Galvanic corrosion direct measurement

The simplest procedure in studying galvanic corrosion is a measurement of the open-circuit potential difference between the metals in a couple in the environment under consideration. This will at least indicate the probable direction of any galvanic effect although no information is provided on the rate. A better procedure is to make similar open-circuit potential measurements between the individual metals and some appropriate reference electrode, which will yield the same information and will also permit obser-... 

The evaluation of field of current density is essential in problems of galvanic corrosion. In many cases the direct measurement of current density is not feasible, while the electric potential can be obtained from experimental measurements. This is particularly true in case of cathodic protection systems in general, where many surveying techniques (for example DCVG and CIS for underground structures) rely in potential measurements at different points at the electrolyte in order to identify the current distribution along the metallic structures. 

There are particular problems when the reinforcing steel is not of the conventional type of steel bars embedded directly into the concrete. There are problems for galvanized steel bars with respect to reference electrode and corrosion rate measurement because the zinc affects the readings in poorly understood ways. However, when the bar is coated in epoxy or the reinforcement is in the form of wires in ducts the problems are multiplied as described later. 

Much of the current that flows between reactions in a galvanic couple will flow between the anodic and cathodic metals, but some also flows within these metab. Internal current is not directly measurable, although it can be inferred from other test methods that use external current that can be measured. Measurable external currents can be predicted from the Evans diagrams as follows. When a metal is corroding by itself, as in Fig. 9, the corrosion current is i,, which flows entirely between anodic and cathodic sites in the metal and cannot be measured. All... 

The remote crevice assembly technique (see Chapter 19) is a research tool that allows one to separate the anode and cathode areas of a crevice corrosion test sample so that the current flowing between them can be measured with a zero-resistance ammeter. This technique is similar to the dual cell method, and it lends itself well to studies of microbial effects on crevice corrosion [7]. It allows direct measurement of microbial effects on both the initiation time and propagation rate for crevice attack, provided again that a suitable control experiment without the microbial influence can be done concurrently. The scime technique of separating the anode and cathode can be used to study the influence of microbes in biofilms on galvanic corrosion [li]. 

For electrical safety reasons, the telecommunication cable plant has to be grounded and bonded. This means that some bare metallic components of the plant have to be directly exposed to corrosive environments. Some of these alloys are not particularly corrosion resistant, and at some locations they require corrosion-control measures. An example of such a condition is where the galvanized-steel support hardware is part of the ground in a flooded manhole. 

A zero-resistance ammeter is connected between two metals, and the galvanic current is directly measured as a function of time. At the same time, a reference electrode can also be used to monitor the galvanic couple potential, which can be used to determine the galvanic corrosion if a third metal is to be connected with this couple. Most commercial potentiostats can be used as a zero-resistance ammeter by changing the electrode connections. 

The main concept that most of the corrosion data interpretation is based on was first introduced by Wagner and Traud (1938), according to which galvanic corrosion is an electrochemical process with anodic and cathodic reactions taking place as statistically distributed events at the corroding surface. The corresponding partial anodic and cathodic currents are balanced so that the overall current density is zero. This concept has proven to be very useful, since it allowed all aspects of corrosion to be included into the framework of electrochemical kinetics. Directly deduced from this were the methods of corrosion rate measurement by Tafel line extrapolation, or the determination of the polarization resistance Rp from the slope of the polarization curve at the open circuit corrosion potential... 

There are a number of ways that galvanic corrosion maybe prevented. These can be used singly or in combination. All of these preventive measures follow directly from the basic mechanism of galvanic... 

The measured currents may not represent actual galvanic corrosion rates, as this form of corrosion is highly dependent on the anode cathode area ratio. An increase in current readings is not always directly associated with an actual increase in corrosion rates. 

The effect of the phosphate layer as a cathodic inhibitor under atmospheric conditions is illustrated in Fig. 12. In this experiment, the Volta potential of a galvanized steel surface is measured as a function of time during a transition from air to Ar atmosphere, as indicated [51]. The measurement is performed with a Kelvin probe, and the Volta potential of the corroding surface is directly proportional to the corrosion potential with appropriate calibration [58]. The potential jump induced by the presence of air is a measure of the sensitivity of the surface to the oxygen reduction reaction. Here, we see that the galvanized steel surface shows a very large potential jump, on the order of 200 mV. However, the phosphated surface shows only... 

Another factor that alters the galvanic position of some metals is the tendency, especially in oxidizing environments, to form specific surface films. These films shift the measured potential in the noble direction. In this state, the metal is said to be passive (see Chapter 6). Hence, chromium, although normally near zinc in the EMF Series, behaves galvanically more like silver in many air-saturated aqueous solutions because of a passive film that forms over its surface. The metal acts like an oxygen electrode instead of like chromium hence, when coupled with iron, chromium becomes the cathode and current flow accelerates the corrosion of iron. In the active state (e.g., in hydrochloric acid), the reverse polarity occurs that is, chromium becomes anodic to iron. Many metals, especially the transition metals of the periodic table, commonly exhibit passivity in aerated aqueous solutions. 

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