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Introduction: The failures of AISI 304L stainless steel and Ta-2.5W alloy observed in factories have spurred in-depth research on the erosion-corrosion behavior of AISI 304L and 316L stainless steels, as well as metals with high corrosion resistance or heat resistance such as Ti, Ta, and Zr. In the corrosion test, the material was immersed in an aqueous solution containing 10 wt% hydrochloric acid, and the test was carried out at room temperature at a speed of 1000 rpm for 168 hours. To investigate the combined effects of pure corrosion and erosion-corrosion, during the experiments, some materials were specifically designed to be protected from impact by solid particles. The results showed that the weight and surface roughness of tantalum (Ta) and zirconium (Zr) remained almost unchanged, whereas the weight loss of titanium (Ti) was an order of magnitude higher than that of Ta and Zr. Stainless steel suffered severe corrosion due to the removal of the oxide layer, which exposed the base metal. Through the analysis of the microstructure using light microscopy and scanning electron microscopy, we were able to clarify the specific mechanisms of erosion-corrosion. Introduction: The economical and efficient operation of fluid handling machinery and plants increasingly relies on the use of materials that are both corrosion-resistant and wear-resistant. Stainless steel is the most commonly used material in industrial environments, but titanium and titanium alloys are also finding increasing use outside of the military and commercial aerospace sectors. All refractory metals possess excellent corrosion resistance, and in the chemical industry, tantalum is the preferred material due to its outstanding corrosion resistance, especially in heat exchangers and pressure vessels that operate under extreme corrosive conditions. The excellent corrosion resistance of tantalum is attributed to the amorphous Ta2O5 film formed on its surface. Zirconium is another refractory metal; due to its ability to form a strongly adherent zirconia passivation surface film, it is being used increasingly in industrial components other than nuclear power plants, exhibiting high corrosion resistance. In recent years, research on the corrosion and wear of materials in corrosive environments has received widespread attention, which is related to the so-called synergistic effects between different processes. Erosion-corrosion can be defined as the interaction that occurs simultaneously between erosion and corrosion, including cavitation erosion and fluid erosion. This synergy can lead to a significant increase in metal loss rates, exceeding those caused by corrosion or erosion acting alone. Erosion-corrosion phenomena are divided into two main categories: erosion-enhanced corrosion (EEC), in which the damaged area is confined to the oxide scale scale ； And corrosion-affected erosion (CAE), where the damaged areas include both scale and the metal itself. As the industrial processes in the chemical industry operate under increasingly stringent conditions to increase production and reduce costs, it is necessary to introduce refractory materials such as superalloys, active metals like titanium or zirconium, tantalum, and their alloys into the components of processing plants. However, the friction-corrosion resistance of these materials (corrosion associated with mechanical degradation) has not received much attention in the past. Even these high-strength materials undergo degradation and failure in harsh environments. Especially under the combined action of strong acids, solid particles in suspensions, and high-speed circulating fluids, material loss occurs through an erosion-corrosion mechanism. Following the investigation of field failures of AISI 304L stainless steel and Ta-2.5W alloy in chlorides and suspended particles, this study presents the results of laboratory erosion-corrosion tests on AISI 304L, AISI 316L, as well as commercially pure Ti, Ta, and Zr metals. Experiments: In these corrosion laboratory tests, we investigated the corrosion resistance of different materials (AISI 300 series with carbon contents of 0.03%, 1.03%, and 1.6%) in a 10% hydrochloric acid solution containing 80 grams per liter of alumina (Al2O3) particles. The experimental conditions simulated the harsh conditions in an industrial environment; the solution was stirred at room temperature at a speed of 1000 revolutions per minute (rpm) for 168 hours, using a magnetic stirrer with a capacity of 1000 cubic centimeters. All samples were polished before testing to ensure a consistent surface condition. To evaluate the wear condition of the material surface, we used an SM7 profilometer to measure the average roughness values of the flow-exposed areas before and after testing; the specific data are shown in Table 1. In addition, we also recorded the average weight changes of the samples before and after the test, and calculated the average percentage mass loss for each material; these data are shown in Table 2 and Figure 2 respectively. Figure 1. (a) Testing. (b) Morphology of alumina particles Table 2 Weight values Figure 2. Weight loss: Results and Discussion 1. Case Study 1.1 On advanced alloys such as stainless steels and nickel-based alloys, the material loss rate due to corrosion in AISI 304L can be significant. Figure 3 shows the failures