Accelerated corrosion of low carbon steel by oscillatory acidic streams generated with a bio-inspired claw device

A mechanical device inspired by the pistol shrimp snapper claw was developed. This technology features a claw characterized by a periodic opening/closing motion, at a controlled frequency, capable of producing oscillating flows at transitional Reynolds numbers. An innovative method was also proposed for determining the corrosion rate of carbon steel samples under oscillating acidic streams (aqueous solution of HCl). By employing very-thin carbon steel specimens, with one side coated with Zn and not exposed to the stream, it became possible to electrochemically sense the Zn surface once the steel sample was perforated, thus providing the average dissolution rate into the most relevant pit on the steel surface. Furthermore, a laser light positioned beneath the metallic sample, along with a camera programmed to periodically capture images of the steel surface, facilitated the accurate counting of the number of newly formed pits. The system consisting of the thin steel sample and the Zn coating can be seen as a type of corrosion sensor. Furthermore, the proposed laser illumination method allows corroborating the electrochemical detection of pits and also establishing their location. The techniques crafted in this study pave the way for developing alternative corrosion sensors that boast appealing attributes: affordability, compactness, and acceptable accuracy to detect in time and space localized damage.

Experimental data on claw motion were obtained by processing videos taken with a high-speed camera (1000 frames per second).  The Excel file "Angular_motion.xlsx" contains the displacement and velocity data developed by the claw. Also attached is the Wolfram Mathematica file "Velocity_pincer.nb" with which the flow velocity plots developed by the device were created (Figure 2 of the paper and Figure S1A in the appendix).

Image analysis to identify and count the pits was performed using two Python programs (Program 1.py; Program 2.py). 

Program 1: This program performs IMAGE segmentation using the thresholding method. Moreover, multiplies grayscale IMAGE by a manually created MASK for each exposure time. The use of a separate mask for each exposure time is necessary to compensate for mechanical device vibrations that prevent the creation of a standard mask. Consequently, a manual creation of a mask using Paint software is required for each image. The output image of this program will feed into program 2.

Program 2: This program detects discontinuities in the segmented image (white pixels) and surrounds them with lines while counting them (which corresponds to the laser dots that indicate local dissolutions).

The Excel program "Image processing parameters.xlsx" contains the results of image analysis such as the number and size of pits located. These data were used to construct Figures 8 and 9 of the manuscript.

The Excel program "Corrosion rate.xlsx" contains the calculations for estimating the corrosion velocity of steel samples.