Electrochemical Method for Studying Localized Corrosion beneath Disbonded Coatings under Cathodic Protection

Abstract

This paper presents a new method for measuring localized corrosion under disbonded coatings by means of an electrochemical sensor, denoted differential aeration sensor (DAS). It measures the distribution of electrochemical currents over an electrode array surface partially covered by a crevice that simulates a disbonded coating. The DAS has been evaluated using immersion tests at open circuit and under cathodic protection (CP) conditions. Under both conditions, anodic as well as cathodic current densities were detected within the crevice. A fundamental understanding for the detection of anodic currents under CP has been explained in terms of basic electrochemistry. Based on the current distribution data provided by the sensor, two different analysis methods have been used to estimate corrosion and its distribution. These methods consisted of a direct application of Faraday's Law to the anodic currents detected by the array, and on a sensor-specific method denoted 'corrected currents' method. It has been demonstrated that under diffusion controlled conditions this latter method produces a better corrosion estimation than the direct application of Faraday's Law. The 'corrected currents' method allowed the estimation of corrosion patterns outside the crevice under CP. Good correlation between electrochemical calculations and surface profilometry results has been obtained. Cathodic protection (CP) is typically applied in combination with barrier coatings to prevent corrosion of steel on buried structures such as in water and energy pipelines. However, this combination is not always effective. Industrial experience has shown that severe localized corrosion issues such as pitting, microbiological induced corrosion and stress corrosion cracking can still occur within these structures, and these occurrences are often related to disbonded coatings. [1][2][3][4][5][6] Coating disbandment is common in buried pipes, particularly when CP is applied, 3,7 leading to the formation of a crevice (or gap) between the metal surface and the disbonded coating layer. Solution from the surrounding environment can access these crevices, resulting in enclosed corrosive environments. Furthermore, the effectiveness of the applied CP is believed to decrease significantly within the crevice due to a phenomenon often referred to as cathodic shielding. This phenomenon is particularly serious for buried pipelines, because the typically resistive soil solution could prevent CP current from reaching deep into the crevice created by the disbondment. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Disbonded coatings are generally considered a 'worst case scenario' for pipeline corrosion. Therefore, early detection of insufficient CP and corrosion within such crevice environments is critical.Potential survey methods, currently used by the pipeline industry, are not able to provide information about the CP effectiveness under a disbonded coating.Whilst the use of in-line metal loss inspection tools can detect pipeline corrosion (includ...

Visualization of Coupling Current and Immersion Potential of Iron Surface Corroding Beneath the Ice in the Temperature Cycling under the Freezing Point

“…In this study, the coupling current method, which has been used to investigate the spatial corrosion distribution, was used to understand the corrosion progression under ice [17][18][19][20][21][22][23][24][25] (Fig. 1).…”

“…The coupling current mapping method can visualize the coupling current distribution on corroding metals by dividing the metal sample into many pieces and measuring the coupling current via an external current meter. This method has been used to investigate nonuniform corrosion, [18][19][20] galvanic corrosion, 17,24) welding corrosion, 22) crevice corrosion, 18,23) atmospheric corrosion, 21,22) coating film evaluation 21,22) and so on. In this study, the time transition of surface coupling current distribution under an ice droplet formed using a 0.1 wt% NaCl aqueous solution was measured using a 100-ch electrode made from iron wires arranged in a 10 × 10 matrix.…”

Visualization of Coupling Current and Immersion Potential of Iron Surface Corroding Beneath the Ice in the Temperature Cycling under the Freezing Point

“…Journal of the Iron and Steel Institute of Japan 1. 緒言 大気腐食は実用金属において重要な研究対象である が 1,2) ，特に積雪寒冷地では，降雪の蓄積や氷雪の凍結／融 解，大量の腐食性融雪剤の散布など過酷な腐食環境となる ため，近年材料保全の観点から暴露試験などが活発化して いる [3][4][5] 。環境中の金属腐食のみならず，コンクリート内部 水の凍結融解によるコンクリートの破壊 [6][7][8][9][10][11] や結氷の付着 抑止のための表面ナノ構造の構築に関する研究 [12][13][14] ，さら に氷結を利用した海水からの脱塩 15,16) [18][19][20] ，ガルバニック 腐食 17,24) ，溶接部の不均一腐食 22) ，すきま腐食 18,23) ，大気腐 食 21,22) ，塗膜評価 21,22) などに用いられている。本研究では， 鉄細線を 10 × 10 マトリクス状に配置した 100 ch 電極 [23][24][25] を用い，0.1 w% NaCl 水溶液より形成した氷ドロップレット…”

In Situ Investigation of Under-Deposit Microbial Corrosion and its Inhibition Using a Multi-Electrode Array System

“…For instance, visualizing local electrochemical events on the metal surface in non-homogeneous surfaces, such as those created by biofilms and deposits ( Tan et al, 2011 ; Tan and Revie, 2012 ; Zhang et al, 2014 ). MEA systems have been used to study general and localized corrosion ( Sun and Yang, 2009 ; Tan, 1998 ; Tan, 2009 ; 2011 ), erosion-corrosion ( Xu and Tan, 2018 ; 2019 ), coating evaluation ( Varela et al, 2015 ; Mahdavi et al, 2016 ; Tan et al, 2017 ), UDC, and corrosion inhibition ( Muster et al, 2009 ; Turnbull et al, 2009 ; Hinds and Turnbull, 2010 ; Tan et al, 2011 ; Zhang et al, 2014 ) and most recently, to study MIC ( Dong et al, 2011 ; Liu et al, 2019 ).…”