Reasons for inter-process rusting due to pre-treatment
The problem of rust return between processes in the pre-treatment of workpiece painting has become the most important concern of our customers. After rusting, it directly affects the film quality of pretreatment, and then affects the painting effect. Therefore, how to do a good job of rust prevention between processes is one of the key tasks of pre-treatment.
Typical process flow of phosphate-free pretreatment process (including pickling process).
Pre-degreasing - Main degreasing - Wash 1 - (pickling) - Wash 2 - (Wash 3) - Silane / vitrified treatment - Wash 4 - Wash 5 - Drain - Electrophoresis / Powder / Paint
The causes of inter-process rust return are mainly the following.
I. Overflow water opening after the main degreasing wash 1 is too large
Reason analysis: Before the workpiece enters the silane/vitrified tank, if the overflow water is too large, it may cause the pH of the tank liquid to be weakly acidic and cause the workpiece to return to rust.
Countermeasures: Adjust the flow rate of overflow water appropriately to control the pH of the tank solution; add rust inhibitor appropriately.
II. Pre-treatment process with pickling process, water washing2 is not sufficient
Reason analysis: Before the workpiece enters the silane/vitrified tank, if the water wash 2 is not sufficient, the acid remaining on the surface of the workpiece will cause the workpiece to return to rust.
Countermeasure: Add rust inhibitor appropriately.
Workshop temperature is too high
Cause Analysis: High workshop temperature in summer will cause the water temperature of the washing tank to rise and flash rust will appear on the surface of the workpiece when it exceeds 35℃.
Countermeasures: Keep the workshop ventilated and dry, and keep the temperature within 35℃; increase the flow rate of the overflow water of the washing tank to reduce the water temperature; add rust inhibitor appropriately.
IV. Too long washing time after main degreasing (3 water washing)
Reason analysis: The temperature is very high in summer and the humidity is high, so if 3 water washes are taken, the workpiece may return to rust if it is exposed to humid air for a long time.
Countermeasures: Add rust inhibitor appropriately.
V. Drain time is too long
Cause Analysis: Long leaching time causes the workpiece to be exposed to humid air for a long time, resulting in rusting of the workpiece.
Countermeasures: Increase the air blowing device in the leaching area or reduce the residence time of the workpiece in the leaching area; add a small amount of rust inhibitor in Washing 5.
Sixth, emergency stop line
Cause Analysis: When the line is stopped in an emergency, the workpiece is exposed to air for a long time, resulting in rusting.
Countermeasures: Resume production by manually polishing the surface rust of the workpiece and then reprocessing it.