Pitted Corrosion Detection of Thermal Sprayed Metallic Coatings Using Fiber Bragg Grating Sensors

Abstract

Metallic coatings using thermal spraying techniques are widely applied to structural steels to protect infrastructure against corrosion and improve durability of the associated structures for longer service life. The thermal sprayed metallic coatings consisting of various metals, although have higher corrosion resistance, will still corrode in a long run and may also subject to corrosion induced damages such as cracks. Corrosion and the induced damages on the metallic coatings will reduce the effectiveness of the coatings for protection of the structures. Timely repair on these damaged metallic coatings will significantly improve the reliability of protected structures again deterioration. In this paper, an inline detection system for corrosion and crack detection was developed using fiber Bragg (FBG) grating sensors. Experimental results from laboratory accelerated corrosion tests showed that the developed sensing system can quantitatively detect corrosion rate of the coating, corrosion propagations, and cracks initialized in the metallic coating in real time. The developed system can be used for real-time corrosion detection of coated metal structures in field.

Corrosion Behavior Evaluation of Coated Steel Using Fiber Bragg Grating Sensors

“…In Figure 14, it was observed that there were multiple spikes on curve of Sample #A1 at range from 95 h to 200 h (4 days to 8 days), indicating the coating structure had been severely corroded. From Figure 14 and the previous study [48], it can be seen that the corrosion progress of the wire-arc Al-Zn coating can be divided into three different phases: Phase 0 (P0 in Figure 14) is the corrosion initialization phase when corrosion products start to fill the pores between adhesive and FBG sensor; Phase 1 (P1 in Figure 14) is the high corrosion rate phase, at which the slope of wavelength change curve in Phase 1 reflects the production rate of corrosion products; and Phase 2 (P2 in Figure 14) is the stabilized corrosion progress, when oxygen further diffuses through the corrosion products and strain caused by corrosion products slowly evanesce, and the remaining sensor reading on the wavelength change curve is primarily remnant stress. Thus, all the experiments stopped after the coating corrosion stayed in Phase 2 for 168 h (7 days).…”

“…This will result in a shift in Bragg wavelength. The amount of Bragg wavelength change with strains or temperatures can be calculated as below [48]:…”

“…From Equation 1, it is known that the reduction at cathodes will change the iron into oxidized iron with size more than six times larger the original iron particles [46][47][48]. Thus, detecting the material volume or expansion change using the embedded FBG sensors in the coatings of steel components can potentially reveal the corrosion mechanism.…”

“…However, when corrosion occurs in the steel substrate, as shown in Figure 1, the corrosion products will push the coating up, inducing a strain on the FBG sensor, ε i , that can be monitored by the Bragg wavelength change of the FBG sensors, λ i , where i is the corrosion time step. If the corrosion production is accumulated within a relatively small area, as compared to the total span of the packaged FBG sensor and the expansion of corrosion productions that mainly occur in a vertical direction, the corrosion rate of the coated steel can be obtained through monitoring of the Bragg wavelength changes of the embedded FBG sensors as below [48]:…”

“…A few recent attempts to apply FBG sensors in corrosion monitoring of steel rebar in reinforced concrete had shown the possibility of FBG sensors in corrosion monitoring [36,[44][45][46][47]. Recently, the authors group investigated using FBG sensors to detect corrosion in High-Velocity Oxygen Fuel (HVOF) thermally sprayed metallic coatings [48]. However, limited studies have been performed on using FBG sensors to monitor corrosion for steel components coated with polymeric and other metallic coatings.…”