Application Note | Pitting Corrosion
Pitting Corrosion Measurement Using 3D Profilometry
Counting, Sizing and Mapping Corrosion Pits on Stainless Steel
Research & Experimental Testing
Craig Leising
Visual Design & Editorial
Andrew Shore
Introduction
In the form of holes, pitting corrosion is localized and can penetrate into materials very quickly, and it is one of the most frequent forms of corrosion in stainless steels. The quantification of pitting corrosion has been a main focus of study in recent years. Pitting corrosion is quantified in different ways, e.g. average pit depth, maximum pit depth, volume, etc.
Importance of 3D Non-Contact Profilometry for Pitting Study
Unlike other techniques such as touch probes or interferometry, the 3D Non-Contact Profilometer, using axial chromatism, can measure nearly any surface; sample sizes can vary widely due to open staging and there is no sample preparation needed. Nano through macro range is obtained during surface profile measurement with zero influence from sample reflectivity or absorption, with advanced ability to measure high surface angles and no software manipulation of results. It easily measures any material: transparent, opaque, specular, diffusive, polished, rough, etc. The technique of the non-contact profilometer provides an ideal, broad and user friendly capability to maximize surface studies when pitting analysis is needed, along with the benefits of combined 2D and 3D capability.
ℹ️ Learn more about surface profiling lab services.
Measurement Objective
In this application, the NANOVEA ST400 Profilometer is used to measure the surface of a corrosion-pitted stainless steel coupon. The area measured was selected at random and assumed large enough to be extrapolated to a much larger surface. Density, area, volume, size and shape are used here to quantify the level of corrosion.
NANOVEA ST400 Standard
Optical 3D Profilometer
Results
3D Surface
The 3D view and false color view of a randomly selected area on the sample surface after pitting corrosion show pits of various shapes and depths. They provide users a straightforward tool to directly observe the distribution and morphology of the pits from different angles. Several pits exhibit an elongated shape parallel to the direction of the machine marks, indicating the importance of surface finishing to the corrosion resistance of metals.
2D Surface Analysis
The pits can be quantified in numerous ways using built-in software tools, as shown in the following figures as examples. It can be observed that ~55% of the pits have an open area below 0.03 mm² and a pit depth ranging from 60 to 90 µm. One quarter of the pits possess a volume of 4000–5000. Such statistical information allows us to compare the pitting resistance of different materials and provides more insight into the mechanism of pit formation and development.
Conclusion
In this application, we have shown how the NANOVEA 3D Non-Contact Profilometer can precisely characterize the corrosion-pitted surface of stainless steel. The data shows 95 pits found in the area measured, with a density of 0.3110 pits/mm². The pit density, together with statistical information such as the volume loss and pit size distribution generated by the software, could be used to compare the pitting resistance against other test coupons developed under different conditions. The data shown here represents only a portion of the calculations available in the analysis software; this statistical data can be calculated over the entire surface measured, or each pit can be analyzed individually by selecting it.
Frequently Asked Questions About Pitting Corrosion Measurement
Why quantify pitting corrosion?
Pitting is localized, can penetrate materials very quickly and is one of the most frequent forms of corrosion in stainless steels; quantifying pit depth, size and volume lets pitting resistance be compared.
How many pits were found?
95 pits in the measured area, a density of 0.3110 pits/mm².
How large and deep were the pits?
About 55% had an open area below 0.03 mm² and depths of 60 to 90 µm, and a quarter had volumes of 4000 to 5000.
Why were some pits elongated?
Several pits ran parallel to the machining marks, showing how surface finishing affects the corrosion resistance of metals.
Which parameters can be reported for each pit?
Area, diameter, depth, perimeter and volume, for each pit individually or as statistics over the whole measured surface.
Does measurement disturb the corrosion?
No. The non-contact profilometer measures without touching, so loose corrosion product and pit shapes are preserved.
