Wet Cleaning Methods
Wet cleaning is a generic term that means aqueous solutions are used to prepare the surface of the strip. This term
incorporates both alkaline and acid processes. The wet cleaning section of a coating line has a relatively low initial cost,
compared with the cost of large, gas-fired furnaces, but requires a high degree of maintenance. In addition, the cost of the
chemicals and of the disposal of spent chemicals results in high operating expenses.
Alkaline Cleaning. In the dip operation of continuous strip coating lines, alkaline cleaning is used to remove protective
oils, soil, and the residues of rolling oils. Although it is possible to remove thin oxide layers using alkaline cleaning, these
layers are usually removed during subsequent pickling operations.
Alkaline cleaners are blends of various inorganic salts with small amounts of inhibitors and surfactants to promote
different cleaning mechanisms, such as saponification and emulsification.
Saponification refers to the chemical action whereby an oil is converted to a water-soluble soap. This reaction depends on
concentration, temperature, pH, and time of immersion. This mechanism will clean the majority of light rolling oils and
rust-preventative oils that are common on flat, rolled steel strip. Care must be taken to ensure that the soil level in the
cleaner does not become high enough to cause redeposition of the soil or to interfere with proper rinsing.
Emulsification is the process whereby surfactants penetrate soils and break them into sufficiently small particles to allow
dispersion and suspension in the cleaning solution.
The capabilities of the alkaline cleaner can be augmented by adding brushes to the continuous strip coating line in order
to mechanically loosen and remove surface soil. An abrasive material can be bonded to these brushes to enhance their
scouring or scrubbing action. The brushes are usually placed in the processing line with a steel roll on the opposite side of
the strip. This arrangement allows the contact pressure of the brush to be increased without deflecting the pass-line of the strip. The cleaner is often applied through the brush to provide cooling at the contact area and to act as a medium for
floating the loosened soil away and preventing its redeposition.
Electrolytic Cleaning. Cleaners can be designed to work with electric current. The strip can be either cathodic or
anodic. The recommended practice for continuous lines is either the anodic configuration or a switching current, where
the strip is switched between being positively and negatively charged. In anodic electrocleaning, the strip is positively
charged and oxygen is discharged at the strip surface to create a mechanical scrubbing action. One disadvantage of anodic
cleaning is the tendency to build up sludge on the electrodes. In cathodic electrocleaning, the strip is negatively charged,
producing the release of a much larger volume of hydrogen and better mechanical scrubbing at the strip surface. This
condition also increases the possibility of hydrogen being charged into the strip and the unwanted plating out of metallic
contaminants onto the strip surface, which can cause quality problems with the subsequent coating operation. Periodic
reversing, or switching, electrocleaning is a combination of both anodic and cathodic methods that produces a better
cleaning of the strip surface without excessive contamination, hydrogen charging, or electrode buildup.
Maintenance. The concentration of the cleaner must be maintained and any additives that tend to be used up at a faster
rate than the basic caustic components must be controlled in terms of proportion. The cleaner system should allow for the
periodic settling and removal of solid soils, especially metallic particles such as the iron fines that form during cold
rolling. Oil needs to be cleaned from the system by frequent or continuous skimming. Magnetic separators and other
devices are effective in removing soil burden and prolonging the useful life of the cleaner.
Chemical Pickling. Most pickling operations use an acid to dissolve metal oxides. The resulting reactions are affected
by concentration, temperature, and agitation. These reactions will naturally slow down as the acid solution becomes more
concentrated in the metal being dissolved.
Sulfuric acid is the most common acid used in pickling iron-base materials because of its low cost. After the acid has
removed the oxide scale, it will begin to attack the steel substrate. Therefore, inhibitors are commonly used to retard the
attack of the base metal without affecting oxide scale removal. As with most chemical reactions, an increase in
temperature increases the pickling rate. As the acid is consumed, the pickling time must be increased to achieve the same
level of descaling. In continuous coating lines, it is not possible to simply increase the pickling time to compensate for the
reduced effectiveness of the acid pickling. Pickling effectiveness is also decreased as iron builds up in the pickling bath.
Therefore, both acid and iron contents must be adjusted by replenishment.
Hydrochloric acid is an excellent pickling agent for the removal of oxides from steel strip. Although it is more
expensive and its fumes much more corrosive than sulfuric acid, it has a longer useful life, can be operated at lower
temperatures, and is not affected by the iron concentration to the same degree experienced in sulfuric acid pickling.
Electrolytic Pickling. Electrolytic action can be used to increase the speed of pickling, which is a very important
aspect of continuous coating lines (Fig. 2). The electric current can be either alternating or direct, and the strip can be
either anodic or cathodic. Cathodic pickling is faster than anodic, but this advantage decreases with increasing
concentration and bath temperature. Cathodic pickling charges the steel with hydrogen much faster than straight chemical
pickling does. In modern continuous coating lines, the current is often switched, which makes the strip cathodic and then
anodic.

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