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Electrolytic Coating

U. S. Steel Electrolytic Zinc and Zinc-Iron Alloy Coated Sheet products are manufactured by the U. S. Steel-developed CAROSEL^® (Consumable Anode Radial One-Side Electrolytic) process. This technology was developed originally at U. S. Steel Gary Works in Indiana. Today, we manufacture these products at both Gary Works and at Double Eagle Steel Coating Company, Dearborn, Michigan.

Like in the hot-dip process, the CAROSEL^® Process involves the coating of steel sheet in a continuous coil processing line. The sheet is fed from a coil at the entry end of the line into an electrolytic cleaning unit, where a combination of fast-flowing alkaline solution and high-density electrical current remove oils, lubricants and other surface contaminants. Brushes in scrubber units remove any remaining soil. After hot water rinsing and drying by rubber-squeegee rolls and hot air, the sheet is pickled in hydrochloric acid, then pre-wet with plating solution prior to entry into the plating section.

In the CAROSEL^® process, the sheet passes around a series of large-diameter rubber/metal conductor rolls, each immersed in electrolyte solution in a plating cell. The coating thickness on the steel surface continually increases as the strip passes through each plating cell. Deposition of the coating is accomplished by the use of an electric current to deposit zinc (or in the case of the zinc-iron alloy coating, deposit zinc and iron) from the liquid solution onto the steel surface. Adhesion between the coating and the steel is achieved by an intimate atomic bonding between the zinc and steel. There is no metallurgical alloying reaction like that which occurs in the hot-dip process. To achieve good adhesion in the electrolytic coating process requires that the steel surface be very clean and oxide-free.

The final thickness of the coating is controlled by the total amount of electric current applied to each plating cell. Consumable zinc anodes are positioned close to the steel in the plating cells to maintain the required zinc content of the plating solution. After the steel passes through a series of plating cells and is uniformly coated on one side, it may be passed through a similar set of plating cells to coat the opposite surface. For GALVA-ONE^®, only one side is coated, and for GALVA-TWO^®, both sides are coated.

After the coating is applied and the steel has passed through all the plating cells, the sheet is thoroughly rinsed and dried. After the water is dried, the sheet is inspected very carefully (most applications for electrolytic coated steel sheet require a very high quality surface finish), and usually oiled to prevent staining during storage and transit. Alternatively, a prelubricant can be applied which aids in subsequent forming of the product. The prelubricant also acts to prevent staining during storage and transit.

One significant advantage of the CAROSEL^® process is the ability to optimize coating uniformity on each side of the sheet.  Another is its versatility in providing differential coating thicknesses to meet end-use requirements. Since the coating is applied to each side separately, it is very easy to produce differential thicknesses of  coating. This highly sophisticated process is monitored throughout each section of the coating line by a computerized network of controls to provide precise coating thicknesses to each side of the strip.