Introduction

Lubrication is crucial in the wire drawing process. The lubricant is applied to the wire prior to drawing, through a die for diameter reduction. Phosphating plays an important 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 carrier coating onto which the lubricants (lime, soap, polymer-based) are applied prior to drawing, imparting a smooth, long-lasting film with corrosion protection.

Phosphate coatings are applied either in-line or by means of a batch process. In either case, the zinc phosphate layer is deposited upon contact with a phosphating bath consisting essentially of specially formulated phosphating solutions optimised for substrate, plant lay-out, and process sequence. Although this process is both reliable and easy to control, it does have one main disadvantage: the formation of iron phosphate sludge. This sludge adversely affects the economy of the process in two ways: reduced die life, and increased downtimes. Both of these retard efficiency and raise processing costs. In addition, the sludge has to be removed manually, which is very labor-intensive. Disposal costs are thus quite high.

Most attempts to overcome the formation of this sludge by changing the chemical formulation of the phosphate bath have failed. Now, however, a new phosphating process has been developed: electrolytic phosphating (e-Phos®). The technology has been developed in Japan and is covered under the patents PCT/US00/05458 and EP 0972862.

Electrolytic phosphating: e-Phos®

The new e-Phos technology works by combining the accelerating effect of an electrical current with phosphating chemicals. It is only applicable to in-line pretreatment of the wire. Compared with conventional phosphating, the e-Phos process offers several striking advantages.

Avoidance of sludge formation
Electric current is applied to the wire during the coating process. This prevents the wire from acid etching. No iron is dissolved into the bath solution and formation of iron-containing phosphate sludge cannot occur. Figure 1 shows the difference between a conventional phosphating bath and an e-Phos bath. While the conventional bath is cloudy and milky in appearance due to precipitated iron phosphates, after six weeks of use the e-Phos sample is perfectly clear and shows no evidence of sludge formation.

Figure 1: Comparison between e-Phos® and conventional phosphating bath

Improved surface morphology
Wire treated with e-Phos has a much smoother surface compared with that treated with a conventional phosphate. This is attributable to the absence of acid etching, a result of the electrical current applied to the wire (Figure 2). The REM image of the cross-section of a conventionally treated wire shows an uneven surface with relatively large crystals attached. In contrast, the e-Phos treated wire exhibits a smoother surface (no etching) with a denser coating of zinc phosphate crystals, resulting in an overall improved surface. The friction values are reduced, which leads to extended die life.

Figure 2: Cross-section through an e-Phos (left) and conventional (right) coating

Coating weight and application times
The coating weight of the e-Phos treated wire can easily be adjusted by varying the current density applied to the wire (Figure 3). The higher the current density, the higher the coating weight. There is an almost linear correlation between these two parameters, which makes it very easy to adjust the coating weight for a given treatment time (i.e. line speed).

Figure 3: Coating weight and process time

In the e-Phos process, it takes typically 3 to 5 seconds for the phosphate coating to be applied to the surface of the wire. In contrast, a traditional high-acid-point bath requires 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. Obviously, e-Phos promises an advantage here.

Pure zinc phosphate layers
The zinc phosphate crystals deposited during the e-Phos process clearly show a characteristic hopeitic structure. Figure 4 shows an XRD of an e-Phos treated wire. The large peak in the reflection pattern corresponds to the hopeite structure of zinc phosphate. Within this diffraction pattern there is no evidence of phosphophyllite-type crystals of composition Zn2Fe(PO4)2*4 H2O, which are undesirable for carrier coatings in the wire drawing processes.

Figure 4: XRD analysis of e-Phos zinc phosphate layers

Stainless steel
Stainless steel cannot be phosphated using a conventional process because of its passive nature. Instead, toxic or non-reactive carrier coatings must be used. Again, the e-Phos process offers an excellent alternative. An REM picture of a phosphated SS wire (AlSI 309; pickled with Cleanox® prior to e-Phos) is shown in Figure 5. Drawing results show no apparent difference between common, high carbon steel and stainless steel in terms of surface quality and residue.

Figure 5: e-Phos coating on AISI 309 after pickling with Cleanox

Summary

The main difference between the new electrolytic phosphating process e-Phos and conventional phosphating is that, in the former, electrical current supports the phosphate deposition. A comparison of process times is given in Table I. For in-line phosphating processes, e-Phos provides many advantages over traditional phosphate baths. These are:

• Process time reduction (shorter application times, typically 3 to 5 seconds);
• Reproducible coating weights (direct function of the applied current density);
• Extended die life (smaller crystals, smoother coating surfaces, lower friction values);
• Significant process cost reduction (no sludge formation, no downtime due to sludge removal, no sludge disposal costs);
• Zinc phosphate coatings for stainless steel possible.

Table I: Process time comparison

About Henkel Technologies

Henkel Technologies, of Germany, is a leading global supplier of products, systems, and services for bonding, sealing, and surface treatment of metal surfaces and other substrates. As a license partner of Nihon Parkerizing, the company offers its customers the new e-Phos technology including phosphating chemicals under the trade name Granodraw®.

Acknowledgement

The author wishes particularly to acknowledge Nihon Parkerizing Japan, Nihon Parkerizing Co., Ltd., Tokyo for the development of the e-Phos electrolytic zinc phosphating process.