Introduction
Traditional phosphating has excellent performance in metal corrosion protection and is widely used in the pretreatment process of coating. However, phosphating treatment is facing an increasingly severe situation because it contains harmful heavy metals such as zinc, nickel and manganese, and has a higher treatment temperature, and the treatment of wastewater and waste residue is more complex.
Silanization treatment is a relatively mature technology which can replace phosphating pretreatment. Compared with traditional phosphating, silane treatment has many outstanding advantages: no harmful heavy metal ions such as nickel, zinc and manganese, no phosphorus and no heating; no slag, short treatment time and simple control in silane treatment process; less treatment steps, which can save adjustment and passivation process, long service life of bath liquid and simple maintenance; effectively improve the adhesion of paint to the substrate, and can coplanar iron plate treatment. Galvanized sheet, aluminium sheet and other substrates.
Mechanism of Silane Treatment on Metal Surface
Silane has been widely used in glass or ceramic reinforced polymer composites as adhesives before it was found to have excellent corrosion resistance. Systematic and comprehensive study on the anti-rust 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 hybrids containing silicon group. Its basic molecular formula is R'(CH2)nSi(OR)3. Among them, 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
Silane is rapidly adsorbed on the metal surface through the shrinkage reaction between its SiOH group and the MeOH group (Me for metal) of the metal surface after hydrolysis.
SiOH+MeOH=SiOMe+H2O
On the one hand, silane forms Si-O-Me covalent bonds at the metal interface. Generally speaking, the interaction force between covalent bonds can reach 700 kJ/tool, and the bonding between silane and metal is very strong. On the other hand, the residual silane molecules form silane films with three-dimensional Si-O-Si network structure on the metal surface through the condensation reaction between SiOH groups [2]


 Silane film formation model on metal surface
During drying, the silane film is combined with electrophoretic paint or powder spraying in the back channel through cross-linking reaction to form a firm chemical bond. In this way, a stable film structure can be formed between the base material, silane and paint through chemical bonds.
Characteristics of Silane Treatment on Metal Surface
(1) Silane treatment does not contain zinc, nickel and other harmful heavy metals and other harmful components. Nickel has been proved to be harmful to human health. The World Health Organization (WHO) stipulates that nickel should reach zero emission after 2016, requiring that nickel should not be contained in phosphating wastewater, phosphating vapor and phosphating polishing dust.
(2) Silane treatment is slag-free. The cost of slag treatment is zero, which reduces the cost of equipment maintenance.
Phosphating residue is an inevitable companion of traditional phosphating reaction. For example, in an automobile production line using cold rolled sheets, every vehicle (calculated in 100m2) treated will produce about 600 g of phosphating slag with 50% moisture content, and 60 t of phosphating slag will be produced annually in a production line of 100,000 vehicles.
(3) No nitrite accelerant is needed 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) Silane treatment has no surface adjustment, passivation and other process, less production steps and shorter processing time can help to improve the plant's capacity, shorten the new production line, save equipment investment and occupy an area.
(6) It is feasible at room temperature and can save energy. Silane tank liquid does not need to be heated. Traditional phosphating generally requires 35-55 C.
(7) It does not conflict with the existing equipment technology and can replace phosphating directly without revamping the equipment. It is compatible with the existing painting process and can match all kinds of paint and powder coating currently used.