Stainless steel does not resist corrosion by being thick; it resists by carrying a chromium oxide film two or three nanometres deep. Chloride attacks that film locally, at a flaw, and once a pit initiates it feeds itself. The pitting resistance equivalent number is the industry's shorthand for how well a given composition holds the film.

$\mathrm{PREN} = \%\mathrm{Cr} + 3.3\,\%\mathrm{Mo} + 16\,\%\mathrm{N}$ — P-R-E-N equals percent Cr, plus three point three percent Mo, plus sixteen percent N. $\mathrm{PREN}$ is a bare index. $\%\mathrm{Cr}$, $\%\mathrm{Mo}$ and $\%\mathrm{N}$ are the chromium, molybdenum and nitrogen contents by mass percent, read straight off the mill certificate with no conversion. Chromium enters at face value — it is the baseline the other two are weighed against. Solve it forward for the index, or backwards for the molybdenum a target demands, which is the question a mill is actually asked.

The weights are the entire content of the index, and the 3.3 forgotten is its classic error. A percent of molybdenum is worth more than three of chromium against chloride, and a percent of nitrogen sixteen — nitrogen being both the cheapest and the most potent of the three, which is why modern duplex grades are alloyed with it deliberately. Familiar landmarks: 304 about 18, 316 about 24, 2205 duplex about 35, super duplex 2507 about 42, and the 6 % molybdenum austenitics about 43. The conventional seawater threshold of PREN 40 exists because 2507 clears it.

Now the honesty, which rhymes with the indices at the start of this chapter. PREN is composition arithmetic, not a measurement. It ranks alloys and it predicts nothing. It has no temperature in it, though pitting resistance collapses above a critical pitting temperature that differs by grade. It has no crevice in it, though a gasket face or a tube-to-tubesheet gap is where high-PREN alloys still fail. It says nothing about the weld metal, which is usually the weakest composition in the joint unless the filler was deliberately over-alloyed. And it cannot tell you a rate: pitting rates are not calculable at all, and a component perforates at its deepest pit, never at its average loss.

Use it as it was meant to be used — to order candidates and to hold a mill to a number — and then test the shortlist in the actual water.