Application Note | Coating Corrosion
Corrosion Resistance of Coating After Scratch Testing
Finding the Scratch Load at Which a Protective Coating Stops Preventing Corrosion
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
Corrosion resistant coatings protect metal parts against corrosion caused by various harsh environments or chemicals, such as moisture, salt spray, oxidation and acids. They protect metal surfaces by acting as a barrier to inhibit the contact of corrosive media. Targeting the performance requirements of specific applications, there are a number of different corrosion resistant coatings available on the market, including but not limited to epoxy, fluoropolymer, phosphate, PTFE [1].
Why Scratches Matter for Corrosion Protection
Corrosion resistant coatings should possess sufficient mechanical strength as they are often exposed to abrasive and erosive application environments. For example, the abrasive oil sands constantly wear away the inside of pipe, which progressively compromises the pipe’s integrity and potentially results in failure. In the auto industry, corrosion takes place at the location of scratches on the auto paint, especially during freezing winter when salts are applied on roads. Therefore, a quantitative and reliable tool for measuring the influence of scratches on protective coatings and their corrosion resistance is needed, in order to select the most proper coating for the intended application.
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Measurement Objective
In this study, the NANOVEA Mechanical Tester was used to generate scratches at different constant loads on a coated steel sheet. This was followed by corrosion tests to determine the critical load at which the corrosion resistant coating failed to protect the metal substrate. We would like to showcase that the NANOVEA Mechanical Tester in combination with the corrosion test is an ideal tool for quantitatively evaluating the performance of protective coatings used for abrasion and corrosion protection.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Conditions
The Macro Scratch Module of the NANOVEA Mechanical Tester was employed to perform the scratch tests on a coated steel sheet. A conical diamond stylus of apex angle 120° with tip radius of 200 µm scratches the surface of a coated steel sheet sample along a linear path under a constant normal force with a constant speed. The evolution of coefficient of friction (COF) and true depth were recorded in situ during the scratch test. The test parameters are summarized in Table 1.
| Test parameter | Value |
|---|---|
| Normal force | 5, 10, 15, 20, 25, 30, 35, 40, 50, 75, 100 N |
| Sliding speed | 6 mm/min |
| Sliding distance | 3 mm |
| Atmosphere | Air |
| Temperature | 24 °C (room) |
Table 1: Test parameters of the scratch measurement.
After the scratch test, the coated steel sample was immersed and corroded in 125 g/L Hydrochloric Acid (HCl) for 3 hours as shown in Fig. 2, in order to investigate the effect of scratches of the coated metal surface on its corrosion resistance. The 3D profile of the scratch tracks before and after the corrosion test was measured using the NANOVEA Non-Contact Optical Profilometer.
Results and Discussion
The images of the scratch tracks on the coated steel surface tested at different loads are shown in Fig. 3. The scratches possess progressively increased volume as the constant normal load increases from 5 N to 100 N. The evolution of COF and true depth of the coated steel sample at 5 and 100 N are shown in Fig. 4 as examples. The coating shows a low COF and depth of ~0.3 and 22 µm, respectively at 5 N, compared to ~0.55 and 130 µm for that at 100 N. Such information provides insight of mechanical failures taking place during scratching, enabling users to detect mechanical defects and further investigate the scratch behavior of the tested material.
The 3D profiles of the scratch tracks before and after the corrosion test are compared in Fig. 5, and the calculated scratch track volumes are summarized in Fig. 6. It is evident that the scratch tracks created at loads below 20 N exhibit comparable volume before and after the corrosion test. However, the volume of the scratch tracks generated at loads higher than 25 N increases substantially in the corrosion test, indicating the failure of the polymer coating and exposure of the metal substrate during the scratch tests at loads above 25 N.
The scratch tests at different constant loads in combination with the corrosion test provide a quantitative testing tool for evaluating the coating’s scratch and corrosion resistance with good precision and repeatability. It is useful for quality control and development of new protective coatings or paints for corrosion protection.
Conclusion
In this study, we showcased the capacity of the NANOVEA Mechanical Tester in performing macro scratch tests at different constant loads. Combined with the corrosion test, the scratch test at constant loads provides a tool for quantitatively assessing the scratch and corrosion resistance of protective coatings. In addition, the NANOVEA Mechanical Tester monitors the evolution of coefficient of friction, acoustic emission and true depth during the scratch tests. The NANOVEA non-contact optical profilometer can precisely measure the scratch track volume.
The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system.
References
Metal Coaters, Corrosion resistant coatings, http://www.metcoat.com/corrosion-resistant-coatings.htm
Frequently Asked Questions About Scratch and Corrosion Testing
Why combine scratch testing with corrosion testing?
Protective coatings face abrasion and erosion in service, and corrosion starts where they are scratched, for example on car paint in winter road salt. Scratching at known loads and then corroding shows the load at which the coating stops protecting the metal.
How was the test done?
Constant-load scratches of 5 to 100 N were made with a 120° diamond cone of 200 µm tip radius over 3 mm at 6 mm/min; the sample was then immersed in 125 g/L hydrochloric acid for 3 hours, and the scratch tracks were scanned before and after.
At what load did the coating fail?
Scratches made below 20 N had comparable volume before and after corrosion; above 25 N their volume increased substantially, indicating that the coating failed and exposed the steel substrate.
What do friction and depth show during scratching?
At 5 N the coating showed a COF of about 0.3 and a depth of about 22 µm; at 100 N about 0.55 and 130 µm, giving insight into the mechanical failures during scratching.
How is scratch volume measured?
A non-contact optical profilometer scans the 3D profile of each scratch track before and after the corrosion test and calculates its volume.
Which coatings can be assessed?
Corrosion-resistant coatings such as epoxy, fluoropolymer, phosphate and PTFE, as well as protective paints.
