Zinc Coating of Suspension Parts

Zinc Coating of Suspended Parts Power is supplied to suspended parts during galvanizing through contact points between the suspension and the parts. Attaching parts to suspensions is usually done manually, rarely mechanized or automated, and therefore is highly labour-intensive and uneconomical. For this reason, efforts are made to limit suspension galvanizing to parts that cannot be galvanized in a barrel due to their shape or size. The most important part of suspension galvanizing is the suspension itself, to which the parts are attached. The type of connection affects current distribution across the surface to be galvanized. All important surfaces, for example all visible areas, should be selected so that they are within the optimum current density range. Otherwise, high current density areas may experience burning, while low current density areas may have insufficient coating thickness, no coating, or loss of brightness.

1. Suspensions

1.1 Effect on Metal Distribution

It is known that the homogeneity of metal distribution and consequently many other properties of the zinc layer depend on primary and secondary current distribution. Primary distribution is a function of the shape of the parts and their position within the electrolyte, namely their position and distance from anodes and tank walls. Secondary current distribution depends on the properties of the electrolyte. Figures 1 to 3 demonstrate the effect of primary current distribution. Figure 1 shows the effect of part shape on metal distribution at corners and at angular and rounded exterior and interior edges. While distribution can be improved by rounding edges, i.e. by modifying the part design, and at least in disadvantageous spots the extreme case of zinc accumulation no longer occurring is avoided, the result is not satisfactory. If cathode and anode in an electrolyte are parallel to each other, current lines progress as shown in Figure 2. The practical effect of this can be seen on the left side of Figure 3 (a), while on the right side (b) the position of the object's main points has been modified in favour of a more favourable current line distribution due to a suitably shaped anode. [caption id="attachment_158772" align="aligncenter"] Figure 1: Metal accumulation at angular and rounded exterior and interior edges[/caption] [caption id="attachment_158773" align="aligncenter"] Figure 2: Current line distribution between parallel cathode and anode surfaces[/caption] [caption id="attachment_158774" align="aligncenter"] Figure 3: Galvanized hollow part: a: Parallel, b: Shaped anode[/caption] While the homogeneity of accumulation can be improved by appropriately arranging main surfaces, it is usually not possible to change to an electrolyte with better throwing power. Therefore, despite all reservations, there will always be situations where, for example, the use of a well-throwing alkaline electrolyte instead of a weakly acidic electrolyte with poorer throwing properties becomes necessary. Optimum arrangement of parts in the electroplating tank is primarily the responsibility of suspension technology. When designing the suspension, it should not be forgotten that it must also perform a number of other tasks in addition to this. The suspension not only determines the position and arrangement of parts in the tank (and is therefore also important for its use), but must also provide good and stable contact and support the technological process as a whole. For this reason, there are several basic rules that must always be followed: • Main surfaces and points of parts must always face the anode, • Protruding surfaces and points must not face the anode directly; they must either shield each other or be secured away from the anode, • Actual contact should be made at "insignificant" points of non-galvanized areas, • Parts on the suspension or suspensions in the bath must be sufficiently far apart from each other so as not to shield each other. Optimum distance should be determined experimentally. For cylindrical parts, the basic rule is that it should be the same as the diameter in alkaline electrolytes and 1.5 to 2 times the diameter in acidic electrolytes, • Gaps must be avoided in which evolved hydrogen or air can accumulate and thus prevent or hinder zinc accumulation; bucket points should be avoided due to electrolyte carry-over, • Good rinsing of parts must be ensured, i.e. rinsing water must reach all points and be easily discharged. Additional measures on the frame to improve current line distribution during galvanizing are generally not required. As a rule, neither auxiliary anodes nor special current distributors are necessary.

1.2. Components of Suspensions

A suitable frame for these tasks generally consists of: • Frame hooks or frame heads (usually swallow-tail shaped) used to secure the frame to the busbar, • the actual frame or strip on which the contacts are attached, • and the contacts. When placing contacts on the frame, care must be taken to ensure that the surfaces to be galvanized do not shield each other. If possible, the part should always cover the contact, so that as little metal as possible accumulates on it. It is also important that parts do not fall from the suspension when they are moved or when the electrolyte is stirred. For this reason, hook contacts are more suitable for heavy parts, as contact is guaranteed by the part's own weight. Spring contacts are designed for smaller and especially lighter parts. Examples of different frame shapes are shown in Figure 4. Apart from the contacts, all parts of the frame must be insulated. As a rule, modified PVC sealants that are resistant to both zinc electrolytes and pre- and post-treatment solutions are used for this purpose today. If insulation is lacking, the amount of current absorbed by the suspension may, in unfavourable conditions, be more than required for the actual galvanizing process. In any case, power losses occur.

1.3. Maintenance of Suspensions

The fundamental importance of good contact in galvanizing makes periodic maintenance of suspensions necessary. They are subject to wear, contacts can break or loosen, insulation can be damaged and require repair, etc. [caption id="attachment_158775" align="aligncenter"] Figure 4: Galvanizing suspensions[/caption] Therefore, maintenance of suspensions is an important part of quality assurance. For this reason, it is one of the primary duties of the master at the galvanizing facility. Facilities with high work volumes usually have their own workshops dealing with suspension maintenance. However, suspension maintenance is often also outsourced to companies specializing in suspension manufacturing.

1.4. Busbars

In semi-automatic and fully automatic systems, several suspensions are usually mounted on a product carrier, referred to as a busbar, and are thus transported from bath to bath by robots. The size of product carriers and the number of suspensions on a product carrier affect efficiency and quality. When selecting the size and number of suspensions above the baths, the following important criteria should be considered: • The number of parts to be galvanized per unit time on a cathode bar of a given length, • The number of parts on a suspension and the shape of the parts, • The required properties of the zinc coating, particularly its thickness at critical points.   Sources Techn. Galvanotech. 3rd edition, Eugen G. Leuze Verlag Metal Finishing, Hull, Hinter Handbook of Electroplating, Canning Metal Finishing, Geduld Metal Finishing, Roberts   İzzet Aydın General Manager Hillebrand Chemicals Chemical Marketing Ltd. Co.