3.3. Effects of process parameters on corrosion resistance
of phosphating layer
3.3.1. Effect of pH value of phosphating solution
The samples were immersed in phosphating solution with
various pH values ranging from 1.7 to 3.0 for 7 min at 20 C. After
natural drying for 2 h, qualified colorful phosphating layers were
formed. The relationship between the pH value and the corrosion
time of the phosphating layer is illustrated in Fig. 3. The corrosion
time reached maximum value of 120 s at pH of 2.5. When pH value
was lower than 1.9, the phosphating solution could corrode the
steel specimen at a fast rate. Phosphate solids were produced in the
phosphating layer when pH value was higher than 3.0, which
affected the quality of the phosphating layer. Therefore, the optimal
pH value of the phosphating solution was set at 2.5.
3.3.2. Effect of phosphating time
The effect of phosphating time on the corrosion resistance of the
phosphating layer was evaluated. The steel samples were
immersed into the phosphating solution at pH of 2.5 at 20 C or 3 C
and the phosphating time ranged from 3 to 25 min. After 2 h of
natural drying, qualified colorful phosphating layers were formed
on the steel samples. The relationship of phosphating time with the
corrosion time in the copper sulfate spot test for the phosphating
layers is shown in Fig. 4. From Fig. 4 it can be seen that the corrosion
time of the phosphating layer increased with an increase in phosphating
time. When the corrosion time was larger than 100 s, the
phosphating time needed was more than 6 min.