by Dr E Ramanathan
Generally, thickness of different types of phosphate coating is reported as coating weight. Weight per unit surface area of phosphate coatings is calculated as follows: The coated specimen is weighed to the nearest mg (w1); the coating is removed by immersing the coated specimen in a 50 g/L chromic acid stripping solution at 165 deg C for 15 minutes; then the specimen is cleaned in running water, dried and reweighed (w2); this procedure is repeated until a constant weight is obtained.
Coating weight in g/m^2 is given by
The iridescent blue amorphous iron phosphate layer has the coating weight between 30 and 100 mg/ft^2. Spray type phosphate coating forms coating weight of 100 to 400 mg/ft^2, medium hot immersion zinc phosphate coating forms 450 to 1000 mg/ft^2 and heavy zinc phosphate coating forms 1000 to 3000 mg/ft^2. Manganese phosphate coating weight ranges from 1000 to 3000 mg/ft^2. Heavy zinc phosphate coating and manganese phosphate coating exhibit lot of pores on their thick layer deposited on the metal.
Though they are not suitable for painting due to their more porous and rough surface, the heavy phosphate treated metal surface is ideal for oiling, waxing and lubrication and hence such type of heavy phosphating is employed on the metal components where painting is not required. The cathodic and anodic phosphate coating on mild steel components have been reported to improve the adhesion of paints applied on the metal.
Thickness of phosphate coating ranges from 1 to 30 μm. However, it is commonly referred to as coating weight in g/m^2. By stripping the phosphate coating, the coating weight is found to vary from 6 to 12 g/m2with different immersion times.
By knowing the density (d) of the coating material (3.99 g/cm^3) and coating weight (w), coating thickness (t) was calculated in microns. Their values obtained for different panels were reported in the table using the following equation.
Lorin ratio is given as the range between 1.5 and 3.5 and it is the ratio between coating weight in g/m^2 and coating thickness in microns. The values obtained from the density were comparable with the coating thickness obtained from the Lorin ratio and are shown in the table.
