A hydrophilic substance is a polar substance that attracts water molecules and has a strong affinity for water. The term hydrophilic actually means any substance that loves water, and is the opposite of hydrophobic, which is a substance that is water fearing, i.e., repels water in a practical sense.
The angle that the edge of the water droplet creates with the surface is known as the contact angle of the surface, and helps to determine if the surface is hydrophilic or hydrophobic for the particle. If the contact angle is less than 90 degrees then it is considered a hydrophilic surface, in which case the water droplet spreads across a larger area due to the surface's affinity for the water. When the contact angle is almost zero then the surface is called superhydrophilic.
In contrast, on a hydrophobic surface the contact angle will be greater than 90 degrees because the surface will resist wetting by water molecules.
Copper corrosion affects materials made of copper or copper alloys. When exposed to the atmosphere, copper oxidizes, causing bright copper surfaces to tarnish. After a few years, this tarnish gradually changes to dark brown or black, and finally to green.
Copper does not corrode in any significant way in an unpolluted environment. However, it is prone to rapid degradation due to contact with certain chemicals such as oxidizing acids, ammonia, sulfur and some of their compounds. (Copper corrosion is explored in the article If Copper is a Noble Metal then Why Are My Pipes Corroding?)
Underground copper pipes used for the water supply are prone to pitting corrosion and crevice corrosion. They can also corrode due to aggressive constituents in soils or the presence of stray currents in their proximity.
Corrosion could be uniform across the surface or it could be localized. Inner pipe surfaces may show signs of non-uniform corrosion due to turbulent water flow or particulate contaminants in the water, which can often result in localized pitting corrosion.
The corrosion resistance of copper in seawater depends on its ability to form a protective oxide film. High velocity and turbulence can erode the oxide film and allow the onset of corrosion.
Corrosion Resistance Affected by Alloying Compositions
The high-copper alloys have corrosion resistance that is comparable to pure copper, meaning they have fair resistance to saltwater corrosion but are susceptible to erosion corrosion at high velocities and turbulence. Alloys become more prone to stress corrosion cracking as the zinc content increases.
The addition of tin can increase the corrosion resistance of brasses. Brasses used in marine applications have better corrosion resistance due to addition of nickel, lead and tin. When the alloy contains aluminum, an aluminum oxide layer forms on the surface and provides enhanced protection against erosion corrosion and impingement damage due to turbulent seawater. Copper nickel alloy (C70600) is preferable for heat exchanger and steam condenser applications due to its good corrosion resistance.
Some of the methods that enhance the hydrophilicity (wettability) of a copper surface include:
Hydrophilic coatings are used to reduce the friction and effectively increase the lubricity (slipperiness) of the surface. These coatings readily get wet (i.e., allow water droplets to wet and spread on their surfaces). Typical hydrophilic coatings are made of polymers cross-linked in a loose structure so that they act like a microscopic sponge to attract water and achieve low friction while in contact with other surfaces, provided that the surfaces remain wet.
When these coatings are used on medical devices, the coefficient of friction (sliding) is as low as 0.01, due to the lubricity provided by moisture. The lubricity helps the device to move with minimum patient discomfort, minimizing inflammation of tissues. In catheters for example, the coating enables smooth navigation of the device through the designated pathway and minimizes the risk of tissue damage.
Hydrophilic coatings are used in the medical device industry, where the coating helps increase lubricity to reduce the friction between the patients’ skin and the surface of the device. This reduction in friction helps reduce the patient's discomfort while the components such as needles are inserted under the skin or other procedures are performed.
Hydrophilic Copper Coatings
Proprietary technologies are used to ensure that hydrophilic copper coatings bond to the copper at the nanoscale to create a functional high-lubricity (low friction) surface. These hydrophilic coatings created at the nano scale provide a water-loving surface with spontaneous affinity to water droplets and the capability to allow water to spread across the surface via wicking action. (For more about nano coatings, read The Advantages of Smart Coatings and Nanotechnology for Corrosion Prevention.)
The proprietary chemical process ensures superhydrophilicity of the surface with water droplets forming a contact angle of approximately 10 degrees with the surface. These coatings are designed to be durable and biocompatible to facilitate easy application.
