Published: September 11th, 2018
Here, we present a protocol to measure the tribocorrosion rate and wear-corrosion synergy of thin film and bulk Al alloys in simulated sea water at room temperature.
The increasing complexity and severity of service conditions in areas, such as aerospace and marine industries, nuclear systems, microelectronics, batteries, and biomedical devices, etc., impose great challenges on the reliable performance of alloys exposed to extreme conditions where mechanical and electrochemical attack co-exist. Finding ways for alloys to mitigate the combined attack of wear and corrosion (i.e., tribocorrosion) under such extreme conditions is thus highly critical for improving their reliability and service lifetime when used in such conditions. The challenge lies in the fact that wear and corrosion are not independent of each other, but rather work synergistically to accelerate the total material loss. Thus, a reliable method to evaluate the tribocorrosion resistance of metals and alloys is needed. Here, a protocol for measuring the tribocorrosion rate and wear-corrosion synergy of Al-based bulk and thin film samples in a corrosive environment under room temperature is presented.
Tribocorrosion is a material degradation process caused by the combined effect of wear and corrosion1,2. Tribocorrosion takes place both in nature and in industrial applications where mechanical contact and a corrosive environment are simultaneously present. The complexity of tribocorrosion lies in the fact that chemical and mechanical degradation mechanisms are not independent of each other. A combination of mechanical and chemical attack often leads to accelerated failure, due to synergetic effects. Thus, the total material loss can be calculated as T = C0 + W0 + S (eqn. 1), where C0 is the material loss resulted from corrosion in the absence of wear, W0 is the material loss due to mechanical wear in the absence of corrosion, and S is the material loss due to wear-corrosion synergy3,4. The synergetic effect is prominent for passive alloys such as aluminum, titanium, and stainless steels, which spontaneously form a protective thin (a few nanometer in thickness) oxide film (passive film) when in contact with oxygen or water5,6. During corrosion, and if this passive film is locally disturbed by mechanical wear, depassivation could lead to localized corrosion and unexpected failures1,3,7,8,9.
As an example of the economic impact of tribocorrosion in our society, wear and corrosion are estimated to cost nearly $300 billion per year in the United States10. In Florida, tribocorrosion phenomena of structural alloys in seawater is of interest given its ocean economy (fishing, marine transportation, and coastal construction), which contributes around 4% of Florida's total Gross Domestic Product11. Thus, a better understanding of tribocorrosion of metals and alloys will lead to better guidelines for application and usage of alloys in harsh environment service conditions. Such understanding will also serve to improve design principles for manufacturing new alloys and coatings against tribocorrosion and enhancing durability.
Tribocorrosion studies require integration of a tribometer and an electrochemical measurement system. The tribometer provides controlled mechanical loading and relative motion, and measures the friction force and sample surface height change. The electrochemical measurement system includes a potentiostat/galvanostat with a zero-resistance ammeter (optional) that determines open-circuit potential (OCP) and electrochemical polarization measurements. Such techniques provide a quick and inexpensive method to obtain the electrochemical properties of a material, where the corrosion rate of a metal can be measured by observing the response of the charge-transfer process to a controlled electrochemical disturbance. Here, we present a testing protocol for determining the tribocorrosion rate and wear-corrosion synergy of Al alloys, mostly following the ASTM standard G1192. This protocol includes sample preparation, machine setup, tribocorrosion testing, and post-testing calculation procedures. We hope this effort will benefit those new to the field to perform reliable and repeatable tribocorrosion tests to evaluate the deformation and degradation behavior of bulk as well as thin film metallic samples.
CAUTION: Please consult all relevant materials safety data sheets (MSDS) before use. Some chemicals used in the protocol are toxic. Please use all appropriate safety practices when performing experiments, including the use of engineering controls (fume hood) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, and closed-toe shoes). The CNC (Computer Numerical Control) machine must be operated by trained personnel. Hydrofluoric acid must be handled inside a fume hood that is identified with a sign stating "Danger, Hydrofluoric Acid Used in this Area" or similar.
1. Sample Preparation
NOTE: Proper surface preparation of samples prior to tribocorrosion tests is critical to ensure good reliability of the performed test and enhance test repeatability. In this protocol, a commercial Al 3003 alloy (Si: 0.1, Fe: 0.4, Cu: 0.08, Mn: 1.1 wt.%, balance Al) is used as an example.
