Tribocorrosion is a degradation phenomenon of material surfaces subjected to the combined action of mechanical loading and corrosion attack caused by the environment. Although corrosive chemical species such as materials like chloride atoms, chlorides, and perchlorates have been detected on the Martian surface, there is a lack of studies of its impact on materials for landed spacecraft and structures that will support surface operations on Mars. Here, we present a series of experiments on the stainless-steel material of the ExoMars 2020 Rosalind Franklin rover wheels. We show how tribocorrosion induced by brines accelerates wear on the materials of the wheels. Our results do not compromise the nominal ExoMars mission but have implications for future long-term surface operations in support of future human exploration or extended robotic missions on Mars.
Tribocorrosion is a surface damage phenomenon resulting from the synergistic action of mechanical wear and (electro)chemical reactions. It can imply corrosion accelerated by wear, or wear accelerated by chemical reactions [
The surface of Mars is rich in corrosive chemical species. Martian regolith contains abundant chloride [
Perchlorates (ClO4−) have been found planet-wide on Mars [
A recent work [
The argument used by Schroeder et al. (2016) to justify the absence of corrosion on mechanical parts on Mars actually supports the hypothesis that environmental conditions have an impact on the fate of the rover wheels. The wheels of Opportunity and Curiosity were similar in terms of composition and effective ground pressure supported (see Materials and Methods). The fact that Opportunity showed no signs of chemical weathering or corrosion after more than 14 Earth years of operation on Mars (from 2004 until the middle of 2018), while Curiosity rover faced extreme corrosion and related punctures and tears just months after landing on Mars, maybe an indication of the impact of the local environment on wheels weathering. Actually, the details of this wear and tear on the wheels of Curiosity rover are missing from Schröder et al. [
Corrosion of the surface of a metal is the degradation that results from its chemical interaction with the environment. On Mars, any metal facing the sky, as the observable side of a meteorite, would only be exposed to the air, and thus only ambient oxygen would produce this damage. This is the case analysed by Schroeder et al. (2016), which considers only the chemical weathering rates of metals exposed to the Martian atmosphere. However, in the case of metals in contact with the regolith, as it is the case of the metals used in the wheels of the rovers operating on Mars, these materials may be eventually directly exposed to Cl and potential brines, which are formed naturally under Martian conditions at the interface of the regolith and the atmosphere [
Previous preliminary experiments, designed to look at the interaction between aerospace aluminium alloy (AA7075-T73) and the gases present in the Mars atmosphere, at 20°C and a pressure of 700 Pa with only 0.13% of oxygen, showed that there is an interaction between the small amount of oxygen present in the Mars gas and the alloy, when there is a scratch that removes the protective aluminium oxide film [
Here, we present laboratory studies that show the impact of tribocorrosion (the combination of mechanical and corrosion wearing) on materials in contact with naturally formed brines under current environmental conditions on Mars. Although tribocorrosion is a well-known phenomenon since nearly 150 years ago, there is a lack of analysis of its impact on materials for landed spacecraft and structures that will support surface operations on Mars. Mechanical parts, like the wheels of the rovers operating on Mars, in contact with corrosive brines on the surface, could be affected by the combination of chemical corrosion and mechanical loading. Nevertheless, to our knowledge, no single research on tribocorrosion research on Mars has ever been published, even though corrosive materials have been found on the surface of Mars.
The European Space Agency ExoMars 2022 Rosalind Franklin rover will have a nominal lifetime of 218 sols (around 7 Earth months). Its mass is 310 kg, with an instrument payload of 26 kg (excluding payload servicing equipment such as the drill and sample processing mechanisms). The rover's kinematic configuration is based on a six-wheel, triple-bogie concept with locomotion formula 6 · 6 · 6 + 6, denoting six supporting wheels, six driven wheels, and six steered wheels, plus six articulated (deployment) knee drives. This system enables the rover to passively adapt to rough terrains, providing inherent platform stability without the need for a central differential [
Here, we have performed laboratory studies on how the material (stainless spring steel) used in ExoMars 2022 Rosalind Franklin rover wheels is affected first by corrosion and then by tribocorrosion under environmental conditions on Mars. We suggest that similar studies should be conducted on other materials used for wheels and structures in contact with the Martian regolith.
