Galvanic corrosion:

Copper and aluminum have widely different electrochemical potentials, so when they are combined in a cooling system, galvanic corrosion is likely. Galvanic corrosion (also called dissimilar metal corrosion) erodes the metal, causing leaks over time.

In a cooling loop, metallic materials in electrochemical contact can form a galvanic cell, or battery (fig. 1). In a galvanic cell, when two metals with different electrical potentials are connected, there is a potential difference across them. The metal with the higher electrical potential becomes the anode, and the lower, the cathode. A current will flow from the anode to the cathode. The anode dissolves, or corrodes, to form ions. These ions drift into the water where they either stay in solution or react with other ions in the electrolyte. This process is known as galvanic corrosion.

A galvanic cell requires three elements:

Two electrochemically dissimilar metals, An electrically conductive path between the two metals, and An electrolyte to allow the flow of metal ions.

In a typical liquid cooling circuit, the plumbing provides the electrically conductive path, and the aqueous

coolant provides the electrolyte. In the copper/aluminum scenario mentioned above, the aluminum is the

anode, the copper is the cathode and the cooling fluid is the electrolyte. Over time, the aluminum corrodes

as it dissolves into the water.

The galvanic corrosion rate depends on the electrical potential between the two metals. The Galvanic

Series (fig. 2) orders metals based on the potential they exhibit in flowing seawater. The most reactive are

at the top of the table, and the least reactive at the bottom.

Figure 2: Galvanic Series

Fig 2. Galvanic Series*

Magnesium Zinc Aluminum (most types) Iron, plain carbon and low alloy steels Lead, high lead alloys Tin plate, tin/lead solder Chromium plated materials, chromium alloys, chromium type-steels Brass Copper Nickel Stainless steels Silver Gold

*adapted from MIL-STD-889

Elevated temperatures, which are likely in cooling loops, accelerate galvanic corrosion. A 10°C increase

in temperature can approximately double the corrosion rate. Corrosion inhibitors can be added to the

cooling water. This retards, but does not eliminate, galvanic corrosion. Corrosion inhibitors bind with the

ions in solution to neutralize them. The inhibitors are consumed in this process so they need replacing

regularly. Non-aqueous coolants, such as oils, eliminate galvanic corrosion because they do not support

ions. However, thermal performance is sacrificed, as the thermal conductivities of heat transfer oils are

generally significantly lower than water-based coolants.

To avoid galvanic corrosion, we highly recommend using the same materials, or materials with similar

electrical potential, throughout your cooling loop. You should ensure that the plumbing, connectors and

other components do not introduce a reactive metal into the system.

Using the same materials throughout your circuit does not mean that you have to sacrifice performance.

Lytron offers high performance heat exchangers and cold plates with aluminum, copper and stainless steel

fluid paths.

Erosion-Corrosion in Cooling Systems

Heat exchangers and cold plates are used in cooling applications to remove and transfer heat from one place to another using a heat transfer fluid such as water, ethylene glycol and water solution, oil, etc. There are thousands of combinations of fluids and fluid path materials used in these applications. One of the main criteria for selecting the fluid path materials in these components should be the materials' ability to resist corrosion. Corrosion comes in many different forms, including "erosion-corrosion". It is important to know the fluids' properties as well as the materials' properties in order to minimize erosion-corrosion and optimize system performance and life.

What is erosion-corrosion?

Erosion-corrosion is the acceleration in the rate of corrosion in metal due to the relative motion of a fluid and a metal surface. It typically occurs in pipe bends (elbows), tube constrictions, and other structures that alter flow direction or velocity. The mechanism for this type of corrosion is the continuous flow of fluid, which removes any protective film or metal oxide from the metal surface. It can occur both in the presence and in the absence of suspended matter in the flow stream. In the presence of suspended matter, the effect is very similar to sandblasting, and even strong films can be removed at relatively low fluid velocities. Once the metal surface is exposed, it is attacked by the corrosive media and eroded away by the fluid friction. If the passive layer of metal oxide cannot be regenerated quickly enough, significant damage may occur.

Some materials are more resistant than others to erosion-corrosion under the same fluid conditions. Erosion-corrosion is most prevalent in soft alloys, such as copper and aluminum. Although increasing the flow rate of the fluid in your cooling application may increase its performance, it may also increase erosion-corrosion. Therefore, it is important to determine how great an impact increasing the flow rate will have on your thermal performance, as you may see minimal improvement in performance with a significant drop in the longevity of your heat exchanger or cold plate.

