Open Circuit Potential (OCP) and Its Evolution During a Tribocorrosion Test

The degradation of various materials is caused due to the synergistic action of electrochemical and mechanical components. Open circuit potential (OCP) and its evolution during a tribocorrosion test provide a better understanding about this degradation process.

Bruker’s UMT TriboLab™ combines electrochemistry and tribology capabilities into a single device, and unlike conventional devices, allows a user to monitor the evolution of OCP during a tribocorrosion test. The device is developed to assist engineers and scientists to create new materials that resist tribocorrosion and are more durable and dependable in aggressive environments.

Figure 1. UMT TriboLab

OCP in Tribocorrosion

The development of new materials for various purposes, including chemical, marine, petrochemical, biochemical, dental, and mining applications involve tribocorrosion as a significant R&D activity. A tribocorrosion test entails electrochemical and mechanical interactions between tribological contacts.

Abrasion, sliding wear, fretting, solid particle erosion, and cavitation damage are the mechanical components involved in tribocorrosion. The degradation process of a material is enhanced synergistically by the presence of these chemical and mechanical components.

For example, a mechanical interaction may periodically break the protective passive film of a material surface, increasing the rate of degradation.

The study of tribocorrosion involves electrochemistry and tribology. In order to discover the electrochemical potential at which the anodic and cathodic reaction levels are balanced, it is necessary to observe the OCP during all electrochemical processes.

The net electrical current in such a state is zero. A tribocorrosion module capable of tracking the evolution of OCP during a tribocorrosion test is employed by Bruker’s UMT TriboLab test device.

Bruker’s Tribocorrosion Evaluation Tool

A potentiostat and a tribometer are essential to conduct electrochemical measurements during tribocorrosion research. The tribometer offers relative motion and controlled mechanical loading between a counter-surface, like a ceramic ball, and a metallic sample.

The UMT TriboLab, Bruker’s universal tribocorrosion test device, employs coated or bulk metallic test samples. The tests can be conducted in the presence of chemical solutions, such as oxidants, acidic, ionic lubricants, body fluids, salt solutions, alkaline, etc.

The normal force (Fz) and friction force (Fx) are measured by the tribometer. This force data provides the value of coefficient of friction (COF), as a function of time.

The potentiostat, integrated with the tribometer, conducts electrochemical polarization tests. Results in the form of corrosion current (i_corr) data, rate of materials removed due to corrosion, and corrosion potential (E_corr) are also provided by the potentiostat.

In addition to the data in force channels, the OCP data can also be recorded by the potentiostat during the tribocorrosion test.

The modular design of Bruker’s TriboLab provides it with flexibility to encompass a range of test parameters. It also offers precision control of speed, position, and load.

There are three main drive systems in the tester - Slider for the X- motion, Y-stage for the Y- motion, and Carriage for the Z- motion. The presence of integrated intelligent software and hardware interfaces, TriboScript™ and TriboID™, make TriboLab a versatile, productive, and user-friendly device.

TriboID configures and automatically detects the numerous components that are connected to the device and are essential for smooth operation. A secure, enhanced scripting interface is provided by TriboScript to compile test sequences from the in-built test blocks. A real-time control and data analysis software is also incorporated within the device.

Measurement of OCP Evolution

Figure 2 displays the OCP (V₀) plot as a function of time before, during, and after a tribocorrosion test conducted on an AISI 316 stainless steel in sodium chloride solution. No load was employed during the first 300 seconds and the OCP value was approximately -0.220 V. A 5 newtons load was employed after the first period and for the following 900 seconds the tribocorrosion test was conducted in the sliding mode.

A gradual change in the OCP towards the direction of the cathode and a matching increase in friction force were observed when the tribocorrosion test was initiated. After 400 seconds, the OCP and friction force were stabilized. A passive film, preventing any further corrosion of the stainless steel was fully removed at this stage, and a dynamic equilibrium of OCP and friction was achieved.

The maximum change recorded in the OCP, from the initiation to the stabilization, was approximately 0.080 V. The removal of the passive oxide film from the sample surface, which resulted in a galvanic coupling between the un-passivated and the passivated areas, was the cause for the change in OCP during tribocorrosion test.

The OCP came back to the original level that it was at the beginning of the test, when the sliding movement was ended and the load was released. AISI 316 stainless steel creates a tenacious, self- healing, adherent passive chromium oxide film that can protect it from corrosion.

Figure 2. Plot of evolution of OCP (V₀) during a tribocorrosion test on AISI 316 stainless steel in sodium chloride solution; initial 300s there is no load and no sliding; next 900s there is sliding with a normal force of 5 N; next 300s the sliding stopped and the load was removed.

The film is affected due to the mechanical wear during the tribocorrosion test. The protective film covers the wear track region when the mechanical sliding is stopped. This ensures that the OCP returns to its normal level, eventually.

A Bruker ContourGT^® 3D optical microscope was employed to evaluate the wear track formed on the specimen. The profile of the full wear scar is shown in Figure 3A. Figure 3B shows the depth profile of the wear scar along the horizontal line shown in Figure 3A. The width value of the wear scar was 188 µm, while its depth value was 0.436 µm.

Figure 3. (A) Wear scar profile on the stainless steel surface after the tribocorrosion test, (B) depth profile of the wear scar along the horizontal line in 3(A).

The tribocorrosion test was conducted again in the cathodic condition. In order to keep the electrochemical corrosion rate low, 1 V cathodic potential with respect to OCP was applied to the sample.

The material removal rate, in the cathodic condition, is often caused by the sliding wear. The profile of the full wear scar captured by the ContourGT optical microscope a under cathodic condition, is shown in Figure 4A.

Figure 4B shows the depth of the wear scar in along the horizontal line shown in Figure 4A. The width of the wear scar was approximately 232 µm, while the depth was approximately 0.217 µm.

Under cathodic condition, the depth of the wear scar was about half of that seen in the normal tribocorrosion test (Figure 3B). The results reaffirm that, under the cathodic condition, the materials degradation rate caused due to the combined chemical and mechanical components are low. This shows that under cathodic condition, tribocorrosion occurs at a reduced rate.

Figure 4. (A) Wear scar profile on the stainless steel surface after the tribocorrosion test under cathodic condition; (B) depth profile of the wear scar along the horizontal line in (A).

The evolution of OCP during tribocorrosion test can be observed successfully using Bruker’s tribocorrosion module. In applications where material degradation is caused due to tribocorrosion, leading to reliability and durability problems, similar data can be generated and used for comparing numerous materials.

Conclusion

Examining metallic materials for their vulnerability to degradation caused by the synergistic effect of wear and corrosion is a necessary step in the process of selecting and developing novel materials for various purposes including chemical, marine, petrochemical, biochemical, dental, and mining applications.

Bruker’s TriboLab test system is suitable for conducting tribocorrosion tests and observing the evolution of OCP during tribocorrosion evaluation.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.