Galvanic corrosion fluid velocity

The life of equipment subjected to corrosive environments can be increased by proper attention to design details. Equipment should be designed to drain freely and completely. The internal surfaces should be smooth and free from crevasses where corrosion products and other solids can accumulate. Butt joints should be used in preference to lap joints. The use of dissimilar metals in contact should be avoided, or care taken to ensure that they are effectively insulated to avoid galvanic corrosion. Fluid velocities and turbulence should be high enough to avoid the deposition of solids, but not so high as to cause erosion-corrosion. 

Corrosion tests of metals under static conditions reveal nothing relating to erosion-corrosion susceptibilities. It is entirely possible that a metal tested under static conditions will fail in service when sufficient fluid velocity produces erosion-corrosion. Similarly, it has been observed that galvanic corrosion between coupled, dissimilar metals may be accelerated or even initiated under flow conditions when little or no galvanic corrosion is observed under static conditions (see Chap. 16, Galvanic Corrosion ). 

Generai description. Galvanic corrosion refers to the preferential corrosion of the more reactive member of a two-metal pair when the metals are in electrical contact in the presence of a conductive fluid (see Chap. 16, Galvanic Corrosion ). The corrosion potential difference, the magnitude of which depends on the metal-pair combination and the nature of the fluid, drives a corrosion reaction that simultaneously causes the less-noble pair member to corrode and the more-noble pair member to become even more noble. The galvanic series for various metals in sea water is shown in Chap. 16, Table 16.1. Galvanic potentials may vary with temperature, time, flow velocity, and composition of the fluid. 

Galvanic corrosion typically involves two or more dissimilar metals. It should be recognized, however, that sufficient variation in environmental and physical parameters such as fluid chemistry, temperature (see Case History 16.3), flow velocity, and even variations in degrees of metal cold work can induce a flow of corrosion current even within the same metal. 

The possible effects of fluid velocity on galvanic corrosion are sometimes overlooked. Fluid velocity can affect the apparent potential of metals in a given environment. Depending on the environment, a metal under the influence of relatively rapid flow may assume either a more noble or a more active character than that indicated by the galvanic series. Occasionally, this shift in potential may result in galvanic corrosion that would not occur under stagnant or low-flow conditions. 

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

Design (avoidance of dissimilar metals, galvanic couples, improper materials, high fluid velocities in inappropriate places, caulking or seal welding of areas prone to crevice corrosion, roof design, etc.)... 

Although copper and brass typically have good corrosion resistance in aqneous solntions, they may be subject to corrosion in plant environments, depending upon the process stream. The presence of sulfides and ammonia componnds in wastewater can lead to dissolntion of cuprous compounds. Further, if copper is coupled to a less noble metal like steel or alnminnm, galvanic corrosion of the less noble metal may resnlt. Because copper is a fairly soft material, it is also subject to erosion. This type of corrosion is accelerated by high fluid velocities, high temperatures, and abrasive particulate matter. 

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