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Materials Selection In Corrosive Environment

Material/Coating selection is a step in the process of designing any physical object. In the context of corrosion, the main goal of material selection is to minimize corrosion risk while meeting product performance goals. Systematic selection of the best material for a given application begins with corrosion performance and costs of candidate materials. For example, nickel alloys would provide superior pitting corrosion resistance than Type 304 stainless steel . What used to take /Coating several years of exposure to marine conditions to gain definitive information on materials selection can now be accomplished in few days or less by ASTM G61 testing. See the following examples which clearly indicates nickel alloys (Alloy-C276 and UNS N10276) are the choice for salt containing environment. This method makes it easy for an engineer to select materials and evaluate susceptibility to localized corrosion.

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An indication of the susceptibility to initiation of localized corrosion in this test method is given by the potential at which the anodic current increases rapidly. The more noble this potential, obtained at a fixed scan rate in this test, the less susceptible is the alloy to initiation of localized corrosion. In general, once initiated, localized corrosion can propagate at some potential more electropositive than that at which the hysteresis loop is completed. In this test method, the potential at which the hysteresis loop is completed is determined at a fixed scan rate. In these cases, the more electropositive the potential at which the hysteresis loop is completed the less likely it is that localized corrosion will occur. This techniques also exhibits the extent of passivity of various alloys in corrosive environment.

 

Electrochemical impedance spectroscopy (EIS) For Coating Selection

EIS combined with accelerated environmental exposure tests, such as salt spray (fog) tests, humidity tests and ultraviolet light (QUV) exposure tests, can help to predict the time-to-failure of painted or coated samples and their performance in barrier properties (1-4). When used with accelerated environmental exposure tests, EIS acts as a quantitative detector of coatings quality. The organic coatings resistance generally degrades with time. The degradation is associated with corrosive ions and water penetration into the coatings, transport of ions through the coating, and subsequent corrosion reactions at the polymer-metal interface.

Even though an accelerated environmental exposure test with EIS is faster than real- world exposure, it still takes a long time (from hundreds or thousands of hours). Furthermore, in some cases, for instance, high performance durable coatings with high thickness, the failure of coatings can not be achieved after a long time exposure using this method. New technologies, which can rapidly assess high performance durable coatings, have been developed by incorporating new concepts, terminology, theoretical calculation and unique electrochemical setups into traditional EIS testing. The details are described below.

MethodologyTraditional EIS Theory-Accelerated exposure testing can be complemented with electrochemical impedance spectroscopy (EIS). In the EIS technique capacitance and electrical properties of the coating are measured as a function of time. If the impedance ratio does not change as a function of time, then one can with high degree of confidence conclude that the coating is not altered and performs very well under actual service conditions. The advantage of this technique is that it permits a reduction in severity of test conditions such that these conditions will not result in the complete failure of the coating system. The test will provide reliable data in a short time of exposure. The data from this test can also provide data for calculating permeation rate and minimum coating thickness and cure schedule requirements for adequate corrosion resistance.

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Electrochemical impedance is measured by applying an AC potential to an electrochemical cell and measuring the current. The response to this potential is an AC current signal, containing the excitation frequency and its harmonics. This current signal can be analyzed as a sum of sinusoidal functions (a Fourier series). Data obtained from electrochemical impedance measurements is commonly analyzed by fitting it to an equivalent electrical circuit model. Most of the circuit elements in the model are common electrical elements such as resistors, capacitors, and inductors. Each of these elements has a phenomenological basis for the electrochemical system. The most important elements are resistors and capacitors.

A metal covered with a good (non-diffusive), un-damaged coating usually displays very high impedance at low frequencies (log 9-10).

It is important to note that as the frequency decreases, the impedance increases steadily in a straight line. As stated above, this is the behavior shown by a capacitor. This shows that the overall impedance of the equivalent circuit is almost exclusively dependent on the capacitor (coating capacitance) with insignificant contribution from the resistor (solution resistance). The degradation of a purely capacitive coating can be modeled with the same resistor/capacitor set up. The first step to coating degradation is uptake of the solution into the coating. Then, the resistor (solution resistance) in the electrochemical system significantly contributes to the overall impedance. Physically the solution has started to penetrate the coating, and as a result the current flow through the system is increasing. This increasing current value causes the calculated impedance to decrease (at low frequencies). Other information determined from this technique includes resistance polarization, the quantity of water absorbed in the organic coating and dielectric constant.

The primary EIS measure used for comparisons between coatings as EIS measurements are made during the course of an environmental exposure, such as soil or cyclic salt spray, is the “percent of ideal protection.” This is the area under the log (impedance) – log (frequency) plot expressed as a fraction of the initial area, which is measured before initiating the environmental exposure.

 

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EIS-based Innovative Test Program for Rapid Assessment of Coating Performance. 

Percentage of Ideal Protection-The new terminology of primary EIS measure used for comparisons between coatings as EIS measurements are made during the course of an environmental exposure is the “percent of ideal protection.” This is the area under the log (impedance) – log (frequency) plot expressed as a fraction of the initial area, which is measured before initiating the environmental exposure.

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The percentage of protection measure is used to summarize the changing frequency- dependent impedance of a coating, which reflects both the water penetration and the low frequency impedance which approximates the pore resistance. In traditional EIS measurement, an estimate of the low frequency impedance is given. Therefore, the percentage of ideal protection forms a reliable method of high performance durable coating evaluation and rating. It has been successful for us to rate and rank the high performance coatings in most cases using this method during the course of an environmental exposure. In general, 80% or above EIS protection indicates a sufficient protection according to our experience. Three examples are provided in later section.

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