Hydrophilic Copper Coatings and the Treatment of Copper Substrates
The damage to heat exchangers occurs due to corrosion induced by a harsh environment. The heat, wastewater and salt-laden air from the seacoast can initiate corrosion within a short time and affect unprotected copper pipes, fins and pipe joints (elbows and unions). Often the various components in HVAC units contain dissimilar metals, and the coating must take these factors into account. (To understand why, read Why Do Two Dissimilar Metals Cause Corrosion?)
Hydrophilic coatings spread the condensate evenly all over the substrate's surface, enabling better heat transfer through unimpeded airflow. However, in the case of surfaces facing a turbulent flow of salt water, copper coils coated with epoxy offer better corrosion protection compared to a hydrophilic coating.
In heat exchanger applications, a hydrophilic coating should be used wherever we anticipate that the corrosion caused by the condensate will be greater than the corrosion caused by salts and acids and where the coating's surface is easily wetted and water flows easily from the surface. In cases where condensate is excessive, hydrophilic coatings protect heat exchanger coils; however the uncoated parts may corrode due to accumulated condensate.
A coating manufacturer has developed a hydrophilic copper coating for HVAC-R condensers, with good wettability to permit higher fan speeds. This coating passes the salt spray test.
Aluminum fins generally used in air conditioners often support microorganisms such as bacteria and fungus, which contaminate the air that is supposedly being conditioned.
Copper can be considered as a replacement for aluminum because it inhibits bacterial growth, but copper surfaces suffer from poor wettability (hydrophilicity).
Researchers have experimented with a hydrophilic treatment for copper surfaces to provide a water-loving coating. Copper fin tube evaporators and individual copper fins were treated with a solution (mix) of potassium per sulfate (K2S2O8) and sodium hydroxide (NaOH) on the surface.
In separate trials air heating was individually provided for the fins and fin tube evaporators to enhance hydrophilicity (wettability). Wettability of the treated copper surfaces was evaluated according to the contact angle of the water droplets.
The researchers concluded that solution mix of NaOH and K2S2O8, coated on the surface, can enhance copper surface wettability on both the fins and fin tube evaporators.
An air heat treatment of the copper surface was found to be beneficial for copper fins. (In some set of experiments copper foils were used as copper fins and similar outcomes were observed.)
Hydrophilic and Oleophobic Membranes
A hydrophilic and oleophobic type of membrane has been developed to separate oil from water with high separation efficiency. The membrane was formed on a copper-mesh by using polyvinyl alcohol (PVA) and Chitosan (CTS) along with glutaraldehyde (GA) as a cross-linking chemical agent. By adding nanoparticles of titanium dioxide (TiO2) to the coating, wettability increased and the surface became superhydrophilic and superoleophobic.
The coating process also improved the copper mesh substrate's corrosion resistance and abrasion resistance.
High Performance Heat Exchangers are Now Possible
It is now possible to design and develop high performance heat exchangers using plate fin construction thanks to a hydrophilic coating material. The coating is effective on heat exchanger (evaporator) surfaces designed to transfer heat into space in hot environments.
Hot air treatments and hot water treatments were found to improve the wettability of nanostructured mesh copper substrates.
Copper has good corrosion resistance and heat conductivity — exactly the properties needed for heat exchanger applications. However, copper it is not hydrophilic. The poor hydrophilicity of copper impedes efficient airflow, which is essential for heat exchanger performance.
Various surface treatment methods and coatings are being developed to make the surface hydrophilic as well as corrosion resistant. Some of the methods used to accomplish this include:
- Treating the copper surface with a silica solution and an acrylate solution to improve the hydrophilicity (wettability) and corrosion resistance.
- Air heating of the copper fin surface to improve the hydrophilicity.
- Applying treatments containing a mixture of sodium hydroxide and potassium persulfate on the copper fins and copper tube of heat exchangers to improve hydrophilicity.
- Developing proprietary hydrophilic coatings for copper surfaces (e.g., heat exchangers such as HVAC-R condensers). These coatings have good corrosion resistance in salty and humid environments.
- Applying a hydrophilic-oleophobic membrane on the copper-mesh by using polyvinyl alcohol (PVA) and Chitosan (CTS) along with glutaraldehyde (GA) as a cross-linking chemical agent. By adding nanoparticles of titanium dioxide, wettability is increased and the surface becomes superhydrophilic and superoleophobic. The coating process also improves the copper mesh surface's corrosion resistance and abrasion resistance.