| Organic coatings are widely used to control the corrosion of steel structure in marine environment due to form a layer of organic membranes which isolates the metal from the corrosive environments. However, organic coatings often peel off from the substrate during service which weakens their protective capability. To improve the adhesion strength between the coating and metal substrate, people usually make pretreatment on metal surface before coating. Appropriate surface pretreatment technologies can effectively remove pollutants and osteoporosis from metal surface, increase the surface roughness, improve wettability and surface free energy, and as a result, adhesion between the coating and metal is improved.In this paper, surface of the hull steel was modified by atmospheric pressure air plasma jet, and the effects on wettability of the hull steel, adhesion strength and aging effect for adhesion strength of coatings after seawater immersion, corrosion resistance ability of coatings were researched. The main results and progresses of this work are outlined as following:(1) Influence of atmospheric pressure plasma jet modification on wettability of hull steel. The results of contact angle test showed that atmospheric pressure air plasma jet treatment can improve the wettability and surface free energy of hull steel sharply, and the calculation of the surface free energy indicated that the increase of polar components (ysp) leaded to the increase of surface free energy. The mechanism of improving the hull steel’s wettability after the surface modifications was characterized with the methods of SEM, AFM, EDAX, FTIR, XPS analysis technique, and the results indicated that the improvement of surface wettability was mainly due to introducing a large of polar functional groups to material surface and the surface cleaning effect of air plasma. Besides, the influence of the modification conditions on wettability of surface, such as the discharge current, processing time and treatment distance were experimentally studied. The optomized modification condition was as follows, discharge current3.5A, processing time2.7s(nozzle moving speed400mm/min), treatment distance10mm. Finally, after long-term storage in ambient air, it was observed that the contact angle of the processed hull steel partially recovers, and the longer processing time, the weaker aging effect.(2) The effects of atmospheric pressure plasma jet treatment on improving adhesion strength and adhesion residue of coatings in seawater immersion were experimented. The results indicated that appropriate increase of substrate temperature and surface roughness contributed to improve the adhesion strength between the coating and substrate, the curve of the adhesion strength versus different immersion time in seawater indicates that, during the80days, adhesion strength of the coating which was treated by air plasma had a slower decline when compared with the untreated。(3) The impact on corrosion resistance ability of coatings was tested by neutral salt spray test and corrosion electrochemical experiments (EIS and potentiodynamic polarization). According to the neutral salt spray test, comparing with the untreated, coatings treated by air plasma showed relatively good corrosion resistance capability, especially the Aging time-10min samples, only one of them had mild corrosion phenomenon after immersing a cycle (1000hours), but the untreated had appeared red rust only after soaking240hours. According to the EIS characteristic, three differently perfect coatings (related to defected coatings)nearly had no difference. However, for three differently defected coatings, there were distinct differences. The impedance values for treated coatings were significantly higher than the untreated, especially the Aging time-10min of which impedance values were10times higher than the untreated. Moreover, potentiodynamic polarization results suggested that the Aging time-10min sample had a slower anodic current density and its polarization resistance (31.46×105Ω·cm2) was significantly higher than the unprocessed (5.57×105Ω·cm2). |
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