Evaluating Single Phase Velocity Sensitive Corrosion

Corrosion can be affected by the rate at which fluid passes by an alloy surface. Having the laboratory capability of predicting Velocity Sensitive Corrosion can be important to making appropriate decisions about compatibility between an alloy and an environment. This section discusses laboratory techniques that could be applied to evaluating velocity sensitivity when the environment is a single liquid phase without entrained particles. The choice of which technique to use depends on the test protocol required, if correlation to field results is possible, and which technique best enables that correlation to be established. Four techniques described below are:

Flow Loop

Perhaps the simplest system from a conceptual standpoint is the flow loop shown in the following picture.

The actual design and equipment requirements are dictated by the needs of the user and safety requirements. In all systems, fluid is pumped from a reservoir through a test section meant to simulate the actual geometry and back to the reservoir. The test section can be a pipe section, an annulus, an expansion joint, etc. The ability to provide one-to-one correspondence between the flow parameters in flow loop to that in field depends on how closely the test section copies the actual field geometry or can be scaled up to the field geometry. The effect of heat transfer on corrosion can be examined by using a pipe with heat transferred through it as the test section or by inclusion of an actual heat exchanger. Further information can be found at http://www.argentumsolutions.com/tutorials/heat_tutorialpg1.

One drawback with flow loops is the complexity of the additional equipment and instrumentation needed for safe and successful operation. For example, the tank requires appropriate connections and instrumentation for drainage, cleaning, overfill protection, relief valving. The pump can be subject to leakage at seals. Heaters and insulation would be required to maintain a constant temperature. Filters might be required to remove particulates. Finding appropriate materials of construction that are compatible with the environment can sometimes be difficult and expensive. Some advantages include

  • certain geometries have well developed correlations among mass transfer, fluid shear stress, fluid properties, and flow rate
  • ease of correlation of results to the field
  • coupons can be installed to mimic flow past a flat plate at various angles
  • apparatus can be incorporated in pilot plant or direct field applications to enable real-time acquisition of information

Some disadvantages include:

  • expense both in construction and 24 hour continuous operation
  • leakage especially at pumps and seals
  • possibility if spills and floods require specialized emergency instrumentation for periods when system is not supervised
  • possible non-uniformity of flow field in test section resulting in non-availability of correlation
  • space requirements for all necessary equipment and instrumentation

Rotating Disk Electrode

The rotating cylinder electrode is one of the oldest laboratory tools for studying fluid velocity effects in electrochemical systems. A simple schematic of the electrode is shown in this figure.

The alloy to be studied is usually machined into a cylindrical shape and then glued, often using epoxy, into a body which serves a shaft. An electrical connection is made to the specimen. The connection and lead are not in contact with the fluid. Both the electrical connection and the specimen are electrically isolated from the shaft. Only the surface of the alloy is exposed to the environment. The exposed round profile is in the shape of a disk. This shaft containing the disk rotates around the central axis. This shaft is connected to the apparatus that generates the rotation. The rate of rotation can be varied.

The current distribution across the surface is uniform when the electrochemical corrosion reactions are limited by the rate of mass transfer between the fluid environment and the surface. The limiting current or mass transfer rate can be calculated analytically. These characteristics have made the rotating disk popular to electrochemists. An excellent discussion of the fluid mechanics and analytical equations is provided in J. Newman, Electrochemical Systems, 2nd ed., Prentice Hall, Engelwood Cliffs, N. J., 1991. The combined effect of heat and mass transfer on corrosion has been successfully examined using a modified rotating disk electrode (http://www.argentumsolutions.com/tutorials/heat_tutorialpg7.html)

The rotating disk electrode operates under laminar flow conditions to high rotation rates. But, if the disk becomes very large, i.e. has a large diameter, the disk can be exposed to a combination of laminar flow toward the center and turbulent flow toward the periphery with a transition region between them. Such exposure can result in more localized corrosion because of the large differences in rates of mass transfer across the surface.

Some advantages include:

  • well-defined hydrodynamics especially when laminar flow is maintained
  • analytical expressions are readily available for the rate of mass transfer and limiting current density.
  • fairly easy fabrication
  • fairly easy set-up and operation
  • both electrochemical and mass loss evaluation methods can be used

Some disadvantages include:

  • data obtained when mass transfer is not limiting corrosion do not lend themselves to easily manipulated equations
  • a portion of the disk still operates under laminar flow at high rotation rates that support turbulent flow at the periphery. The result is uneven corrosion across the surface.
  • the entire surface operates under laminar flow conditions at lower rotation rate. Laminar flow is inappropriate if the modeled flow is turbulent.

