Introduction
Traditional phosphating has excellent performance in metal anticorrosion, and is widely used in the process of coating pretreatment. However, phosphating treatment is facing a more and more serious situation because of its high temperature and complex treatment of wastewater and waste residue because of its harmful heavy metals such as zinc, nickel and manganese.
Silanization is a mature pretreatment technology which can replace phosphating. Compared with traditional phosphating, silane treatment has many outstanding advantages: no harmful heavy metal ions such as nickel, zinc and manganese, no phosphorus, no need for heating; no slag in silane treatment process, short treatment time, simple control; less treatment steps, eliminating surface adjustment and passivation process, long service life of tank solution, simple maintenance; effectively improve the adhesion of paint to substrate, and can collinear process iron plate , galvanized plate, aluminum plate and other base materials.
Mechanism of silane treatment on metal surface
Before the discovery of the excellent anticorrosive properties of silane, silane was widely used as adhesive in glass or ceramic reinforced polymer composites. A systematic and comprehensive study on the antirust properties of silane began in the early 1990s [1]. It is found that silane can be effectively used for corrosion protection of metals or alloys.
Silane is a kind of organic / inorganic hybrid with silicon group. Its basic formula is R '(CH2) NSI (or) 3. Where or is a hydrolyzable group and R 'is an organic functional group.
Silane usually exists in aqueous solution in the form of hydrolysis:
-Si(OR)3+H2OSi(OH)3+3ROH
After hydrolysis of silane, it is rapidly adsorbed on the metal surface through the shrinkage reaction between its SiOH group and the MeOH group on the metal surface (me represents metal).
SiOH+MeOH=SiOMe+H2O
On the one hand, silane forms si-o-me covalent bond on the metal interface. Generally speaking, the force between covalent bonds can reach 700 kJ / tool, and the combination between silane and metal is very strong; on the other hand, the remaining silane molecules form a silane film with a three-dimensional Si-O-Si network structure on the metal surface through the polycondensation reaction between SiOH groups [2] (see Figure 1).