For example Types 444 (ferritic), 316 (austenitic) and 2304 (duplex) have similar resistance to pitting corrosion in chloride environments.
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 like physical properties, fabricability, availability and cost are considerations.