Phosphating Plant/PT Line

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Enhance Paint Adhesion and Corrosion Resistance with Effective Pretreatment Solutions

A new steel sheet and paint covering don’t usually adhere well to one another. This is the reason that in order to achieve good long-term performance, it is common to apply some kind of intermediate layer, or pretreatment. Better resistance to blistering and paint adhesion loss after exposure to corrosive environments are two benefits of pretreatment, in addition to improving the mechanical adhesion between the paint and the zinc coating. Pretreatments are nonetheless helpful to produce good resistance to loss of paint adhesion when the product is exposed to a corrosive environment, even if they are not as necessary to create effective initial paint bonding in sheets of steel.

Zinc phosphate is the paint preparation that is most frequently used to create a strong connection between paint and the steel sheet. Apart from its superior impact on paint adherence, phosphate coatings also help to significantly reduce the likelihood of paint disbonding when exposed to air again in a corrosive environment.

The end-user manufacturer can apply phosphate coatings to the cut sheet or fabricated product, or the steel producer or a coil coater (maker of prepainted sheet) can apply them to the steel sheet while it is still coil form.

The typical phosphate process entails many phases, regardless of whether the coating is done by the component producer or on a coil line. In the event that oil is found on the steel’s surface, degreasing is the initial step in removing the oil. This might entail degreasing with solvents in various ways or cleaning with an aqueous, alkaline cleaning solution. If the phosphating is carried out on a coil processing line, the following procedure should ideally incorporate the usage of a For improved corrosion prevention, chromates are usually added to the final rinse.

The steps in a 10-stage zinc phosphating operation might include:
• Hot Water Rinse
• Knock-off-Degrease
• Degrease
• Water Rinse – I
• Water Rinse – II
• Activation
• Phosphating
• Rinse – III
• Rinse – IV
• Drying

It is crucial to observe the prescribed temperatures, durations, and chemical concentrations in each of the aforementioned phases in order to achieve the growth of the desired fine phosphate crystalline surface.

The final paint adhesion is significantly enhanced by correct phosphating. The first method the product is enhanced is by the phosphate film’s porosity and little roughness, which enable mechanical keying between the paint and the phosphate. Chemical interaction, or hydrogen bonding, between the paint and phosphate coating is also made possible by the phosphate film’s significant oxygen content. Furthermore, and possibly most significantly, the phosphate coating lessens the rate of paint undercutting where the paint’s integrity is compromised, as around sheared edges, scratches in the paint, etc