Corrosion Resistant Coatings
Coatings to resist corrosion are commonly sought, as they provide essential protection to metal components and other items that come into contact with potentially damaging conditions.
What Is Corrosion?
Before understanding the potential coating solutions, you must have a grasp of corrosion. Corrosion occurs when a metal has a chemical or electrochemical reaction that consumes the metal to form a metallic compound such as a sulfide, hydroxide, or oxide. To put this in more understandable terms, it is when contact with its environment causes a metal to deteriorate.
What Causes Corrosion?
Metal corrodes through a reaction that typically involves oxygen, hydrogen, chlorine, and/or sulfur from its surroundings. The aggressiveness of corrosion can increase greatly in the presence of heat, electrical current, or mechanical stress. Stress-corrosion cracking on steel is a well-known example of mechanical stress increasing corrosion attack that has resulted in boiler explosions, gas pipeline ruptures, and bridge collapses.
What are Some Types of Corrosion?
- Uniform or general attack corrosion occurs evenly over the surface of the material.
- Pitting corrosion is localized and occurs if a local point turns into a corrosion cell.
- Crevice corrosion takes place in areas on a material where access to the surrounding environment is limited. It is often found under gaskets and bolts or between fittings
- Intergranular corrosion takes place at a metal alloy’s grain boundaries. These boundaries have a different chemistry than the bulk alloy and are more susceptible to corrosion.
- Stress corrosion cracking is the growth of cracks in a corrosive environment on particular alloys under mechanical stress. Without the mechanical stress the environment would have little corrosion attack on the base material, but under the mechanical stress, the base material can corrode rapidly and catastrophically.
- Galvanic corrosion or bimetallic corrosion is when different metals contact each other and are exposed to a corrosive environment. The less “noble” metal will suffer accelerated corrosion, while the more “noble” metal will corrode more slowly.
- High-temperature corrosion is a chemical attack from gases, solid or molten salts, or molten metals, typically at temperatures above 400°C (750°F).
- High-temperature oxidation is one of high-temperature corrosion process involving the reaction between the metallic alloys or coating sand the atmospheric oxygen at elevated temperatures.
- Hot corrosion is an accelerated oxidation of metallic alloys or coating induced by molten salts in an oxidizing atmosphere at temperature typically between 1200°F and 1700°F when metals and alloys.
What are Some Ways to Prevent Corrosion?
There are three main categories of anti-corrosion coatings that offer protection.
- Barriers are non-porous and protective. The barrier protective layer is applied directly on top of the metal itself forming a protective layer leaving the metal unexposed to a corrosive environment. The coating used to protect the underlying material will depend on the type of metal as well as the type of damage that you want to avoid.
- Inhibitive coatings create a passive layer on the substrate. They are common in primers and the protection reduces over time.
- Sacrificial coatings corrode preferentially as a way of protecting the material underneath. If you remember the description of galvanic corrosion, a sacrificial coating creates deliberate galvanic corrosion where the less noble coating corrodes instead of the more noble material underneath.
Potential Coating Solutions to Provide Corrosion Resistance or Protection?
Among organic materials, Fluoropolymers are unique to combine at the same time different properties that can be usually found only separately in different and distinct classes of polymers: among other different properties, they exhibit:
- universal chemical inertness (only a very limited of exotic fully fluorinated chemicals can chemically attack them at high temperature)
- high temperature resistance, up to 260 °C
- a fully ductile and tough behavior in a temperature range between cryogenic conditions and the maximum operating temperature
- full resistance to weathering conditions with outstanding performance at all long-term outdoor tests (Arizona and Florida tests, QUV test cabinets, etc)
- very low and negligible leach-out, so that purity of process fluids in contact with fluoropolymers can be ensured
Because of that, they have been largely used and deployed in the chemical, pharma, energy, semiconductor Industries, wherever corrosion in harsh conditions must be fought.
Particularly, the role of fluoropolymers coatings has become essential to provide to process, maintenance and production engineers the only viable solution and alternative to high Nickel alloys (Inconel 625, Hastelloy C, etc.) to protect their process equipment (regardless whether it is made of carbon steel), wherever strong acidic conditions are present, particularly over 60 °C (when rubber lining does not provide any longer a solution) or wherever a combination of solvents and acids are present and mixed together, or when the purity of process fluids from any metallic cation, even at ppb level, must be provided, as typically required in the pharma or in the semiconductor Industries.
High Performance Slurry Coatings
Inorganic metallic-ceramic basecoat consists of an aqueous inorganic binder combined with aluminum particles that provide galvanically-sacrificial corrosion protection. It is sprayed onto prepared substrate material followed by drying, curing and burnishing process to form protection layer. Chromate-phosphate, phosphate or silicate-based binder is often used to provide chemical binding to substrate. Sometimes sealing is utilized on top of basecoat to create additional barrier from corrodents by filling pores, increasing corrosion resistance, and increasing operating efficiencies in more aggressive environments. Seal coat or sometimes called topcoat consists of either inorganic binder or organic binder systems with or without pigments to enhance corrosion protection of already coated metallic substrate.
Diffusion coatings consist of a substrate alloy surface layer enriched with the protective oxide scale formers Al, Cr, Si or their combination to a depth of 15 to 100 μm. These elements combined with the primary constitutes of the substrate alloy to form new chemistry with significant high levels of Al, Cr, Si, or their combinations. Various diffusion coatings have been developed for different applications. For protection against high-temperature oxidation, the diffusion coating of choice are the aluminides, which form a protective alumina scale on high-temperature exposure in air. For protection against hot corrosion, platinum or chromium modified aluminide coatings, and chromizing coating are more beneficial.
Diffusion coating is essentially a type of surface enrichment by chemical vapor deposition process. It can be typically produced by pack cementation process, slurry process, above or beside the pack process, and “true” chemical vapor deposition process.
Thermal Spray Coatings
Laser Cladding and Hot Wire TIG Overlays
The right coating for your application
Coatings are one of the most powerful tools available for protecting components in corrosive operating environments. We offer an unmatched variety of coatings and treatments to protect your parts.
|Coating Family||Performance Characteristics|
Prevention of corrosion, oxidation and wear
Aluminide, silicon-aluminide and cobalt-based MCrAly alloys
Protection against hot corrosion
Improved corrosion and galling resistance
Protection from galvanic corrosion up to 1000°F / 538°C
Environmental barrier to mitigate corrosion with cathodic protection
We’ll work with you to evaluate your part's function, dimensions, weight, alloy and temperature exposure to select or develop the coating that addresses your corrosion prevention needs.