Karakterizacija korozijskih procesov pod kapljico elektrolita s sklopljeno mrežo elektrod

Pavlič, Simon (author), Strlič, Matija (mentor), Legat, Andraž (comentor)

Atmosferska korozija je kompleksen in dinamičen pojav, odvisen od številnih dejavnikov. Korozija pod kapljico je osnovni sestavni del omenjenih procesov. V magistrskem delu sem preučeval vpliv različnih koncentracij NaCl na prostorsko in časovno dinamiko korozijskih procesov pod kapljico elektrolita na površini ogljikovega jekla. Procese sem spremljal s pomočjo sklopljene mreže elektrod (ang. coupled multi-electrode array), s katero sem meril anodne in katodne tokove. Velikost in obliko kapljice sem vzporedno spremljal z dvema digitalnima kamerama: z vrha (tloris) in s strani (naris). Uporabil sem 4 različne koncentracije NaCl. Rezultati so pokazali, da sta obseg in oblika korozijske poškodbe pri 1. kapljici v grobem podobni ne glede na koncentracijo NaCl. Istočasno je bilo iz tokov izmerjenih z mrežo sklopljenih elektrod razvidno, da so se prostorsko-časovne dinamike omenjenih procesov zelo razlikovale, vključno z njihovim trajanjem. Analiza slik zajetih s kamerama je pokazala t.i. »pripeto« vrsto kapljice (ang. sessile droplet), kjer stična površina s kovino ostaja enaka po velikosti in obliki. Za to vrsto kapljic sta značilna kapilarni in Marangonijev tok. Največja kumulativna korozijska poškodba je bila izmerjena pod kapljico 0,5 % NaCl, največja trenutna korozijska hitrost v 0,1 % NaCl. Pri vseh meritvah sem opazil drugi vrh povečanja gostote električnega toka, pred posušitvijo kapljice. Vrhovi so bili relativno ozki, razen pri 3,5 % NaCl, kjer je bil omenjeni vrh bistveno širši. Pri vseh nanosih 2. in 3. kapljice so se pojavili izraziti, a kratki začetni vrhovi. Vrhovi pred posušitvijo kapljic so postali izrazitejši: višji in širši. V nekaterih primerih se je pojavilo celo več zaporednih vrhov. Omenjeni vrhovi so nakazali na daljšo življenjsko dobo 2. in 3. kapljice v primerjavi s 1. kapljico. Eden glavnih ciljev mojega magistrskega dela je bil oceniti prostorsko-časovno dinamiko korozijskih procesov pri različnih kapljicah. Meritve s sklopljeno mrežo elektrod so pokazale, da so korozijski procesi v začetnem delu obstoja kapljice v grobem sledili Evansovemu modelu (anodni center v sredini s katodnim obročem). S tanjšanjem debeline sloja elektrolita so se pojavila odstopanja od omenjenega modela, pri čemer so se anodna mesta praviloma preselila bližje robu kapljice. Sicer so pri vseh koncentracijah NaCl nekatere elektrode ob sušenju kapljice spremenile predznak (določene tudi večkrat), pri čemer sam mehanizem prehajanja anod v katode in nasprotno, ni popolnoma jasen. Predvidevamo, da imata pomemben vpliv kapilarni in Marangonijev tok. Pri kapljici 3,5 % NaCl je slaba topnost kisika v kombinaciji z Marangonijevim tokom verjetno povzročila, da je večina začetnih katodnih področij proti koncu sušenja postala anodnih.

• Language: Slovenian
• Keywords: sklopljena mreža elektrod (CMEA), pripeta kapljica, digitalna kamera, ogljikovo jeklo
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2025
• PID: 20.500.12556/RUL-168077
• COBISS.SI-ID: 234135043
• Publication date in RUL: 28.03.2025

Secondary language

• Language: English
• Title: Characterization of Corrosion Processes Under Electrolyte Droplet with Coupled Multielectrode Array

Abstract

Atmospheric corrosion is a complex and dynamic phenomenon influenced by multiple factors, where the fundamental part are electrochemical processes under an electrolyte droplet. In my MSc Thesis the evolution of corrosion process under droplet along with its evaporation on carbon steel was monitored by means of coupled multi-electrode array (CMEA). Simultaneously, digital imaging in vertical and horizontal directions was performed, in order to follow shape and volume of a droplet. Four different concentrations of NaCl were used. It was indicated by CMEA that the cumulative corrosion damage and the corrosion pattern generated by the 1st droplets were roughly similar regardless of the concentration of NaCl. On the other hand, the spatio-temporal behaviour of corrosion processes and their duration differed significantly. Digitized imaging revealed sessile type of droplet evaporation, with the constant contact area and continuous reduction of the contact angle. It is known that an internal capillary flow and Marangoni flow occur within this type of droplets. The highest cumulative corrosion damage was indicated in a droplet with 0.5 % NaCl concentration, whereas the highest corrosion rate was measured in a droplet with 0.1 % NaCl concentration. In all droplets a secondary corrosion current peak in the period before the final evaporation was observed. These peaks were generally relatively short, but in 3.5 % NaCl was considerably wider and contributed to the droplet duration. At the application of 2nd and 3rd successive droplets, initial short and high current transients were generated. The secondary peaks before the final evaporation of droplets became evident, not only higher, but also wider. In certain cases, a sequence of these peaks appeared. Due to these peaks, the durations of the 2nd and 3rd droplets were longer in comparison to the 1st ones. One of my main interest was the spatio-temporal dynamics of anodic and cathodic sites during the evaporation of droplets. It was indicated by means of CMEA that the corrosion processes generally followed the Evans model (anodic centre and cathodic ring) in the first part of evaporation, but with thinning of the electrolyte film the anodic areas moved closely to the edge of a droplet. In all cases, some electrodes converted from anodes to cathodes, and vice versa (some even a few times). The mechanism of these changes is not quite clear, but it is assumed that the capillary and Marangoni flow played an important role. In 3.5 % NaCl droplets poor oxygen solubility in combination with Marangoni flow probably caused that most of initial cathodic sites changed to anodes before the end of evaporation.

• Keywords: coupled multi-electrode array (CMEA), sessile droplet, digital imaging, carbon steel