The influence of the polyaniline coatings on corrosion protection of mild steel in different environments

Subject and scientific goal of this dissertation was to investigate the corrosion behavior of mild steel as the most widespread and the price very acceptable structural material based on polyaniline coatings in different corrosive environments: corrosion in sea water (3% NaCl), atmospheric corrosion and corrosion in soil (Sahara sand). In the first part was investigated the electrodeposition of polyaniline using galvanostatic technique on mild steel in of sodium benzoate solution in order to determine the corrosion behavior. It was also examined the potential role of polyaniline coating in cathodic protection of mild steel, especially in the case of the failure, and it was found that partial polyaniline coatings provide adequate corrosion protection. It was shown that partial polyaniline films can offer good corrosion protection of mild steel in Sahara sand and against atmospheric corrosion. Based on these studies of a model system, the „switching zone mechanism” has been proposed. For the first time it was discovered that corrosion processes of mild steel covered with polyaniline is affected with light, even the mild illumination was applied (4 mW cm-2). Since the electrochemical method of of synthesis on mild steel is suitable only for relatively small objects, in the second part of the dissertation has investigated the electrochemical and chemical synthesis polyaniline powder. In this sense, it is determined the yield of the synthesis, the obtained form of polyaniline and its morphology. By the chemical dedoping and doping, polyaniline powder was formed in the form of emeraldine salts or emeraldin base, since there is controversy in the literature which form gives better protection against corrosion of the mild steel. With such obtained and characterized powders the composite coatings with different content of PANI powder (1-10 wt%) was prepared, and applied on the mild steel samples, and the corrosion behavior was investigated using electrochemical impedance technique and the method of moist chamber for samples with artificially induced defects. It was found that chemically synthesized polyaniline in the form of emeraldin salts offers the best protection with the optimal concentration of polyaniline about 5 wt.%. The content of phenazine and azane determined by UV-vis spectroscopy in the investigated samples are connected with corrosion protection ability. The proposed mechanism of corrosion protection of mild steel in a real system was also confirmed.