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MATERIALS SELECTION & DESIGN FIGURE 5
ER, galvanic, and LPR sensors have the following challenges: 1) the corrosion rate of the sensor alloy may not correlate to that of other materials, 2) the sensor outputs are easily attenuated by contaminants and chemicals, 3) longevity issues because the sensor corrodes to produce a signal, and 4) corrosion product buildup on the sensor surface that attenuates signals. While several of the above sensor types
PI^M ÆW_V WV IQZKZIN\ IVL XZW^QLML ZMTI- tive indications of the corrosiveness level, there are limited data correlating sensor output to actual corrosion of aircraft structures, or related witness panels. Fur- ther evaluations of long-term sensor du- rability are also needed to better under- stand long-term cost issues.9
Coating Deterioration Sensors Sensor efforts for monitoring coating
deterioration focus on two basic methods. One approach is to simply coat ER, LPR, and galvanic sensors and detect when the coating fails by the onset of corrosion in the underlying metal. The other is elec- trochemical impedance spectroscopy (EIS).10
Such approaches face challenges
in detecting damage before it becomes [QOVQÅKIV\ IVL \PM KWUXTM`Q\a WN KWI\QVO systems and their various degradation mechanisms lead to a multitude of dam- age scenarios. Another approach for coating deterioration monitoring involves coated ER sensors with intentional un- coated areas that represent coating de- fects of various widths. When the coating inhibitor no longer leaches out and pro- \MK\[ \PM M`XW[ML UM\IT KWZZW[QWV [\IZ\[ IVL \PM UM\IT ÅTU ZM[Q[\IVKM QVKZMI[M[ (Figure 4[c]). This unique approach has been deployed on Australian aircraft.11 Many additional sensor types have been pursued but few have reached aircraft trials.12-15
A few companies are focused on
developing the required electronics and making them compatible for a suite of potential sensors.16
NACE International, Vol. 51, No. 3
Block diagram of typical sensor system components. Most sensor element fabrication ap-
XZWIKPM[ QV^WT^M ¹ÆM` KQZK]Q\ \MKPVWTWOaº where planar metal electrodes are depos- ited on durable polymer substrates (e.g., polyimide). Sensors need to be durable because they are usually located in water traps and areas known to be prone to corrosion because most corrosive com- pounds (e.g., chlorides) are not volatile.
Sensor System Diagram Figure 5 contains a block diagram of
the hardware components for a typical sensor system. Communication means can be wired or wireless and data can be downloaded to various information tech- nology (IT) platforms as required. A global database is desirable for managing highly mobile aircraft, and maintenance decisions can be as simple as skipping an inspection if a local sensor has not de- tected any corrosive conditions since the last inspection. More sophisticated prog- nostic models are also being developed that integrate information from various sensor inputs to optimize models over time and make more reliable predictions. Unfortunately, such efforts are beyond the scope of this brief overview. Mainte- nance personnel often prefer a simple red light vs. green light indicator to determine when maintenance should be performed in various locations. Much more informa- tion is available as needed to better un- derstand corrosion maintenance drivers IVL UIVIOM ÆMM\ UIQV\MVIVKM [KPML]TM[ Corrosion sensor data should eventually
help rewrite maintenance manuals, and guide maintenance actions like seal re- placements and the application of corro- sion prevention compounds.
Technology Presentations To help accelerate the implementa-
tion of corrosion sensors and reduce maintenance costs, the author co-chaired a workshop on corrosion sensors with Vinod Agarwala at the 2011 DoD Cor- rosion Conference in La Quinta, Califor- nia. The workshop was attended by 60 M`XMZ\[ QV^WT^ML _Q\P IQZKZIN\ UIQV\M- nance, corrosion science, and sensor ap- plications. The speakers reviewed the state-of-the-art in sensor technology and provided an overview of aircraft corro- sion issues. Focused discussions were then held on the challenges slowing sensor implementation. A summary is provided below.
Conclusions and Recommendations Sensor systems are getting very close
to enabling condition-based maintenance I\ I ÆMM\ TM^MT