Replica Molding of Inner Pipe Corrosion Using 3D Profilometry

Corrosion causes an annual estimated direct loss of $276 billion in the U.S., approximately 3.1% of the nation’s Gross Domestic Product (GDP) according to a breakthrough 2-year study entitled “Corrosion Costs and Preventive Strategies in the United States” [1]. Metal pipe corrosion is a major cause of pipe failure. It is a continuous and variable process determined by various environmental conditions, including but not limited to the water properties, the soil surrounding the pipe and stray electric currents [2].

Surface finish of the metal pipe is critical for its product quality and performance. Rust progressively builds up and pits initiate and grow on the metal surface as the corrosion process takes place, resulting in roughening of the pipe surface. The differential galvanic properties between metals, the ionic influences of solutions as well as the solution pH may all play roles in the pipe corrosion process, leading to corroded metal with different surface features. An accurate surface roughness and texture measurement of the corroded surface provides insight in the mechanisms involved in a specific corrosion process.

Conventional profilometers have difficulty in reaching in and measuring the corroded inner pipe wall. Replica molding provides a solution by replicating the inner surface features in a non-destructive way. It can easily be applied on the inner wall of the corroded pipe and sets in 15 min. We scan the replicated surface of the replica molding to obtain the surface morphology of the inner pipe wall.

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The replica molding was applied to the inner surface of the corroded pipe as shown in Fig. 2 and set in 15 min. It was then easily removed from the pipe, with one face replicated with the 3D inner surface morphology of the corroded pipe. This method significantly facilitates measurement of the inner surface features of the pipes, such as roughness, pits and others. Such information is usually difficult to obtain using conventional profilometer measurement directly on the inner surface, due to the restricted space in the small pipe.

The 3D view of the replica molding is displayed in Fig. 3, which provides users a useful tool to directly observe the surface morphology from different angles in detail. No evident pitting is observed on the replica molding of the pipe.

Fig. 4 shows the 3D View of the inner surface measured directly and the replica molding of the corroded pipe at a larger magnification, in order to further evaluate and compare the shape and roughness of the inner pipe surface and the replica molding. The surface morphology of the inner pipe surface and the replica molding after removing the cylinder form is compared in Fig. 5, revealing more detailed rough surface features. The direct inner surface and the replica molding of the corroded pipe match well in terms of shape and roughness. As summarized in Table 1, they possess comparable average roughness Sa of ~8.7 µm. Such a roughened metal surface is caused by rusting, a common form of corrosion which includes galvanic corrosion, pitting corrosion, and crevice corrosion. Rust usually creates red or brown flaking or pitting on the metal surface [3].

The 2D profiles across the 3D morphology are shown in Fig. 6, allowing us to directly measure the shape and texture of the surface profile at different locations and detect possible corrosion pits with different depth, size, volume and diameter.

In this application, we have showcased that the NANOVEA ST400 non-contact profilometer in combination with replica molding is a useful and convenient tool to evaluate the morphology and roughness of the inner surface of a corroded pipe. The measured 3D profile and roughness of the replica molding is in agreement with the values measured directly on the inner surface of the corroded pipe. The software provides various analysis tools such as roughness and peak and valley profile, enabling users to have a comprehensive understanding of the corroded surface, which was difficult to measure using direct profilometer measurement.

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.

NACE International, Corrosion Costs and Preventive Strategies in the United States, http://www.nace.org/uploadedFiles/Publications/ccsupp.pdf
CDC, Corrosion control (community water fluoridation fact sheet), http://www.cdc.gov/fluoridation/factsheets/engineering/corrosion.htm
InterNACHI, Rust inspection and prevention, https://www.nachi.org/rust-inspection-prevention.htm