Real-time coupled multielectrode array sensor probes were used to measure the localized corrosion rates for Type 1008 carbon steel, Type 110 copper, and Type 316L stainless steel in simulated seawater under creviced and exposed conditions. The maximum localized corrosion rate, for the carbon steel probes measured under a crevice, was 10 to 25 times lower than that measured under exposed conditions; the localized corrosion inside the crevice was found to be in the form of pitting corrosion. The maximum localized corrosion rate for the stainless steel probes was similar to that measured without a crevice. The similarity of the localized corrosion behavior, with and without a crevice for the stainless steel probes, was attributed to the relatively large gap of crevice (0.089 mm) that was not tight enough to have a significant effect on the corrosion. An exposed large electrode, made from the same wire as that of the carbon steel probe electrodes, was placed near the probes and in the same solution, in order to simulate the metal surrounding a creviced area. The corrosion potential of the creviced probe was found to be higher than that of the exposed large metal electrode. When the creviced probe was coupled to the large metal electrodes in the simulated seawater, its corrosion rate decreased essentially to zero, and the sensing electrodes of the creviced probe were cathodically protected by the exposed large electrode.
Crevice corrosion is one of most common types of localized corrosion that cause metallic equipment and engineering structure failures. The online and real-time measurements of crevice corrosion play an important role in corrosion control and mitigation. However, there are only a limited number of methods that can be used to quantitatively measure the propagation rate under crevices. Coupled multielectrode sensors (CMAS) have been recently used as in situ and online monitors for nonuniform corrosion, including localized corrosion?such as pitting corrosion?in cooling water pipes of chemical plants and other laboratory and field systems. Some of the CMAS applications include quantitative and real-time localized corrosion monitoring for cathodically protected systems, coated metal components, metals in concrete, metals in soil, and metals in low conductivity waters. In the present work, coupled multielectrode corrosion probes were used as an online tool for measuring the propagation rates in simulated seawater for Type 316L stainless steel (UNS S31603), Type 110 copper (UNS C11000), and Type 1008 carbon steel (UNS G10080). The measurements of localized corrosion rates under exposed conditions for these metals were also conducted, for comparison.
EXPERIMENTAL PROCEDURES
Coupled multielectrode sensors were used in the experiment. Figure 1 shows the principle of a coupled multielectrode corrosion instrument, which couples the multiple sensing electrodes of the probe to a common joint through small resistors. Under a non-uniform corrosion condition (e.g., localized conditions), some of the electrodes corrode in preference to others and, therefore, dispersion in the measured currents from the sensing electrodes can be observed. Thus, the multiple electrodes in the probe simulate a single piece of metal. If the sensing elements are sufficiently small, so that separation of anodic and cathodic reactions between the different electrodes can be assumed, the maximum localized corrosion rates can be obtained directly from the measured current densities from these electrodes:
CRmax = (1/e)Ia maxWe /(FpA) (1)
Where CRmax is the calculated maximum penetration rate (cm/s), e is the current distribution factor (fraction of the electrons produced on th
