Humidity potential mapping

Fig. 31.3 shows as an example a typical potential map as measured by the scanning Kelvin probe for an iron/adhesive interface activated with 0.5 m NaCl during cathodic undermining in a humid air atmosphere (at about 95% r.h.). The samples for this type of measurement are prepared with a defect area of about 20 mm which is free of the adhesive, adjacent to the adhesive film, and filled with an electrolyte solution the measurement with the SKP is performed above the adhesive film starting from the border of the defect defined as x=0 pm. 

Fig. 22 Typical potential maps above a delaminating organic coating on iron as measured with SKP as a function of time in humid air. The defect (filled with 0.5 M KCl) is located on the left. The mapping covers the area from the defect border to the intact area.

Figure 7-76. Volta potential map of iron modified by 1 monolayer of dimethylhexadecylsilanol. (a) immediately after scratching the surface (b) surface scratched after 20 h corrosion in a humid S02-containing atmosphere (Stratmann et al., 1995).

Sensors are distributed equally in various areas of the stability chamber no less than 2 inches from any wall. A set of sensors should be placed near or at the temperature and/or humidity controller of the chamber, as the controller will maintain the set-point temperature and/or humidity within the chamber during normal use. For a typical walk-in chamber, a minimum of 24 thermocouples and six resistance-transmitting devices are recommended for use in the mapping study. For a benchtop or reach-in chamber, a reduced number of sensors may be used. It is important to note that regardless of the size of the chamber, the placement pattern of the sensors should be such that any potential hot or cold spots are mapped, particularly those areas near the door and comers of the chamber. Typical sensor placement patterns for a reach-in and walk-in chamber are shown in Figures 16.1 and 16.2, respectively. In these examples, the extremities of the chamber (i.e., top and bottom) have a larger number of sensors than the middle of the chamber, since these areas would have a greater probability of either hot or cold spots, due to the airflow pattern within the stability chamber. 

When large areas are to be surveyed the huge number of potential readings can be examined statistically. Half cell potential data represented as cumulative probability (Fig. 8-19) are especially well suited for comparative purposes. The different bridge decks represented in Fig. 8-19 show similar curves but shifted on the potential axis. This shift is due to the influence of concrete temperature and humidity. The San Bernardino curve corresponds to the map in Fig. 8-17 and it is interesting to note that in the... 

Again, the Kelvin probe can be used to monitor the local corrosion potential of a partly coated metal surfece in a humid atmosphere. If, for example, the presence of the organic molecule changes the kinetics of the metal dissolution reaction, then this is reflected in a change of the corrosion potential an acceleration of the metal dissolution will shift the corrosion potential cathodically, a retardation will shift the corrosion potential anodically. As an example. Figure 22 shows a map of the corrosion potential of an iron surface that is partly coated by one monolayer of octadecylsilanol as measured with the Kelvin probe in a humid atmosphere [87]. The corrosion potential changes locally by several 100 mV due to the presence or absence of the film and it has been proved that the anodic potentials correspond to the areas that are coated by the polymer [53]. The potential plot of Figure 22 is therefore a representation of the inhibition of the anodic metal dissolution in a humid atmosphere. 

Figure 22 Map of the Volta potential of one monolayer of octadecylsilanol on iron after expostue to 100% relative humidity for 343h. Area 5 x 5mm [87].

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