Erosion-corrosion resistance of engineering materials in various test conditions

Erosion–corrosion is a complex phenomenon which involves the interaction between the mechanical processes of solid particle erosion and the electrochemical processes of corrosion. A whole range of issues is faced by a designer when trying to obtain relevant information on erosion–corrosion performance of a material. Amongst the constraints are the dispersed test conditions and test rigs available in the literature making comparisons and quantifying erosion–corrosion wear rates of different materials very difficult. The aim of this work is to evaluate the repeatability of erosion–corrosion experiments and to investigate the role of different parameters influencing erosion–corrosion. The materials tested in this work are stainless steel (SS316L/UNS S31603), carbon steel (AISI 1020/UNS G10200) and nickel-aluminium bronze (NAB/UNS C63200). A slurry pot erosion tester was used as the test apparatus and test parameters such as erodent size, erodent concentration, flow velocity and test solutions were varied to study their effect on erosion–corrosion. SEM analysis showed that a similar erosion–corrosion mechanism is seen for SS316L and NAB with formation of multiple extruded lips and platelets typically seen for erosion dominated material. In contrast the surface of AISI 1020 revealed the formation of craters, pits and shallow indentations which suggests that corrosion mechanism has a dominant influence on the material. Error rates in tests were found to have an average of 5.5% which are relatively low indicating good repeatability of test measurements and data from the test rig. The erosion–corrosion resistance of AISI 1020, SS316L and NAB were compared and it was found that SS316L showed the lowest erosion–corrosion mass loss rates in all test conditions followed by NAB and then AISI 1020. However in terms of synergistic ranking, NAB showed the best resistance to the combined action of erosion and corrosion with the highest negative synergy value. Positive synergy was evident for AISI 1020 in 3.5% NaCl and SS316L in 0.3 M HCl. A wear map is presented to evaluate erosion–corrosion trends of the materials. This work combines the assessment of test repeatability, variation in test conditions and comparison of material performance which are key stages in a material selection process

Evaluation of a semi-empirical model in predicting erosion–corrosion

The phenomenon of erosion–corrosion has been studied extensively by various investigators but no accuratemodel has been developed to predict the interactions between erosion and corrosion. This is mainlyattributed to the complexity of the interactions that generate either a synergistic or antagonistic weareffect for a particular material in a certain environment. A semi-empirical model has recently been developedat theUniversity of Southampton which incorporates dynamic Hertzian contact mechanics to modelthe damage during particle impact and accommodates the effect of erodent deforming the surface leadingto an increased corrosion activity. The model was found to have good agreement with erosion–corrosionrates of carbon steel. The aim of this paper is to evaluate the robustness of this semi-empirical model bytesting it on a passive metal. UNS S31603 was chosen due to its inherent passivity to corrosion. A slurrypot erosion tester was used as the test rig to perform the experiments. It was found that this passivemetal produces high synergistic levels when exposed to erosion–corrosion in 0.3MHCl with variation inerodent concentrations and flow velocities. SEM and surface profilometry show typical ductile materialbehaviour with cutting mechanism and deformation mechanism occurring simultaneously. A wear mapis presented and it is observed that the increase in velocity and sand concentration causes the material toshift from a corrosion–erosion dominated region to an erosion–corrosion dominated region. This paperwill also evaluate the semi-empirical model and discuss its applicability in predicting erosion–corrosion

Electrochemical investigation of erosion-corrosion using a slurry pot erosion tester

The aim of this paper is to use a modified slurry pot erosion tester to perform in-situ electrochemical measurements during solid particle impingement to investigate the effects of velocity, sand size and sand concentration on a passive metal (UNS S31603). Samples are subjected to a set of erosion-corrosion experiments. The electrochemical response of UNS S31603 to the test parameters are plotted and compared to develop an understanding of the erosion-corrosion process. The current trend with variation of test parameters has been explained by an erosion enhanced corrosion synergistic effect. The current transients associated with depassivation and repassivation during solid particle impingement are observed through electrochemical noise measurements. It was observed that the increase in velocity and sand concentration increased the current levels during erosion-corrosion. However, the increase in sand size had a more complex response. Single particle impact experiments conducted revealed that the peak corrosion current and the repassivation time increased with increasing velocity. A linear trend was seen between the peak current and kinetic energy. A second order exponential decay was fitted to the repassivation kinetics of the single particle impact. SEM has been used to develop a mechanistic understanding of erosion-corrosion. The surface scars reveal that the depth of the craters and the length of the lips increase with increasing velocity. Micro-cracks also appear on these lips, believed to be due to corrosive action attacking the roots of these lips

