Studies On The Boric-sulfuric Anodizing And Sealing Process Of 2024-T3 Aluminum Alloy

Boric-sulfuric acid anodizing?BSAA?can be used as an environmentally friendly alternative process of chromic acid anodizing.Its oxide film has excellent corrosion resistance without reducing the fatigue property of base material.So,BSAA shows a broad application prospect.Based on the related patent processes,this paper optimized the process specifications of boric-sulfuric acid anodizing,developed and further studied a new chromium-free sealing process for the anodic oxide film.Single factor experiment has been applied to anodize 2024-T3 aluminum alloy under different conditions in terms of sulfuric acid concentration,boric acid concentration and anodizing time.The results show that:the excellent corrosion resistance,dense surface morphology and uniform cross-section thickness of 2024-T3 aluminium alloy were obtained with the following parameters of sulfuric acid?70 g/L?,boric acid?6.7 g/L?,and anodizing time?25 min?.Electrochemical measurement technique of anodized film indicates that the sample?H₂SO₄:70 g/L?has a higher|Z|value in comparison to other samples and has a more dense porous layer of the anodic film with boric concentration of 6.7g/L.The anodic oxide coating was formed on 2024-T3 aluminum alloy with optimized boric-sufuric acid anodizing.Then it was sealed in traditional sealing solution and an alkaline Mn-Mo electrolyte,respectively.The results exhibit that the Mn-Mo sealed sample shows an excellent corrosion resistance,which is comparable to that of traditional potassium dichromate sealing,and better than that of boiling water sealing,nickel fluoride sealing and zirconium salt sealing.Further investigation reveals that the surface morphology of the Mn-Mo sealed sample shows flake-like surface cluster structure.EDS analysis results of the coating cross-section verified the relatively even distribution of the oxygen,aluminium species,but Mo element existed in the outer region of the sealed anodic coating.XPS analysis revealed the presence of hypervalent oxidation state soluble species of Mn and Mo such as MnO_x,MoOx,and formation of pseudoboehmite during sealing process.The electrochemical characterization of the sealed anodic coatings with different sealing recipes was evaluated by electrochemical analysis techniques.The potentiodynamic polarization curves showed that the corrosion potential of the Mn-Mo sealed sample was the most positive?-261mV?.No general corrosion pits were observed on the surface of Mn-Mo sample after salt spray test for 384 h.The formulated equivalent circuit is utilized to study the corrosion mechanism of the oxide film immersed in salt solution.The corrosion process has been summarized into the four stages:general corrosion,corrosive ions penetration into porous and barrier layer,self-healing in the local region,coating damage.