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).




Figure 1 film formation model of silane on metal surface
In the drying process, the silane film combines with the electrophoretic paint or spray powder of the back layer through cross-linking reaction to form a strong chemical bond. In this way, a stable film structure can be formed between substrate, silane and paint through chemical bond.
Characteristics of silane treatment on metal surface
(1) the silane treatment does not contain harmful heavy metals such as zinc, nickel and other harmful components. Nickel has been proved to be harmful to human body. According to the World Health Organization (who), after 2016, nickel shall reach zero emission, and Phosphating Wastewater, phosphating steam and phosphating grinding dust shall not contain nickel.
(2) silane treatment is slag free. The cost of slag treatment is zero, reducing the cost of equipment maintenance.
Phosphating slag is an inevitable companion of traditional phosphating reaction.  For example, an automobile production line using cold-rolled plates will produce about 600 g of phosphating slag with a moisture content of 50% for each treatment of 1 vehicle (calculated by 100m2), and 60 t of phosphating slag will be produced each year for a production line of 100000 vehicles.
(3) no need of nitrite accelerator, so as to avoid the harm of nitrite and its decomposition products to human body.
(4) the product consumption is low, only 5% - 10% of phosphating.
(5) there is no surface adjustment, passivation and other technological processes in silane treatment. Less production steps and shorter treatment time are helpful to improve the capacity of the plant, shorten the new production line, save equipment investment and floor space.
(6) it is feasible at normal temperature and energy saving. Silane bath solution does not need heating, and traditional phosphating generally needs 35-55 ℃.
(7) it does not conflict with the existing equipment process, and can directly replace phosphating without equipment transformation; it is compatible with the original coating process, and can match with all kinds of paint and powder coating currently used.