Origins of pitting corrosion

Abstract

Corrosion of metals and alloys by pitting constitutes one of the very major failure mechanisms. Pits cause failure through perforation and engender stress corrosion cracks. Pitting is a failure mode common to very many metals. It is generally associated with particular anions in solution, notably the chloride ion. The origin of pitting is small. Pits are nucleated at the microscopic scale and below. Detection of the earliest stages of pitting requires techniques that measure tiny events. This paper describes techniques designed to do this and discusses the measurements that result. Some metals show preferential sites of pit nucleation with metallurgical microstructural and microcompositional features de ning the susceptibility. However, this is not the phenomenological origin of pitting per se, since site speci city is characteristic only of some metals. A discussion is presented of mechanisms of nucleation; it is shown that the events are microscopically violent. The ability of a nucleated event to survive a series of stages that it must go through in order to achieve stability is discussed. Nucleated pits that do not propagate must repassivate. However, there are several states of propagation, each with a nite survival probability. Several variables contribute to this survival probability. Examples are shown of several metals and some common features of their behaviour are discussed. It is shown that for some systems, the pit sites can be deactivated.CEST/2124

Corrosion Performance of Stainless Steel and Nickel Alloys in Aqueous Sodium Hydroxide as Revealed from Cyclic Voltammetry and Potentiodynamic Anodic Polarization

“…The formation and development of this type of attack on the metal surface can occur only in solutions containing the aggressive Cl -, Br -and I -ions [4][5][6][7][8][9][10][11][12] . The critical potential at which pitting initiates is reported by several authors and found to depend on a number of factors including the type and concentration of attacking anion, type of metal used and relative concentration of other anions [13][14] .…”

Enhanced Pitting Corrosion Resistance of 304 SS in 3.5 wt% NaCl by Ultrasonic Nanocrystal Surface Modification

“…32,44,45 Three potential levels, i.e., 250, 350, and 450 mV SCE were selected for potentiostatic tests based on CPP results (Figure 7a). At 450 mV SCE , stable pitting corrosion occurred on the control sample (Figure 8a) with high current (mA).…”

A Conceptual Model for the Interaction between Carbon Content and Manganese Sulphide Inclusions in the Short-Term Seawater Corrosion of Low Carbon Steel

“…For passive metals, such as stainless steels and aluminium, the effect of secondary constituents in pit initiation is well-known [21]. For mild and low alloy steels, however, secondary constituents usually are considered in terms of their effect on corrosion products and the consequent effect on corrosion as controlled by oxygen diffusion limitations.…”