Corrosion Protection of Metals by Conductive Polymers III. Improved Performance and Inhibition in NaCl
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and various coating procedures (curing temperature, coating thickness and mild steel surface treatments) were applied. These different polyaniline formulations were assigned AC I to AC7, respectively. The average conductive polymer film coating thickness were 25 gim (AC4, AC5 and AC6) and 50 gim (AC 1, AC2 and AC3). ASTM standard B 117 was used for standard test method of salt spray testing. Polyaniline blend coatings on mild steel coupons overcoated with a high viscosity two-component epoxy resin (e) top-coat were investigated in this study. The coating systems were applied to 2"x2" SAE1010 mild steel panels (s) with appropriate surface pretreatments. The coated coupons were drilled with a 1.18 mm diameter mill (D) to expose a fixed area of bare steel surface. 3.5 % NaCI testing electrolyte was used in immersion and salt fog tests. Sample designations indicate that the sample had a drilled exposed steel surface (D), an epoxy top coat (e), the nature of the conductive polymer coating (ACx), and a mild steel substrate (s). Thus, the designation, D,e/AC4/s, indicates that this sample is a mild steel coupon coated with the polyaniline formulation AC4, overcoated with an epoxy top coat, and contains a drilled exposed bare steel area through the coatings of 1.18 mm diameter. RESULTS AND DISCUSSION Visual Inspections Close-up observations by bare eye and an optical microscope provided valuable corrosion information about passive properties observed in the exposed bare steel areas and protection efficiencies provided by the conductive polymer formulations. Table 1 outlines the corrosion progress of exposed areas in NaCl for various sample configurations. Astonishing, every tested AC formulation avoided the formation of corrosive products at exposure sites for times up to about 3 weeks. After this time the various formulations exhibted different levels of corrosion prevention performance. Some even accelerated corrosion with long exposure times. Table 1 Visual observations on bare steel holes in NaCl immersion tests sample
initial stage
intermediate stage
final stage
D,e/ACI/s
shiny until 7 days inmersion became 2 separated flat and smooth red-brown colored films at centre ofdent (outer: darker& inner:lighter)
red-brown passivated film covered whole drilled hole in two distingished colors and slightly extended out dent edge at week 3
disk center ofdent became black but outer ring area still showed red-brown to yelow brown and filmthickness increased
D,e/ACI*/s
shiny for the first 3-week
thin light brown film appeared at week 4
voluminous yellow brown rust from week 5
D,e/AC4/s
shiny for first 2-week
thin grayish brown filmformed at week 3
thin light brown filmcovered entire dent since week 4
D,e/AC5/s
shiny through first week
deep brown filmat outer edge ofdent and light grayish brown filmat center ofdent
grow-up inner light brown rust & outer deep brown passivated film around the dent edge
D,e/AC6/s
shiny for first 2-week
thin light brown filmat week 3 covered whole dent
two deep brown/yellow outer
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