The calculation of the PREN (pitting resistance equivalent) from the content of the alloying elements chromium, molybdenum and nitrogen is a widely used formalism to rank the resistance to pitting corrosion of stainless steels. However, there are other factors like mechanical and chemical surface treatments which largely influence the functional properties of the metal surfaces and thereby the corrosion resistance. These influences were evaluated in a systematic study and correlated to the critical pitting potential measured in 3 % NaCl-solution. Based on these results a new formula, named PRE-S (Pitting Resistance Equivalent Surface) was developed which can be used to characterize the actual corrosion behavior of parts according to their functional surface properties. This PRE-S formula includes the traditional PREN together with terms for surface roughness, chemical treatment and airpassivation.
Stainless steels owe their functional properties to the formation of a passive layer on the surface. The properties of this passive layer are very much dependent on the alloying content, but can also largely be influenced by mechanical and chemical surface finishing processes. These finishing treatments are made after producing the material in the steel mills and/or after further processing into the final products.1- In both cases mechanical treatments are usually the first step which is followed by a chemical reaction, either by applying chemicals in form of spray, pastes or by immersion, or by reaction of the steel with the oxygen in the air without any further deliberate action. Commonly used chemicals are reducing acids like sulfuric or hydrofluoric, oxidizing acids, especially nitric acid, mixtures of reducing and oxidizing acids and complex formers like citric acids. The reactions with the steels vary with the character of these reagents, and are also influenced by the preceding mechanical treatment, whether grinding, blasting, polishing etc.