Coating activities – electroplating

The materials commonly used for electroplating steel strip because of their special characteristics are: tin, chromium, zinc, copper, lead and some of their alloys. Electrolytic reactions are described at the start of Chapter 2.

Chapter 2

86 September 2005 PT/EIPPCB/STM_BREF_FINAL Electrolytic cells for continuous steel coil

The cleaned and pickled strip is fed through electrolytic cells. The electrolytic cells are the heart of an electrolytic line. The choice, design and sizing of the other line components and supplying sections are dependent on the choice of the electrolytic cell and its components.

The main components of a continuous coil electrolytic cell are:

• conductor roll: this gives the steel strip a negative electrical charge. The voltage is transformed into current by means of a rectifier. The negative pole of the rectifier is connected to the carbon brushes of the conductor roll

• press roll: provides good contact and high electric conductivity between conductor roll and steel strip

• anode: repels the positive ions towards the steel strip (cathode). The positive pole of the rectifier is connected to the anode

• sink roll: turns the steel strip by 180°

• wringer rolls or squeeze rolls: minimise the drag-over (drag-in) into the next cell

• edge masks: prevents zinc edge overthrow (build-up of zinc preferentially at the edge of the coil where charge density is highest)

• conductor roll cleaning device: cleans the surface of the conductor roll to avoid surface defects on the steel strip.

The choice of an electrolytic cell depends on the industry applications the producer intends to supply, on the layer thickness they intend to deposit and on the capacity they intend to install.

Electrolytic cell type is a function of four main parameters:

• cell geometry

• current density

• electrolytic solution type

• anode type.

Cell geometry

The three main types of cell geometry are:

Vertical cell

The two strip sides may be coated simultaneously in one cell. On entry to the cell the strip runs from top to bottom, from the conductor roll through one pair of guiding rolls and the first anode pair down to the sink roll. From there it runs to the exit side, upwards through to the second electrode pair and wringer rolls and on to the next conductor roll.

Conventional cells are filled in with electrolyte and the sink roll and the two pairs of anodes are submerged in the electrolyte bath. In the Gravitel cell, the electrolyte enters via a weir in the narrow gap between the insoluble anode and the strip, holding only a small amount of electrolyte in contact with the strip. In this case, neither the anodes nor the sink roll are submerged in the electrolyte.

Chapter 2

PT/EIPPCB/STM_BREF_FINAL September 2005 87 Figure 2.12: Vertical cell

Radial cell

Only one side of the strip may be coated at a time in one cell. On entry to the cell, the strip runs from the top to the bottom, from the conductor roll through one pair of wringer rolls down to the sink roll. From there the strip runs to the exit side, upwards through the wringer rolls and on to the next conductor roll. Only the lowest part of the sink roll is plunged in the electrolyte bath.

In a variant, the carousel cell, the top rolls have the function of deflector rolls while the sink roll, equipped with a metallic winding, combines the functions of deflector roll and conductor roll.

Figure 2.13: Radial cell

Horizontal cell

Both strip sides may be coated simultaneously in one cell. The strip runs horizontally through the cells. At the entry to the cell, the strip leaves the conductor roll and runs through one pair of wringer rolls, then between a pair of anodes and from there to the exit side, onwards through a another pair of wringer rolls to the next conductor roll. The electrolyte is continuously injected between the two anode pairs, thereby only holding a small amount of electrolyte in contact with the strip at any one time.

Chapter 2

88 September 2005 PT/EIPPCB/STM_BREF_FINAL Figure 2.14: Horizontal cell

Current density

Installations normally working at low current density can be differentiated from those normally working at high current density. The current density will depend on the main industrial application, the normal metal thickness required and the normal steel substrate thickness. Table 2.1 shows thicknesses for zinc and zinc alloy coil coating applications. A high current density allows a thicker metal layer to be plated onto the steel substrate with a shorter anode length.

Current density (A/dm²⁾

Main industrial applications

Zinc layer thickness

(µm)

Steel thickness minimum

(mm)

Electrolyte relative speed

(m/sec)

60 to 120 Vehicle 5 to 12 0.5 1.0 to 4.0

30 to 90 White goods 2.5 to 3.5 0.3 <1.0

30 to 90 Others 2.5 to 3.5 0.3 <1.0

Table 2.1: Zinc and zinc alloy layer thickness as a function of industry application

High current density cells are equipped with systems such as electrolyte injection devices to realise a high relative electrolyte speed (electrolyte speed versus strip speed). These systems assure a sufficient supply of metal ions to the polarisation layer at the steel strip surface to carry the current.

Electrolyte bath

These are described for each process separately, see Sections 2.9.8, 2.9.9 and 2.9.10.

Anode type and gap

Two families of anodes are available: soluble anodes and insoluble anodes, see the introduction of Chapter 2, Electrolytic cells and reactions.

The gap between the anode and the steel strip differs as a function of the cell geometry and of the maximum steel strip width.

Chapter 2

PT/EIPPCB/STM_BREF_FINAL September 2005 89 Electrolytic cell Minimum gap

(mm)

Vertical 16 to 26

Vertical gravitel 7 to 14.5

Radial 7 to 15

Horizontal 10 to 20

Table 2.2: Gaps between anode and steel strip for different electrolytic cell types