For an application which requires an acid resistant coating for metal substrates, what testing can be performed to determine how the product will hold up to such exposures?
Certain epoxies can be formulated to be acid resistant. A good way to conceptualize this would be to expose the epoxy coated substrate to acid over time, measure any variations in weight and record any physical changes. This is a good indication for how well the epoxy can resist harsh acidic environments.
In an effort to illustrate this, Master Bond prepared 5 castings of different weights and sizes of a two part epoxy EP21ARHT, which has excellent acid resistance properties. Each casting was cured overnight at room temperature, followed by a heat cure of 6 hours at 80°C (175°F). The samples were then completely immersed in 95% sulfuric acid at room temperature and monitored over time, up to a total period of 150 days, or 5 months.
The cured samples were observed and their weights were measured after 60 hours, 10 days, 3 weeks (21 days), 6 weeks (42 days) and 5 months. The data collected showed no visual effects nor were there any changes in weight after each of the above time periods. Presumably a weight gain might indicate that the acid was attacking the matrix to cause it to swell; whereas a weight loss might indicate that the acid was dissolving a part of the casting.
These observations are important for applications where acid resistance is needed. Chemically resistant epoxies such as EP21ARHT may be used as a protective coating on substrates such as metals, composites, glass and certain grades of plastics. This epoxy may also be used with applications that require internal corrosion protection such as tank linings or chemical reactor vessels, for example. It is typically used in these industries—aerospace, oil and gas, and chemical processing.
It is important to note that proper processing techniques should be followed to optimize the results. The addition of heat with this system as a post curing step plays a vital role in improving the cross linking and eventually optimizing the chemical resistance over time. In terms of the application process itself, one must also be mindful of the appropriate surface preparation procedures to follow prior to application of the epoxy.
Disclaimer: The observations in this paper are not meant to be used for specification purposes.