ROHRBACK COSASCO SYSTEMS CORRATER 9020 Instrukcja Użytkownika Pobierz pdf (Strona 31)

3.2.10 A two-electrode probe with electrochemical impedance measurement

technique at high frequency short circuits the double-layer capacitance, C

, so

that a measurement of solution resistance R

can be made for application as a

correction. This also extends the operating range over which adequate

determination of polarization resistance can be made."

Imbalance (or Pitting/Index)

In addition to general or uniform corrosion, localized corrosion (pitting) may occur in a system.

This can result in much more rapid failure of a structure than a simple measure of corrosion rate

would indicate. A pit on the metal surface is the result of a localized, high anodic current

density. Positive ions flow away from the pit into the solution and electrons flow away from the

pit into the surrounding metal.

If it were possible to place a zero-impedance ammeter between the pit and the nearby metal

surface, the current in the anode-cathode system of the pit could be measured. Individual

measurements are not practical because the areas are small. Instead, current flow between the

two metallurgically identical electrodes of a CORRATER

probe under short-circuit conditions

can be used to indicate pitting tendency. All Rohrback Cosasco CORRATER

instruments

include a imbalance/pitting reading. The user should note that this is a qualitative measurement

(or index) and utilize it accordingly. It has proven very useful in many applications (e.g. cooling

water treatment) and offers information not generally available about a system except by

coupons which lag behind actual events and offer no way of detecting upsets.

If the pitting reading is low compared to the corrosion reading, the pitting problem will probably

be minimal. On the other hand, a pitting reading which is high compared to the corrosion

reading can indicate that pitting or crevice corrosion will be the main form of corrosive attack.

When the readings are about equal, some pitting is indicated, but the pits will probably be broad

and shallow.

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ROHRBACK COSASCO SYSTEMS CORRATER 9020 Instrukcja Użytkownika Strona 31