The following graph shows the effects of fluid velocity on performance and erosion on a 3/8" copper tube-fin liquid-to-air heat exchanger. The graph shows that in the turbulent region of flow (Re > 4000) and at volumetric flow rates of less than 2 gpm, water velocities are within the recommended values of less than 8 ft/sec (2 gpm) for copper tubing (see Table 1). Given the same diameter tubing, doubling the flow rate in the turbulent region of flow doesn't result in double the thermal performance. However, doubling the flow rate in the laminar and transitional regions can more than double the heat exchangers performance.

Preventing Corrosion in Cooling Systems

Water and water/glycol solutions are common heat transfer fluids used in cooling

systems and recirculating chillers. Although the fluids are the lifeblood for your heat transfer applications,

they can also cause corrosion within your systems. This corrosion can result in a reduction in system

thermal performance due to scaling on the heat transfer surface, decreased flow due to reduced pipe

diameters from corrosion deposits, and ultimately the need for system component replacement due to

corrosion damage.

Corrosion of Stainless Steel

Corrosion is the chemical or electrochemical reaction between materials, usually a metal and its

environment, which results in the deterioration of the metal and its properties. This article will cover

chemical corrosion. (For more information on electrochemical or galvanic corrosion, please see our

application note "Avoiding Galvanic Corrosion.") Corrosion of metallic components is an inherent problem

for water and water/glycol cooling systems because many metals naturally tend to oxidize in the presence

of water. The dissolved oxygen in water accelerates most corrosion processes. In closed loop systems,

the dissolved oxygen is consumed over time and no longer poses a corrosion risk. For open loop

systems, however, the continued exposure to air allows oxygen to dissolve into the coolant. Therefore,

open loop systems often suffer more corrosion problems compared to closed units.

Corrosion is usually classified as either general or localized. General corrosion is the loss of metal

uniformly distributed over an entire surface. It typically does not lead to rapid system failure because the

rate of metal loss can be discovered before the metal ruptures. Localized corrosion, on the other hand, is

not as predictable. It usually shows up in the form of pitting, which can penetrate into the metal very

quickly, forming cavities or holes. Another common form of localized corrosion is cavitation, which occurs

when pockets of vapor form in a liquid. This process occurs when the local pressure near the metal

surface falls below the vapor pressure of the liquid. When these vapor bubbles collapse or implode, they

generate large amounts of energy. This causes severe pitting to system components (such as pumps),

generates a great deal of noise, and results in a decrease in pump efficiency.

Potential corrosion problems

Corrosion can lead to many problems, the most significant being perforation that may result in coolant

leakage. Other problems may include reduced heat transfer caused by surface scaling, which occurs

when the metal reacts with oxygen, chloride, and/or inhibitors in the coolant and precipitates back to the

metal surface, creating a layer that acts as a heat transfer barrier. Additionally, concerns include the

clogging of particulate filters and damage to mechanical seals.

When copper corrodes, it is more often degraded by general corrosion than by pitting. General corrosion

will often attack copper exposed to ammonia, oxygen, or fluids with high sulfur content. Another source of

corrosion affecting copper is dissolved salts in the fluid, such as chlorides, sulfates, and bicarbonates.

For aluminum, pitting is the most common form of corrosion. Pitting is usually produced by the presence

of halide ions, of which chloride (Cl-) is the most frequently encountered in liquid cooling loops. Pitting of

aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is

readily polarized to its pitting potential and the naturally occurring protective oxide layer or film is

penetrated. This film is stable in aqueous solutions when the pH is between about 4.0 and 8.5. The film is

naturally self-renewing and accidental abrasion or other mechanical damage of the surface oxide film is

rapidly repaired. Lytron strongly recommends an inhibitor when using water with aluminum to maintain a

clean heat transfer surface.

Stainless steel is typically used in corrosive environments but, as with aluminum, it is sensitive to high

concentrations of chlorides (>100 ppm) in an oxidizing environment. Pitting remains among the most

common and damaging forms of corrosion in stainless steel alloys, but it can be prevented by ensuring

that the material is exposed to oxygen and protected from chloride wherever possible. Stainless steels

high in chromium, and particularly molybdenum and nitrogen, are more resistant to pitting corrosion.

Corrosion caused by uninhibited ethylene glycol

Studies1 show that uninhibited ethylene glycol will degrade into five organic acids - glycolic, glyoxylic,

formic, carbonic, and oxalic - in the presence of heat, oxygen, and common cooling system metals such

as copper and aluminum. Copper and aluminum act as a catalyst in the presence of uninhibited ethylene

glycol. These organic acids will then chemically attack copper and aluminum in as little as three weeks

under extreme conditions (212°F and oxygen bubbling into the uninhibited ethylene glycol solution) to

form metal organic compounds in the fluid, which can lead to clogging of pipes, pumps, valves, etc.

