Pitting resistance equivalent number (PREN) is a predictive measurement of a stainless steel's resistance to localized

pitting corrosion Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation r ...

based on its chemical composition. In general: the higher PREN-value, the more resistant is the stainless steel to localized pitting corrosion by

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...

. PREN is frequently specified when stainless steels will be exposed to

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appr ...

or other high chloride solutions. In some instances stainless steels with PREN-values > 32 may provide useful resistance to pitting corrosion in seawater, but is dependent on optimal conditions. However,

crevice corrosion Crevice corrosion refers to corrosion occurring in occluded spaces such as interstices in which a stagnant solution is trapped and not renewed. These spaces are generally called crevices. Examples of crevices are gaps and contact areas between pa ...

is also a significant possibility and a PREN > 40 is typically specified for seawater service. These alloys need to be manufactured and heat treated correctly to be seawater corrosion resistant to the expected level. PREN alone is not an indicator of corrosion resistance. The value should be calculated for each heat to ensure compliance with minimum requirements, this is due to chemistry variation within the specified composition limits.

PREN formulas (w/w)

There are several PREN formulas. They commonly range from: : PREN = %Cr + 3.3 × %Mo + 16 × %N to: : PREN = %Cr + 3.3 × %Mo + 30 × %N. There are a few stainless steels which add

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

(W), for those the following formula is used: : PREN = %Cr + 3.3 × (%Mo + 0.5 × %W ) + 16 × %N All % values of elements must be expressed by mass, or weight (wt. %), and not by volume. Tolerance on element measurements could be ignored as the PREN value is indicative only.

Pitting resistance measurement

Exact pitting test procedures are specified in the

ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, a ...

G48 standard.''Full text of standard not available free of charge online but available in libraries''

References

Corrosion Stainless steel {{measurement-stub