2. Tribocorrosion Test
Following the testing protocol described above, the tribocorrosion rate (T) is measured at various potentials. Figure 8 represents the material loss obtained for the Al thin film sample after tribocorrosion at the cathodic (350 mV below OCP), open circuit, and anodic (200 mV above OCP) potential. The test was performed in 3.5 wt.% NaCl aqueous solution for 5 min under 0.5 N normal load, at a 1 Hz sliding frequency and 5 mm stroke length. Prior to each test, the OCP was stabilized for 20 min. Figure 9 shows the summary of all components of eqn. 1, including the tribocorrosion rate (T), wear rate (W0), corrosion rate (C0), and wear-corrosion synergy (S) of Al thin film.
Figure 1. Photo of (a) unpolished and polished Al bulk sample, (b) wired and painted bulk, and (c) thin film Al sample for tribocorrosion testing. Please click here to view a larger version of this figure.
Figure 2. (a) Photo of the front of the Bruker UMT machine without custom-made tribocorrosion cell. (b) schematic of tribocorrosion testing setup. Please click here to view a larger version of this figure.
Figure 3. Photo of custom-made tribocorrosion cell installed on the UMT rotary stage. The cell is made from Teflon with an O-ring at the bottom surface to prevent liquid leakage during tribocorrosion test. Please click here to view a larger version of this figure.
Figure 4. Representative potentiodynamic polarization curves of Al bulk and thin film after 1 hour immersion in 0.6 M NaCl solution. Please click here to view a larger version of this figure.
Figure 5. Photo of tribocorrosion machine during testing where the indentor probe is moving on the sample surface in reciprocal motion. Please click here to view a larger version of this figure.
Figure 6. Scanning electron microscopy image of the wear track after tribocorrosion test. The dashed lines represent the boundaries of the wear track. Please click here to view a larger version of this figure.
Figure 7. Typical wear track profile of Al thin film after tribocorrosion test obtained by profilometer. Please click here to view a larger version of this figure.
Figure 8. Summary of tribocorrosion rate (T) of Al thin films at different applied potential. The arrow bar represents one standard deviation from all repeated test results. Please click here to view a larger version of this figure.
Figure 9. Summary of tribocorrosion rate (T), wear rate (W0), corrosion rate (C0), and wear-corrosion synergy (S) of Al thin films. The arrow bar represents one standard deviation from all repeated test results. Please click here to view a larger version of this figure.
Figure 10. Evolution of corrosion potential during tribocorrosion test of Al thin film at OCP. Please click here to view a larger version of this figure.
Figure 11. Evolution of coefficient of friction (COF) during tribocorrosion test of Al thin film at OCP. Please click here to view a larger version of this figure.
Figure 12. Evolution of corrosion current during tribocorrosion test of Al thin film at 200 mV above OCP. Please click here to view a larger version of this figure.
Figure 13. Summary of mechanical and chemical wear of Al thin film during tribocorrosion test at 200 mV above OCP. Please click here to view a larger version of this figure.
There are several critical steps within this protocol. First, proper surface preparation of the samples prior to the tribocorrosion tests is critical to ensure good reliability of the performed test and enhance test repeatability. Different bulk alloys are to be prepared following different procedures to ensure a controlled surface roughness, and removal of any surface contaminants or scales. The procedure described here consists of solely mechanical grinding and polishing. This method generally applies to alloys with medium to high hardness such as Al, Ti, Ni, Cu alloys and steel. For softer alloys such as Mg alloys, electrochemical polishing or ion milling should be coupled with mechanical polishing to achieve the desired surface roughness. Secondly, for thin film sample sputtering, maintaining an ultra-low (< 10-6 Torr) vacuum in the sputtering chamber is critical to ensure low defect concentration in the deposited film, which would otherwise affect the corrosion resistance significantly. Thirdly, when preparing either bulk or thin film samples into the working electrode, it is important to ensure a good electrical connection between the sample and the connecting (Cu) wire. In this protocol, conductive tape or conductive epoxy is used. Alternatively, soldering, spot welding or similar techniques may be used. However, the effect of heating during welding on the microstructure and eventually the tribocorrosion resistance of samples have to be evaluated carefully. This is especially important for metals and alloys with low melting point. Finally, since tribocorrosion involves a three-body interaction (including the two bodies in contact, and the third body in between), it is important to ensure that a new ball tip (or a new area of the ball tip) is used when performing repeated tribocorrosion test.