For the study of corrosion, we have tested samples of the Sandvik 11R51 stainless steel, used in the ExoMars 2022 Rosalind Franklin rover wheels, and two other control materials: stainless steel SS4301 and regular steel S235. Sandvik 11R51 is austenitic (US and Euro standards: AISI 301, EN 1.4310) stainless steel with excellent spring properties with higher corrosion resistance (due to the addition of molybdenum), mechanical strength, tensile strength and tempering effect, and fatigue and relaxation properties. Every material was exposed to two sets of salt environments inside the SpaceQ chamber at Martian conditions [
Environmental conditions of the experiments performed with 11R51, SS4301, and S235.
Case | Conditions | Salt environments |
---|---|---|
#1 | SpaceQ Martian conditions (pressure = 6 mbar; temperature = 260 K) | 1.5 g NaClO4 (+water spontaneously absorbed from the atmosphere) |
#2 | SpaceQ Martian conditions (pressure = 6 mbar; temperature = 260 K) | 1.5 g MMS soil + 0.15 g NaClO4 salt (+water spontaneously absorbed from the atmosphere) |
Control #1 | Laboratory conditions | NaClO4 (1.5 g salt + 1 g water) brine immersion |
Control #2 | Laboratory conditions | 1.5 g MMS soil + 0.15 g NaClO4 salt |
Control #3 | Laboratory conditions | Indoors (exposed to the air at ambient lab conditions) |
Cases #1 and #2 were exposed for 5 hours to the Martian environment within the SpaceQ chamber, and to the simulated Martian water cycle described in Materials and Methods. The set of control samples #1, #2, and #3 were left on the bench at laboratory conditions during the same time, and the control sample #1 was immersed in a brine of NaClO4 (1.5 g salt + 1 g water) to observe the reactiveness of the material within the liquid already formed. After the experiments, the samples were packed in an airtight bag and studied using a scanning electron microscope (SEM) along with elemental detection via electron dispersive X-ray spectroscopy (EDS) to determine the effects of corrosion. The samples were only temporarily stored for transport in the airtight bag, and the clean metal was exposed during SEM.
The overview of the optical inspection by digital microscopy is shown in Table
Optical inspection of the experiments.
Condition | Sandvik 11R51 | SS4301 | Reg S235 |
---|---|---|---|
Case #1 (SpaceQ brine) | |||
Case # 2 (SpaceQ soil + salt) | |||
Control 1 (ambient brine) | N/A | ||
Control 2 (ambient soil + salt) | |||
Control 3 (ambient) |
In order to identify spectrally the corrosion in S235 under Case #1, we performed the analysis with the SEM-EDS as seen in Figures
(a) Microscope image of 10 × 10 mm regular steel sample. (b) High magnification SEM image showing location of EDS analysed regions. (c) Results of EDS analysis confirming corrosion by detection of Na and O in spectra 3 and 4.
Actually, it is worth remarking that upon exposure to two different environments (Earth and Mars environments), the regular steel was more severely corroded under Martian conditions (Cases #1 and #2) than under ambient conditions (see for comparison Case #2 vs. Control# 2 in Table
As shown in Table
Setup of tribological experiment showing ceramic ball and stainless-steel plate in their respective sample holders.
For the analysis we included the following (see
Figure
Friction data indicates transition to corrosive wear around time of 1800 s when immersed in brine.
Table
Wear data.
Worn volume ( | Max depth ( | Projected area (mm2) | Worn volume per cycle ( | |
---|---|---|---|---|
TN07 brine | 80 466 771 | 41.2 | 3.16 | 1490 |
TN08 water | 54 217 302 | 38.4 | 3.59 | 1004 |
TN09 brine | 78 590 249 | 40.5 | 3.19 | 1455 |
TN10 water | 26 081 611 | 19.6 | 3.53 | 483 |
The wear marks are shown in Figure
Friction data (scale shown in (a)). (a) TN07 brine. (b) TN09 brine. (c) TN08 water. (d) TN10 water.
