Pitting resistance criteria for corrosion-resistant steels

Abstract

Comparative analysis is provided for currently used and proposed criteria for stainless steel pitting resistance, i.e., the potential for forming a salt film. It is shown that the potential for forming a salt film is specified by current oscillations, that is a typical sign of pitting corrosion, and it makes it possible to predict stainless steel pitting corrosion limits more precisely. Comprehensive use of the applied and proposed pitting resistance criteria make it possible to separate clearly the possible conditions of a passive film in three areas: an area of a passive condition or stable passivity, where the probability of formation and development of previously occurring pitting is zero; an area of a metastable condition or unstable passivity within which there is a non-zero probability fore existence of previously occurring pitting and the generation of new repassivating pitting; an area of pitting formation where formation and a stable increase in pitting is possible.Pitting corrosion is one of the most critical and widespread forms of corrosion failure of equipment in the chemical, petrochemical and allied branches. It is due to the fact that the main structural material in these branches is stainless steel, and within the majority of reactive media there are chlorides, bromides and other halogenides. In this connection it is necessary to determine pitting resistance criteria and based upon it to select stainless steels with high pitting resistance for prescribed operating conditions.In order to evaluate the resistance of stainless steel to pitting corrosion (pitting resistance), currently pitting resistance bases are used [1] specifying the degree of resistance of a passive film on metal and the potential limits within which this condition is retained.As the main basis ∆E pr min is used, determined as the difference of minimum galvanostatic pitting corrosion potential E pc min and corrosion potential E cor . Here as the minimum galvanostatic potential we take the potential with which stable occurrence is provided for previously started pitting corrosion. As additional bases of pitting resistance ∆E fp pr and ∆E rp pr we take bases, determined on the basis of the difference in potentials for formation E fp and repassivation E rp of pitting, i.e., potentials with which pitting may already be generated with an increase in potential and may still develop with a reduction of it:∆E pr min = E cp min -E cor ;∆E fp pr = E fp -E cor ;∆E rp pr = E rp -E cor .(1)

Study of Equipment Corrosion Resistance in the Production of Potash by a Halurgical Method

“…moting erosive removal of a passive film, it may be predicted that the corrosion resistance of steel under operating conditions will be higher [7][8][9][10].…”