Literature2 references often state that copper and aluminum are compatible with uninhibited ethylene

glycol, but usually those recommendations are based on a two-week chemical compatibility study of

various metals at different temperatures. The study above indicates that uninhibited ethylene glycol

typically does not begin to degrade until after three weeks under those extreme conditions. In conclusion,

the reported data is based on ethylene glycol's ability to dissolve the metal and ignores the concern of

degraded, acidic uninhibited ethylene glycol and its effects on metals. The latter is much more corrosive

towards metals.

Protecting against corrosion

In general, corrosion can be reduced through pH control and corrosion inhibitor use. The inhibitors attach

to the metal surfaces to passivate them and prevent corrosion. It is also important to maintain a stable

water flow to avoid stagnant zones inside the cooling system, which can cause corrosion.

Quality of water also needs to be considered when trying to prevent corrosion. The corrosive effect of

natural water can vary considerably depending on its chemical composition. As mentioned earlier in this

article, chloride is corrosive and use of tap water should be minimized or avoided if it contains more than

100 ppm of chloride. Hardness of water also needs to be considered because it introduces calcium and

magnesium, which form scale on the metal surfaces. Deionized water, demineralized water, or water that

has been passed through a reverse osmosis process to remove harmful minerals and salts is highly

recommended in order to avoid chloride and scale buildup. A suitable corrosion inhibitor must be used

with deionized or demineralized water.

There are different inhibitors for use with different metals, each with its advantages and disadvantages.

Phosphate is an effective corrosion inhibitor for iron, steel, lead/tin solder, and most aluminum components. It is also a very good buffer for pH control. One disadvantage of phosphate is precipitation with calcium in hard water, which is one reason that deionized water is used for diluting a glycol/water coolant.

Tolyltriazole is a common and highly effective corrosion inhibitor for copper and brass.

Mercaptobenzothiazole also works for copper and brass, but it is not as stable as tolyltriazole.

Nitrite is an excellent corrosion inhibitor for iron. At high concentrations, this inhibitor is corrosive to lead/tin solder.

Silicate is an effective inhibitor for most metals but it tends to form thick deposits in cooling systems. The rust inhibitors in the automotive anti-freeze may cause premature failure of the pump seals.

Chromate and soluble oils have been used in the past, but their use has greatly diminished due to their toxicity. Modern inhibitors have replaced them.

Summary

Although we can't stop corrosion all together, there are ways to significantly limit it. By selecting the

proper fluid path materials, monitoring solution chemistry (specifically pH levels and water quality), and

choosing the appropriate inhibitors, you can minimize the cost impact due to corrosion and ensure the

effective operation of your liquid cooling loop for years.

1"Heading off Corrosion". Process Cooling & Equipment. @July/August 2002.2"Technical Insights into Uninhibited Ethylene Glycol". Process Cooling & Equipment. @July/August 2002.

 

Table 1 - Maximum Recommended Water Velocities for Specific Materials

Water

Low carbon steel 10 ft/sec

Stainless steel 15 ft/sec

Aluminum 6 ft/sec

Copper 8 ft/sec

90-10 Cupronickel 10 ft/sec70-30 Cupronickel 15 ft/sec

Controlling erosion-corrosion

Some methods for minimizing erosion-corrosion include improving the flow lines within the pipe by deburring (i.e. - smoothing out irregularities), allowing bends to have larger angles, and changing pipe diameters gradually rather than abruptly. Other methods include slowing the flow rate (minimizing turbulence), reducing the amount of dissolved oxygen, changing the pH, and switching the pipe material to a different metal or alloy.

In addition to the fluid path material used, it is also important to consider your fluid's temperature. At higher temperatures, flow rates should be lowered to minimize erosion-corrosion. For example, as a general rule, water flow velocities should not exceed 8ft/sec for cold water and 5 ft/sec for hot water (up to approximately 140 °F). In systems where water temperatures routinely exceed 140 °F, flow velocities should not exceed 3 ft/sec. For maximum recommended water velocities in other typical tube materials, refer to Table 1. For other fluids, the maximum allowable fluid velocity can be calculated from:

Allowable velocity for given fluid] = [Allowable velocity for water] x [Density of water/density of given liquid] 1/2

There will always be a trade-off between thermal performance and reliability/longevity in any cooling system. Increasing fluid flow will give you more cooling or performance only up to a point. After that, the increased fluid velocities may rapidly begin to erode and corrode the inside metal surface of the tubing. Designers should consider many different factors, such as the ones discussed above, in order to determine the best solution for their application.