Influence of microstructure on the erosion and erosion–corrosion characteristics of 316 stainless steel

The economic impact of surface damage and component failure arising from solid particle impact in the UK has been estimated in 1997 at around d20 million [1]. The additional complexity associated with erosion in a corrosive environment such as that encountered in the chemical and hydro-carbon extraction industries can significantly accelerate surface wear and material loss. In this study, surface material response of a stainless 316 alloy subject to erosion and erosion–corrosion was investigated by focused ion beam (FIB) and transmission electron microscopy (TEM) techniques. Samples tested in a slurry pot apparatus using 1% uncrushed silica at 7 m/s for 60 min, both in water and 3.5% NaCl solution. Site specific FIB–TEM lamellas showed that solid particle impact resulted in extensive crater and lip formations and a martensitic phase transformation at the surface. The presence of a corrosive fluid resulted in preferential dissolution of the martensitic phase, reducing the work hardening behaviour and promoting greater elongation to failure and thus higher erosion–corrosion rates. These results are discussed in light of the extensive literature on solid particle impact and corrosion by considering the influence of nano-scale phase changes which can often only be observed using transmission electron microscopy

Investigation of erosion-corrosion mechanisms of UNS S31603 using FIB and TEM

Accelerated wear due to synergy during erosion–corrosion of UNS S31603 is extremely complex. It is this reason that current modelling approaches fail to accurately model the physical mechanisms in this wear process. The objective of this work was to perform FIB and TEM analysis on UNS S31603 to investigate the subsurface deformation mechanisms and microstructural changes in the material during erosion–corrosion. FIB investigation revealed a decrease in grain size at the surface and a change in grain orientation towards the impact direction. Networks of cracks were observed near the surface which is believed to be caused by work hardening of the material which increased the material susceptibility to fatigue cracking. Folding of lips is also proposed as an important mechanism for subsurface wear. The large amount of strain imposed on the material also induced martensitic phase transformation. Fragmented erodent particles and oxide film were found embedded into the material which caused formation stress concentrated regions in the material and contributed to crack initiation. A composite structure is formed consisting silicon oxide sand particles and chromium oxide film along with the martensitic phase transformed metal. The corrosive environment is also believed to have played a significant role in the initiation and propagation of cracks. Crack initiation and propagation due to the mechanical and electrochemical processes enhances the material mass loss as the crack networks coalesce and subsequently cause material spalling. Physical models are developed based on these observations to explain the microstructural changes and synergistic mechanisms

A study on the evolution of surface and subsurface wear of UNS S31603 during erosion-corrosion

This paper studies the material response of UNS S31603 to incremental particle impact and evolution of surface and subsurface wear with time during erosion–corrosion. Multiple tests were performed at increasing time duration from 0.5 min to 2 h using a slurry pot erosion tester with 3.5% NaCl and 1 wt.% silica sand at a test velocity of 7 m s?1. SEM, FIB and TEM were used to investigate the mechanisms and microstructural changes that arise during this process. Between 0.5 min and 20 min of testing, when the particles are impacting the fresh uneroded surface, material removal occurs through the formation of prominent lips and deep craters. After a duration of 20 min, when the surface has been completely covered with a layer of lips and craters, a second layer starts forming. Between 0.5 min and 20 min the depth of the nanocrystalline region formed subsurface increases with direct particle impact on the surface. As the top surface layer becomes work hardened, load is transmitted by particle impact to the bulk grains leading to the formation of nano and micro sized grains. TEM investigation on the single particle impact crater revealed that deformed nanograins and twinning are formed immediately beneath the impact crater. TEM analysis of the specimen exposed to erosion–corrosion for 5 min also revealed the formation of deformed nanograins and twinning due to the high strain rates. It is believed that the compact fine grained microstructure makes it difficult for anodic dissolution to occur. However, the depassivation of the oxide film and the formation of micro galvanic cells on the deformed metal will enhance corrosion. A graph of mass loss rate versus time plotted gives good correlation with surface and subsurface features observed. Physical models are developed based on these observations