Micro-galvanic corrosion during formation of epoxy coating

Prediction of long-term service life of an organic coating based on short-term exposure results

“…Thus, the following reasonable assumptions can be made: There are random microdefects/pores in a coating in various directions. Only those defects/pores through the whole coating layer from the outmost surface to the substrate can provide shortcuts for the corrosive solution to diffuse to the substrate. The corrosive water traveling in the through pores can be described by Fick second law. The accumulation of the corrosion products at the interface accelerates the delamination of the coating due to the wedging effect of the products. There is a critical delamination area, above which the coating starts to rupture along the through pores from the interface to the coating outmost surface. The other factors that can be regarded as a reduction of the coating thickness, such as microdefects (Song et al , 2012), UV irradiation (Feng et al , 2020a) and more complicated substrate corrosion (Feng et al , 2020b), are not considered. …”

“…The other factors that can be regarded as a reduction of the coating thickness, such as microdefects (Song et al , 2012), UV irradiation (Feng et al , 2020a) and more complicated substrate corrosion (Feng et al , 2020b), are not considered.…”

Modification, Degradation and Evaluation of a Few Organic Coatings for Some Marine Applications

“…Therefore, when the mixture of the epoxy and amine is applied on the substrate metal, the water dissolved in the curing agent will facilitate the electrochemical reaction of the substrate, resulting in corrosion damage of the substrate. Consequently, the corrosion products, including oxides/hydroxides of the substrate metal and hydrogen bubbles, formed on the substrate under the curing coating will decrease the coating adhesion, increase the coating porosity, and exacerbate the coating permeability [94]. It was found that after 1 and 4 days of salt spray (ASTM B117), no corrosion was visualized on an epoxy coated carbon steel (CS) or a magnetron-sputtered Mg (SM), whereas nearly half of the coating surface area was damaged on a magnetron-sputtered Mg layer covering over the carbon steel coupon (CS+SM) or a magnetron-sputtered iron layer covering over a magnetron-sputtered Mg layer (SM+SI) (see Figure 5).…”

“…It should be noted even through the importance of the substrate surface state or the essentiality of the substrate pre-treatment has been well-known, the "pre-corrosion" of a substrate during coating polymerization [94] has not been realized in the coating industry. According to this new knowledge, a good surface preparation of the substrate before coating is not enough.…”

“…This can enhance the electrical conductivity of the mixture the curing agent and epoxy resin during curing, electrochemically facilitating the micro-galvanic activity of the substrate. It has been reported [94] that an Mg-steel couple immersed in epoxy resin (A) curing agent (B) and their mixture (A 3 B) has galvanic current densities increasing with increasing relative humidity (RH) due to the continuous absorption of water from the environment into the curing agent (see Figure 6). However, as the mixture (A 3 B) of epoxy resin and curing agent polymerizes and water absorption reduces with time, the galvanic current density decreases (see Figure 6c).…”