Research Of ZnAl Coated Rebar And Its Adhesion Properties With Epoxy

Reinforced concrete structure is one of the most widely used engineering materials since thetwentieth century, and its failure problem has caused more and more attention. The main reasonfor the failure of the reinforced concrete structure is due to the corrosion of rebars embedded inthe concrete. The two main reasons caused rebar corrosion are atmospheric carbon dioxide andchloride ions come from external environment. Carbon dioxide in the air can cause concretecarbonization (carbon dioxide reacts with Ca(OH)2and generate to CaCO3), it will lower the pHof the pore solution in the concrete and the corrosion of the rebar will increase. Many studies onpretection of rebar are having been reported all over the world, hot-dip galvanized rebar or epoxyrebar is currently used in many places. A new type of rebar, ZnAl/epoxy duplex coated rebar wasdiscussed in this paper. The corrosion behavior of ZnAl coating in the simulated concrete pore atdifferent pH values and the adhesion problems of duplex coatings were also studied.The strucure of zinc-aluminum coating was characterized firstly, and then the corrosionbehavior of zinc-aluminum coating with different pH9-13.5in a saturated calcium hydroxidesimulated concrete pore solution was studied systematically. The microstructure morphology andcomponents of the corrosion products of coatings after immersion in the pore solution wereinvestigated by means of scanning electron microscopy (SEM), EDS and X-ray diffraction(XRD).The formation mechanism of corrosion products on ZnAl coating in saturated Ca(OH)2solution with different pH value was discussed and the electrochemical corrosion characteristicsof ZnAl coating were investigated by using Tafel polarization test and electrochemical impedanceanalysis. The results were shown as:Zn-Al coating on rebar is an eutectic structure, which is thinner than zinc coating and hasbetter ductile since no brittle Fe-Zn phase is existed in the coating, it also has better salt spraycorrosion-resistant than that of zinc coating under the same conditions. After immersion in thesimulated pore solution for3days, when pH is9and11, the coating surface is mainly generatedZn(OH)2film (after dehydration is ZnO) and little bluk CaHZn; when pH is12.5, although itgenerates CaHZn on the surface of the film, but its incomplete and has grow loose CaHAl; whenpH is13.5, the coating surface has obviously corrosion pits,it mainly contains Zn and O. Afterimmersion in the simulated pore solution for15days, when pH is9, the coating surface is stillmainly covered ZnO film and has better corrosion resistance; when pH is11, the coating surfacehas a small amount of bulk CaHZn except ZnO; when pH is12.5, the CaHZn film growthincompleted on the surface and was almost covered with loose CaHAl, but the loose bulk ofCaHAl dose not have good protection to the substrate; When pH is13.5, the coating surface only generates loose CaHAl that almost can not afford protection to the substrate.To solve the adhesion problem between ZnAl coating and epoxy coating, phosphatingpretreatment before spraying epoxy on ZnAl coating was studied. The results showed thatphosphate pretreatment improved the adhesion ability between the ZnAl coating and epoxycoating when adding F-ion in phosphate solution. F-ion can dissolve Al2O3layer on coatingsurface, avoiding anodic polarization and promoting the formation of phosphate film. The optimalprocess parameters for phosphating pretreatment are as follows: ZnO1.2g/L, NaNO315g/L,H3PO415ml/L, NaF0.4g/L, pH3.0, Time5min, Temperature45℃. ZnAl phosphate film hasgrowed to be wafer and needle-like, the pore structure of phosphate film increased the adhesionbetween ZnAl coating and epoxy coating.