| As a cost-effective surface anticorrosion technology,electroless Ni-P is widely used in many fields,such as electronics,aerospace and petrochemical industry.However,the inherent pinhole of the plating layer can easily lead to local corrosion of the large cathode-small anode in the corrosive medium to induce perforation,which has become one of the bottleneck problems that restrict the development of this industry.In order to solve this problem,the researchers tried corresponding sealing techniques around the organic films and inorganic conversion films.Chemical vapor deposition technology(CVD)has the advantages of high efficiency permeation,fast film formation and uniform coverage.It has been widely applied in the preparation of films.However,there are few studies on the use of CVD for the sealing of pores.This paper systematically studies the corrosion rule of electroless Ni-P coating on carbon steel surface,the sealing technology by CVD and the secondary sealing technology.Firstly,the corrosion resistance of the electroless Ni-P coating was researched in the circulating water medium.Compared with the carbon steel and 304 stainless steel,the corrosion resistance of the Ni-P coating was found to be close to that of the stainless steel.At the same time,the galvanic corrosion rule between carbon steel and Ni-P coating was studied.The galvanic corrosion rate accelerated with the increase of the area ratio of cathode to anode,indicating that the corrosion could be inhibited by the area ratio decrease of cathode to anode besides anode shielding.Subsequently,the PDMS hydrophobic film was fabricated to seal the pores of Ni-P coating via CVD;And an electrochemical stripping method was designed to analysis the through-hole area and calculate the sealing rate of the film.The results show that the intrapore and outside surface of specimens were evenly covered by the film and the pores area of the Ni-P coating was reduced more than 90%.The corrosion resistance of the specimen is improved by 1 to 2 orders of magnitude.The specimen is only slightly rusty spot after 15 days of immersion in circulating water solution.In order to further improve the corrosion resistance and heat shock resistance of the specimens,the effects of secondary sealing technique of polydimethylsiloxane(PDMS)soft film and(3-aminopropyl)triethoxysilane(APTES)modified graphene sealing agent were studied.The results show that the pores of the specimen are completely sealed after secondary sealing of PDMS soft film,and the corrosion resistance is improved by 2 to 4 orders of magnitude.There is still no trace of corrosion on the surface after the specimen is immersed for 30 days.And its corrosion resistance is still 2 to 4 orders of magnitude higher than other specimens,and there is no obvious macroscopical trace of corrosion on the surface after the specimen was subjected to six thermal shocks cycle at 80 ?.This paper discusses the sealing mechanism of CVD to Ni-P coating.On the one hand,the gas-phase components penetrate into the pores occured surface chemical reactions and deposit to form a film,which acts as a shielding layer for the inner metal substrate of the pores;On the other hand,the surface of Ni-P coating is evenly covered with a layer of deposited film,which greatly reduces the area ratio of the cathode and the anode,thereby reducing the driving force leading to galvanic corrosion.In addition,the PDMS elastic layer and the chemical vapor deposition film are well combined,reducing the porosity of coating,the service life of the specimen is significantly prolonged,and the heat shock performance of the film layer is improved.In this work,the corrosion behavior of the electroless Ni-P coating on the carbon steel surface and the new sealing technology are studied.A new method for sealing the pores of the electroless Ni-P coating is developed to provide a new solution method for long-term stable application of electroless Ni-P coating technology. |
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