Corrosion Surface Analysis Using 3D Profilometry

Corrosion of materials is the most important and common failure mechanism in industry, from infrastructure and transportation to production and manufacturing. According to a 2-year study entitled “Corrosion Costs and Preventive Strategies in the United States”¹, released by NACE International and the U.S. Federal Highway Administration, the total annual estimated direct cost of corrosion in the U.S. is a staggering $276 billion, approximately 3.1% of the nation’s Gross Domestic Product (GDP). Therefore, corrosion inspection and monitoring become critical for implementing optimal corrosion control practices to improve lifecycle and asset management.

Electrochemical oxidation of metals is the most common form of corrosion. Oxides or salts grow on the metal surface as the metals are exposed to moisture in air. Corrosion can either take place locally to form a pit or crack, or it can extend uniformly across a wide surface. As a result, proper surface analysis of the corroded sample is vital in the evaluation of corrosion characteristics and mechanism, so as to select the best material, protective coatings and corrosion control measures for the targeted application. Unlike other techniques such as touch probes, the NANOVEA 3D Non-Contact Profilometer measures the surface features without touching the sample. This avoids the risk of removing important loose corrosion product and preserves the true surface features. Moreover, the portable JR25 model enables corrosion inspection on site.

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The false color view and 3D view provide users a straightforward tool to directly observe the morphology of the surface from different angles. Fig. 2 shows that Sample A has been severely attacked by corrosion and erosion at the edge.

Fig. 3 shows an example of 2D profile analysis on the valleys created during the corrosion and erosion attack. The three adjacent valleys possess depths of 265, 318 and 309 µm, respectively, and the total area of the hole is ~2.3 mm². Such 2D profile analysis is helpful in providing a more precise measurement of the shape of the sample.

The volume loss of Sample A is analyzed in Fig. 4, where the severely attacked area was selected and measured. The total material loss in the selected area is 89 mm³, as calculated by the software. This information is important in the process of selecting the best material candidate with the lowest corrosion rate.

Fig. 5 measures the average roughness of the less attacked area of Sample A. The roughness Sa in this region is 1.73 µm.

Pitting corrosion is a localized form of corrosion which leads to the generation of cavities in the material. Pitting is more difficult to detect, predict and design against, and thus more dangerous than uniform corrosion damage. Therefore, a reliable tool for pitting detection and analysis is critical to prevent the catastrophic failures that pitting may provoke.

Fig. 6 shows the false color view and 3D view for Sample B after pitting corrosion attack. Eight pits of different sizes are distributed on the measured surface, while the other areas remain intact, reflecting the extremely localized nature of pitting corrosion.

Pitting can be initiated by several factors, such as localized chemical or mechanical damage to the protective oxide film or the protective coating, as well as the presence of non-uniformities in the metal structure². Fig. 7 exhibits the 2D profile analysis on one of the pits as an example. The measured pit has a “cone” shape with a maximum depth of 43 µm and an area of 5513 µm². Such information is critical in analyzing the corrosion mechanism and development stage of pitting.

Fig. 8 and Fig. 9 analyze the individual and overall pits on the sample surface, respectively. From Fig. 8 we can easily measure the depth, size, volume and diameter of each pit, while in Fig. 9 the statistics of all the pits are evaluated. This in-depth information on the pits enables users to precisely assess the severity of the pitting corrosion of the test sample, so as to make an informed decision in the implementation of optimal corrosion control practices.

In this application, we have shown how the NANOVEA 3D Non-Contact Profilometer evaluates different forms of corrosion, including erosion and pitting corrosion. The axial chromatism technique allows measuring the corroded surface without touching, preserving all the details of the surface features. The analysis software provides measurements such as roughness and volume loss, as well as pit distribution, depth, size and volume, enabling users to obtain more insight into the corrosion characteristics and mechanism.

The data shown here represents only a portion of the calculations available in the analysis software. NANOVEA Profilometers measure virtually any surface in fields including Semiconductor, Microelectronics, Solar, Fiber Optics, Automotive, Aerospace, Metallurgy, Machining, Coatings, Pharmaceutical, Biomedical, Environmental and many others.

1. NACE International, Corrosion Costs and Preventive Strategies in the United States (supplement), nace.org.
2. NACE International, Pitting Corrosion, nace.org.