ABSTRACT
At present, the material of the large mechanical hydraulic rod used in the marine environment is mainly 40Cr steel, which is alloy construction steel and one of the most widely used steels in machinery manufacturing. The mechanical properties of the 40Cr steel can meet the mechanical indicators of the hydraulic rod well after quenching and tempering treatment. But it shows the poor resistance to seawater corrosion in the actual use. The corrosion can cause a decline in mechanical strength of the rod, which is harmful to the safety of the mechanical equipment. The corrosion products may also damage the sealing ring. So, there is an urgent need for studying the corrosion and protection of the hydraulic rod used in the marine environment.
The thesis began to study mainly from the following two aspects. On the one hand,the better corrosion resistance of steel was looked for to replace the 40Cr steel that the hydraulic rod generally used, so that improved the service life of the hydraulic rod. On the other hand, the protective coating was looked for to coat in the rod of 40Cr, the coating should not only meet the mechanical properties, but also improve the corrosion resistance of the material. Moreover, the protective coating was also intended to coat in the alternative material of 40Cr, in order to obtain better service functionality of the hydraulic rod.
Besides 40Cr, 0Cr17Ni4Cu4Nb, 1Cr18Ni9 and 18Cr2Ni4WA were also selected as the bare metals and substrate materials of the tests. The 0Cr17Ni4Cu4Nb and 18Cr2Ni4WA can be used as the candidate materials for new hydraulic rods, and the 1Cr18Ni9 as the contrast material of the corrosion resistance. The four fine spraying schemes of ceramic coating were designed to protect the above four substrate materials,which included DPAT-13 ceramic coating system, DPAT-40 ceramic coating system,ceramet (WCCoCr) coating system and GBAT-20 ceramic coating system. The tests were carried out to in-depth study of the corrosion resistance of the selected materials and the samples of spraying ceramic coatings mainly from the following four aspects, which included laboratory simulation accelerating corrosion research, marine environment corrosion research, electrochemical performance research, galvanic corrosion research.
According to the results and analysis of the tests, the corrosion failure mechanisms of spraying ceramic coatings on metal were proposed firstly.The main results of the study were as follows. (1) the corrosion resistance of the materials were ordered from larger to little as 0Cr17Ni4Cu4Nb > 1Cr18Ni9 >18Cr2Ni4WA > 40Cr. The evolution of the phases of the 40Cr corrosion products as follows: γ-FeOOH → Fe3O4+ α-FeOOH → Fe3O4→ Fe3O4+ Fe2O3+ CrO. The evolution of the phases of the 18Cr2Ni4WA corrosion products as follows: α-FeOOH +Fe3O4→ Fe3O4. In Sanya tidal environment, the rust layer of 40Cr was divided into inner rust layer and outer rust layer, the phase of the inner rust layer was γ-FeOOH, the phases of the outer rust layer were composed of Fe2O3, Fe3O4, α-FeOOH and a very small amount of γ-FeOOH. The bare metals were more inclined to localized corrosion in the marine environment in Sanya than in Qingdao. The number of the passivation film generating on the surface of the materials were ordered from more to less as
0Cr17Ni4Cu4Nb > 1Cr18Ni9 > 18Cr2Ni4WA >> 40Cr. The smoothness of passive film were ranked as follows: 0Cr17Ni4Cu4Nb > 18Cr2Ni4WA > 1Cr18Ni9. There were difference for the capacitance characteristics of the electric double layers compared 40Cr with the other materials. The dispersion effect of 40Cr was very weak, but the dispersion effect of the other materials was great. The accelerated test involving cyclic exposure to salt mist, "dry" and "wet" condition was called the salt spray-dryness-wetness circulation test (SSDWC test) for short. The better the seawater corrosion resistance of steel, the greater the acceleration of SSDWC test. For the metals which have good corrosion resistance and aren't easy to generate rust layer in the tidal environment, the corrosion rates of the metals in the artificial sea water immersion test were greater than in the tidal environment. But for the metals which are easy to generate rust layer in the tidal environment, the corrosion rates of the metals in the artificial sea water immersion test were much smaller than in the tidal environment.
(2) The compactness of four kinds of ceramic coating system were ranked as follows: GBAT-20 ≈ Ceramet (WCCoCr) > DPAT-40 > DPAT-13. The results of the laboratory simulation accelerating corrosion tests showed that: the corrosion resistance of the samples of spraying GBAT-20 ceramic coating system was the best, the samples almost had no hole corrosion and passed the assessment of the SSDWC test of 5000 h.The corrosion resistance of the samples of spraying DPAT-13 ceramic coating system was the worst, the samples appeared hole corrosion quickly during the tests. For the ceramet coating system, the samples of stainless steel substrate had poor corrosion resistance, but the samples of alloy construction steel substrate had good corrosion resistance. The ceramic layer of the ceramet coating system was prone to corrosion in aqueous solution of chlorine ions. For the ceramic coating sprayed on round bar, the ceramic layer would appear axial crack under the tension caused by the curvature and corrosion after severe corrosion. The results of the marine environment corrosion tests also showed that: the corrosion resistance of the samples of spraying GBAT-20 ceramic coating system was the best, the samples of spraying ceramet coating system had poor corrosion resistance in the marine environment. The round bar samples of spraying ceramic coating had better corrosion resistance than the flat samples of spraying ceramic coating. The corrosion resistance of the ceramic coating systems themselves were ordered from larger to little as GBAT-20 > Ceramet (WCCoCr) > DPAT-40 > DPAT-13.
The GBAT-20 coating system could greatly reduce the galvanic corrosion between 40Cr or 0Cr17Ni4Cu4Nb and Aluminum Bronze.
(3) For the samples of spraying DPAT-13 and DPAT-40 ceramic coating systems, the corrosion failure mechanisms were similar which had four kinds. The samples of spraying GBAT-20 ceramic coating system mainly possessed two kinds of corrosion failure mechanisms. For the samples of spraying ceramet (WCCoCr) coating systems,the ceramic layer would be corroded firstly, then the corrosion failure mechanisms mainly had three kinds. The types of the corrosion failure mechanisms obtained through research of the thesis are to have universality, which also are applied to other ceramic coating systems.
(4) In the candidate materials of new hydraulic rod, the 0Cr17Ni4Cu4Nb has best corrosion resistance, which is suitable as the substrate material of new hydraulic rod.The GBAT-20 ceramic coating system can significantly improve the corrosion resistance of the metal substrates and greatly reduce the galvanic corrosion, so which is suitable as the protective coating of the hydraulic rod used in the marine environment.
KEY WORDS: metal, ceramic coating, marine environment, corrosion resistance,failure mechanism
本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://en.sunspraying.com/
|
