| Typically, it is difficult to achieve good adhesion between a paint coating and fresh steel sheet. For this reason, it is customary to apply some type of intermediate layer, a pretreatment, to develop good long-term performance. Not only does the pretreatment improve the mechanical adhesion between the paint and the zinc coating, but it also improves the resistance to blistering and loss of paint adhesion during exposure to corrosive environments. In the case of steel sheet, the need for a pretreatment to achieve good initial paint bonding is less critical, but a pretreatment is still beneficial to achieve good resistance to loss of paint adhesion when the product is exposed to a corrosive environment. |
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The most commonly applied paint pretreatment for developing good bonding qualities between paint and the steel sheet is zinc phosphate. In addition to the excellent effect that a phosphate coating has on paint adhesion, it also aids to decrease, often dramatically, the tendency for paint disbondment during subsequent atmospheric exposure in a corrosive environment.
Phosphate coatings can be applied to the steel sheet while it is in coil form by either the steel manufacturer or a coil coater (manufacturer of prepainted sheet), or it can be applied to the cut sheet or fabricated article by the end-use manufacturer.
The usual phosphate process involves several steps, both if the coating is applied on a coil line or by the part manufacturer. If there are oils present on the surface of the steel, the first step is the removal of the oil by degreasing. This might involve cleaning with the use of an aqueous, alkaline cleaning solution or by other forms of degreasing with the use of solvents. Preferably, if the phosphating is done on a coil processing line, the next step involves the use of a
Typically, the final rinse contains chromates for enhanced corrosion protection.
To accomplish the development of the preferred fine phosphate crystalline surface, it is important to pay attention to the specified temperatures, times, and chemical concentrations in each of the above stages.
When phosphating is done properly, the resultant paint adhesion is greatly improved. The first way the product is improved is that the somewhat rough and porous phosphate film allows for mechanical keying between the phosphate and the paint. The substantial quantity of oxygen in the phosphate film also allows for chemical bonding (hydrogen bonding) to occur between the paint and phosphate coating. But, perhaps more importantly, the phosphate coating helps to reduce the rate of paint undercutting at areas where the integrity of the paint is destroyed, such as along sheared edges, scratches in the paint, etc
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