Metal production ultimately requires exposure of an untreated surface to the corrosive oxygen and humidity in our atmosphere. To prevent surface corrosion, oil is frequently applied to the product prior to storage or delivery to seal the strip surface. Several oil application methods are available, from direct contact application to using electrostatic forces to direct the oil to the product surface. These two methods are described below and each presents its own benefits and drawbacks.
Rolling oil onto the surface provides the lowest up-front cost. The mechanics are minimal; in effect, a partially-submerged roller contacts the surface of the passing strip, which causes the roller to rotate into a pool of oil. Imagine a paint roller on a wall and you have a general principle. While simple, this style of oiling presents some obvious problems. First, the coating weight cannot be tightly controlled. At times, too much oil or too little oil is applied. Too little oil leads to uncoated portions of the strip, while too much oil leads to wastage and spillage which may further lead to personnel and fire hazards.
Electrostatic oiling provides a method to tightly control coating weight but requires a significant initial capital cost. Electrostatic oilers apply a high voltage direct current (HVDC) charge to the oil. Positive-displacement pumps deliver the oil to nozzles located above and/or below the product. The discharge point is electrified to several thousand volts causing the oil to atomize into fine droplets. The oil is attracted to and adheres to the grounded strip. Since each droplet is of identical polarity, each droplet repels its neighbor, preventing the formation of large droplets. The result is precise control of coating weight so long as the line speed is known. That is, as line speed increases pump speed increases. The high voltage power supplies will increase their current proportionally. The use of positive displacement pumps allows the electrostatic oiler to precisely meter the coating application rate (weight) in proportion to line speed.
The best method for your facility depends on several factors specific to your site. In most cases, the capital cost is the driver. It is important to consider potential coating wastage (more common with contact oilers) and the engendered hazards (HVDC, fire hazards and spillage) against up-front cost when considering which coating method fits your site.