encountered by AISI 304L pipes during operation. Figure 3a shows the thinning of the inner wall of the pipe due to erosion corrosion, a mechanism of material loss caused by impacting particles and/or turbulence. Figure 3b shows the thinning of the wall and leakage issues near the weld, which are typically core problems arising when the material’s inertness is compromised by mechanical factors. No corrosion signs were observed on the outer surface of the pipe, which further indicates that the failure was caused by internal erosion-corrosion. Although the chemical plant that provided the samples did not supply complete information on the operating conditions, factors that may cause erosion include high-speed flow, the presence of particles in the suspension, and protruding welds. The erosion pattern indicates that cavitation also exacerbates the erosion process. Furthermore, corrosion can be accelerated by relative motion or by an aggressive liquid that increases the rate of attack on the metal. This type of attack can be identified by the appearance of waves, grooves, circular holes, valleys, and ridges. Another characteristic feature of this type of combined attack is the fan-shaped appearance resulting from the formation of overlapping horseshoe-shaped depressions. Figure 4 shows the wear and corrosion of the Ta-2.5W alloy in actual use. The low-magnification optical micrograph in Figure 4a reveals erosion-wear marks caused by suspended particles, which are distinct damages on the material surface resulting from particle impacts. Figure 4b shows a low-magnification optical micrograph of the fan-shaped surface of the Ta-2.5W alloy, in which characteristics of horseshoe-shaped erosion-corrosion marks can be observed, which are caused by the combined action of corrosive agents and abrasive particles. Compared to stainless steel, tantalum has lower hardness, which makes it more prone to failure when subjected to impact particles. The addition of tungsten, such as in Ta-2.5W alloys, increases hardness compared to pure tantalum, thereby enhancing the wear resistance of the material. However, even with increased hardness, the combined effect of corrosive agents and abrasive particles still causes wear and corrosion failure on the surface of the Ta-2.5W alloy. This is clearly evident from the macroscopic views of the Ta-2.5W reactor wall (Figure 4), which correspond to those of the AISI 304L tubes from the same plant mentioned earlier. The wear marks and erosion effects can be seen in the macro view shown in Figure 4a. The low-magnification image of another area on the inner wall of the Ta reactor (Figure 4b) shows the horseshoe-shaped characteristics of the erosion-corrosion process. Figure 5. AISI 304L testing. (a) Optical micrograph of the interface between the flow exposure zone (left) and the erosion protection zone (right). (b) The SEM micrographs of the flow exposure area show the grain microstructure. (c) Pitting that erodes the surface of the protected area. (d) Magnification of the cavities formed by pitting. 2 Laboratory Tests 2.1 Roughness Measurement The order of roughness increases after testing is as follows: AISI 304L > AISI 316L > Ti > Zr > Ta. The roughness of stainless steel increased by an order of magnitude, while the change in Ti was much smaller; in Zr and Ta, the surface roughness remained almost unchanged. 2.2 Weight reduction: The weight loss of stainless steel is 2% or more (Figure 2). The weight loss of Ti is more than one order of magnitude lower than that of stainless steel, while the weight differences of Ta and Zr are negligible. 2.3 Microstructural study During the erosion-corrosion test, the specimen surface was exposed to fluid flow and impact from alumina particles; aside from the area around the pores (Figure 1a), the position of the specimen was maintained to protect it from the abrasive particles, but it was susceptible to corrosion by the acid solution. In this way, for the same material, there is an interface between the water flow exposure area and the erosion protection area. 2.4 AISI 304L Figure 5a shows the interface between the flow-exposed area and the erosion-protected area on the AISI 304L specimen after the erosion-corrosion test. The surface of the exposed area shows a combined effect of erosion and corrosion. The microstructure of the region exposed to flow (Figure 5b) shows grain boundaries and grain fracture surfaces, with the surface morphology indicated by the degree of crystal filling within each grain. The erosion test areas (Figures 5c and d) demonstrate the typical pitting process caused by hydrochloric acid solution, which is able to penetrate between the test piece and the Teflon sheet. These observations are consistent with erosion mechanisms severely affected by corrosion, in which the damage includes oxide scale and base metals. 