Design Decisions: Use Epoxies to Protect Against Metal Corrosion

Metals are ubiquitous in a wide array of structural applications and corrosion is of paramount concern in this regard. Corrosion can occur when metals are exposed to the environment along with reactions to water, acids, salts, and other chemicals.

One of the best approaches to preventing and inhibiting corrosion is to create a barrier over the metal surface, which can be achieved using protective coatings. With their excellent adhesion, convenient application, and performance properties, epoxies are highly effective as corrosion protection coatings for numerous applications.

Types and Sources of Corrosion

The type and rate of corrosion is determined by the metal, the types and concentrations of the reactive chemicals, and the environment, e.g. temperature and humidity. For example, in oil and gas production, there are corrosion issues. Sweet and sour corrosion occur when carbon dioxide (CO₂) and hydrogen sulfide (H₂S) are dissolved in water generating acidic solutions that attack the metal. Both can result in pitting, or the formation of small cavities in the metal surface. The presence of oxygen accelerates this process. Another example is galvanic corrosion, which occurs when two metallic materials with different electrochemical potentials are in contact in the presence of an electrolyte. Crevice corrosion generally occurs in areas where the surface is partially hidden from the environment and narrow, deep cracks are present, such as where two parts meet or underneath a screw head. There are numerous other scenarios where corrosion can occur. Another important example is erosion and fretting (constant wear due to movement against another surface).

Prevention Through Protection

With so many possible mechanisms by which corrosion can occur, it is imperative to consider it in the design phase. Obviously, it is easier to prevent corrosion rather than to deal with it after it has occurred. Epoxies are highly effective entities for preventing corrosion. In addition, their performance properties, such as their resistance to moisture, acids, and temperature, can be tailored to meet the needs of different applications.

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Selecting the Right Epoxy System

The successful performance of epoxy-based systems depends on proper selection and formulation of the key components: the epoxy resin and the curing agent.

In epoxy resins, the epoxy ring reacts readily with curing agents to yield crosslinked films that act as effective barriers against reactive chemicals and moisture. Aromatic groups impart toughness, rigidity, and high temperature resistance, while the oxygen substituents provide enhanced adhesion performance. Bis A, Novolac, are some of the commonly used epoxy resins. Multifunctional epoxies are attractive because they have additional sites for crosslinking, leading to higher crosslinking densities and even greater thermal and chemical resistance properties for coatings based on these resins.

Choosing the Best Curing Agent

The selection of the curing agent for an epoxy coating intended for a specific application is probably the most critical in the selection of the epoxy system. Curing agents not only affect the viscosity and reactivity of a formulation, they are incorporated into the applied coating and determine the type of crosslinking bonds that are formed, which has a direct impact on the properties of the coating. There are numerous types of curing agents that are effective for epoxy resins, thus allowing the formulator to tailor the properties of the coating system to meet the corrosion protection needs of many different applications.

The most important types of curing agents for epoxy coatings include nitrogen-based compounds (aliphatic amines, polyamides, cycloaliphatic amines, aromatic amines), polysulfides, and catalyzed systems such as imidazoles.

With the growing demand for new-age applications, corrosion has become an inherent problem in oil & gas, electronics, medical, optical, and many other industries. In order to prevent corrosion in such situations, epoxies are widely utilized. Not only do they confer excellent corrosion resistance, they are also easy to handle and process. However, one must be mindful of selecting the right epoxy system along with the appropriate application techniques for best results.

In order to improve the corrosion resistance properties of epoxy systems, constant research is being carried out. One such area of interest is the addition of various fillers in epoxy systems. Some specialized nanofillers have attracted particular attention in this regard.

Going forward, the objective for formulators is to enhance the capacity of such materials to withstand even more aggressive chemicals, temperatures, and different hostile environments.

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Venkat Nandivada is manager of technical sales at Master Bond Inc.