Corrosion in cooling systems is a problem faced by all, but some types of corrosion are trickier to avoid or repair than others. Here, common causes and ways to prevent erosion corrosion and galvanic corrosion will be discussed.
One of the main criteria for selecting the fluid path materials in these cold plates should be the materials’ ability to resist corrosion.
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, oil or ethylene glycol and water solution. Literally thousands of combinations of fluids and fluid path materials are used in these applications. One determining factor used for selecting the fluid path materials in process heating components should be the materials’ ability to resist corrosion. Corrosion comes in many different forms, including erosion corrosion, so it is important to know the fluid’s properties as well as the materials’ properties in order to minimize erosion corrosion and optimize system performance and life.
Controlling 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, 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 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 a heating or cooling application may increase its performance, it also may increase erosion corrosion. Therefore, it is important to determine how great an impact increasing the flow rate will have on overall thermal performance. In some applications, the increased flow rate may produce only a minimal improvement in performance, yet the erosion corrosion will cause a significant drop in the longevity of the heat exchanger or cold plate.
Some methods for minimizing erosion corrosion include:
- Improving the flow lines within the pipe by deburring.
- Allowing bends to have larger angles.
- Changing pipe diameters gradually rather than abruptly.
- Slowing the flow rate (minimizing turbulence).
- Reducing the amount of dissolved oxygen.
- Changing the pH.
- Switching the pipe material to a different metal or alloy.
In addition to the fluid path material used, it also is important to consider the 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 8 ft/sec for cold water and 5 ft/sec for hot water (up to approximately 140°F [60°C]). In systems where temperatures routinely exceed 140°F (60°C), flow velocities should not exceed 3 ft/sec. Maximum recommended water velocities in other typical tube materials can be calculated using the formula:
Allowable Velocity for a 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 users more cooling performance-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 to determine the best solution for an application.
Avoiding Galvanic Corrosion
Heat exchangers are used in cooling applications to remove and transfer heat from one place to another. If not kept free of corrosion, the process will suffer.
When selecting components for the cooling loop, it is important to consider material compatibility as well as individual performance. Although an aluminum cold plate paired with a copper-tube heat exchanger might meet the thermal requirements, it is not a reliable cooling circuit. Copper and aluminum have widely different electrochemical potentials, so when they are combined in a cooling system, galvanic corrosion is likely to occur.
Galvanic corrosion - also called dissimilar metal corrosion - erodes metal, causing leaks over time. In a cooling loop, metallic materials in electrochemical contact can form a galvanic cell or battery. 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 while the lower becomes 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.
- 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, 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.
Elevated temperatures, which are likely in cooling loops, accelerate galvanic corrosion. An 18°F (10°C) increase in temperature can approximately double the corrosion rate. Corrosion inhibitors can be added to the cooling water to retard, but 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, the thermal performance is sacrificed as the thermal conductivities of heat transfer oils are generally lower than water-based coolants.
Using the same materials, or materials with similar electrical potential, throughout the cooling loop is the recommended action to avoid galvanic corrosion. It should be ensured that piping, connectors and other components do not introduce a reactive metal into the system.
The galvanic corrosion rate depends on the electrical potential between two metals. Adapted from MIL-STD-889, the galvanic series orders metals based on the potential they exhibit in flowing seawater. The most reactive are at the top of the list and the least reactive at the bottom.
- 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.
- Stainless steels.