1173Electrochemical Corrosion Tests

Electrochemical techniques are used for a variety of purposes, not least to provide the engineer with information of the corrosion rate of a metal/alloy in a given environment. Only a brief summary is possible here, and the reader is advised to refer to references [18 and 37], which provide further descriptions of the different types of corrosion tests currently available.

Assessment of corrosion rate may be conducted using standard polarization methods as described in Section 11.3.4. Alternatively, a technique based upon applying a small sinusoidal alternating current or potential perturbation to the corroding system may be employed. This is commonly known as electrochemical impedance spectroscopy (EIS) and has been applied to systems where a metal is coated or where the metal is passive, for example, stainless steel or aluminum [38, 39]. Impedance measurements have gained in popularity and are used to determine the polarization resistance value, hence corrosion rate, particularly in poorly conducting media and on coated samples, such as paints and lacquers. As with the polarization resistance methods alternating current (ac) impedance is used to evaluate the "general" uniform corrosion rate and provides no information on localized corrosion rates.

Galvanic corrosion may also be determined by superimposing individual anodic and cathodic polarization curves for the respective anodic and cathodic corrosion reactions that occur on each of the two metals making up the galvanic couple [40, 41]. An example of this is shown in Fig. 11.17.

Figure 11.18 presents an alternative experimental set up for determining the magnitude of galvanic corrosion (current) using zero resistance ammetry.

In addition polarization curves may also be used to determine the pitting resistance and potential regimes within which SCC may occur. Figure 11.19 presents the polarization curves for three different stainless steels: AISI 304L, 316L, and 2025 duplex stainless steels, in an artificial seawater environment [42]. Highlighted in Fig. 11.19 are the pitting potentials and regions where SCC may be encountered (}). As can be seen from this figure the pitting resistance is in the order duplex > 316L > 304L.

Current density (A/cm2)

FIGURE 11.17 Composite polarization plot: pure aluminum (anodic branch) and steel (cathodic branch) (after [41]).

Current density (A/cm2)

FIGURE 11.17 Composite polarization plot: pure aluminum (anodic branch) and steel (cathodic branch) (after [41]).

Potentiostat

Potentiostat

FIGURE 11.18 Application of potentiostat as a zero resistance ammeter for measurement of galvanic current (corrosion rate) from two different metals.
FIGURE 11.19 Polarization curves highlighting different pitting potentials for duplex, 316L, and 304L stainless steel grades.

As previously pointed out, the above techniques are primarily used to assess the general corrosion behavior of a material. Recent developments in scanning methods and rapid data acquisition have given rise to electrochemical scanning probe techniques. An example is the scanning reference electrode technique (SRET) [43-45]. These techniques are based upon measurement of corrosion activity via a twin platinum wire probe, which moves in discrete steps, from 0.5 ^m, across the surface of a corroding sample. The result is a two-dimensional map of the localized corrosion activity on the surface. When scans are repeated over selected intervals in time, the result is a time-lapsed progression of corrosion activity. Figure 11.20a shows a typical SRET map showing galvanic corrosion. In this case the galvanic couple is that of an explosively bonded (kelocouple) joint containing steel/pure A1 and A1 alloy. It can be seen from Figure 11.20b that by changing the conductivity of the solution, corrosion activity (dark areas) moves from the A1 alloy to the pure A1 [46]. Such spatial information concerning corrosion activity cannot be derived from those conventional techniques previously described.

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