Pit Geometries Research Articles

Evolution of Surface Pits on Stainless Steel Strip in Cold Rolling and Strip Drawing

Theoretical models are presented for describing the evolution of pits in the inlet and work zone during cold rolling and strip drawing of shot-blast stainless steel under ‘mixed’ lubrication. Results shows that the rough shot-blast surface is flattened rapidly in a short inlet zone, thereby entrapping the lubricant in surface pits. The subsequent evolution of these surface pits in the work zone can be explained by micro-plasto-hydrodynamic-lubrication (MPHL) models described previously. A development of these models is presented which takes into account the effects of the oil film entrained in the inlet, an oil film penetrating from adjacent pits and the finite depth of the pits. The role of an inlet oil film and penetrating MPHL oil film is to limit the potential reduction of pit size. Lubrication regime maps are constructed which describe the evolution of the surface pits for a range of pit geometries. Results explain the experimental observation that some pits survive even after a multi-pass schedule. Predictions of the pit area show good agreement with measurements on samples obtained in strip drawing or rolled under industrial conditions.

Effect of pit geometry and bulk stress on near- and far-side calculated MFL signals*

Nonlinear anisotropic materials were used to simulate the local effects of tensile stress concentrations on magnetic flux leakage (MFL) signals using three-dimensional finite element analysis (FEA). FEA was used to calculate the effects of near- or far-side pit depth and line-pressure induced simulated bulk tensile stress on radial MFL signals for pipelines with corrosion pits. It was found that the percentage changes increased with increasing degrees of anisotropy, simulating increasing stress concentrations, and also with greater far- or near-side pit depth for the three types of corrosion pit geometries studied.

Factors Influencing the Transition from Metastable to Stable Pitting in Single-Crystal Beryllium

The pitting potential of single-crystal beryllium in 0.01 M sodium chloride decreased with crystallographic orientation in the order Orientations which were associated with the lowest and were characterized by crystallographically oriented parallel plates of unattacked Be on the pit interior and square pit walls. While pit geometries in the (0001) surface appeared to be random, corrosion propagation was often in the and families of directions. Potentiostatic current/time data revealed that metastable pitting was characterized by two types of transients: (i) transients with a lifetime <1 s, similar in shape to the current/time data observed in the repassivation process, and (ii) transients whose lifetimes lasted 5-140 s and were associated with physical damage on the sample surface. The nucleation frequency of repassivation events was directly proportional to passive current density and inversely proportional to applied anodic potential. Although the frequency and magnitude of damage events increased with increasing applied anodic potential for any given orientation, the total accumulated damage was greatest for orientations with the highest Prepassivation resulted in a decrease in the frequency of repassivation events but had only minimal effect on the frequency and magnitude of the damage events. The results are discussed in terms of physical bulk metal properties as well as critical pitting environments (i.e., ohmic and mass transport models). © 2000 The Electrochemical Society. All rights reserved.

Multiple-Pit Breakwaters

Linearized shallow-water wave theory is used to investigate the interaction of surface waves with multiple rectangular submarine pits in water of otherwise uniform depth. The solution is obtained by a boundary element technique using a two-dimensional Green's function. It is shown that appropriate selection of pit dimensions and placement may lead to a significant reduction in wave heights behind these structures. Numerical results have been presented that illustrate the influence of the various pit characteristics on the diffracted wave field. Two pits can provide a shadow region in which wave heights are reduced to 10–20% of the incident wave height. The shadow region, with wave heights reduced to 30% of the incident wave height, is approximately the width of the pits and five wavelengths long. Guidance is given on how to select pit geometries and placement for optimal breakwater performance. An example is also presented that shows how to select a pit to reduce wave heights in a navigation channel.

Etch Pit Studies on ZnS Crystals

Etch pits and x-ray studies have been made on several ZnS crystals. Etch pit geometries have indicated, in some cases, a mixed cubic-hexagonal structure. This has been confirmed by x-ray analysis. A model to explain the x-ray and etch pit data is presented.