The current protocol evaluates tribocorrosion rate by measuring material loss. Modifications of this protocol can be easily made to evaluate the depassivation and repassivation kinetics of tribocorrosion. This is achieved by tracking the current, potential, and coefficient of friction (COF) evolution during the test. As an example, Figure 10 and 11 show representative results of the evolution of corrosion potential and COF respectively, of Al thin film after tribocorrosion test at OCP. The arrows in Figure 10 represent the start and finish of scratching. It can be seen that for passive alloys such as Al, the mechanical disruption during tribocorrosion leads to local breakdown/removal of the passive film on the wear track and exposing a depassivated area which leads to a decrease in the corrosion potential by ~ 20 mV. Our earlier work16 showed that the magnitude of corrosion potential reduction is strongly related to the microstructure of the metal given the testing parameters (e.g., applied load, sliding speed, temperature) are the same. For Al with higher hardness and finer microstructure, the same applied load may lead to a smaller depassivated area, hence smaller change in corrosion potential. It is also noted that during the open circuit mode, the current is too low to be detected as the circuit is 'open'. However, current evolution during tribocorrosion test at imposed cathodic or anodic potentials can be monitored. An example can be found in our previous work16. Figure 12 shows the current evolution of Al thin film during tribocorrosion at an imposed anodic potential of 200 mV more positive than OCP. This anodic potential was selected within the passive region yet well below the pitting potentials. This result can be used to quantify the wear accelerated corrosion. In this case, the total material loss can be evaluated as T = Vmech + Vchem, where Vmech and Vchem corresponds to the contribution of mechanical and electrochemical material loss, respectively. Specifically, Vchem can be regarded as resulting from metal oxidation under anodic applied potential. Thus it can be calculated by Faraday's law as 17,18,19 , where Q is the electrical charge (calculated by multiplying the difference between the average anodic current during and before sliding by the time), M is the molecular weight, n is the oxidation valence, F is Faraday's constant, and ρ is the density of Al. Figure 13 shows the typical result of both terms for Al thin films. From the above discussion, it can be seen that an evaluation of the electrochemical parameters change, in addition to the weight loss, will thus offer critical insight to the depassivation kinetics during tribocorrosion.
The protocol presented here also bears several limitations. First, the corrosion cell is made from Teflon (polytetrafluoroethylene) or similar material. Thus, all tests were performed near room temperature. For applications that require higher temperature (e.g., above 400 °C for nuclear reactor cores), a special corrosion cell and tip have to be manufactured that will withstand high temperature creep and corrosion. Additional safety is also needed for handling molten salt electrolyte and metallic samples at high temperatures. Secondly, the attachment of a reference electrode near the working electrode (sample) has limited the wear motion to be linear reciprocal. In applications where a rotational motion of the sample is required, a special tribocorrosion setup has to be designed. Thirdly, in the present setup, the wear scratch rate is much faster than the corrosion rate. Hence the contribution of C0 is negligible compared to all other terms. While corrosion itself did not lead to significant material loss during the limited testing time, its effect on S is significant. In real world applications where mechanical scratch occurs at much lower frequencies, this trend might change where C0 may become dominant. Finally, special care has to be paid to errors generated during testing. This is especially important for evaluating the wear-corrosion synergy (S), which is derived from tribocorrosion rate (T), wear rate (W0), and corrosion rate (C0). Thus errors can be accumulated. To minimize errors generated in T and W0, a non-contact 3D optical profilometer (instead of the contact 2D profilometer) can used to determine the total material loss volume. To minimize error in C0, PD tests can be coupled with non-destructive EIS (electrochemical impedance spectroscopy) test to evaluate corrosion rate20.
As a final note, tribocorrosion rate is not a material property, but rather a system's response that depends on the testing parameters (applied load, sliding speed, etc.), the environment (temperature, pH, salt concentration, etc.), and material properties (hardness, surface roughness, etc.). The protocol presented here is demonstrated using only one set of condition. The readers should consider the differences and adopt appropriate changes in sample preparation, testing setup, and data analysis when dealing with different systems. Alternative testing setup including pin on plate (reciprocating), microabrasion, cylinder on bar, et al. can be found in 21. Tribocorrosion is an emerging interdisciplinary subject. It is hoped that this protocol will facilitate both the evaluation of existing engineering materials as well as the design of new materials resistant to both wear damage and corrosion degradation. Such materials become increasingly demanded in future applications such as advanced implantable medical devices, next generation nuclear power plants, and high capacity fast charging batteries, etc., which all require not only a strong and tough material, but one that is robust and reliable when interacting with some very extreme environment.
The authors declare no competing financial interests.
This work was supported by the US National Science Foundation Grant DMR-1455108 and CMMI-1663098.
|UMT (universal mechanical testing) machine
|NANO2-1010-06 (1 um)
NANO2-1003-06 (0.3 um)
NANO2-1005-06 (0.05 um)
|Ag/AgCl Reference electrode
|SYC Technologies, Inc.
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