2.5 AISI 316L As expected, AISI 316L exhibits less material removal compared to AISI 304L, although the two materials show similar behavior. Figure 6a shows the interface between the flow exposure zone and erosion in Figure 6. AISI 316L testing. (a) Optical micrograph of the interface between the flow exposure zone (left) and the erosion protection zone (right). (b) The SEM micrographs of the flow exposure area show the grain microstructure. (c) Pits and cracks located at the outer edge of the AISI 316L specimen. Figure 7. Titanium testing. (a) Scanning electron micrograph of the interface between the erosion protection zone (left) and the flow exposure zone (right). (b) Shows details of the flow-exposed areas subjected to erosion, corrosion, and wear. Figure 8. Tantalum testing. (a) Scanning electron micrograph of the interface between the erosion protection zone (light gray) and the flow exposure zone (dark gray). (b) Detailed information on the erosion protection area showing grinding marks prior to the test. (c) Optical micrograph of the wear scar in the flow exposure zone. (d) Scanning electron microscopy images show the oxide particles on the surface of the flow-exposed area. Widespread erosion and corrosion attacks were observed in the flow-exposed areas, while the erosion-protected areas showed signs of localized attacks. The combined effect of erosion and corrosion reveals the microstructure of the particles in the exposed areas, with the most severe attacks occurring at the outer edges of the specimen, resulting in cavities and cracks. 2.6. Titanium: For the active metal titanium, the oxide scale formed on its surface provides effective protection against corrosion attacks, as no signs of corrosion were observed on the surface of the protected area. Erosion-corrosion in the exposed areas only leads to an increase in roughness and the formation of surface cracks. The main mechanism is that erosion exacerbates corrosion; in other words, the damage is confined within a thicker oxide layer, while the base metal itself remains unaffected. 2.7. Tantalum: For tantalum, there is almost no change in weight or surface roughness, but the surface does show some localized signs of erosion and corrosion. The scanning electron microscope images of the tantalum sheets after the test showed that the eroded regions remained unchanged, retaining their original polishing marks. On the surface of the areas exposed to flow, some localized erosion-corrosion traces can be detected, but the attack primarily affects the oxide layer. Scanning electron microscopy observation of the exposed area revealed the accumulation of very small oxide particles, and energy dispersive spectroscopy (EDS) of these particles showed only tantalum and oxygen peaks. In the case of tantalum, the presence of abrasive particles in the suspension is a decisive factor, as no material loss was observed in the areas where particle impact was prevented. These observations indicate that the enhanced corrosion process caused by erosion is due to the damage inflicted by abrasive particles. 2.8. Zircon: The behavior of zircon is similar to that of tantalum. The eroded protection area showed no signs of corrosion, whereas the flow-exposed area exhibited signs of corrosive wear resulting from material removal. Severe erosion and some zirconia deposits are mainly visible at the edges. The oxidized particles that are eroded away are likely to be captured by new substances, leading to oxidation-scale growth in the Enhanced Erosion Corrosion (EEC) mechanism, which primarily occurs on these active metals. Figure 9. Zirconium testing. (a) SEM micrograph of the interface between the flow exposure zone (top) and the erosion protection zone (bottom). (b) The outer edge of the coupon and the enlarged details (right) show material removal by erosion-corrosion. (c) Oxide particles aggregate at the outer edges. Conclusion: 1. The corrosion effects of stainless steels AISI 304L and 316L: Stainless steels AISI 304L and 316L, in the presence of hydrochloric acid and alumina particles, suffer loss of oxide scale and erosion of the base metal through the corrosion-affected corrosion (CAE) mechanism. This mechanism indicates that under the combined action of the corrosive medium and abrasive particles, the damaged areas of the material include both the scale and the metal itself. 2. Pitting corrosion of stainless steel in acidic media: Even in the absence of abrasive alumina particles, stainless steel can suffer from severe pitting corrosion due to acidic media. This indicates that the corrosion behavior of stainless steel in HCl solution is affected by the pH value; as the pH value increases, the corrosion current density decreases and the corrosion resistance improves. 3. Corrosion resistance of reactive metals Ti, Ta, and Zr: The reactive and refractory metals Ti, Ta, and Zr show that, in the presence of hydrochloric acid and alumina particles, the changes in roughness and weight loss are negligible compared to stainless steel. This indicates that these active metals have good corrosion resistance. 4. Acid resistance of Ti, Zr, and Ta in the absence of alumina particles: In the absence of alumina particles, Ti, Zr, and Ta exhibit good acid (10 wt% hydrochloric acid) corrosion resistance. This further confirms the high corrosion resistance of these materials in the absence of abrasive particles. 5. Enhanced corrosion mechanism due to erosion of reactive or refractory metals: For these reactive or refractory metals, the combined action of hydrochloric acid and alumina particles removes only the outer protective oxide layer, via the enhanced corrosion (EEC) mechanism. In this mechanism, the damaged area is confined to the oxide scale range, leaving the base metal unaffected. Statement: This article was first published on the WeChat official account; the original title is: [Corrosion Parameters of Stainless Steel, Titanium, Tantalum, and Zirconium]