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MATERIALS SELECTION & DESIGN Corrosion Sensor Systems for Aircraft Applications FIGURE 3
Different TOW sensor designs can pro- vide significant variations in reported TOW values, primarily due to differences in surface wettability and electrode grid dimensions, so care must be taken in sen- sor selection.5
Figure 3 shows tempera- TOW and temperature sensor output over a 12-day period showing daily cycles. FIGURE 4
ture and TOW measurements for an outdoor coastal test site, where the daily temperature and dew cycle are apparent. The area under the TOW/conductivity curve is integrated to determine cumula- tive corrosion damage. Measurements are typically made every 10 to 30 min, stored, and the cumulative amount of damage is calculated.
Metal Corrosion Sensor
Examples of: (a) LPR sensor, (b) TOW/conductivity sensor, and (c) coating inhibitor ER sensor.
sprayed into aircraft compartments to help prevent corrosion, which attenuate sensor outputs, as well as the stability and durability challenges created by the ther- mal, mechanical, chemical, and electrical stresses a sensor experiences aboard air- craft. Sensors should last several years with minimal attention, ideally until de- pot-level overhaul and repair visits.
Sensor Approaches The two main approaches to measur-
ing corrosiveness of aircraft environ- ments are to measure the corrosion rate of a proxy material or the environmental factors that drive corrosion. Key envi- ronmental factors are: 1) time of wetness (TOW), 2) concentration of corrosive species (e.g., chlorides [Cl–
] and sulfur dioxide [SO2 ]), and 3) temperature.
Models similar to the Ibero-American Map of Atmospheric Corrosiveness (MICAT)3
equation for aluminum can
then be used to predict corrosion rates. See Equation (1):
(1) 58 MATERIALS PERFORMANCE March 2012
where CAl = annual corrosion (g/m2 = chloride deposition (mgCl–
/m2 = SO2 deposition (mg SO2/m2 ), Cl *d), SO2 *d), TOW
= time of wetness (annual fraction), and T = annual average temperature (°C).
Environmental Sensors One advantage of environmental sen-
sors is that they measure the physics- based parameters that drive corrosion, which can be useful for building models and predicting corrosion for a variety of materials. They also can be very durable. Environmental sensors, however, have issues that include: 1) sensing all the cor- rosive species, 2) sensor stability/calibra- tion over time, and 3) developing correla- tions for a variety of corrosion damage modes. Currently, there are no chloride, SO2
, or pH sensors proven to be stable
and durable for long periods of time in aircraft environments. Some more recent environmental sensor approaches there- fore use TOW sensors to also measure electrolyte conductivity, which is propor- tional to ionic concentration; the ionic concentration is then used as a proxy for the concentration of aggressive species.4
where ba and bc Due to the challenges associated with
environmental sensors, most sensor ap- proaches are focused on direct measure- ments of the corrosion rate of a proxy material. Specific sensor designs em- ployed for this approach include:
1) electrical resistivity (ER) measurements WN \PQV UM\IT ÅTU[ \PI\ QVKZMI[M QV ZM[Q[- tance as they corrode,6
2) galvanic sensors
that measure the corrosion current be- tween dissimilar metal couples as the active metal corrodes,7
and 3) linear po-
larization resistance (LPR) sensors, fabri- cated out of the alloy of interest.8
The
LPR method involves polarizing the working proxy electrode from ~ –20 mV to +20 mV relative to the open circuit potential. The slope of the resulting curve near the corrosion potential is the polar- ization resistance Rp and is related to the corrosion rate by Equation (2):
(2) are the Tafel slopes of the
anodic and cathodic reactions. LPR and galvanic sensors provide in- stantaneous measurements of corrosion rate, which are recorded and analyzed to evaluate the overall corrosion risk. ER sensors, on the other hand, provide a cu- mulative measure of corrosion damage.
NACE International, Vol. 51, No. 3