Is it possible to compare the resistance to
chloride attack of several materials of construction?
The Pitting Resistance Equivalent Index (PRE) can be used for such a comparison if the
chemical make-up of each material is known.
The formula for the PRE is:
PRE = % Cr + (3.3 x % Mo) + (30 x % N)
As an example, let's say that one wanted to
compare the resistance to pitting of 316 SS, Incoloy 825, and 254 SMO. The major
components of each material is: 316 SS : 17% Chromium, 2.5% Molybdenum, and 0.10% Nitrogen
Incoloy 825:21.5% Chromium, 3.0% Molybdenum, and no N2 254 SMO:19.5% Chromium, 6.0%
Molybdenum, and 0.18% Nitrogen.
PRE (316SS) = 28
PRE (Incoloy 825) = 31
PRE (254 SMO) = 45
So, we can confirm from this index that under
most circumstances, 254 SMO will be more resistant to cloride pitting attack that both 316
SS and Incoloy 825.
There are a couple of PRE online calculators
that you can use at:
under the "Toolbox" menu item.