Research Of Anti-corrosion Of Al Coatings By Low Pressure Cold Spray Technology

The steel structures of ships and ports have been severely eroded in marine environment, ithad been reported that spraying metal protective coating technology is an important means，whichcan provide the long-lasting protection for the steel structures in the marine environment. theanti-corrosion coating of aluminum, being an anode coating, are getting increasingly commonbecause that aluminum coating not only can separate the steel substrate from corrosion medium,but also has the effect of cathode proteetion for the steel substrate, and has a wide applicationprospect in anti-corrosion under marine environment. The low pressure cold spray technology is anew type of surface engineering technology, which its principe is the smae with high-pressure coldspray technology; a coating is deposited on the substrate through plastic deformation in acompletely solid state with high velocity impact. The low pressure cold spray technology canprepare most of protective coating of metals or alloys, and more researchers are concerningabout the technology, taking into account its portability, cost-effective.In this Paper, aluminum-alumina coatings were prepared using low pressure cold spraytechnology. The surface properties of the coating and its corrosion resistance in the marineenvironment were investigated, using moden methods for materials analysis and eleetroehemicaltest; the influence that the content of ceramic phase Al₂O₃on the properties of the coating wasdiscussed.In addation, Compared with the high-pressure cold spray and thermal spray aluminumcoatings, it can supply data reference for preparation of coatings being low-cost andhigh-performance anti-corrosion coating.With the process parameters in optimization, coatings of Al-10%Al₂O₃、Al-20%Al₂O₃、Al-30%Al₂O₃、Al-40%Al₂O₃and Al-50%Al₂O₃which contain five different volume content ofalumina, were made by the equipment of low pressure cold spray, called DYMET-314. Thesurface and cross-section characteristics of the coatings were observed and studied by ScanningElectron Microscopy （SEM）, X-ray diffraction （XRD）, Optical Microscopy, and pull test, theresults show that coating was deposited through intensive plastic deformation of the sprayparticles by the way of mechanical bite with matrix; the coatings are denser, and alumina as thesecend phasse is uniformly distributed in the coating; The higher the alumina content in theoriginal spraying powder, the alumina content of the coating is relatively high. the embedding ofalumina results in the coating surface of a certain interface defects and porosity being increased; the more alumina content in coating, the porosity is relatively large; However, the porossity ofAl-Al₂O₃coating is entirely low, being about1%～5%. Through the bonding strength test of thecoating, coating cohesion is greater than between the coating and the substrate; the more aluminacontent in coating, bonding strength is relatively high between coating and substrate; the bondingstrength of Al-Al₂O₃coating is up to31.2Mpa. When the content of Al₂O₃in the spraying powderbeing about10%～20%, porosity of coatings is low and bonding strength is similar.The coatings of low pressure cold spray and plasma spray were immersed in the seawater forthe test of the corrosion potential, linear polarization, potentiodynamic polarization andelectrochemical impedance,by means of the analysis of the coating corrosion products, combinedwith scanning electron microscopy, XRD, the results show that the corrosion products of lowpressure cold spray is more denser than that of thermal spray; With the analysis of Ecorr, it wasfound that all the coatings can provide effective and reliable protection for the steel substrate;thestable potential of Al-10%Al₂O₃coating is more negative, being about-0.88V; The Ecorr of4#and5#coating fluctuated in the range of-0.735V～-0.75V; The potential of thermal sprayAluminum coatings stabilized at about-0.88V, and can provide cathodic protection for thesubstrate. With the research of the liner polarization curves, it was found that the Rp of the lowpressure cold-sprayed aluminum coating increased rapidly with time in seawater；the Rp of lowpressre cold spray is entirely higher than that of thermal-spraying aluminum coating; Rp of thefirst coating is the biggest at a maximum of6×10⁴Ω·cm², having a better anti-corrosionperformance. With the analysis of potentiodynamic polarization curves, the results show that theIcorr of the low pressure cold-sprayed aluminum coating increased firstly and then decrease earlyin the immersion; Though, the stable corrosion current density of low pressure cold sprayaluminum coating is entirely higher than thermal sprayed aluminum coating, both can provideeffective protection for the steel substrate；As for the first coating，its current density is smallerthan that of other coatings, being about at0.3μA/cm².By electrochemical impedance test, theresults show that it presents a obvious diffusion process early and late in the immersion，owing toprotection of the surface passivation layer or the corrosion product film.Simulating the marine environment, all the coatings were tested in the splash zone in themarine environment for180days, and tested by neutral salt spray for1000hours; the experimentalphenomena shows that the steel substrate are all protected by low pressure cold spray and themalspray Al coatings in the experiment period; corrosion products are distributed uniformly andcompactly in the coating surface; The more content Al₂O₃in the coating,surface of the coatingpresents a largely crude salvation and a relatively fast corrosion rate; The Al-20%Al₂O₃coatingprepared by plasma spraying can also have a preferable anticorrosive property. Due to the defectsof the thermal-spraying coatings organizational structure, the protective properties of thelow-pressure cold spray aluminum coating is more long-term stability. By test of Al coatings in physical properties, electrochemical performance and anti-corrosionunder marine environment, learned that when content of ceramic Al₂O₃in the spraying powderbeing about10%～20%, Al coatings can protect steel substrate exhaustively; not only have anisolation of seawater electrolyte, but also can provide cathodic protection. For the low pressurecold spray Al Coatings, which its thickness is about300μm，the protective life can be up to20years.