Pitting Corrosion, PREN and Critical Pitting Temperature

Stainless steel is not affected by a corrosive environment in the same way as ordinary steel is. While the latter is susceptible to general corrosion, stainless steel is not, due to a very thin protective chromium-rich oxide film. It doesn't mean stainless steel is completely resistant to corrosion though, as various types of local corrosion still do great damage to poorly engineered stainless steel structured around the world, with pitting corrosion being an important example.

Pitting corrosion - definition

The protective film is being continuously disturbed and reformed. It means when the film is reformed faster than it disintegrates, the steel part can resist the corrosion for a very long time. However when the film starts to disappear, due to contamination or higher temperature for example, then corrosion centres start to appear, called pits. Most unfortunately, those pits are electrochemically self-sufficient, so it's really hard to stop pitting, once the process begins.

Pitting corrosion - causes

The main factor is the presence and concentration of halides, primarily chlorides, but also bromides, fluorides and iodides. The higher the concentration, the more severe the environment. Another important factor is the temperature - the lower, the better. A secondary factor is the acidity of the environment - the lower the pH, the pitting is more likely.

Pits usually appear there, where the protective film is less strong, due to being contaminated by small particles of ordinary steel, or due to mechanical damage, or in small cavities.

Therefore, a good design should take into account not only the correct choice of stainless steel grade but also of surface finish.

But which grades are most pitting resistant? How pitting resistance can be estimated?

Pitting Resistance

The are various methods allowing to estimate the pitting corrosion resistance of a steel grade. The most common are:

• Pitting Resistance Equivalent Number (PREN)
• Critical Pitting Temperature (CPT)
   

Let's discuss these indicators one by one.

Pitting Resistance Equivalent Number

The PREN is based on the chemical composition of a grade and may serve as a general indicator of pitting resistance. It has limited precision and should be used only for preselection of a material.

The higher the content of chromium, molybdenum and nitrogen, the better the overall pitting resistance.

PREN is obtained through the formula:

PREN = %Cr + 3.3% Mo + 16%N

The PREN of selected grades:

PREN of more than 30 could be viewed as a good indicator of above-average pitting resistance.

It should be noted that PREN does not take surface finish nor heat treatment into account, which also contribute to overall pitting resistance.

Critical Pitting Temperature

The pitting corrosion is accelerated by temperature. Critical pitting temperature means minimum temperature, at which the alloys start to corrode. No corrosion occurs below this point. This is a more reliable method of classifying alloys in terms of resistance to pitting corrosion.

There are several standards describing the details of several test procedures. The most important are ASTM G150 and ASTM G48. Critical pitting temperatures determined by different methods should not be compared.

ASTM G48 CTP values for selected grades:

• X5CrNiMo17-12-2 - 20°C
• X2CrNiMoN22-5-3 - 35°C
• X1NiCrMoCu25-20-5 - 40°C
• X2CrNiMnMoN25-18-6-5 - 90°C

Note: According to the description, the results "can be used to rank the relative resistance of stainless steels (...) to pitting and crevice corrosion in chloride-containing environments. No statement can be made about resistance of alloys in environments that do not contain chlorides".

ASTM G150 CTP values for selected grades:

• X2CrMnNiN21-5-1 - 17°C
• X2CrNiMoN22-5-3 - 52°C
• X1CrNiMoCuN20-18-7 - 87°C

Note: According to the description, "the test is not intended for design purposes since the test conditions accelerate corrosion in a manner that does not simulate any actual service environment."