K. Sasaki

Effect of impact angle on the slurry erosion–corrosion of 304L stainless steel

Abstract This communication describes an investigation of the effect of impact angle on slurry erosion–corrosion of stainless steel using a new slurry erosion rig. With the new apparatus, it is possible to measure the individual erosion events under impact as both electrochemical current/time transients, and mechanical transients through acoustic emission (AE) simultaneously. Each sharp rise observed in the electrochemical current transient under particle impact is accompanied by an AE event. These sharp current rises are attributed to the rupture or removal of the oxide film on the surface by the abrasive particles. The correlation between the current rise and the maximum of the AE event, although scattered, shows an increase with decreasing impact angle. The current transients due to individual erosion events show that on the average, the current rise is greater and the rise time is longer at oblique angles compared with those at normal incidence. The degree of denudation of the metal surface by individual particle impingements, the process which strips the surface of its passivity, increases at more oblique angles. Weight loss measurements demonstrate that the synergistic effect between erosion and corrosion is enhanced by a more oblique angle of impact. The origin of the synergism is discussed.

The generation of surface roughness during slurry erosion-corrosion and its effect on the pitting potential

Abstract Results of experiments performed to isolate the effect of surface roughness generated during slurry erosion-corrosion of 304L stainless steel are presented. Surfaces prepared by grinding to different finishes show that increasing the roughness lowers the pitting potential in chloride solution. Subsequent erosion by an aqueous sand slurry then alters this pitting potential (Ep), as measured in the solution phase alone, in a manner dependent on the initial surface roughness. Surfaces which were initially rougher show a pronounced rise in p after erosion, while those with an initially smoother finish show a fall in Ep after erosion. Measurement of Ep during erosion-corrosion does not fit either process, with values of Ep lower than those predicted from the surface roughness alone.

The birth of corrosion pits as stimulated by slurry erosion

Abstract A novel technique has been developed to examine the details of the slurry erosion–corrosion behaviour of stainless steel in aqueous silica slurry using a modified slurry erosion rig with a two-electrode system. Measurement of current transients of 304L stainless steel in chloride solution shows the propagation of metastable pits immediately after a short duration of erosive particle impingement below the pitting potential. Metastable pit events after the particle impingement are more numerous than those under non-erosive conditions. Erosive impact by solid particles thus stimulates pitting corrosion of stainless steel in chloride solution by nucleation of more metastable pits. It is argued that the effect arises both from exposure of fresh active sites during erosion as well as by altering their geometry.

Detecting electrochemical transients generated by erosion–corrosion

Abstract Methods of measuring current transients due to solid particle impingement are presented. The processes involved form the basis of slurry erosion–corrosion, whereby the rate of wastage of a metal is greater than the sum of the separate erosion rate and the corrosion rate, when under exposure to a flowing solid/liquid slurry. The measurements are electrochemical in nature, backed up by a coupled acoustic emission sensor to detect slurry impact on the metal surface. Evidence is shown that it is possible to measure a threshold energy of the impacting slurry above which the passivating surface oxide is ruptured by erosion, and below which it is not. Some consequences of depassivation are discussed, and it is shown that depassivation of stainless steels by the impacting slurry can stimulate the formation of metastable pits in a chloride containing environment.

Electrochemically induced annealing of stainless-steel surfaces

Modification of the surface properties of metals without affecting their bulk properties is of technological interest in demanding applications where surface stability and hardness are important. When austenitic stainless steel is heavily plastically deformed by grinding or rolling, a martensitic phase transformation occurs that causes significant changes in the bulk and surface mechanical properties of the alloy. This martensitic phase can also be generated in stainless-steel surfaces by cathodic charging, as a consequence of lattice strain generated by absorbed hydrogen. Heat treatment of the steel to temperatures of several hundred degrees can result in loss of the martensitic structure, but this alters the bulk properties of the alloy. Here we show that martensitic structures in stainless steel can be removed by appropriate electrochemical treatment in aqueous solutions at much lower temperature than conventional annealing treatments. This electrochemically induced annealing process allows the hardness of cold-worked stainless steels to be maintained, while eliminating the brittle martensitic phase from the surface. Using this approach, we are able to anneal the surface and near-surface regions of specimens that contain rolling-induced martensite throughout their bulk, as well as those containing surface martensite induced by grinding. Although the origin of the electrochemical annealing process still needs further clarification, we expect that this treatment will lead to further development in enhancing the surface properties of metals.

Observation of a threshold impact energy required to cause passive film rupture during slurry erosion of stainless steel

The impact of solid particles during the erosion of stainless steel by a slurry jet reveals a sequence of anodic current transients due to individual impacts. Measurement of these current transients shows that there is a minimum particle energy below which no transients can be detected. The result is interpreted as a threshold energy required to rupture the passivating oxide film on the metal surface. If this energy is exceeded, the current transients arise from electrochemical reaction of the depassivated metal surface after denudation. Below this threshold no rupture occurs and there are no current transients.

Lattice changes generated by electrochemically induced surface annealing of austenitic stainless steel

Measurement of the X-ray diffraction patterns of 304L stainless steel after subjecting it to an electrochemically induced surface annealing process in concentrated nitrite solutions shows that the austenite lattice contracts during processing relative to the untreated alloy. The surface annealing procedure, which involves alternate anodic and cathodic potential pulses, thereby induces a small increase in the density of the metal surface. The observations are not due to processes involving ingress of reduced hydrogen. The results of comparative experiments in alternative electrolytes show that ingress of hydrogen into the steel expands the austenite lattice.