Application of Scanning Kelvin Probe in the Study of Protective Paints

Abstract

Industrial coatings are composed of layers of different polymers (top coats, primers) containing pigments, corrosion inhibitors, and fillers as well as additives. For corrosion protection, it is vitally important to preserve the strong adhesion and long-term stability of the metal-polymer interface in corrosive environments. In recent decades, the performance of painted materials increased, which requires the application of advanced methods for quick assessing, ranking and predicting corrosion stability. Scanning Kelvin probe (SKP) is a highly sensitive and non-invasive technique to analyze in situ the metal-polymer interface of high-performance industrial coatings. SKP is able to monitor the adhesion and corrosion underneath different kinds of paints without the need for long-term corrosion tests. SKP is a localized electrochemical technique with a spatial resolution in the range of 70-100 µm. Hence, it is possible to obtain information about the intact and corroding portions of the interface at defect sites, corrosion blisters, contaminants, and intermetallics, quality of pretreatments, and the development of galvanic couples that lead to corrosion de-adhesion of the polymeric coatings. This article reviews the application of SKP to the determination of the mechanisms of corrosion deadhesion of model paints, thick marine paints, coatings with zinc rich primers, automotive paints, and coil coatings applied on galvanized steel substrates.

Powder and High-Solid Coatings as Anticorrosive Solutions for Marine and Offshore Applications? A Review

“…Coating failure normally initiates at local defects, which can be the result of application errors, chemical heterogeneities in the coating or other defects, such as bubbles, under film deposits or mechanical damages. The corrosion of steel at a defect can lead to cathodic delamination of the surrounding coating or be the result of anodic undermining or filiform corrosion [1,4,20,48,64,65]. For submerged steel, cathodic delamination will be the predominant degradation mechanism whereas in atmospheric weathering conditions alternative mechanisms of de-adhesion can be expected [64].…”

“…The corrosion of steel at a defect can lead to cathodic delamination of the surrounding coating or be the result of anodic undermining or filiform corrosion [1,4,20,48,64,65]. For submerged steel, cathodic delamination will be the predominant degradation mechanism whereas in atmospheric weathering conditions alternative mechanisms of de-adhesion can be expected [64]. Another minor mechanism but that can be important is a mechanical delamination due to the accumulation of corrosion products which can expose fresh steel.…”

“…Disbonding from damage is the result of bonds breaking at the coating-metal interface coming from the alkalinity of the cathodic products [1,4,19,63]. They are a result of electrochemical reactions occurring at the coating-steel interface, where solid iron is oxidized to ferrous ions and oxygen is reduced to hydroxyl ions at the metal-paint interface, see Figure 6 [64]. In addition to the production of OH -, cations should also be available to maintain electroneutrality.…”

“…Now, when the anodic reaction is taking place under the coating, chloride ions (Cl − ) will migrate to Figure 6. Schematic for corrosion-driven coating de-adhesion in the case of cathodic delamination [64].…”

“…Oxygen diffusion through the thick electrolyte layer is the limiting factor and to remove this limitation, the cathodic front spreads from the defect underneath the polymeric coating. In humid conditions (thin electrolyte), the defect area is well aerated and works mostly as a cathode where oxygen reduction reaction can take place, while anodic reaction occurred underneath the coating and spread from the defect, Figure 7 [1,4,64,70,71]. Now, when the anodic reaction is taking place under the coating, chloride ions (Cl − ) will migrate to balance the Fe 2+ ions that are formed by the anodic reaction.…”