Aluminum and Erosion-Corrosion
July 1, 2008
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 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 can 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 might increase thermal performance, it might 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 might see minimal improvement in performance with a significant drop in the longevity of your heat exchanger or cold plate.
Some effective 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 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). In systems where water temperatures routinely exceed 140°F, flow velocities should not exceed 3 ft/sec.
There will always be a tradeoff 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 might 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 their application.
This article was provided by Lytron Inc., Woburn, Mass., a manufacturer of cold plates, cooling systems and heat exchangers. For more information, call (781) 933-7300, e-mail firstname.lastname@example.org or visit www.lytron.com.