ABSTRACT
The localized corrosion resistance of stainless steels and nickel alloys is primarily due to alloying elements. The most important elements, as indicated by the Pitting Resistance Equivalent (PRE) are Cr, Mo, N and sometimes W. This paper compares the effect of these elements in pitting and crevice corrosion. Using ASTM G48 Methods C and D, the critical pitting temperature and critical crevice temperature are determined for several superaustenitic stainless steels (UNS N08366, N08367, N08830 and N08031) and a nickel alloy (UNS N06625). The varying effect of the elements on the two forms of corrosion and its relation to localized corrosion mechanisms are discussed. Differences between pitting corrosion and crevice corrosion are discussed. Most notably, the CCT of N06625 was the same as that of N08367 (6Mo) although the CPT of N06625 was at least 25 °C greater than that of N08367 (6Mo).
INTRODUCTION
Pitting corrosion resistance is often represented by a pitting resistance equivalent number (PRE). Although several PRE variations have been created, the elements Cr, Mo, and N are the primary factors. The most common PRE is Cr + 3.3Mo + 16N. This equation is defined in industrial standards such as NORSOK M-001 [1] and MR0175/ISO15159-3 [2]. The equation in MR0175 also contains a 1.65W component, which is used in the PRE calculations of this paper.
The concept of a PRE was first presented by Lorenz and Medawar [3] in 1969 and developed further by others including the addition of the N component [4,5]. Since then, PRE has been widely used to estimate the localized corrosion resistance of stainless steels. Although PRE can be a useful comparative tool, it should only be used as a limited guide. An obvious limitation of PRE is that the microstructural and surface effects are not accounted for. Surface roughness is significant in localized corrosion, especially so for crevice corrosion [6,7]. An additional restriction of PRE is that it may not be selective enough to rank alloys within an alloy type. For example, Cleland [8], Manning [9] and Richard-Minier et al. [10] have pointed out the shortcomings of the conventional PRE when evaluating corrosion resistant alloys (CRAs) such as superaustenitic stainless steels.
