Advanced materials for corrosion resistant coatings

In the recent years, we have learned about the discovery and development of exciting new materials and structures that possess and display many interesting properties and useful phenomena. Many of these advances have been achieved by innovative materials synthesis and processing. Hence, in our quest for attenuating or arresting corrosion, we often look to these new technologies for the possibility of developing superior corrosion resistant coatings. This special issue of Corrosion Reviews will examine just some of these new technologies. The promises and challenges of utilizing advanced hybrid ceramic-polymer coatings, superhydrophobic coatings, and graphene coatings for corrosion protection are reviewed in this special issue.

Hybrid ceramic-polymer, or ceramer coatings can be synthesized to have some of the advantages of both ceramic and polymer coatings. Ceramics are generally UV resistant, hard, impervious to moisture, and abrasion resistant, but can be brittle, have high processing temperatures, and be difficult to top coat. Polymers are softer, flexible, have lower processing temperatures, and generally easier to top-coat, but are more easily breached and less UV resistant. The general goal in using ceramers is to obtain coatings that are UV and abrasion resistant, impervious to moisture, flexible, have the ability to be top coated, and have processing parameters closer to ambient conditions. The hybrid ceramers consist of an organic polymer or monomer segmented with inorganic moiety. Tiwari provides an overview of the extent of the current volume of research on ceramer coatings. He covers examples of state-of-the art research on various types of traditional ceramer coatings; those with blended polymers, nanoparticles, and pigments; and touches on the types of catalysts that are used for curing. He also identifies unresolved issues and current short comings in ceramer coatings, and increasing environmental regulations that may limit or prohibit constituents that are currently used.

Super hydrophobic coatings (SHCs) are characterized as coatings that have extreme water repellence, such as seen, for example, on a lotus leaf, with wetting contact angles exceeding 150°. The goal of using SHCs is to prevent the aqueous phase and corrosive constituents (e.g. chlorides) from making contact with the metal substrate to prevent corrosion. The review by Bahgat Radwan et al. covers the fundamentals of wettability, methods for synthesizing SHCs, the challenges of using SHCs, and the mechanisms of corrosion associated with SHCs. In the fundamentals section, the wettability of smooth and rough surfaces is discussed, as well as hysteresis effects. The section on SHC fabrication methods is comprehensive and includes sol-gel, electrospinning, etching, electrodeposition, anodization, hydrothermal reaction, and spray coating. In the section on challenges in using SHCs, Bahgat Radwan et al. discuss the effects of UV, pH, long-term water immersion, and mechanical durability on the effectiveness of SHCs. Loss of adhesion, and chemical, thermal, and mechanical stability are major concerns. In the section on corrosion, the authors have compiled a convenient table on the corrosion protection efficiency of SHCs categorized by fabrication method, substrate, hydrophobic material, and wetting contact angles.

Graphene is a monolayer of carbon atoms arranged in a honeycomb or hexagonal structure having a theoretical pore diameter of approximately 0.6 Å. The small pore size makes a flawless graphene layer impervious to molecules such as H₂O. However, the pore size can be tailored and most graphene sheets are not flawless; hence, variability in corrosion protectiveness can be expected. Gergely has compiled a comprehensive review that covers an introduction to graphene films; and single-layer graphene, layered graphene, and composite graphene coatings for corrosion protection. The general properties of graphene and applications in a wide variety of areas are covered in the introduction. A summary of graphene synthesis methods and corrosion testing and properties are discussed. For example, as graphene can catalyze the oxygen reduction reaction, if the films lack adequate protection and or adhesion, corrosion can be enhanced. Layered graphene and composite graphene have been investigated and developed to impart various characteristics to the coatings for the improvement of corrosion protection.

Ceramer, superhydrophobic, and graphene coatings offer many exciting properties. A plethora of properties and characteristics can be tailored and engineered into these coatings. One of the greatest challenges, however, is improving coating durability in the harsh corrosive environments where corrosion resistance is needed. It is anticipated that much research is needed in the coming years to fully take advantage of these exciting materials as the possibilities and variations appear to be endless. It is very likely that future coatings may incorporate graphene into a composite ceramer-based coating with superhydrophobic structure and properties.