Electrolytic phosphating line

Lubrication is crucial to the wire drawing process. The lubricant is applied to the wire prior to drawing using a die for diameter reduction. Phosphating also plays an essential role in this multi-stage process. Tight bonding to the substrate in the application of the phosphate coating ensures optimal performance, even in multi-drawing operations. The crystalline phosphate layer acts as a carrier coating onto which the lubricants (lime, soap, polymer-based) are applied before drawing, imparting a smooth, long-lasting film with corrosion protection.

Phosphate coatings are either applied in-line or through the means of a batch process. In either case, the phosphate layer is deposited upon contact of the wire with a phosphating bath that is essentially made of specially formulated phosphating solutions optimised for substrate, plant layout, and process sequence. Although this process is both reliable and easily controllable, it does have one major drawback: the formation of iron phosphate sludge. This sludge badly affects the economy of the process in two ways: reduced die life, and increased downtimes, both of which lessen efficiency and inflate processing costs. In addition, the sludge must be removed manually, which is very labour-intensive. Thus, disposal costs prove very high.

The electrolytic phosphating technology works by combining the accelerating effect of an electrical current with phosphating chemicals. It is only applicable to in-line pre-treatment of the wire. Compared with conventional phosphating, the electrolytic process offers several remarkable advantages.

Avoidance of sludge formation

Electric current is applied to the wire during the coating process, thus preventing the wire from acid etching. Iron is never dissolved in the bath solution, which means that the formation of iron-containing phosphate sludge cannot take place.

Improved surface morphology

Wire treated with electrolytic phosphating has a much smoother surface compared with that treated with a conventional phosphate. This is due to the absence of acid etching, which results from the electrical current applied to the wire. The friction values are reduced, leading to an extended die life.

Coating weight and application times

The coating weight of the electrolytic phosphating treated wire can easily be adjusted by varying the current density applied to the wire. The higher the current density, the higher the coating weights. There is a virtually linear correlation between these two parameters, which makes the adjustment of the coating weight for a given treatment time (i.e. line speed) incredibly easy. In the electrolytic process, 2 to 5 seconds are usually necessary for the phosphate coating to be applied to the wire surface. In contrast, a traditional high-acid-point bath will require at least 10 seconds. And it can take up to several minutes to complete the coating, depending on the desired coating weight, the nature of the substrate, and other process parameters.

General view of an electrolytic phosphating line
General view of an electrolytic phosphating line

View of insoluble anodes used in electrolytic phosphating process
View of insoluble anodes used in electrolytic phosphating process