A study of the influence of osmotic and corrosion effects on the working capacity of nonmetallic coatings in chemically active media

Abstract

Using the model representations for the mechanism of destruction of nonmetallic coatings in chemically active media proposed in previous papers by the authors a description of the osmotic and corrosion effects that influence the destruction of coatings is given. We study the loss of ability of coatings to function due to destruction of cohesion and adhesion.In a previous paper [1] the authors delineated the fundamental stages in the process of wear of nonmetallic coatings in electrochemically active media up to the loss of protective properties. In that paper we also gave computations of the time required for defects (cavities) under the coating to be filled with a water solution of the contacting medium and the geometric dimensions of the corrosion sites at various stages of weal'.In this paper we proposed a mathematical description of the osmotic and corrosion effects at sites under the coating, making it possible to use the results obtained earlier [1] to determine the characteristic points in the wear up to the loss of working capacity of the coating due to destruction of cohesion and adhesion.

Mathematical description and analysis of the corrosion of metals under lacquer coatings in seawater

“…The products of reaction (2) are also partially dissolved in the volume of the blister thus decreasing the molar concentration of water in a solution, intensifying osmosis, and enhancing the level of pressure in the center. If we now take into account the adsorption of water in the course of reactions (2), then, according to the approach proposed in [2], the behavior of the mass of water for z > Xcor is described by the following differential equation:…”

“…where mw is the inflow of water due to osmosis, m r is the mass of the rust in the center, mp and m a are, respectively, the masses of the passivator and other admixtures per unit of area of the coating, pp and p,/are the coefficients of their washing out into the center, Cr is the concentration of dissolved corrosion products, 7p, Ta, and Tr are, respectively, the ratios of the molar mass of water to the molar masses of the dissolved passivator, admixtures, and corrosion products, oc and D are, respectively, the reduced solubility coefficient of water [2] and its diffusion coefficient in the deformed lacquer film, R('c) is the radius of the corrosion center, and h is the thickness of the coating.…”

“…It is shown that, for the examined type of coatings, the corrosion rate first linearly increases with time but then, on attainment of a certain level, its subsequent increase is substantially decelerated. If the corrosion rate of the uncoated metal is higher than 0.05 mm/yr, then the rate of corrosion under coatings is lower by at least an order of magnitude.Some model representations of the mechanism of aging and fracture of lacquer coatings in seawater, the corresponding boundary-value problems formulated on the basis of the relevant mathematical model, and analytic-numerical methods for their solution can be found in [1,2]. These results were obtained for small deflections (blisters) of a lacquer film over the corrosion center and an almost linear law of increase in the corrosion rate to a certain fixed value.…”