This article was published in the May issue (pgs. 14-16) of Materials Performance Magazine. The full text can be found here.
By Kathy Larsen
Offshore oil and gas assets are continuously exposed to corrosive atmospheric conditions such as moisture and chlorides, and often at temperatures above ambient. To protect metal components in these environments against corrosion, a protective zinc coating is often applied. A zinc coating protects the underlying substrate in two ways—by creating a barrier that prevents oxygen and moisture from reaching the surface of the steel, and by providing localized cathodic protection of the steel when the zinc coating is damaged. To improve the corrosion protection provided by a zinc coating, Modumetal, Inc. (Seattle, Washington) developed an electrochemical process that creates a nanolaminated zinc coating, and this emerging coating technology is being evaluated by the industry on its ability to enhance corrosion resistance and extend the service life of offshore equipment.
According to Christina Lomasney, president, CEO, and co-founder of Modumetal, nanolaminated metallic coatings enable more efficient use of the coating material because of the architecture of the coating. “It’s a concept that is very similar to plywood in terms of the ability to architect the material, but much more profound in terms of the implications for the material’s properties,” she says. “There is a dramatic shift in material properties that we can affect through the nanolayering process.”
Nanolaminated coatings are comprised of nanometer-scale metal particles (ions) that are electrochemically deposited onto a substrate in nanolayers that vary in composition and material microstructure. Coating attributes achieved by nanolamination include enhanced corrosion resistance as well as improvements in strength, toughness, wear characteristics and embrittlement resistance as compared to other commercial coating systems.
The nanolamination process is similar to electroplating in that the component to be coated (the cathode) is immersed in an electrolyte that contains the zinc and other raw metal materials (the anodes). An electric field is used to reduce metal ions from the metals in the electrolyte into a metallic form that bonds to the surface of the component. The process is unique in that the electric field is modulated to create different conditions in the electrolyte for very short time periods during the coating process, which results in the controlled deposition of a distinct zinc alloy nanolayer during each of these time periods. All parameters of the electric field—voltage, current, and frequency—are carefully controlled to achieve a very specific chemistry and microstructure for each snapshot of time. These laminated layers cannot be seen optically because they are smaller than the wavelength of visible light.
The resulting microns-thick zinc alloy coating comprises several 10- to 100-nm layers that form the coating’s nanolaminated architecture, with each nanolayer possessing a unique microstructure. The process can be used to apply nanolaminated coatings to complex parts and surfaces such as fastener threads. For thicker protective zinc coatings, the coating deposition process is repeated—each time applying one coat of the nanolaminated coating with its specific nanolayer architecture—until the required coating thickness is attained. Nickel-based nanolaminated coatings that isolate downhole components from hydrogen sulfide (H2S) and carbon dioxide (CO2) also can be applied using this technology.
The coating’s properties not only incorporate the ability to resist particular corrosive environments due to the alloy’s inherent composition and microstructure, but also include enhanced performance resulting from the nanolamination architecture, says Lomasney. Corrosion proceeds laterally along the interface of the nanolayers rather than toward the substrate, and fracture propagation also follows the lamination planes. Because the discontinuities tend to move along a layer boundary rather than down into the layers themselves, corrosion and fatigue resistance are improved.
Manipulating the architecture of the nanolayers also can enhance the coating’s corrosion- and fatigue-resistant properties, and yield a coating that is very different from a single-layer coating made of the same base material. “It’s the interface between layers in the nanolaminated architecture that creates the properties of the coating and provides the mechanism to take the coating’s performance to the next level,” Lomasney notes. When building the architecture of the coating, the nanolayers can be changed and arranged to achieve the desired coating properties. “We’re not changing the average alloy composition or mixing the zinc with something else to change how it corrodes; we are creating an interface that enables a dramatic change in performance,” she says.
Lomasney cites galvanic coupling in the zinc-based alloy as an example. “We have found that if we create a potential difference between the nanolaminated layers that comprise the coating, we can enhance the corrosion protection behavior of the zinc. In the process we use, we can actually control that galvanic couple, which creates the improvement in performance. That is an architectural parameter that doesn’t exist in a homogeneous material,” she explains.
By creating small galvanic cells within the zinc coating’s nanolayers instead of between the zinc coating and the substrate, Lomasney continues, oxidation occurs within the coating itself if the coating is scratched or damaged, which delays oxidation of the underlying steel surface and makes it less susceptible to pitting corrosion. At the same time, the zinc’s consumption rate decreases, which provides a longer coating life. This more efficient use of the zinc is possible because of the coating’s nanolayered architecture. The strength of the coating also can be enhanced, she adds, by creating a nanolayer interface that prevents dislocation movement in the alloy.
Nanolaminated components using this coating process are commercially available for the oil & gas, aviation, defense, and highway construction industries. Coated products include small-scale tubular joints, valves, connectors, and fasteners.
To evaluate the performance of the zinc-based nanolaminated coating in terms of improved corrosion resistance and extended service life of offshore assets, laboratory tests have been conducted on metallic test coupons as well as steel fasteners—important components on offshore assets that corrode in the marine environment and often need to be replaced, Lomasney says. The tests included three types of accelerated corrosion tests and seven different mechanical tests. Additionally, field trials are currently in progress in a tropical offshore environment to verify the results of the laboratory tests.
The zinc-based nanolaminated coating consistently demonstrated good results in the standardized coatings tests and the offshore field trial. The experimental methods and results are reported in paper 5735 presented at CORROSION 2015 in Dallas, Texas.1
Contact Christina Lomasney, Modumetal—e-mail: email@example.com.
1 M. W. Joosten, J. Vander Laan, S. Lomasney, C. Lomasney, L. Collinson, J. St. Clair, “Nano-Laminated, Metallic Coatings for Corrosion and Abrasion Resistance” CORROSION 2015, paper no. 5735 (Houston, TX: NACE International, 2015).
Photos from Figure1
A fastener assembly coated with a zinc-based nanolaminated coating.
Photos from Figure 12
Shown are black oxide bolts (left) and bolts with a zinc-based nanolaminated coating (right) after 24 months of service during an independent analysis and field trial of a zinc-based nanolaminated coating system conducted by the United States Coast Guard.
Photos from Figure 2
Panels with a zinc-based nanolaminated coating are shown prior to salt fog exposure (left) and after 4,344 h of salt fog exposure (right).