Abstract
Pitting resistance equivalent number (PREN or PRE) has been introduced by suppliers as a tool for comparing stainless steels for their relative resistance to pitting. The user of stainless steels has only a very limited knowledge for PREN. Important questions for users are: “Which steel quality is necessary to be applied in a given environment?”, “Up to which temperature can a given stainless steel be applied in a certain environment?” or “Can the plant be operated with an increased chloride concentration up to … ppm?. To answer this questions based on literature data two formulas are proposed. The first is based on a purely linear approach and is limited therefore to chloride concentrations of 500 ppm and higher. The second formula uses a mixed hyperbolic and linear relation for the parameters PREN, chloride concentration and pH.
Practical electrochemical measurements have been carried out to supply additional data for validation of the equation. Limitations and presumtions of the presented formula are discussed. It results in highly accurate critical pitting temperatures (CPTs) when using steels below a PREN of 30. Higher PRENs yields a conservative CPT. Influence of more parameters such as inhibition or acceleration by presence of additional ions other than Cl- and H+ in the solution, purity and amount of segregates of the alloys as well as effect of potential should be included in further research work to obtain a better predictive model.