Rotating Cylinder Electrode

The rotating cylinder electrode has been replacing the rotating disk electrode for evaluating the effect of fluid flow on corrosion when trying to simulate field conditions. A simplified schematic diagram of the electrode itself is shown in this figure.

Numerous electrode designs exist. The alloy to be studied is machined into a cylindrical shape. Often the electrode is fabricated so as to sit in between plastic spacers above and below the exposed surface. The electrode itself is hollow and sits on a liner that itself sits on an inner shaft. The top spacer can be set at the fluid interface to prevent hydrodynamic end effects. The bottom spacer provides electrical isolation. The electrode rotates around the axis of the central shaft. The inner shaft which is attached to the spinning apparatus is electrically isolated from the outer electrode. The rotation rate can be varied. Note that sometimes the spacers are eliminated and the entire outer cylindrical shaft becomes the electrode.

The current distribution on the electrode is uniform when the electrochemical corrosion reaction is limited by mass transfer to or from the surface. The apparatus operates under turbulent flow conditions to extremely low rotation rates. Well-defined mass transfer correlations exist that characterize the rate of mass transfer in terms of fluid properties. These characteristics have made the apparatus more popular because most velocity effects in the field occur under turbulent flow conditions. Similarity in hydrodynamics can sometimes enable the rotating cylinder electrode to model the field system of interest. Further information on the rotating cylinder electrode, the hydrodynamics of the system, mass transfer correlations, and modeling other geometries can be found at http://www.argentumsolutions.com/tutorials/cylexpert_tutorialpg3.html. Examination of the combined effect of heat and mass transfer on corrosion has been reported (http://www.argentumsolutions.com/tutorials/heat_tutorialpg7.html)

Some advantages include:

  • well-defined hydrodynamic flow with well defined mass transfer and shear stress correlations
  • the current is one-dimensional under conditions of complete mass transfer control
  • fairly easy fabrication
  • fairly easy set-up and operation
  • both electrochemical and mass loss evaluation methods can be used
  • turbulent flow conditions at low rotation rates can enable modeling of other geometries

Some disadvantages include:

  • localized corrosion caused by fluid velocity cannot be studied
  • leakage is possible behind the spacers
  • electrode surface preparation is best done prior to assembly
  • surface roughness as often occurs in corroding systems can prevent modeling of other geometries

Jet Impingement Rig

The jet impingement rig has proven to be another alternative for determining if corrosion is sensitive to fluid motion. A diagram showing the main features is shown below.

The apparatus requires a storage tank for the environment, a pump to accelerate flow, a tank to hold the jet, and the alloy specimen against which the jet flows at a 90o angle. This specimen tends to be disk shaped. The nozzle height can often be varied. The disk is often fitted with flush mounted electrodes at various locations along the surface. Additional equipment can include recirculation lines, filters, and controls to ensure safe operation when unattended.

The picture below is a simplified diagram of the jet region and the disk upon which the jet impinges. The flow is somewhat complicated.

The axial velocity within the stagnation region is independent of radial position. The boundary layer begins to develop further from this region into a region called the wall jet region. This boundary layer becomes thicker as the position moves farther from the stagnation region. The radial velocity is zero at the center of the disk and increases with distance. Most corrosion studies use a disk that has the stagnation region, the wall jet region, and the transition region between the other two. If the disk encompasses all three regions, the complex flow creates a complication. Differential oxygenation cells can be created between various regions of the alloy specimen. This effect can influence corrosion and make data interpretation more difficult. An excellent reference showing pictures of the flow pattern is in C. O. Popiel and O. Trass, "Visualization of a Free and Impinging Jet", Experimental Thermal and Fluid Science, Vol. 4, p. 253 (1991).

Some advantages include:

  • fluid flow being well-characterized in the stagnation region
  • mass transfer to the disk being uniform if the disk encompasses only the stagnation region
  • the wall jet region consisting of a well-defined boundary layer in which mass transfer and shear stress have been correlated to fluid velocity and position

Some of the disadvantages are:

  • differential aeration influencing results when all flow regimes are present so field corrosion is not duplicated.
  • leakage especially at pumps and seals and possibility of spills or floods requiring specialized emergency instrumentation for periods when system is not supervised
  • possible non-uniformity of the flow field in the test section making correlations between flow rate and mass transfer rate unavailable

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