Reference samples (scale shown in the figures). (a) Reference sample 1 (ambient conditions) at low magnification. (b) Reference sample 1 (ambient) at high magnification. (c) Reference sample 2 (SpaceQ brine) at high magnification.
SEM overview of experiments: (a) TN08 (water). (b) TN07 (brine).
Detection of Cl in test with brine. (a) TN07 brine, edge of wear mark. (b) TN07 brine, high magnification.
No Cl detected from water test. (a) TN08 water, edge of wear mark. (b) TN08 water, high magnification.
The results shown in Figures
The main conclusion of these experiments is that brine accelerates wear by chemical reaction leading to a sacrificial wear mechanism. In the sacrificial wear mechanism, Cl reacts with the steel surface (Fe) to reduce the toughness of the surface material. The consequence is that the resulting metal can wear more easily than the original did. This is a well-known concept in tribology—it is the basic principle relied on in antiseizure (also called extreme pressure) additives—where it is sometimes necessary to sacrifice material to reduce friction.
The experiments clearly demonstrate that wear under briny conditions result in a combination of a damaged passive layer and the presence of chlorides, which are the two factors usually associated with an increased risk of pitting corrosion. The duration of the presented experiment was about 5 hours, which is much shorter than the lifetime of the rovers on Mars. It can reasonably be assumed that the rovers’ wheels, after a long exposition to the surface and Martian atmosphere, may get damaged. In that case, an oxide layer damaged by wear will not regenerate, due to the lack of available oxygen. Additionally, the experiments presented here are a conservative case (low corrosion case) of the real conditions of Mars, as other perchlorates, like MgClO4 and CaClO4, are the main perchlorates present on Mars [
From the surface analysis (SEM-EDS), it was clear that Cl was present in the steel surface material after the wear process in brine, and therefore the corrosion risk should be further investigated.
Although the materials used in Mars exploration may be resistant to chemical corrosion from brines in static conditions, mechanical wear only has to remove a few nanometres of oxide to accelerate the corrosion [
Further studies are needed to consider other important components of the Mars surface environment that can affect this interaction, such as the effect of oxidants, the effect of radiation on their oxidising properties, the possible catalytic effects of the minerals present in the Martian regolith, the diurnal thermal changes, and variation in ambient humidity, and in particular to develop more detailed studies including all the perchlorates existing on the surface of Mars to quantity the impact of tribocorrosion. Once on Mars, it would be useful to perform regular monitoring of the wheels with the robotic arm camera of the rovers. That would allow to observe and characterise the impact of tribocorrosion and also to infer the occurrence of brines on Mars.
All the data are available in the main text or the supplementary materials.
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
JMT was responsible for conceptualisation, supervision, investigation, writing the original draft, funding acquisition, and resources. MPZ contributed to conceptualisation, methodology, supervision, investigation, writing the original draft, and funding acquisition. EN carried out the tribocorrosion and corrosion experiments and reviewed and edited the manuscript. AVR conducted the corrosion experiments; provided experimental support to the tribocorrosion experiments; and reviewed and edited the manuscript. AB reviewed and edited the manuscript.
The authors thank the ExoMars Project Team, European Space Agency (ESA), for reviewing the manuscript. The SpaceQ chamber has been developed in collaboration with Kurt J. Lesker Company and was funded by the Kempe Foundation. MPZ’s contribution has been partially supported by the Spanish State Research Agency (AEI), Project No. MDM-2017-0737, Unidad de Excelencia “María de Maeztu”–Centro de Astrobiología (CSIC-INTA).
The Supplementary Materials contain a detailed explanation of the steps followed in the brine corrosion and tribocorrosion tests and how the water and temperature cycle in the Martian near-surface have been performed. As complementary information, we give an estimate of the effective ground pressure of the Curiosity, Spirit, and Opportunity rovers. .