Components of Continuous Steel Strip Plating Lines

Figures 6, 7, 8, and 9 show schematic diagrams of continuous steel electroplating lines for tin, chromium, and zinc coatings. Figure 9 may be consulted in the following discussion of the general features of these lines. The line in Fig. 9 is also designed for painting over the metallic coated strip, which for economic reasons may become the trend, particularly for zinc and zinc alloy coating lines whose lower speeds can allow roll coating of the organic films.

Fig. 6 Schematic diagram of a typical electrotinning line, using vertical plating cells. Source: Ref 1
Fig. 7 Schematic diagram of a typical two-step chromium plating line, also called a tin-free steel line. Source: Ref 1
Fig. 8 Schematic diagram of a typical electrogalvanizing line with vertical plating cells. Source: Ref 1
Fig. 9 Schematic diagram of an 1830 mm (72 in.) combination electrogalvanizing and coil coating line. Source: Ref 18

A typical continuous plating line has five main sections: payoff, pretreatment, plating, post-treatment, and delivery. The functions and key pieces of equipment of each of these sections are described below.

The payoff section feeds the strip into the line. It may consist of payoff reels, a shear, a strip welder, an edge-notcher, a burr masher, a degreasing section, and an accumulator tower. Coils of cold-rolled, fully annealed steel are loaded onto the entry reels and are fed into the continuous line. The head end of a new coil is welded to the tail end of the previous coil. The edges of the weld are notched or cut out to eliminate loose flaps when two different widths are welded together, and any steel edge burr is mashed. The strip passes through a precleaning and rinse station where the bulk of the rolling and protective oils are removed, then moves into the entry strip accumulator or looper. The looper, which can be either vertical or horizontal, accumulates extra strip ahead of the plating section and provides it to that section when the entry end is stopped to load a new coil.

The pretreatment section is where residual oil, surface carbon, and any light surface oxide are removed prior to plating. It normally consists of one or more alkaline cleaning and electrocleaning stations, brushing or scrubbing stations, pickling or electropickling stations, and rinsing stations. Many modern, high-speed lines have a tension leveler within the pretreatment section to flatten the strip. This guarantees the uniform anode-to-strip spacing needed to produce a highly uniform coating. The leveler location is such that strip coming into the unit is fairly clean, but additional cleaning follows the leveling process.

The plating section, the heart of the process, always consists of multiple plating cells located in a row (Fig. 10). At the beginning of the plating section, there is frequently a conditioning or preplating cell. A conditioning cell may simply contain the process electrolyte, to wet the strip surface prior to the application of current, but a preplating cell may also contain a slightly different or completely different electrolyte to deposit a thin initial layer. This layer can be used either to enhance the adherence of the main coating (e.g., in the case of alloys of zinc with nickel or iron) or to control a postplating process (e.g., a thin nickel layer may be deposited to limit tin-iron alloy growth during reflowing or subsequent processing of tinplate).

Fig. 10 Twenty vertical plating cells in a continuous plating line that applies zinc and zinc-nickel alloy coatings on sheet steel, primarily for the automotive market

The full thickness of the main coating is built up gradually as the strip moves from cell to cell. Associated with the cells are electrolyte distribution tanks, pumps, filters and heat exchangers, and electrolyte chemical replenishment reactors and systems. Upon exiting the last cell, the coated sheet is immediately rinsed and dried to prevent streaking or staining of the coated surface. Following the dryer, there is normally a coating thickness gage (x-ray fluorescence) that continuously monitors and records the edge-to-edge distribution of the coating on both sides of the strip.

The post-treatment section is present on most, although not all, strip plating lines. There are three basic types of post-treatments:

• Surface-stabilizing post-treatments: Examples are the etching solutions used to clean and stabilize the uncoated side of one-side electrogalvanized sheet (Ref 19, 20, 21)

• Property-enhancing post-treatments chemically convert the surface to accept further coatings, such as paint. In addition, post-treatments can impart certain surface properties, such as lower friction or change in ohmic surface resistance (which are important properties for forming and welding) or changes in surface brightness or appearance. Examples are the chromium-containing chemical conversion coatings that slow down surface oxide growth on tin (Ref 5, 6), zinc coatings that serve as pretreatments for the adhesion of paints for zinc and Zn-Ni (Ref 15), and enamels for tinplate (Ref 6).

• Proprietary rinses, phosphates, and dry film lubricants may be used to aid in the final manufacturing of parts (Ref 22).

Melting and reflowing of the tin coatings can also be characterized as a post-treatment. It is almost always followed by a chromate conversion or electrolytic post-treatment.

The delivery section starts with a delivery accumulator, which allows the processing sections to continue running while the delivery reels are stopped to remove coils. Several additional stations and pieces of equipment are included in the delivery section, such as for edge trimming, inspecting, marking, oiling, and sampling the final strip before it is coiled again onto the rewind reels (Fig. 11).

Fig. 11 Delivery section of a continuous tinning line
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