Research On The Corrosion Resistance Of Alloy Construction Steels, Stainless Steels And Their Surface Ceramic Coatings

At present, the material of the large mechanical hydraulic rod used in the marineenvironment is mainly40Cr steel, which is alloy construction steel and one of the mostwidely used steels in machinery manufacturing. The mechanical properties of the40Crsteel can meet the mechanical indicators of the hydraulic rod well after quenching andtempering treatment. But it shows the poor resistance to seawater corrosion in the actualuse. The corrosion can cause a decline in mechanical strength of the rod, which isharmful to the safety of the mechanical equipment. The corrosion products may alsodamage the sealing ring. So, there is an urgent need for studying the corrosion andprotection 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 the40Cr steel that thehydraulic rod generally used, so that improved the service life of the hydraulic rod. Onthe other hand, the protective coating was looked for to coat in the rod of40Cr, thecoating should not only meet the mechanical properties, but also improve the corrosionresistance of the material. Moreover, the protective coating was also intended to coat inthe alternative material of40Cr, in order to obtain better service functionality of thehydraulic rod.Besides40Cr,0Cr17Ni4Cu4Nb,1Cr18Ni9and18Cr2Ni4WA were also selected asthe bare metals and substrate materials of the tests. The0Cr17Ni4Cu4Nb and18Cr2Ni4WA can be used as the candidate materials for new hydraulic rods, and the1Cr18Ni9as the contrast material of the corrosion resistance. The four fine sprayingschemes of ceramic coating were designed to protect the above four substrate materials,which included DPAT-13ceramic coating system, DPAT-40ceramic coating system,ceramet (WCCoCr) coating system and GBAT-20ceramic coating system. The tests werecarried out to in-depth study of the corrosion resistance of the selected materials and thesamples of spraying ceramic coatings mainly from the following four aspects, whichincluded laboratory simulation accelerating corrosion research, marine environmentcorrosion research, electrochemical performance research, galvanic corrosion research.According to the results and analysis of the tests, the corrosion failure mechanisms ofspraying 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 as0Cr17Ni4Cu4Nb>1Cr18Ni9>18Cr2Ni4WA>40Cr. The evolution of the phases of the40Cr corrosion products asfollows: γ-FeOOHâ†'Fe3O4+α-FeOOHâ†'Fe3O4â†'Fe3O4+Fe2O3+CrO. Theevolution of the phases of the18Cr2Ni4WA corrosion products as follows: α-FeOOH+Fe3O4â†'Fe3O4. In Sanya tidal environment, the rust layer of40Cr was divided intoinner rust layer and outer rust layer, the phase of the inner rust layer was γ-FeOOH, thephases of the outer rust layer were composed of Fe2O3, Fe3O4, α-FeOOH and a verysmall amount of γ-FeOOH. The bare metals were more inclined to localized corrosion inthe marine environment in Sanya than in Qingdao. The number of the passivation filmgenerating on the surface of the materials were ordered from more to less as0Cr17Ni4Cu4Nb>1Cr18Ni9>18Cr2Ni4WA>>40Cr. The smoothness of passive filmwere ranked as follows:0Cr17Ni4Cu4Nb>18Cr2Ni4WA>1Cr18Ni9. There weredifference for the capacitance characteristics of the electric double layers compared40Crwith the other materials. The dispersion effect of40Cr was very weak, but the dispersioneffect of the other materials was great. The accelerated test involving cyclic exposure tosalt mist,"dry" and "wet" condition was called the salt spray-dryness-wetness circulationtest (SSDWC test) for short. The better the seawater corrosion resistance of steel, thegreater the acceleration of SSDWC test. For the metals which have good corrosionresistance and aren’t easy to generate rust layer in the tidal environment, the corrosionrates of the metals in the artificial sea water immersion test were greater than in the tidalenvironment. But for the metals which are easy to generate rust layer in the tidalenvironment, the corrosion rates of the metals in the artificial sea water immersion testwere much smaller than in the tidal environment.(2) The compactness of four kinds of ceramic coating system were ranked asfollows: GBAT-20≈Ceramet (WCCoCr)> DPAT-40> DPAT-13. The results of thelaboratory simulation accelerating corrosion tests showed that: the corrosion resistanceof the samples of spraying GBAT-20ceramic coating system was the best, the samplesalmost had no hole corrosion and passed the assessment of the SSDWC test of5000h.The corrosion resistance of the samples of spraying DPAT-13ceramic coating systemwas the worst, the samples appeared hole corrosion quickly during the tests. For theceramet coating system, the samples of stainless steel substrate had poor corrosionresistance, but the samples of alloy construction steel substrate had good corrosionresistance. The ceramic layer of the ceramet coating system was prone to corrosion inaqueous 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 andcorrosion after severe corrosion. The results of the marine environment corrosion testsalso showed that: the corrosion resistance of the samples of spraying GBAT-20ceramiccoating system was the best, the samples of spraying ceramet coating system had poorcorrosion resistance in the marine environment. The round bar samples of sprayingceramic coating had better corrosion resistance than the flat samples of spraying ceramiccoating. The corrosion resistance of the ceramic coating systems themselves wereordered from larger to little as GBAT-20> Ceramet (WCCoCr)> DPAT-40> DPAT-13.The GBAT-20coating system could greatly reduce the galvanic corrosion between40Cror0Cr17Ni4Cu4Nb and Aluminum Bronze.(3) For the samples of spraying DPAT-13and DPAT-40ceramic coating systems, thecorrosion failure mechanisms were similar which had four kinds. The samples ofspraying GBAT-20ceramic coating system mainly possessed two kinds of corrosionfailure mechanisms. For the samples of spraying ceramet (WCCoCr) coating systems,the ceramic layer would be corroded firstly, then the corrosion failure mechanismsmainly had three kinds. The types of the corrosion failure mechanisms obtained throughresearch of the thesis are to have universality, which also are applied to other ceramiccoating systems.(4) In the candidate materials of new hydraulic rod, the0Cr17Ni4Cu4Nb has bestcorrosion resistance, which is suitable as the substrate material of new hydraulic rod.The GBAT-20ceramic coating system can significantly improve the corrosion resistanceof the metal substrates and greatly reduce the galvanic corrosion, so which is suitable asthe protective coating of the hydraulic rod used in the marine environment.