Effect Of Different Acid Doped PANI-epoxy Coatings On Corrosion Resistance Of Several Metals

The emeraldine base polyaniline (PANI) and hydrochloric acid (HCl), camphorsulfonic acid (CSA) and hydrofluoric acid (HF) doped PANI were synthesised by chemicaloxidation method and characterized by FTIR, XRD and four-probe measurements. Theresults of FTIR indicated that the synthesized powders were PANI in emeraldine state oracid doped state. The acid doped location was mainly on nitrogen atoms in the quinoid ringsrather than in the benzenoid ring of PANI. The results of XRD indicated that the crystaldegree of PANI powders was low. The crystal degree and the conductivity of PANI wereincreased by the acid doped anoins. The hydrofluoric acid doped PANI has the highestconductivity among four kinds of PANI powders.The epoxy resin coatings containing different PANI powders were prepared on thesurface of steel, pure aluminum, aluminum alloy and magnesium alloy. Corrosionprotection of PANI coatings for several metals was studied by EIS in3.5%NaCl solution.The LEIS was used to study the corrosion protection of defected PANI coatings. The metalsurface beneath coatings was observed by the camera and SEM after immersion. The resultswere as follows:(1)The impedance values of varnish coating on the surface of carbon steel were lessthan10~6Ω·cm~2after100day immersion, which indicated that the coating lost itsanticorrosion properties. After175day immersion, the impedance values of emeraldinebase PANI and acid doped PANI coatings was10~7Ω·cm~2and10~8Ω·cm~2, respectively. Theresults indicated that PANI coating showed high anticorrosion properties. The impedancevalues of CSA doped PANI coating were higher than that of other coatings. After400dayimmersion, lots of pit corrosion and loose corrosion product were formed on the surface ofcarbon steel beneath epoxy varnish. The carbon steel surface beneath acid doped PANIcoating was predominated by filiform corrosion, but a compact and continuous productlayer was detected on the corrosion spots.(2) The impedance values of varnish coating on the surface of pure aluminium andaluminium alloy were10~7Ω·cm~2after100day immersion and30day immersion,respectively. But the impedance values of emeraldine base PANI were above10~8Ω·cm~2.The impedance values of acid doped PANI coating maintained10~8Ω·cm~2~109·cm~2after 215day immersion. The results indicated that acid doped PANI coating showed highcorrosion protection for pure aluminium and aluminium alloy. The impedance values of HFdoped PANI coating were higher than that of other coatings. The impedance values ofdefected HF acid doped PANI coating on the surface of aluminium alloy were higherthan that of defected emeraldine base PANI coating and defected epoxy varnish coatingduring all immersion time.(3) The impedance values of varnish coating on the surface of magnesium alloy were10~6Ω·cm~2after42day immersion, which indicated that coating lost its anticorrosionproperties. After400day immersion, the impedance values of emeraldine base PANIcoating and acid doped PANI coating were10~7Ω·cm~2and10~8Ω·cm~2, respectively. Theresults indicated that PANI coating showed high anticorrosion properties. The impedancevalues of HF doped PANI coating were higher than that of other coatings. Compactprotective layer was formed on the surface of magnesium alloy beneath PANI coating. Theimpedance values of defected HF acid doped PANI coating on the surface ofmagnesium alloy were higher than that of defected epoxy varnish coating during allimmersion time.The polyaniline/organophilic montmorillonite (PANI/OMMT) compound powderswhich were synthesized via in-situ emulsion polymerization increased the tortuosity of thediffusion pathways and improved the barrier properties to the electrolyte solution. Therefore,the corrosion protective properties of PANI coating was improved.The corrosion protection mechanisms of PANI coating were discussed by the analysisof DSC and SEM of coating, EIS measurements of coatings in deaerated or aerated3.5%NaCl solution, and the SEM, EDS and XPS analyses of the surface of metal beneathcoatings after immersion experiments. The result indicated:(1) The barrier properties of coating were improved because of the increaseing ofcrosslink density by the addition of PANI powders.(2) The compact protective layer was formed by the reduction-oxidation reaction ofPANI in the case of oxygen presence. The corrosion protection of PANI decreased rapidly inthe solution without O2. Therefore, the main protection mechanism of PANI coating wasreduction-oxidation reaction of PANI. (3) The formative speed of protective layer was improved by increased conductance ofacid doped PANI. The doped anions released concomitant redox reaction of PANI andformed protective layer with metal ions. Therefore, the protective effect of coating wasimproved.